Sensory versus Motor Levels
A dermatome is a patch of skin that is innervated by a given spinal cord level. Figure 2 is taken from the ASIA classification manual, obtainable from the ASIA web site. Each dermatome has a specific point recommended for testing and shown in the figure. After injury, the dermatomes can expand or contract, depending on plasticity of the spinal cord.
C2 to C4. The C2 dermatome covers the occiput and the top part of the neck. C3 covers the lower part of the neck to the clavicle (the horizontal bone that goes to the shoulder. C4 covers the area just below the clavicle.
C5 to T1. These dermatomes are all situated in the arms. C5 covers the lateral arm at and above the elbow. C6 covers the forearm and the radial (thumb) side of the hand. C7 is the middle finger, C8 is the lateral aspects of the hand, and T1 covers the medial side of the forearm.
T2 to T12. The thoracic covers the axillary and chest region. T3 to T12 covers the chest and back to the hip girdle. The nipples are situated in the middle of T4. T10 is situated at the umbilicus. T12 ends just above the hip girdle.
L1 to L5. The cutaneous dermatome representating the hip girdle and groin area is innervated by L1 spinal cord. L2 and 3 cover the front part of the thighs. L4 and L5 cover medial and lateral aspects of the lower leg.
S1 to S5. S1 covers the heel and the middle back of the leg. S2 covers the back of the thighs. S3 cover the medial side of the buttocks and S4-5 covers the perineal region. S5 is of course the lowest dermatome and represents the skin immediately at and adjacent to the anus.
Ten muscle groups represent the motor innervation by the cervical and lumbosacral spinal cord. The ASIA system does not include the abdominal muscles (i.e. T10-11) because the thoracic levels are much easier to determine from sensory levels. It also excludes certain muscles (e.g. hamstrings) because the segmental levels that innervate them are already represented by other muscles.
Arm and hand muscles. C5 represents the elbow flexors (biceps), C6 the wrist extensors, C7 the elbow extensors (triceps), C8 the finger flexors, and T1 the little finger abductor (outward movement of the pinky finger).
Leg and foot muscles. The leg muscles represent the lumbar segments, i.e. L2 are the hip flexors (psoas), L3 the knee extensors (quadriceps), L4 the ankle dorsiflexors (anterior tibialis), L5 the long toe extensors (hallucis longus, S1 the ankle plantar flexors (gastrocnemius).
The anal sphincter is innervated by the S4-5 cord and represents the end of the spinal cord. The anal sphincter is a critical part of the spinal cord injury examination. If the person has any voluntary anal contraction, regardless of any other finding, that person is by definition a motor incomplete injury.
It is important to note that the muscle groups specified in the ASIA classifications represent a gross oversimplication of the situation. Almost every muscle received innervation from two or more segments. There is an excellent web site (Wheeless’ Textbook of Orthopedics) that gives details of the muscle innervation.
In summary, the spinal cord segments serve specific motor and sensory regions of the body. The sensory regions are called dermatomes with each segment of the spinal cord innervating a particularly area of skin. The distribution of these dermatomes is relatively straightforward except on the limbs. In the arms, the cervical dermatomes C5 to T1 are arrayed from proximal radial (C5) to distal (C6-8) and proximal medial (T1). In the legs, the L1 to L5 dermatomes cover the front of the leg from proximal to distal while the sacral dermatomes cover the back of the leg.