Somatic nervous system

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Somatic nervous system
Somatic Nervous System Image.svg
1. (Brain) Precentral gyrus: the origin of nerve signals initiating movement.

2. (Cross section of Spinal cord) Corticospinal tract: Mediator of message from brain to skeletal muscles.

Contents

3. Axon: the efferent nerve fiber that carries the command to contract muscles.

4. Neuromuscular junction: muscle cells are stimulated to contract at this intersection
Details
Part of Peripheral nervous system
Identifiers
FMA 9904
Anatomical terminology

The somatic nervous system (SNS) is made up of nerves that link the brain and spinal cord to voluntary or skeletal muscles that are under conscious control as well as to skin sensory receptors. Specialized nerve fiber ends called sensory receptors are responsible for detecting information within and outside of the body.

The somatic nervous system, or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles. [1] [2] The movements of our arms, legs, and other body parts are among the functions that the somatic nervous system is in charge of and that we can consciously control. The somatic nervous system consists of nerves carrying afferent nerve fibers, which relay sensation from the body to the central nervous system (CNS), and nerves carrying efferent nerve fibers, which relay motor commands from the CNS to stimulate muscle contraction. [3]

The a- of afferent and the e- of efferent correspond to the prefixes ad- (to, toward) and ex- (out of).

Structure

There are 43 segments of nerves in the human body. [4] With each segment, there is a pair of sensory and motor nerves. In the body, 31 segments of nerves are in the spinal cord and 12 are in the brain stem. [4] Besides these, thousands of association nerves are also present in the body.[ citation needed ]

Thus the somatic nervous system consists of two parts:

Function

The somatic nervous system's principal goal 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 (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 binds to nicotinic acetylcholine receptors of alpha-motor neurons. [3]

The somatic nervous system controls all voluntary muscular systems within the body, and the process of voluntary reflex arcs. [9]

The basic route of nerve signals within the efferent somatic nervous system involves a sequence that begins in the upper cell bodies of motor neurons (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, to lower motor neurons (alpha motor neurons) in the brainstem and ventral horn of the spinal cord: upper motor neurons release a neurotransmitter called glutamate from their axon terminal knobs, which is received by glutamate receptors on the lower motor neurons: from there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.

Reflex arcs

A reflex arc 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 next simplest reflex arc is a three-element chain, beginning with sensory neurons, which activate interneurons 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 central 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, and Guillain-Barre syndrome are a few instances of them. [10]

Charcot-Marie-Tooth (CMT)

The Charcot-Marie-Tooth (CMT) disease group comprises diverse hereditary illnesses that manifest as chronic, progressive neuropathy that affects both the motor and sensory neurons. [11]

Myasthenia Gravis (MG)

An autoimmune neurological condition called myasthenia gravis (MG) is typified by impaired neuromuscular junction communication. [12]

Guillain-Barré syndrome (GBS)

A rare but dangerous post-infectious immune-mediated neuropathy is Guillain-Barré syndrome (GBS). 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. [13]

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. [14]

Damage to the motor nerves shows as:

The following signs could be present if the sensory system is damaged:


Other animals

In invertebrates, 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 vertebrates, however, the response of a skeletal striated muscle fiber to a neurotransmitter – always acetylcholine (ACh) – can only be excitatory.

See also

Related Research Articles

<span class="mw-page-title-main">Nerve</span> Enclosed, cable-like bundle of axons in the peripheral nervous system

A nerve is an enclosed, cable-like bundle of nerve fibers in the peripheral nervous system.

<span class="mw-page-title-main">Peripheral nervous system</span> Part of the nervous system excluding the brain and spinal cord

The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brain and the spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a relay between the brain and spinal cord and the rest of the body. Unlike the CNS, the PNS is not protected by the vertebral column and skull, or by the blood–brain barrier, which leaves it exposed to toxins.

<span class="mw-page-title-main">Vagus nerve</span> Cranial nerve X, for visceral innervation

The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that carries sensory fibers that create a pathway that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right vagus nerves—but they are typically referred to collectively as a single subsystem. The vagus is the longest nerve of the autonomic nervous system in the human body and comprises both sensory and motor fibers. The sensory fibers originate from neurons of the nodose ganglion, whereas the motor fibers come from neurons of the dorsal motor nucleus of the vagus and the nucleus ambiguus. The vagus was also historically called the pneumogastric nerve.

<span class="mw-page-title-main">Motor neuron</span> Nerve cell sending impulse to muscle

A motor 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 or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

<span class="mw-page-title-main">Autonomic nervous system</span> Division of the nervous system supplying internal organs, smooth muscle and glands

The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, its force of contraction, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response.

<span class="mw-page-title-main">Parasympathetic nervous system</span> Division of the autonomic nervous system

The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.

<span class="mw-page-title-main">Sympathetic nervous system</span> Part of the autonomic nervous system which stimulates fight-or-flight responses

The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.

<span class="mw-page-title-main">Trigeminal nerve</span> Cranial nerve responsible for the faces senses and motor functions

In neuroanatomy, the trigeminal nerve (lit. triplet nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chewing; it is the most complex of the cranial nerves. Its name (trigeminal, from Latin tri- 'three', and -geminus 'twin') derives from each of the two nerves (one on each side of the pons) having three major branches: the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). The ophthalmic and maxillary nerves are purely sensory, whereas the mandibular nerve supplies motor as well as sensory (or "cutaneous") functions. Adding to the complexity of this nerve is that autonomic nerve fibers as well as special sensory fibers (taste) are contained within it.

<span class="mw-page-title-main">Motor nerve</span> Nerve located in the central nervous system

A motor nerve is a nerve that transmits motor signals from the central nervous system (CNS) to the muscles of the body. This is different from the motor neuron, which includes a cell body and branching of dendrites, while the nerve is made up of a bundle of axons. Motor nerves act as efferent nerves which carry information out from the CNS to muscles, as opposed to afferent nerves, which transfer signals from sensory receptors in the periphery to the CNS. Efferent nerves can also connect to glands or other organs/issues instead of muscles. In addition, there are nerves that serve as both sensory and motor nerves called mixed nerves.

<span class="mw-page-title-main">Muscle spindle</span> Innervated muscle structure involved in reflex actions and proprioception

Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.

<span class="mw-page-title-main">Afferent nerve fiber</span> Axonal projections that arrive at a particular brain region

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

<span class="mw-page-title-main">Efferent nerve fiber</span> Axonal projections that exit a particular region

Efferent nerve fibers refer to axonal projections that exit a particular region; as opposed to afferent projections that arrive at the region. These terms have a slightly different meaning in the context of the peripheral nervous system (PNS) and central nervous system (CNS). The efferent fiber is a long process projecting far from the neuron's body that carries nerve impulses away from the central nervous system toward the peripheral effector organs. A bundle of these fibers is called an efferent nerve. The opposite direction of neural activity is afferent conduction, which carries impulses by way of the afferent nerve fibers of sensory neurons.

<span class="mw-page-title-main">Reflex arc</span> Neural pathway which controls a reflex

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.

<span class="mw-page-title-main">Type Ia sensory fiber</span> Type of afferent nerve fiber

A type Ia sensory fiber, or a primary afferent fiber is a type of afferent nerve fiber. It is the sensory fiber of a stretch receptor called the muscle spindle found in muscles, which constantly monitors the rate at which a muscle stretch changes. The information carried by type Ia fibers contributes to the sense of proprioception.

A sensory nerve, or afferent nerve, is an anatomic term for a nerve that contains exclusively afferent nerve fibers. Nerves containing also motor fibers are called mixed. Afferent nerve fibers in a sensory nerve carry sensory information toward the central nervous system (CNS) from different sensory receptors of sensory neurons in the peripheral nervous system.

<span class="mw-page-title-main">Alpha motor neuron</span>

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

<span class="mw-page-title-main">General visceral afferent fiber</span> Part of the visceral nervous system

The general visceral afferent (GVA) fibers conduct sensory impulses from the internal organs, glands, and blood vessels to the central nervous system. They are considered to be part of the visceral nervous system, which is closely related to the autonomic nervous system, but 'visceral nervous system' and 'autonomic nervous system' are not direct synonyms and care should be taken when using these terms. Unlike the efferent fibers of the autonomic nervous system, the afferent fibers are not classified as either sympathetic or parasympathetic.

<span class="mw-page-title-main">Mesencephalic nucleus of trigeminal nerve</span>

The mesencephalic nucleus of trigeminal nerve is one of the sensory nuclei of the trigeminal nerve. It is located in the brainstem. It receives proprioceptive sensory information from the muscles of mastication and other muscles of the head and neck. It is involved in processing information about the position of the jaw/teeth. It is functionally responsible for preventing excessive biting that may damage the dentition, regulating tooth pain perception, and mediating the jaw jerk reflex.

Pallesthesia, or vibratory sensation, is the ability to perceive vibration. This sensation, often conducted through skin and bone, is usually generated by mechanoreceptors such as Pacinian corpuscles, Merkel disk receptors, and tactile corpuscles. All of these receptors stimulate an action potential in afferent nerves found in various layers of the skin and body. The afferent neuron travels to the spinal column and then to the brain where the information is processed. Damage to the peripheral nervous system or central nervous system can result in a decline or loss of pallesthesia.

<span class="mw-page-title-main">Axon reflex</span>

The axon reflex is the response stimulated by peripheral nerves of the body that travels away from the nerve cell body and branches to stimulate target organs. Reflexes are single reactions that respond to a stimulus making up the building blocks of the overall signaling in the body's nervous system. Neurons are the excitable cells that process and transmit these reflex signals through their axons, dendrites, and cell bodies. Axons directly facilitate intercellular communication projecting from the neuronal cell body to other neurons, local muscle tissue, glands and arterioles. In the axon reflex, signaling starts in the middle of the axon at the stimulation site and transmits signals directly to the effector organ skipping both an integration center and a chemical synapse present in the spinal cord reflex. The impulse is limited to a single bifurcated axon, or a neuron whose axon branches into two divisions and does not cause a general response to surrounding tissue.

References

  1. ↑ "Somatic nervous system". qbi.uq.edu.au. 2018-10-09. Retrieved 2021-04-22.
  2. ↑ How does the nervous system work?. Institute for Quality and Efficiency in Health Care. 2016-08-19.
  3. 1 2 Akinrodoye MA, Lui F (2022). "Neuroanatomy, Somatic Nervous System". StatPearls. StatPearls Publishing. PMID   32310487 . Retrieved 12 December 2022.
  4. 1 2 3 Rea P (2014-01-01). "Introduction to the Nervous System". Clinical Anatomy of the Cranial Nerves. Academic Press. pp. xv–xxix. doi:10.1016/b978-0-12-800898-0.00019-1. ISBN   978-0-12-800898-0.
  5. 1 2 Felten DL, O'Banion MK, Maida MS (January 2016), "9 - Peripheral Nervous System", in Felten DL, O'Banion MK, Maida MS (eds.), Netter's Atlas of Neuroscience (Third ed.), Philadelphia: Elsevier, pp. 153–231, doi:10.1016/b978-0-323-26511-9.00009-6, ISBN   978-0-323-26511-9
  6. ↑ Kaiser JT, Lugo-Pic JG (2024). "Neuroanatomy, Spinal Nerves". StatPearls. Treasure Island (FL): StatPearls Publishing. PMID   31194375 . Retrieved 2024-01-26.
  7. ↑ Sonne J, Lopez-Ojeda W (2024). "Neuroanatomy, Cranial Nerve". StatPearls. Treasure Island (FL): StatPearls Publishing. PMID   29261885 . Retrieved 2024-01-26.
  8. ↑ Traylor KS, Branstetter BF (August 2022). "Cranial Nerve Anatomy". Neuroimaging Clinics of North America. Neuroimaging Anatomy, Part 1: Brain and Skull. 32 (3): 565–576. doi:10.1016/j.nic.2022.04.004. PMID   35843663. S2CID   250568029.
  9. ↑ Betts JG, Desaix P, Johnson E, Johnson JE, Korol O, Kruse D, et al. (July 16, 2023). Anatomy & Physiology. Houston: OpenStax CNX. Introduction:The somatic nervous system. ISBN   978-1-947172-04-3.
  10. ↑ Waxenbaum JA, Reddy V, Varacallo M (2024). "Anatomy, Autonomic Nervous System". StatPearls. Treasure Island (FL): StatPearls Publishing. PMID   30969667 . Retrieved 2024-01-26.
  11. ↑ Szigeti K, Lupski JR (June 2009). "Charcot-Marie-Tooth disease". European Journal of Human Genetics. 17 (6): 703–710. doi:10.1038/ejhg.2009.31. PMC   2947101 . PMID   19277060.
  12. ↑ Dresser L, Wlodarski R, Rezania K, Soliven B (May 2021). "Myasthenia Gravis: Epidemiology, Pathophysiology and Clinical Manifestations". Journal of Clinical Medicine. 10 (11): 2235. doi: 10.3390/jcm10112235 . PMC   8196750 . PMID   34064035.
  13. ↑ Nguyen TP, Taylor RS (2024). "Guillain-Barre Syndrome". StatPearls. Treasure Island (FL): StatPearls Publishing. PMID   30335287 . Retrieved 2024-01-26.
  14. ↑ "Somatic Nervous System: What It Is & Function". Cleveland Clinic. Retrieved 2024-01-26.
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