Organization of the Nervous System

The nervous system has two main subdivisions, the central nervous system (CNS) and peripheral nervous system (PNS). The central nervous system is the major processing center in the animal body, which is composed of the two major parts, the brain and spinal cord.

The nervous tissue, except the brain and spinal cord, is known as the peripheral nervous system (PNS). PNS consists of the nerves, ganglia, and sensory receptors.

number of neurons in the enteric nervous system is more than the entire spinal cord.

Know More......

Nucleus: Nucleus is the collection of neuron cell bod­ies in the CNS.

Tract: Nerve tract is the collection of axons in the CNS.

Ganglia: Ganglia is the collection of neuron cell bodies in the PNS.

Nerve: Nerve is the collection of axons of neurons in the PNS.

The CNS and PNS act together in a synchronous pattern with each other. The sensory receptors are present through­out the body, which continuously monitor the external as well as the internal environment and send the information to the CNS via PNS. The information is then analyzed in the CNS which sends signals to the target organ through PNS. Then the particular organ takes necessary action according to the need. Some of the functions are completely restricted within the CNS, viz. dreaming, thinking, and storage of information.

The two main subdivisions of PNS are somatic nervous system and autonomic nervous system. The somatic nervous system is associated with the voluntary movement of skeletal muscles, whereas the autonomic nervous system regulates the involuntary functions of organs and tissues.

11.2.1 Central Nervous System

Central nervous system is composed of brain and spinal cord. Brain is the main part of CNS, covered and protected by the skull. Morphologically, the brain is divided into three parts, forebrain or prosencephalon, midbrain or mesencephalon, and hindbrain or rhombencephalon. The major parts of fore­brain are cerebrum, thalamus, and hypothalamus (part of the limbic system). The main parts of midbrain are the tectum and tegmentum, and the hindbrain consists of the cerebellum, pons, and medulla oblongata. The midbrain, pons, and medulla together are considered as the brain stem.

11.2.1.1 Meninges

The intact CNS is enclosed by the connective tissue covering known as meninges. Meninges has three layers, viz. dura mater, arachnoid mater, and pia mater (from outside to inside). Dura mater is composed of an outer endosteal layer and an inner meningeal layer, and the dural sinus is present in between the layers. Arachnoid mater covers the brain just above the pia mater. Pia mater is attached to the brain by astrocytes and wraps brain tightly. The space between the arachnoid mater and dura mater is referred to as subdural space, whereas the space between arachnoid mater and pia mater is known as subarachnoid space. Both the spaces are filled with cerebrospinal fluid (CSF). Subdural space and subarachnoid space are frequent sites of intracranial hemor­rhage. In the spinal cord, the dura covering is single layered. The space underneath the dura is known as subdural space, and the space external to it is called epidural space.

11.2.1.2 Ventricles of Brain

There are four ventricles present in the brain (Fig.

11.2.1.3 ChoroidPlexuses

The choroid plexus is formed by capillaries as a complex network. It is lined by specialized types of ependymal cells. These cells produce cerebrospinal fluid (CSF). Choroid plexus acts as a barrier and separates the blood from the CSF and is thus known as the blood-CSF barrier. Choroid plexus also secretes different growth factors, which maintain the stem cell pool in the sub-ventricular zone. It is involved in brain development and gives protection against pathogenic microorganisms and toxic materials.

11.2.1.4 CerebrospinalFluid

Fig. 11.5 Ventricles of brain (from the lateral side)

Cerebrospinal fluid (CSF) is a colorless fluid found in the ventricles of the brain, central canal of the spinal cord, and subarachnoid space surrounding the outer surface of the brain and spinal cord. The CSF contains very less quantity of protein and almost no blood cells due to selective tight junction barrier.

11.2.1.4.1 Formation, Absorption, and Composition of CSF

CSF is secreted by the ependymal cells located in the choroid plexus. The plexuses consist of tufts of capillaries covered by a layer of ependymal cells. These ependymal cells unlike the cells lining the rest of the ventricle form a selective tight junction barrier to the secretions of the leaky capillaries and to other surrounding fluids. Membrane transporters and selec­tive channels regulate the passage of ions and molecules across the ependymal cell barrier, effectively controlling the composition of the CSF being synthesized in the ventricle.

CSF is absorbed and returns to the venous system mainly into dural venous sinuses, which are present intracranially between the endosteal layer and meningeal layers of the dura mater. Majority of the CSF is absorbed from the subarach­noid space into the dural sinus through arachnoid villi. The absorption occurs due to the difference of pressure between the arachnoid mater and venous sinuses. CSF is also drained into lymphatic vessels. Reabsorption of CSF occurs through sheaths of cranial and spinal nerve and through ependymal cells.

11.2.1.4.2 FunctionsofCSF

The major function of CSF is to act as a cushion and provide supports to different structures of CNS. CSF protects the brain and spinal cord from any physical injury and in any significant variation in the local environment.

11.2.2 Cerebrum

Cerebrum is the topmost and largest part of brain. It is composed of two cerebral hemispheres separated incom­pletely by the median longitudinal fissure, and the hemispheres are joined with each other by the corpus callosum. Cerebrum controls the higher mental functions, which include the conscious thoughts and experience. Cere­brum processes the somatic sensory and motor information. The surface layer of cerebrum is of grey matter (cerebral cortex). Cerebrum is attached with the rest of the brain through the cerebral peduncles. Cerebral peduncles are interconnected in between them and also the subarachnoid cisternae through the foramina of Luschka and Magendie.

The surfaces of hemisphere are highly convoluted having numerous elevations and depressions known as gyri and sulci, respectively. These convoluted appearances increase the surface area for accommodation of large number of cells. The deep grooves are called fissures. The superficial layer of cerebral hemisphere is rich in cell bodies of neurons, known as cerebral cortex. The cerebral hemispheres regulate the major functions of the body. It is also the site for integra­tion of different somatic functions. Cerebral cortex is grey in appearance due to the presence of large numbers of cell bodies of the neurons, so the cortex is known as grey matter. Beneath the cerebral cortex, the nerve fibers are grouped together and form the white matter.

Know More.......

Corpus callosum: Corpus callosum is a broad band of nerve fibers that join the left and right hemispheres. It is the largest white matter structure in the brain and allows the two hemispheres to communicate.

11.2.2.1 Lobes of the Brain

Each of the cerebral hemispheres has been divided into four lobes, viz. frontal lobe, parietal lobe, temporal lobe, and occipital lobe. Each lobe is associated with different functions. Frontal lobe is located at the front part or anterior part of cerebral hemispheres of the brain and extends up to the central sulcus. This lobe is associated with reasoning, planning, making decisions, and controlling the behavior, parts of speech, movement, emotions, etc. Parietal lobe is located just behind the frontal lobe and is associated with the sensory information like touch, spatial awareness, and navi­gation. This lobe controls the movement, orientation, recog­nition, and perception of stimuli. Temporal lobe is located on either side of the brain and just above the ears in human. This lobe is related to perception and recognition of auditory stimuli, memory, and speech. Occipital lobe is present at the back portion of brain and is associated with visual processing. Basal ganglia are the masses of grey matter, which are stacked lateral to the hypothalamus inside each hemisphere below the lateral ventricles but lateral to the third ventricle.

11.2.3 Diencephalon

Diencephalon is a derivative of prosencephalon and located just below the cerebral hemispheres. It is mainly composed of the thalamus, epithalamus, hypothalamus, and third ventricle. Diencephalon connects the cerebrum with the rest of the brain.

11.2.3.1 Thalamus

The thalami are two egg-shaped structures present on either side of the midline. It acts as an important relay center for the nerve fibers to connect the cerebral hemisphere with the brain stem, cerebellum, and spinal cord. All the sensory informa­tion passes through the thalamus and reaches the cerebral cortex for processing; the only exception is the smell sensa­tion, which does not pass through the thalamus. The thalami also involve to process the information.

11.2.3.2 Epithalamus

The epithalamus is located just dorsal to the thalamus and forms the roof of the third ventricle. It contains the pineal gland, an endocrine gland that secretes melatonin hormone. Melatonin plays an important role in the regulation of circa­dian rhythms and sleep-wake cycle. It also regulates the breeding seasons in seasonal breeding animals. Epithalamus also includes the choroid plexus of the third ventricle and involves in the formation of the CSF.

11.2.3.3 Hypothalamus

The hypothalamus is present just ventral to the thalamus and surrounds the ventral part of the third ventricle. It terminates in a sharp angle where the pituitary gland is attached. The hypothalamus forms the floor of the diencephalon. Hypothal­amus is an important center of autonomic nervous system. Hence, hypothalamus is termed as the “captain of autonomic nervous system.” Hypothalamus plays an important role in the regulation of body temperature, water balance, metabo­lism, and emotions. Thus, it is an essential part of the limbic system. Hypothalamus also controls the endocrine system and plays an indispensable role in the regulation of homeo­stasis. The hypothalamic hormones regulate the secretion of pituitary gland. Hypothalamus is discussed in details in Sect. 11.2.5.

11.2.3.4 Mammillary Bodies

The mammillary bodies are small round-shaped paired structures present just inferior to hypothalamus. It is a part of diencephalon and connected to hippocampus, thalamus, and tegmental nuclei of the midbrain. Mammillary bodies act as a relay center for olfaction and are associated with memory.

11.2.4 BrainStem

The brain stem, like true stem, holds the cerebrum at its top and cerebellum posteriorly. Brain stem processes information between spinal cord and cerebrum or cerebellum. It controls autonomic behavior necessary for survival. It is divided into midbrain, pons, and medulla oblongata, from top to down­wards. It is associated with 10 of the 12 pairs of cranial nerves. Outer surface of the brain stem, unlike the cerebrum and cerebellum, contains white matter which opens up poste­riorly for the fourth ventricle. There are many grey matter masses in its substance serving various important functions. Some of these nuclei connect with the central nervous system.

11.2.4.1 Mesencephalon or Midbrain

Mesencephalon or midbrain is a small component of brain, which remains in between the thalamus and pons. Midbrain has two major parts, i.e., the tectum and tegmentum. Tectum from the roof and tegmentum form the floor of midbrain. Four round-shaped protrusions or colliculi (singu­lar = colliculus) are present on the dorsal side of the midbrain known as corpora quadrigemina. They consist of right and left rostral colliculi and right and left caudal colliculi. The rostral colliculi are associated with orienting the eyes to a sound or touch stimulus, and the caudal colliculi are respon­sible for auditory reflex. Two cerebral peduncles, also called crura cerebri, are present in the midbrain. They consist of bundles of nerve fibers, which connect the spinal cord and brain stem to the cerebral hemispheres. These peduncles mainly comprise descending motor fiber tracts.

11.2.4.2 Cerebellum

The cerebellum (Latin for “little brain”) is located caudal to the cerebral cortex and dorsal to the brain stem and serves important functions in the regulation of balance and equilibrium. It is connected by three pairs of peduncles with the brain stem. Cerebellum is the second largest part (almost 10%) of the brain; but, due to the highly convoluted structure, it contains more than half of all the brain’s neurons. The outer layer of cerebellum is called cerebellar cortex, which consists of grey matter and highly regular arrangement. Two large pairs of white matter stacks called cerebellar peduncles mainly carry axons into the cerebellum, and a third pair of cerebellar peduncles carries axons out from the cerebellum. A number of cerebellar nuclei are present within the cerebellar white matters, which are the principal origin of the axons leaving the cerebellum. Cerebellum controls the timing and coordination of movement by adjusting and modulating the output of the motor cortices, corticospinal tract, descending brain stem motor pathways, and spinal cord. The intended movement is continuously compared with the actual move­ment by the cerebellum, and then suitable adjustment is done. The vestibulocerebellum adjusts the coordination of vestibu­lar reflexes. This part of the cerebellum was the first to appear in vertebrate evolution; hence, it is sometimes called the archicerebellum. The spinocerebellum extends rostrocaudally through the medial portion of the cerebellum, which helps to coordinate muscle tone as well as limb move­ment. It recognizes and predicts subconsciously (as does all processing that occurs outside the cerebral cortex). In some of the motor learning processes which require balance, the cere­bellum activity is high during learning, and when they become automatic, cerebellum is no longer involved. Any disease in cerebellum results in abnormalities of movement, which further illuminates cerebellar function.

11.2.4.3 Pons

Pons (Latin for “bridge”) is a part of brain stem located in between the midbrain and medulla oblongata. The white matter on the anterior surface forms the bridge, whereas the grey matter which remains beneath the white matter is the extension of the tegmentum from the midbrain. The grey matter contains neurons which receive descending inputs from the prosencephalon and send it to the cerebellum. Pons is associated with the somatic and visceral motor con­trol. Four cranial nerve nuclei are present in the pons, viz. V, VI, VII, and VIII. Pons also contains the nucleus for reticular formation. Several nuclei and tracts to cerebellum and other parts of CNS pass through the pons. The nerves associated with pons are involved in the regulation of hearing, mainte­nance of equilibrium, taste, and facial sensations of touch and pain.

11.2.4.4 Medulla Oblongata

It is the most inferior part that connects brain to spinal cord. It plays an important function in the transmission of signals between the spinal cord and the higher parts of the brain and controls some autonomic functions like heartbeat and respiration. The medullary pyramids are two longitudinal ridges formed by corticospinal tracts. Medulla oblongata regulates autonomic functions. It regulates arousal, heart rate, blood pressure, pace for respiration, and digestion. Cra­nial nerves, IX, X, XI, and XII, come off or enter from the medulla oblongata. The nerve fibers related to the reticular formation pass through the medulla oblongata.

11.2.4.4.1 MedullaryNuclei

Some regulating centers are present in medulla. These are the following:

1. Cardiovascular control center: It adjusts force and rate of heart contraction.

2. Respiratory center: It controls the rate and depth of breathing.

3. Additional center: It regulates vomiting, hiccupping, swallowing, coughing, and sneezing.

Know More.......

Basal ganglia: This is involved in the control of vol­untary motor movements, procedural learning, and decisions about which motor activities to carry out. Diseases that affect this area include Parkinson’s disease and Huntington’s disease.

Broca’s area: This small area on the left side of the brain (sometimes on the right in left-handed individuals) is important in language processing. When damaged, an individual finds it difficult to speak but can still understand speech. Stuttering is sometimes associated (Trusted Source) with an underactive Broca’s area.

11.2.5 Limbic System

Limbic system is referred to the entire neuronal components, which control the olfaction, emotional behavior, motivational drives, and memory. The limbic system is composed of several parts of the brain present on both sides of the thala­mus, just under the cerebrum. The structures are hypothala­mus, hippocampus, amygdala, and other parts of the brain in nearby areas. The major function of limbic system is control of emotions and formation of memories. It is also involved in the regulation of homeostasis, olfaction, and many other psychologic functions.

11.2.5.1 Hypothalamus

Hypothalamus is the major part of the limbic system and regulates most of the vegetative and endocrine functions and emotional behaviors (anger and aggressive). It is involved in the homeostasis. The thermoregulatory, appetite, satiety, and thirst centers are located in the hypothalamus. Hypothalamus regulates different functions of the body in two ways, either through the autonomic nervous system or via secretions of different hormones from the pituitary gland. Hypothalamus controls different vital functions like blood pressure, heart rate, breathing, digestion, sweating, and arousal in response to emotional situations through auto­nomic nervous system. It also regulates all the sympathetic and parasympathetic functions. Hypothalamus controls the secretion of pituitary hormones by discharging the releasing or inhibitory hormones. Hypothalamus also regulates many important functions, like response to pain, levels of pleasure, and sexual satisfaction.

11.2.5.2 Hippocampus

The hippocampus is present in the temporal lobe and medial to the inferior horn of the lateral ventricle of each cerebral hemisphere. The name hippocampus came from the Greek word for seahorse because of its structure. Hip­pocampus is associated with different important functions; but it is best known for its role in memory. The hippocam­pal formation, a prominent C-shaped structure located in the temporal lobe, consists of hippocampus and the adja­cent cortex called the parahippocampal gyrus and a strip of grey matter in between the two structures known as the dentate gyrus.

The hippocampal gyrus areas are called the entorhinal cortex and subiculum, which are both involved in the flow of information through the hippocampus. The hippocampus is anatomically subdivided into four regions, i.e., CA1 through CA4 (the CA stands for cornu ammonis). The hippo­campus receives information from the rest of the cerebral cortex primarily via the perforant pathway, which originates in the entorhinal cortex and projects to the dentate gyrus. Fibers then leave the dentate gyrus, which is part of the hippocampal formation, and project to neurons in the CA3 region of the hippocampus. Neurons in CA3 then send axons to neurons in the CA1 region, which projects to neurons in the subiculum. The subiculum can be considered the main output region of the hippocampal formation. Fibers from the subiculum project back upon neurons in the entorhinal cor­tex. Fibers from the entorhinal cortex travel out to a variety of areas in the cerebrum. Output fibers also leave the subiculum and hippocampus and enter the fornix, a fiber bundle that connects the hippocampus with a variety of subcortical areas like the thalamus and hypothalamus.

The sensory information causes activation of hippocam­pus and distributes signals to the anterior thalamus, hypothal­amus, and other parts of the limbic system, especially through the fornix, a major communicating pathway. So, the hippo­campus acts as an additional channel which transmits incom­ing sensory signals that can initiate behavioral reactions related to different purposes. Hippocampus is associated with different behavioral patterns like other parts of the limbic system.

11.2.5.3 Amygdala

The amygdales, two almond-shaped structures, are the col­lection of neurons present deep in each temporal lobe on either side of the thalamus and adjacent to the hippocampus. It is connected with the hypothalamus as well as other parts of the limbic system. It receives information from different parts of the limbic system and from the neocortex of the temporal, parietal, and occipital lobes specially from the auditory and visual association areas. Amygdala is primarily associated with emotion, memory, and fight-or-flight response. Because of these multiple connections, the amygdala has been called the “window” through which the limbic system sees the place of the individual in the world. When amygdala is stimulated, animals respond with aggression.

11.2.5.4 Other Related Parts of Limbic System

The limbic system is intimately connected with some nearby structures, like cingulated gyrus, ventral tegmental area, basal ganglia, and prefrontal cortex. Cingulate gyrus is a part of the cerebrum and is present just above the corpus collosum. It makes a pathway between the thalamus and hippocampus. Cingulate gyrus is associated with emo­tional events and associating memories to smells and to pain.

The ventral tegmental area is a part of the brain stem present just below the thalamus. It is a part of the limbic system. Ventral tegmental area has dopamine pathways, which are associated with pleasure. The basal ganglia are also a part of the limbic system and remain over and to the sides of the limbic system. Basal ganglia are firmly connected with the cerebral cortex above them. Basal ganglia consist of caudate nucleus, putamen, globus pallidus, and substantia nigra. These parts are responsible for repetitive behaviors, reward experiences, and focusing attention.

Another part of limbic system is the prefrontal cortex. It is a part of the frontal lobe of cerebrum located in front of the motor area. Prefrontal cortex is associated with thinking about the future, making plans, and taking action. It is also involved in the dopamine pathways as the ventral tegmental area and plays a part in pleasure and addiction.

11.2.6 ReticularFormation

The reticular formation is the phylogenetically primitive network of small neurons and occupies the midventral por­tion of the medulla and midbrain, extending throughout the brain stem and into the spinal cord. The reticular formation is a very complex structure, which contains a number of nuclei, which are interconnected, and nerve tracts. The nerve fibers form a netlike appearance in the central core of the brain stem. But the part is not anatomically well defined as the nuclei are present in different parts of the brain. It is also known as the reticular activating system (RAS). The neurons form a complex network, and they are extended from the upper part of the midbrain to the lower part of the medulla oblongata. The ascending pathway of reticular formation toward the cortex is known as ascending reticular activating system (ARAS), whereas the descending pathway toward the spinal cord is known as reticulospinal tracts.

Various neural clusters and fibers of it have discrete functions. For example, it contains the cell bodies and fibers of many of the serotonergic, noradrenergic, adrenergic, and cholinergic systems. It also contains many of the areas concerned with regulation of heart rate, blood pressure, and respiration. Some of the descending fibers in it inhibit trans­mission in sensory and motor pathways in the spinal cord; various reticular areas and the pathways from them are concerned with spasticity and adjustment of stretch reflexes. The RAS is a complex polysynaptic pathway arising from the brain stem reticular formation with projections to the intralaminar and reticular nuclei of the thalamus, which, in turn, project diffusely and nonspecifically to wide regions of the cortex. The system is therefore nonspecific, whereas the classic sensory pathways are specific in that the fibers in them are activated by only one type of sensory stimulation. The axons of the reticulospinal tract are associated with spinal reflex activity and can modulate the sensory input by controlling the gain at synapses in the spinal cord. The reticulospinal tract also carries axons that modulate auto­nomic activity in the spinal cord.

Reticular formation is the major regulator of the state of consciousness and arousal. Several neurons of reticular for­mation are serotonergic; that is, they use serotonin as neuro­transmitter. Fibers from these nonspecific thalamic nuclei extend to most of the cerebral cortex and thus control the activity of large numbers of neurons. Reticular formation is mainly associated with the regulation of arousal and con­sciousness of animals. A variety of sensory stimuli like auditory, visual, somatosensory, and visceral sensory stimuli excite the neurons of the reticular formation. Different regions of the cerebral cortex are associated with the arousal. As different features of the external environment (viz. color, shape, location, sound of various external stimuli) are represented in different areas of the cortex, it has been suggested that “binding” of neural activity in these different areas is involved in consciousness. The reticular system is also associated with the regulation of respiration, heart rate, and blood pressure and modulation of nonspecific sensory information. Reticular system is also associated with the adjustment of reflexes and postural control.

11.2.7 Blood-BrainBarrier

The CNS has some special kind of barrier, which isolates neuronal tissue of CNS from the general circulation known as the blood-brain barrier (BBB) (Fig. 11.6). The existence of the BBB has been observed using the dye like trypan blue. The trypan blue can stain all tissues in the body except tissues of brain and spinal cord when infused intravenously. But if the stain is injected directly in the ventricles, then brain tissue takes the stain.

11.2.7.1 Structure of BBB

The barrier is formed by a network of tight junctions between endothelial cells of CNS capillaries and by feet of astrocyte processes. Entry of the substances is generally restricted through BBB due to special morphological features of the capillary endothelium, the structural basis of the blood-brain barrier. The endothelial cells interact with the surrounding layer of astrocytic “end feet” to form the special barrier (Fig. 11.6).

The blood capillaries of the CNS have some unique characteristics, like the following: (1) the capillaries have continuous tight junctions, which seal neighboring endothe­lial cells; (2) absence of fenestrations; and (3) presence of very few numbers of small pinocytotic vesicles. The capillaries of the brain have a more number of mitochondria, which helps in the operation of the transporters.

11.2.7.2 Restricted Movement Through BBB

The body water-soluble compounds easily pass through open clefts present between capillary endothelial cells. However, most of the compounds that pass through intercellular clefts of various tissues are blocked by tight junctions, and entry of brain capillary blood solutes is very much selective. Gener­ally, the molecules which are relatively small, uncharged, lipid soluble, and unbound to plasma proteins (e.g., O₂, CO₂, N₂O, ethanol, nicotine) can easily pass through the capillary endothelium of the BBB and glucose, and some amino acids are able to pass by specific carrier-mediated transport mechanisms.

Some smaller neutral amino acids such as glycine, ala­nine, serine, cysteine, proline, and γ-aminobutyric acid (GABA) are synthesized in the CNS, and these are transported mainly from the brain to the blood circulation. The transport of these amino acids requires an energy­dependent and Na⁺-dependent symport carrier located at the abluminal side of the endothelial cell membrane. The lopsided transport of the neurotransmitters across the blood-brain barrier ensures that neurotransmitters will not accumulate in the brain, preventing the potential neu­rotoxic glutamate effect and unwanted inhibition of neurons by glycine and GABA. Few degradative enzymes like monoamine oxidase are expressed by brain capillary endothelium, which gives an additional restriction on substances to pass the BBB.

11.2.7.3 Non-barrier Regions in the Brain

There are some specialized areas in the brain, like choroid plexus, hypophysis, median eminence, pineal gland, and area postrema, where blood capillaries are fenestrated, with lack of interaction between astrocytes and endothelial cells. These areas are considered as non-barrier regions. It facilitates the related organs to maintain their normal functions, like release of hormones into the circulation and monitoring circulating molecules. The interaction between astrocytes and endothe­lial cells is disturbed in some pathological conditions, viz. neoplasia, hypertension, dementia, epilepsy, infection, multi­ple sclerosis, and trauma.

Fig. 11.6 Blood-brain barrier. In brain, the cells of the capillary walls are joined by tight junctions, which restrict the passage of different materials. End feet astrocytes also help in the formation of tight junction and limit the entry of different materials in the brain

11.2.8 SpinalCord

The spinal cord is the caudal continuation of the medulla oblongata protected in the vertebral canal. Spinal cord contains the central canal in the middle around which there is a mass of grey matter, and in the periphery, there is the white matter. The spinal cord has 31 functional segments, each of which is connected with a pair of spinal nerves. In the spinal cord, the dorsal and ventral roots unite and come together as a nerve at the point where the axons exit and enter the vertebral canal (Fig. 11.7). Sensory neuronal cell bodies are present as a group called dorsal root ganglia lateral to the spinal cord. The neurons within these ganglia are pseudounipolar. They give rise to processes that enter the dorsal horn of the spinal cord.

Other fibers which unite with motor fibers form the ventral horn neurons to become the spinal nerve extending into the periphery. The processes which extend from the spinal nerve to the spinal cord form the dorsal root.

The ventral root of the spinal nerve consists of motor fibers that arise from the nerve cells primarily in the ventral horn of the spinal cord. The dorsal and ventral roots unite to form the spinal nerve close to the intervertebral foramen between adjacent vertebrae. The dorsal root ganglion remains very close to the joining of dorsal and ventral roots. The spinal cord is extended throughout the full length of the back and carries information between the brain and the other parts of the body. Throughout the length, the spinal cord is connected with different nerves of peripheral nervous system. The sensory information from different tissues runs through the spinal cord to reach the brain. The motor commands from the brain run in the course of the spinal cord to the muscle. The spinal cord is also associated with reflexive responses: for example, the sudden involuntary movement of the arm takes place if fingers come in contact with a flame.

11.2.9 PeripheralNervousSystem

The peripheral nervous system (PNS) is composed of the nerves and ganglia present outside the CNS. This includes the 12 pairs of cranial nerves and 31 pairs of spinal nerves along with different ganglia and plexuses. The function of PNS is to convey sensory information to the brain and spinal cord and to produce movement of muscle as well as secretion from glands via its motor nerves. Function of PNS is to connect different parts of the body with CNS. The nerves in the PNS are the processes of the neurons whose cell bodies are situated mostly in the CNS, and some are situated in the dorsal root ganglia of spinal nerves, ganglia of cranial nerves, autonomic ganglia, etc.

The enteric nervous system (ENS) is another division of peripheral nervous system (PNS), which controls the activity of gastrointestinal tract. ENS is the largest section of the autonomic nervous system. The ENS is able to control the gastrointestinal function independently of central nervous system (CNS) input.

11.2.9.1 Ganglia

A ganglion is the collection of neuron cell bodies in the PNS. On the basis of the primary functions, ganglia are of two types, sensory ganglia and autonomic ganglia. The dorsal (posterior) root ganglion is the most common sensory gan­glion. In ganglia, cell bodies are present and the axons of the neurons act as sensory fiber endings in the periphery, like in the skin. It extends into the CNS through the dorsal nerve root. In dorsal root ganglion, the neurons are unipolar and small round nuclei of satellite cells are also seen.

Fig. 11.7 Cross-sectional view of the spinal cord and spinal nerves

11.2.9.2 Nerves

A number of axons form a bundle in the PNS called nerve. In CNS, collection of axons is called a tract. Nerves have connective tissues in their structure. Blood vessels supply nourishment to the nerve tissues. Epineurium is the fibrous connective tissue layer surrounding the outer surface of a nerve. Within the nerve, a number of axons form a bundle called the fascicles. Each fascicle is surrounded by fibrous connective tissue layer called perineurium. Individual axons are surrounded by a loose connective tissue known as the endoneurium. These layers of connective tissue surrounding a nerve are similar to the connective tissue coverings of muscles. Nerves are of two types depending upon the regions to which they are connected, i.e., cranial nerves that are connected to the brain and spinal nerves which are connected to the spinal cord.

11.2.9.2.1 CranialNerves

The nerves associated with the brain are called cranial nerves. These nerves come out from the part of CNS present within the cranium. These nerves are mainly responsible for the sensory and motor functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of the parasympa­thetic nervous system). Twelve pairs of cranial nerves are present in the nervous system. The first and second are connected to the forebrain, and rest of the nerves are connected to the brain stem.

The nerves are classified as sensory nerves, motor nerves, or a combination of both (known as mixed nerve). Within the 12 pairs of cranial nerves, 3 pairs of the nerves (cranial nerves I, II, and VIII) are exclusively composed of sensory fibers; 5 pairs of nerves (III, IV, VI, XI, XII) are strictly motor; and the remaining 4 pairs (V, VII, IX, X) are mixed nerves (Table 11.1).

11.2.9.2.2 SpinalNerves

The nerves which arise from the spinal cord are called spinal nerves. Their arrangement is much more regular than that of the cranial nerves. Each nerve has both sensory and motor fibers, which separate into two nerve roots. Generally, a pair of spinal nerves (one right and another left) come out caudal to the vertebra of the same number and name (exceptions are cervical and caudal nerves). For example, the first pair of thoracic nerves originate from the intervertebral foramina between the last thoracic and first lumbar vertebrae, and the first pair of lumber nerves origi­nate from the foramina between the first and second lumbar vertebrae.

The sensory axons of the spinal nerve enter into the spinal cord as the dorsal root, and the motor fibers of the spinal nerve, both somatic and autonomic, come out as the ventral root. The dorsal root ganglion is an enlargement of the spinal nerve. There are 31 pairs of spinal nerves attached to the spinal cord by two routes. The nerves are named according to the name of the structure from where it originates, viz. cervi­cal, thoracic, lumbar, sacral, and coccygeal nerve. There are 8 pairs of cervical nerves (C1-C8), 12 pairs of thoracic nerves (T1-T12), 5 pairs of lumbar nerves (L1-L5), 5 pairs of sacral nerves (S1-S5), and 1 pair of coccygeal nerves.

Table 11.1 Cranial nerves with their origin and functions

11.2.9.2.3 NervePlexuses

Spinal nerves originate from the spinal cord, come out through the vertebral column, and enervate the periphery. The nerve fibers of different spinal nerves come together to form a bundle and again come out and give rise to systemic nerves. These nerve bundles are called nerve plexus. The formation of nerve plexus is seen at four places in the body. The other spinal nerves directly correspond to nerves at their respective levels. So, nerve plexus is the network of nerve fibers without any associated cell bodies.

Out of the total four nerve plexuses, two are found at the cervical region, one at the lumbar region, and one at the sacral region. The axons of cervical nerve C1-C4 form the cervical plexus and again branches are distributed in the head and neck region. Spinal nerves C3, C4, and C5 together form the phrenic nerve, which is connected with diaphragm. The C5, C6, C7, C8, and T1 come together and form the brachial plexus. Brachial plexus innervates the arms. Three nerves originating from the brachial plexus are the medial nerve, radial nerve, and ulnar nerve. The spinal nerves L1 through L4 form the lumbar plexus and give rise to the nerves enervating the pelvic region and thigh region of legs. Femoral nerve is a major nerve that arises from the lumbar plexus, which gives rise to saphenous nerve that extends through the lower part of legs. The sacral plexus is formed by the lumbar nerves L4 and L5 and the sacral nerves S1-S4. The most important systemic nerve which arises from this plexus is the sciatic nerve. It is a combination of the tibial nerve and the fibular nerve. The sciatic nerve runs across the hip joint. The sacral plexus supplies nerves to the posterior leg.

The systemic nerves that arise from the different nerve plexuses have fibers for the function of both the sensory and motor activities. The sensory fibers extend from cutaneous or other peripheral sensory surfaces and send information to the CNS. The sensory neurons in the dorsal root ganglia enter the spinal cord through the dorsal nerve root. The motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root, send action potentials to cause skeletal muscles to contract in their target regions. The spinal nerves of the thoracic region, T2 through T11, do not form any plexuses, but they give rise to the intercostal nerves present in the intercostal space.

11.2.8 MyelinSheath

Several nerve fibers are covered by a lipid envelope called myelin sheath, which acts as an insulator and helps in the transport of the action potential faster (saltatory conduction).

The myelin sheath is deposited by Schwann cells in PNS and by oligodendrocytes in CNS. The process by which myelin sheath is formed or deposited on nerve fibers is called myelinogenesis, which occurs as follows: the nerve fiber to be myelinated is first invaginated by the cell membrane of the Schwann cell and thus a double-layered mesaxon is formed. Then, the Schwann cell rotates around the nerve fiber and several layers of this mesaxon (actually two layers of the cell membrane).

11.2.9 Sensory Receptors

Receptors which are related to the nervous system are called sensory receptors. Sensory receptors are biological transducers, which can convert various forms of energy into action potential in the sensory nerves to which they are connected. Sensory receptors are present in everybody’s tis­sue except the nervous system itself. These receptors receive different types of information from inside and outside the body to maintain homeostasis. The sensory receptors correctly sense other types of changes, which occur inside and outside the body.

11.2.11.1 Classification of Sensory Receptors

Based on structure, sensory receptors are of three types:

1. Free nerve endings: Most sensory receptors are axon terminals of primary sensory neurons. Those receptors which do not have any modification are called free nerve endings. Free nerve endings are nonmyelinated and widely distributed in the body. They form many branches in the tissue. They are receptors for pain, touch, temperature, etc. Some sensory receptors have expanded nerve endings; that is, their nerve endings are thickened to form a specialized structure to detect the sensory stimuli. For example, in Merkel’s corpuscle, a mechanoreceptor detects pressure.

2. Encapsulated nerve endings: In some sensory receptors,

sensory terminals are covered by a connective tissue cap­sule, classified as encapsulated nerve endings. They are mechanoreceptors and are not myelinated. They are mainly seen in the inner dermis, fasciae, mesenteries, skeletal muscles, and some viscera and comprise Pacinian corpuscles, Meissner’s corpuscles, and Ruffini’s

corpuscles.

3. Specialized receptors: These specialized receptors have distinct structural components for interpreting particular types of stimuli, for example, retinal photoreceptor cells. Some special mechanoreceptors, Golgi tendon organs, are present in skeletal muscle tendons and muscle spindles. They are responsible for the awareness of kinesthesia (i.e., joint position direction and velocity of joint movements). They are called proprioceptors.

The receptors can also be classified into three types based on the origin of stimuli:

1. Exteroceptor: For stimuli outside the body. For example, somatosensory receptors on the skin.

2. Interoceptor: For stimuli inside the body, i.e., from the internal tissue and organs. For example, receptors sense the blood pressure in the aorta or carotid sinus.

3. Proprioceptors: Proprioceptors respond to the stimuli originating in the muscles and tendons.

According to the nature of the stimulus, sensory receptors are of the following types:

(1) Mechanoreceptors: Physical stimuli, such as pressure and vibration, and sensation of sound and body position (bal­ance), are interpreted through a mechanoreceptor. (2) Nociceptors: Receptors for pain. The potential damage to the tissues by noxious stimulation gives rise to the sensation of pain. The pain receptors in the skin and other tissues are free nerve endings. Pain sensation may be of three types, i.e., pricking pain, burning pain, and aching pain. (3) Chemoreceptors: Receptors for chemical changes. Important chemoreceptors are (a) receptors for taste buds; (b) receptors of the olfactory epithelium for detection of smell; and (c) chemoreceptors in the carotid body and aortic arch.

(4) Thermoreceptors: Thermoreceptors are stimulated by temperature changes. When responding to decreased tempera­ture, some of these thermoreceptors are called cold receptors, and others that respond to increased temperature are called warm receptors. (5) Osmoreceptors: Receptors for osmotic changes. They respond to solute concentrations of body fluids. (6) Photoreceptors: The photoreceptors are found within rod and cone cells of the retina. When light falls on the photoreceptors, the photopigments are transformed leading to changes in the membrane potential of photoreceptors.

11.2.10 ReflexAction

The sudden rapid involuntary effector response to a sensory stimulus is a reflex. Reflexes are the simplest example of the general function of the nervous system: a collection of sen­sory input, integration, and motor output. Marshall Hall, an English physician, first observed this type of action in 1833. Neural reflex involves sensory fibers to CNS and motor fibers to effectors. Reflexes help maintain homeostasis of different autonomic functions like heart rate, breathing rate, BP, and digestion. Reflexes also perform other important automatic actions like swallowing, sneezing, coughing, and vomiting. Reflexes also help in maintaining the balance and posture of the body.

11.2.12.1 Properties of Reflexes

Reflexes have the following properties:

1. Reflexes are spontaneous reactions.

2. Reflexes are automatic.

3. Reflexes are a short-lived response.

4. Reflexes are a mechanical action.

5. Spinal cord is predominately involved in this (though the brain is also involved, e.g., cranial reflex).

11.2.12.2 ReflexArc

The path through which the reflex action takes place is known as a reflex arc (Fig. 11.8). The reflex arc is essential for maintaining the posture and locomotion of the animal and is very useful for the clinical diagnosis of nervous disorders. Reflexes are vital for both survival and different critical behaviors. A reflex is composed of five primary components, and abnormality in any one of these components alters reflex response.

Fig. 11.8 Basic components of the reflex arc

11.2.12.3 Basic Components of the Reflex Arc

1. Receptors: Reflex arcs begin with a sensory receptor that receives the stimulus and generates impulses. These receptors send the signal to the next component of the reflex arc, i.e., sensory neuron. Different sensory receptors are present in the body. Still, all have a common function,

i. e., transduction of different types of stimuli (like light, heat, cold, pressure, taste) into a cellular response that directly or indirectly produces action potentials along the sensory neuron.

2. Sensory neuron: Sensory neuron or afferent neuron car­ries the nerve impulse in the form of action potential to the interneurons of the brain or spinal cord. Sensory neurons enter the spinal cord through the dorsal roots or enter the brain through cranial nerves.

3. Interneuron: Interneurons act as a processing center, process the information, and generate responses. In this part of the reflex arc, synapse formation is seen. The majority of the reflexes are polysynaptic. However, a few reflex arcs originating from muscle spindles are monosynaptic.

4. Motor neuron: Motor neurons or efferent neurons trans­mit the brain or spinal cord response to the effector organs. These neurons leave the spinal cord through the ventral roots and leave the brain through the cranial nerves.

5. Effectors: The last part of a reflex arc is the effector. The effector shows the effect of the reflex action. It may be an organ, muscle, or gland. In “knee jerk,” the effector is the quadriceps muscle of the leg.

11.2.12.4 Types of Reflex Action

Reflexes can be classified in several ways as follows:

1. Based on the control center reflex, actions are of two types: cranial reflexes and spinal reflexes.

(a) Cranial reflexes: It is under the control of cranial nerves and takes place in the facial or head area. Cranial reflexes are slow in response, and hence there is no emergency. The brain generally regulates cranial reflex. Constriction of the pupil in response to light is an example of cranial reflex. The release of saliva after the sight or smell of food is another example.

(b) Spinal reflexes: It involves only the spinal nerves, and the response in spinal reflex is quick. Examples are the stretch reflex, knee-jerk, or patel­lar reflex.

2. Based on the previous experiences, reflex actions are classified into two types, i.e., unconditional reflexes and conditional reflexes:

(a) Unconditional reflex: These are innate responses that are the same among the same species members. It is inborn, and they do not require any previous experi­ence. This type of response helps the particular animal to adapt to a stable living condition. Examples are sneezing, coughing, hiccupping, and yawning.

(b) Conditional reflexes: It is also called an acquired reflex. This type of reflex develops after birth through conditioning or learning: for example, secretion of saliva after seeing known food.

(c) According to the number of synapses in the reflex path

According to the number of synapses in the reflex path, the reflex action is classified into monosynaptic reflex and polysynaptic reflex:

1. Simple monosynaptic: There is only one synapse in the reflex path; that is, it involves only sensory and motor neurons, for example, knee-jerk reflex.

2. Complex polysynaptic reflex: There is more than one synapse in the reflex path; that is, it involves sensory interneuron and motor neuron, for example, cycling and swimming.

11.2.12.4.1 Clinical Classification

Clinical classification is essential and is used in clinical practice to examine the nervous system to diagnose any abnormality in the nervous system.

(1) Superficial reflexes: planter reflex, abdominal reflex, etc. (2) Deep reflexes.

Generally, the majority of the reflexes are polysynaptic. Even if a sensory neuron participates in a monosynaptic reflex arc, it will often give off branches in the CNS that participate in polysynaptic reflex circuits. Even the simplest mammalian reflex responses often involve the excitation of a given muscle or muscles and the inhibition of another (usu­ally antagonistic) muscle or muscles. For example, in the knee-jerk reflex, some sensory neurons make excitatory monosynaptic connections with motor neurons that activate the quadriceps muscle. In addition to that, other terminal branches of that same sensory neuron participate in a disynaptic circuit that inhibits motor neurons innervating the antagonistic hamstring muscle.

Reflexes are seen in most parts of the nervous system. So, reflexes are vital for clinical examination of animals for any diseases involving the nervous system, for example, pupillary light reflex, muscle stretch (knee jerk) reflex, and flexor reflex. Any abnormality of any component of the reflex arc results in an altered reflex action.

11.3