Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

11. Introduction to Neuroanatomy

11.1 Central Nervous System (CNS)

A Central nervous system, In situ and In isolation

a Central nervous system in situ, left lateral view, b Isolated central nervous system, anterior view.

The nervous system is concerned with the perception of processes that take place inside (enteroception) or outside the body (exteroception) and with internal and external communication. Given the diversity of these interrelated tasks, the body is endowed with a complex nervous system that can be subdivided in various ways. One basic principle of classification is to divide the nervous system morphologically into a peripheral nervous system (PNS) and a central nervous system (CNS). The central nervous system consists of the brain and spinal cord, which are seamlessly interconnected and comprise a functional unit. The peripheral nervous system is formed by the nerves that emerge from the brain and spinal cord (cranial nerves and spinal nerves) and ramify in the periphery of the body. Macroscopically, the brain and spinal cord consist of gray matter and white matter (see B). The surface of the brain is gray because of the presence of nerve cell bodies. The surface of the spinal cord is white because of the presence of nerve cell processes (axons) and their insulating myelin sheaths (= axons, see C). The CNS communicates with the rest of the body through the cranial nerves and spinal nerves, whose sites of emergence are shown in b.To shield the CNS from external injury, the brain and spinal cord are encased by bone (cranial bones and vertebrae). Situated between the bones and CNS are the coverings (meninges) of the brain and spinal cord, which are the first structures encountered when the overlying bone is removed. Having already described the bony anatomy in an earlier chapter, we now proceed to a description of the brain and spinal cord.

В Distribution of gray and white matter in the CNS

a Coronal section through the cerebrum (telencephalon, see p. 198ff). b Cross-section through the spinal cord.

Even on gross inspection, sections of the brain and spinal cord differ markedly in their appearance due to differences in the distribution of gray and white matter. In the cerebrum (a), most of the gray matter is concentrated superficially in the cerebral cortex. The cerebrum also contains more deeply situated islands of gray matter (e.g., the basal ganglia) in addition to other gray-matter structures that are not specifically addressed in this overview. The white matter of the cerebrum lies directly beneath the cortex and also surrounds more deeply placed groups of gray matter. Section a additionally shows part of the internal cavity system of the brain, the ventricles (see p. 192 ff). The gray/white matter arrangement is reversed in the spinal cord (b), in which the gray matter is placed centrally, forming a butterfly-shaped figure, while the white matter is external to it.

C Histological appearance of the gray and white matter

The gray matter is made up of the cell bodies (perikarya or somoto) of neurons, which are interconnected to form neuronal networks (neuron histology is described on p. 174 ff). The white matter, on the other hand, contains the processes (axons) of neurons that interconnect different areas of the brain and spinal cord. It derives its white color from the lipid content of the myelin sheaths. Many axons running in the same direction are collected to form fiber pathways or tracts. Because the processing of neural information begins in the perikaryon (soma) and ends at the synapse of the axon, these tracts are often named for their sites of origin and termination, e.g. the corticospinal tract. The perikarya of this tract are located in the cerebral cortex, and its axons terminate in the spinal cord. This flow of information is also described macroscopically as a “projection," i.e., the corticospinal tract projects from the cortex to the spinal cord. The brain does not function as a “hard-wired computer,” however. Learning processes like those that occur during puberty can alter the patterns of impulse transmission within the brain. An example is the physical awkwardness that is common during puberty, such as overturning a water glass at the dinner table. As the individual matures, these accidents become less frequent. Some time is needed for position sense to adapt to changes in body size and proportions.

11.2 Neurons

A The neuron (nerve cell)

The neuron is the small est functional unit of the nervous system. It consists of a cell body, called the soma or perikaryon, from which two fundamentally different types of processes arise:

 Dendrites: Dendrites are called the receptor segment of the neuron because they conduct impulses to the cell body that they have received at synapses with other neurons. One neuron may have multiple dendrites, which may undergo very complex arborization to increase their surface area (see C). Dendrites, unlike axons, are not insulated by a myelin sheath. 

• Axons or nerve fibers: The axon is the projecting segment of the neuron because it relays impulses to other neurons or other cells (e.g., skeletal muscle cells). Each neuron has only one axon. Axons in the CNS are generally covered by a myelin sheath (the axons plus their myelin sheaths constitute the white matter). The myelin sheath may be absent in the peripheral nervous system (see details in C, p.177).

Either excitatory or inhibitory neurotransmitters are released at synapses. These substances produce either an excitatory or inhibitory postsynaptic potential at the target neuron. In this way the transmitters released at synapses modulate the potential in the perikaryon of the neuron. The excitatory and inhibitory impulses are integrated in the axon hillock. When the potential exceeds the depolarization threshold of the neuron, the axon “fires,” i.e., the hillock initiates an action potential that travels along the axon and triggers the release of a transmitter from its presynaptic knob (bouton). Although this simple characterization applies to most neurons, connections in the CNS can be much more complex than described here (see C, D, and E).

В Electron microscopy of the neuron

The organelles of neurons can be resolved with an electron microscope. Neurons are rich in rough endoplasmic reticulum (protein synthesis, active metabolism). This endoplasmic reticulum (called Nissl substance under a light microscope) is easily demonstrated by light microscopy when it is stained with cationic dyes (which bind to the anionic mRNA and nRNAof the ribosomes). The distribution pattern of the Nissl substance is used in neuropathology to evaluate the functional integrity of neurons. The neurotubules and neurofilaments that are visible by electron microscopy are referred to collectively in light microscopy as neurofibrils, as they are too fine to be resolved as separate structures under the light microscope. Neurofibrils can be demonstrated in light microscopy by impregnating the nerve tissue with silver salts. This is important in neuropathology, for example, because the clumping of neurofibrils is an important histological feature of Alzheimer's disease.

C Basic forms of the neuron and its functionally adapted variants

The horizontal line marks the region of the axon hillock, which represents the initial segment of the axon. (The structure of a peripheral nerve, which consists only of axons and sheath tissue, is shown on p. 180.)

a Multipolar neuron (multiple dendrites) with a long axon (= long transmission path). Examples are projection neurons such as alpha motor neurons in the spinal cord.

b Multipolar neuron with a short axon (= short transmission path). Examples are interneurons like those in the gray matter of the brain and spinal cord.

c Pyramidal cell: Dendrites are present only at the apex and base of the triangular cell body, and the axon is long. Examples are efferent neurons of the cerebral motor cortex (see pp. 180 and 200). d Purkinje cell: An elaborately branched dendritic tree arises from one circumscribed site on the cell body. The Purkinje cell of the cerebellum has many synaptic contacts with other neurons (see p. 241). e Bipolar neuron: The dendrite arborizes in the periphery. The bipolar cells of the retina are an example (see C, p. 131). f Pseudounipolar neuron: The dendrite and axon are not separated by the cell body. An example is the primary afferent (sensory) neuron in the spinal (dorsal root) ganglion (see pp. 180,272, and 274ff).

D Electron microscopic appearance of the two most common types of synapse in the CNS

Synapses are the functional connection between two neurons. They consist of a presynaptic membrane, a synaptic cleft, and a postsynaptic membrane. In a “spine synapse" (1), the presynaptic terminal (bouton) is in contact with a specialized protuberance (spine) of the target neuron. The side-by-side synapse of an axon with the flat surface of a target neuron is called a parallel contact or bouton en passage (2). The vesicles in the presynaptic expansions contain the neurotransmitters that are released into the synaptic cleft by exocytosis when the axon fires. From there the neurotransmitters diffuse to the postsynaptic membrane, where their receptors are located. A variety of drugs and toxins act upon synaptic transmission (antidepressants, muscle relaxants, nerve gases, botulinum toxin).

E Synaptic patterns in a small group of neurons

Axons may terminate at various sites on the target neuron and form synapses there. The synaptic patterns are described as axodendritic, axosomatic, or axoaxonal. Axodendritic synapses are the most common (see also A). The cerebral cortex consists of many small groups of neurons that are collected into functional units called columns (see p.201 for details).

11.3 Neuroglia and Myelination

A Cells of the neuroglia in the CNS

Neuroglial cells surround the neurons, providing them with structural and functional support (see D). Various staining methods are used in light microscopy for more or less selectively defining specific portions of the neuroglial cells:

a Cell nuclei demonstrated with a basic stain, b Cell body demonstrated by silver impregnation.

Neuroglial cells constitute the vast majority of cells in the CNS, outnumbering the neurons by approximately 10-to-1 (1 trillion neuroglial cells to 100 billion neurons by recent estimates). The neuroglia have an essential role in supporting the function of the neurons. For example, astrocytes absorb excess neurotransmitters from the extracellular milieu, helping to maintain a constant internal environment. While neurons are, almost without exception, permanently post-mitotic some neuroglial cells continue to divide throughout life. For this reason, most primary brain tumors originate from neuroglial cells and are named for their morphological similarity to normal neuroglial cells: astrocytoma, oligodendroglioma, and glioblastoma. Developmentally, most neuroglial cells arise from the same progenitor cells as neurons. This may not apply to microglial cells, which develop from precursor cells in the blood from the monocyte lineage.

В Myelinated axon in the PNS

Most axons in the peripheral nervous system are insulated by a myelin sheath, although unmyelinated axons are also found in the PNS (see C).

The myelin sheath enables impulses to travel faster along the axon as they “jump” from one node of Ranvier to the next (saltatory nerve conduction), rather than travel continuously as in an unmyelinated axon.

C Myelination differences in the PNS and CNS

The purpose of myelination is to insulate the axons electrically. This significantly boosts the nerve conduction velocity as a result of saltatory conduction. While almost all axons in the CNS are myelinated, this is not the case in the PNS. The axons of the PNS are myelinated in regions where fast reaction speeds are needed (e.g., skeletal muscle contraction) and unmyelinated in regions that do not require rapid information transfer (e.g., the transmission of muscle spindle and tendon tension sensation). The very lipid-rich membranes of myelinating cells are wrapped around the axons to insulate them. There are differences between the myelinating cells of the central and peripheral nervous systems. Schwann cells (left) myelinate the axons in the PNS, whereas oligodendrocytes (right) form the myelin sheaths in the CNS.

Note: In the CNS, one oligodendrocyte always wraps around multiple axons; however, Schwann cells ensheath either one myelinated axon or multiple unmyelinated axons.

This difference in myelination has important clinical implications. In multiple sclerosis, the oligodendrocytes are damaged but the Schwann cells are not. As a result, the peripheral myelin sheaths remain intact in MS while the central myelin sheaths degenerate.

D Summary: Cells of the central nervous system (CNS) and peripheral nervous system (PNS) and their functional importance

Cell type

Function

Neurons (CNS and PNS) (see p. 179)

1. Impulse formation

2. Impulse conduction

3. Information processing

Glial cells

 

Astrocytes (CNS only) (also called macroglia)

1. Maintain a constant internal milieu in the CNS

2. Help to form the blood brain-barrier

3. Phagocytosis of nonfunctioning synapses

4. Scar formation in the CNS (e.g., after cerebral infarction or in multiple sclerosis)

5. Absorb excess neurotransmitters and K+

Microglial cells (CNS only)

Cells specialized for phagocytosis and antigen processing (brain macrophages, part of the mononuclear phagocyte system); secrete cytokines and growth factors

Oligodendrocytes (CNS only)

Form the myelin sheaths in the CNS

Ependymal cells (CNS only)

Line cavities in the CNS

Cells of the choroid plexus (CNS only)

Secrete cerebrospinal fluid

Schwann cells (PNS only)

Form the myelin sheaths in the PNS

Satellite cells (PNS only) (also called mantle cells)

Modified Schwann cells; surround the cell body of neurons in PNS ganglia

11.4 Sensory Input, Perception and Qualities

A Schematic diagram of information flow in the nervous system

We began this chapter (p. 172) by dividing the nervous system into the CNS and PNS. The nervous system can also be divided based the direction of information flow. Nerves that transmit impulses toward the brain or spinal cord are called afferent fibers (left), and nerves that transmit impulses away from the brain or spinal cord are called efferent fibers (right). The terms afferent and efferent are also used within the CNS to describe the connections between nuclei. The structure of the neuron is important in this scheme, because the dendritic tree and its processes are afferent while axons and their synapses are efferent. Another possible classification scheme shown here is to divide the nervous system into a somatic and autonomic (visceral, vegetative) nervous system (upper and lower parts of the diagram, respectively). The somatic nervous system is responsible for communication between the organism and its environment, and it coordinates locomotion. The autonomic (visceral) nervous system coordinates the function of the internal organs. Using the scheme pictured here, we can subdivide axons, as well as nerves and fiber tracts, into four different modalities: somatic afferent, somatic efferent, visceral afferent, and visceral efferent. Further subdivisions of the afferent and efferent fibers (e.g., special visceral afferent or secretomotor fibers) are omitted here in the interest of clarity.

В Special sensory qualities

The ability of the nervous and sensory system to perceive a great variety of stimuli is called sensation. This communication with the environment is mediated by specialized perceptual organs that are located at anatomically defined sites. The special sensory qualities include taste, smell, vision, hearing, and the sense of balance. All of these sensory perceptions are transmitted to the CNS by cranial nerves.

C General sensory qualities

A basic distinction is drawn between external perception (exteroception) and internal perception (proprioception) depending on the source of the stimulus. Because the stimulus in exteroception comes from the external environment and is perceived by “exteroceptors” in the skin, this sense is also known as superficial sensation. In proprioception, the source of the stimulus lies “deep” within a muscle, tendon, or joint {information on the relative position of the body parts), and so this mode of perception is also called deep sensation. Moreover, two sensory qualities are distinguished in exteroception, which may both be perceived at the same location: (1) epicritic perception (light touch, vibration, two-point discrimination) and (2) protopathic perception (pain and temperature), which includes an emotional component (pain is distressing, for example). Exteroception, then, is largely a conscious mode of perception that is mediated by the gracile and cuneate fasciculi (epicritic) and the anterior and lateral spinalothalamic tracts (protopathic). Proprioception, on the other hand, is largely unconscious and is integrated chiefly by the cerebellum.

Testing superficial sensation:

 Vibration sense: tested with an alternately vibrating (64 or 128 Hz) and nonvibrating tuning fork, which may be placed on the shin, for example. The patient should be able to perceive the difference between the vibrating and nonvibrating states.

 Pressure and touch sensation: The skin is touched with a cotton swab.

 Pain perception: The skin is pricked with a sterile hypodermic needle. This test can also be used to test two-point discrimination.

 Heat and cold sensation: Test tubes containing warm or cold water are placed in contact with the skin.

Testing deep sensation (proprioception): With the patient’s eyes closed, the examiner moves the distal phalanges of the toes, for example, and asks the patient to describe the position of the digits without looking at them.

D Receptors In the muscles and tendons

The receptors in the muscles (muscle spindles), tendons (Golgi tendon organs), and joints (not shown) give the brain information about the position of joints, muscular force, and movements. This information is known collectively as proprioception. For example, we know when our hand is clenched into a fist even when it is behind our back. The brain receives additional information on the position of the head and limbs from the vestibular apparatus (sense of balance), the eyes, and mechanical sensors (mechanoreceptors) in the skin.

E Different types of sensory receptors

Proprioception as described above is mediated by specialized peripheral endings of primary sensory neurons whose cell bodies are in spinal (dorsal root) and cranial sensory ganglia. Other (exteroceptive) sensations involve receptor cells that are situated within special sense organs. These receptors can be neurons with axons that synapse onto secondary neurons (as in a, an olfactory receptor situated in the olfactory epithelium, which sends its axon into the olfactory bulb [CNS]). Other specialized receptor cells (b, vestibular hair cell) may have no axon, but instead participate in local synapses with neurons that, in turn, transmit the information to higher centers. The neurons that synapse with vestibular hair cells have cell bodies in the vestibular ganglion. The central processes of these ganglion cells travel in the vestibulocochlear nerve to the brainstem.

11.5 Peripheral and Central Nervous Systems

A Peripheral nerve

Information travels in the PNS along nerves, which are the equivalent of tracts in the CNS. Like the tracts, the nerves consist of bundles of axons (neurites or nerve fibers). But whereas the axons in the CNS tracts are routed in an afferent or efferent direction (e.g., toward or away from the cortex), a typical peripheral nerve carries both afferent and efferent fibers and is therefore called a mixed nerve. Afferent and efferent fibers may be myelinated or unmyelinated (lacking a myelin sheath). It will be recalled that the peripheral nerves are myelinated by Schwann cells (see C, p. 177).

Note: The perikarya of neurons in the PNS are located in ganglia (see B).

В Ganglia

As noted above, the perikarya of neurons in the PNS are located in ganglia. Two main types of ganglion can be distinguished:

a Spinal ganglia are located at the dorsal root of spinal nerves and contain pseudounipolar neurons. These neurons convey sensory information from the periphery (e.g., pressure, temperature, pain) into the spinal cord, where the impulses are relayed to another neuron. The perikaryon has a T-shaped connection with the axon (see C, p. 175); thus, no synaptic relay in the spinal ganglion. Since the peripheral process receives sensory impulses, this neuron is called a primary afferent neuron. The sensory cranial-nerve ganglia also contain pseudounipolar neurons, which correspond functionally to the spinal ganglia.

b Autonomic ganglia are part of the autonomic nervous system. The efferent fibers to the (internal) organs are relayed in these ganglia (see p. 316).

Intramural ganglia in the intestinal wall (not shown) are part of the enteric nervous system (see p. 324).

C Somatomotor Integration

This greatly simplified circuit diagram shows how the sensory and motor systems work together during ordinary activities. An example: Placing the foot tentatively on the lower rung of a ladder to see if the ladder is stable initiates a flow of signals through a chain of neurons (the sensory neurons are shown in blue, the motor neurons in red). The sensation of the foot touching the rung is conveyed by the primary sensory neuron to the spinal cord. This neuron synapses with a secondary sensory neuron at the upper end of the spinal cord (dorsal column nuclei), which synapses with a third neuron in a specialized nucleus in the diencephalon. From there the information is relayed to the sensory cortex. Inter neurons in the brain then give rise to flow of impulses from the sensory cortex to the upper motor neuron in the motor cortex, which relays the motor command down to a motor interneuron. Finally this interneuron activates the lower motor neuron, which causes the muscle to contract and enables the individual to start climbing the ladder.

Note: Most diagrams omit the interneurons and show only the first and second motor neurons, which are called the “upper motor neuron” in the cortex and the “lower motor neuron” in the spinal cord. The distinction between these two neurons is very important clinically: A lesion of the upper motor neuron causes spastic paralysis, whereas a lesion of the lower motor neuron causes flaccid paralysis (see p. 343 for details).

11.6 Nervous System, Development

A Neural tube and neural crest (after Wolpert)

The tissues of the nervous system originate embryonically from the dorsal surface ectoderm. The notochord in the midline of the body induces the formation of the neural plate, which lies above the notochord, and of the neural crests, which are lateral to the notochord. With further development, the neural plate deepens at the center to form the neural groove, which is flanked on each side by the neural folds. Later the groove deepens and closes to form the neural tube, which sinks beneath the ectoderm. The neural tube is the structure from which the central nervous system (CNS)—the brain and spinal cord—develops (further development of the spinal cord is shown in B, further brain development in D). Failure of the neural groove to close completely will leave an anomalous cleft in the vertebral column, known as spina bifida. The administration of folic acid to potential mothers around the time of conception can reduce the incidence of spina bifida by 70%. Cells that migrate from the neural crest develop into various structures, including cells of the peripheral nervous system (PNS) such as Schwann cells and the pseudounipolar cells of the spinal ganglion (see C).

В Differentiation of the neural tube in the spinal cord during development

Cross-section, superior view.

a Early neural tube, b intermediate stage, c adult spinal cord.

The neurons that form in the basal plate are efferent (motor neurons), while the neurons that form in the alar plate are afferent (sensory neurons). In the future thoracic, lumbar, and sacral spinal cord, there is another zone between them that gives rise to sympathetic (autonomic) efferent neurons. The roof plate and floor plate do not form neurons.

C Development of a peripheral nerve

Afferent axons (blue) and efferent axons (red) sprout separately from the neuronal cell bodies during early embryonic development.

a Primary afferent neurons develop in the spinal ganglion, and alpha motor neurons develop from the basal plate of the spinal cord,

b The interneurons (black), which functionally interconnect the sensory and motor neurons, develop at a later stage.

D Development of the brain

a Embryo with a greatest length (CL) of 10 mm at the beginning of the second month of development. Even at this stage we can see the dif- ferentation of the neural tube into segments that will generate various brain regions.

 Red: telencephalon (cerebrum)

 Yellow: diencephalon

 Dark blue: mesencephalon (midbrain)

 Light blue: cerebellum

 Cray: pons and medulla oblongata

Note: The telencephalon outgrows all the other brain structures as development proceeds.

b Embryo with a CL of 27 mm near the end of the second month of development (end of the embryonic period). The telencephalon and diencephalon have enlarged. The olfactory bulb is developing from the telencephalon, and the primordium of the pituitary gland is developing from the diencephalon.

c Fetus with a GL of 53 mm in approximately the third month of development. By this stage the telencephalon has begun to cover the other brain areas. The insula is still on the brain surface but will subsequently be covered by the hemispheres (compare with d). d Fetus with a GL of 27 cm (270 mm) in approximately the seventh month of development. The cerebrum (telencephalon) has begun to develop well-defined gyri and sulci.

E Brain vesicles and their derivatives

The cranial end of the neural tube expands to form three primary brain vesicles for the

 forebrain (prosencephalon),

 midbrain (mesencephalon), and

 hindbrain (rhombencephalon).

The telencephalon and diencephalon develop from the prosencephalon. The mesencephalon gives rise to the superior and inferior colliculi and related structures. The rhombencephalon differentiates into the pons, cerebellum, and medulla oblongata. The pons and cerebellum are also known collectively as the metencephalon. Some important structures of the adult brain are listed in the diagram at left to illustrate the dérivâtes of the brain vesicles. They can be traced back in the diagram to their developmental precursors.

11.7 Brain, Macroscopic Organization

A Left lateral view of the brain

The cerebrum is divided macroscopically into four lobes:

 Frontal lobe

 Parietal lobe

 Temporal lobe

 Occipital lobe

The surface contours of the cerebrum are defined by convolutions (gyri) and depressions (sulci). An example is the central sulcus, which separates the precentral gyrus from the postcentral gyrus. These two gyri are functionally important because the precentral gyrus is concerned with voluntary motor activity while the postcentral gyrus is concerned with the conscious perception of body sensation. Deep within the lateral sulcus is the insular lobe, often called simply the insula (see B, p.173). The sulci are narrowed and compressed in brain edema (excessive fluid accumulation in the brain), but they are enlarged in brain atrophy (e.g., Alzheimer's disease) because of tissue loss from the gyri. The brains that are available for dissection in medical school courses frequently manifest signs of brain atrophy. Often the atrophy is predominantly frontal in males and predominantly occipital in females, but the reason for this disparity is unknown.

В Basal view of the brain

The spinal cord has been sectioned in its upper cervical portion. This view demonstrates the sites of emergence of most of the cranial nerves (yellow) from the brainstem (see p.66ff). The frontal lobes, temporal lobes, pons, medulla oblongata, and cerebellum are the principal structures that can be identified on the base of the brain. This view clearly displays the two hemispheres and the longitudinal cerebral fissure between them. The gyri vary considerably in different individuals, and even the convolutions of a single brain may show marked side-to-side differences, presumably due to the specialization of the hemispheres.

C Midsagittal section of the brain showing the medial surface of the right hemisphere

The brain has been split along the longitudinal cerebral fissure. Developmentally, the brain can be divided into several major parts (see p. 183), all of which are visible in this section:

 Telencephalon (cerebrum)

 Diencephalon

 Mesencephalon (midbrain)

 Pons

 Medulla oblongata

 Cerebellum

The medulla oblongata is continuous interiorly with the spinal cord, with no definite anatomical boundary between them. The mesencephalon, pons, and medulla oblongata are collectively referred to as the brainstem based on their common embryological and functional features. The brainstem lies near the anterior surface of the cerebellum.

D Terms of location and direction in the central nervous system

Midsagittal section viewed from the left side. Repeated references are made in subsequent units to two different axes of the brain: the Meynert axis, which is used to designate locations in the brainstem, and the Forel axis, which describes the topography of the diencephalon and telencephalon.

• The Meynert axis (1 ) passes through the brainstem and corresponds roughly to the longitudinal body axis.

 The Forel axis (2) runs horizontally through the diencephalon and telencephalon.

The following chapters on the CNS begin with the cerebrum and proceed downward to other brain structures and the spinal cord. Our approach to CNS topography also proceeds from outside to inside, following the order in which the structures are encountered in a dissection. Neuroanatomy is particularly challenging because we cannot directly infer the function of a structure from its appearance as we can with muscle tissue, for example. Our presentation of the CNS therefore ends with a chapter on functional systems. In describing the functional systems, we will use a peripheral-to-central approach (i.e., from the simple to the complex) so that the reader may better understand the path followed by a stimulus from its source to its various relay stations in the CNS.