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Concept
Nerve root
Summary
A nerve root (radix nervi) is the initial segment of a nerve leaving the central nervous system. Nerve roots can be classified as:
Cranial nerve roots: the initial or proximal segment of one of the twelve pairs of cranial nerves leaving the central nervous system from the brain stem or the highest levels of the spinal cord.
Spinal nerve roots: the initial or proximal segment of one of the 31 pairs of spinal nerves leaving the central nervous system from the spinal cord. Each spinal nerve is formed by the union of a sensory dorsal root and a motor ventral root, meaning that there are 62 dorsal/ventral root pairs, and therefore 124 nerve roots in total, each of which stems from a bundle of nerve rootlets (or root filaments).
Cranial nerves originate directly from the brain's surface: two from the cerebrum and the ten others from the brain stem. Cranial roots differ from spinal roots: some of these roots do not separate into individual sensory (dorsal) and motor (ventral) roots, but can emerge from one fusion root instead; of the eleven cranial nerves, four express this concept of fusion. The remaining eight nerve roots only express one of the two types of connections. Five of these are exclusive motor roots, and the remaining three are all sensory.
Spinal nerve roots are much more uniform than cranial nerves, one emerging from each level of the spinal column. These roots look extremely similar to one another, and form separate sensory and motor root connections to the central nervous system. Sensory nerves all enter the column as dorsal nerve roots, while motor nerves enter as ventral roots. They are expressed uniformly on both sides of each vertebra along the spinal column.
The specific vertebrae classify spinal nerve roots they originate from. These are separated into three sections: cervical, thoracic, lumbar, and sacral. The cervical is separated into eight vertebrae named C1-C8. The thoracic segment consists of T1-T12; the lumbar is L1-L5; and sacral S1-S5.
Damage to nerve roots can cause paresis and paralysis of the muscle innervated by the affected spinal nerve.
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