Questions and Answers
What are descending tracts?
Pathways by which motor signals are sent from the brain to lower motor neurons.
Which of the following are groups that the motor tracts can be divided into?
What is the role of pyramidal tracts?
Responsible for the voluntary control of the musculature of the body and face.
What do extrapyramidal tracts control?
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What are the two types of pyramidal tracts?
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What is the primary function of corticospinal tracts?
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Identify the two pathways of corticospinal tracts.
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Where do the fibers of the lateral corticospinal tract decussate?
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What is unique about the anterior corticospinal tract?
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What is the role of corticobulbar tracts?
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Upper motor neurons for the facial nerve innervate the muscles bilaterally.
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What happens during an upper motor neuron lesion affecting the corticospinal tracts?
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What is the effect of a unilateral lesion on the corticobulbar tract?
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What is the function of interneurons?
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What do Renshaw interneurons do?
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Study Notes
Descending Tracts Overview
- Descending tracts are pathways that convey motor signals from the brain to lower motor neurons, which directly innervate muscles to facilitate movement.
- The system consists entirely of upper motor neurons, whose cell bodies are located in the cerebral cortex or brain stem, with axons remaining within the central nervous system (CNS).
- There are no synapses along the descending pathways until they reach lower motor neurons.
Functional Groups of Motor Tracts
- Motor tracts are categorized into two main groups:
- Pyramidal tracts
- Extrapyramidal tracts
Pyramidal Tracts
- Originate in the cerebral cortex and convey motor fibers to the spinal cord and brain stem.
- Control voluntary movements of body musculature and facial muscles.
Extrapyramidal Tracts
- Originate in the brain stem and also convey motor fibers to the spinal cord.
- Responsible for involuntary and automatic controls, such as muscle tone, balance, posture, and locomotion.
Types of Pyramidal Tracts
- Comprised of two primary types:
- Corticospinal tracts
- Corticobulbar tracts
Corticospinal Tracts
- Control the flexor motor system and fine motor movements.
- Originates from various regions of the cerebral cortex, including the primary motor cortex, premotor cortex, and supplementary cortex.
- Receives inputs from the somatosensory area to help regulate ascending tract activity.
- Descends through the internal capsule, crus cerebri of the midbrain, pons, and into the medulla, where the pathway divides.
Pathways of Corticospinal Tracts
- Two key pathways:
- Lateral corticospinal tract
- Anterior corticospinal tract
Lateral Corticospinal Tract
- Fibers decussate into lateral and anterior tracts, descending into the spinal cord.
- Terminates in the ventral horn of the spinal cord, where lower motor neurons supply body muscles.
Anterior Corticospinal Tract
- Remains ipsilateral, descending into the spinal cord before decussating.
- Terminates in the ventral horn at cervical and upper thoracic levels.
Corticobulbar Tracts
- Emerge from the primary motor cortex, passing through the internal capsule to the brainstem.
- Terminate on motor nuclei of cranial nerves, synapsing with lower motor neurons that innervate facial and neck muscles.
- Many fibers innervate motor neurons bilaterally, but notable exceptions exist, such as with the facial and hypoglossal nerves, which have contralateral innervation.
Clinical Relevance: Upper Motor Neuron Lesions
- Lesions in the corticospinal tracts are particularly vulnerable at the internal capsule, a common stroke site.
- Unilateral lesions in corticospinal tracts lead to contralateral symptoms.
Clinical Relevance: Corticobulbar Lesions
- Unilateral lesions in the corticobulbar tract often result in mild muscle weakness due to bilateral input.
- Exceptions include lesions affecting:
- Hypoglossal nerve (CN XII) causing spastic paralysis and tongue deviation to the contralateral side.
- Facial nerve (CN VII) affecting spastic paralysis in the lower quadrant of the face on the contralateral side.
Interneurons
- Interneurons connect with other neurons to indirectly influence sensory or motor activities.
Renshaw Interneurons
- Glycinergic interneurons that are activated by collateral processes of excited alpha motor neurons.
- Function as an "off switch," inhibiting the originating alpha motor neuron to prevent excessive firing, a phenomenon known as negative feedback or recurrent inhibition.
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Description
Explore the essential concepts of descending tracts with this comprehensive set of flashcards. Each card features key definitions, functions, and important details about how motor signals travel from the brain to muscles. Perfect for students of neuroscience and anatomy!
