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Questions and Answers
What is the primary function of the descending tracts in the nervous system?
What is the primary function of the descending tracts in the nervous system?
Which tract is specifically associated with controlling muscles of the head?
Which tract is specifically associated with controlling muscles of the head?
Where do the descending tracts primarily originate?
Where do the descending tracts primarily originate?
What are the anterior horn cells in the spinal cord primarily associated with?
What are the anterior horn cells in the spinal cord primarily associated with?
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Which of the following best describes the cortico-spinal tract?
Which of the following best describes the cortico-spinal tract?
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What structure represents the majority of fibers in the cortico-spinal tract?
What structure represents the majority of fibers in the cortico-spinal tract?
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In which area of the cerebral cortex is the foot represented?
In which area of the cerebral cortex is the foot represented?
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What type of neurons are primarily involved in the efferent (motor) pathway?
What type of neurons are primarily involved in the efferent (motor) pathway?
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At which level do the fibers collect to form the pyramid in the cortico-spinal tract?
At which level do the fibers collect to form the pyramid in the cortico-spinal tract?
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Which part of the brain stem contains the middle third of the crus cerebri for the cortico-spinal tract?
Which part of the brain stem contains the middle third of the crus cerebri for the cortico-spinal tract?
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Study Notes
Descending Tracts
- Descending tracts are bundles of nerve fibers that relay motor information, muscle tone, and posture from the brain to the spinal cord and muscles.
- These tracts originate primarily from the cerebral cortex.
- The pyramidal tract is a major descending tract that controls voluntary movements and muscle tone, arising from the motor cortex and premotor cortex.
- Corticospinal tract is a part of the pyramidal tract and descends to the spinal cord controlling voluntary movements of the body's muscles.
- Corticonuclear tract (corticobulbar) is another part of the pyramidal tract that descends to the brainstem, controlling voluntary movement of the head and facial muscles through cranial nerves.
- Upper motor neurons reside in the cerebral cortex and project to the spinal cord (corticospinal) or brainstem (corticonuclear) through the pyramidal tract.
- Lower motor neurons reside in the anterior horn cells of the spinal cord or in the motor nuclei of cranial nerves.
- Betz cells are giant pyramidal neurons in the motor cortex that contribute significantly to the corticospinal tract.
- The body map in the motor cortex is organized upside down, with the face at the bottom and the foot at the medial surface.
- Descending fibers of the corticospinal tract converge in the corona radiata and pass through the internal capsule, where they are bundled together.
- At the level of the medulla oblongata, most corticospinal fibers cross over to the opposite side of the body (motor decussation).
- The lateral corticospinal tract controls muscles of the limbs, while the anterior corticospinal tract controls trunk muscles.
- Upper motor neuron lesions lead to loss of voluntary movements (paralysis of movement), while lower motor neuron lesions cause loss of all movements (paralysis of muscle) and loss of muscle tone.
- Extrapyramidal tracts are descending pathways that influence motor neurons but are not part of the pyramidal tract.
- These tracts contribute to automatic motor control and help regulate muscle tone and posture.
- Corticonuclear tract innervates motor nuclei of cranial nerves 3, 4, 6, and 5, 7, 9, 10, 11, 12, controlling muscles of the head and face.
- The corticonuclear tract innervates most cranial nerve nuclei on both sides of the body (bilateral innervation), except for the lower part of the facial nerve (VII) and the hypoglossal nerve (XII) that receive only contralateral innervation.
- Upper motor neuron lesions lead to spasticity, an increase in muscle tone due to interruption of inhibitory nerve fibers.
- Lower motor neuron lesions lead to hypotonia (flaccidity), a decrease in muscle tone, and disuse atrophy of muscles.
References
- Richard S. Snell, Clinical Neuroanatomy (page 137-166)
- Gray's Anatomy, 40th Edition, 2008
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Description
Explore the vital role of descending tracts in the nervous system. This quiz covers the origins and functions of these nerve fiber bundles, including the pyramidal tract, corticospinal tract, and corticonuclear tract. Test your understanding of how motor information is relayed from the brain to the muscles.
