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| Somatic nervous system | |
|---|---|
 1. (Brain)Precentral gyrus: the origin of nerve signals initiating movement. 2. (Cross section ofSpinal cord)Corticospinal tract: Mediator of message from brain toskeletal muscles. 3.Axon: theefferent nerve fiber that carries the command tocontract muscles. 4.Neuromuscular junction:muscle cells are stimulated to contract at this intersection | |
| Details | |
| Synonym | Voluntary nervous system |
| Part of | Peripheral nervous system |
| Identifiers | |
| FMA | 9904 |
| Anatomical terminology | |
Thesomatic nervous system (SNS), also known asvoluntary nervous system, is a part of theperipheral nervous system (PNS) that linksbrain andspinal cord toskeletal muscles under conscious control, as well as tosensory receptors in the skin.[1][2] The other part complementary to the somatic nervous system is theautonomic nervous system (ANS).[3]
The somatic nervous system consists ofnerves carryingafferent nerve fibers, which relay sensation from the body to thecentral nervous system (CNS), and nerves carryingefferent nerve fibers, which relay motor commands from the CNS to stimulatemuscle contraction.[4] Specialized nerve fiber ends calledsensory receptors are responsible for detecting information both inside and outside the body.[3]
Thea- ofafferent and thee- ofefferent correspond to theprefixesad- (to, toward) andex- (out of).
There are 43 segments of nerves in the human body.[5] 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.[5]Interneurons, also known as association neurons, are present throughout the central nervous system forming links between the sensory and motor fibres.[6]Thus the somatic nervous system consists of two parts:
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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.[11]
Theprimary motor cortex, orprecentral gyrus, is home to the higher motor neurons that make up the basic motor pathway.[12] These neurons transmit signals to the lower motor neurons in the spinal cord through axons known as the corticospinal tract. These impulses move to theneuromuscular junction (NMJ) of skeletal muscle via peripheral axons after synapsing with thelower 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 binds tonicotinic acetylcholine receptors of alpha-motor neurons.[4]
The somatic nervous system controls allvoluntary muscular systems within the body, and the process of voluntaryreflex arcs.[13]
The basic route ofnerve signals within theefferent somatic nervous system involves a sequence that begins in the uppercell bodies ofmotor neurons (upper motor neurons) within theprecentral gyrus (which approximates theprimary motor cortex).[14] Stimuli from the precentral gyrus are transmitted from upper motor neurons, down thecorticospinal tract, tolower motor neurons (alpha motor neurons) in thebrainstem andventral horn of thespinal cord: upper motor neurons release aneurotransmitter calledglutamate from their axon terminal knobs, which is received byglutamate receptors on the lower motor neurons: from there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynapticreceptors (nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.[15]
Areflex arc is aneural 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 thepatellar reflex. The next simplest reflex arc is a three-element chain, beginning with sensory neurons, which activateinterneurons 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.[16]
A medical condition known asperipheral 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 themyelin 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 thecentral nervous system, 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, andGuillain–Barré syndrome are a few instances of them.[17]
TheCharcot–Marie–Tooth disease group comprises diverse hereditary illnesses that manifest as chronic, progressive neuropathy that affects both the motor and sensory neurons.[18]
An autoimmune neurological condition calledmyasthenia gravis is typified by impaired neuromuscular junction communication.[19]
A rare but dangerous post-infectious immune-mediated neuropathy isGuillain–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.[20]
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.[21]
Damage to the motor nerves shows as:
The following signs could be present if the sensory system is damaged:
Ininvertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory. Forvertebrates, however, the response of askeletal striated muscle fiber to a neurotransmitter – alwaysacetylcholine (ACh) – can only be excitatory.
