Sympathetic fibres are not conveyed from the brain or brain stem in cranial nerves, but are found in distal branches of some cranial nerves. They are not usually considered components of cranial nerves, but they appear here for the sake of completeness.

Functions of the sympathetic system in the head

These are similar to those in the rest of the body: secretomotor to sweat glands, vasomotor (especially important for cerebral vessels), muscles of the hair follicles and so on. In addition, various structures in the eye receive a sympathetic innervation, particularly dilator pupillae and part of levator palpebrae superioris muscle.

Sympathetic pathways to cranial structures

Sympathetic nerve impulses leave the central nervous system only in the thoracolumbar region of the spinal cord. This means that if their destination is the head, they leave the spinal cord in upper thoracic spinal nerves and thence pass back up to the head. The sympathetic chain is the redistribution system by which means they ascend.

Preganglionic axons: T1, neck of first rib, cervical chain, synapse in superior cervical ganglion

• Preganglionic axons arise in lateral grey horn of T1 and/or T2 segments of spinal cord, and possibly also C8.

• Ventral roots of appropriate spinal nerves, spinal nerve, anterior primary ramus, white ramus communicans.

• Sympathetic chain at T1 ganglion near neck of first rib.

• Preganglionic axons for cranial structures do not synapse immediately, but ascend in sympathetic chain, posterior to carotid sheath, on surface, or in substance of prevertebral muscles.

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