Nucleus of accessory nerve

At a glance

English

Nucleus of accessory nerve

Latin (TA2)

Nucleus nervi accessorii

System

Nerve

Region

Head

Side

Paired, left and right

About the nucleus of accessory nerve

The nucleus of the accessory nerve, also called the spinal accessory nucleus, is a long, thin column of motor neurons in the upper cervical spinal cord, running from about the first to the fifth cervical segment and sometimes a little lower. It sits in the posterolateral part of the anterior (ventral) horn of grey matter, slightly apart from the main motor cell groups for the neck muscles. At its upper end it lies close to the lower limit of the nucleus ambiguus in the medulla.

Its axons take an unusual route. Rather than leaving through the anterior roots with other motor fibres, they run sideways through the lateral funiculus and leave the side of the cord as a row of rootlets between the anterior and posterior roots. The rootlets join into a trunk that climbs alongside the cord, enters the skull through the foramen magnum and leaves again through the jugular foramen with the vagus and glossopharyngeal nerves, before descending in the neck to supply sternocleidomastoid and trapezius.

Classical accounts described a second, cranial part of the accessory nerve arising from the nucleus ambiguus and joining the vagus. Recent work suggests that these fibres belong to the vagus from the start, and many authors now regard the accessory nerve as purely spinal, although the older two-part description is still in common use. The classification of the nucleus is also debated: its target muscles behave partly like neck muscles and partly like branchial arch muscles, so it is sometimes grouped with the special visceral efferent column.

The neurons receive descending input from the motor cortex through corticospinal and corticobulbar fibres. Input to the part for sternocleidomastoid comes mostly from the same hemisphere, whereas trapezius is driven mostly by the opposite one, so a stroke may weaken head turning and shoulder shrugging in a mixed pattern. Disease of the nucleus itself, as in motor neuron disease or a cavity in the upper cord, wastes both muscles, leaving the shoulder drooping and turning of the head against resistance weak.

Course and supply

Supplies

Holds the lower motor neurons for sternocleidomastoid and trapezius, whose axons form the spinal accessory nerve.

Blood supply

The anterior spinal artery, with contributions from radicular branches of the vertebral arteries in the upper cervical cord.