Motor fibres

Motor fibres are present in all cranial nerves except I, II and VIII.

Classification of motor components in cranial nerves

In spinal nerves, it is useful to distinguish between somatic and visceral motor fibres. This is based on the embryological origin of the muscle innervated.

Somatic motor (voluntary) fibres innervate muscles which develop from somites: striated muscle. Cell bodies are the ventral horn cells of the spinal cord grey matter. These muscles are under voluntary control.

Visceral motor (autonomic, involuntary) fibres innervate muscles which develop in association with the gut tube and its derivatives (e.g. bronchial tree), in glands, hair follicles and the heart. Except for cardiac muscle, it is smooth or non-striated. It is involuntary.

Thus, in the trunk and limbs voluntary may be loosely equated with striated and somatic, and involuntary with smooth and visceral.

Additional component in cranial nerves: for branchial arches

In the head and neck there is an additional group of muscles which are striated and are under voluntary control, but are classed as visceral because they develop in association with the cranial end of the gut tube. These are derivatives of the branchial or pharyngeal arches. Branchial arch muscles are concerned only with the cephalic end of the gut tube and have no equivalents below the neck; they are innervated by branchiomotor fibres, found only in cranial nerves, which originate from branchiomotor nuclei in the brain stem.