Somatic Motor Pathways in the Central Nervous System Impulses must be conducted from its motor areas to skeletal muscles by relays of neurons referred to as somatic motor pathways. Consist of motor neurons that conduct impulses from the central nervous system to somatic effectors such as skeletal muscles. There are two methods used to classify somatic motor pathways: one- divides them into pyramidal and extrapyramidal tracts. Two- classifies them as facilitatory and inhibitory. Pyramidal tracts are those whose fibers come together in the medulla to form the pyramids. Extrapyramidal tracts are much more complex, they consist of all motor tracts from the brain to the spinal cord anterior horn motor neurons except the corticospinal tracts. (1)
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Somatic Motor Pathways in the Central Nervous System
Impulses must be conducted from its motor areas to skeletal muscles by relays of neurons referred to as somatic motor pathways. Consist of motor neurons that conduct impulses from the central nervous system to somatic effectors such as skeletal muscles.
There are two methods used to classify somatic motor pathways: one- divides them into pyramidal and extrapyramidal tracts. Two- classifies them as facilitatory and inhibitory.
Pyramidal tracts are those whose fibers come together in the medulla to form the pyramids.
Extrapyramidal tracts are much more complex, they consist of all motor tracts from the brain to the spinal cord anterior horn motor neurons except the corticospinal tracts. (1)
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Structure of Spinal Nerves
Each spinal nerve attaches to the spinal cord by means of two short roots, ventral and dorsal.
The dorsal is easily recognized by a swelling called the dorsal root ganglion, or spinal ganglion. The roots and dorsal ganglia lie within the spinal cavity.
Each spinal nerve splits into a distinct dorsal ramus and ventral ramus. (1)
Nerve Plexuses
The ventral rami of most spinal nerves subdivide to form complex networks called plexuses. Four Major Pairs:
1. The Cervical Plexus – found deep within the neck; innervates muscles and skin of the neck, upper shoulder, and part of the head
2. The Brachial Plexus – plexus located deep in the shoulder that innervates the lower part of the shoulder and the entire arm
3. The Lumbar Plexus - spinal nerve plexus located in the lower back
4. The Sacral Plexus – plexus formed by fibers from the fourth the fourth and fifth lumbar nerves and the first four sacral nerves
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Dermatomes and Myotomes
At first glance of the distribution of spinal nerves does not appear to follow an ordered arrangement, but detailed mapping of the skin surface has revealed a close relationship between the spinal origin of each spinal nerve and the region of the body it innervates.
Dermatomes – skin surface areas supplied by a single spinal nerve
Myotomes – skeletal muscle or group of muscles that receives motor axons from a given spinal nerve.
There is some overlap among myotomes also, thus some skeletal muscle organs may be innervated by motor axons from more than one spinal nerve
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Cranial Nerves
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Afferent and efferent
Afferent division of the nervous system consists of all of the incoming sensory or afferent pathways.
Efferent division of the nervous system consists of all the outgoing motor or efferent pathways.
Phrenic nerve
Vagus nerve
The autonomic nervous system is influenced by the conscious mind, its autonomous of voluntary control.
Sympathetic and Parasympathetic
Sympathetic division prepares the body to deal with immediate threats to the internal environment. It produces the “fight-or-flight” response.
Parasympathetic division coordinates the body’s normal resting activities. Sometimes called the “rest-and-repair” division.
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