Nerve impulses travel much faster in nerves with a myelin sheath than in those without one. Nerves that directly connect the brain and the brain stem with the eyes, ears, nose, and throat and with various parts of the head, neck, and trunk are called cranial nerves. There are 12 pairs of them see Overview of the Cranial Nerves Overview of the Cranial Nerves Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk.
Some of the cranial nerves are involved in the special senses such as seeing Cranial nerves transmit sensory information, including touch, vision, taste, smell, and hearing. Nerves that connect the spinal cord with other parts of the body are called spinal nerves.
The brain communicates with most of the body through the spinal nerves. There are 31 pairs of them, located at intervals along the length of the spinal cord see Overview of Spinal Cord Disorders Overview of Spinal Cord Disorders Spinal cord disorders can cause permanent severe problems, such as paralysis or impaired bladder and bowel control urinary incontinence and fecal incontinence. Sometimes these problems can Several cranial nerves and most spinal nerves are involved in both the somatic and autonomic parts of the peripheral nervous system.
Spinal nerves emerge from the spinal cord through spaces between the vertebrae. Each nerve emerges as two short branches called spinal nerve roots : one at the front of the spinal cord and one at the back. Motor nerve root anterior nerve root : The motor root emerges from the front of the spinal cord. Motor nerve fibers carry commands from the brain and spinal cord to other parts of the body, particularly to skeletal muscles.
Sensory nerve root posterior nerve root : The sensory root enters the back of the spinal cord. Sensory nerve fibers carry sensory information about body position, light, touch, temperature, and pain to the brain from other parts of the body.
The sensory nerve fibers in each sensory nerve root carry information from a specific area of the body, called a dermatome see figure Dermatomes Dermatomes Nerve root disorders result from sudden or long-term pressure on the spinal nerve root.
Nerve root disorders usually result from a herniated disk or osteoarthritis in the spine. These disorders After leaving the spinal cord, the corresponding motor and sensory nerve roots join to form a single spinal nerve. Some of the spinal nerves form networks of interwoven nerves, called nerve plexuses. In a plexus, nerve fibers from different spinal nerves are sorted and recombined so that all fibers going to or coming from one area of a specific body part are put together into one nerve see figure Nerve Junction Boxes: The Plexuses Nerve Junction Boxes: The Plexuses Plexuses networks of interwoven nerve fibers from different spinal nerves may be damaged by injury, tumors, pockets of blood hematomas , or autoimmune reactions.
Some spinal nerves divide into smaller branches, without forming a plexus. A plexus is a group of nerves that combine with each other. There are five main plexi formed by the spinal nerves:. There are numerous described variants of spinal nerve anatomy, but these are generally discovered during pre-operative testing or during surgery for an injury to the spine, spinal cord, or spinal nerve. A study evaluating the spinal nerve anatomy of 33 cadavers deceased people identified spinal nerve plexus variants in The spinal nerves have small sensory and motor branches.
Each of the spinal nerves carries out functions that correspond to a certain region of the body. These are muscle movement, sensation, and autonomic functions control of internal organs.
Because their function is so well understood, when a particular spinal nerve becomes impaired, the resulting deficit often pinpoints which spinal nerve or nerves are affected.
Motor messages to the spinal nerves originate in the brain. The motor strip homunculus in the brain initiates a command for muscle control. This command is sent to the spine through nerve impulses and then travels through the motor root to the spinal nerve.
Motor stimulation is very specific, and it may activate the whole spinal nerve or just one of its branches to stimulate a very small group of muscles—depending on the command from the brain.
The distribution of spinal nerve control throughout the body is described as a myotome. Each physical movement requires one or more muscles, which is activated by a branch of a spinal nerve. For example, the biceps muscle is controlled by C6 and the triceps muscle is controlled by C7. There are fewer autonomic branches of the spinal nerves than there are the motor and sensory branches. The spinal nerves receive messages including touch, temperature, position, vibration, and pain from the small nerves in the skin, muscles, joints, and internal organs of the body.
Each spinal nerve corresponds to a skin region of the body, described as a dermatome. For example, sensation near the belly button is sent to T10 and sensation from the hand is sent to C6, C7, and 8. The sensory dermatomes do not match up perfectly with the motor myotomes. Spinal nerves can be affected by a number of conditions. The diagnosis of a spinal nerve problem involves several steps.
Reflexes also correspond to spinal nerves, and they are usually diminished in these situations as well, further helping to identify which nerves are involved. These tests help in identifying which spinal nerves are involved and how extensive the impairment is.
Conditions affecting spinal nerves include the following. Usually, the first symptoms include neck pain or tingling down the arm or leg. A herniated disc can be a medical emergency because it can cause permanent damage to the spinal cord. Treatment includes oral anti-inflammatory medications, therapy, injections of pain medication or anti-inflammatory medication, and possibly surgical repair and stabilization of the spine. Foramen Narrowing The foraminal openings through which spinal nerves travels are not much larger than the nerves themselves.
Inflammation and bony degeneration can compress a spinal nerve as it travels through the foramen, producing pain and tingling. This is often described as a pinched nerve. Weight gain and swelling can cause or exacerbate a pinched nerve. During pregnancy, for example, many women experience the symptoms of a pinched nerve. This can resolve after weight loss or even with the redistribution of weight—some women notice improvement of symptoms even before having their baby, and most have a complete resolution after the baby is born.
There are a number of treatments for foraminal narrowing, including anti-inflammatory medication and physical therapy. Interventional procedures such as surgery or injections are not usually necessary. A very common condition, shingles is reactivation of the virus that causes chicken pox, herpes zoster. Shingles are characterized by severe pain and are sometimes accompanied by a rash. If you have ever had a chickenpox infection, the virus remains in your body, in a nerve root, after recovery from the illness.
When it reactivates—usually due to a weak immune system— it causes pain and skin lesions in the region supplied by a nerve root or a whole spinal nerve. There is an immunization that can prevent shingles, however, and it may be recommended if you are susceptible to developing a reactivation of the virus.
GBS, also called acute demyelinating polyneuropathy, causes weakness of the peripheral nerves, and it can affect many spinal nerves at a time. Typically, GBS initially causes tingling in the feet, followed by weakness in the feet and legs, which advances to weakness of the arms and chest muscles.
It can eventually impair the muscles that control breathing. Respiratory support with a mechanical ventilator is usually necessary until the condition resolves.
This disease is caused by demyelination, which is a loss of the protective myelin fatty layer that surrounds each nerve. The myelin is eventually replaced and the nerves can function again, but medical support is necessary in the interim. This plexus lies within the psoas major muscle. The sacral plexus is formed by the ventral rami of L4-S3, with parts of the L4 and S4 spinal nerves. It is located on the posterior wall of the pelvic cavity. The coccygeal plexus serves a small region over the coccyx and originates from S4, S5, and Co1 spinal nerves.
It is interconnected with the lower part of sacral plexus. Brachial plexus : Cervical C5—C8 and thoracic T1 nerves comprise the brachial plexus, which is a nerve plexus that provides sensory and motor function to the shoulders and upper limbs.
Lumbar plexus : The lumbar plexus is comprised of the ventral rami of the lumbar spinal nerves L1—L5 and a contribution from thoracic nerve T The posterior green and anterior yellow divisions of the lumbar plexus are shown in the diagram. The intercostal nerves are part of the somatic nervous system and arise from anterior divisions rami anteriores, ventral divisions of the thoracic spinal nerves T1 to T The intercostal nerves are distributed chiefly to the thoracic pleura and abdominal peritoneum.
Intercostal nerves : An image of the intercostal brachial nerves. They differ from the anterior divisions of the other spinal nerves in that each pursues an independent course without plexus formation. These are limited in their distribution to the parietes wall of the thorax. The anterior divisions of the second, third, fourth, fifth, and sixth thoracic nerves, and the small branch from the first thoracic, are confined to the walls of the thorax and are named thoracic intercostal nerves.
Near the sternum, they cross in front of the internal mammary artery and transversus thoracis muscle, pierce the intercostales interni, the anterior intercostal membranes, and pectoralis major, and supply the integument of the front of the thorax and over the mamma, forming the anterior cutaneous branches of the thorax.
The branch from the second nerve unites with the anterior supraclavicular nerves of the cervical plexus. The seventh intercostal nerve terminates at the xyphoid process, at the lower end of the sternum.
The anterior divisions of the seventh, eighth, ninth, tenth, and eleventh thoracic intercostal nerves are continued anteriorly from the intercostal spaces into the abdominal wall; hence they are named thoraco-abdominal nerves or thoracicoabdominal intercostal nerves. Unlike the nerves from the autonomic nervous system that innervate the visceral pleura of the thoracic cavity, the intercostal nerves arise from the somatic nervous system.
This enables them to control the contraction of muscles, as well as provide specific sensory information regarding the skin and parietal pleura. This explains why damage to the internal wall of the thoracic cavity can be felt as a sharp pain localized in the injured region. Damage to the visceral pleura is experienced as an unlocalized ache. A dermatome is an area of skin that is supplied by a single spinal nerve, and a myotome is a group of muscles that a single spinal nerve root innervates.
A dermatome is an area of skin that is supplied by a single spinal nerve. There are eight cervical nerves, twelve thoracic nerves, five lumbar nerves and five sacral nerves.
Each of these nerves relays sensation, including pain, from a particular region of the skin to the brain. Dermatomes : Dermatomes are areas of skin supplied by sensory neurons that arise from a spinal nerve ganglion. Dermatomes and the associated major cutaneous nerves are shown here in a ventral view. Along the thorax and abdomen, the dermatomes are like a stack of discs, with each section supplied by a different spinal nerve.
Along the arms and the legs, the pattern is different. The dermatomes run longitudinally along the limbs, so that each half of the limb has a different dermatome. Although the general pattern is similar in all people, the precise areas of innervation are as unique to an individual as fingerprints. Dermatomes have clinical significance, especially in the diagnosis of certain diseases.
Symptoms that follow a dermatome, such as pain or a rash, may indicate a pathology that involves the related nerve root. Examples include dysfunction of the spine or a viral infection. Viruses that remain dormant in nerve ganglia, such as the varicella zoster virus that causes both chickenpox and shingles, often cause either pain, rash, or both in a pattern defined by a dermatome.
Shingles rash : The shingles rash appears across a dermatome. In this patient, one of the dermatomes in the arm is affected, restricting the rash to the length of the back of the arm. Shingles is one of the only diseases that causes a rash in a dermatomal pattern, and as such, this is its defining symptom. The rash of shingles is almost always restricted to a specific dermatome, such as on the chest, leg, or arm caused by the residual varicella zoster virus infection of the nerve that supplies that area of skin.
Shingles typically appears years or decades after recovery from chickenpox. A myotome is the group of muscles that a single spinal nerve root innervates. The myotome is the motor equivalent of a dermatome. The testing of myotomes provides the clinician with information about the level in the spine where a lesion may be present.
During testing, the clinician looks for muscle weakness of a particular group of muscles. Results may indicate lesions to the spinal cord nerve root, or intervertebral disc herniation that presses on the spinal nerve roots. The peripheral nervous system PNS consists of the nerves and ganglia outside of the brain and spinal cord. Unlike the CNS, the PNS is not protected by the bones of the spine and skull, or by the blood —brain barrier, leaving it exposed to toxins and mechanical injuries.
The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. Spinal nerve : This diagram indicates the formation of a typical spinal nerve from the dorsal and ventral roots. Numbers indicate the types of nerve fibers: 1 somatic efferent, 2 somatic afferent, 3—5 sympathetic efferent, 6—7 sympathetic afferent. The peripheral nervous system includes 12 cranial nerves and 31 pairs of spinal nerves that provide communication from the CNS to the rest of the body by nerve impulses to regulate the functions of the human body.
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body.
Each pair of spinal nerves roughly correspond to a segment of the vertebral column: 8 cervical spinal nerve pairs C1—C8 , 12 thoracic pairs T1—T12 , 5 lumbar pairs L1—L5 , 5 sacral pairs S1—S5 , and 1 coccygeal pair. The anterior divisions of the lumbar, sacral, and coccygeal nerves form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic. For descriptive purposes, this plexus is usually divided into three parts: lumbar plexus, sacral plexus, and pudendal plexus.
The sympathetic division typically functions in actions that need quick responses. The parasympathetic division functions with actions that do not require immediate reaction. The sympathetic system is often considered the fight or flight system, while the parasympathetic system is often considered the rest and digest or feed and breed system.
Autonomic nervous sytem : The functions of the parasympathetic and sympathetic nervous systems are detailed. The somatic nervous system consists of afferent and efferent nerves and is associated with the voluntary control of skeletal muscle movements.
The afferent nerves are responsible for relaying sensations from the body to the central nervous system CNS , while the efferent nerves are responsible for sending out commands from the CNS to the body to stimulate muscle contraction. Upper motor neurons release acetylcholine.
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