Spinal cord External features

Objectives Describe the gross anatomical features of the spinal cord. Describe the level of the different spinal segments comparing to the level of their respective vertebrae. Identify important gross features of spinal cord, nerve roots, and spinal ganglia. Describe the internal features of spinal cord (gray matter and white matter) in the different regions.

Structure of the Spinal Cord A cylinder of gray & white matter In the upper 2/3 of vertebral canal Extends From foramen magnum to L1 (or L2) Covered with meninges & CSF A typical adult spinal cord ranges between 42 and 45 centimeters .

The spinal cord is associated with 31 pairs of spinal nerves that connect the CNS to muscles, receptors, and glands. Each side of the spinal cord contains : 8 cervical nerves (called C1–C8), 12 thoracic nerves (T1–T12), 5 lumbar nerves (L1–L5), 5 sacral nerves (S1–S5), and 1 coccygeal nerve

Structure of the Spinal Cord The spinal cord is shorter than the vertebral canal that houses it. Extends only to L1/ L2 Has two enlargements, cervical & lumbar due to cells and fibers of limbs. Ends inferiorly in a tapering conus medullaris Anchored to the coccyx by a meningeal ( non neuronal) extension (filum terminale) Held to dura by denticulate ligament

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The Spinal Cord Provides a vital link between the brain and the rest of the body. Exhibits some functional independence from the brain. The spinal cord and its attached spinal nerves serve some important functions :-

Functions of the Spinal Cord Receives afferent fibers from sensory receptors of the trunk and limbs Controls movements of the trunk and limbs Provide autonomic innervation for most of the viscera. Responsible for many loop reflexes It conveys afferent information to higher centers and mediate their controlling influence over spinal mechanisms.

Spinal Meninges Are continuous with the cranial meninges. Structures that encircle the spinal cord, listed from outermost to innermost are: 1-vertebra 2-epidural space: Contains blood vessels, areolar connective tissue and fat. 3-dura mater 4-subdural space: a potential cavity between the dura and arachnoid mater, contains a small volume of serous fluid 5-arachnoid 6-subarachnoid space: Contains cerebrospinal fluid (CSF) and blood vessels 7-pia mater

The cervical enlargement consists of cord segments C3-T1 and provides innervation for the upper limb via the brachial plexus. The lumbar enlargement is made up of segments L1-S3 and is associated with innervation of the lower limb via the lumbar plexus (L1-L4) and the sacral plexus (L4-S2)

Surfaces Surface marked by furrows A deep ventral (anterior) median fissure A dorsal (posterior) median sulcus 2 dorsolateral & 2 ventrolateral sulci 2 dorsal intermediate sulci

Cross Section of Spinal Cord Anterior median fissure and posterior median sulcus deep clefts partially separating left and right halves Gray matter: neuron cell bodies, dendrites, unmylinated axons Divided into horns Posterior (dorsal) horn Anterior (ventral) horn Lateral horn

White matter Myelinated axons Divided into three columns (funiculi) Ventral Dorsal lateral Each of these divided into sensory or motor tracts Commissures: connections between left and right halves Gray with central canal in the center White Roots Spinal nerves arise as rootlets then combine to form dorsal and ventral roots Dorsal and ventral roots merge laterally and form the spinal nerve

Grey matter Consists of nerve cell bodies and their processes, neuroglia, and blood vessels The nerve cells are multipolar and are of three main categories: Sensory neurons (Tract cells), which receive impulses from the periphery of the body and whose axons constitute the ascending fasciculi of the white matter, are located in the dorsal horns Lower motor neurons, which transmit impulses to the skeletal muscles, are located in the ventral horns (similar neurons in the lateral horn are the preganglionic neurons of the autonomic system) Interneurons (connector neurons) : linking sensory and motor neurons, at the same or different levels, which form spinal reflex arcs.

Different cord levels vary in the relative amounts and configuration of grey and white matter. Higher levels contain greater amounts of white matter. Why? Because ascending tracts gain fibres at each successive level, whereas the opposite is true of descending tracts.

The size and shape of the dorsal and ventral horns varies according to the level. Both dorsal and ventral horns are, therefore, particularly well developed at cervical and lumbar levels in association with innervation of the upper and lower limbs.

The Gray Matter Contain categories of neurons: The smallest interneurons Motor neurons of ventral horn (large  & smaller ) Lateral horns contain autonomic neurons Tract (or relay) cells in dorsal horn (related to sensory system)

Gray Matter The grey matter of the spinal cord may be divided, on the basis of its cytoarchitecture, into ten zones, known as Rexed's laminae, which are numbered sequentially from dorsal to ventral. Some of these laminae are equated with cell groupings of particular functional types.

Corresponding nucleus Gray Matter Corresponding nucleus Lamina I II III & IV V VI VII VIII IX Posteromarginal nucleus Substantia gelatinosa Nucleus properius Zone anterior to lamina IV Zone at base of posterior horn Zona intermedia Zone in anterior horn Medial nuclear column and lateral nuclear column

The White matter 3 columns or funiculi (ventral, Dorsal, Lateral) Mainly of myelinated axons Form definite functional tracts (fasciculi) The sensory ones are the ascending tracts. The motor ones are the descending tracts.

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