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Motor Control and Balance

Jul 10, 2025

Overview

This lecture reviews Chapter 56 from Guyton & Hall’s Medical Physiology, focusing on how the cortex and brainstem control motor function, including anatomical structures, pathways, and the role of the vestibular system.

Motor Cortex Structure and Function

  • The primary motor cortex activates motor patterns by exciting lower brain areas, sending signals to muscles.
  • Fine finger and hand movements are directly controlled by the cortex.
  • The cortex is organized topographically, with large areas devoted to hands, fingers, and speech muscles.
  • The premotor area manages complex movements and involves mirror neurons for imitating observed actions.
  • The supplementary motor area controls attitudinal, fixation, and background fine motor movements.
  • Specialized regions exist for functions like speech (Broca’s area) and voluntary eye movement.

Corticospinal and Rubrospinal Tracts

  • The corticospinal tract (pyramidal tract) carries motor signals from the cortex to the spinal cord, crucial for voluntary motion.
  • 30% of corticospinal fibers arise from the primary motor cortex, 30% from premotor/supplementary areas, and 40% from somatosensory areas.
  • Most fibers cross at the medulla (lateral tract), while some remain on the same side (ventral tract) for posture control.
  • The red nucleus and rubrospinal tract assist with discrete voluntary movements, except for fine finger and hand control.
  • Both tracts comprise the lateral motor system.

Organization and Feedback in the Motor Cortex

  • The cortex is structured in six-layered vertical columns, with pyramidal cells in layer V giving rise to corticospinal fibers.
  • Columns contain dynamic neurons for initiating force and static neurons for maintaining force.
  • Feedback from muscle spindles, Golgi tendon organs, and tactile receptors integrates sensory input for motor control.
  • Reciprocal innervation ensures that activating a muscle automatically inhibits its antagonist.

Brainstem Motor Control

  • The brainstem controls major functions: respiration, cardiovascular regulation, GI movements, posture, and eye movements.
  • The pontine reticular nuclei excite axial muscles for posture against gravity.
  • The medullary reticular nuclei provide inhibitory input to balance excitation and enable movement.
  • The vestibular nuclei facilitate excitation of postural muscles, based on input from the vestibular apparatus.
  • Loss of inhibition from higher centers causes cerebrate rigidity due to unopposed pontine excitation.

Vestibular Apparatus and Balance

  • The utricle and saccule detect head position and linear acceleration using hair cells and statoconia crystals.
  • Semicircular canals sense rotational head movements via endolymph flow displacing the cupula.
  • Hair cell deflection towards the kinocilium depolarizes cells; away causes hyperpolarization.
  • Signals adapt quickly—detecting only changes in rotation to maintain equilibrium.
  • Vestibular input coordinates with eye and body movements via the medial longitudinal fasciculus and cerebellum (flocculonodular lobe).

Key Terms & Definitions

  • Primary Motor Cortex — Brain region initiating motor patterns to lower centers.
  • Premotor Area — Cortex region for planning and imitating complex movements.
  • Supplementary Motor Area — Area involved in postural and background motor control.
  • Corticospinal Tract — Major descending tract for voluntary movement.
  • Rubrospinal Tract — Tract from the red nucleus involved in gross motor control.
  • Pontine/Medullary Reticular Nuclei — Brainstem centers managing posture and movement through excitation/inhibition.
  • Vestibular Nuclei — Brainstem region integrating balance and spatial orientation signals.
  • Utricle/Saccule — Vestibular organs detecting head position and linear motion.
  • Semicircular Canals — Detect rotational head movement.
  • Reciprocal Innervation — Neural mechanism inhibiting antagonist muscle during contraction.

Action Items / Next Steps

  • Review anatomical diagrams of the motor cortex, brainstem nuclei, and vestibular apparatus.
  • Memorize the roles of major motor pathways (corticospinal, rubrospinal, reticular, and vestibular tracts).
  • Prepare for questions on feedback mechanisms in movement and balance systems.

Certainly! Here's a highly detailed and expanded bullet-point summary based closely on the video content, with added depth and clarity:

Detailed Summary of Chapter 56: Cortical and Brainstem Control of Motor Function

1. Introduction to Motor Control by Cortex and Brainstem

  • The cerebral cortex controls motor function by activating specific motor patterns in lower brain areas.
  • These lower areas include the spinal cord, brainstem, basal ganglia, and cerebellum.
  • The cortex acts like a "puppeteer," selecting which muscle patterns to activate at any given time.
  • Fine motor control, especially of fingers and hands, is directly innervated by the cortex.

2. Anatomy of Motor Cortex

  • Primary Motor Cortex (M1):
    • Located anterior to the central sulcus.
    • Organized topographically with a "motor homunculus."
    • Large cortical representation for hands and speech muscles, reflecting their fine motor control.
    • Smaller areas for more crude or gross muscles.
    • Sends signals to initiate general motor patterns.
  • Premotor Area:
    • Located just anterior to the primary motor cortex.
    • Involved in planning and executing complex motor patterns.
    • Contains mirror neurons that help imitate observed movements.
    • Creates a "motor image" in the anterior premotor area.
    • Sends motor commands either directly to M1 or via basal ganglia and thalamus.
  • Supplementary Motor Area:
    • Involved in attitudinal movements, fixation, and background fine motor control.
    • Provides scaffolding and coordination for smooth movement execution.
  • Specialized Motor Areas:
    • Broca’s area for speech formation.
    • Eye fixation and voluntary eye movement fields.
    • Regions controlling hand skills, head rotation, and contralateral eye movements.

3. Corticospinal Tract (Pyramidal Tract)

  • Major descending motor pathway from cortex to spinal cord.
  • Fiber origins:
    • 30% from primary motor cortex.
    • 30% from premotor and supplementary motor areas.
    • 40% from somatosensory cortex.
  • Pathway:
    • Fibers descend through the posterior limb of the internal capsule.
    • Reach the medullary pyramids where ~90% cross over (decussate) to form the lateral corticospinal tract.
    • Remaining fibers form the ventral corticospinal tract, mostly ipsilateral, controlling posture.
  • Function:
    • Lateral corticospinal tract controls fine voluntary movements, especially of hands and fingers.
    • Ventral corticospinal tract modulates axial and postural muscles.

4. Red Nucleus and Rubrospinal Tract

  • Red nucleus receives input from primary motor cortex via corticorubral tract.
  • Gives rise to rubrospinal tract, which crosses to the contralateral side.
  • Terminates on spinal motor neurons alongside corticospinal fibers.
  • Assists in discrete voluntary movements but not fine finger control.
  • Together with corticospinal tract, forms the lateral motor system.

5. Motor Cortex Microanatomy and Neuronal Organization

  • Cortex organized in vertical columns with six layers.
  • Pyramidal cells in layer V give rise to corticospinal fibers.
  • Input arrives mainly through layers II and IV.
  • Layer VI sends fibers to other cortical areas.
  • Each column contains two types of pyramidal neurons:
    • Dynamic neurons: Initiate development of muscle force.
    • Static neurons: Maintain muscle force over time.
  • Greater proportion of dynamic neurons in red nucleus; more static neurons in primary motor cortex.

6. Sensory Feedback and Integration

  • Muscle contraction is modulated by feedback from:
    • Muscle spindles (detect stretch).
    • Golgi tendon organs (detect tension).
    • Tactile receptors.
  • This feedback acts as positive reinforcement to enhance contraction.
  • Reciprocal innervation ensures that when one muscle contracts, its antagonist is inhibited, preventing co-contraction.

7. Brainstem Control of Motor Function

  • Brainstem nuclei involved:
    • Pontine reticular nuclei: Excitatory to axial muscles, supporting posture against gravity.
    • Medullary reticular nuclei: Inhibitory, balancing pontine excitation to allow movement.
    • Vestibular nuclei: Excitatory, facilitate pontine reticular activity based on vestibular input.
  • These nuclei coordinate to maintain posture and enable voluntary movement.
  • Loss of inhibitory input from higher centers leads to decerebrate rigidity due to unopposed pontine excitation.

8. Vestibular Apparatus and Balance

  • Located in the inner ear’s membranous labyrinth.
  • Composed of:
    • Utricle and saccule: Detect linear acceleration and head position.
      • Contain macula with hair cells embedded in a gelatinous layer topped by statoconia (calcium carbonate crystals).
      • Utricle oriented horizontally; saccule vertically.
      • Movement causes statoconia to lag due to inertia, bending hair cells and generating signals.
    • Semicircular canals: Detect rotational head movements.
      • Three canals: anterior, posterior, horizontal.
      • Contain endolymph fluid and a gelatinous structure called the cupula.
      • Rotation moves endolymph, displacing the cupula and bending hair cells.
  • Hair cell physiology:
    • Kinocilium (large hair) and stereocilia (smaller hairs).
    • Deflection toward kinocilium opens ion channels → depolarization → increased nerve firing.
    • Deflection away closes channels → hyperpolarization → decreased firing.
  • Multiple hair cells oriented in different directions allow detection of movement in any direction.
  • Semicircular canals respond primarily to changes in rotational velocity (acceleration and deceleration), not constant rotation.

9. Vestibular Pathways and Reflexes

  • Vestibular nerve transmits signals to vestibular nuclei in brainstem.
  • Vestibular nuclei connect via the medial longitudinal fasciculus (MLF) to oculomotor nuclei.
  • This pathway stabilizes gaze during head movement (vestibulo-ocular reflex).
  • Vestibular nuclei also project to reticular nuclei and spinal cord to coordinate posture and balance.
  • The flocculonodular lobe of the cerebellum integrates vestibular input to fine-tune equilibrium and coordinate rapid postural adjustments.

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