Lateral intermediate substance

At a glance

English

Lateral intermediate substance

Latin (TA2)

Substantia intermedia lateralis

System

Nerve

Region

Whole body

Side

Single, on the midline

About the lateral intermediate substance

The lateral intermediate substance is the part of the spinal grey matter that forms the lateral horn, also called the lateral grey column or intermediolateral column. Viewed in cross-section, the grey matter has the outline of a butterfly with anterior and posterior horns; from T1 to L2 a small triangular point juts sideways between them, on the back and outer edge of the anterior horn. This point is the lateral horn. It is clearest from the first thoracic to the second lumbar segment.

Its cells, the intermediolateral nucleus, are medium-sized and spindle- or star-shaped. They are the preganglionic neurons of the sympathetic system, and every sympathetic pathway in the body begins with one of them. Their thinly myelinated axons leave through the anterior roots and pass in the white rami communicantes to the sympathetic trunk. There they either synapse in a trunk ganglion, whose postganglionic fibres return to the spinal nerves through grey rami, or run on without stopping in the splanchnic nerves to prevertebral ganglia around the aorta and to the adrenal medulla. The preganglionic neurons release acetylcholine, and most postganglionic cells release noradrenaline. A similar intermediate cell group in the second to fourth sacral segments gives rise to the pelvic parasympathetic outflow.

The column is organised by level. Upper and middle thoracic segments serve the head, neck, upper limb and thoracic organs; segments from the lower chest down to L2 serve the abdomen, pelvis and lower limb. The neurons are driven by descending fibres from the hypothalamus and brainstem and by visceral sensory input. Their output raises the heart rate, widens the airways and pupils, narrows most skin and gut vessels, slows digestion, raises the blood glucose, makes hairs stand and starts sweating.

Interruption of the outflow to the head, anywhere from the hypothalamus through the upper thoracic lateral horn to the cervical sympathetic chain, produces Horner syndrome: a small pupil, a slightly drooping upper lid and reduced sweating of the face. Loss of these neurons also contributes to autonomic failure in degenerative disorders such as multiple system atrophy.

Course and supply

Supplies

Holds the preganglionic sympathetic neurons of the body, controlling heart rate, blood vessel tone, sweating, pupil size and gut activity.

Blood supply

Supplied mainly by the sulcal (central) branches of the anterior spinal artery.