Nucleus ambiguus

At a glance

English

Nucleus ambiguus

Latin (TA2)

Nucleus ambiguus

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the nucleus ambiguus

The nucleus ambiguus is a slender, broken column of large motor nerve cells in the medulla oblongata. It lies deep within the reticular formation of the lateral medulla, behind the inferior olivary nucleus, and extends through the upper (rostral) medulla. Its name, meaning the doubtful nucleus, reflects how hard its scattered cell groups are to outline in ordinary sections. It was named by the anatomist Jacob Lockhart Clarke.

Its main output is branchial motor. Axons leave it and run backwards and laterally to join the roots of the glossopharyngeal and vagus nerves, together with the fibres older books assign to a cranial accessory root. Through the glossopharyngeal nerve it supplies stylopharyngeus. Through the vagus, mainly via its pharyngeal and laryngeal branches, it reaches the palatal muscles such as levator veli palatini and musculus uvulae, the constrictors of the pharynx, all the intrinsic muscles of the larynx, and the striated muscle of the upper oesophagus. Its cells receive upper motor neuron input through corticobulbar fibres from the motor cortex of both hemispheres, and it works with the solitary nucleus in the reflexes of swallowing, coughing and gagging.

A separate outer group of cells, the external formation, holds preganglionic parasympathetic neurons that use acetylcholine. Their fibres travel in the vagus to the cardiac ganglia and slow the heart, providing a fast brake on heart rate when blood pressure rises, working against the sympathetic outflow. Fibres from this region also narrow the bronchi, a reflex that may contribute to airway narrowing in asthma and chronic obstructive lung disease, where inhaled anticholinergic drugs are used.

Damage to one nucleus, for example in the lateral medullary syndrome, weakens the palate, pharynx and larynx on the same side. The result is a hoarse voice, nasal speech and difficulty swallowing, and the uvula is pulled away towards the healthy side when the patient says ah.

Function and supply

Function

Motor supply, through the vagus, of the palate, pharynx, larynx and upper oesophagus, and of stylopharyngeus through the glossopharyngeal nerve; parasympathetic slowing of the heart and narrowing of the bronchi.

Blood supply

Branches of the vertebral artery and the posterior inferior cerebellar artery supplying the lateral medulla.