There are 31 pairs of spinal nerves that leave the vertebral column through the intervertebral foramina. These foramina are formed by the adjacent vertebrae. These nerves are named according to the vertebral region with which they are associated; that is:

• 8 cervical
• 12 thoracic
• 5 lumber
• 5 sacral
• 1 coccygeal

Although there are only 7 cervical vertebrae, but there are 8 nerves; as the 1st nerve leaves the vertebral canal between the occipital bone and atlas(C1). The lumber, sacral and coccygeal nerves leave the spinal cord at the level of the 1st lumber vertebrae.

NERVE ROOTS

Spinal nerves arises from both sides of the spinal column, and leaves through the intervertebral foramina. A typical spinal nerve has two connections to the cord- a posterior root and the anterior root. Both these posterior and anterior roots unite to form a spinal nerve at the intervertebral foramen. Since the posterior root contains sensory axons and the anterior root contains motor neuron; all spinal nerves are classified as mixed nerve. The posterior root contains the posterior root ganglia in which the cell bodies of sensory neurons lies.

BRANCHES

Just after leaving the intervertebral foramen, the spinal nerves divides into several branches. These branches are also called as rami. There are several types of rami

• The posterior rami, supplies to the muscles and skin of posterior surface of trunk.
• The anterior rami supplies to the muscles and skin of lateral and anterior surface of trunk, and upper and lower limbs.
• The meningeal branch(rami) supplies to the vertebrae, vertebral ligaments, blood vessels of spinal cord and meninges.
• The rami communicantes is one of the component of ANS.

PLEXUSES

Axons from the anterior rami of spinal nerves, except from the thoracic nerves; do not directly go to the body surfaces they supply.  Instead they form networks on both the left and right side of the body by joining with various axons of adjacent rami. Such a networks is known as plexuses. The various kinds of plexuses are as follows:

• Cervical plexus:- This is formed by the anterior rami of 1st four cervical nerves. It lies within the neck opposite to 1st four cervical vertebrae. This plexus is protected by the sternocleidomastoid muscles.

Its superficial branches supplies at the back structure and the side of head; deep branches supply muscles of neck; and phrenic nerve supplies the diaphragm, hence initiates inhalation.

• Brachial plexus:- It is formed by the anterior rami of C5-C8 and T1 and T2 nerves. It is situated deeply within the neck and shoulder above. The branches of this plexus supply the skin and muscles of upper limbs and chest muscles.

Nerves that emerges from this plexus which are- axillary nerve, radial nerve, musculocutaneous nerve, median nerve, ulnar nerve and medial cutaneous nerve.

• Lumber plexus:- It is formed by the anterior rami of 1st three and some part of 4th lumber nerves. It lies in front of the transverse processes of lumber vertebrae and behind the psoas muscle. The main branches of this plexus are- iliohypogastric nerve, ilioinguinal nerve, genitofemoral nerve, lateral cutaneous nerve, femoral nerve, obturator nerve and lumbosacral nerve.

• Sacral plexus:- This plexus is formed by the anterior rami of lumbosacral trunk and the S1-S3 nerves. The lumbosacral trunk is formed by the 5th and part of L4. It lies in the posterior wall of pelvic cavity.

Sacral plexus is divided into a number of branches; these branches supplies the skin and muscles of pelvic floor, muscles around hip joint and pelvic organs. It also provides the sciatic nerve; the largest nerve in the body.

• Coccygeal plexus:- This plexus is very small and is formed by the part of S4 and S5 and also the coccygeal nerve. The nerve of this plexus supplies to the skin and muscles around the coccyx and also the anal area.

Multiple choice questions(MCQs)

1. Spinal nerves comes under which part of nervous system?
A. central nervous system B. peripheral nervous system
C. somatic nervous system D. autonomic nervous system

2. How many pairs of spinal nerves are present in the body?
A. 31 B. 12
C. 15 D. 30
3. Which of the following is a branch of lumber plexus?
A. femoral nerve B. ilioinguinal nerve
C. obturator nerve D. all of the above

4. Match the following-
a. cervical plexus 1. Phrenic nerve
b. brachial plexus 2. Median nerve
c. lumber plexus 3. Iliohypogastric nerve
d. sacral plexus 4. Sciatic nerve

5. Anterior root contains which kind of nerve fiber?
A. sensory fibers B. motor fibers
B. mixed fibers D. does not contain nerve fibers
6. Which branch supplies the lower and upper limbs?
A. anterior rami B. posterior rami
C. meningeal rami D. rami communicantes

7. Which of the following statement is true?
A. posterior rami supplies lower limb
B. spinal nerves is part of CNS
C. branches of spinal nerves occurs after reaches body structure
D. all spinal nerves are mixed nerves

8. Where do the cervical plexus lies?
A. above the neck B. below the neck
C. within the neck d. in front of neck

9. Which one is the branch of sacral plexus?
A. axillary nerve B. radial nerve
C. ulnar nerve D. none of the above

10. Through which structure, the spinal nerves leave the spinal cord?
A. intervertebral foramen B. intervertebral ligaments
c. grey matter of spinal cord D. white matter of spinal cord

ANSWERS:-

1. peripheral nervous system
2. 31
3. all of the above
4. a – 1 b – 2 c – 3 d – 4
5. motor fibers
6. anterior rami
7. all spinal nerves are mixed nerves
8. within the neck
9. none of the above
10. intervertebral foramen

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REFERENCE:

1. Ross and Wilson-Anatomy and physiology in health and illness; 12th edition; page no.-: 165-169
2. Gerard J. Tortora -Principles of anatomy and physiology; edition twelfth ; page no.-: 468-475.