Human Neuroanatomy is the division of Human Anatomy which deals with of Human Nervous System. The Nervous System is defined as the “Master of all Systems” or the “Master System” of the body, because it controls or regulates all bodily functions performed by other systems of the body, for example locomotor system, gastrointestinal system, respiratory system.
PRINCIPLES OF FUNCTIONS OF NERVOUS SYSTEM (FIG. 1.1)
When nervous system exerts its action over the other systems of body, most simplified form of its action is manifested basically as—
- Contraction of muscles.
- Secretion of exocrine glands.
It may be noted here that the secretion of endocrine glands is mostly under the hormonal control.
The muscles, whose contraction is regulated by nervous system, may be voluntary (striated or skeletal) or involuntary (nonstriated or smooth). Contraction of the voluntary muscles results in movement of a joint. The involuntary muscles may be in the wall or in the substance of viscera, which are specifically called ‘Visceral muscle’, e.g. in the wall of the gastrointestinal tract, or tracheobronchial tree or in the substance of any solid viscera. Again, the involuntary muscle may be in the wall of the cardiovascular channel, e.g. in the wall of the heart (myocardium) or in the wall of blood vessel (tunica media). It may be also in the dermis of the skin named the Arrectores pili.
The exocrine glands influenced by the activity of the nervous system may be single and solitary like any salivary gland or the lacrimal gland, or it may be multiple and minute, like the mucous glands of the wall of GI tract, or respiratory tract.
So result of functions of nervous system may be summarized as follows—
- Contraction of voluntary muscle(s): Resulting movement of a joint. It may result movement of some organs, like tongue, eyeball.
- Contraction of involuntary muscle(s) present in:
- a) Viscera: It is called visceral muscle.
- b) Wall of the cardiovascular system: Myocardium of heart or smooth muscle in the wall of the blood vessels.
- c) Dermis of skin called Arrectores pili: It is attached to the root of hair follicle.
- Secretion of exocrine glands like:
- a) Salivary glands or lacrimal gland: Large and solitary.
- b) Mucous secreting glands: In the wall of GI tract or respiratory tract-many and minute.
But it is to be noticed that the functions of nervous system do not mean only the effects as mentioned above, but, in gist it also performs the followings: (Fig. 1.1).
- It receives and carries different information from its periphery to center, which are related to change in external and/or internal environment.
- It perceives or acknowledges the informations at its center.
- It analyzes, integrates and coordinates the informations or inputs.
- It commands for some effect after reception and, integration or coordination of informations.
- It stores the informations for the memory, intelligence, learning and emotion of an individual.
A. Topographical Subdivision
- Central: Part situated in the central axis of the body, known as Central Nervous System. These are Brain and Spinal cord. Brain is the proximal expanded part situated inside the cranial cavity. Distal, narrow, tubular and elongated part is the spinal cord which is lodged in the upper two-third of the vertebral canal. Grossly brain is divided into three parts-Forebrain, Midbrain and Hindbrain. Spinal cord is divided into 31 segments, which are classified regionally as Cervical-8, Thoracic-12, Lumbar-5, Sacral-5 and Coccygeal-1.
- Peripheral: This is known as Peripheral Nervous System. This is peripheral outflow or peripheral extensions from Central Nervous System in the form of peripheral nerves. The peripheral nerves are divided into two groups as–
- a) Proximal (Cranial): Cranial nerves, 12 pairs arising as outflow from brain.
- b) Distal (Caudal): Spinal nerves, 31 pairs, each pair arising from each segment of spinal cord.
Central Nervous System may be compared as the Director of an office, and Peripheral Nervous System as the Field Staff. Like the Director, Central Nervous System gathers information from and gives direction to the Peripheral Nervous System, whose duty is to convey information and also to carry out the order from its Director, i.e. Central Nervous System, for action.
- Fundamental difference between the Cranial and Spinal nerves:All the spinal nerves contain sensory (incoming) fibers carrying impulse (information) towards the central nervous system and motor fibers (outgoing) carrying impulse (directions) away from the central nervous system to the effector organ, that is why all the spinal nerves are mixed nerves. But some cranial nerves are mixed like spinal nerves and some are either purely motor or purely sensory.
B. Functional Subdivision
It is already understood that nervous system controls various bodily functions. The simplified form of functions controlled by nervous system are the followings:
- Contractions of voluntary muscles to move the joints or to move some organs like tongue, eyeball.
- Contractions of involuntary muscles like—
- a) Visceral muscles.
- b) Smooth muscles in the wall of cardiovascular channel.
- c) Smooth muscles in the root of hair follicle of skin, known as Arrectores pili.
- Secretions of exocrine glands which may be either single, large, solitary, e.g. Salivary glands or tiny innumerable, for example-mucous glands of gastrointestinal and respiratory tract.
Out of these different functions—The contractions of voluntary muscles is controlled or regulated as per one's own desire and is known as voluntary function, whereas others are not under one's own control, called involuntary function.
With the help of this background knowledge, it is to be noted that — functionally the nervous system is classified as — Somatic and Autonomic (Figs 1.4A and B).
- Somatic Nervous System: It is that division of nervous system which controls or regulates the voluntary functions, i.e. functions which can be performed as well as controlled as per one's own desire. It is contraction of voluntary or skeletal muscles.
- Autonomic Nervous System: It is that division of nervous system which controls or regulates involuntary functions, e.g. functions which can neither be preformed nor can be regulated as per one's own desire. These are contraction of involuntary or smooth muscles and secretion of exocrine glands.
Two parallel components of autonomic nervous system:
They are called sympathetic and parasympathetic nervous system. These two systems have anta-gonistic actions on the “same” target organ, e.g. Parasympathetic nervous system contracts the muscles in wall of hollow viscera like GI tract (peristaltic movements), but relaxes the sphincters; whereas the sympathetic nervous system causes the opposite action on the same target organ. Again in some cases either of them has the influence, e.g. mucous glands of respiratory or alimentary tract are under control of parasympathetic nervous system, whereas secretion of sweat glands are controlled by sympathetic system.
CENTERS OF NERVOUS SYSTEM
Center for somatic nervous system extend throughout the whole length of brain and spinal cord, lying in central axis of body.
That's why it is called Central nervous system.
Centers for sympathetic and parasympathetic components of autonomic nervous system are situated in some of the levels of brain and spinal cord.
- a) Sympathetic center is located in first to twelfth thoracic and first and second lumbar (T1–L2) segments of spinal cord.
- b) Center of parasympathetic system situated partly in brain in the form of nuclei of some cranial nerves (3rd, 7th, 9th, 10th). Again part of its center is occupied in 2nd, 3rd, 4th sacral segments of spinal cord (S2–4). These centers for sympathetic and parasympathetic components of autonomic nervous system are of course, finally controlled by posterior and anterior parts of hypothalamus of brain respectively.
COMPOSITION OF NERVOUS SYSTEM
Nervous system is composed of very delicate and sensitive tissue known as nervous tissue. In general, it is known that a tissue is composed of cells and intercellular substance. The intercellular substance may be little or minimum as in epithelial tissue, or may be maximum as in connective tissue. But everywhere it is noncellular.
The structural and functional units of nervous system are cells, known as Neuron. It is noteworthy that in the nervous system, the intercellular substance is not noncellular, rather made up of cells called Neuroglia. The neuroglia, proportionately more in number and primarily acting as supporting element occupying the interstitial spaces between neurons.
The structural as well as functional units of nervous system is Neuron.
It has two special properties—
- Irritability— It is the power of a cell(neuron) by which it is able to respond or react to change in the environment (known as stimulus), which may be external or internal (outside or inside the body).
- Conductivity— It is the power of a cell (neuron) by which the excited state (known as impulse) is propagated from the site of stimulus for a distance to get the desired effect through ‘hand to hand’ contact of thread-like protoplasmic processes of chain of neurons.
Structure of a Typical Neuron (Fig. 1.5)
A typical neuron is composed of—
- Cell body: It is known as Soma or Perikaryon and
- Processes: Thread-like protoplasmic prolongations.
The processes are of two types known as Dendrites and Axons. Dendrites are the processes through which impulse is transmitted towards the cell body. Axons transmit impulse away from the cell body. So, when an impulse passes through a chain of neurons, it passes from axon of one neuron to the dendrite of the next neuron of the chain (Fig. 1.6).
Number of processes in a neuron— A neuron always posseses at least one process, which is axon, the number of which is always single. A neuron may or may not have the Dendrites. If it is present, it may be one or multiple (Fig. 1.7).
It is the main expanded mass of cell with a centrally placed nucleus containing a prominent nucleolus.
Cytoplasm has following unique characteristics:
- a) Nissl bodies (Nissl granules/Nissl substance): These are nothing but large aggregations of prominently stained rough endoplasmic reticulum. These are concerned with synthesis of enzymes which are required for productions of chemical substances known as neurotransmitters. Nerve impulse is transmitted over the junction of adjacent neurons (synapse) through those neurotransmitters. Nissl substances are absent not only in the axons but also in the base of axons known as axon-hillock.
- b) Neurofibrils: These are ultramicroscopic thread-like or fibrillar structures homologous to microfilaments of other cells. Neurofibrils are concerned with maintenance of architecture of neuron and acts as a storehouse of protein called Tubulin.
- Dendrites: They are fibrillar protoplasmic extensions of neuron with the following characteristics —
- These are the processes through which impulse travels towards the cell body.
- Narrower in width.
- Highly branched.
- Branching of the dendrites are short known as “Dendrite Tree”.
- Terminal ends of dendrite tree are known as “Dendrite Spines”.
- Dendrites may be absent, if present it may be single or multiple.
- Axon: It is the fibrillar protoplasmic extension of neuron with following characteristics—
It is important to notice that dendrites and axons cannot be differentiated by their relative length. Some neurons may have long axon. Again some may have long dendrite. Fibers of median or ulnar nerve supplying small muscles of hand are example of long axon. Whereas fibers of saphenous nerve carrying sensation from skin of foot are the example of very long dendrite. In both the cases cell bodies are located in or very close to spinal cord.
- These are the processes through which impulse travels away from the cell body.
- Basal end is conical-known as “axon-hillock”.
- Wider in breadth.
- It has no terminal branching, but from the middle of the axon branching at right-angle may takes place known as “Collaterals”.
- Terminal end is expanded known as “Telodendria” showing knob-like or button-like endings called axon terminals or Terminal buttons.
- Number of axon in neuron is always constantly one.
Neuronal (Axonal) Transport
Chemical substances synthesized in the neuronal cell body are required to be transported through the axon at its distal end. This is known as “Orthograde transport” (Fig. 1.8A). These chemical substances are either concerned with the nerve conduction, when these pass through the interneuronal junction (synapse) or these may be concerned with desired function of nerve impulse when these reach the effector organ. Sometimes chemical substances (may be neurotoxins) liberated at the tissue level, absorbed by axon terminals, are carried back towards the cell body of the neuron. This is known as “Retrograde transport” (Fig. 1.8B).
Classification of Neurons
It is to be noted that, at one initial phase of development, neurons used to have no process. However, this phase is followed by gradual appearance of number of processes which will classify the neurons as follows:
a. Unipolar neurons
These are developmentally primitive variety of neurons with single process which is the axon. It is devoid of any dendrites.
Fig. 1.8B: Retrograde transport, – toxins liberated in tissue pass in opposite direction through axon toward cell body
b. Bipolar neurons
These are the fusiform or spindle-shaped neurons with one dendrite and one axon arising from opposite poles. These are specialized neurons found in the pathways of special senses, e.g. Retina (visual pathway), nasal epithelium (olfactory pathway) and in the vestibulocochlear nerve (auditory pathway for hearing and equilibrium).
c. Pseudounipolar neurons
These are neurons with round or oval shape with a common short stem of process dividing into peripheral (dendrite) and central (axon) limbs. These neurons are called pseudounipolar because apparently they seem to have two poles. Classical example of these are the dorsal root ganglion cells of spinal nerve lying just outside and close to the spinal cord carrying sensory impulse from periphery towards the spinal cord.
d. Multipolar neurons
These neurons present single axon with multiple dendrites. Their shape will vary from triangular or pyramidal to polygonal depending upon numbers of dendrites so also the polarity. For example, the neurons of motor area of cerebral cortex are pyramidal or triangular whereas the motor neurons of spinal cord are polygonal and neurons of cerebellum are flask-shaped.
2. According to length of axon (Figs 1.10A and B)
a. Golgi type I (Fig. 1.10A)
Axons of these neurons are long in comparison to their multiple short dendrites, viz. ‘Pyramidal Neurons’ of motor area of cerebral cortex, ‘Anterior horn cells’ of spinal cord, ‘Purkinje cells’ of cerebellum.
Axons of pyramidal cells of cerebral cortex form long descending tracts passing through the spinal cord. Axons of anterior horn cells of spinal cord form long peripheral nerves supplying voluntary muscles. Purkinje cells axons form efferent fibers from cerebellar cortex to relay in cerebellar nuclei situated in its white matter.
b. Golgi type II (Fig. 1.10B)
Axons of these neurons are short, similar to the length of the dendrites.
Figs 1.10A and B: A. Golgi type I neuron (with long axon), B. Golgi type II neuron (with short axon)
Classical example of these neurons are stellate cells of cerebellar cortex, which have short axon and multiple short dendrites giving a star-shaped appearance. It forms synaptic connection with too many neurons.
It is important to note that some of the neurons may have single long dendrite. For example, fibers present in the sensory nerves carrying sensory impulse from the periphery are the long dendrites of sensory neurons present in the posterior root ganglia of spinal nerve.
3. According to function of neuron
a. Sensory neuron (Fig. 1.11)
These neurons carry sensory impulse from a receptor (sensory end organ) through the dendrite towards the center of nervous system finally through axon. From the sensory end organ or receptor situated at the periphery of the body, the sensory nerve impulse needs to pass through a chain of neurons as the relay system to reach the center of nervous system. The participating neurons in this “chain” are classified as primary, secondary and tertiary neurons (Fig. 1.11).
- Primary sensory (First order) neurons: They start from the receptor or sensory end organ to enter the central nervous system. Their cell bodies are situated outside the central nervous system. Only exception is the cell group of mesencephalic nucleus of trigeminal nerve, whose cell bodies lie inside central nervous system.
- Secondary sensory (Second order) neu-rons: They are situated at the level of spinal cord which receive impulse from 1st order of neurons.
- Tertiary sensory (Third order) neurons: They relay the sensation from the secondary neurons to the final target, i.e. cerebral cortex. First group of these neurons are situated in the thalamus. The second or final group is situated in the sensory area of cerebral cortex.
b. Motor neuron (Fig. 1.10A)
These neurons carry outgoing motor impulse from central nervous system to the peripherally situated effector organs which are either muscles or glands.
- Types of motor neuron:In somatic nervous system—Upper motor neuron and Lower motor neuron.
- Lower motor neuron: These motor neurons are situated in spinal cord.
4. Classification of neurons in relation to neuronal junction(synapse) – (Fig. 1.12)
Functions of a neuron depends upon the transmission of impulse through a chain of successive neurons. The junction of neuronal chain is known as Synapse or Ganglion (pl. Ganglia). When related to a particular synapse, the neurons are classified as—
- a) Presynaptic (Preganglionic) neuron: Proximal to a synapse
- b) Postsynaptic (Postganglionic) neuron: Distal to the synapse.
In somatic nervous system, both the pre and postganglionic neurons are situated inside the central nervous system except the first order of sensory neuron which lies outside of central nervous system, e.g. Posterior root ganglia cells of spinal nerve. But in autonomic nervous system the preganglionic neuron is situated inside the central nervous system and postganglionic neuron is situated outside the central nervous system.
NEURONAL JUNCTION (SYNAPSE) (FIG. 1.12)
It has already been noticed that, when a neuron is stimulated due to change in the environment, external or internal, impulse or action potential is generated. But activity of nervous system depends on transmission or conduction of nerve impulse or action potential through a chain of neurons. In the chain neurons are approximated or apposed closely to each other. This site of apposition or contact between two neurons is known as synapse. Though, it is simple to understand, but in 1891, neuronal theory of Waldeyer first established that at the synapse or neuronal junction of two successive neurons are contiguous, but not continuous to each other. It was then detected that some chemical substances called “Neurotransmitters” jump across the synaptic junction to carry the nerve impulse or action potential of the neuronal chain.
Fundamental points to remember regarding the synapse:
- Two successive neurons are contiguous in the synapse but not continuous.
- Chemical substance released in the proximal neuron (presynaptic neuron) passes to distal or postsynaptic neuron, through which impulse is transmitted.
- Impulse under physiological condition travels through the synapse in one direction only.
- Single end of an axon, known as axon terminal will form synapse with single dendritic spine.
- Multiple end button of one presynaptic neuron may form synapse with dendrites of multiple neurons or multiple dendrites of single neuron.
Types of Synapse (Fig. 1.12)
So far, it is already understood that axon of presynaptic neuron forms synapse with the dendrons of postsynaptic neuron. But truly speaking axon of a neuron may form synapse with any component of another neuron, e.g. dendrite, cell body, even the axon also. So, the synapses are grossly classified as–
- Axodendritic: Synapse between axon of presynaptic and dendron of postsynaptic neuron.
- Axosomatic: Synapse between axon of presynaptic and cell body or soma of postsynaptic neuron.
- Axoaxonic: It is considered as a lateral synapse. In this type, axon of lateral neuron form synaptic connection with axon of another neuron which is lying in the regular neuronal chain.Besides the above mentioned commoner types of synapses, other types are somatodendritic, somatosomatic and dendrodendritic.
Structure of a typical axodendritic synapse (Fig. 1.13)
A typical axodendritic synapse is composed of following three parts. These are—
- Presynaptic membrane of axon of proximal neuron.
- Synaptic cleft between axon and dendrite.
- Postsynaptic membrane of dendrite of distal neuron.
Thickened cell membrane of the axon terminal at the site of synapse is called presynaptic membrane. Beneath this membrane the axoplasm shows some specialized features. The cytoplasm is condensed with presence of number of mitochondria. It also contains many membrane bound vesicles which contain chemical substances known as neurotransmitter. The vesicles are very tiny, 40–50 nm (nanometer) in diameter. One µm (micrometer) is 1/1000 of a millimeter and one nm (nanometer) is 1/1000 of a µm (micrometer). During transmission of nerve impulse, neurotransmitter is released from presynaptic vesicles into synaptic cleft by exocytosis to stimulate postsynaptic membrane of the distal neuron.
It is the gap measuring 20–30 nm between pre and postsynaptic membranes. It contains interstitial fluid rich in polysaccharides. Through the process of exocytosis neurotransmitters are released across the presynaptic membrane into synaptic cleft.
This is the thickened plasma membrane of dendrite spine at the site of synapse. This membrane shows specialization known as receptors which are to uptake neurotransmitters passing across the synaptic cleft. The dense cytoplasm beneath postsynaptic membrane is segmented and known as synaptic web which contains a network of filame-ntous structure.
Impulse Transmitted Across the Synapse
Nerve impulse transmitted through presynaptic neuron causes release of neurotransmitter from presynaptic vesicles. Neurotransmitter passing across the synaptic cleft act as chemical impulse to stimulate receptors of postsynaptics membrane. Chemical impulse reaching synaptic web beneath postsynaptic membrane is again converted into nerve impulse to stimulate postsynaptic neuron.
There are varieties of chemical substances acting as neurotransmitter. Mostly found neurotransmitters are Acetylcholine and Norepinephrine. Acetylcholine is liberated as neurotransmitter in many synapses of central and peripheral nervous system including those of parasympathetic nervous system. Norepinephrine is released in most of the synapses of sympathetic nervous system. Glycine is the neurotransmitter discharged in the synapses of spinal cord. Dopamine is the transmitter found in basal ganglia and substantia nigra. Serotonin and Gumma-amino-butyric acid (GABA) are other examples of commonly known neurotransmitters.
Deactivation or cessation of action of neurotransmitters
After desired effect, influence of neurotransmitters is withdrawn in either of two different ways. In case of Acetylcholine, it is broken down by the enzyme Acetylcholinesterase at synaptic cleft. But in case of transmitters like norepinephrine, its effect is restricted by its reuptake back through presynaptic membrane.
These are the chemical substances which enhance, prolong, restrict or inhibit the effect of the neurotransmitter on postsynaptic membrane. They are stored in separate presynaptic vesicles.
Broadly, the neuroglia can be defined as group of cells of nervous system which are other than the neurons. So the cells of this family do not posses two basic characteristics of neurons, i.e. irritability and conductivity. That is why none of them can generate and conduct the nerve impulse. Both in central as well as peripheral nervous system fundamentally they act as intercellular (interneuronal) supportive material. In addition, each type of neuroglia is characterized by its independent specific function.
Size of neuroglia is much smaller than neurons, but their number is far more proportionately, may be as many as 50 times the number of neurons. When the number of neurons are fixed after birth, the neuroglia can multiply throughout life. In case of injury or disease of nervous tissue, area of damaged or dead neurons, are occupied by multiplying neuroglia. This process is known as replacement gliosis.
Types of Neuroglia
In central nervous system
- Ependymal cell
- Macroglia – a) Astrocytes b) Oligodendrocytes
In peripheral nervous system
- Schwann cells.
- Satellite cells.
Ependymal cell (Fig. 1.14A)
These are single-layered cubical or columnar cells lining the cavities (ventricles and central canal) of central nervous system (brain and spinal cord). They represent the original cells lining the neural tube of embryonic life. The free surface of the cells present ultramicroscopic finger-like prolongations which are nonmotile in nature. These are known as stereocilia.
- Stereocilia of free surface of ependymal cells increase surface area, so help in absorption of cerebrospinal fluid circulated in cavity of central nervous system.
- Specialized area of ependymal lining of ventricles is also concerned with formation of cerebrospinal fluid (CSF).
These cells are so-called because they are star-shaped with radiating cytoplasmic processes. Astrocytes are of following two types.
Protoplasmic astrocytes (Fig. 1.14B)
The radiating processes of these types of astrocytes are thicker containing more amount of cytoplasm inside.
They are related in relation to cell bodies of neuron (in gray matter of central nervous system). Terminal ends of the processes present foot-like expansions known as end-feet. These types of astrocytes are intermediate in position between cell bodies of neuron and blood capillary. End-feet come in contact in one side with neuronal cell body and in another side with wall of capillary, thus helping in selective transport of substance like nutritive substance or metabolites from blood capillary to neuron. This media may prevent transport of some unwanted or toxic materials, for which it is known as ‘blood brain barrier’, some drugs having action on central nervous system posses the ability to cross this blood brain barrier.
Fibrous astrocytes (Fig. 1.14C)
The cell bodies of these types of astrocytes are smaller with thinner and more branching processes. They are predominantly distributed in between pro-cesses of nerve cells (in white matter of central nervous system).
- Astrocytes posses supportive function acting as packing material of central nervous system.
- Astrocytes transport nutritive materials from blood capillaries to neurons.
- It forms the ‘blood brain barrier’.
Oligodendrocytes (Fig. 1.15A)
These are smaller round or spherical cells with lesser number of processes. They are found in white matter of central nervous system where expanded end of their processes wrap around the length of nerve fibers. This wrapping (ensheathment) or insulation of nerve fibers is known as Myelination. The myelination prevent the nerve impulse to be dissociated to the surrounding tissue and thus facilitate the full conduction of impulse towards the destination.
- Oligodendrocytes primarily provide supportive functions around neurons of central nervous system.
- They form myelin sheath around nerve fibers (processes of neurons) inside central nervous system.
Microglia (Fig. 1.15B)
Microglia are so-called because they are smaller in size. The cells present finer tortuous processes. Microglia are identified by following three characters which are related to the letter ‘M’.
- Microglia are Mesodermal in origin which develops from circulating Monocytes of fetal blood.
- Microglia function as Macrophages of central nervous system to engulf toxic substances, microorganism and damaged CNS tissue debris.
- For phagocytic function, microglia are Migratory in nature. Many microglia, acting as scavenger cells are localized at the site of damaged or degenerated nerve tissue and may fuse together to form a Multinucleated giant cell of central nervous system.
- Function: Microglia, as already stated above, are phagocytic in nature to act as scavenger cells or macrophages of central nervous system.
Schwann cells (Fig. 1.16)
These are the neuroglial cells found in peripheral nervous system, related to peripheral nerve fibers. The cells are flattened with adequate amount of cytoplasm surrounding nucleus. The surface of the cell is invaginated by processes of neuron. The nerve fiber, following invagination, undergoes spiral movement to be finally wrapped by layers of Schwann cells which finally acts as myelin sheath.
- Function: Many Schwann cells in a row, take part in formation of myelin sheath of peripheral nerves outside central nervous system.
Satellite cells (Fig. 1.17)
These are another variety of glial cells related to peripheral nervous system. These cells are related to surface of cell body of the neurons which are present outside the central nervous system, e.g. neurons of posterior root ganglia of spinal nerves and neurons of sympathetic ganglia. The satellite cells are flattened in shape and small in size. A good number of these cells form an encapsulation around the surface of the above mentioned neurons present outside the central nervous system.
- Function: Satellite cells are also known as capsular cells as they form a covering around the cell bodies of neurons of peripheral nervous system.
Developmental Source of Neuroglial Cells
- Ependymal cells: These represent the original parent lining cells of primitive neural tube (ectodermal).
- Macroglia (Astrocytes and oligodendrocytes): from the spongioblasts of mantle zone of neural tube (ectodermal).
- Microglia: From the circulating monocytes of fetal life (mesodermal).
- Schwann cells and satellite cells: From cells of neural crest (ectodermal).
A nerve fiber, either in peripheral or in central nervous system carries nerve impulse towards destination. This impulse must reach the destination to the full extent with full velocity without being dissociated in the surrounding tissue. For this, the nerve fiber needs insulation (nonconductive coating). This insulation is formed by formation of sheath around the fiber, known as myelin sheath. In the nervous system, only the supporting cells are available to form this myelin sheath. In peripheral nervous system, the Schwann cells and in central nervous system, the oligodendrocytes take part in formation of myelin sheath.
In peripheral nervous system, many Schwann cells come in relation to the length of a single nerve fiber in a row. These many Schwann cells are invaginated by a single nerve fiber. Due to invagination, Schwann cell give rise to formation of mesoaxon (Fig. 1.16A). Now the Schwann cell rotates around axon in a spiral fashion. The mesoaxon turn around several times around the fiber, thus squeezing out the cytoplasm at the periphery of Schwann cell. The turns of cell membrane of Schwann cell around nerve fiber form the myelin sheath (Fig. 1.18A). The multiple layered membranous sheath is white in color due to presence of white lipid– protein. More peripherally rim of cytoplasm of Schwann cell form an additional sheath which is known as Schwann cell sheath or neurilemmal sheath. Intermittent gap between the segments of myelin sheath formed by adjacent Schwann cells is known as nodes of Ranvier (Fig. 1.18B). An unit of sheath formed by a single Schwann cell, in between nodes of Ranvier is known as internode.
Myelin sheath of central nervous system is formed by oligodendrocytes. But it is important to note following important points at this stage (Fig. 1.15A).
- It is not the whole cell, but only the processes of oligodendrocyte take part in formation of myelin sheath.
- Many processes of one oligodendrocyte take part in the formation of myelin sheath of many nerve fibers.
- Again processes of many oligodendrocytes take part in formation of myelin sheath of single nerve fiber.
- Myelin sheath helps in conduction of impulse through nerve fiber to the full extent and with full velocity to the destination.
- Thus myelin sheath prevents dissociation of impulse to the surrounding tissue.
- It acts as a support to the nerve fiber.
- It prevents ionic interchange between the cytoplasm of nerve fiber and the surrounding tissue.
Tumors of Central Nervous System
A tumor is formed as a result of abnormal cell division (mitosis) of a tissue. It is important to note at this stage that nervous system is composed of neurons, neuroglia as well as blood vessels and meninges (covering of central nervous system made up of connective tissue). Among these, only the neurons are fixed in number as they do not multiply after birth. So a tumor in nervous system cannot originate from neuron. Tumors formed due to abnormal proliferation or multiplication of neuroglia and cells of connective tissue of meninges and cells of wall of blood vessels are known as Glioma, Meningioma and Angioma respectively.
Neuronal Damage or Injury
Injury may affect neuronal cell body and/or processes. Initially it leads to loss of function. But ultimate effect will depend upon serverity of the injury and duration of action of injurious agents. It is important to note at this stage that if neuronal death occurs quickly, within a few minutes, no morphological changes are found. But if the neuron manages to survive for 6–12 hours, morphological changes are characterized by swelling of cell cytoplasm and nucleus, and displacement of Nissl granules to the periphery. This is followed by recovery of the neuron.
When a nerve cell process (axon) is a cut or injured, it will lead to change in nerve cell which is known as axonal reaction or axonal degeneration. This change is noticed within 24–48 hours. The change is more rapid if axon is injured close to cell body. Axonal injury in peripheral nervous system is followed by an attempt for repair in cell body. In central nervous system, degeneration is not followed by regeneration.
Activities of Neuroglia Following Neuronal Damage
Neurons show some stage of cell death. Initially they are characterized by dark stained cytoplasm with ill-defined nucleus. But the neurons finally get dissolved passing through a stage of appearance of vacuoles in cytoplasm and disintegration of cell organelles. By this time microglia, being migratory in nature, rush to the site of lesion to act as scavenger with their phagocytic activity. Monocytes from the neighboring bloodstream also join with the microglia to help in scavenging activity. It is now the astrocytes which undergo hyperplasia and hypertrophy to occupy the space of disintegrated neurons. This procedure is known as replacement gliosis.
Spread of Some Viral Infection Through Neuronal Process
Rabies is a viral disease which causes acute attack on central nervous system. The virus is transmitted through the bite of rabied dog or some other wild animal. From the site of bite virus spread centrally towards central nervous system through retrograde direction (retrograde axonal transport) via axoplasm. Therefore, it is clear that onset of the disease will be quicker if the site of the wound (due to bite) is more close to central nervous system (i.e. in trunk or head and neck) than if it is away (e.g. in distal part of limbs).
Axonal transport also play important role in spread of some other viral diseases affecting nervous system, e.g. poliomyelitis, and herpes simplex and herpes zoster.
Chemical Agents Acting on Synaptic Transmission
Neurotransmitters jumping through synaptic cleft from presynaptic neuron to postsynaptic neuron are responsible for conduction of nerve impulse through chain of neurons to the destination. Chemical agents acting on autonomic ganglia may interfare with neurotransmission is either of two ways. Some agents like procaine, simply inhibit release of Acetylcholine (neurotransmitter) from presynaptic neuron. The other group, like nicotine, hexamethonium do not give chance to Acetylcholine to act on postsynaptic membrane, because these drugs occupy the receptor site of postsynaptic membrane. Some drugs which can cross the blood brain barrier, like atropine, scopolamine, are able to act on synapses of central nervous system. In myasthenia gravis there is a profuse deficiency of synaptic transmission due to absence of Acetylcholine in synaptic cleft.
Caffeine present in tea or coffee act as neuromodulator which enhance the activity of neurotransmitters stimulating central nervous system.
Multiple Sclerosis – A Disease Causing Demyelination
Multiple sclerosis is a degenerative disease of central nervous system. Exact cause of the disease is not known. Probable cause is the imbalance between some viral infection and immune response of the individual. Young adults between the age of 20–40 years are most commonly affected. Fibers of optic nerve, spinal cord and cerebellum are affected usually. The myelin sheath of nerve fibers are degenerated during active phase of the disease. The myelins are scavenged by microglia with subsequent formation of replacement gliosis. The disease is sometimes typically characterized by exacerbations and remissions.