The Spinal Nerves
Each spinal nerve has a dorsal (posterior) root that passes dorsally toward the back of the spinal cord and a ventral (anterior) root that passes ventrally toward the front of the spinal cord.
Each spinal nerve is composed of numerous nerve fibers (axons) bound together by connective tissues. Each nerve fiber is surrounded by a basal lamina and connective tissue called endoneurium. The nerve fibers are in bundles and are wrapped in a sheath called the perineurium. The perineurium is composed of up to 20 layers of squamous epithelial-like cells. Several nerve bundles (fascicles) are then bundled together and wrapped in a dense irregular fibrous connective tissue called epineurium.
Ganglia
A ganglion is a cluster of cell bodies (somas) outside the CNS. It is enveloped in an epineurium. Among the somas are bundles of nerve fibers leading into and out of the ganglion
Spinal Nerves
There are 31 pairs of spinal nerves:
8 cervical (C1 - C8)
12 thoracic (T1 - T12)
5 lumbar (L1 - L5)
5 sacral (S1 - S5)
1 coccygeal (Co1)
Cutaneous Innervation and Dermatomes
Each spinal nerve except C1 receives sensory input from a specific area of skin called a dermatome as demonstrated below. Spinal nerve damage is assessed by testing the dermatomes with pinpricks and noting areas in which the patient has no sensation.
Plexuses
There are four major spinal nerve plexuses: the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus.
Cervical Plexus
The cervical plexus is formed by the ventral rami (branches) of C1–C4. Most of these rami are branches of the cutaneous nerves of the neck, ear, back of head and shoulders.
It also contains the major motor and sensory nerves of the diaphragm.
Brachial Plexus
The brachial plexus is formed by C6 - C8 and T1, although C4 and T2 may also contribute. This plexus gives rise to nerves that innervate the upper limbs.
Lumbar Plexus
The lumbar plexus arises from L1 - L4 and innervates the thigh, the abdominal wall and psoas muscles. It also forms the femoral nerve and obturator nerve.
Sacral Plexus
The sacral plexus arises from L4 - S4 and serves the buttock, lower limb, pelvis structures and the perineum. Its major nerve is the sciatic nerve, the longest and thickest nerve of the body.
Reflexes
- require stimulation
- are quick
- are involuntary
- are stereotyped
Visceral Reflexes
- responses of glands, cardiac muscle, and smooth muscle
- controlled by the autonomic nervous system
Somatic Reflexes
- responses of skeletal muscle
- controlled by the somatic nervous system
Reflex Arc
Identify on the spinal cord the various spinal nerve plexus
Test Yourself
