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 out from the brain and spinal cord to other parts of the body. It includes the cranial nerves, spinal nerves, peripheral nerves, and 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 vice versa. On the other hand, ganglia are made up of groups of neurons; and are found outside the brain and spinal cord. 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 stimuli from the environment. 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 This 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 the ones that transmit information from the nerves in the central nervous system. While motor (or efferent) neurons carry information from the brain and spinal cord to organs, muscle fibers, as well as glands in 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, breathing and digestion. Thanks to the autonomic nervous system , we can perform these functions without consciously thinking about their execution. This system is divided into sympathetic system and parasympathetic system. The sympathetic system regulates the response to stress 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 increasing the heart rate, increasing breathing, blood pressure, as well as the secretion of sweat and the dilation of the pupils. These responses help us act quickly in the face of threats. In addition, it helps us feel hot or cold, dilates the bronchial tubes and inhibits intestinal motility and urine production. On the other hand, the parasympathetic system is responsible for maintaining body functions and conserving physical resources. It starts in the brainstem and regulates the internal organs. Basically this system allows us to return to a normal or resting state, slowing down the heart rate, breathing 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 performs necessary tasks 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 sensory, motor or mixed. Thus, 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 cranial nerves and their functions are listed below: I. Olfactory nerve: is a sensory nerve that carries odor impulses to the brain. II. Optic nerve: is responsible for 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: is a motor nerve that carries impulses to the major 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 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 ramus, it is linked with the surfaces of the jaws, the lower teeth, and the lower mucosa of the mouth. As well as the taste in the anterior part of the tongue. The trigeminal nerve in its motor function is related to the digastric muscle (jaw movement). SAW. Abducent nerve: it is also a mixed nerve, although mainly motor. It carries impulses to the external rectus muscle of the eye. VII. Facial nerve: it is a mixed nerve and carries taste sensations from the tongue. It also controls impulses in various muscles of the face. Like the lacrimal, submandibular, and sublingual glands. VII. Vestibulocochlear or auditory nerve: it is a very important nerve since it is responsible for transferring auditory impulses to the brain. Although it also handles the sensations of balance. The cells involved are the hair cells 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 external ear and the mucous membranes of the pharyngeal region. As well as the middle ear, 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 with 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 (sacral bone) and 1 coccygeal (coccyx). Each spinal nerve joins the spinal cord through two roots: a dorsal (posterior) sensory root and a ventral (anterior) root. Sensory root fibers transmit pain, temperature, touch, and position sense impulses from joints, tendons, and body surfaces. In addition, they send sensory information from the trunk and extremities through the spinal cord, reaching the central nervous system. 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 pair of spinal nerves 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. Diseases of the peripheral nervous system The peripheral nerves are an extensive and complicated network that constitutes a very fragile system. The nerves of this system can be damaged by pressure, syndromes or neurological problems. There are people who are born with affections of this type while others are acquired. In short, there is a wide 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. It seems that high blood sugar can affect the nerves. This produces high heart rate, dizziness, muscle weakness, changes in vision, pain in the extremities, loss of sensitivity, among others. Nerve problems can also occur from consuming high amounts of alcohol, producing alcoholic neuropathy. Brachial Plexus Injury The brachial plexus is a bundle 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 delivering 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 This is a disorder characterized by pressure on the nerves of the hand. This causes the palm of the hand, the fingers and the palmar side to lose sensation. It usually occurs in people who use computers all 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 it 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.