Anatomy of the spinal cord Prof. Mujahid Khan

Objectives Objectives of the lecture: At the end of the lecture, the students should be able to: • Describe the external features of the spinal cord regarding position, beginning, termination, segments and enlargements • Define the terms “cauda equina” and “ filum terminale” • Describe the sites of exit of spinal nerves from vertebral column • Describe in brief the blood supply of the spinal cord • List the important nuclei in the grey matter of the spinal cord and identify their location • List the important tracts in the white matter of the spinal cord and identify their location and types of fibres

Spinal Cord Function: Provides innervation of trunk and limbs via spinal nerves and peripheral branches Receives primary afferent fibres from peripheral receptors and visceral structures Sends motor axons to skeletal muscle Houses preganglionic neurons of sympathetic and parasympathetic nervous system

External Anatomy of the Spinal Cord From: 4 External Anatomy of the Spinal Cord The spinal cord is a segmental structure, giving rise to 31 paired spinal nerves Note the two enlargements CERVICAL, related to the upper limb LUMBAR, related to the lower limb

External Anatomy of the Spinal Cord A series of rootlets emerge from the dorsal and ventral aspects of each segment of the cord and coalesce to form a dorsal and ventral root respectively These roots pass through the subarachnoid space until they reach the appropriate intervertebral foramina As they pass through the foramina the dorsal root is enlarged by the dorsal root ganglion The roots fuse to form the mixed spinal nerve, which produces dorsal and ventral rami

External Anatomy of the Spinal Cord The spinal cord is continuous with the medulla oblongata at a site just below the foramen magnum it terminates in a tapered cone-shape called the conus medularis which in turn, continues as a thin connective tissue cord called the filum terminale which is anchored to the dorsum of the coccyx

External Anatomy of the Spinal Cord Spinal cord ends at the L1 vertebral level (L2 at birth) It is thus necessary to differentiate the spinal segment from the vertebral level For example, the L2 spinal segment is the segment that provides the L2 spinal nerve and innervates the L2 dermatome, BUT, it is actually found in the vertebral canal associated with the T11 vertebra

Dermatomes Dermatome is a segment of skin supplied by one spinal nerve Cutaneous areas supplied by adjacent spinal nerves overlap No sensory loss after interruption of a single spinal nerve or dorsal root Al examination

External Anatomy of the Spinal Cord: Meninges The spinal meninges are continuous with the cranial meninges via the foramen magnum include all 3 layers, the dura mater, arachnoid mater and pia mater The spinal cord is suspended in the canal by a ribbon of tissue on the lateral aspects of the cord called the denticulate ligament This ligament is formed of pia and arachnoid tissue and attaches to the dura at points along the length of the cord Note that just as with the brain, the subarachnoid space is filled with CSF

External Anatomy of the Spinal Cord

Blood supply Three longitudinal vessels run the length of the spinal cord These are: A single anterior spinal artery and Paired posterior spinal arteries

The anterior spinal artery arises from the two vertebral arteries at the level of the medulla

The posterior spinal arteries arise from either the vertebral arteries or the posterior inferior cerebellar arteries

Anastomosis with radicular arteries derived from segmental vessels, including the ascending cervical, intercostal and lumbar arteries Radicular arteries pass through the intervertebral foramina and divide into anterior and posterior branches, which run with the dorsal and ventral spinal nerve roots, respectively

The venous drainage of the cord follows a basically similar pattern to the arterial supply Anterior Posterior

Internal Features

Internal Anatomy of the Spinal Cord The cord is made up of an outer layer of white matter and an inner, ‘H’ shaped core of grey matter Dorsal median sulcus and septum A small central canal extends the length of the spinal cord. Rostrally it opens into the 4th ventricle, Caudally it is a blind ending Ventral median fissure

Internal Anatomy of the Spinal Cord Cervical The relative sizes and proportions of grey and white matter vary significantly in different regions Grey matter is greatest at cervical and lumbar enlargements White matter is greatest at cervical levels and is progressively less at more caudal levels Thoracic Lumbar Sacral

Posteriorly by median septum The spinal cord is incompletely divided into two equal parts: Posteriorly by median septum Anteriorly by median fissure WM GM

Two lateral horns are added in thoracic & upper lumbar segments The arrangement of grey matter resembles the shape of the letter H, having two dorsal and two ventral horns/columns Two lateral horns are added in thoracic & upper lumbar segments The area of grey matter lying transversely across the midline is thin grey commissure, that surrounds the central canal and White Commissure: Lies ventral to the gray commissure The white matter consists of three funiculi: dorsal, lateral & ventral Dorsal horn DF LF VF Ventral horn Grey commissure

Neuronal Architecture of Spinal Grey Matter Cells are clustered into groups, which occur in long columns In transverse section, these columns appear as layers, especially within the dorsal horn These layers are called the laminae of Rexed laminae of Rexed are numbered consecutively by Roman numerals, starting from the tip of the dorsal horn and moving ventrally into the ventral horn

Organization of Gray Matter 1. Dorsal horns: contain somatic sensory (SS) and visceral sensory (VS) cell bodies 2. Ventral horns: contain somatic motor cell bodies (SM) 3. Lateral horns: located ONLY in T1 to L2-3 and S2-4 segments, contain visceral motor cell bodies (VM)

Nerve Cell Groups in Dorsal Horn 4 main groups Substantia gelatinosa Nucleus proprius Nucleus dorsalis (Clark’s column, nucleus thoracis) Visceral afferent nucleus

Motor neurons are organized in 3 groups: Medial: present in most segments, innervates muscles of neck and trunk (including intercostal and abdominal muscles) Central: smallest, present in some cervical (phrenic C3-5, spinal accessory C1- 6) and lumbosacral (L2- S1) segments Lateral: present in cervical and lumbosacral segments, innervates muscles of the limbs

Nerve Cell Groups in Lateral Horn Small column composed of small neurons Extends from T1 to L2-3 segments: sympathetic fibers Extends from S2-4 segments: parasympathetic fibers

White Matter Consists of mixture of nerve fibers, neuroglia and blood vessels. White color is due to high proportion of myelinated nerve fibers The nerve fibers are arranged as bundles running vertically through the cord A group of nerve fibers (axons) that share a common origin, termination and function form a tract or fasciculus Tracts are often named according to their points of origin and destination, e.g. spinothalamic, corticospinal Depending on their function, the spinal tracts are divided into ascending and descending tracts

Organization of White Matter Divided into three funiculi (columns): posterior, lateral & anterior. Anterior white commissure Dorsolateral tract of Lissauer (at the tip of dorsal horn)

Most of the fibers divide into ascending and descending branches: ascending sensory fibers terminate in the gracile and cuneate nuclei in the medulla The descending fibers terminate in the dorsal horn

References Clinical Neuroanatomy for Medical Students, Richard S. Snell-6th Edition. Clinical Neuroanatomy and related neuroscience. M.J.T. FitzGerald, Jean Folan-Curran, Fourth Edition. Crossman, AR and Neary D, Neuroanatomy: An Illustrated Colour Text. Haines, DE, Neuroanatomy: An Atlas of Structures, Sections and Systems.