Somatic nervous system

1. CLASSIFICATION OF NERVOUS SYSTEM – SOMATIC NERVOUS SYSTEM
Sneha Thiagarajan
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Nervous system:
The nervous system is a complex network of nerves and cells that carry messages to and from the
brain and spinal cord to various parts of the body. (Dr Ananya Mandal, 2018)
The nervous system is further divided into central and peripheral nervous system. The central
nervous system (CNS) consists of the brain and the spinal cord. The peripheral nervous system (PNS) as
the name suggests, consists of the nerves outside the brain and spinal cord. The PNS is further divided into
somatic and autonomic nervous system.
Somatic Nervous System:
The somatic nervous system, or voluntary nervous system, is that part of the peripheral nervous
system that regulates body movement through control of skeletal (voluntary) muscles and also relates the

2. organism with the environment through the reception of external stimuli, such as through the senses of
vision, hearing, taste, and smell. ("Somatic nervous system - New World Encyclopedia", 2018)
Parts of the Somatic Nervous System:
The somatic nervous system contains two major types of neurons:
1. Sensory neurons, also known as afferent neurons, are responsible for carrying information from
the nerves to the central nervous system.
2. Motor neurons, also known as efferent neurons, are responsible for carrying information from the
brain and spinal cord to muscle fibers throughout the body.
In humans, there are 31 pairs of spinal nerves and 12 pairs of cranial nerves.
The 31 pairs of spinal nerves emanate from different areas of the spinal cord and each spinal nerve has a
ventral root and a dorsal root. The ventral root has motor (efferent) fibers that transmit messages from the
central nervous system to the effectors, with the cell bodies of the efferent fibers found in the spinal cord
gray matter. The dorsal root has sensory (afferent) fibers that carry information from the sensory receptors
to the spinal cord.
The Spinal nerves are classified into 5 categories on the basis of their origin. They are:
 Cervical nerves: Eight pairs of nerves emerge from within the vertebral column between the neck
and the vertebrae. (C1-C8)
 Thoracic nerves: Twelve pairs of nerves that emerge between the vertebrae to which ribs are
attached. (T1-T12)
 Lumbar nerves: Five nerves that lie between the vertebrae of the lower back (L1-L5)
 Sacral nerves: Five nerves that lie between the vertebrae of hip cage, or pelvis (S1-S5)
 Coccygeal nerve: A pair of nerves which emerge from the rudiment of the tail.
The 12 pairs of cranial nerves transmit information on the senses of sight, smell, balance, taste and hearing
from special sensory receptors. They also transmit information from general sensory receptors in the body,
largely from the head. This information is received and processed by the central nervous system and then

3. the response travels via the cranial nerves to the skeletal muscles to control movements in the face and
throat, such as swallowing and smiling.
S.No Name Function Symptoms of Dysfunction
1. Olfactory Smell Loss of sense of smell (anosmia)
2. Optic Vision Loss of vision (apnosia)
3. Oculomotor Lateral Eye movements Double vision, drooping eyelids,
deviation of eye outwards.
4. Trochlear Upward and Downward
eye movements
Double Vision, defect of downward gaze
5. Trigeminal Masticatory movements
and Facial sensation
Decreased sensitivity or numbness of
face, brief attacks of severe pain,
weakness and wasting of facial muscles,
asymmetrical chewing
6. Abducens Eye Movements Double vision, inward deviation of the
eye
7. Facial Facial movement and
Sensation
Facial paralysis, loss of taste over anterior
two-thirds of tongue
8. Auditory
Vestibular
Hearing and Balance Deafness, Disequilibrium, feelings of
Disorientation and space
9. Glossopharengeal Tongue and Pharynx
Movement and sensation
Partial dry mouth, loss of taste, anesthesia
and paralysis of upper pharynx
10. Vagus Heart, blood vessels,
Viscera, movement of
Larynx and pharynx
Hoarseness, lower Pharyngeal anesthesia
and paralysis, indefinite visceral
disturbances
11. Spinal Accessory Neck Muscles Wasting of neck with weakened rotation,
inability to shrug.
12. Hypoglossal Tongue Muscles Wasting of tongue with deviation to side
of lesion on protrusion.
Nerve signal transmission
The basic route of nerve signals within the efferent somatic nervous system involves a particular sequence.
1. Precentral gyrus (brain): The origin of nerve signals initiating movement.
2. Corticospinal tract (cross-section of spinal cord): Mediator of message from brain to skeletal
muscles.
3. Axon: The messenger cell that carries the command to contract muscles.
4. Neuromuscular junction: The messenger axon cell tells muscle cells to contract at this
intersection.

4. Reflex Arcs and the Somatic Nervous System:
In addition to controlling voluntary muscles movements, the somatic nervous system is also associated
with involuntary movements known as reflex arcs. During a reflex arc, muscles move involuntarily without
input from the brain.
This occurs when a nerve pathway connects directly to the spinal cord. Some examples of reflex arcs
include jerking your hand back after accidentally touching a hot pan or an involuntary knee jerk when your
doctor taps on your knee.
Sensory nerves carry signals to the spinal cord, often connect with inter-neurons in the spine, and then
immediately transmit signals down the motor neurons to the muscles that triggered the reflex. Reflex arcs

5. that impact the organs are called autonomic reflex arcs while those that affect the muscles are referred to as
somatic reflex arcs.
References
Dr Ananya Mandal, M. (2018). What is the Nervous System?. News-Medical.net. Retrieved 6 February
2018, from https://www.news-medical.net/health/What-is-the-Nervous-System.aspx
Somatic nervous system. (2018). En.wikipedia.org. Retrieved 13 February 2018, from
https://en.wikipedia.org/wiki/Somatic_nervous_system
Somatic Nervous System - Definition, Function & Examples | Biology Dictionary. (2018). Biology
Dictionary. Retrieved 6 February 2018, from https://biologydictionary.net/somatic-nervous-system/
Somatic nervous system - New World Encyclopedia. (2018). Newworldencyclopedia.org. Retrieved 6
February 2018, from http://www.newworldencyclopedia.org/entry/Somatic_nervous_system
The Role of the Somatic Nervous System. (2018). Verywell. Retrieved 6 February 2018, from
https://www.verywellmind.com/what-is-the-somatic-nervous-system-2795866
Bryan Kolb, I. Q. Fundamentals of Human Neuropsychology (5th ed.).