Anterior horn of spinal cord

At a glance

English

Anterior horn of spinal cord

Latin (TA2)

Cornu anterius medullae spinalis

System

Brain and spinal cord

Region

Whole body

Side

Single, on the midline

About the anterior horn of spinal cord

In a cross-section of the spinal cord the grey matter forms a butterfly or H shape, and the anterior horn is the broad front wing on each side. Seen along the length of the cord, each horn is a continuous ridge, which is why it is also called the anterior grey column, ventral horn or motor horn. It is one of three grey columns: the posterior horn behind deals with incoming sensation, and the small lateral horn appears only in the thoracic and upper lumbar segments.

The horn is thick and roughly rounded or four-sided in outline. Its deeper part, nearest the central canal, is called the base and its forward part the head, though no clear narrowing separates the two. A rim of white matter lies between the head and the surface of the cord, and the rootlets of the anterior (ventral) roots cross this rim on their way out. The horn is largest in the cervical and lumbosacral enlargements, where the limbs need many motor cells, and smallest in the thoracic cord.

The large cells of the horn are the alpha motor neurons, whose axons leave in the anterior root and end on skeletal muscle fibres. Smaller gamma motor neurons set the sensitivity of muscle spindles. The cells are grouped in columns that follow a rough map of the body: neurons for the trunk lie medially and those for the limbs laterally, with flexors placed further back than extensors. In Rexed's scheme the motor pools form lamina IX, set within laminae VII and VIII.

Because these are the final common pathway to muscle, damage here gives a lower motor neuron picture: weakness, wasting, reduced tone, absent reflexes and twitching (fasciculation). Poliomyelitis, West Nile virus infection, the spinal muscular atrophies and amyotrophic lateral sclerosis all strike these cells. Some muscle relaxants, such as cyclobenzaprine, are thought to act at this level by damping motor neuron activity.

Function and supply

Function

Holds the alpha and gamma motor neurons whose axons leave through the anterior roots to supply the skeletal muscles of the trunk and limbs; it is the last relay in every voluntary and reflex movement.

Blood supply

Mainly from the anterior spinal artery through its central (sulcal) branches, which also supply most of the rest of the grey matter.