Spinal cord

The spinal cord is an integral part of the central nervous system, resembling a cylindrical structure roughly 42-45 cm long in adults and slightly shorter in females. It extends from the foramen magnum at the base of the skull to the level of the first lumbar vertebra (L1) in adults and the third lumbar vertebra (L3) in neonates.

Location and Extent:

Enclosed within the vertebral canal, the spinal cord is protected by three layers of meninges: dura mater (outermost), arachnoid mater (middle), and pia mater (innermost). The spinal cord begins where the medulla oblongata of the brainstem ends, passing through the foramen magnum and extending down to L1 in adults.

Within the vertebral column, the spinal cord exhibits two enlargements:

• Cervical Enlargement: Located between C4 and T1 spinal segments, this enlargement accommodates the nerve cells that innervate the upper limbs.

• Lumbosacral Enlargement: Found between L1 and S2 spinal segments, this area hosts the nerves serving the lower limbs.

The spinal cord ends in the form of a tapered, conical structure known as the conus medullaris. Extending from the conus medullaris, a delicate strand of pia mater called the filum terminale stretches down to the coccyx, serving to anchor the spinal cord. Additionally, the spinal cord is stabilized by 21 pairs of denticulate ligaments, which are pia mater extensions running along its length, attaching firmly to the surrounding dura mater.

Surface Features:

The spinal cord's external surface displays notable landmarks:

• Anterior Median Fissure: A deep groove along the front of the spinal cord.

• Posterior Median Sulcus: A shallower furrow on the dorsal side of the spinal cord.

• Anterolateral and Posterolateral Sulci: These grooves mark the exit and entry points of the ventral (motor) roots and dorsal (sensory) roots, respectively.

Internal Composition:

Upon transverse sectioning, the spinal cord is seen to comprise an inner core of gray matter surrounded by white matter. The gray matter, rich in neuron cell bodies, forms an H-shaped structure with anterior (ventral), posterior (dorsal), and, in the thoracic and upper lumbar regions, lateral horns. The functions of the gray matter horns are:

• Anterior Horns: Contain motor neurons responsible for voluntary muscle control.

• Posterior Horns: House sensory neurons that receive and process incoming sensory information.

• Lateral Horns: Present only in the T1-L2 segments, these contain neurons of the autonomic nervous system.

The gray matter is connected across the midline by the gray commissure, containing the central canal.

The white matter, which comprises myelinated axons, surrounds the gray matter and is divided into three columns (or funiculi):

• Anterior Funiculus

• Lateral Funiculus

• Posterior Funiculus

These columns contain both ascending sensory and descending motor tracts that facilitate communication between the spinal cord and the brain.

Vascular Supply:

The spinal cord receives arterial blood from:

Anterior Spinal Artery: Runs along the anterior median fissure.

Posterior Spinal Arteries: A pair of arteries that run along the posterolateral sulci.

Venous drainage occurs via longitudinal veins that drain into radicular veins and subsequently into segmental veins.

Spinal Nerves:

The spinal cord gives rise to 31 pairs of spinal nerves, categorized as 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each spinal nerve is formed by the merging of two roots: a ventral (motor) and a dorsal (sensory) root. The spinal nerves exit the vertebral column through the intervertebral foramina and are responsible for sensory and motor innervation of the body.

Inferior End and Cauda Equina:

At the lower end of the spinal cord, the conus medullaris gives rise to a bundle of spinal nerves called the cauda equina, which resemble a horse's tail. These nerves descend to their respective exit points in the vertebral column.

Meninges and CSF:

The spinal cord is encased in three meninges. The dura mater is the tough outer layer, followed by the web-like arachnoid mater, and the delicate pia mater adheres closely to the spinal cord. The subarachnoid space, between the arachnoid and pia mater, contains cerebrospinal fluid (CSF) which cushions the spinal cord.

This intrinsic anatomy of the spinal cord allows it to function effectively in transmitting neural signals between the brain and the rest of the body, and in executing reflex actions independently. Understanding this anatomy is crucial for diagnosing and treating spinal cord injuries and diseases.