Spinal nerve is generally defined as a mixed nerve, which carries motor, sensory and autonomic signals between spinal cord and the body. There are 31 pairs of spinal nerves, including 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
Area of skin supplied by a single segment of spinal cord is called a dermatome. Each spinal nerve is connected with the spinal cord by two roots, a ventral root which is motor, and a dorsal root which is sensory.
The dorsal root is characterized by the presence of a spinal ganglion at its distal end. In the majority of nerves the ganglion lies in the intervertebral foramen.
The ventral and dorsal nerve roots unite together within the intervertebral foramen to form the spinal nerve. The nerve emerges through the intervertebral foramen, gives off recurrent meningeal branches, and then divides immediately into a dorsal and a ventral ramus.
The dorsal ramus passes backwards and supplies the intrinsic muscles of the back, and the skin covering them.
The ventral ramus is connected with the sympathetic ganglion, and is distributed to the limb or the anterolateral body wall.
In case of a typical (thoracic) spinal nerve, the ventral ramus does not mix with neighboring rami, and gives off several muscular branches, a lateral cutaneous branch, and an anterior cutaneous branch. However, the ventral rami of other spinal nerves are plaited to form the nerve plexuses for the limbs, like the brachial plexus, lumbar plexus, etc.
Nerve Plexuses for Limbs:
All nerve plexuses are formed only by the ventral rami, and never by the dorsal rami. These supply the limbs. Against each plexus the spinal cord is enlarged, e.g. ‘cervical enlargement’ for the brachial plexus, and ‘lumbar enlargement’ for the lumbosacral plexus. Plexus formation resembles a tree.
Each nerve root of the plexus (ventral ramus) divides into a ventral and dorsal division.
The ventral division supplies the flexor compartment, and the dorsal division, the extensor compartment, of the limb.
The flexor compartment has a richer nerve supply than the extensor compartment. The flexor skin is more sensitive than the extensor skin and the flexor muscles (antigravity, bulkier muscles) are more efficient and are under a more precise control than the coarse extensor muscles.
The plexus formation is a physiological or functional adaptation, and is perhaps the result of the following special features in the limbs.
1. Overlapping of dermatomes
2. Overlapping of myotomes
3. Composite nature of muscles
4. Possible migration of muscles from the trunk to the limbs
5. Linkage of the opposite groups of muscles in the spinal cord for reciprocal innervation.