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Somatic Reflex

What Is It, Types, and More

Author:Corinne Tarantino, MPH

Editors:Alyssa Haag,Józia McGowan, DO

Illustrator:Jillian Dunbar

Copyeditor:David G. Walker


What is a somatic reflex?

A somatic reflex is an involuntary response to a stimulus, such as pulling one’s hand away after touching a hot stove. The nervous system is split into the central nervous system (i.e., the brain and spinal cord) and the peripheral nervous system (i.e., nervous system outside of the brain and spinal cord). The peripheral nervous system is further divided into the autonomic nervous system, which controls organs and glands, and the somatic nervous system, which controls voluntary skeletal muscle movements. The somatic nervous system is made up of afferent (sensory) neurons and efferent (motor) neurons. The somatic reflex is a motor response to a sensory stimulus.

What is a somatic reflex arc?

A somatic reflex arc is the neural pathway that occurs from the initial sensing of a stimulus to the response, such as the moving of a limb. The basic pathway relays information from a sensory organ to muscle cells by passing through afferent, or sensory, neurons to the CNS. This information is then relayed to the efferent, or motor, neurons. 

The first part of a reflex arc is a sensory input from the environment, like touching a hot stove. This sensory information is then relayed through at least one afferent neuron. The information travels through a neuron as a result of action potentials, which are electrical signals that occur due to ion movement in and out of the neuron. When the information reaches the end of a neuron, it is passed to the next neuron through a synapse (i.e., a small gap between neurons) by chemical messengers called neurotransmitters. 

At the integration center in the spinal cord, the information may connect directly from an afferent neuron to an efferent neuron or may pass through an interneuron (a neuron that transmits information between other neurons). The afferent neuron enters the spinal cord through the back (i.e., dorsal) horn, and the efferent neuron exits through the front (i.e., ventral) horn of the spinal cord. The efferent neurons transmit information via neurotransmitters to the muscle cells through the neuromuscular junction, thereby causing muscle contraction and movement. In the case of the hot stove, this movement may involve removing one’s hand from the stove.

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What are the somatic reflexes?

There are several somatic reflexes, with the most common categories of somatic reflexes including the stretch reflex, inverse stretch reflex, and the withdrawal reflex.

Stretch reflex

The stretch reflex, or tendon reflex, is one of the simplest monosynaptic reflexes (reflexes that have only one synapse between the afferent and efferent neurons). The stretch reflex occurs when muscle spindles, or afferent neurons along the length of skeletal muscles, are stretched, causing muscle contraction. The impulses from the muscle spindles are transmitted to the central nervous system, followed by direct transmission to motor neurons, which return to the same muscle. 

The most commonly identified type of stretch reflex is the knee jerk reflex, where tapping the patellar tendon (i.e., the tendon located at the front of the knee) causes the foot to kick forward. This reflex is a type of deep tendon reflex tested during a neurological examination to identify abnormalities within this reflex arc.

Inverse stretch reflex

The inverse stretch reflex is the relaxation response to high tension when stretching a muscle. The Golgi tendon organ is a web of nerve endings that surrounds the tendon fibers and stimulates the reflex during both passive stretching and active muscle contraction. 

Withdrawal reflex 

The withdrawal, or flexor, reflex is an important example of a polysynaptic reflex (reflexes with 2 or more synapses), where an extended body part is quickly flexed, or bent, in response to a harmful stimulus. This reflex protects an individual from harmful situations, such as the fast retrieval of a hand off a hot stove. 

The reflex can also include a cross-extensor response, whereas if one limb is withdrawn the opposite limb is extended. This cross-extensor reflex is important when the body needs to shift weight to the other side of the body

What differentiates an autonomic reflex from a somatic reflex?

The autonomic reflex and somatic reflex often differ in their efferent branches of the reflex arc and their effector targets. In an autonomic reflex, the efferent branch typically contains a two-step pathway, where the efferent neuron enters a ganglion (i.e., a bundle of nerve cell bodies) before reaching the effector target. In addition, autonomic reflex axons are only myelinated (i.e., insulated with a fatty sheath) in the CNS. Lastly, the effector target in the autonomic reflex is often smooth muscle cells (e.g., stomach, intestines, and bladder). In contrast, the somatic reflex pathway contains a myelinated axon connecting from the spinal cord directly to the effector target, which is often skeletal muscle

Why are somatic reflexes routinely assessed?

It is important to assess somatic reflexes because they demonstrate the ability of nerves to respond to stimuli. Each reflex is associated with a few cranial or spinal nerves. There are twelve cranial nerves that each connect from the brain to a specific area of the head, neck, and trunk. Therefore, a lack of response in a specific reflex may indicate an underlying issue with a nerve, such as a herniated disk. Additionally, if multiple reflexes are affected, a systemic disease, such as diabetes mellitus and thyroid disorders, may be present.

What are the most important facts to know about a somatic reflex?

A somatic reflex is an involuntary movement in response to a stimulus. To produce the action, the somatic reflex arc is activated when a signal from the stimulus is sent to the muscle cells, passing through afferent neurons to the CNS, and finally, to the efferent neurons. The most common categories of somatic reflexes include the stretch reflex, the inverse stretch reflex, and the withdrawal reflex. Somatic reflexes have myelinated axons connecting the CNS to the skeletal muscle cells. In contrast, the autonomic reflex has a ganglion, the CNS axons are the only myelinated axons, and the nerves affect smooth muscle cell movement. Somatic reflexes are routinely assessed to indicate underlying issues involving one or more nerves. 

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Related links

Spinal cord reflexes
Neuromuscular junction and motor unit
Cranial nerves rap

Resources for research and reference

Akinrodoye, M.A., & Lui, F. (2021). Neuroanatomy, Somatic Nervous System. (2021). In: StatPearls [Internet]. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK556027/

Biga L. M., Dawson S., Harwell A., Hopkins R., Kaufmann J., LeMaster M., Matern P., Morrison-Graham K., Quick D., & Runyeon J. Anatomy & Physiology. OpenStax/Oregon State University. Retrieved from https://open.oregonstate.education/aandp/chapter/16-2-autonomic-reflexes-and-homeostasis/

Ju, W. (2020). Neuroscience, (2nd ed.). Pressbooks. Retrieved from https://ecampusontario.pressbooks.pub/neurosciencecdn2/chapter/anatomy-physiology-the-autonomic-nervous-system/

Reflex & voluntary control of posture & movement. (2019). In: Barrett, K. E., Barman S. M., Brooks, H.L., & Yuan, J.J.(Eds.), Ganong's Review of Medical Physiology, (26th ed.). McGraw-Hill. 

The spinal cord. Waxman, S.G.(Ed.), (2020). In: Clinical Neuroanatomy, (29th ed.). McGraw Hill. Retrieved from https://accessmedicine-mhmedical-com.ezproxylocal.library.nova.edu/content.aspx?bookid=2850§ionid=242763051