Classification of the Nervous System: Definition, Functions and Structure of This Body Regulator

It consists of the cranial, spinal, and peripheral nerves, along with their motor and sensory endings.

The nervous system comprises the central nervous system, which consists of the brain, spinal cord, and the peripheral nervous system.

Central Nervous System

The central nervous system is made up of millions of nerve and glial cells , along with blood vessels and some connective tissue.

Nerve cells, or neurons, are characterized by many processes and are specialized for the reception and transmission of signals.

Glial cells, called neuroglia , are characterized by short processes that have special relationships with neurons, blood vessels, and connective tissue.


The brain is the enlarged upper end of the central nervous system; occupies the skull.

The brain has three main divisions:

  • The forebrain ( forebrain ).
  • The midbrain.
  • The rhombencephalon.

The forebrain in turn has two subdivisions:

  • The telencephalon (brain of the end).
  • The diencephalon (between brain).

The rhombencephalon also has two subdivisions:

  • The metancephalon (after the brain).
  • The melencephalon (bone marrow).

Most of the brain is made up of two cerebral hemispheres, which are derived from the telencephalon. The hemispheres are distinguished by convolutions, which are separated by furrows.

The diencephalon is located between the hemispheres. It forms the upper part of the brain stem, an unpaired stem that descends from the base of the brain.

The brainstem is made up of the diencephalon, midbrain, pons, and melenbrain, or medulla oblongata. The latter is continuous with the spinal cord at the foramen magnum.

The cerebellum is a cracked mass of gray matter that occupies the posterior cranial fossa and is attached to the brain stem by three pairs of peduncles. Twelve pairs of cranial nerves exit from the base of the brain and the brainstem.

The cerebral cortex, which is the most superficial part of the hemispheres and measures only a few millimeters, is made up of gray matter, in contrast to the interior of the hemispheres, which is made up in part of white matter.

Gray matter largely consists of the bodies of nerve and glial cells, while white matter largely consists of the processes or fibers of the nerves and glial cells.

The interior of the cerebral hemispheres, including the diencephalon, contains not only white matter but also large masses of gray matter known collectively as the basal ganglia.

This term is inappropriate as the term “ganglion” should be reserved for collections of nerve cell bodies outside the central nervous system and nuclei should be used for collections of neurons within it.

Therefore, it would be more appropriate to call these “basal nuclei”, however, that term is reserved for another structure.

The cerebellar cortex, like the cerebral cortex, is made up of a thin cortex of gray matter. The interior of the cerebellum is composed primarily of white matter, but it also contains nuclei of gray matter. The brain stem, on the other hand, contains nuclei and diffuse masses of gray matter within it.

The inside of the brain also contains cavities called ventricles, which are filled with cerebrospinal fluid.


The highest mental and behavioral activities characteristic of humans are mediated by the cerebral hemispheres, in particular by the cerebral cortex.

Important aspects of these functions are learning and language. In addition, there are association mechanisms for the integration of motor and sensory functions.

Some areas of the cerebral hemispheres control muscle activity and their nerve cells send processes to the brain stem and spinal cord, where they are connected to motor neurons, the processes of which exit through the cranial nerves or the ventral roots of the spinal cord.

Other areas are sensory and receive impulses that have reached the spinal cord via peripheral nerves and dorsal roots, and have ascended through the spinal cord and brainstem by pathways consisting of a succession of nerve cells and their processes.

The ascending and descending fibers in the brain and spinal cord often segregate into bundles that have similar courses and functions, known as “tracts,” generally grouped in extensions.

Tracts are usually named according to their origin and destination, for example, corticospinal.

The brainstem contains, in addition to tracts that descend and ascend through it, collections of cells that comprise important integrating centers of motor and sensory functions, form the nucleus of most of the cranial nerves (all the cranial nerves except the first are attached to the brainstem)

They also form centers related to the regulation of a variety of visceral, endocrinological, behavioral and other activities, they are functionally associated with most of the special senses.

In the same way they are associated with the control of muscular activity in the head and part of the neck and supplying structures of the pharyngeal arch, and are connected with the cerebellum.

The cerebellum is responsible for the automatic regulation of movement and posture, and the learning of new motor patterns. It works closely with the cerebral cortex and brain stem.

Spinal cord

The spinal cord is a long, cylindrical mass of nerve tissue, oval or rounded in cross section. It occupies the upper two thirds of the vertebral canal. In contrast to the cerebral hemispheres, gray matter is found on the inside, surrounded by white matter.

The neurons of the spinal cord include somatic motor cells, whose axons exit through the ventral roots and supply skeletal muscles; Autonomous motor cells, whose axons exit through the ventral roots and go to the autonomic ganglia.

Likewise, the transmission neurons that give rise to ascending projections to the brain and connections with other levels of the spinal cord; and interneurons, which connect with other neurons at the spinal level and deal with sensory and reflex mechanisms.

White matter contains ascending and descending tracts. Some ascend or descend from the brain, while others connect cells at various levels of the cord.

Attached to the spinal cord on each side are a series of spinal roots, called dorsal and ventral depending on their position. There are generally 31 pairs, comprising 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

The corresponding dorsal and ventral roots unite to form a spinal nerve. Each spinal nerve is divided into a dorsal and a ventral branch, and these are distributed in various parts of the body.

The spinal cord performs the following functions:

  • Sensory.
  • Integrative and motor : which can be categorized as a reflex activity.
  • Reciprocal (when one activity starts, another stops).
  • Monitoring and modulation of sensory and motor mechanisms.
  • Transmission of impulses to the brain.

Meninges and cerebrospinal fluid

The brain and spinal cord are surrounded and protected by layers of non-nervous tissue, collectively called the meninges.

These layers, from the inside out, are the dura, arachnoid, and pia mater, and are described in more detail elsewhere. The space between the arachnoid and the pia mater, the subarachnoid space, contains cerebrospinal fluid (CSF).

The ventricles of the brain contain vascular choroid plexuses, from which CSF, a nearly protein-free liquid, is formed.

This fluid circulates through the ventricles, enters the subarachnoid space, and finally leaks into the venous system. The CSF protects the brain which basically floats. It serves to minimize damage from blows to the head and neck.

Blood supply

The brain is supplied by the cerebral branches of the internal and vertebral carotid arteries, the meninges mainly by the middle meningeal branch of the maxillary artery.

The spinal cord and spinal roots are supplied by the vertebral arteries and by the segmental arteries. The peripheral nerves are supplied by several small branches along the course of the nerves.

Peripheral nervous system

A nerve is a collection of nerve fibers that is visible to the naked eye. The constituent fibers are held together by connective tissue. Each fiber is microscopic in size and surrounded by a sheath formed by a neurilemic cell (comparable to the glial cells of the central nervous system).

Hundreds or thousands of fibers are present in each nerve. Therefore, according to the number of constituent fibers, a rib may be barely visible, or it may be quite thick.

A nerve as a whole is surrounded by a covering of connective tissue, the epineurium. Connective tissue fibers run inward from the sheath and enclose bundles of nerve fibers.

Such bundles are called fasciculi (funiculi); the connective tissue that encloses them is called the perineurium. Very small nerves may consist of a single bundle derived from the parental nerve.

Peripheral nerve fibers can be classified according to the structures they supply, that is, according to function. A fiber that stimulates or activates skeletal muscle is called a motor (efferent) fiber.

A fiber that carries impulses from a sensory terminal is called a sensory (afferent) fiber. The fibers that activate the glands and smooth muscle are also motor fibers, and several types of sensory fibers arise from endings in the viscera.

Consequently, a more detailed classification of functional components is sometimes required.

Spinal nerves

The spinal roots, which are anchored to the spinal cord, consist of a dorsal root, attached to the dorsal aspect of the spinal cord, and a ventral root, attached to the ventral aspect of the cord.

Each dorsal root (which contains sensory fibers from the skin, subcutaneous and deep tissues, and often also from the viscera) is made up of neural processes that carry afferent impulses to the spinal cord and that arise from neurons that join together to form an enlargement called the ganglion. spinal (dorsal root).

The peripheral processes of the ganglionic neurons of the dorsal root arise directly within the organ or structure from which they transmit sensation.

Each of the ventral roots (which contain skeletal muscle motor fibers, and many of which contain preganglionic autonomic fibers) are formed by processes of neurons in the gray matter of the spinal cord.

While the projections of motor neurons to skeletal muscle go directly to their termination in muscle, autonomous motor axons synapse on neurons in a ganglion (hence the term preganglionic).

Neurons in the ganglion (postganglionic neurons) have axons that reach their target in glands or smooth muscles.

Basically, the dorsal roots are afferent, the ventral roots efferent. The corresponding dorsal and ventral roots join to form a spinal nerve. Each spinal nerve then divides into a primary dorsal and ventral branch.

Distribution of spinal and peripheral nerves

The primary dorsal branch (or simply dorsal branch) of the spinal nerves supplies the skin and muscles of the back. The ventral primary branches (ventral branches) supply the limbs and the rest of the trunk.

The ventral branches supplying the thoracic and abdominal wall remain relatively separate throughout their course. In the cervical and lumbosacral regions, however, the ventral rami intermingle to form plexuses, from which the main peripheral nerves emerge.

When the ventral branch of a spinal nerve enters one plexus and joins other branches, the funicular components or fascicles eventually enter several of the nerves exiting the plexus.

Therefore, as a general principle, each spinal nerve that enters a plexus contributes to several peripheral nerves, and each peripheral nerve contains fibers derived from several spinal nerves.

This arrangement leads to two fundamental and important types of distribution. Each spinal nerve has a segmental or dermatomal distribution.

A dermatome is the area of ​​the skin supplied by the sensory fibers of a single dorsal root through the dorsal and ventral branches of its spinal nerve.

Cranial nerves

The 12 pairs of cranial nerves are special nerves associated with the brain. The fibers in the cranial nerves are of various functional types. Some cranial nerves are made up of a single type, others of several.

The cranial nerves differ significantly from the spinal nerves, especially in their development and relationship to the special senses and because some cranial nerves supply structures to the pharyngeal arch.

They are attached to the brain at irregular rather than regular intervals; they are not formed by the dorsal and ventral roots; some have more than one node, while others have none; and the optic nerve is actually a tract of the central nervous system rather than a peripheral nerve.

Characteristic features of peripheral nerves

The branches of the main peripheral nerves are usually muscular, cutaneous (or mucous), articular, vascular (to the adjacent blood vessels) and terminal (one, several or all of the previous types).

The muscle branches are the most important: severing even a small muscle branch results in complete paralysis of all muscle fibers supplied by that branch and can be severely disabling.

The significance of sensory loss varies, but sensory loss is most disabling in the hand, head, and face.

Peripheral nerves vary in their course and distribution, but not as much as blood vessels. Adjacent nerves can communicate with each other.

Such communications sometimes account for residual sensation or movement after nerve damage above the level of a communication.

Autonomic nervous system

The autonomic nervous system regulates the activity of the heart muscle, smooth muscle, and glands.

The autonomous system can be thought of as a series of hierarchical levels, with higher levels producing more generalized and general functions.

The highest level is the cerebral cortex, certain areas of which control or regulate visceral functions. These areas send fibers to the next lower level, the hypothalamus, located at the base of the brain.

The hypothalamus is a focal point for the motor control of visceral activity. One of its many functions, for example, is the regulation of body temperature.

The hypothalamus has nerve and vascular connections with the pituitary gland (pituitary), by virtue of which it influences the pituitary gland and, through the pituitary gland, the other endocrine glands.

The hypothalamus also sends nerve fibers to lower centers in the brain stem that deal with even more specific functions, for example, reflex regulation of breathing, heart rate, and circulation.

These centers function through connections to even lower centers, which are collections of nerve cells in the brain stem and spinal cord that send their axons to certain cranial and spinal nerves.

It is characteristic of these axons that, unlike skeletal muscle motor fibers, they synapse with multipolar neurons located in ganglia outside the central nervous system before they reach the structure to be supplied.

The axons that pass from the central nervous system to these ganglion cells are called preganglionic fibers.

The axons of ganglion cells are called postganglionic fibers and they function in the reflex regulation of respiration, heart rate, and circulation.

Sympathetic system

The comprehensive or thoracolumbar part of the autonomic system comprises the preganglionic fibers that originate at the thoracic and upper lumbar levels of the spinal cord.

These fibers reach the spinal nerves by means of ventral roots and then leave the spinal nerves, reaching the adjacent ganglia by means of communicating branches.

These ganglia are contained in long nerve strands, the sympathetic trunks, one on each side of the spinal column, extending from the base of the skull to the coccyx.

Some preganglionic fibers synapse in the ganglia that are studded along this nerve trunk, others continue in ganglia located anterior to the vertebrae, along the aorta (prevertebral or aortic plexuses) and other synapses with cells in the medulla of the Kidney glands.

Postganglionic fibers go directly to the viscera and adjacent blood vessels or return to the spinal nerves via other communicating branches.

In the area of ​​distribution of these nerves, they supply the skin with secretory fibers to sweat the glands, motor fibers to smooth the muscle attached to the hair follicles (arrectores pilorum), and vasomotor fibers to the blood vessels of the extremities.

Parasympathetic system

The parasympathetic or craniosacral part of the autonomic system comprises the preganglionic fibers emerging from the brainstem (cranial nerves III, VII, IX, X, XI) and the sacral part of the spinal cord (segments S2,3 or S3,4).

The ganglion cells with which these fibers synapse are in or near innervated organs. Postganglionic fibers are very short: apparently none go to blood vessels, smooth muscle, or glands in the extremities or the wall of the body.

Most viscera, however, have dual sympathetic and parasympathetic motor supplies, often with opposite roles.

Functions of the autonomic nervous system

Due to its role in the central integrative mechanisms, the autonomic system is involved in behavioral and neuroendocrinological mechanisms, and in the processes by which the body maintains its internal environment constant, that is, it maintains the temperature, water balance and ionic composition of the blood.

The parasympathetic system takes care of many specific functions, such as digestion, intermediate metabolism, and excretion. The sympathetic system is an important part of the stress reaction mechanism.