Anterior horn of spinal cord

The anterior horn of spinal cord is part of the spinal gray matter which is situated ventral to the central canal. It is motor in function and is the place where motor information exits from the central nervous system through efferent neurons to reach out to the effector organs, including muscles and glands.

The anterior horn of spinal cord extends throughout the length of the cord. It comprises of distinct laminae, based on the cytoarchitectural details of cells in this region. These include lamina VIII and lamina IX.

Three distinct groups of nuclei can be found within the anterior horn of spinal cord, including the medial, central and lateral groups. The medial group of cells innervates structures closer to midline, whereas the lateral group innervates structures further away from midline, such as the limb musculature. The central group lies somewhere in the middle.

The anterior horn of spinal cord has distinct features in the cervical, thoracolumbar and sacral regions of the spinal cord. For instance, the thoracic part of spinal cord–unlike the cervical and lumbosacral spinal regions–has smaller anterior horns. It does not require additional gray matter (or the lateral group nuclei) to innervate the upper and lower limb musculature. There is no prominent central group of cells either. However, the medial group of cells are evident, which innervate structures closer to midline including the trunk musculature.

The cervical and lumbosacral parts of spinal cord have larger anterior horns as they have more gray matter due to additional neurons required for innervating upper and lower limb musculature, via the brachial and lumbosacral plexuses, respectively. This explains the presence of prominent lateral group nuclei in the cervical and lumbosacral anterior horns. In addition, the cervical anterior horn also contains a prominent medial group of cells which innervate the neck muscles. Moreover, a prominent central group lies in the center, which comprises of phrenic nucleus (to innervate diaphragm) and spinal accessory nucleus (to innervate trapezius and sternocleidomastoid). The latter gives off rootlets which combine to form the spinal accessory nerve, which ascends upwards through the foramen magnum to combine with the cranial accessory nerve.

An additional prominent central group of cells in the anterior horn of spinal cord includes the lumbosacral nucleus, which lies in the second lumbar to first sacral (L2 to S1) spinal segments. Its axonal distribution is not clearly known though. The sacral anterior horns also contain prominent medial gray matter nuclei for innervating the perineal muscles.