Nerve Supply

Introduction

The nerve supply refers to the different types of nerves supplying different parts of the body. The nervous system is divided into the central nervous system and peripheral nervous system of the body. The central nervous system consists of the brain and the spinal cord. The peripheral nervous system consists of different types of nerves that carry and receive information from different organs and organ systems of the body. 

In the peripheral nervous system, a nerve is an enclosed cable-like bundle of fibers called axons that transmit electrical impulses. In the human nervous system, there are two types of nerves: cranial nerves and spinal nerves. Cranial nerves originate from different parts of the brain and mostly innervate parts of the head and neck. The spinal nerves arise from the spinal cord and innervate the thorax, abdomen, and limbs. There are 12 cranial nerves and 31 pairs of spinal nerves in the human body.

Types

Different parts of the body receive different types of nerve supply. The nerve supply can be divided into the somatic nerve supply and the autonomic nerve supply of the organs.

Somatic nerve supply

The somatic nerve supply is associated with the voluntary control of the skeletal muscles. The skeletal muscle sends sensory information to the brain and receives motor impulses through the somatic nerves. The upper parts of the human body such as the head, eyes, ears, nose, mouth, neck, and other parts get their somatic nerve supply through cranial nerves. The thorax, abdomen, upper limb, lower limb, and various other organs in the middle and lower part of the body get somatic nerve supply through the spinal nerves. 

The somatic nerve supply controls all the voluntary muscle movements. Stimuli from the precentral gyrus of the brain come down to the spinal cord through the corticospinal tract. It is then transmitted via the axons of motor neurons to the muscles of the body. The somatic system also controls the process of a voluntary reflex arc. The reflex arc is a neural circuit between sensory input and specific motor output. It is a pathway followed by the nerves which carry sensory information just to the spinal cord and not to the brain. The spinal cord generates a response to effector organs which results in a much quicker reaction.

The somatic nerves are divided into two categories: the sensory nerves and the motor nerves.

  1. Sensory nerve supply

The sensory nerve supply or the afferent nerve supply of the organs carries sensory information to the central nervous system. The sensory nerves are all those nerves that can sense or recognize internal or external stimuli. The sensory nerves link different sensory neurons throughout the body. These nerves get information from the sensory receptors and enter the dorsal root ganglia of the spinal cord. The information is then transferred to the brain. 

  1. Motor nerve supply

The muscles of the human body receive information through motor nerves. Motor nerves are also called efferent nerves because they carry information from the central nervous system to the effector muscles. Motor nerves are divided into three subtypes based on the type of motor neurons associated with them. The motor nerves associated with alpha neurons target extrafusal muscles and are responsible for their contraction. The beta neurons innervate intrafusal fibers while the gamma motor neurons are not directly involved in any type of muscle contraction. 

Autonomic nerve supply

The autonomic nerves are part of the autonomic nervous system which acts unconsciously and regulates body functions such as heart rate, respiratory rate, and digestion, etc. Different organs of the body get their autonomic nerve supply through cranial and spinal nerves. 

The autonomic nerve supply is divided into sympathetic nerve supply, parasympathetic nerve supply, and enteric system. Most of the organs of the body are dually innervated by both sympathetic and parasympathetic nerves except adrenal medulla, sweat glands, arrector pili muscles, and most blood vessels which only get sympathetic nerve supply. 

  1. Sympathetic nerve supply

The sympathetic nerves arise from the thoracolumbar region of the spinal cord starting from the first thoracic vertebra to the second or third lumbar vertebra. They arise in the lateral grey column of the spinal cord. The sympathetic nerve fibers leave the spinal cord and become a part of the sympathetic chain. 

The sympathetic chain extends from the upper neck to the coccyx. It has 22-23 pairs of ganglia which include 3 cervical ganglia, 11 thoracic ganglia, 4 lumber ganglia, and 4-5 sacral ganglia. The sympathetic nerve supply prepares the body for a “fight and flight” response by increasing heart rate, respiratory rate, and blood supply to the brain, and skeletal muscles. It decreases urinary output and promotes emission before ejaculation in Ductus Deferens.

  1. Parasympathetic nerve supply

The parasympathetic nerve fibers arise from the brain in the form of some cranial nerves and also from the sacral region of the spinal cord. The cranial parasympathetic nerves fibers come from the third, seventh, ninth, and tenth cranial nerves and synapse at four parasympathetic ganglia. These four ganglia are ciliary, pterygopalatine, otic, and submandibular. From these ganglia, the parasympathetic nerves reach their target tissues through trigeminal nerve branches. 

In the sacral region, the axons of pelvic splanchnic nerves exit the vertebral column as S2-S4 spinal nerves. These parasympathetic nerve fibers innervate the urinary bladder, ureters, urinary sphincters, prostate gland, vagina, and penis. The parasympathetic system works when the body is at rest and there is no danger. It increases digestion, decreases the respiratory rate, constricts the pupil, and increases salivation and lacrimation. The parasympathetic system is also involved in sexual activity, micturition, and defecation. 

  1. Enteric system

The enteric nerve supply controls the autonomic function of the gastrointestinal tract. The neurons of the enteric system are collected into two types of ganglia. The Auerbach’s plexus lies between the layers of muscularis externa of the gastrointestinal tract and supplies motor input to both layers. The submucosal plexus lies in the submucosal layer of the gastrointestinal tract and supplies the mucosal layer. 

Nerve supply of Different Organs

  1. Eyes

The muscles of the eye are innervated by six cranial nerves are optic nerve, oculomotor nerve, trochlear nerve, trigeminal nerve, abducens nerve, and facial nerve. The sympathetic nerve supply comes from carotid plexuses through the nasociliary nerve. The eye gets its parasympathetic innervation from the ciliary ganglion through ciliary nerves. 

  1. Ears

There are four sensory nerves that supply the ear. The external ear gets its nerve supply through the auriculotemporal (fifth cranial) nerve and the great auricular nerve. The middle ear is supplied by the fifth cranial nerve, the tympanic branch of the ninth cranial nerve, and the auricular branch of the vagus. The vestibulocochlear nerve contains afferent fibers from the internal ear, and it is involved in hearing and balance. 

  1. Lungs

The lungs are innervated by the nerves coming from the pulmonary plexus which contain sympathetic, parasympathetic, and sensory fibers. The parasympathetic supply comes from the vagus nerve while the sympathetic supply comes from sympathetic trunks and causes bronchodilation. 

  1. Heart

The nerve supply to the heart is autonomic and contains both sympathetic and parasympathetic fibers. The sympathetic fibers arise from the superior, middle, and inferior cervical ganglia while the parasympathetic fibers arise in the medulla oblongata and come to the heart through the vagus nerve. 

  1. Stomach

The vagus nerve contains parasympathetic fibers for the stomach and supplies the stomach through anterior and posterior vagal trunks. The parasympathetic nerve supply arises from the sixth to the ninth thoracic vertebra of the spinal cord. The gastrointestinal system gets its nerve supply from the enteric nervous system. 

  1. Liver

The parenchyma of the liver is supplied by the hepatic plexus which gets parasympathetic supply from the vagus nerve and sympathetic supply from the coeliac plexus. The exact role of liver nerve supply is not known, although it may be involved in the vasoconstriction. 

  1. Kidneys

The parasympathetic fibers innervate the kidney through the renal branches of the vagus nerve. The afferent fibers that convey the pain originating from the kidney stones and the viscera travel along the sympathetic pathway. The role of the parasympathetic system in the kidney is unclear. 

  1. Skin

Specific cutaneous nerves supply the different areas of the skin. There are four main types of sensory receptors present in the skin. Meissner receptors detect light touch while the Pacinian corpuscles detect deep pressure and vibrational changes. The Ruffini endings detect the stretching of the collagen fibers of the skin. The Free nerve endings in the epidermis respond to pain, light touch, and variations in temperature.

  1. Small intestine

The duodenum receives sympathetic and parasympathetic innervation through the splanchnic and the vagus nerve, respectively. The lower section of the small intestine gets sympathetic supply from the 9th to 10th thoracic spinal cord segments via the lesser splanchnic nerves and parasympathetic supply from the vagus nerve. 

10. Large intestine

The parasympathetic innervation of the large intestine comes from the vagus nerve and the pelvic splanchnic nerves. The superior and inferior mesenteric, and inferior hypogastric plexus contain sympathetic fibers for the large intestine. 

Diseases/Disorders

Peripheral Neuropathy

Peripheral neuropathy is a general term that describes the dysfunction of one or more nerves. It may result in impairment of sensation, movement, or the function of the involved organ. Several systemic diseases are known to cause peripheral neuropathy including diabetes mellitus, leprosy, celiac disease, sarcoidosis, and amyloidosis.

Mononeuropathy affects a single nerve and is commonly caused because of local trauma or infection. In Polyneuropathy, many nerves get damaged most likely due to systemic diseases such as diabetes mellitus. When nerves of the non-voluntary autonomic nervous system get damaged, it is referred to as autonomic neuropathy. The sign and symptoms of autonomic neuropathy may include urinary retention, dysphagia, abdominal pain, gastroparesis, and disturbance in the cardiovascular system. Dysfunction of the respiratory system is also a symptom of autonomic neuropathy. 

Neuritis

Neuritis refers to the inflammation of a single nerve or general inflammation of the whole peripheral nervous system. There are several types of neuritis including brachial neuritis, cranial neuritis, and optic neuritis. Some common symptoms are pain, paresthesia, paralysis, hypoesthesia, and loss of reflexes. Physical and chemical injuries, infections such as diphtheria infection, multiple sclerosis, diabetes mellitus, Hypothyroidism, alcoholism, cancer, and celiac disease can cause neuritis. 

Summary

Different types of nerves supply different parts of the body. The peripheral nervous system is divided into the somatic and the autonomic nervous system. The somatic nerve supply consists of both sensory and motor supply and it is associated with voluntary control of the skeletal muscles. The sensory nerves are distributed throughout the body and they recognize stimuli and carry the information to the central nervous system. The motor nerve supply carries information from the central nervous system, and it causes a response in muscles and glands. 

The autonomic nervous system is divided into the sympathetic, parasympathetic nervous system and enteric division and acts unconsciously. The sympathetic nerves arise from the thoracolumbar region of the spinal cord and synapse at the ganglia of the sympathetic chain. The autonomic nerve fibers arise in the brain and some spinal nerves such as pelvic splanchnic nerves also act as parasympathetic nerves. 

The eye is innervated by six cranial nerves that are optic, oculomotor, trochlear, trigeminal, abducens, and facial nerves. The ear is innervated by facial, the tympanic branch of the ninth nerve, and the vestibulocochlear nerve. The heart is innervated by the parasympathetic fibers of the vagus nerve and sympathetic fibers of the cervical ganglia. The lungs are innervated by nerves of the pulmonary plexus which contain sympathetic, parasympathetic, and sensory fibers. The liver is supplied by the hepatic plexus and the kidneys are innervated by the renal branches of the vagus nerve. 

The term neuropathy refers to damage to one or more nerves and it results in impairment of sensation and function. Diabetes mellites, leprosy, celiac disease, sarcoidosis, and amyloidosis are some common causes of neuropathy. Neuritis refers to inflammation of one more nerve and it results in paresthesia, hyperesthesia, pain, and paralysis of the involved area. 

References