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define motor homunculus
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.).
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.
We usually consider two general motor pathways--the pyramidal motor pathways and the extrapyramidal motor pathways.
Page 2 A Pyramidal Motor Pathway
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.
Page 3 Cerebral Motor Cortex
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.
Page 4 Motor Neurons
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.
Page 5 Corticospinal Pathways
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, the axons cross to the opposite side of the CNS (spinal cord). The axons then descend as the lateral corticospinal tract, within the lateral funiculus (Figure 12-6). Thus, the left cerebral hemisphere commands the right side of the body, and the right cerebral
hemisphere controls the left side of the body.
Page 6 Extrapyramidal Motor Pathways
The extrapyramidal motor pathways are concerned with automatic (nonvolitional) control of body parts. This particularly includes patterned, sequential movements or actions.
Thus, the major command system of the human nervous system uses these pathways. There are several extrapyramidal motor pathways.
Having multisynaptic circuits throughout the CNS, they use many intermediate relays before reaching the effector organs. The cerebellum of the brain plays a major role in extrapyramidal pathways; the cerebellum is the major center for coordinating the patterned sequential actions of the body, such as walking.
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