CHAPTER PREVIEW
Now it is time to dig into the nervous system and explore the cells that make up the nervous system, including their form and function.
In addition, we will explore diseases related to these cells.
In this chapter, we will . . .
■ Review the history of research regarding cells and neurons
■ Discuss the structure of molecules, cells, tissues, organs, and systems
■ Survey the different cells that make up the nervous system
■ Examine the form and function of neurons
■ Explore nervous system disorders that involve nervous system cells
LEARNING OBJECTIVES
■ The learner will define the following: molecule, cell, tissue, organ, and system.
■ The learner will list and briefly describe each nervous system cell.
■ The learner will accurately label the parts of a neuron and synapse.
■ The learner will list and briefly describe the steps in neuron function.
■ The learner will list and briefly describe select nervous system disorders involving nervous system cells.
■ Introduction
■ Historical Considerations
■ Cell Structure and Function
• Molecules
• The Cell
• Cells, Tissues, and Systems
■ An Overview of Nervous System Cells
• Neurons
• Glial Cells
■ Neuron Form and Function
• Neuron Form
• Important Aspects of Neuron Function
• The Firing Neuron: The Analogy of a Gun
• The All-or-None Principle
■ Select Disorders of Nervous System Cells
• Intellectual Disability
• Brain Tumors
• Amyotrophic Lateral Sclerosis
• Multiple Sclerosis
• Myasthenia Gravis
• Guillain-Barre Syndrome
■ Conclusion
■ Summary of Learning Objectives
■ Key Terms
■ Draw It to Know It
■ Questions for Deeper Reflection
■ Case Studies
■ Suggested Projects
■ References
► Introduction
Neurons and glial cells form the foundation for the nervous system. Historically, neuroscientists have been more interested in neurons than glial cells; however, a newfound respect has been found for glial cells through the work of R. Douglas Fields and others. Neurons are still vitally important, and their form and function will be the focus of this chapter along with various nervous system disorders that arise from neuron dysfunction. Before discussing neurons, we will briefly survey how we have come to know what we do about neurons. In addition, we will set the context for talking about neurons by talking about the structure and function of cells in general.
► Historical Considerations
The 18th and 19th centuries led to many advances in understanding the nervous system. The same is true about biology in general. Many of these advances were made possible by the invention of the microscope. Although microscopes had been around since the 16th century, Anton van Leeuwenhoek (1632-1723) is considered the father of microscopy. He made many biological discoveries through the use of microscopes, such as the detection of bacteria.
In the 1830s, Matthias Schleiden (a botanist) and Theodor Schwann (a zoologist) made observations using microscopy; from their conclusions, they proposed the cell theory. This theory states that all organic beings (humans, animals, and plants) are composed of individual cells (Wallace, King, & Sanders, 1986). As microscopists continued to examine cells, they found this theory to be true of all cells except one type—nervous system cells. These cells looked strange, with all the odd-looking projections around them. It seemed that the nervous system was the exception to the cell theory in that the fundamental building blocks were cells plus these projections. Because of limited techniques, scientists in the first half of the 19th century did not know that these projections were an actual part of nervous system cells.
In 1873, an Italian scientist named Camillo Golgi (1843-1926) developed a way to stain a whole neuron using silver nitrate (FIGURE 4-1). This method, the Golgi method, is still in use today. Through this method, Golgi could see the entire neuron, which included the cell body, dendrites, and axon (FIGURE 4-2). He also noticed something else—the neurons appeared connected to each other. This observation was problematic at first because it violated the cell theory, which stated that cells were individual units and not dependent on each other. Neurons appeared connected and dependent on each other, which Golgi described in his reticular theory.
FIGURE 4-1 Camillo Golgi.
FIGURE 4-2 A pyramidal neuron stained using the Golgi method.
FIGURE 4-3 Santiago Ramon y Cajal.
Around Golgi’s time, a Spanish physician and scientist named Santiago Ramon y Cajal (1852-1934) further refined Golgi’s method and discovered that neurons did not actually physically touch each other (FIGURE 4-3). There was a space between them, which modern microscopy has since confirmed. Cajal had upheld the cell theory, and his discovery gave rise to the neuron doctrine, the belief that each neuron is a separate cell and the fundamental building block of the nervous system.
During the latter half of the 19th century, other important discoveries were made about the nervous system. Motor and sensory neurons were discovered. Motor neurons take movement information from the brain to the body, and sensory neurons take sensory information from the body to the brain. In addition, in terms of the spinal cord and spinal nerves, it was found that sensory neurons enter the spinal cord dorsally through a dorsal nerve root and that motor neurons leave the spinal cord through a ventral nerve root.
In 1833, Johannes Muller (1801-1858) put forth the law of specific nerve energies. The law revolves around our sensory experience. It states that the origin of the sensation (e.g., visual or tactile) does not determine our sensory experience, but rather the pathway it is carried on. For example, whether you see the night sky or mechanically press your finger against your eye, your sensory experience is visual (i.e., seeing stars). This occurs because the optic tracts are visual tracts and connect to the visual centers of the brain. The whole system is visual, so sensory experience, no matter the stimulus (e.g., gently pressing the eye), is visual.
Cajal built further on the law of specific nerve energies and proposed that connections between neurons were not random, but rather were specific and predetermined. Cajal also proposed another law on the dynamic polarization of neurons. He theorized that information flows only one way through a neuron, beginning with dendrites, then through the cell body, and finally through the axon. In other words, dendrites always send information to the cell body, and axons always send information away from the cell body. This theory has been found to be generally true, but there are exceptions. For example, sometimes synapses are bidirectional (Jessell & Kandel, 1993).
A few final pieces of the nervous system picture need to be mentioned. Both involve Sir Charles Sherrington (1857-1952), an English scientist and Nobel Prize winner. In 1897, he described how the spaces between neurons functioned and called them synapses. He also described how our reflexes work.
This walk through history has given us the pieces we need to discuss the form and function of neurons. Before we do that, we will survey molecules and cells in general to give a context for neurons and other nervous system cells.
► Cell Structure and Function
Molecules
Molecules consist of two or more atoms held together by a chemical bond (McQuarrie & Rock, 1984). Cells are mostly composed of four families of molecules: simple sugars, fatty acids, amino acids, and nucleotides
(Alberts et al., 1983). We will briefly look at each of these molecules.
First, simple sugars are also known as carbohydrates. They can be small molecules (like table sugar) or chains of small sugar molecules (like glycogen). Sugars are stored until they are needed as energy to power cellular functions of small sugar molecules. Second, fatty acids, or lipids, are more commonly known as fats. These molecules are important in the cell’s architecture, making up the cell membrane. In the nervous system, lipids make up a large percentage of myelin, a substance that coats axons and speeds transmission of signals. Third, amino acids are organic compounds made up of amine (-NH2) and carboxylic acid (-COOH). In the brain, two amino acids—glutamate and gamma-aminobutyric acid (GABA)—are important excitatory and inhibitory neurotransmitters. Proteins consist of chains of amino acids. Some are structural in nature (globular proteins), whereas others act as enzymes, which facilitate chemical reactions. Structural proteins can be found in the cell membrane; they act as gates, allowing some materials in and out of the cell. Proteins that are enzymes perform a variety of chemical reactions, one being the breakdown of unneeded neurotransmitters in synapses. Finally, nucleotides (or nucleic acids) are large molecular chains in the cell’s nucleus. Two nucleotides of note are deoxyribonucleic acid (DNA), the genetic code for our bodies (FIGURE 4-4), and ribonucleic acid (RNA), the mechanism for building our body’s structure, including our nervous systems (Alberts et al., 1983).
FIGURE 4-4 Diagram of a DNA molecule.
The Cell
Cells are the fundamental units of an organism. The analogy of a city is helpful in understanding the different components of a cell (FIGURE4-5). In ancient times, cities had walls for protection, and the cell membrane acts as a protective wall for the cell. The cell membrane is a double-walled structure (bilipid membrane) made up of lipids and proteins that when bonded together are called lipoproteins (FIGURE 4-6). Proteins form the innermost and outermost walls of the membrane, and lipids fill the center. Globular proteins act as gates in the membrane that allow substances, like ions (small molecules with electric charges), into and out of the cell. The cell’s nucleus acts like the mayor of the cell. It contains DNA, which is the genetic code that regulates the maintenance of the cell and production of new cells. Inside the nucleus is the nucleolus, which contains RNA. The nucleolus can be thought of as the general contractor of the cell that directs the creation of proteins needed to build and repair the cell. The endoplasmic reticulum with its ribosomes then acts as a production plant for proteins needed by the cell. The mitochondria are the cell’s energy factories, where oxygen and sugars are metabolized by enzymes, and their energy powers the cell. The Golgi apparatus (named after Camillo Golgi) is the cell’s mail office that packages and sends sugars and proteins out of the cell. Lysosomes are the garbage collectors of the cell and use enzymes to break down and recycle used molecules. The centrosome is like the city planning department, directing the growth of the cell through cell division. Lastly, the cytoskeleton is like a city’s transportation system. It is made up of microtubules that transport molecules around the cell. These cell components are summarized in TABLE 4-1.
FIGURE 4-5 A cell.
Cells, Tissues, and Systems
In our bodies, cells are only one level of organization. When groups of similar cells come together to carry out certain functions, we call the product a tissue (e.g., muscle tissue, nervous tissue). When various tissues are brought together to carry out certain functions, we are now at the level of organs (e.g., heart, brain). As organs are grouped together to carry out certain functions, we have now reached the systems level. There are numerous systems in the human body (e.g., circulatory, digestive, reproductive), including the one under discussion in this text, the nervous system.
► An Overview of Nervous
System Cells
There are two basic types of nervous system cells in the human body, neurons and glial (or neuroglial) cells. TABLE 4-2 displays examples of different nervous system cells and their functions (FIGURE 4-7). No one really knows how many total cells (neurons and glial cells) make up the human nervous system. Many sources report that humans have 100 billion neurons and 1 trillion glial cells (Allen & Barres, 2009; Kandel, Schwartz, Jessell, Siegelbaum, & Hudspeth, 2013) but do so without reference to a source (Herculano- Houzel, 2009).
FIGURE 4-6 A bilipid membrane.
|
TABLE 4-1 Summary of Cell Components |
||
|
Cell Structure |
Cell Function |
City Analogy |
|
Membrane |
Serves as selectively permeable barrier/protector |
City walls |
|
Golgi apparatus |
Stores and delivers proteins |
Post office |
|
Cytoplasm |
Contents of cell (except nucleus) |
The city itself |
|
Mitochondria |
Produce energy for the cell |
Energy factory |
|
Nucleus |
Acts as genetic control center |
Mayor's office |
|
Nucleolus |
Produces ribosomes |
Construction (builds factories) |
|
Ribosomes |
Produce proteins |
Factory |
|
Centrioles |
Assist in cell division and microtubule formation |
Planning department |
|
Lysosomes |
Digest cell debris and bacteria |
Sanitation |
|
Microtubules |
Contribute to cell framework and movement of cell parts |
Infrastructure |
|
Endoplasmic reticulum (ER) |
Rough ER produces proteins; smooth ER produces fatty acids, calcium, and enzymes |
Factory |
|
TABLE 4-2 Some Cells of the Nervous System |
|||
|
Category |
Cell Types |
Location |
Examples of Functions |
|
Neuronal cells |
Neurons |
CNS and PNS |
Communicate within nervous system and with organs, muscles, and glands |
|
Glial cells |
Astrocytes |
CNS |
Maintain neural environment, repair/feed neurons, modulate neural transmission, modulate breathing |
|
Oligodendroglia |
CNS |
Produce myelin |
|
|
Microglia |
CNS |
Scavenge debris and defend against foreign substances |
|
|
Schwann cells |
PNS |
Produce myelin |
|
|
Satellite cells |
PNS |
Maintain neuronal environment |
|
Abbreviations:
CNS, central nervous system; PNS, peripheral nervous system.
Data from Brodal, P. (2010). The central nervous system (4th ed.). New York, NY: Oxford University Press, pp. 19-27; Snell, R. S. (2001). Clinical neuroanatomy for medical students (5th ed.).
Baltimore, MD: Lippincott Williams & Wilkins, pp. 34-66.
FIGURE 4-7 Cells of the central nervous system.
© Alila Medical Images/Shutterstock.
FIGURE 4-8 A fluorescent micrograph of a neuron in culture, showing neuronal cell body and the dendritic extensions.
Neurons
The most well-known nervous system cell is the neuron. A neuron (Greek for “nerve”) is a cell with specialized projections that transfers information throughout the body via an electrochemical process (FIGURE 4-8). Neurons are found in both the peripheral nervous system (PNS) and the central nervous system (CNS).
There are approximately 85 billion to 100 billion neurons in the human brain that range in size between 4 and 100 microns (1 micron is one-thousandth of a millimeter) (Herculano-Houzel, 2009).
Glial Cells
Glial (Greek for “glue”) cells were once thought to be simple support cells to neurons that anchored, nourished, insulated, and protected them. These cells were thought to play no role in the transmission of information throughout the nervous system. Research has challenged this assumption, showing that glial cells do modulate neurotransmission (Fields, 2009) and may even play a role in regulating basic life functions, like breathing (Gourine et al., 2010).
Astrocytes
Astrocytes (Greek for “star-shaped cell”) are starshaped cells that nourish neurons and help to maintain the neuronal environment (Kandel et al., 2013). Fields (2009) reports that astrocytes do more than first thought. In fact, they work with neurons to control the activity that occurs in synapses by regulating ions, neurotransmitters, and other molecules. Astrocytes also secrete substances that stimulate the formation of new synapses.
Oligodendroglia and Schwann Cells
Oligodendroglia (Greek for “few tree glue”) and Schwann cells (named after physiologist Theodor Schwann) both produce and coat axons with myelin. Oligodendroglia perform this function in the CNS;
Schwann cells are the PNS equivalent. Both coat only segments of the axon because the axon is not continuously coated. The uncoated areas, called nodes of Ranvier, help to facilitate the propagation of electrical signals down the axon due to the signal jumping from one node to the next. The thickness of the myelin coating is proportional to the thickness of the axon. Myelin is 70% lipid and 30% protein. Lipids act as insulators that keep the signal in the axon, and proteins provide structural stability for the myelin sheath (Kandel et al., 2013).
Microglia
Microglia (Greek for “small glue”) are CNS cells that are produced in the bone marrow. Their function is not well understood, but they are thought to defend nervous system structures by warding off foreign invaders. This role means that microglia are part of the immune system (Kandel et al., 2013). In addition, they rally to areas of damage in the brain or spinal cord, producing a scar tissue-like substance on injuries. This process is known as gliosis (Webster, 1999).
Satellite Cells
Satellite (Latin for “attendant”) cells are the astrocytes of the PNS that surround neurons, helping to nourish them. They also maintain the neuronal environment by taking up neurotransmitters, like GABA. Satellite cells also appear to respond to neuron injury, a similar function to astrocytes in the CNS (Hanani, 2010).
► Neuron Form and Function
Now that we have surveyed different nervous system cells, we will focus on neurons because they are important in neural transmission. We will first learn the form or structure of neurons and then turn our attention to how they work.
Neuron Form
Discussing neuron form involves two structures, the neuron itself and something called a synapse. We will look at the structure and types of neurons first and then explore the structure of a synapse. This will prepare us to consider neuron function since both structures are crucial to the sending of neural messages.
The Neuron
Neurons are unique cells in the human body, with their specialized projections and electrochemical communication process (FIGURE 4-9). Unlike other cells in the human body, neurons have projections conduct these signals away from the cell body. Dendrites and axons are also known as nerve fibers. Some dendrites have spines on them that are involved in chemical transmission of signals (Bear, Connors, & Paradiso, 2007). These spines come in different shapes and sizes and are critical pieces to forming cortical circuits (Arellano, Benavides, DeFelipe, & Yuste, 2007) (FIGURE4-11). Their shape also appears to isolate connections from each other, so there is no interference (Araya, Eisenthal, & Yuste, 2006; Araya, Jiang, Eisenthal, & Yuste, 2006).
FIGURE 4-9 A neuron.
BOX 4-1 What Is the Longest Axon in the World?
The longest axon in the world belongs to the giant squid (FIGURE 4-10) . This sea creature's axons run down the sides of its mantle and power its jet propulsion mechanism. The axons can be as long as 16 feet and as thick as a dime (about 16 mm). What about humans? What is the longest axon? This honor goes to the sciatic nerve, which runs from the end of the spinal cord in the lower back to the big toe. If pressure is applied to this nerve through a herniated vertebral disc or a narrowing of the vertebral column, a person may experience a condition called sciatica, in which pain radiates down the leg.
FIGURE4-10 A giant squid.
(e.g., axons) that can be quite long, sometimes up to several feet (see BOX 4-1) and also Nissl bodies where protein synthesis occurs. Neurons contain two basic parts—a cell body or soma (Greek for “body”) and projections called neurites. Neurites can be further divided into two main types—dendrites (Greek for “tree”), which receive signals and pass them toward the cell body, and axons (Greek for “axis”), which
FIGURE 4-11 Types of dendritic spines.
Axons also have unique features. As mentioned earlier, a fatty, white coating called myelin coats axons. This coating is made up of water, proteins, and lipids. Functionally, myelin not only keeps electrical signals in the axon but also speeds up the transmission of signals. An analogy for myelin is the rubber coating on electrical cords in our homes. Myelin is essential to a properly working nervous system; demyelinating diseases, like multiple sclerosis, can result in severe neurological deficits. The production of myelin is called myelination. In humans, this process begins during the 14th week of development and continues into adolescence. It can be observed behaviorally through the improved motor skills of babies. Oligodendroglia cells produce myelin in the CNS, whereas Schwann cells produce it in the PNS (Snell, 2001).
Neurons can be classified in several different ways; one of the most common ways is by the number of neurites the neuron has. Using this system, there are three basic types of neurons: unipolar, bipolar, and multipolar (FIGURE 4-12). Unipolar (or pseudopolar) neurons have a single projection that functions as an axon and comes off the cell body. These neurons are typically sensory in nature (e.g., touch, pain). Bipolar neurons have two projections, one dendritic and the other axonic. These neurons are located in structures devoted to special senses, like hearing, smell, and vision. Lastly, multipolar neurons are motor in nature and have multiple projections coming off the cell body, most of which are dendrites along with a single axon.
Neurons can also be classified based on their connections. Sensory neurons connect to sensory structures in the body. In contrast, motor neurons connect to body structures involved with movement, like muscles. Interneurons connect neurons together and transmit signals between them. Other classification schemes include classifying neurons based on dendrite shape, axon length, and neurotransmitter chemistry (Bear, Connors, & Paradiso, 2007).
The Synapse
Neurons connect with each other in order to pass signals to each other; these places of connection are called synapses (Greek, “to join together”) (FIGURE 4-13). The ends of axons house terminal buttons (or boutons). Within these buttons are vesicles or sacs that hold chemicals called neurotransmitters; when stimulated, the presynaptic membrane vesicles release the neurotransmitters, which pass through the synaptic space or cleft and connect to receptor sites on the postsynaptic membrane. The result is that the signal is passed from one neuron to the other neuron through this chemical process. After the signal is passed from one neuron to another, some of the neurotransmitters are reabsorbed by the presynaptic membrane (or a neighboring glial cell) and returned to the vesicles. Other neurotransmitters may be broken down by enzymes in the synaptic cleft.
There are three types of synapses (FIGURE 4-14). An axodendritic synapse involves the axon of one neuron connecting and sending a chemical signal to the dendrite of another neuron. These connections are usually excitatory in nature. Axons can also connect directly to the soma of the neuron. These connections are called axosomatic synapses, and they are usually inhibitory. The last type of synapse involves one axon connecting with another axon. This is called an axoaxonic synapse, and it is typically modulatory, meaning it regulates a signal (Seikel, King, & Drumright, 2010).
Neurotransmitters
As mentioned earlier, neurotransmitters are chemical messengers that transmit messages from a pre- synaptic membrane to a postsynaptic membrane through the synaptic cleft. There are over 100 known neurotransmitters, and many of these chemicals are crucial to functions that speech-language pathologists (SLPs) and audiologists care about, including speech, language, hearing, and swallowing.
FIGURE 4-12 Types of neurons.
© Alila Medical Images/Shutterstock.
In order to be a neurotransmitter, Purves (2004) states three conditions must be met:
■ It must be present in the presynaptic membrane.
■ It must be released in response to presynaptic depolarization.
■ There must be specific postsynaptic receptors to receive it.
In terms of structure, there are two basic categories of neurotransmitters: large molecule and small molecule. Large molecule neurotransmitters (neuropeptides) are considered large because they consist of 3 to 36 amino acids. In terms of effect, they are long lasting. Small molecule neurotransmitters (amines and amino acids) consist of single amino acids and are short lasting. It was once thought that each neuron held only one specific neurotransmitter, but now we know that a neuron can hold multiple types. Small molecules are held in synaptic vesicles on the presynaptic membrane, and large molecules are stored in secretory granules on the axon terminals.
FIGURE4-13 A synapse.
FIGURE4-14 Three types of synapses.
Functionally, neurotransmitters mediate transmission between neurons by exciting (starting), inhibiting (stopping), or modulating (regulating a signal) postsynaptic action potentials. Second, neurotransmitters modulate synaptic communication in small groups of neurons (neuromodulation). This means they sometimes help to control communication (e.g., make fast, make slower, keep going over a period of time).
The following list presents a brief overview of the most well-known neurotransmitters, which are also summarized in TABLE 4-3.
■ Acetylcholine (ACh): ACh was the first neurotransmitter to be discovered. It was first identified in 1951 by Henry Hallett Dale through his work on the nervous system’s relationship to the heart. It is a rapid-fire neurotransmitter of the PNS neuromuscular junction that causes muscle tissue to contract. It is also released at the neuromuscular junction between the vagus nerve and cardiac tissue. Neurons that release ACh are called cholinergic neurons. After being released at the neuromuscular junction, ACh’s action is stopped by an enzyme called acetylcholinesterase (AChE) that breaks it down so it can be reused. ACh has a restricted role in the CNS, being found only in the brainstem, the base of the forebrain, and the basal ganglia. It is thought to regulate CNS neuronal activity, especially alertness, attention, memory, and learning. The degeneration of cholinergic neurons in the CNS is thought to be behind the memory issues in Alzheimer disease (Bear, Connors, & Paradiso, 2007; Blumenfeld, 2010; Nolte, 2002; Purves, 2004).
|
Acetylcholine |
ACh |
Acetylcholine |
Excitatory: stimulates skeletal muscle contraction; involved in memory and learning Inhibitory: cardiac tissue |
Axon terminal |
Severe muscle spasms; difficulty in thinking and learning |
Cognitive issues, like memory loss; associated with dementia and Alzheimer disease |
Nicotine |
Curare |
|
Glutamate |
Glu |
Amino acid |
Excitatory: memory, learning, movement |
Axon terminal |
Possible epileptic seizures (with low GABA levels) |
Fatigue, cognitive impairment |
Theanine |
Alcohol |
|
Gamma- aminobutyric acid |
GABA |
Amino acid |
Inhibitory: blocks action of excitatory neurotransmitters; sleep-wake cycle |
Axon terminal |
Sleeping problems, impaired thinking |
Anxiety and other mood disorders |
Diazepam, alprazolam |
Benzodiazepines |
|
Dopamine |
DA |
Monoamine |
Inhibitory: indirect motor pathway Excitatory: direct motor pathway, heart rate, blood pressure, reward/ pleasure |
Axon terminal |
Psychosis, bipolar disorder (manic stage), schizophrenia, addictions |
Parkinson disease |
L-dopa, cocaine |
Chlorpromazine |
|
Epinephrine (adrenaline) |
E, Ad |
Monoamine |
Excitatory: fig ht- or-flight response (increases blood flow, heart rate, and blood sugar; dilates pupils) |
Axon terminal |
Anxiety |
Fatigue, poor concentration |
Ephedrine |
Alpha and beta blockers |
(continues)
Neuron Form and Function
|
TABLE 4-3 Some Important Neurotransmitters and Their Characteristics |
(continued) |
|||||||
|
Name |
Abbreviation |
Group |
Function |
Storage |
Surplus Effect |
Deficit Effect |
Agonist Drug |
Antagonist Drug |
|
Norepinephrine (noradrenaline) |
NE, NAd |
Monoamine |
Excitatory: fight- or-flight response (increases arousal; focuses attention; enhances memory; increases blood flow, heart rate, blood pressure, and blood sugar) |
Axon terminal |
Anxiety, panic attacks |
Depression, low energy levels, attentiondeficit/ hyperactivity disorder |
Caffeine, amphetamines |
Lithium |
|
Serotonin |
5-HT |
Monoamine |
Regulatory: mood, sleep, appetite |
Axon terminal |
Agitation, autism |
Depression, mood disorders |
Selective serotonin reuptake inhibitors |
Clozapine |
|
Substance P |
SP |
Peptide |
Excitatory: stimulates perception of pain; stimulates inflammation in response to injury |
Soma |
Inflammatory disorders (e.g., skin conditions like eczema, psoriasis) |
Decreased levels associated with Alzheimer disease, type 1 diabetes |
Hemokinin 1 |
Aprepitant |
Data from: Bl umenfeld, H. (2010). Neuroanatomy through clinical cases. Sunderland, MA: Sinauer Associates; Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S. A., & Hudspeth, A. J. (2013). Principles of neural science (5th ed.). New York, NY: McGraw-Hill; Guyton, A. C, & Hall, J. E. (2006). Textbook of medical physiology (11th ed.). Philadelphia, PA: Elsevier Saunders.
■ Glutamate: Glutamate is the ACh of the CNS. It is the major excitatory chemical of synaptic activity in the CNS and plays a role in synaptic plasticity. It is involved in both learning and memory. There is some evidence that imbalances in glutamate play a role in schizophrenia (Kandel et al., 2013). High levels of the chemical can lead to epileptic seizures in combination with low GABA levels.
■ Gamma-aminobutyric acid (GABA): GABA is the main inhibitory neurotransmitter of the CNS. It acts by binding to postsynaptic receptor sites, blocking the action of other neurotransmitters. Neurons that contain GABA are called GABAer- gic neurons. In addition to controlling information flow in the nervous system, GABA plays a role in the sleep-wake cycle (Blumenfeld, 2010). Low levels of GABA have been linked to depression and other mood disorders, and high levels are associated with insomnia.
■ Dopamine: Dopamine plays a role in motor control as well as our reward system. It is involved in three CNS pathways (FIGURE 4-15). The first is the mesostriatal pathway that begins in the substantia nigra and projects to the basal ganglia. Damage to this pathway can lead to movement disorders such as Parkinson disease. The second pathway is the mesolimbic pathway that originates from an area in the brainstem called the tegmentum and projects to the limbic system, which is our emotional system. This pathway is important in reward and addiction. Dysfunction in this pathway can lead to positive schizophrenic symptoms such as delusions or hallucinations. The third pathway is the mesocortical pathway, which also arises from the tegmentum (area of the brainstem), but projects to the prefrontal cortex. Impairment to this pathway leads to the negative symptoms of schizophrenia, like flat affect and emotion, lack of speech, and lack of motivation (Blumenfeld, 2010).
■ Epinephrine: Also known as adrenaline, epinephrine is involved in regulating heart rate, blood pressure, and breathing and in the fight-or-flight response. Epinephrine-containing neurons originate in the brainstem and project to the thalamus and hypothalamus (Purves, 2004). High levels are associated with anxiety; low levels are associated with fatigue and poor concentration.
■ Norepinephrine: Norepinephrine is also known as noradrenaline. Neurons containing this neurotransmitter arise out of the brainstem and project to the entire forebrain. Norepinephrine modulates attention, sleep-wake cycle, and mood and is involved in our fight-or-flight response. Impaired levels of it have been linked to attention- deficit/hyp er activity disorder, narcolepsy (sleep attacks), and mood disorders like depression, manic-depressive disorder, and anxiety (Blumenfeld, 2010).
FIGURE 4-15 The three dopaminergic projection systems.
■ Serotonin: Serotonergic neurons (i.e., neurons that contain serotonin) originate in the brainstem and project to the entire forebrain. They have both excitatory and inhibitory effects on the nervous system and regulate functions such as mood, sleep, and appetite. Low levels of this neurotransmitter relate to depression, anxiety, obsessive-compulsive disorders, and eating disorders. Serotonin also plays a role in arousal in the sleep-wake cycle, and thus a surplus can disrupt this cycle (Blumenfeld, 2010).
■ Substance P: The perception of pain is an important protective biological mechanism. For example, imagine how much damage we could do to our hand around a hot burner if we did not perceive pain. Substance P sensitizes us to pain and also causes inflammation at an injury site, which leads to healing (Kandel et al., 2013). Chronically increased levels of substance P can lead to inflammatory skin disorders, such as eczema, whereas low levels have been associated with Alzheimer disease and type 1 diabetes.
Neuron Damage and Repair
Neurons in the CNS or PNS are sometimes damaged through injury. This damage can occur through cutting or crushing an axon, a phenomenon known as an axotomy. When an axotomy occurs, the neuron enters an active degeneration process called Wallerian degeneration (FIGURE 4-16), named after British physiologist Augustus Waller (1856-1922). In this process, that part of the axon that is distal from the soma and anterograde from the injury degenerates and the axon terminal pulls away from the synapse. Microglia scavenge the debris left by the Wallerian degeneration. Axons in the CNS regenerate poorly because of a buildup of glial cells as scar tissue at the end of the damaged axon; axons in the PNS, in contrast, retain some ability to regenerate (FIGURE 4-17). Why is there a difference in regeneration properties between the two parts of the nervous system? It may be because the CNS needs stability once the brain is fully developed in order to be precise in its functioning, and to do so means giving up regeneration abilities (Kandel et al., 2013).
Important Aspects of Neuron Function
As stated earlier, neurons and glial cells are the primary cells of the nervous system, but we will be considering only neurons here. The main job of neurons is communication, which they do through tracts, or neural pathways. A neural pathway consists of a series of neurons connected together to make communication between the brain and the body possible. We will discuss specific neural pathways when we discuss the internal organization of the spinal cord as well as the motor speech system.
The communication between the brain and the body takes two basic forms. First there is efferent communication, which is top-down or descending communication through neural pathways from the brain to the body. Second there is afferent communication, which is bottom-up or ascending communication through neural pathways from the body to the brain. Overall, the communication of neurons in these pathways involves two phases: an electrical phase involving the dendrites, soma, and axon of the neuron and a chemical phase involving the synaptic cleft and neurotransmitters. Thus, the communication of neurons is said to be electrochemical in nature, a word that captures both of these phases. This being said, how does a neuron work?
FIGURE4-16 Injury effects on neurons. A. An intact, normal neuron. B. A neuron undergoing Wallerian degeneration after axonal damage.
FIGURE 4-17 Axon regeneration in the nervous system. A. Axon regeneration occurs in the peripheral nervous system. B. Axons in the central nervous system are more likely to undergo Wallerian degeneration.
FIGURE 4-18 Water tower illustration of gradients and transport (see text for details).
Neuron function is complicated, but we will briefly describe neuron function in enough detail for SLPs and audiologists to understand the basics of it. We will also explore how neuron dysfunction can lead to various neurological conditions, many of which affect speech and hearing.
Two concepts are important to understanding how a neuron works: transport and gradient (i.e., an imbalance). In regard to the first concept, transport, there are two types: (1) active transport, in which energy is used to move something from point A to point B, and (2) passive transport, in which no energy is used to move something from point A to point B. Both of these ideas can be illustrated in the example of a water tower (FIGURE 4-18). To get water from point A to point D, energy has to be expended (i.e., active transport) using a pump B because of gravity. However, to get the water in D to point C, one only has to open a valve and allow gravity to push the water down through the pipes. No energy is used in this second process (i.e., passive transport) (Seikel, King, & Drumright, 2010).
The second concept to understand is a gradient. A gradient is a sloping or imbalance of some sort. For example, a mountain road has a grade (or gradient) to it. Membranes, like the ones found in neurons, are semipermeable, meaning they allow some substances (in this case, ions) in and keep others out. Because of the neuron membrane’s semipermeable nature, gradients or imbalances of two types occur—concentration gradients and charge (or electrical) gradients (Seikel, King, & Drumright, 2010). These will be further explained in the next section.
The Firing Neuron: The Analogy of a Gun
Neuron function is like a gun. First, the gun must be loaded. Second, something must trigger the gun to fire. Third, the gun is reloaded in order to be used again.
Step 1: The Loaded Neuron (Polarization)
In a state of rest, the neuron is said to be polarized, meaning there are two imbalances. First, there is an imbalance of sodium (Na+), with a large concentration of it on the outside of the neuron membrane and a small concentration on the inside. The + following the symbol for sodium (Na) indicates we are talking about a sodium ion. Ions are atoms that have either gained or lost an electron, causing them to gain either a positive or a negative charge. Like a magnet, positive charges will repel other positive charges but will be attracted to negative charges. In addition to the Na+ imbalance, there is a large concentration of potassium (K+) on the inside of the membrane and a smaller amount on the outside. Again, the neuron’s semipermeable membrane is responsible for maintaining these imbalances. Second, there is an electrical or charge gradient, with the interior of the neuron being more negative (-70 mV) than outside the neuron. FIGURE 4-19 illustrates these two imbalances. The concentration and charge gradients put the neuron in a position of being ready to fire. All that is needed is the right circumstance for the trigger to be pulled and the neuron to fire.
Step 2a: The Chemical Firing of the Neuron (Chemical Transmission)
Chemical transmission occurs at the synapse between a neuron and another neuron’s dendrites, soma, or axon (or a muscle or gland). Neurotransmitters are released from the synaptic vesicles from the presynap- tic membrane. From here, they venture into the synaptic cleft and attach to receptors on the postsynaptic membrane. The neurotransmitter acts like a key and the receptor on the postsynaptic membrane is like a lock. When the key unlocks the lock, the action signal is transmitted to the receiving neuron, as illustrated in FIGURE4-20. Common neurotransmitters include ACh (muscle contraction) and dopamine (smoothness of muscle movement), which were discussed earlier.
FIGURE4-19 Two imbalances in a polarized neuron.
There are two types of postsynaptic receptors that recognize neurotransmitters (FIGURE 4-21). The first is known by its physiological name of ionotropic receptors. These receptors are also known by their anatomical name, ligand-gated ion channels. Their physiological name (ionotropic) means “ion feeding.” These receptors directly open and close ion gates and do so in a rapid manner. If the ion gates are opened, then ions quickly feed into the neuron, causing it to fire. If the ion gates are closed, then ions are quickly stopped, and the signal is stopped. Both GABA and glutamate use this type of receptor; glutamate opens ion channels and propagates signals, whereas GABA closes ion channels and stops signals. The second type of postsynaptic receptors is metabotropic receptors (physiological name meaning “feeding” or “nourishing” through metabolization), or G protein-linked receptors (anatomical name). When a neurotransmitter connects to one of these receptors, a molecule (G protein) in the postsynaptic cell is mobilized and either directly or indirectly, through a series of reactions, opens and closes ion channels. These receptors are slower than ionotropic receptors because they are less direct. Overall, the sending of a signal down the neuron is slower and can last anywhere from milliseconds to days. Dopamine is an example of a neurotransmitter that uses these kinds of receptors.
FIGURE 4-20 The lock and key system of postsynaptic receptors and neurotransmitters.
FIGURE 4-21 Two types of postsynaptic receptors.
A. An ionotropic receptor. B. A metabotropic receptor.
Step 2b: The Electrical Firing of the Neuron (Depolarization)
When neurotransmitters attach to receptor sites on the postsynaptic membrane, small molecular ion gates at the nodes of Ranvier open. (Note: The nodes of Ranvier are unmyelinated sections of neuron.) The opening of these gates causes sodium to rapidly and passively transport into the neuron. Previous gradients are equalized. This sudden change in polarity from the influx of positive sodium ions triggers an action potential, a rapid change in membrane polarity, which moves or propagates like a wave down the axon (Guyton & Hall, 2006). This process is called depolarization. Zemlin (1998) states that “every sensation we experience, every thought we have, every movement we execute is dependent upon the generation and transmission of electrical energy called an action potential” (p. 385). FIGURE 4-22 illustrates this process. It is at the nodes of Ranvier where depolarization occurs. The signal that has been created jumps from node to node instead of the whole membrane (i.e., myelinated portions) depolarizing. This process of jumping speeds the transmission of the signal down the axon (Guyton & Hall, 2006).
FIGURE 4-22 An action potential.
Step 3: The Reloading of the Neuron (Repolarization)
At this point, sodium gates close and a different set of molecular gates open that allow potassium ions to rush out of the neuron. Because these ions are also positively charged, their absence causes the interior of the axon to become negative again, stopping the depolarization process. The sodium ions are pumped more slowly outside of the neuron membrane by the sodium-potassium (Na+/K+) pump, which uses the hydrolysis of ATP to provide the needed energy (BOX 4-2) and is thus a form of active transport (FIGURE 4-23). This pump exports three sodium ions for every two potassium ions that are imported. The neuron is then put into a polarized state again (see Step 1) as the resting membrane potential is reestablished, and the neuron is ready to fire when needed.
FIGURE4-23 The sodium-potassium pump.
Modified from © Alila Medical Media/Shutterstock.
BOX 4-2 Adenosine Triphosphate and the Sodium-Potassium Pump
Adenosine triphosphate (ATP) releases energy when it is broken down by the enzyme adenosine triphosphatase (ATPase) (FIGURE 4-24). The by-product of this process is adenosine diphosphate (ADP) and a single phosphate, which recombine with an ADP molecule and form ATP again, making ATP a reusable molecule. ATPase is found in the plasma membrane of cells and is responsible for powering the sodium-potassium (NA+/K+) pump through this breakdown process, called ATP hydrolysis. This pump uses one ATP molecule for every three Na+ ions sent out of the neuron and every two K+ ions that are pumped into the neuron. By doing so, the NA+/K+ pump reestablishes the neuron’s resting membrane potential. Because the NA+/K+ pump uses ATP as energy, this pumping process is considered active rather than passive. ATP is synthesized through three processes: glycolysis in the cytoplasm of cells (FIGURE 4-25A), the citric acid cycle in a cell’s mitochondria (FIGURE 4-25B), and the electron transport chain that occurs through the inner mitochondrial membrane (FIGURE 4-25C).
(continues)
BOX 4-2 Adenosine Triphosphate and the Sodium—Potassium Pump
(continued)
FIGURE4-24 Adenosine triphosphate (ATP). ATP is the primary high-energy molecule produced in human cells. Bonds between the phosphate groups are hydrolyzed to liberate energy, which is applied to cellular processes.
FIGURE4-25A Glycolysis. The breakdown of one glucose molecule yields two pyruvate molecules, a net of two adenosine triphosphate (ATP) and two nicotinamide adenine dinucleotide (NADH) molecules. The two NADH molecules shuttle pairs of high-energy electrons to the electron transport chain for ATP production. Glycolytic reactions do not require oxygen, and some steps are irreversible.
FIGURE4-25B The citric acid cycle. This circular pathway accepts one acetyl coenzyme A (CoA) molecule and yields two carbon dioxide (CO2), three nicotinamide adenine dinucleotide (NADH), one flavin adenine dinucleotide hydroquinone (FADH2), and one guanosine triphosphate (GTP). (GTP is readily converted to adenosine triphosphate [ATP].) The electron shuttles NADH and FADH2 to carry high-energy electrons to the electron transport chain for ATP production.
(continues)
BOX 4-2 Adenosine Triphosphate and the Sodium—Potassium Pump
(continued)
FIGURE4-25C The electron transport chain pathway produces most of the adenosine triphosphate (ATP) available from glucose.
The All-or-None Principle
For the visually minded, the graphs shown in FIGURE 4-26 might help illustrate the whole process of neuron function in one picture. A threshold (around -55 mV) needs to be crossed in order for the neuron to fire (see the middle dotted line on the graph in Figure 4-26A[b]). When this threshold is reached, the neuron will depolarize and fire at a fixed strength. If the threshold is not met, the neuron will not fire. Thus, neurons function in an all-or-none manner, meaning either they fire or they do not, much like a light switch.
This is called the all-or-none principle.
FIGURE4-26 A. The development of the absolute and relative refractory periods. B. Summary graph showing the absolute and relative refractory periods.
In addition, there are two refractory periods. (The term refractory denotes the responsiveness of the neuron.) If another stimulus is given to the neuron during the absolute refractory period (1—2 milliseconds), nothing will happen because Na+ channels are inactivated. During the relative refractory period, the neuron will respond to another stimulus, but that stimulus must be stronger than normal due to Na+ channels still being in recovery mode. The whole refractory period lasts approximately 4 to 5 milliseconds but can last longer in some neurons.
► Select Disorders of Nervous System Cells
Intellectual Disability
What Is It?
Intellectual disability is currently the preferred term for what was once called mental retardation (FIGURE 4-27). The Individuals With Disabilities Education Act of 2004 (PL 108-446) defines intellectual disability as “significantly sub-average general intellectual functioning, existing concurrently with deficits in adaptive behavior and manifested during the developmental period, that adversely affects a child’s educational performance” (Individuals With Disabilities Education Improvement Act [IDEA] of 2004, PL 108-446).
Causes
Neurologically, what causes this “significantly subaverage general intellectual functioning”? Bear, Connors, and Paradiso (2007) report that intellectual disability is associated with a smaller number of dendritic spines as well as morphological differences. Specifically, the spines are long and thin instead of short and rounded (FIGURE 4-28). This anatomical difference can be caused by external environmental factors (e.g., maternal alcohol or drug consumption, mercury poisoning) or internal factors such as gene and chromosomal abnormalities (Shapiro & Batshaw, 2013).
FIGURE4-27 Three children with intellectual disabilities.
© kali9/Getty Images
FIGURE 4-28 Normal and abnormal dendrites.
Signs and Symptoms
People with intellectual disabilities have intellectual and adaptive functioning deficits. Intellectual deficits involve intelligence quotient (IQ) scores lower than 70 and issues with reasoning, problem solving, learning, and abstract language. Adaptive functioning refers to how well a person meets the common demands of daily life as well as how independent he or she is as compared to peers. Deficits in adaptive functioning can include not meeting developmental or social milestones as well as difficulty functioning in two or more of the following: communication, self-care, home living, social skills/participation, use of the community, self-direction, health, safety, academics, leisure activities, and work.
Assessment and Treatment
Assessment of people with suspected intellectual disability involves a psychologist’s assessment of intellectual functioning through an intelligence test. In addition, SLPs would assess both symbolic (e.g., words) and nonsymbolic (e.g., gestures) communication, play, social communication and interaction, receptive and expressive language skills, speech, and oral motor skills. An audiologist would assess hearing abilities.
Treatment by the SLP is indicated if a person’s language abilities lags behind his or her cognitive abilities. For example, a 12-year-old child with the cognitive level of a 5 year old but language abilities of a 3 year old would be a prime candidate for speech language therapy because there is a gap between cognition and language. However, if the child’s cognitive and language levels matched, then speech language treatment might not be indicated because the child would be doing as well as he or she could. For those who qualify for treatment, therapy would be individualized for each person, but could focus on treatment targets like turn-taking, social interaction through play, speech and language production, and/or literacy.
Brain Tumors
What Are They?
Brain tumors, also known as neoplasms (Greek for “new growth”), are abnormal growths of nervous system cells (FIGURE 4-29). Sometimes one of these cells will cease functioning as it should and will replicate it self, forming a mass of abnormal cells. The names of brain tumors are taken from the type of cell from which they originate. For example, astrocytomas develop from astrocytes, and neuromas develop from nerve tissue (TABLE 4-4). As the mass grows, it can impair brain function in various ways, usually through putting pressure on normal neural tissue (FIGURE 4-30).
FIGURE4-29 A primary brain tumor.
Courtesy of the National Library of Medicine.
Tumors of the brain can be benign (approximately 46% of cases) or malignant (approximately 54% of cases) (Centers for Disease Control and Prevention, 2004). Malignant tumors tend to grow quickly and spread to other parts of the body, but benign tumors are slow growing and do not spread. Both are dangerous and potentially lethal because they create increased pressure inside the skull. Brain tumors can originate in the brain (primary brain tumor) or somewhere else in the body (metastatic brain tumor).
Causes
The cause of these masses is unknown, but it is thought that at least one factor is tumor suppressor genes being turned off, thus allowing cells to go haywire and form masses (Newton, 1994). There is a long list of possible environmental factors, but none, other than ionizing radiation exposure from situations like radiotherapy during childhood, has been definitively linked to brain tumor development. Although epidemiological evidence suggests a connection between cell phone use and brain tumors, no definitive connection has been established at this time (Bondy et al., 2008; Khurana, Teo, Kundi, Hardell, & Carlberg, 2009; Peter, Linet, & Heineman, 1995).
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TABLE 4-4 Select Examples of Brain Tumors 1 |
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Name |
Description |
Where Arises From |
|
Schwannoma (or acoustic neuroma) |
Slow-growing tumor, typically on the 8th cranial nerve, but can affect other cranial or spinal nerves |
Sheath around nerve fibers |
|
Astrocytoma |
Fast- or slow-growing, invasive, cancerous tumor that rarely spreads outside the brain |
Astrocytes |
|
Chordoma |
Rare cancerous tumor that occurs at lower spine or base of skull; can metastasize |
Leftover cells from fetal development |
|
Craniopharyngioma |
Slow-growing tumor near base of skull and optic nerves |
Leftover cells from fetal development |
|
Ependymoma |
Slow- or fast-growing tumor located near or in the brain ventricles |
Ependymal cells that line areas with cerebrospinal fluid |
|
Medulloblastoma |
Cancerous tumor found in cerebellum or near brainstem; more common in children |
Cells from embryonic stage of development |
|
Meningioma |
Benign or cancerous; slow- or fast-growing tumor that compresses brain tissue as it grows, causing neurological symptoms |
Cells in the meninges |
|
Oligodendroglioma |
Benign or cancerous; slow- or fast-growing tumor found typically in frontal or temporal lobes |
Oligodendroglia |
Data from: Gould, D. J., & Brueckner-Collins, J. K. (2016). Neuroanatomy (5th ed.). Philadelphia, PA: Wolters Kluwer; Bhatnagar, S. C. (2013). Neuroscience for the study ofcommunicative disorders.
Philadelpia, PA: Lippincott Williams & Wilkins.
FIGURE4-30 A magnetic resonance imaging (MRI) scan of the brain showing a large tumor. © DeanAustinPhotography/iStock/Getty Images.
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TABLE 4-5 The World Health Organization's Grading System for CNS Tumors |
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Tumor Grade |
Description |
|
|
Low Grade |
Grade I tumor |
A benign, slow-growing tumor whose cells look almost normal under a microscope. Associated with long-term survival. Least likely to reoccur. |
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Grade II tumor |
A benign or malignant, slow-growing tumor whose cells look slightly abnormal under a microscope. Has some tendency to spread. Can reoccur as a higher grade tumor. |
|
|
Grade III tumor |
A malignant, fast-growing tumor whose cells look abnormal under a microscope. Has a tendency to spread and a tendency to reoccur as a higher grade tumor. |
|
|
High Grade |
Grade IV tumor |
A malignant, fast-growing tumor whose cells look very abnormal under a microscope. Tumor forms own vascular system to power its growth and invasiveness. Tumor has necrosis in its center. |
Data from: Eckley, M., & Wargo, K. A. (2010). A review of glioblastoma multiforme. U.S. Pharmacist, 35(5), 3-10.
Signs and Symptoms
Symptoms of brain tumors can include headache, seizures, personality changes, motor (e.g., paralysis) impairment, and sensory impairment (e.g., visual changes). Patients can also experience speech, language, hearing, and cognitive problems. Other problems include nausea, vomiting, fatigue, and drowsiness.
Diagnosis and Treatment
Tumors are diagnosed through consideration of the patient’s signs and symptoms and through neuroimaging studies such as computed tomography and magnetic resonance imaging. Part of the diagnostic process is grading the tumor with the World Health Organization’s grading system for CNS tumors (TABLE 4-5). Grading a tumor means determining how serious it is. Treatment typically involves tumor resection (i.e., surgery) for both benign and cancerous tumors and radiation, and/or chemotherapy for cancerous tumors (Weiner & Goetz, 2004). The treatment of choice depends on the tumor’s grade.
Amyotrophic Lateral Sclerosis
What Is It?
Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig disease, is the most well-known type of motor neuron disease (MND). Other MNDs include pseudobulbar palsy, primary lateral sclerosis, progressive muscular atrophy, and progressive bulbar palsy (TABLE 4-6). As the name implies, MND is a problem with motor neurons. In ALS, both the upper and the lower motor neurons wither, whereas in other MNDs either the lower or the upper motor neurons are affected (FIGURE 4-31). ALS typically has an onset between the ages of 40 and 60 years and is more common in men than in women. The condition affects about 3.9 out of 100,000 people (Mehta et al., 2014).
Causes
About 10% of ALS cases occur due to familial inheritance, but the remaining 90% of cases occur for unknown reasons (National Institute of Neurological Disorders and Stroke [NINDS], 2018). One recent study has implicated prions (misfolded proteins) as a possible cause of familial ALS (Grad et al., 2011).
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TABLE 4-6 Motor Neuron Disorders |
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|
Name |
Description |
|
Amyotrophic lateral sclerosis |
Progressive neurological disease of the upper and lower motor neurons |
|
Pseudobulbar palsy |
Progressive neurological disease of the upper motor neurons that has many of the symptoms of progressive bulbar palsy, a condition affecting the lower motor neurons |
|
Primary lateral sclerosis |
Progressive neurological disease of the upper motor neurons |
|
Primary muscular atrophy |
Progressive neurological disease of the lower motor neurons in the spinal cord |
|
Progressive bulbar palsy |
Progressive neurological disease of the lowest motor neurons in the brainstem |
Data from: National Institute of Neurological Disorders & Stroke. (2018). Motor neuron diseases fact sheet. Retrieved from https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education
/Fact-Sheets/Motor-Neuron-Diseases-Fact-Sheet
FIGURE4-31 Healthy neuron versus neuron with amyotrophic lateral sclerosis (ALS). Note how the ALS neuron is withered and smaller than the normal neuron.
Signs and Symptoms
ALS is characterized by progressive weakness that often begins in the hands, feet, or mouth. Most patients die from respiratory failure within 3 to 5 years, but a small percentage may live 10 or more years. In addition to progressive weakness, patients experience dysarthria that progresses to speechlessness, dysphagia, and dyspnea (i.e., breathing difficulty).
Diagnosis and Treatment
ALS is diagnosed through a combination of clinical presentation and ruling out other MNDs. There is no cure or effective treatment at this time (NINDS, 2018), but a drug, riluzole, has been used successfully in some patients. The drug slows the progression of symptoms and delays the need for ventilator support. It may add a few months to a person’s life, but not years (Carlesi et al., 2011).
Multiple Sclerosis
What Is It?
In multiple sclerosis (MS; “multiple scarring”), the myelin sheath around the axon is damaged, impairing the ability of neurons to communicate with other neurons and muscles. The condition has an onset between 20 and 50 years of age and affects 150 out of 100,000 people (Dilokthornsakul et al., 2016).
Causes
The cause of MS is the body’s own immune system, which attacks the myelin, resulting in progressive scarring of the brain’s white matter (FIGURE 4-32). Why does the body attack itself? The answer to that question is still unknown, though some have posited it has polygenic origins (i.e., from multiple genes) (Compston & Coles, 2002). MS is more common in women than in men, usually first appearing in the person’s 30s. It can appear in one of four different forms (FIGURE 4-33). The first form is relapsingremitting (RR) MS, in which the patient either has an attack with full recovery or has a full attack with some residual deficits. Second, primary progressive (PP) MS progresses steadily without recovery or remission. Third, secondary progressive (SP) MS begins like RR MS, but over time transitions into the steady decline of PP MS. Fourth, progressive-relapsing (PR) MS shows steady progression with ongoing attacks (Lublin & Reingold, 1996).
FIGURE4-32 Healthy neuron versus neuron with multiple sclerosis (MS). Note how the myelin has been depleted on the axon. The jagged lines below the picture show the relative firing strength of each neuron.
Modified from © Alila Medical Media/Shutterstock.
FIGURE4-33 The four types of multiple sclerosis.
Signs and Symptoms
Almost any symptom can be manifested because of MS. Patients report a “pins and needles” prickling sensation in the fingers and/or toes, numbness, and weakness. Both speech and swallowing problems can result. Patients also have issues with walking and balance. Once thought to be uncommon, hearing problems in patients with MS are now receiving greater attention. Lewis et al. (2010) reported that people with MS had poorer hearing thresholds than a control group (also see Saberi, Hatamian, Nemati, Banan, & Honarmand, 2012).
Diagnosis and Treatment
The diagnosis of MS is made through clinical presentation and neuroimaging. There is no cure for this disease, but there are medications, such as corticosteroids or interferon beta-1a, that can lessen inflammation and the symptoms of the condition (Compston & Coles, 2002). Rehabilitative therapy is also beneficial for speeding recovery after attacks.
Myasthenia Gravis
What Is It?
Myasthenia gravis (MG; “grave muscle weakness”) is a progressive autoimmune disease of the neuromuscular junction that affects women in their 30s and men in their 50s (Weiner & Goetz, 2004). The body’s antibodies block postsynaptic ACh receptors at the neuromuscular junction, resulting in muscle weakness and fatigue, which affects muscles involved with speaking and swallowing. One of the hallmark signs of the disorder is ptosis or droopy eyelids (Conti-Fine, Milani, & Kaminski, 2006).
Causes
In MG, the immune system malfunctions and sends antibodies that attack or block ACh receptors on the postsynaptic membrane. It is unknown why some people have an impaired immune system and what the specific antigen is, but genetics may play a role in developing the disease.
Signs and Symptoms
Two of the primary signs of MG are increasing weakness as muscles are used, but improvement of muscle function after rest, and drooping of both eyelids (ptosis). Patients may also complain of double vision (diplopia), dysarthria, and dysphagia.
Diagnosis and Treatment
MG is diagnosed through clinical presentation as well as the presence of ACh antibodies. Drug therapy, such as the use of steroids, can reduce the severity of symptoms. Thymectomy, which involves the removal of the thymus gland from the chest, has been a common surgical technique for the condition with mixed results (Weiner & Goetz, 2004).
Guillain-Barre Syndrome
What Is It?
Guillain-Barre syndrome (GBS), named after the French physicians who first described it, is a rapid, progressive demyelinating disease of the PNS. There are different types of GBS, but the most commonly occurring form involves the body having an autoimmune response to the Schwann cells. When damaged, these cells cannot lay down the myelin needed by the PNS. Paralysis usually begins in the feet and hands, and progresses to the trunk. The maximum impact of the disease happens at about 1 month and may affect the respiratory muscles. Patients will then experience partial or full recovery over the course of weeks to months.
Causes
The disease’s cause is unknown, but it is thought to be caused by an autoimmune response, botulism poisoning, or a viral infection (Kemp, Burns, & Brown, 2008; Weiner & Goetz, 2004). The thought is that one of these conditions can trigger an autoimmune response that results in GBS.
Signs and Symptoms
The primary sign of GBS is progressive weakness that begins in the feet and progresses to the upper body or weakness that begins in the face and hands and progresses to the lower body. Sensory issues also often appear in the form of a pins and needles sensation. As the weakness progresses, patients will have trouble walking and controlling their bowel and bladder functions. When the disease is at its worst, the patient will struggle with breathing and will typically need ventilator support.
Diagnosis and Treatment
GBS is difficult to identify in its early stages and is typically diagnosed through clinical presentation as the condition worsens. Because of GBS’s effect on the respiratory muscles, the disease is a medical emergency requiring prompt intervention to maintain respiratory support. The patient will rapidly lose the ability to speak and swallow, so the SLP must be involved to assess when the patient will need alternate means of communication and nutrition. The SLP should also be assessing the patient’s readiness for oral hydration and nutrition as he or she recovers from GBS. There are drug treatments that can shorten recovery times, but there is no cure for GBS at this time (Weiner & Goetz, 2004).
► Conclusion
Neurons are tiny cells, and a powerful microscope is required to see them. Even though they are small, damage to neurons can lead to serious neurological disorders, such as intellectual disability. In this chapter, the form and function of neurons have been reviewed as well as some diseases involving neurons. Many people with communication disorders suffer from these disorders; having a foundational understanding of neurons will help the SLP and audiologist have a better appreciation of people and their struggles with disorders caused by diseases of nervous system cells.
SUMMARY OF LEARNING OBJECTIVES
The following were the main learning objectives of this chapter. The information that should have been learned is below each learning objective.
1. The learner will define the following: molecule, cell, tissue, organ, and system.
• Molecule: two or more atoms held together by a chemical bond
• Cell: the fundamental unit of an organism
• Tissue: groups of similar cells that come together to carry out certain functions
• Organ: various tissues brought together to carry out certain functions
• System: organs grouped together to carry out certain functions
2. The learner will list and briefly describe each nervous system cell.
• Neuron: a cell with specialized projections that transfers information throughout the body via an electrochemical process
• Astrocytes: star-shaped cells that nourish neurons and help to maintain the neuronal 5. environment
• Oligodendroglia: cells that produce and coat CNS axons with myelin
• Schwann cells: cells that produce and coat PNS axons with myelin
• Microglia: cells that defend nervous system structures by warding off foreign invaders
• Satellite cells: the astrocytes of the PNS that surround neurons, helping to nourish them; also function in neurotransmitter uptake
3. The learner will accurately label the parts of a neuron and synapse.
• Refer to Figures4-9 and 4-13.
4. The learner will list and briefly describe the steps in neuron function.
• The loaded neuron (polarization): The neuron is in a polarized state due to chemical and electrical imbalances.
• The firing neuron (depolarization): Neurotransmitters are released into the synaptic cleft and fit into postsynaptic receptors. As a result, molecular gates open, allowing Na+ in and erasing the former imbalances. These actions cause an action potential to run down the neuron.
• The reloading of the neuron (repolarization): The molecular gates close, and through the sodiumpotassium pump, polarization is reestablished.
• The all-or-none principle: Neurons function in an all-or-none manner, meaning they either fire or they do not.
5. The learner will list and briefly describe select nervous system disorders involving nervous system cells.
• Intellectual disability: significantly subaverage general intellectual functioning, existing concurrently with deficits in adaptive behavior and manifested during the developmental period, that adversely affects a child’s educational performance
• Brain tumors: abnormal growths of nervous system cells
• Amyotrophic lateral sclerosis: a disease of the motor neurons leading to increasing weakness leading to eventual paralysis
Multiple sclerosis: when the myelin sheath around the axon is damaged due to an autoimmune response, impairing the ability of neurons to communicate with other neurons and muscles
• Myasthenia gravis: a progressive autoimmune disease of the neuromuscular junction that leads to increasing weakness
• Guillain-Barre syndrome: a rapid, progressive demyelinating disease of the PNS from which patients usually improve
KEY TERMS
Absolute refractory period Acetylcholine (ACh) Action potential Afferent communication All-or-none principle Amyotrophic lateral sclerosis (ALS)
Astrocytes Astrocytomas Axoaxonic synapse Axodendritic synapse Axons
Axosomatic synapse Cells
Cell theory Centrosome Cytoskeleton Dendrites Depolarization Dopamine Dynamic polarization of neurons
Efferent communication Endoplasmic reticulum Epinephrine
Gamma-aminobutyric acid (GABA)
Glial cells
Glutamate
Golgi apparatus
Gradient
Guillain-Barre syndrome (GBS)
Intellectual disability
Interneurons
Ionotropic receptors
Law of specific nerve energies
Lysosomes
Membrane
Metabotropic receptors
Metastatic brain tumor
Microglia
Microtubules
Mitochondria
Molecules
Motor neurons
Multiple sclerosis (MS)
Myasthenia gravis (MG) Myelin
Myelination
Neoplasms Neurites Neuromas Neuron Neuron doctrine Neurotransmitters Norepinephrine Nucleolus Nucleus Oligodendroglia Organs
Primary brain tumor
Relative refractory period Ribosomes
Satellite cells
Schwann cells
Sensory neurons Serotonin Soma
Substance P
Synapses
Systems
Tissue
Transport
Wallerian degeneration
DRAW IT TO KNOW IT
1. Draw a neuron (see Figure 4-9) and label the following parts: dendrite, cell body, axon, and axon terminals.
2. Draw a synapse (see Figure 4-13) and label the following parts: vesicles, presynaptic membrane, synaptic cleft, postsynaptic membrane, and receptors.
QUESTIONS FOR DEEPER REFLECTION
1. Write an essay describing neuron function.
2. Explain what the absolute and relative refractory periods are, and include graphs and other pictures to illustrate these concepts.
CASE STUDIES
Janet is a department secretary at a university. She has noticed over the last few months difficulty hearing faculty and students in her left ear. She compensates by turning her right ear to people when they speak to her. In addition to this left ear hearing loss, Janet has begun to experience a ringing in her left ear and some dizziness. Janet was referred to an audiologist by her primary care physician who diagnosed her with a high-frequency sensorineural hearing loss. The audiologist referred Janet to a neurologist who ordered an MRI. The MRI revealed a brain tumor on the eighth cranial nerve.
1. What type of brain tumor is the likely type in this case?
2. Is this type of brain tumor fast or slow growing?
3. What type of treatment might Janet receive for her brain tumor?
Mark is a 23-year-old graduate student who experienced flu-like symptoms for a week followed by a 1-month decline in his motor and sensory abilities. His respiratory function was even affected, requiring a ventilator. After this 1-month period, Mark began to recover his motor and sensory abilities. His physician believes he will make a full recovery. Which of the following conditions is the most likely? Explain why you chose this answer.
1. Multiple sclerosis
2. Guillain-Barre syndrome
3. Myasthenia gravis
4. Amyotrophic lateral sclerosis
SUGGESTED PROJECTS
1. Pick one of the neurotransmitters mentioned in this chapter and write a two- to three-page paper about what it is, what it does, and disorders associated with either too much or too little of it.
2. Search through scholarly journals and find a case study on one of the neurological disorders mentioned in this chapter. Present the case to your class.
3. Pick one of the neurological disorders mentioned in this chapter and write a two- to three- page paper with the following sections: cause, signs/symptoms, diagnosis, treatment, speech/ language/hearing issues.
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