Intermediolateral nucleus

At a glance

English

Intermediolateral nucleus

Latin (TA2)

Nucleus intermediolateralis

System

Brain and spinal cord

Region

Whole body

Side

Single, on the midline

About the intermediolateral nucleus

The intermediolateral nucleus, often shortened to IML, is a slender column of nerve cells forming the lateral horn, the small third horn of grey matter that juts outwards between the anterior and posterior horns. It belongs to Rexed lamina VII, the intermediate zone of the grey matter. The lateral horn, and the nucleus in it, is present only from about the T1 to the L2 or L3 segment, which is why this stretch of cord is called the thoracolumbar outflow.

Its cells are the preganglionic neurons of the sympathetic nervous system. Each sends a thin myelinated axon out through the anterior root and into the spinal nerve, then into a white ramus communicans to reach the sympathetic trunk. In the trunk the fibre may end in a ganglion at the same level, run up or down to another ganglion, or pass straight through to a prevertebral ganglion in the abdomen or to the adrenal medulla. All sympathetic control of the body therefore starts in this one column of cells.

Lamina VII also holds other well-defined groups: the posterior thoracic nucleus (Clarke's column), a relay for the posterior spinocerebellar tract, lies medially near the base of the posterior horn, and in the sacral cord a separate column, the sacral parasympathetic nucleus, occupies a similar position and supplies the pelvic organs.

The nucleus is driven by descending fibres from the hypothalamus and brainstem. When those pathways are interrupted, as in a high spinal cord injury, the cells lose their control from above; the result may be a drop in blood pressure and heart rate, or, later, the surges of autonomic dysreflexia. A lesion of the upper thoracic outflow can also produce Horner syndrome on one side.

Function and supply

Function

Contains the preganglionic sympathetic neurons whose axons leave in the anterior roots and white rami to reach the sympathetic ganglia and adrenal medulla, controlling heart rate, blood vessels, sweating, the pupil and the gut.

Blood supply

Supplied by central branches of the anterior spinal artery, reinforced in the thoracic cord by segmental medullary arteries.