Functions of the Spinal Cord

Functions of the Spinal CordThe spinal cord subserves two functions:

(1) it is a centre for reflex actions;
(2) it is a great conducting path to and from the higher centres.

In discussing reflex actions it was pointed out that the reflex pathways in the higher animals are all of a complex type. Some of the main modification of the arcs, as seen in the spinal cord, may now be indicated. For convenience they may be divided into:

(a) those of the afferent portion of the arc;
(b) those of the efferent portion of the arc.

(a) The entering axon of a sensory neuron sends a long branch downwards in the cord. Collaterals from this arborise round cells at the apex of, and in the substance of, the posterior born. The axons of these in turn arborise round one or more motor cells of the anterior horn of the same or opposite side of the cord.

The outstanding modification introduced thus is the interposition of an intermediate neuron between the motor and sensory neurons. Other modifications are (1) the linking up of the sensory neuron with several of these intermediates and (2) the communication of each intermediate with several motor neurons (Fig. 41).

These modifications are still more strikingly evident when the main stems of the entering sensory axons are traced. These run up in the columns of Goll and Burdach, towards the medulla where they arborise round cells in this part of the brain. From these in turn fibres arise, cross to the opposite side of the brain and pass on to the great ganglia at the base of the brain.

A third set of fibres runs thence up to the surface of the cerebral hemispheres. All these fibres give off side branches, so that the communications they form are multitudinous.

(b) Some of the twigs of the sensory collaterals pass directly to arborise round the motor cells of the anterior horn (Fig. 41), and this represents thE simplest type of reflex pathway found in the higher animals. In most, how ever, the intermediate neuron is interposed.