Section VIII. Motor Pathways in the Human Ner...

Section VIII. MOTOR PATHWAYS IN THE HUMAN NERVOUS SYSTEM

12-28. INTRODUCTION

The CNS receives information through the sensory pathways and collates this

information against information stored in memory. This results in a decision. If the

decision is to do something, then the CNS sends out commands through the motor

pathways to the effector organs (muscles, glands, etc.).

a. The motor pathways descend in the neuraxis and transmit the commands to

the motor neurons. The processes of motor neurons leave the CNS by way of the

peripheral nerves. The somatic motor neurons activate striated muscle fibers. The

visceral motor neurons activate smooth muscle tissue, cardiac muscle tissue, and

glands.

b. We usually consider two general motor pathways--the pyramidal motor

pathways and the extrapyramidal motor pathways.

12-29. PYRAMIDAL MOTOR PATHWAYS

A pyramidal motor pathway is primarily concerned with volitional (voluntary)

control of the body parts, particularly with the fine movements of the hands. Since such

a pathway is concerned with volitional actions, it is suitable for neurological screening

and testing.

a. Cerebral Motor Cortex. The pyramidal motor pathway begins in the

precentral gyrus of the cerebral hemisphere. As we have already seen with the sensory

pathways, the neurons making up the cerebral cortex of the precentral and postcentral

gyri are arranged in a pattern (motor homunculus) corresponding to the various parts of

the body to which they are connected.

b. Motor Neurons. From the precentral gyrus, the axons of these upper motor

neurons (UMN) pass into the neuraxis of the CNS and descend. At the level of the

appropriate segmental nerve, the UMN synapses either directly or indirectly with a lower

motor neuron of the segmental nerve. Direct synapses (monosynaptic) provide the

most rapid reactions. Such direct synapses are used in particular for the fine

movements of the hands.

c. Corticospinal Pathways. The medulla is the lowest part of the brainstem.

On the underside of the medulla, the axons of the UMNs form a pair of structures known

as the pyramids. Immediately below the pyramids, at the beginning of the spinal cord,

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