Clinical Neuroscience Week 5 – Descending Motor Pathways (Hard)
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Questions and Answers

A patient presents with hyperreflexia and hypertonicity in their left lower extremity. Which of the following is the most likely location of the lesion?

  • Left internal capsule
  • Right middle cerebral artery
  • Left posterior limb of the internal capsule
  • Right anterior cerebral artery (correct)

    • A patient with a complete spinal cord injury at the T10 level would likely exhibit which of the following?

  • Unilateral loss of sensation in the lower extremities
  • Bilateral weakness and hyperreflexia in the lower extremities (correct)
  • Unilateral weakness and hypertonicity in the upper extremities
  • Bilateral loss of sensation in the upper extremities

    • What is the mechanism by which the corticospinal tract influences a muscle?

  • Directly innervating muscle fibers
  • Stimulating α and γ motor neurons (correct)
  • Inhibiting interneurons in the spinal cord
  • All of the above

    • Which of the following is a characteristic of a central lesion that distinguishes it from a peripheral lesion?

    <p>All of the above (D)</p> Signup and view all the answers

    A patient with a stroke affecting the right middle cerebral artery (MCA) would exhibit which of the following motor deficits?

    <p>Right facial weakness and left arm weakness (C)</p> Signup and view all the answers

    Which of the following statements accurately describes the effect of aging on skeletal muscle?

    <p>Decreased motor neurons (C)</p> Signup and view all the answers

    What is the primary cause of central fatigue?

    <p>Increased neuronal activity in the central nervous system (C)</p> Signup and view all the answers

    A patient with a lesion in the left corticospinal tract at the level of the medulla would exhibit which of the following symptoms?

    <p>Right-sided weakness and hyperreflexia (A)</p> Signup and view all the answers

    Which of the following structures is NOT a part of the corticospinal tract?

    <p>Thalamus (A)</p> Signup and view all the answers

    Which of the following accurately describes the location where the corticospinal tract decussates?

    <p>At the level of the medulla oblongata (D)</p> Signup and view all the answers

    Why is a lesion in the basal ganglia NOT considered a central lesion in the context of muscle physiology?

    <p>Because it does not directly affect the corticospinal tract (B)</p> Signup and view all the answers

    What is the primary role of the corticospinal tract in maintaining muscle health and function?

    <p>Directing voluntary movement and maintaining appropriate muscle activation (C)</p> Signup and view all the answers

    Which statement best describes the difference between central and peripheral fatigue?

    <p>Central fatigue is caused by depletion of neurotransmitters, while peripheral fatigue is caused by muscle exhaustion (C)</p> Signup and view all the answers

    A patient with a complete spinal cord injury at the C5 level will likely have which of the following symptoms?

    <p>Complete paralysis of the lower limbs and partial weakness in the upper limbs (C)</p> Signup and view all the answers

    Which of the following myotomes are primarily involved in shoulder abduction?

    <p>C5-C6 (C)</p> Signup and view all the answers

    Which of the following is NOT a function of the extrapyramidal tract?

    <p>Initiating voluntary movement (C)</p> Signup and view all the answers

    What is the primary function of the corticospinal tract?

    <p>Control simple voluntary movements (B)</p> Signup and view all the answers

    Which part of the corticospinal tract does the majority of fibers decussate?

    <p>In the brainstem at the medulla (B)</p> Signup and view all the answers

    What distinguishes the anterior corticospinal tract from the lateral corticospinal tract?

    <p>It does not decussate at the medulla (B)</p> Signup and view all the answers

    Which of the following best describes the structure through which the corticospinal tract passes as it descends?

    <p>Internal capsule (C)</p> Signup and view all the answers

    What type of muscle fibers does the corticospinal tract primarily synapse with?

    <p>α and γ motor neurons (B)</p> Signup and view all the answers

    Which cranial nerves are targeted by the corticobulbar pathway?

    <p>3, 5, 7, 9, 10, 11 (C)</p> Signup and view all the answers

    What percentage of corticospinal tract fibers typically remains ipsilateral?

    <p>10-20% (B)</p> Signup and view all the answers

    What is the significance of the internal capsule in relation to corticospinal fibers?

    <p>It acts as a gatekeeper between the cerebral cortex and subcortical structures (C)</p> Signup and view all the answers

    Which statement is true regarding the impact of a lesion along the corticospinal tract?

    <p>The contralateral side will show significant weakness, while the ipsilateral side may show subtle weakness (C)</p> Signup and view all the answers

    What is a key characteristic of the corticobulbar tracts compared to the corticospinal tracts?

    <p>They do not descend into the spinal cord. (C)</p> Signup and view all the answers

    Which of the following cranial nerves receives predominantly unilateral influence from the opposite motor cortex?

    <p>Facial nerve (VII) (A)</p> Signup and view all the answers

    What is the effect of a left-sided stroke in the motor cortex on the lower half of the face?

    <p>Right side weakness with preserved forehead wrinkling. (B)</p> Signup and view all the answers

    How do lesions in the cortical bulbar tract generally affect muscle function?

    <p>They generally do not produce significant clinical effects due to bilateral influence. (D)</p> Signup and view all the answers

    Which extrapyramidal pathway is noted for crossing the midline?

    <p>Rubrospinal (C)</p> Signup and view all the answers

    What mechanism defines spasticity following a cortical spinal tract injury?

    <p>Increased involuntary muscle resistance and inability to relax. (C)</p> Signup and view all the answers

    What type of weakness is typically experienced in case of an ACA stroke?

    <p>Major trunk and leg weakness. (B)</p> Signup and view all the answers

    Which extrapyramidal tract specifically adjusts head position with respect to gravity?

    <p>Vestibulospinal (C)</p> Signup and view all the answers

    In the case of a right-sided cortical bulbar tract lesion, which side will the tongue deviate upon protrusion?

    <p>Right side. (C)</p> Signup and view all the answers

    Which pathway is primarily responsible for locomotion and postural control?

    <p>Reticulospinal tract (C)</p> Signup and view all the answers

    What clinical feature is least likely to be observed with unilateral lesions in the cortical bulbar tract?

    <p>Bilateral weakness in facial muscles. (B)</p> Signup and view all the answers

    What effect would a unilateral lesion typically have on the cortico-bulbar tracts?

    <p>Minimal or no observable clinical effects. (A)</p> Signup and view all the answers

    Which cranial nerves are exceptions to the bilateral input from the cortical bulbar tract?

    <p>Cranial nerves VII and XII (C)</p> Signup and view all the answers

    Which aspect of movement do the extrapyramidal pathways influence?

    <p>Walking and posture. (A)</p> Signup and view all the answers

    Which extrapyramidal pathway starts in the red nucleus?

    <p>Rubrospinal (D)</p> Signup and view all the answers

    Flashcards

    Descending Motor Pathways

    Neural pathways transmitting signals from the brain to the spinal cord to control movement.

    Pyramidal Tracts

    Corticospinal and corticobulbar tracts responsible for voluntary movements.

    Corticospinal Tract

    Major motor pathway starting from the cortex, controls voluntary muscle movements.

    Lateral Corticospinal Tract

    Part of the corticospinal tract that decussates and controls limb movements.

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    Anterior Corticospinal Tract

    Part of corticospinal tract that remains ipsilateral and influences neck and trunk muscles.

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    Internal Capsule

    A structure in the brain containing both ascending and descending nerve fibers.

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    Corticobulbar Pathway

    Pathway that connects the motor cortex to cranial nerve nuclei for face and neck movements.

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    Decussation

    The crossing over of nerve fibers from one side of the brain/spinal cord to the other.

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    Upper Motor Neuron

    Neurons that originate in the motor cortex and project down to the spinal cord.

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    Motor Neurons

    Neurons that carry signals from the spinal cord to skeletal muscles.

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    Contralateral deficits

    Weakness or sensory loss on the opposite side of the body from the stroke.

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    ACA Stroke Strength

    Weakness primarily in the lower extremity more than the upper extremity on the contralateral side.

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    MCA Stroke Strength

    Weakness more pronounced in the upper extremity than the lower extremity on the contralateral side.

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    Hypertonicity

    Increased muscle tone resulting from upper motor neuron injuries like strokes.

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    Hyperreflexia

    Increased reflex responses due to upper motor neuron lesions.

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    Complete Spinal Cord Injury

    Total loss of signals below the level of injury, leading to muscle weakness and sensory loss.

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    Incomplete Spinal Cord Injury

    Some signals can still pass through the spinal injury, varying impairment level.

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    Peripheral Lesion

    Lesions affecting peripheral nerves leading to dysfunction distal to the injury site.

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    Central Lesion

    Lesions in the central nervous system affecting motor control but preserving spinal and peripheral function.

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    Skeletal Muscle Aging Effects

    Age-related changes leading to decreased type II fibers, increased fat, and overall muscle weakness.

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    Central Fatigue

    Fatigue stemming from the central nervous system's impaired ability to drive muscle function.

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    Peripheral Fatigue

    Fatigue related to muscle use and breakdown during physical activity.

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    Exaggerated Muscle Responses

    Increased muscle response due to lesions rather than normal reflexes.

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    Reinnervation of Denervated Fibers

    Process where nerve fibers regain connection to muscles after injury.

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    Corticobulbar Tract

    Pathway that carries motor signals from the cortex to the brainstem without going to the spinal cord.

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    Cranial Nerve 7 & 12

    Cranial nerves that receive unilateral input, causing exceptions in facial and tongue muscle innervation.

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    Unilateral Stroke Effects

    Strokes affect opposite side facial muscles; forehead wrinkling remains unaffected.

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    Hypoglossal Nerve

    Innervates tongue muscles, primarily influenced unilaterally by the opposite motor cortex.

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    Extrapyramidal Pathways

    Pathways that support movement and do not pass through the pyramids in the brain.

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    Reticulospinal Tract

    Starts in the reticular formation; controls locomotion and posture.

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    Tectospinal Tract

    Originates from the tectum in the midbrain; aids in head orientation to auditory stimuli.

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    Vestibulospinal Tract

    Starts from the vestibular nuclei; adjusts head position concerning gravity.

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    Rubrospinal Tract

    A spinal tract starting in the red nucleus, primarily controlling flexor movements.

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    Spasticity

    Involuntary muscle resistance with quick muscle stretch, often due to damaged corticospinal tracts.

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    Motor Homunculus

    Topographical representation of the body within the brain that maps motor functions.

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    ACA Stroke Effects

    Weakness primarily in the trunk and leg areas due to damage to the anterior cerebral artery.

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    MCA Stroke Effects

    Common stroke affecting motor control of the face, hand, and arm, with contralateral weakness.

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    Bilateral Influence in Corticobulbar

    Most cranial nerve nuclei receive bilateral input, allowing compensation for unilateral lesions.

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    Clinical Significance

    Unilateral lesions of corticobulbar tract usually do not cause significant clinical symptoms.

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    Study Notes

    Descending Motor Pathways

    • Descending pathways carry motor signals from the brain to the muscles.
    • Major descending pathways include pyramidal (corticospinal) tracts and extrapyramidal tracts.

    Corticospinal Tract (CST)

    • The CST is responsible for voluntary movements.
    • It consists of two main parts: lateral and anterior (ventral).
    • Pathways: cortex → corona radiata → internal capsule → midbrain → spinal cord → α and γ motor neurons → skeletal muscle.
    • ~90% of fibers cross midline (decussate) at the medulla forming the lateral corticospinal tract impacting lumbar and sacral regions for limbs.
    • ~10-20% of fibers remain ipsilateral forming the anterior corticospinal tract (impacting cervical and thoracic spinal regions)
    • Other brain regions contributing to the CST include supplemental motor area, premotor cortex, and parietal lobe.
    • For a stroke not impacting the CST, ipsilateral weakness can result, but most often won't be clinically significant.

    Anterior/Ventral Corticospinal Tract

    • This tract impacts primarily cervical and thoracic regions.
    • Fibers do not cross the midline in the medulla, instead continuing ipsilateral through the spinal cord.
    • Pathway: cortex → corona radiata → internal capsule → midbrain → spinomedullary junction → α and γ motor neurons → skeletal muscle.

    Corticobulbar Pathway

    • Homologous to the CST, but terminates in cranial nerve motor nuclei (CNs 5, 7, 9, 10, 11).
    • It accompanies the CST through the internal capsule and cerebral peduncles.
    • It mainly controls voluntary movements of the head and neck.

    Corticobulbar Tracts

    • These fibers originate in the motor cortex and control cranial nerve nuclei.
    • These tracts provide bilateral input except for CNs 7 (facial) and 12 (hypoglossal).
    • CN VII (facial) and XII (hypoglossal) muscles mainly receive contralateral input (unilateral)
    • Damage to a specific corticobulbar tract usually doesn't have significant clinical implications, due to compensations from the contralateral tract.

    Extrapyramidal Pathways

    • These pathways support movement and locomotion and originate in the brainstem.
    • Includes:
      • Reticulospinal: influences locomotion and posture. Originates in the reticular formation of the pons and medulla.
      • Rubrospinal: controls mainly flexor muscle movements. Originates at the red nucleus of the midbrain and mainly travels ipsilateral.
      • Vestibulospinal: adjusts posture with respect to gravity. Originates from vestibular nuclei in the pons.
      • Tectospinal: involved in head orientation in response to auditory stimuli. Originates in the superior colliculus of the midbrain.

    Spinal Cord Injury

    • Complete SCI: signals cannot pass the lesion.
    • Incomplete SCI: some signals still pass the lesion.
    • Injuries can affect strength, sensation, muscle tone, and reflexes below the site of injury bilaterally.
    • Lesions will result in diminished or absent signals to the muscles

    Clinical Application: Spasticity

    • Spasticity is involuntary muscle resistance caused, typically, by damage in the corticospinal tracts.
    • The extrapyramidal pathways continue providing excitatory input.
    • Treatments are often multi-factorial.

    Stroke

    • Middle Cerebral Artery (MCA) stroke often impacts upper extremities more than lower, and contralaterally.
    • Anterior Cerebral Artery (ACA) stroke usually results in more lower extremity weakness than upper.

    Central vs. Peripheral Lesions

    • Central Lesion: The brain's signal to the muscles is blocked (damage to the CST). Muscle weakness and/or spasticity are typical responses.
    • Peripheral Lesion: The signal can get to the spinal cord, but muscle is affected; weakness is common. Muscles distal to the lesion typically degenerate and atrophy.
    • Aging results in decreased type II muscle fibers, motor neurons, and an overall loss of muscle with increased intramuscular fat.

    Fatigue

    • Central fatigue involves the brain's decreased capacity to stimulate muscles (related to a stroke for example).
    • Peripheral fatigue involves the muscles themselves losing function.

    Summary: Tracing the CST

    • Starts at the primary motor cortex, traverses the corona radiata, internal capsule, midbrain, medulla, spinal cord and ends at the targeted muscle.

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    Description

    Explore the intricacies of descending motor pathways in this quiz. This content focuses on the corticospinal tract and its role in voluntary movement, detailing the cross-section of motor signals from the brain to muscles. Dive into their anatomical pathways and implications of impairments like stroke effects.

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