Motor Tracts and Upper Motor Neurons

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Questions and Answers

Which of the following structures does the corticospinal tract pass through in the midbrain?

  • Pons
  • Substantia nigra
  • Medulla oblongata
  • Crus cerebri (correct)

Which of these is a characteristic of the anterior limb of the internal capsule?

  • Located between the thalamus and the basal nuclei
  • Contains fibers from the premotor cortex
  • Houses the corticospinal tract
  • Located between the caudate nucleus and the putamen (correct)

What does the term "somatotopic distribution" refer to in relation to upper motor neurons?

  • The size and shape of neurons based on their function
  • The arrangement of neurons in the cortex based on their function
  • The direction of nerve impulses within the motor cortex
  • The specific location of neurons in the cortex corresponding to different body parts (correct)

What is the function of the corona radiata?

<p>Relays motor signals from the cortex to the spinal cord (C)</p> Signup and view all the answers

Where are upper motor neurons primarily located?

<p>The primary motor cortex (A)</p> Signup and view all the answers

What is the location of the pyramids in the central nervous system?

<p>The medulla oblongata (A)</p> Signup and view all the answers

Which of the following statements about corticospinal tracts is true?

<p>They are influenced by feedback loops involving the basal nuclei, cerebellum, thalamus, and pons (A)</p> Signup and view all the answers

What is the name of the structure that connects the thalamus and the basal nuclei in the brain?

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

What percentage of fibers decussate at the pyramidal decussation to form the lateral corticospinal tract?

<p>85% (B)</p> Signup and view all the answers

Where do the fibers of the anterior corticospinal tract synapse?

<p>Both ipsilaterally and contralaterally with LMNs (D)</p> Signup and view all the answers

Which tract is responsible for controlling gait and posture?

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

Which of the following motor tracts begins in the midbrain from the red nucleus?

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

How are the axons arranged in the lateral corticospinal tract with respect to their target areas?

<p>Lower extremity axons are more peripheral (D)</p> Signup and view all the answers

Flashcards

Motor Tracts

Tracts that descend from the cerebral cortex to the spinal cord, controlling contralateral movement.

Upper Motor Neurons (UMN)

Neurons that originate in the cortex and transmit signals to the spinal cord.

Lower Motor Neurons (LMN)

Neurons that run from the spinal cord to the muscles, executing movement commands.

Corticospinal Tracts

Motor pathways between the cerebral cortex and spinal cord involved in voluntary movement.

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Internal Capsule

A structure that contains motor fibers; maintains somatotopic distribution for body movement.

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Crus Cerebri

Part of the midbrain where corticospinal tracts condense, also known as cerebral peduncles.

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Pyramids

Structures formed by the coalescing corticospinal fibers at the pontomedullary junction.

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Somatotopic Distribution

Arrangement of neurons in a way that reflects the organization of the body regions they control.

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Pyramidal Decussation

Point where ~85% of motor fibers cross to become the lateral corticospinal tract.

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Lateral Corticospinal Tract

Motor tract that controls distal upper extremity and lower limb movements, decussates in the pyramids.

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Anterior Corticospinal Tract

Tract that controls trunk and girdle musculature, synapsing with LMNs ipsilaterally and contralaterally.

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Corticobulbar Tract

Tract that runs parallel to corticospinal tracts, responsible for head and neck muscle innervation.

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Rubrospinal Tract

Tract from the red nucleus in midbrain, controls contralateral muscles after decussation.

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Study Notes

Motor Tracts

  • Motor tracts are pathways that transmit signals from the cerebral cortex to the spinal cord controlling contralateral movement
  • These tracts are influenced by feedback loops involving basal nuclei, cerebellum, thalamus, and pons
  • Motor tracts involve two types of neurons: upper motor neurons (UMN) and lower motor neurons (LMN)
  • UMNs originate in the cortex and project to the spinal cord
  • LMNs originate in the spinal cord and project to muscles
  • Motor tracts run in either lateral or anterior white columns of the spinal cord
  • Key tracts include the lateral corticospinal tract and anterior corticospinal tract

Upper Motor Neurons

  • Originate primarily in the primary motor cortex and sometimes in the premotor area
  • Arranged in a somatotopic distribution (organized according to the body part they control)
  • Large pyramidal neurons located in layer V of the cerebral cortex

Upper Motor Neuron Axons in the Cerebrum

  • Axons leave the cortex and enter the corona radiata (radiating crown)
  • The corona radiata contains axons ascending or descending from the cortex, sometimes also known as projection fibers
  • Axons in the corona radiata form the internal capsule as they pass between the thalami and basal nuclei
  • The internal capsule has distinct sections: anterior limb, genu, and posterior limb

Internal Capsule

  • Maintains the somatotopic organization of the motor tracts (e.g. leg, trunk, arm, face representation)

Motor Tracts in the Midbrain

  • Corticospinal tracts condense in midbrain, becoming the crus cerebri
  • Situated anterior to the substantia nigra

Motor Tracts in the Pons

  • Corticospinal fibers descend from the crus cerebri and pass through the basilar portion of the pons

Motor Tracts in the Medulla Oblongata

  • Corticospinal fibers converge in the pons and form pyramids
  • Most fibers (85%) decussate (cross over) at the pyramidal decussation, becoming lateral corticospinal tract
  • Remaining 15% become the anterior corticospinal tract

Motor Tracts in the Spinal Cord

  • Anterior corticospinal tracts descend through anterior white columns
  • They synapse with lower motor neurons (LMNs), controlling trunk and girdle muscles (ipsilateral and contralateral)
  • Lateral corticospinal tracts descend through contralateral lateral white columns
  • They synapse with LMNs, controlling musculature
  • Both tracts show somatotopic organization, with distal upper extremity axons centralized relative to lower extremity neurons

Other Motor Tracts

  • Corticobulbar tract: parallels the corticospinal tract but only reaches the brainstem, controlling head & neck muscles
  • Rubrospinal tract: originates in midbrain's red nucleus, controlling contralateral muscles
  • Tectospinal tract: originates in superior colliculus, controlling head and eye movements
  • Vestibulospinal tract: originates in brainstem vestibular nuclei, positioning head and neck/maintaining balance
  • Reticulospinal tract: originates in reticular formation, controlling gait and posture

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