The spinal cord and brain make up the central nervous system while the spinal nerves (branching to the spinal cord) and cranial nerves (branching to the brain) make up the peripheral nervous system. There are 31 sets of nerves that extend out of the spinal cord and are connected to it by the nerve root. Each nerve branches out about a half inch from the spinal cord before dividing into smaller branches. The dorsal rami are on the posterior side of the branch while the larger ventral rami are on the anterior side.
- Cervical plexus, C1 – C4, innervates the diaphragm, shoulder, and neck
- Brachial plexus, C5 – T1, innervates the upper limbs
- Lumbar plexus, T12/L1 – L4, innervates the thigh
- Sacral plexus, L4 – S4, innervates the leg and foot.
What are Myotomes and Dermatomes?
A group of muscles that are innervated by the motor fibers that stem from a specific nerve root is called a myotome. An area of the skin that is innervated by the sensory fibers that stem from a specific nerve root is called a dermatome. These patterns of myotome and dermatome are almost always identical from person to person. There are occasionally variances, but that is rare.This consistency allows doctors to treat nerve pain in patients. If a certain area is hurting, they know that it is attributed to a certain myotome or dermatome, whichever the case may be, and its corresponding nerve root. Problems with nerve damage are often the result of stretching the nerve or compressing it. When the nerves are injured in specific areas like the lumbosacral or brachial plexus, it presents as sensory and motor deficits in the limbs that correspond to them. Myotomes and dermatomes are used to assess the extent of the damage.
How are Myotomes and Dermatomes used to Assess Nerve Damage?
- Hypoesthesia (decreased sensation).
- Hyperesthesia (excessive sensation).
- Anesthesia (loss of sensation).
- Paresthesia (numbness, tingling, burning sensation).
- 5 – Normal – Complete range of motion against gravity with full resistance
- 4 – Good – Complete range of motion against gravity with some resistance
- 3 – Fair – Complete range of motion against gravity with no resistance, active ROM
- 2 – Poor – Complete range of motion with some assistance and gravity eliminated
- 1 – Trace – Evidence of slight muscular contraction, no joint motion evident
- 0 – Zero – No evidence of muscle contraction