Spinal nerves are bundles of nerve fibers connected to the spinal cord. These nerves carry information to and away from the spinal cord. Spinal nerves supply all the areas of the body except the head and neck region. Some structures in the head and neck region are also innervated or supplied by the spinal nerves such as muscles of the neck, etc.
Spinal nerves are essential for the control of body parts by the higher centers of the nervous system. If any spinal nerve is cut, trapped, injured, or is involved in a disease process, the areas of the body supplied by that nerve escape the control of CNS.
When any area of the body escapes the control of the CNS, its functions are not regulated. As a result, that particular area of the body loses its functional ability and dies soon. In this article, we will study different aspects of the spinal nerves. We will discuss their formation, distribution, functions, diseases and a lot more. So, keep reading.
Number and Distribution
In our body, there are 31 pairs of spinal nerves. This means that from both sides of the spinal cord, 31 spinal nerves arise and are distributed to different body parts. There are 31 spinal nerves on the right side, and 31 spinal nerves on the left side.
These spinal nerves are distributed in different regions of the body as follows;
Cervical nerves (C1 to C8)
There are 8 pairs of spinal nerves in the cervical region. These nerves mainly supply the muscles, joints, and skin of the upper limbs. The cervical nerves also supply some areas of the neck region, including some muscles of the neck. They also supply some patch of skin over the head and chest and some extrinsic muscles of the chest.
Thoracic nerves (T1 to T12)
There are 12 pairs of spinal nerves that originate from the thoracic segments of the spinal cord. These nerves mainly supply the skin over the chest and intrinsic muscles of the thoracolumbar region. The thoracic nerves also supply some of the areas of skin over the abdomen and axilla.
Lumbar nerves (L1 to L5)
The lumbar segments of the spinal cord give rise to 5 pairs of spinal cord. These nerves mainly supply the muscles of the abdomen and pelvis. Some muscles of the thigh are also supplied by the lumbar nerves. These nerves also supply the skin of the abdominal region, the skin of the back, skin of the anterior thigh, and skin of the inguinal region.
Sacral nerves (S1 to S5)
There are five pairs of spinal nerves that originate from the sacral segments of the spinal cord. These nerves provide innervation mainly to the lower limbs. The skin, muscles, and joints of the lower limb are supplied by the sacral nerves. However, these nerves also provide innervation to some muscles of the pelvic region.
Coccygeal nerve (Co1)
It is the last pair of spinal nerves that arises from the tapering part of the spinal cord, the conus medullaris. This nerve is distributed mainly to the skin over the coccyx. It has no other distribution in the body.
Formation of Spinal Nerve
The formation of the spinal nerve means the way a spinal nerve originates from the spinal cord. In this section, we will say how nerve fibers leave the spinal cord and join together to form the spinal nerve.
Each spinal nerve is made up of two roots. These are;
- Anterior or Ventral root
- Posterior or Dorsal root
Both these roots emerge from the spinal cord and later join together to form a single spinal nerve.
In order to understand the formation of spinal nerves, let us understand how these roots originate and join together.
Anterior or Ventral Root
The anterior root of spinal nerve consists of the motor fibers. It contains the axons of the motor neurons located in the anterior gray horn of the spinal cord. These motor fibers represent the efferent fibers carrying information away from the spinal cord. These efferent nerve fibers emerge from the anterolateral aspect of the spinal cord and form the anterior root of a spinal nerve.
Posterior or Dorsal Root
The posterior root of spinal nerve is made up of sensory fibers. These nerve fibers carry sensory information towards the spinal cord and thus, are called afferent nerve fibers.
These afferent fibers are actually the central process of the neurons having cell bodies in the dorsal root ganglia of spinal nerves. The afferent sensory nerve fibers enter the spinal cord from the posterolateral wall and form the posterior root of the spinal nerve.
It should be kept in mind that the dorsal root ganglion is also considered a part of the posterior root of spinal nerve. The peripheral process of the neurons located in the dorsal root ganglia then continue forwards to form the rest of the spinal nerve.
The nerve fibers in the posterior root of spinal nerve curve anteriorly while the nerve fibers in the anterior root curve posteriorly. Both these roots then join together to form a single spinal nerve.
A pair of spinal nerves originate from each segment of spinal cord.
In addition to the roots, the spinal nerve also receives fibers in the form of white ramus and gray ramus communicans.
White Ramus Communicans
These rami contain the preganglionic fibers. The preganglionic fibers leave the spinal nerve through white ramus communicans and then join the sympathetic ganglia located in the paravertebral chain. These white rami are present only in thoracic nerves (T1-T12) and first two lumbar nerves (L1-L2).
Gray Ramus Communicans
These rami contain the postganglionic fibers of the sympathetic ganglia. The gray rami are present in all the spinal nerves. Every spinal nerve receives postganglionic fibers of sympathetic trunk via the gray rami.
Division of Spinal Nerve
Just after receiving the gray ramus, each spinal nerve divides into two trunks or rami. These are the anterior or ventral trunk and a dorsal or posterior trunk. The anterior or ventral trunk comes forward and supplies the anterior aspects of the body. The posterior or dorsal trunk curves backward and supplies the posterior parts of the body.
As we have studied the formation of spinal nerves, we know that the spinal nerve contains both sensory as well as motor fibers. That’s why spinal nerves are called mixed nerves. In addition, they also contain autonomic fibers. The functions performed by the spinal nerves are dependent on these nerve fibers.
Thus, a spinal nerve performs the following functions.
Transmission of Sensory Information
The spinal nerves carry sensory information from the peripheral body parts to the spinal cord. The sensory neurons are located in the dorsal root ganglia. The peripheral fibers of these sensory neurons form the sensory fibers present in the spinal cord.
The nerve endings of these fibers perceive stimuli such as heat and cold, touch, etc. This information is carried in the form of nerve impulses to the dorsal root ganglia and from there to the spinal cord via the central process of these neurons.
Conduction of Motor Impulses
The spinal nerves carry motor impulses to the muscles of the body. This function is dependent on the motor fibers present in spinal nerves. These motor fibers are the axons of the motor neurons and originate from the spinal cord in the form of the anterior root of spinal nerve.
The motor impulses are generated by these motor neurons and are carried by spinal nerves to the muscles of peripheral body parts.
Conduction of Autonomic Signals
The spinal nerves also carry autonomic signals to the peripheral body parts. The autonomic fibers enter the spinal nerves in the form of gray rami and are carried by the spinal nerves to the visceral organs of the body. This function is essential for the regulation of physiological signals performed by the body.
This is very interesting and important function performed by the spinal nerves. In this action, both the motor and the sensory fibers of the spinal nerve are involved. A reflex action is an involuntary response to a painful stimulus. For example, when you touch a hot object, the temperature sensations are carried by the sensory fibers to the spinal cord.
The interneurons in the gray matter of spinal cord receive this information and pass a nerve impulse to motor neurons of the same spinal nerve. The motor fibers in the spinal nerve carry the motor impulse to the muscles of the arm and the hand is pulled away from the hot object. The pathway in this action involves both sensory and motor fibers and is called reflex arc.
Spinal nerves carry information towards and away from the spinal cord.
There are 31 pairs of spinal nerves in the human body.
Each spinal nerve originates from the spinal cord in the form of anterior and posterior roots. Later, these two roots join to form a single spinal nerve.
The anterior root contains motor nerve fibers, whereas the posterior root contains sensory nerve fibers.
Soon after the formation, the spinal nerve divides into anterior and posterior trunks or divisions.
The functions performed by the spinal nerve includes the following;
- Conduction of sensory information
- Conduction of motor impulses
- Conduction of autonomic signals
- Reflex action
- Anatomy, descriptive and surgical: Gray’s anatomy. Gray, Henry. Philadelphia : Courage Books/Running Press, 1974
- Clinically Oriented Anatomy. Moore, Keith L. Philadelphia : Wolters Kluwer Health/Lippincott Williams & Wilkins, 2010 (6th ed)
- Human Neuroanatomy. Carpenter, Malcolm B. Baltimore : Williams & Wilkins Co., 1976 (7th ed)