Spinal pia

Spinal pia refers to the innermost layer of meninges which envelops the spinal cord. It is essentially a continuation of the cranial pia mater, which surrounds the brain, extending downwards through the foramen magnum and entering the vertebral canal. inside the canal, it closely invests the spinal cord as the spinal pia mater.

In adults, although the spinal cord ends at the lower border of the first lumbar (L1) vertebra, the spinal pia continues its descent beyond L1. It traverses through the arachnoid membrane at the second sacral vertebral level and eventually attaches to the bony coccyx. This extension of the spinal pia is called filum terminale. Its main purpose is to anchor the spinal cord vertically, preventing it from floating within the subarachnoid space. The part of the filum terminale within the subarachnoid space, from L1 to S2, is referred to as the filum terminale internum. The part outside the space, from S2 to coccyx, is known as the filum terminale externum.

In addition, there are several lateral, triangular-shaped, extensions of the spinal pia. These extensions, known as the denticulate ligaments, run along the entire length of spinal cord. These extend from the cord–between the ventral and dorsal nerve roots–to tightly adhere to arachnoid and the dura mater. Their purpose is to anchor the spinal cord horizontally and provide support.