PERIPHERAL NERVOUS SYSTEM: PARTS AND FUNCTIONS - GEOGRAFÍA - 2024

The peripheral nervous system is a collection of nerves and ganglia that control motor and sensory functions. It transmits information from the brain and spinal cord throughout the body.

The human nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord, while the peripheral nervous system is the one outside of it. In fact, "peripheral" in anatomy has a meaning opposite to "central."

The peripheral nervous system comprises all the nerves that branch from the brain and spinal cord to other parts of the body. It includes the cranial nerves, the spinal nerves, the peripheral nerves, and the neuromuscular junctions.

Nerves are cords of white matter that branch into axons and / or dendrites. These transmit sensory and motor information from the brain to the periphery and in reverse. On the other hand, ganglia are made up of groups of neurons; and they are outside the brain and spinal cord.

Ganglia and nerves

The main function of the peripheral nervous system is to connect the central nervous system with the organs, limbs, and skin. This allows the brain and spinal cord to both receive and send information to other areas of the body. In this way, it allows us to react to environmental stimuli.

In the peripheral nervous system, information is transmitted by bundles of nerve fibers or axons. In some cases these nerves are very small, however, in others they can reach a size that the human eye can capture.

Parts of the peripheral nervous system

The peripheral nervous system is divided into two components, the somatic nervous system and the autonomic nervous system. Each one has very important functions:

Somatic nervous system

Somatic nervous system diagram. Source: Medium69, Jmarchn / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

The somatic nervous system is responsible for both sending and receiving sensory and motor information to the central nervous system. The somatic nervous system contains two types of neurons: sensory neurons and motor neurons.

Sensory (or afferent) neurons are those that transmit information from nerves in the central nervous system.

Whereas motor (or efferent) neurons carry information from the brain and spinal cord to organs, muscle fibers, as well as glands on the periphery of the body. These neurons allow a physical response to stimuli.

Autonomic nervous system

It is responsible for regulating the involuntary functions of the body. For example, heart rate, respiration, and digestion. Thanks to the autonomic nervous system, we can perform these functions without consciously thinking about their execution.

This system is divided into the sympathetic system and the parasympathetic system. The sympathetic system regulates the stress response produced by hormones. These are typical fight or flight reactions. That is, it prepares us to face potential threats from our environment.

When that threat is presented, the body responds by accelerating the heart rate, increasing breathing, increasing blood pressure, as well as the secretion of sweat and the dilation of the pupils. These responses help us to act quickly in the face of threats.

In addition, it helps us feel cold or heat, dilates the bronchi and inhibits intestinal motility and urine production.

On the other hand, the parasympathetic system is responsible for maintaining the functions of the body and conserving physical resources. It starts in the brain stem and regulates the internal organs.

Basically this system allows us to return to a normal or resting state, slowing down the heart rate, respiration and blood flow.

Thus, the pupils contract, saliva production increases, gastrointestinal movements are increased, blood pressure and heart rate are reduced, it makes us more resistant to infections, etc.

In short, it develops necessary tasks but that do not require an immediate response as occurs with the sympathetic nervous system.

Nerves of the peripheral nervous system

The peripheral nervous system is made up of 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Cranial nerves

They originate in the brain, and are part of the head and neck. Its function can be sensitive, motor or mixed.

In this way, some of these nerve pairs are exclusively sensory cells. For example, those that detect information from smell and vision.

Other pairs of nerves are exclusively motor cells, such as those in the eye muscles. There are also pairs of nerves that have both sensory and motor cells, for example those involved in taste or swallowing.

The following are the cranial nerves and their functions:

I. Olfactory nerve: it is a sensory nerve that carries odor impulses to the brain.

II. Optic nerve: it is in charge of sending visual stimuli to the brain.

III. Oculomotor nerve: transmits information to the external eye muscles, which helps direct the position of the eyeball. They are also the constrictor muscles of the iris and the ciliary muscles.

IV. Trochlear nerve: it is a motor nerve that carries impulses to the greater oblique muscle of the eye.

V. Trigeminal nerve: it is a mixed nerve that produces general sensations of touch, temperature and pain. It has different branches.

In the ophthalmic branch it is related to the forehead, the eye and the upper nasal cavity. In the maxillary branch it is associated with the sensation of the lower nasal cavity, the face, the upper teeth and the mucosa of the upper part of the mouth.

And in the mandibular branch, it is linked to the surfaces of the jaws, the lower teeth, and the lower mucosa of the mouth. As well as the taste on the front of the tongue.

The trigeminal nerve in its motor function is related to the muscles of the jaws. In addition to working as a tensor of the eardrum, the palate and the digastric muscle (movement of the jaw).

SAW. Abducent nerve: it is also a mixed nerve, although mainly motor. It carries the impulses to the external rectus muscle of the eye.

VII. Facial nerve: it is a mixed nerve and carries the taste sensations of the tongue. It also controls impulses in various muscles of the face. Like the lacrimal, submandibular, and sublingual glands.

VIII. Cochlear or auditory vestibule nerve: it is a very important nerve as it is responsible for transferring auditory impulses to the brain. Although it also handles the sensations of balance. The cells involved are the ciliates of the organ of Corti and those of the vestibular apparatus.

IX. Glossopharyngeal nerve: it is mixed and carries information from the skin of the outer ear and from the mucous membranes of the pharyngeal region. As well as the middle ear, and the posterior third of the tongue. In its motor function, it is related to the striated muscle of the pharynx, which helps to swallow.

X. Vagus nerve: it is a mixed nerve that carries impulses from the pharynx, larynx and other more internal organs to the brain. The motor fibers of this nerve transmit information to the intestine, to the heart, to the respiratory structures. As well as the striated muscles of the palate, pharynx and larynx.

XI. Accessory nerve: has a motor function. It is associated with the muscles of the thoracic and abdominal viscera, as well as the muscles of the back (sternocleidomastoid and part of the trapezius).

XII. Hypoglossal: it is mainly a motor nerve, and transmits impulses to the muscles of the tongue and throat.

Spinal or spinal nerves

They branch from the spinal cord to the rest of the body. As mentioned above, there are 31 pairs. They are distributed in 8 cervical (neck), 12 thoracic (chest), 5 lumbar (lower back), 5 sacral (sacrum) and 1 coccygeal (coccyx).

Each spinal nerve joins the cord through two roots: a dorsal (posterior) sensory root and a ventral (anterior) root.

The fibers of the sensory root transmit impulses of pain, temperature, touch and sense of position from the joints, tendons and surfaces of the body.

In addition, they send sensory information from the trunk and extremities through the spinal cord, reaching the central nervous system. The nerves carry information about the skin to specific regions of the body called dermatomes.

The ventral roots are those with motor fibers. They transmit information about the state of the joints and control the skeletal muscles.

Each spinal nerve pair has the same name as the segment of the spinal cord to which it connects, plus its corresponding number. Thus, the cervical goes from C1 to C8, the dorsal from D1 to D12, the lumbar from L1 to L5 and the coccyx, corresponding to the coccygeal nerve.

Peripheral nervous system ganglia

A ganglion is a group of cell bodies of neurons in the periphery. They can be classified into sensory ganglia or autonomic ganglia, according to their primary functions.

The most common sensory ganglion is the dorsal (posterior) root ganglion. Another type of sensory ganglion is the cranial nerve ganglion. The roots of the cranial nerves are inside the skull, while the ganglia are outside the skull.

Other categories of ganglia are those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic systems.

The ganglia of the sympathetic chain form a row along the spinal column. They arise from the lateral horn of the lumbar and upper thoracic spinal cord.

While the parasympathetic ganglia are located next to the organs where they act. Although there are some parasympathetic ganglia in the head and neck.

Peripheral nervous system diseases

The peripheral nerves are an extensive and complicated network that constitutes a very fragile system. The nerves in this system can be damaged by pressure, syndromes, or neurological problems. There are people who are born with affectations of this type while others are acquired.

In short, there is a great variety of pathologies that can affect the peripheral nervous system. Some of them are:

Neuropathy

It is usually a consequence of another condition and there are many types. It involves damage to any nerve or nerves in the body. The symptoms it causes usually consist of tingling and numbness.

For example, one type is diabetic neuropathy. Apparently, high blood sugar content can affect the nerves. This produces a high heart rate, dizziness, muscle weakness, vision changes, pain in the extremities, loss of sensation, among others.

Nerve problems can also occur due to consuming high amounts of alcohol, producing alcoholic neuropathy.

Brachial plexus injury

The brachial plexus is a set of nerves that send information from the spine to the shoulders, arms, and hands. Most brachial plexus injuries are caused by trauma. This can be due to traffic accidents, injuries, tumors, among others.

There is also the so-called obstetric brachial plexus palsy that occurs in at least 1% of births. It is common when there is difficulty in removing the baby's shoulder at birth.

In this way, the nerves of the brachial plexus are injured. This results in a loss of movement around the shoulder and an inability to flex the elbow.

Carpal tunnel syndrome

It is a disorder characterized by pressure on the nerves of the hand. This causes the palm of the hand, fingers and the palmar side to lose sensation.

It usually occurs in people who use computers throughout the day, as well as carpenters, assembly line workers, musicians, and mechanics.

Ulnar nerve compression

The ulnar nerve runs from the shoulder to the fingers, and is very superficial. Putting pressure on it can cause damage, which can lead to loss of sensation. It is commonly reflected in tingling, burning, or numbness.

Guillain Barre syndrome

In this disorder, the immune system fails by mistakenly attacking part of the peripheral nervous system. In this way, inflammation appears in some nerves, pain, tingling, loss of coordination, and muscle weakness.

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