The somatic nervous system (SNS), also known as voluntary nervous system, is a part of the

peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of Bilateria, bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside t ...

(PNS) that links

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

and

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

s under conscious control, as well as to

sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduc ...

s in the skin. The other part complementary to the somatic nervous system is the

autonomic nervous system The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...

(ANS). The somatic nervous system consists of

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the Electrochemistry, electrochemical nerv ...

s carrying

afferent nerve fiber Afferent nerve fibers are axons (nerve fibers) of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections ''arrive'' at a particular brain region. In the peripheral nerv ...

s, which relay sensation from the body to the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

(CNS), and nerves carrying

efferent nerve fiber Efferent nerve fibers are axons (nerve fibers) of efferent neurons that exit a particular region. These terms have a slightly different meaning in the context of the peripheral nervous system (PNS) and central nervous system (CNS). The efferen ...

s, which relay motor commands from the CNS to stimulate

muscle contraction Muscle contraction is the activation of Tension (physics), tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in musc ...

. Specialized nerve fiber ends called sensory receptors are responsible for detecting information both inside and outside the body. The ''a-'' of ''afferent'' and the ''e-'' of ''efferent'' correspond to the

prefix A prefix is an affix which is placed before the stem of a word. Particularly in the study of languages, a prefix is also called a preformative, because it alters the form of the word to which it is affixed. Prefixes, like other affixes, can b ...

es ''ad-'' (to, toward) and ''ex-'' (out of).

Structure

There are 43 segments of nerves in the human body. With each segment, there is a pair of sensory and motor nerves. 31 segments of nerves are in the spinal cord and 12 are in the brain stem.

Interneuron Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are neurons that are not specifically motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enab ...

s, also known as association neurons, are present throughout the central nervous system forming links between the sensory and motor fibres. Thus the somatic nervous system consists of two parts: * Spinal nerves: They are mixed nerves that carry sensory information into and motor commands out of the spinal cord. The spinal nerves serve as a bridge between the environment and the central nervous system (CNS). These neurons work together to transfer autonomic, sensory, and motor impulses from the spinal cord to the body's other systems. The spinal nerves are arranged into 31 pairs according to the regions of the spinal cord. To be more precise, there are eight pairs of cervical nerves (C1–C8), twelve pairs of thoracic nerves (T1–T12), five pairs of lumbar nerves (L1–L5), five pairs of sacral nerves (S1–S5), and one pair of coccygeal nerves. Peripheral nerves are included in the category of peripheral nervous system. *

Cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

: They are the nerve fibers that carry information into and out of the brain stem. They include smell, eye muscles, mouth, taste, ear, neck, shoulders, and tongue. Partially innervating the head and neck structures are the cranial nerves, which supply afferent and efferent functions. Neural processes connected to certain brainstem nuclei and cortical regions make up cranial nerves, in contrast to spinal nerves, which have neural fibers originating from the spinal grey matter as their roots. The cranial nerves VIII (vestibulocochlear), II (optic), and I (olfactory) are regarded as entirely afferent. The exclusively efferent cranial nerves are XI (spinal accessory), XII (hypoglossal), VI (abducens), IV (trochlear), and III (oculomotor). The remaining cranial nerves, X (vagus), IX (glossopharyngeal), VII (facial), and V (trigeminal), have mixed sensory and motor functions. Anatomically, cranial nerves are numbered from I to XII, which indicates their sequential origin from the caudal to the ventral brainstem. Alternatively, they can be postulated in groups based on the developmental functions they perform (sensory, motor, mixed).

Function

The somatic nervous system's principal function is to facilitate the organs and striated muscles of the central nervous system so that we can carry out our daily responsibilities. The primary motor cortex, or precentral gyrus, is home to the higher motor neurons that make up the basic motor pathway. These neurons transmit signals to the lower motor neurons in the spinal cord through axons known as the corticospinal tract. These impulses move to the

neuromuscular junction A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...

(NMJ) of skeletal muscle via peripheral axons after synapsing with the lower motor neurons through the ventral horn of the spinal cord. A signal that travels to the NMJ, which innervates muscles, is produced by the release of acetylcholine by upper motor neurons.

Acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

binds to nicotinic acetylcholine receptors of alpha-motor neurons. The somatic nervous system controls all voluntary muscular systems within the body, and the process of voluntary

reflex arc A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons synapse in the spinal cord and the signal then travels through it into the brain. This allows for faster reflex actions to occur by activating spinal mo ...

s. The basic route of nerve signals within the efferent somatic nervous system involves a sequence that begins in the upper cell bodies of

motor neuron A motor neuron (or motoneuron), also known as efferent neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly o ...

s ( upper motor neurons) within the precentral gyrus (which approximates the primary motor cortex). Stimuli from the precentral gyrus are transmitted from upper motor neurons, down the

corticospinal tract The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neu ...

, to lower motor neurons ( alpha motor neurons) in the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

and ventral horn of the

: upper motor neurons release a

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

called

glutamate Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that ...

from their axon terminal knobs, which is received by

glutamate receptor Glutamate receptors are synaptic and non synaptic receptors located primarily on the membranes of neuronal and glial cells. Glutamate (the conjugate base of glutamic acid) is abundant in the human body, but particularly in the nervous system ...

s on the lower motor neurons: from there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic

receptor Receptor may refer to: * Sensory receptor, in physiology, any neurite structure that, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and respond ...

s (

nicotinic acetylcholine receptor Nicotinic acetylcholine receptors, or nAChRs, are Receptor (biochemistry), receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the c ...

s) of muscles, thereby relaying the stimulus to contract muscle fibers.

Reflex arcs

is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output. Reflex circuits vary in complexity—the simplest spinal reflexes are mediated by a two-element chain, of which in the human body there is only one, also called a monosynaptic reflex (there is only one synapse between the two neurones taking part in the arc: sensory and motor). The singular example of a monosynaptic reflex is the

patellar reflex The patellar reflex, also called the knee reflex or knee-jerk, is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord. Many animals, most significantly humans, have been seen to have the patellar reflex, including dogs, cat ...

. The next simplest reflex arc is a three-element chain, beginning with sensory neurons, which activate

interneuron Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are neurons that are not specifically motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enab ...

s inside of the spinal cord, which then activate motor neurons. Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex ("knee jerk") activated by tapping the patellar tendon, contribute to ordinary behavior.

Clinical significance

A medical condition known as peripheral neuropathy affects the somatic nervous system's peripheral nerve fibers. They can be divided into congenital and acquired disorders based on the causes. They can also be categorized based on whether the myelin sheath (demyelinating neuropathy) or axons (axonal neuropathy) have the predominant disease. There is a wide range of causes for axonal peripheral neuropathy, most of which are toxic-metabolic in origin and include group B vitamin deficiencies and diabetes. Demyelinating neuropathies do not vary with length. They are frequently immune-mediated, which causes a more widespread involvement of sensorimotor function and an early loss of deep tendon reflexes. When joint position and vibratory sensory loss are present, sensory participation is more selective. Defects in the

, peripheral nervous system, or muscle itself are the cause of numerous congenital illnesses of sensory and motor function. Owing to the vast territory encompassed by the somatic nerve system, these ailments may manifest as localized in nature, or as broad and systemic. Charcot-Marie-Tooth disease,

Myasthenia gravis Myasthenia gravis (MG) is a long-term neuromuscular junction disease that leads to varying degrees of skeletal muscle weakness. The most commonly affected muscles are those of the eyes, face, and swallowing. It can result in double vision, ...

, and Guillain–Barré syndrome are a few instances of them.

Charcot–Marie–Tooth

The Charcot–Marie–Tooth disease group comprises diverse hereditary illnesses that manifest as chronic, progressive neuropathy that affects both the motor and sensory neurons.

Myasthenia gravis

An autoimmune neurological condition called

myasthenia gravis Myasthenia gravis (MG) is a long-term neuromuscular junction disease that leads to varying degrees of skeletal muscle weakness. The most commonly affected muscles are those of the eyes, face, and swallowing. It can result in double vision, ...

is typified by impaired neuromuscular junction communication.

Guillain–Barré syndrome

A rare but dangerous post-infectious immune-mediated neuropathy is Guillain–Barré syndrome. It is brought on by an autoimmune reaction that destroys peripheral nervous system nerves, leading to symptoms including tingling, weakness, and numbness that can become paralysis.

Signs of somatic nervous system problems

Depending on whether the damage is to the motor nerves, which regulate movement, or the sensory nerves, which affect the senses, the symptoms of a somatic nervous system problem can differ. Damage to the motor nerves shows as: * Loss of movement control * Spasms or cramps in muscles * Tremors or twitching * Wasting of muscles ( muscle atrophy) The following signs could be present if the sensory system is damaged: * Inability to feel things touched * Numbness or tingling * Sharp or burning pain in the damaged area

Other animals

invertebrates Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum ...

, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory. For

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s, however, the response of a

skeletal striated muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

fiber to a neurotransmitter – always

acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

(ACh) – can only be excitatory.

References

{{DEFAULTSORT:Somatic Nervous System Sensory systems Peripheral nervous system