The somatic nervous system(SNS) is the part of theperipheral nervous systemthat controls voluntary movement and processes external stimuli through two types of neurons: motor neurons (activating muscles) and sensory neurons (relaying information to the central nervous system).
Take-home Messages
Somatic Nervous System Function
The SNS has two main functions:
Below are the steps involved in how the SNS works:
Step 1: Environmental detection
Receptors in the skin and sense organs (e.g., mouth, nose, and ears) detect changes in the environment, such as temperature, pressure, or texture.
Step 2: Signal creation
Once environmental changes have been detected, impulses are created within the sensory neurons, which then carry signals through thespinal nervesto the spinal cord.
Step 3: Brain processing
These signals will then travel up the spinal cord to the brain.
The brain will then integrate this sensory information and determine an appropriate response.
Step 4: Response generation
This response is then transmitted from the brain to the spinal cord, reaching motor neurons.
The impulses will then be carried through the motor neurons out of the spinal cord and continue to the skeletal muscles, causing them to contract if needed.
There are two pathways involved in the SNS:The afferent pathwaywill carry sensory information from sensory organs to the CNS. In contrast,the efferent pathwaywill carry motor information from the CNS to skeletal muscles to regulate motor functions.
Reflex Arcs
As well as controlling all voluntary muscular systems of the body, the SNS also processes muscle reflex arcs.
Reflex arcs are neural pathways that produce involuntary movements, typically in response to stimuli perceived as imminent danger.
This happens when sensory neurons sense something within the environment and carry this signal directly to the spinal cord, but this is not transmitted to the brain.
Instead, the spinal cord will transmit signals through the motor neurons to the muscles to trigger a reflex movement. This way,the muscles move without any input from the brainto generate a response that is so fast that it is completed almost automatically.
An example of a reflex arc being used is moving a hand away after touching a hot surface. Another instance is the ‘knee jerk’ reaction.
Part of the SNS
The SNS contains two main types ofneurons. These neurons function to transmit signals throughout the body:
Other components of the SNS include:
Somatic vs. Autonomic nervous system
The SNS and the autonomic nervous system (ANS) are both part of the peripheral nervous system (PNS), but they have different functions:
Somatic Nervous System (SNS)Autonomic Nervous System (ANS)Controls voluntary movementsControls automatic behaviorsRequires conscious thoughtFunctions without conscious thoughtControls skeletal muscle movementControls breathing and heart rateProcesses external sensory informationRegulates internal organ functionManages reflex actionsMaintains automatic body processes
Working Together:
Both systems collaborate tomaintain homeostasis(“homo-” meaningsameand “-stasis” meaningstate of equilibrium), with the SNS handling external responses to environmental changes while the ANS manages internal bodily functions.
Damage
As the SNS is responsible for receiving sensory information and motor movements, symptoms associated with damage to this system includenumbness, muscle weakness, and pain.
Diabetes is the most common cause of neuropathy in the PNS, but damage can also be the result of autoimmune diseases, infections, and trauma. Damage to the nerves through injury can affect the functions of the afferent and efferent pathways of the SNS.
Some conditions could be the result of issues with the axons of the neurons (axonal neuropathy) or could be the result of issues with the myelin sheath (demyelinating neuropathy), which is the protective layer of the neurons (Akinrodoya & Lui, 2020).
Motor neuron disease is the result of the death of neurons, making it a neurodegenerative disease. Eventually, the muscles of people with this disease will deteriorate, resulting in a loss of function.
Multiple sclerosis is an autoimmune disease that causes the destruction of peripheral nerves, resulting in a variety of sensory and motor problems.
Whilst these diseases are not always preventable, there are some lifestyle changes that can aid in stopping the preventable symptoms of SNS damage/weakening.
This can include sticking to a healthy diet in order to maintain a healthy weight, not smoking, avoiding alcohol, regular exercise, and correcting vitamin deficiencies to limit the chance of developing issues.
ReferencesAkinrodoye MA, Lui F.Neuroanatomy, somatic nervous system.StatPearls[Internet]. Treasure Island (FL): StatPearls Publishing. Updated April 2, 2020.Cuevas, J. (2015).The Somatic Nervous System.Reference Module in Biomedical Sciences. Elsevier.Dorland, W. A. N. (2011).Dorland’s Illustrated Medical Dictionary E-Book. Elsevier Health Sciences.Fukudo, S. (2012).Hypothalamic-pituitary-adrenal axis in gastrointestinal physiology. In Physiology of the gastrointestinal tract(pp. 791-816). Elsevier Inc.Rea, P. (2014).Introduction to the nervous system. Clinical Anatomy of the Cranial Nerves; Rea, P., Ed.; Academic Press: Cambridge, MA, USA.
References
Akinrodoye MA, Lui F.Neuroanatomy, somatic nervous system.StatPearls[Internet]. Treasure Island (FL): StatPearls Publishing. Updated April 2, 2020.
Cuevas, J. (2015).The Somatic Nervous System.Reference Module in Biomedical Sciences. Elsevier.
Dorland, W. A. N. (2011).Dorland’s Illustrated Medical Dictionary E-Book. Elsevier Health Sciences.
Fukudo, S. (2012).Hypothalamic-pituitary-adrenal axis in gastrointestinal physiology. In Physiology of the gastrointestinal tract(pp. 791-816). Elsevier Inc.
Rea, P. (2014).Introduction to the nervous system. Clinical Anatomy of the Cranial Nerves; Rea, P., Ed.; Academic Press: Cambridge, MA, USA.
Karina Ascunce GonzálezNeuroscience B.A. (Hons), Harvard UniversityPhD Neuroscience Student, Yale University
Karina Ascunce González
Neuroscience B.A. (Hons), Harvard University
PhD Neuroscience Student, Yale University
PhD Student at the Yale Biological & Biomedical Sciences' Interdepartmental Neuroscience Program interested in neurodegeneration, stem cell culture, and bioethics. AB in Neuroscience with a Secondary in Global Health & Health Policy from Harvard University. Karina has been published in peer reviewed journals.
Saul McLeod, PhD
BSc (Hons) Psychology, MRes, PhD, University of Manchester
Saul McLeod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.
Olivia Guy-Evans, MSc
BSc (Hons) Psychology, MSc Psychology of Education
Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.
