Textbook of Clinical Neuroanatomy, 2 ed.

2. Organization and Functions of the Nervous System

Neuroanatomy is the study of the nervous system. The nervous system is the most complex, widely investigated and least understood system in the body. It along with endocrine system regulates the functions of all other systems of the body. Hence nervous system is also called master system of the body.

The functions of the nervous system include:

• Reception of sensory stimuli from internal and external environments.

• Integration of sensory information.

• Coordination and control of voluntary and involuntary activities of the body.

• Assimilation of experiences, a requisite to memory, learning and intelligence.

• Storage of experiences to establish pattern of responses in future, based on prior experience.

• Programming of basic instincts.*

N.B. The brain, “the divinest part of the body” is involved in much more than these functions such as thoughts and aspirations. The thought processes of brain have devised technology for making computer, launching rockets into space, etc.

The nervous system consists of three basic functional types of neurons: sensory, motor and interneurons. The sensory neurons detect stimuli and motor neurons send commands to the effector organs. The interneurons confer on the nervous system its prodigious capacity to analyse, integrate and store information.

The mechanism of functioning of the nervous system is as follows: The sensory stimuli (afferent impulses) received from inside or outside the body are correlated within the nervous system and then coordinated motor response (motor impulses) is sent to the effector organs (muscles, glands, etc.) so that they work harmoniously for the well-being of the individual (Flowchart 2.1).

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FLOWCHART 2.1 Mechanism of working of the nervous system.

Divisions of Nervous System

Anatomical

Anatomically the nervous system is divided into two parts, the central nervous system and the peripheral nervous system (Fig. 2.1).

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FIG 2.1 Anatomical divisions of the nervous system. The central nervous system consists of brain and spinal cord. The peripheral nervous system consists of cranial nerves which arise from the brain, and spinal nerves which arise from the spinal cord. (CP = cervical plexus, BP = brachial plexus, LP = lumbar plexus, SP = sacral plexus, CxP = coccygeal plexus, CN = cranial nerves.)

• The central nervous system (CNS) consists of brain and spinal cord. The brain is located within the cranial cavity and the spinal cord within the vertebral canal. The CNS is responsible for integrating, processing, and coordinating sensory data, and giving appropriate motor commands. It is also the seat of higher functions such as intelligence, memory, learning, and emotions.

• The peripheral nervous system (PNS) includes all the neural tissues outside the CNS, such as 12 pairs of cranial nerves, 31 pairs of spinal nerves, and ganglia associated with cranial and spinal nerves. The PNS provides sensory information to the CNS and carries its motor commands to the peripheral tissues and systems.

Functional

Functionally also the nervous system is divided into two parts, the afferent division and the efferent division (Fig. 2.2).

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FIG. 2.2 Functional subdivisions of the nervous system.

• The afferent division brings sensory information to the CNS.

• The efferent division carries motor commands to the muscles and glands.

The efferent division has somatic and visceral components constituting somatic and autonomic nervous systems, res pectively.

– The somatic nervous system (SNS) provides the voluntary control over the skeletal muscle contraction.

– The autonomic nervous system (ANS) innervates involuntary structures, such as heart, smooth muscle and glands and thus provides an involuntary regulation of smooth muscle, cardiac muscle, and glandular activity.

N.B. The afferent division brings sensory information to the CNS. The CNS interprets the sensory information and sends commands through the efferent division to produce a response.

Cellular Organization of the Nervous System

The highly specialized and complex nervous system consists of only two principal categories of cells, (a) neurons, and (b) neuroglia.

• Neurons form the basic structural and functional units of the nervous system. They are excitable cells which are specialized for reception of stimuli and the conduction of nerve impulses.

• Neuroglia or glial cells are supportive cells that support the neurons both structurally and functionally. The neu-roglia are five times more abundant than the neurons and account for more than half of the weight of the brain.

Neurons (Neuro, Nerve)

The neurons are the structural and functional units of the nervous system. They are specialized for reception, integration, interpretation and onward transmission of information. They conduct nerve impulses over long distances at great speeds. The nervous system consists of vast number (about 1012) of neurons.

Each neuron consists of a cell body (also called soma or perikaryon or nerve cell body) and its processes called neurites.

The typical neuron has a single long process called axon and many short processes called dendrites (Fig. 2.3).

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FIG. 2.3 A neuron. Note that Nissl substance is distributed throughout the cytoplasm of the cell body except in the region close to axon called axon hillock. It extends into the dendrites but is locking in the axon.

The axon does not branch freely except at its termination; although it does give off side branches as collaterals by which it establishes interconnections with the other neurons. The axon conducts impulses away from the cell body.

The dendrites receive stimuli and conduct nerve impulses to the nerve cell body. The dendrites often branch profusely and form a major part of the receptive area of the neuron.

N.B. The collections of nerve cell bodies within the CNS are called nuclei, and outside the CNS ganglia. The axons are generally referred to as nerve fibres.

Classification of neurons (types of neurons)

The neurons exhibit considerable diversity in form and function. Therefore, they are classified structurally as well as functionally.

Anatomical (morphological) classification

According to polarity (Fig. 2.4)

• Pseudounipolar neurons. These neurons possess oval or rounded cell body. A single process emerges from the cell body and after a short convoluted course bifurcates at a T-junction into peripheral and central processes. They are called pseudounipolar neurons because it is thought that the two processes of the bipolar neurons, during the process of differentiation, are approximated and finally fused near the cell body to form a single process. Thus, it appears that the neurons possess a single process bifurcating in a T-shaped manner, a short distance from the cell body. Such neurons are found in dorsal root ganglia of spinal nerves and sensory ganglia of some cranial nerves.

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FIG. 2.4 Three basic morphological types of neurons. The arrows indicate the usual direction of impulse transmission.

• Bipolar neurons. They possess spindle-shaped cell body, from each end of which a single neurite (process) emerges. Thus, bipolar neurons have two processes, one dendrite and one axon, with the soma between them. Such neurons are found in olfactory epithelium of nasal cavity, retina of eyeball and sensory ganglia of cochlear and vestibular nerves.

• Multipolar neurons. Have multipolar cell body from which emerges several dendrites and a single axon. Most of the neurons in the body especially those in CNS belong to this category. For example, all the motor neurons that control skeletal muscles are multipolar neurons.

In fact multipolar neurons make up almost entire neuronal population of the CNS. Due to presence of several dendrites and their elaborate primary and secondary dendritic branches, these neurons enormously increase their synaptic surfaces.

N.B. In addition to three main morphological types of neurons (vide supra), there are unipolar neurons, which are found only in the mesencephalic nucleus of the Vth cranial nerve.

According to relative lengths of axons and dendrites

• Golgi type I neurons. These neurons have long axons that may be one metre long in extreme cases and connect different parts of the nervous system. The axons of these neurons form the long fibre tracts of the brain and spinal cord, and the nerve fibres of the peripheral nerves. The pyramidal cells of the cerebral cortex, Purkinje cells of the cerebellum and motor anterior horn cells of spinal cord are Golgi type I neurons. Their dendrites are short and numerous.

• Golgi type II neurons (microneurons). Axons of these neurons are morphologically similar to that of dendrites. This gives these cells a star-shaped appearance. They establish synaptic contacts with large number of neurons in their neighbourhood.

They are found in large numbers in cerebral cortex, cere-bellar cortex and in the retina.

Table 2.1 summarizes the morphological (anatomical) classification of neurons.

Table 2.1

Morphological classification of neurons

Morphology

Location and example

According to polarity

 

• Unipolar/pseudounipolar

Posterior root ganglia of spinal nerves, sensory ganglia of cranial nerves

• Bipolar

Olfactory epithelium, retina, sensory ganglia of cochlear and vestibular nerves

• Multipolar

Central nervous system (motor cells forming fibre tracts of brain and spinal cord and peripheral nerves), autonomic ganglia

According to size of nerve fibre

 

• Golgi type I

Pyramidal cells of cerebral cortex, Purkinje cells of cerebellum, anterior horn cells of spinal cord

• Golgi type II

Cerebral cortex, cerebellar cortex (stellate cells forming synaptic contacts with other neighbouring neurons)

Functional classification

• Sensory neurons

They carry impulses from the receptor organs to the CNS.

Types of sensory neurons

In relation to the general sensory pathways, they are classified into three types:

1. Primary sensory neurons: The cell bodies of these neurons lie outside the CNS except those of mesencephalic nucleus of fifth cranial nerve which lie within the CNS.

2. Secondary sensory neurons: The cell bodies of these neurons lie in the CNS.

3. Tertiary sensory neurons: The cell bodies of these neurons lie in the thalamus. For details seeChapter 17.

• Motor neurons

They transmit impulses from the CNS to the muscles and glands. The cell bodies of these neurons lie within the CNS except those of postganglionic neurons of autonomic nervous system.

Types of motor neurons

In the somatic nervous system they are divided into two types:

1. Upper motor neurons have their cell bodies located in the cerebral hemisphere, viz. motor area of the cerebral cortex. They form the descending pathways of the brain and synapse with the motor neurons of the cranial nerve nuclei in the brainstem and motor neurons of the spinal nerves in the anterior horns of the spinal cord. The upper motor neurons are involved in the voluntary control of muscular activity.

2. Lower motor neurons have their cell bodies located in the brainstem and spinal cord.

The skeletal muscles are supplied by the motor neurons of the anterior horns in the spinal cord and in the motor nuclei of cranial nerves. These neurons form the final common pathway (Sherrington) for determining the muscle action and are collectively known as lower motor neurons.

In the autonomic nervous system also the motor neurons are divided into two types:

1. Preganglionic neurons:The cell bodies of these neurons lie in the brain and spinal cord.

2. Postganglionic neurons:The cell bodies of these neurons lie outside the CNS in lateral, collateral and terminal autonomic ganglia.

The common anatomical terms used for describing the nervous system are mentioned in Table 2.2.

Table 2.2

Terms commonly used for describing nervous system

Terms

Definition

Nerve fibre

Axon

Nerve

Bundle of nerve fibres outside the CNS

Tract

Bundle of nerve fibres inside the CNS

Ganglion

Collection of nerve cell bodies outside the CNS

Nucleus

Collection of nerve cell bodies inside the CNS

Sensory neuron

Neuron that transmits impulses from a sensory receptor to the CNS

Motor neuron

Neuron that transmits impulses from the CNS to the effector organ, e.g. muscle

Somatic motor nerve

Nerve that stimulates contraction of skeletal muscles

Autonomic motor nerve

Nerve that stimulates contraction/inhibition of smooth and cardiac muscles; and that stimulates secretion of glands

Nerve plexus

Network of intercalated nerves

CNS = central nervous system.

Fine structure of a typical neuron (Fig. 2.5)

A typical neuron consists of three principal components: (a) a cell body, (b) dendrites, and (c) an axon.

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FIG. 2.5 Fine structure of a neuron. Note that the cytoplasm of the body is rich in rough and smooth endoplasmic reticulum and contains following organelles and inclusions: (a) Nissl substance, (b) Golgi apparatus, (c) mitochondria, (d) neurotubules, (e) neurofilaments, (f) lysosome, (g) centrioles, and (h) lipo-fuchsin, and melanin, glycogen and lipid.

• The cell body is an enlarged portion of the neuron. It consists of a mass of cytoplasm, surrounded by a plasma membrane. The cytoplasm contains a single relatively large and centrally located nucleus with prominent nucleolus.

The two main characteristic features of the cytoplasm of a neuron are: (a) the presence of Nissl substance (also called Nissl bodies or granules), and (b) neurofibrils.

The Nissl substance is composed of large aggregations of rough endoplasmic reticulum. The high concentration of rough endoplasmic reticulum is thought to be necessary for the production of enzymes involved in neurotransmitter synthesis. The Nissl substance extends into the dendrites but are absent in axon hillock and axon.

The neurofibrils represent the microfilaments and microtubules of the other cells of the body.

The electron microscopy reveals the presence of neuro-tubules and neurofilaments in the cytoplasm of a neuron. The neurotubules are made up of protein tubulin and course through the cell body into the neurites. These are concerned with the transport of large molecules along the neurites in either direction.

N.B. The centrosomes (centrioles) usually a feature of dividing cells has been observed in mature neurons incapable of division. They are possibly associated with the formation or maintenance of neurotubules.

• The dendrites are highly branched short tapering processes which either end in the specialized sensory receptors as in primary sensory neurons, or form synapses with neighbouring neurons from which they receive stimuli. In some neurons the smaller processes of dendrites bear numerous minute projections called dendritic spines or gemmules. The dendrites conduct the nerve impulse towards the cell body—the law of forward conduction or the law of dynamic polarity.

• The axon arises from a cone-shaped portion of the cell body called axon hillock. The axon extends as a cylindrical process of uniform diameter of variable length terminating on other neurons or effector organs by a variable number of small branches the telodendria which end in small swellings called terminal boutons or presynaptic terminals (Fig. 2.3).

The plasma membrane (plasmalemma) forms the continuous external boundary of the cell body and its processes.

In the neuron it is the site for the initiation and conduction of the nerve impulse.

The plasmalemma bounding the axon is called axo-lemma. The cytoplasm of the axon is called axoplasm.

N.B. The initial segment of axon (50-100 jxm) after it leaves the axon hillock is the most excitable part of the axon and is the site at which an action potential originates.

Axon transport

The axon transports substances in both the directions in its axoplasm, i.e. away from the cell body, called orthograde transport (anterograde flow), and towards the cell body, called retrograde transport (retrograde flow). Thus, substances produced in the nerve cell body having many of the characteristics of a secretory cell can be passed along the axon to the area or tissue which it innervates, for example dopamine produced in the substantia nigra of midbrain is transported to the corpus striatum by nigros-triate fibres. Similarly, the materials absorbed from extracellular fluid by the axon terminals (by pinocytosis) can be transported to the cell body. This explains how the cell bodies of neurons respond to changes in the distal ends of the axons—a mechanism which may control the activity of nerve cell in relation to that of tissue which it innervates.

N.B. It is an amazing feat of biological engineering that different substances can move in different directions and at different rates through a very-very narrow tube—the axon.

Neuroglia

The neuroglia are the interstitial or supporting cells of the nervous system. They do not contribute to the propagation of impulses or the processing of the perceived information but support the neurons both structurally and functionally.

Neuroglia in the central nervous system

There are four main types of neuroglia (glial cells) in the CNS: (a) astrocytes, (b) ependymal cells, (c) oligodendrocytes, and (d) microglia (Fig. 2.6A).

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FIG. 2.6 (A) Four types of neuroglia found in the central nervous system. (B) The perivascular feet of astrocytes forming a sleeve around a capillary.

• Astrocytes are the largest and most numerous, and form the main supporting tissue of the nervous system. They are star-shaped as the name implies and possess many fine dendrite-like processes. At the ends of processes there are small swellings called foot-processes.

Astrocytes are of two types: protoplasmic astrocytes and fibrous astrocytes.

– The protoplasmic astrocytes are found in the grey matter. Their processes are thicker and more branched than fibrous astrocytes.

– The fibrous astrocytes are found mainly in the white matter. Their processes are long, slender, smooth and less branched. Further, they contain more filaments in their cytoplasm as compared to the processes of protoplasmic astrocytes.

The astrocytes fill up most of the extracellular spaces among the neurons and their processes contact the surfaces of neurons and capillaries of the CNS. They are involved in the exchange of metabolites between the neurons and capillaries. The astrocytes are thought to be primary glycogen storehouse in the brain.

Astrocytes are found in large numbers adjacent to the blood capillaries with their foot processes, perivascular feet forming a sleeve around them (Fig. 2.6B). Thus, blood is separated from neurons by the capillary wall (endothe-lial cells) and a layer of astrocytic foot processes, which together constitute the blood-brain barrier (BBB) (for details seepage 183).

Because of blood-brain barrier, only certain substances can pass from blood into the neurons, hence protecting them from toxic substances in the blood.

The astrocytes thus help regulate the passage of molecules from the blood to the capillaries.

• Ependymal cells line the ventricles of the brain and central canal of the spinal cord. Ependymal cells are of three types: (a) ependymocytes, (b) choroid epithelial cells, and (c) tanycytes. The ependymocytes are cuboidal or columnar in shape with tuft of cilia on their luminal surfaces and constitute the majority of the ependymal cells. The specialized ependymal cells in choroid plexuses (choroidal epithelial cells) secrete cerebrospinal fluid. The cilia of ependymal cells assist in moving cerebrospinal fluid through the cavities of the brain.

The ependymal cells lining the floor of the fourth ventricle have long basal processes are termed ‘tanycytes’.

• Oligodendrocytes are smaller than astrocytes and as the name implies have fewer processes. They are found (a) in clusters around the neurons of grey matter, and (b) adjacent to and along the length of myelinated nerve fibres in the white matter.

Oligodendrocytes form myelin sheath around axons in the CNS, having same function as Schwann cells in peripheral nervous system. A Schwann cell forms myelin sheath around a portion of one axon only whereas an oligodendrocyte, through its processes myelinates portions of several axons.

• Microglias are the smallest of the glial cells, and are capable of migrating through the surrounding neural tissue. Microglia do not develop in the neural tissue. They are derived from phagocytic white blood cells (fetal monocytes) that migrate from the blood into the nervous system before birth.

The microglia enlarges and become phagocytic in areas of inflammation and cell destruction. They remove cell debris, wastes and pathogens that invade the CNS by phagocytosis.

N.B. All the neuroglia (glial cells) are derived from ectoderm except microglia, which are derived from mesoderm.

Clinical Correlation

• Following death of the neurons the astrocytes proliferate and fill the spaces previously occupied by the neurons. This process is called replacement gliosis.

• The ‘glioblastoma multiforme’, the most fatal tumour of brain with life expectancy of only 2 or 3 months, arises from astrocytes.

• Numerous microglia migrate to the areas of CNS that are damaged by infection, trauma or stroke to phagocytose the necrotic tissue. A pathologist therefore can identify these damaged areas of CNS during an autopsy, as large number of microglia is found in them.

Neuroglia in the peripheral nervous system

There are two types of glial cells in the PNS: satellite cells, and Schwann cells.

• Satellite cells or amphicytes, surround the nerve cell bodies in peripheral ganglia and provide support and nutrition to them.

• Schwann cells or neurolemmocytes form myelin sheath around axons in the peripheral nervous system. It is important to note that Schwann cells form neurilemma around all axons in PNS whether they are unmyelinated or myelinated.

N.B. Both neurilemma and myelin sheath are components of Schwann cells.

The types and functions of glial cells are summarized in Table 2.3.

Table 2.3

Glial cells in the central nervous system and peripheral nervous system

Cell type

Functions

Central nervous system

 

• Astrocytes

– Maintain blood-brain barrier, regulate ion, nutrient, and dissolved gas concentrations

 

– Form scar tissue after injury

• Oligodendrocytes

Form myelin around CNS axons

• Microglia

Remove cellular debris, and pathogens in CNS by phagocytosis

• Ependymal cells

Line ventricles of the brain and central canal of the spinal cord. Assist in production, circulation and monitoring of cerebrospinal fluid

Peripheral nervous system

 

• Satellite cells

Surround nerve cell bodies in peripheral ganglia

• Schwann cells

– Surround all axons in PNS

 

– Responsible for myelination of axons in PNS

 

– Participate in repair process after injury

Synaptic Transmission

In the nervous system, information moves from one location to another in the form of action potentials. An action potential travelling along an axon is called nerve impulse.

The nerve impulse is akin to a tiny electrical charge and forms the physiological unit of the nervous system.

Synapse

There is always more than one neuron involved in the transmission of a nerve impulse from its origin to its destination, whether it is sensory or motor. The neurons form long chains along which the impulses are conducted.

N.B. All the neuroglia are derived from ectoderm like neurons except microglia which are derived from mesoderm.

The point at which the nerve impulse passes from one neuron to another is called synapse.

It is important to know that at synapse, the contact between the neurons is by contiguity and not by continuity (neuron theory of Waldeyer, 1891), and the impulse is transmitted across a synapse by a specific neurotransmitter.

The synaptic communication is the process by which neurons communicate among themselves and with the muscles and glands.

Classification of synapses

Depending upon the parts of two neurons forming a synapse, the synapses are of the following types:

1. Axodendritic: synapse between an axon and a dendrite.

2. Axosomatic: synapse between an axon and a soma.

3. Axoaxonal: synapse between two axons.

4. Somatodendritic: synapse between a soma and a dendrite.

5. Somatosomatic: synapse between two somas and soma.

6. Dendrodendritic: synapse between two dendrites.

N.B. The most common synapse is between an axon of one neuron and the soma or dendrite of another neuron (i.e. axosomatic or axodendritic). The axodendritic synapse is generally termed typical synapse.

Structure of a synapse

Figure 2.7 presents the structure of an axodendritic synapse.

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FIG. 2.7 (A) The structure of a typical synapse as seen under electron microscope. (B) The synaptic transmission. The neurotransmitter diffuses from the presynaptic terminal across the synaptic cleft to the receptors on the postsynaptic membrane.

At its free end the axon breaks up into minute branches which terminate in small swellings called presynaptic knobs or boutons. They lie in close proximity to the dendrites of the other neurons. The region of dendrite receiving the axon terminal is called postsynaptic process. The membrane opposed to the presynaptic knob is called postsynaptic membrane. The space between presynaptic knob and post-synaptic membrane is termed synaptic cleft, which is about 20 nm wide.

Thus, the essential anatomical components of a synapse are: the presynaptic knob, the synaptic cleft, and the post-synaptic membrane (Fig. 2.7A).

The granular material or delicate fibres may be seen within the synaptic cleft. On either side of the cleft there is a region of dense cytoplasm. On the presynaptic side the dense cytoplasm is broken up into several bits, whereas on the postsyn-aptic side the dense cytoplasm is continuous and is associated with a meshwork of filaments called synaptic web.

The thickened areas on the pre- and postsynaptic membranes constitute the active zone/zones of synapse for neurotransmission.

When the synaptic web is thick, the synapse is called asymmetrical, and when it is thin, the synapse is called symmetrical.

In most locations the inhibitory synapses are symmetrical and the excitatory synapses are asymmetrical.

Within the presynaptic knob are synaptic vesicles containing chemical transmitter called neurotransmitter which carry nerve impulses across the synaptic cleft. The neu-rotransmitter is secreted by nerve cells, actively transported along axon and stored in synaptic vesicles. Synaptic knob in addition to vesicles, contains endoplasmic reticulum and mitochondria.

The postsynaptic membrane contains the receptors for the neurotransmitter.

Mechanism of transmission of nerve impulse

Arrival of nerve impulse at terminal knob causes release of neurotransmitter into the synaptic cleft, which binds with receptors on the postsynaptic membrane. This binding produces response in the postsynaptic membrane, in the form of depolarization or hyperpolarization. The excitatory nerve impulse causes depolarization of postsynaptic membrane while an inhibitory impulse causes its hyperpolarization. If depolarization reaches threshold, an action potential is produced in the synaptic neuron. In this way, action potentials are transferred from one neuron to another neuron.

Neurotransmitters

Mostly the synaptic transmission is carried out by a chemical substance called neurotransmitters. The neurotransmitters produce either depolarization or hyperpolarization of post-synaptic membrane and their effects are termed excitatory or inhibitory respectively.

There are a number of neurotransmitters (Table 2.4) but acetylcholine (ACh) and noradrenaline (epinephrine) are the main ones.

Table 2.4

Neurotransmitters and their effects

Neurotransmitter

Typical effects

Acetylcholine

Excitatory

Amines

 

• Noradrenaline

Excitatory

• Adrenaline

Excitatory

• Serotonin

Inhibitory

• Dopamine (DOPA)

Inhibitory

• Histamine

Inhibitory

Amino acids

 

• Gamma-aminobutyric acid (GABA)

Inhibitory

• Glycine

Inhibitory

• Glutamate

Excitatory

• Aspartate

Excitatory

Neuropeptides

 

• Substance P

Excitatory

• Endorphins

Inhibitory

• Enkephalins

Inhibitory

The synapses releasing the acetylcholine are known as the cholinergic synapses and those releasing the noradrena-line, the adrenergic synapses.

The sequence of events at a typical cholinergic synapse is as follows:

• An arriving action potential depolarizes the presynaptic membrane.

• The calcium (Ca2+) ion channels open and the calcium ions enter the cytoplasm of synaptic knob.

• The calcium ions cause synaptic vesicles to fuse with the presyn-aptic membrane and release their content (ACh) into synaptic cleft through exocytosis.

• ACh diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.

• The sodium (Na+) ion channels on the postsynaptic membrane open and sodium ions enter the cytoplasms of postsynaptic cell causing its depolarization.

• ACh release ceases because calcium ions are removed from the cytoplasm of synaptic knob.

• The depolarization ends as ACh is broken down into acetate and choline by an enzyme, acetylcholine esterase (AChE).

• The synaptic knob reabsorbs choline from the synaptic cleft and uses it to resynthesize ACh.

N.B. Adrenaline usually has an excitatory depolarizing effect on postsynaptic membrane, but the mechanism is quite distinct from that of ACh. For details consult textbooks on neurophysiology.

Properties of synapse

• The nerve impulse passes only in one direction, i.e. passage of impulse is unidirectional

• The passage of nerve impulse is slightly delayed at the synapse.

• The synapse is susceptible to fatigue.

• Certain substances released in the CNS can inhibit or facilitate the release of neurotransmitter by presynaptic inhibition or facilitation.

• The mechanism of transmission of nerve impulse at synapse is susceptible to certain drugs/chemical agents.

N.B. Knowledge of the actions of different neurotransmitters is important because drugs are available which may neutralize or prolong their effects.

Clinical Problems

1. A histopathologist while examining a nervous tissue under microscope can determine the sex of an individual.

2. The time of occurrence of rabies following a bite by an animal is less if the person is bitten in the body part nearer to the central nervous system, viz. in the face.

3. The people involved in skilled activities are very fond of taking tea or coffee.

4. The aged heart has decreased ability to pump faster and harder during exercise.

5. The tumour of neurons in the central nervous system is rare in adult individuals.

Clinical Problem Solving

1. The neurons of female individuals are characterized by the presence of a small stainable body of chromatin (Barr body) on the inner surface of the nuclear membrane. It represents one of the two X-chromosomes present in the female. It is not seen in neurons of male as they contain only one X-chromosome. Thus, the presence or absence of the Barr body enables the histopathologists to determine the sex of an individual from whom the tissue has been taken.

N.B. The presence of Barr body was first noticed by Barr and Bertram in 1949.

2. The rabies is a fatal viral disease of the central nervous system. The virus is transmitted by the bite of an infected wild or domestic animal like dog. The virus is present in the saliva of the infected animal and following a bite, it travels to the CNS by way of axonal transport in nerves. The incubation period (i.e. period between the time of bite and appearance of symptoms) is related to the length of the peripheral nerves. The longer the nerve, the longer is the duration of the incubation period.

N.B. The virus causing poliomyelitis also travel from the gastrointestinal tract to the anterior horn cells of the spinal cord by an axonal transport.

3. The synaptic transmission is affected by various drugs. The caffeine present in the coffee and tea increases the rate of transmission at synapse with subsequent stimulatory effect on the central nervous system.

4. The number of Ca2+ ion channels in the presynaptic knobs of the nerve fibres that stimulate the heart decreases with age. As a result, less number of Ca2+ ions enter into the presynaptic knobs, causing a decreased release of neurotransmitter, which causes less stimulation of the heart, hence in old age the heart is not able to pump faster and harder during an exercise.

5. A tumour is an expanding lesion (swelling) due to uncontrolled proliferation of the cells. Since neurons are incapable of division in the postnatal life the tumours cannot arise from neurons in the adults (the mitotic activity of the nerve cells is completed during prenatal development).


*The basic instincts in humans are survival, eating, drinking, voiding, and sex (following puberty).



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