The spinal cord is the elongated, almost cylindrical part of the central nervous system, which is suspended in the vertebral canal surrounded by the meninges and cerebrospinal fluid.the spinal cord is continues above with the medulla obligate and extends from the upper border of the atlas the lower border of the 1st lumber vertebra. It is approximately 45 cm long in adult males, and is about the thickness of the little finger. A specimen of cerebrospinal fluid can be taken using a procedure called lumber puncture.
The spinal cord is the nervous tissue link between the brain and the rest of the body. Nerves conveying impulses from the brain to the various organ and tissue descend through the spinal cord. At the appropriate level they leave the cord and pass to the structure they supply. Similarly, sensory nerves from organs and tissue enter and pass upwards in the spinal cord to the brain.
Some activities of spinal cord are independent of the brain and are controlled at the level of the spinal cord by spinal reflexes. There are extensive neurones connection between sensory and motor neurones at the same or different levels in the cord.
The spinal cord is incompletely divided into two equal parts, anteriorly by a short, shallow median fissure and posteriorly by a deep narrow septum, the posterior median septum.
A cross section of the spinal cord shows that it is composed of grey matter in the centre surrounded by white matter supported by neuroglia.
Spinal Cord Grey and White Matter
Grey matter
The arrangement of grey matter in the spinal cord resembles the shape of the latter H , having two posterior, two anterior and two lateral columns . The area of grey matter lying transversely is the transverse commissure and it is pierced by the central canal, an extension from the fourth ventricle, containing cerebrospinal fluid. The nerve cell bodies may belonged to :
1- sensory neurones
2- lower motor neurones
3- connector neurones
Posterior column of grey matter
These are composed of cell bodies that are stimulated by sensory impulses from the periphery of the body. The nerve fibres of theses cells contribute to the white matter of the cord and transmit the sensory impulses upwards to the brain.
Anterior column of grey matter
These are composed of the cell bodies of the lower motor neurones that are stimulated by the upper motor neurones or the connector neurones linking the anterior and posterior column to form reflex arcs.
White matter
The white matter of the spinal cord is arranged in three column or tracts; anterior , posterior and lateral. The tracts are formed by sensory nerve fibres ascending to the brain, motor nerve fibres descending from the brain and fibres of connectors neurones.
Sensory nerve tract in the spinal cord
Neurones that transmit impulses towards the brain are called sensory. There are two main sources of sensation transmitted to the brain via the spinal cord.
1- the skin : sensory receptors in the skin are stimulated by pain, touch , heat and cold.
2- the tendon, muscles and joints: sensory receptors are specialised nerve endings in these structures, called proprioceptors and they are stimulated by stretch.
Motor nerve tracts in the spinal cord
Neurones that transmit nerve impulses away from the brain are motor neurones. Stimulation of the motor neurones results in :
1- contraction of skeleton muscle
2- contraction of smooth muscle, cardiac muscle and the secretion by glands controlled by nerves of the autonomic nervous system.
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