Human Anatomy & Physiology (9th Edition)

Published by Pearson
ISBN 10: 0321743261
ISBN 13: 978-0-32174-326-8

Chapter 12 - The Central Nervous System - Review Questions - Page 481: 33


There are two kinds of fiber pathways in the spinal cord,the somato-sensory pathways and the motor pathways. The somatosensory pathways carry impulses from spinal cord to the brain; the motor pathways transmit instructions from the brain to the spinal cord. . Further sub-classification of these spinal neural path ways divides descending motor pathways into direct and indirect descending motor pathways. The neurons of the direct pathways originate in the precentral gyri and pass through the brain stem by way of the pyraamidal corticospinal tracts. In the spinal cord these neurons synapse with ventral neurons directly or with interneurons and then ventral horn neurons. It is important to member that these neural fibers travel through the brain and spinal cord without synapsing until they reach their destined positions in the ventral horn. There, they synapse with ventral horn neurons. Indirect descending pathways are multi-synaptic pathways. These neurons originate in the superior colliculus, the vestibular nucleus , nuclei of the reticular formation ( and other nuclei). The axons travel by the tectospinal, the vetibulospinal, the reticulospinal and the rubrospianal descending tracts. The terminal synaptic connections are made on the interneurons and/or the ventral horn motor neurons. The direct pathway neurons control fast and skilled skeletal muscle activities like writing,typing and playing a piano. The indirect descending neurons are concerned with muscle tone , body posture and balance, and certain visceral muscle activities.

Work Step by Step

The direct and the indirect spinal pathways are both pathways of motor fibers that travel from brain to synapse on neurons in the ventral horn of the spinal cord. Nevertheless, there are some differences between the two pathways. 1. Both originate in upper motor neurons in the brain: The direct pathway neurons originate in the precentral gyrus. The indirect path neurons originate in superior colliculi. vestibular nucleus, the red nucleus, and the nuclei of the reticular formation. 2 . Neurons of both pathways travel by spinal descending pathways to synapse terminally on neurons in the ventral horn of the spinal cord. An important fact to remember the axons of the direct path way travel through the brain without synapsing until they reach their destined positions in the ventral horn. In contrast, the neurons of the indirect path are multi-synaptic . That means they may synapse with other neurons before reaching the ventral horn motor neurons. . Neurons of the direct system manage mainly skeletal muscle activity. Fibers are crossed (decussate) in this system, so the activation of the skeletal muscles is contra-lateral. These pathways are vital for the conscious management of fast and skilled muscle activities like knitting, keyboarding and playing the piano Indirect pathway neurons are mixed, that is some decussate (cross)and some do not. They are important in the control of muscles that control the head in the process of visual tracking. The subconscious maintenance of muscle tone, posture and balance are under indirect pathway management .The stimulating some visceral muscle also one of their functions.
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