VAN 211: Veterinary Histology and Embryology (2+2)

SPINAL CORD

• Spinal cord is surrounded by spinal piamater. It is made up of outer white and inner gray matter. Surrounding the central canal of the spinal cord is the central mass of gray matter, the cross section of which is like ‘H’.
• The transverse bridge connecting the lateral halves of this is called the gray commissure. The central canal is lined by ependyma, enclosed by a layer of glia. The gray matter is covered all round by white matter.
• The gray matter on each side consists of a dorsal horn (column), which extends almost to the surface of the cord and a ventral horn (column), which is shorter and broader and does not reach the surface of the cord.
• The portion of gray matter connecting the dorsal and ventral horn is termed as intermediate part. The jelly-like substance called gelatinous substance of Rolando covers the summit of the dorsal horn.
• In the concavity between the ventral and the dorsal horn processes of gray matter extend into the white matter where they interlace with the longitudinally directed fibres form the reticular formation. In the thoracic and anterior lumbar region a small projection of gray matter on the upper part of ventral horn is present and is known as the lateral horn.
• White matter encloses the gray matter on all sides and is divided into two lateral halves by ventral median fissure and dorsally by a dorsal median septum, which contains a sheet of piamater.
• The outgoing ventral motor root and the entering dorsal sensory root into ventral, lateral and dorsal funiculi further divide each of these halves. A bridge of white matter - white commissure, connects the two ventral funiculi. At the entrance of the dorsal root on either side there is dorsolateral groove or sulcus.
• The gray matter of the cord consists of nerve cell and nerve fibres held together by neuroglia. The nerve cells are in different group,
• Motor cells - large multipolar nerve cells upto 150 microns in size, present throughout the ventral gray (horn) column. Their dendrites extend into the dorsal gray horn and into the ventral and lateral funiculi of white matter. Their axon traverses the white matter where it receives a myelin sheath and enters into formation of ventral root of spinal nerve.
• Column cells - make up the great majority. They are smaller than motor cells and distributed throughout gray matter but most abundant in the dorsal horn. Their axons pass out into the white matter.
• White matter is composed of predominantly myelinated and few non-myelinated nerve fibres neuroglia and blood vessels, most of the myelinated nerve fibres are directed longitudinally except in the region of the nerve roots and white commissure where they are radial or transverse. White matter contains neither nerve cells nor dendrites.
• In transverse section, spinal cord appears oval, more flattened on its ventral surface than on the dorsal. If the section has been cut through a dorsal nerve root, a bundle of dorsal root fibres can be seen entering the white matter dorsal and medial to the dorsal gray horn.

Different regions of spinal cord in transection

• Cervical region: Section is large and oval. White matter greater in amount than in any region. Ventral gray horn large and dorsal, slender.
• Thoracic region: Section smaller and circular. Both gray horns are slender Opposite the gray commissure on both sides, is a pointed projection called lateral horn. The lateral horn is present only in thoracic and upper lumbar segments of the cord.
• Lumbar region: Section is larger than in thoracic region and more of gray matter than white. Both dorsal and ventral gray horns are made of white matter.
• Sacral region: White matter is very small in amount. Section is smallest, gray matter is two large oval masses