Present in spinal canal or vertebral canal of vertebral column. It is extended from foramen magnum to between I and II lumber vertebra. Spinal cord is swollen in cervical and lumber region which are called cervical and lumber enlargement.
Structure of spinal cord:
Conus medullaris : It is last tapering ends of spinal cord, its ciliated central canal is called Vth ventricle.
Cauda equine : Nearly upto birth the length of spinal cord corresponds the length of V.C. but after birth there is vertically no growth of spinal cord but vertebral column grow upto I lumber vertebra in adult. Spinal nerve come out through their respective intervertebral foramen, form horse tail hair like cluster below conus medullaris it is called cauda equine.
Filum terminales : It is extension of piamater below conus up to coccyx. In frog spinal cord also extends upto end of vertebral column.
Cisterna terminalis : It is last dilation of subarachnoid space below 1st lumbar vertebra. It is a proper site for lumber puncture or spinal tap, which is done to drain C.S.F out (5 to 10 ml). This C.S.F is used in diagnosing many diseases of CNS like meningitis, cyphalis, inter cranial pressure, menningococcal inferaction etc.
Meninges : Like brain, spinal cord is also enclosed with in three membranes. In this case duramater does not remain attached with the vertebra, instead there is a space between duramater and vertebra called epidural space. The epidural space is filled with a fluid. The distribution of duramater and piamater in spinal cord is the same as that of brain.
The cross section of spinal cord reveals the following structures:
- Central canal : In the centre of spinal cord, there is a canal called central canal. It is filled with cerebrospinal fluid.
- Dorsal fissure : In the mid dorsal line, there is a groove extending throughout its length.
- Ventral fissure : It is also a groove situated in the mid ventral line throughout the length of spinal cord.
- Dorsal septum : It is a partition extending from dorsal fissure to central canal.
- Grey matter : It lies around the central canal in the form of a butterfly.
- Dorsal horns : It is like horn of grey matter on the dorsal side.
- Ventral horns : On the ventral side of the grey matter are horn like structures the ventral horns.
- Lateral horns : These are horns on the lateral side of grey matter.
- White matter : White matter is present around grey matter. Spinal cord provides pathway for the impulses from the brain and to the brain.
First of all Marshal Hall (1833) studied the reflex action. Best and Taylor defined reflex action “simplest form of irritability associated with the nervous system is reflex actions or a reflex reaction is an immediate involuntary response to a stimulus.” The reflex actions are involuntary actions because these are not under the conscious control of the brain. The spinal cord and brain stem are responsible for most of the reflex movements. A few examples of the reflex actions are withdrawal of hand or leg if pricked by a pin, secretion of saliva as soon as one thinks of delicious food or mere its sight causes salivation, if the body part is touched with acid or hot object it is automatically, without thinking and planning is withdrawn, cycling, motor driving etc. Central nervous system is responsible for the control of reflex action.
Reflex arc is formed by the neurons forming the pathway taken by the nerve impulses in reflex action. The simplest reflexes are found in animals involving a single neuron and the following pathway —
The reflex areas in all the higher animals than coelenterates, include at least two neurons, an afferent or sensory neuron carrying impulses from a receptor towards aggregation of nervous tissue which may be a ganglion, nerve cord or central nervous system and an efferent or motor neuron carrying impulses away from the aggregation to an effector.
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