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HLTH2013 – Lecture 2 Notes (Neuromotor & Sensory Basis for Motor Control)

Lecture Objectives

  • Part A (Neuromotor system)
    • Understand components & functions of CNS and PNS
    • Describe neuron anatomy, information flow & the various neuron types
    • Identify key brain structures (cerebrum, diencephalon, cerebellum, brain-stem) and spinal cord elements
    • Explain motor units and principles of motor-unit recruitment
  • Part B (Sensory inputs)
    • Explain how vision, touch, proprioception & vestibular information contribute to motor control
    • Describe investigative techniques (eye-tracking, temporal/spatial occlusion)
    • Relate sensory feedback to movement accuracy, consistency & learning

Neuromotor System — Overview

  • Motor control emerges from a distributed network spanning the CNS & PNS
  • CNS (brain + spinal cord)
    • Central integration & coordination hub
    • Receives, integrates & interprets sensory input; generates & sends motor commands
  • PNS (nerves outside CNS)
    • Links CNS to body
    • Afferent division: carries sensory (input) signals to CNS
    • Efferent division: carries motor (output) commands to effectors
    • Further split into
    • Somatic nervous system: directs voluntary skeletal-muscle movement
    • Autonomic nervous system (ANS): regulates involuntary functions; subdivided into sympathetic (\"fight-or-flight\") & parasympathetic (rest-and-digest)

The Neuron — Structure & Function

  • Basic unit of nervous system (≈ 200\text{ billion} in humans)
  • Common architecture
    • Cell body (soma): metabolic centre housing nucleus
    • Dendrites: up to \text{thousands}; receive incoming signals
    • Axon (nerve fibre): single elongated projection; branches terminate in axon terminals
    • Myelin sheath speeds conduction of the action potential (saltatory conduction)
    • Axon terminals release neurotransmitters at synapses to relay the signal
  • Neural transmission: electro-chemical propagation of an action potential from dendrites → soma → axon → synapse → next neuron/muscle

Functional Types of Neurons

  1. Sensory (afferent) neurons
    • Convey information from sensory receptors to CNS
  2. Motor (efferent) neurons
    • Convey commands from CNS to skeletal muscle
    • Alpha motor neurons (α-MNs): innervate extrafusal muscle fibres; reside mainly in spinal ventral horn; generate muscle contraction and movement
    • Gamma motor neurons (γ-MNs): innervate intrafusal fibres (muscle spindles); regulate spindle sensitivity to length/velocity changes
  3. Interneurons
    • Confined to CNS; connect sensory and motor pathways; crucial for reflexes & complex circuits (e.g., Renshaw cells inhibit α-MNs to modulate output)

CNS Structures Relevant to Motor Control

Cerebrum

  • Largest brain portion; two hemispheres connected via corpus callosum
  • Cerebral cortex (highest processing centre)
    • Four lobes: Frontal, Parietal, Occipital, Temporal
    • Frontal lobe: voluntary movement planning & initiation
    • Parietal lobe: somatosensory perception & integration
    • Occipital lobe: visual perception
    • Temporal lobe: memory & auditory processing
    • Central sulcus separates frontal (motor) from parietal (sensory) areas
Functional Cortical Areas
  • Sensory (Primary Somatosensory Cortex)
    • Posterior to central sulcus; receives touch, pain, temperature, proprioceptive info
  • Motor areas (all anterior to central sulcus)
    1. Primary Motor Cortex (M1)
    • Final cortical output; initiates & executes specific muscle contractions
    1. Premotor Cortex (PMC)
    • Plans & organises movements, especially in response to external cues; rhythmic & sequential actions (e.g., piano)
    1. Supplementary Motor Area (SMA)
    • Internally generated movement plans; bimanual coordination; memory-driven sequences
  • Association areas (posterior parietal cortex)
    • Integrate multisensory inputs; transform perception into action goals relayed to frontal motor regions

Sub-cortical Motor Centres

  • Basal Ganglia (caudate nucleus, putamen, substantia nigra, globus pallidus)
    • Movement initiation, parameter scaling (speed, force, direction), posture; dopamine deficiency here ↔ Parkinson’s disease
  • Diencephalon
    1. Thalamus: relay station to cortex; attention, mood, pain perception
    2. Hypothalamus: endocrine regulation & homeostasis (temperature, hunger, thirst)
  • Cerebellum
    • Error-detection & correction (compares intended vs actual movement); timing, balance, hand-eye coordination; critical for motor learning & adaptation
  • Brainstem (pons, medulla, reticular formation)
    • Pons: bridge cortex↔cerebellum; chewing, balance
    • Medulla: crossover of sensory/motor tracts; respiration & heart rate regulation
    • Reticular formation: arousal, attention; modulates sensory & motor activity

Spinal Cord

  • Grey matter (H-shaped): dorsal horns (sensory input), ventral horns (motor output), interneurons (e.g., Renshaw)
  • White matter: ascending & descending tracts
    • Ascending (sensory): dorsal column (touch/pressure/proprioception), spinothalamic (pain/temp)
    • Descending (motor):
    • Pyramidal (corticospinal) tracts: \approx90\% fibres decussate at brain-stem → fine voluntary control
    • Extrapyramidal tracts: \approx10\% uncrossed; posture, proximal limb & hand/finger fine control
  • Functions: reflexes, real-time timing tweaks, conduit for commands & feedback

Motor Units

  • Motor unit = one α-MN + all muscle fibres it innervates
  • All-or-none: if α-MN fires, every connected fibre contracts fully
  • Innervation ratio reflects functional demand
    • Fine control (e.g., ocular muscles): 1\text{ fibre/unit}
    • Gross force (e.g., posture): up to 700\text{ fibres/unit}

Fibre & Motor-Unit Types

  • Slow-twitch (Type I): low force, fatigue-resistant
  • Fast-twitch (Type IIa): higher force, moderately fatigue-resistant
  • Fast-twitch (Type IIb): highest force, quick fatigue
  • Recruitment principle (Henneman’s \"size principle\")
    • Activate smallest (Type I) units first → progressively larger (IIa, IIb) as force demand increases → smoother, efficient force scaling
    • Practical: explosive tasks (kicking, batting) require rapid Type II recruitment

From Intention to Action — Hierarchical Flow

  1. Decision/intention in association areas → motor planning (PMC/SMA)
  2. Basal ganglia & cerebellum refine parameters; thalamus relays
  3. Primary motor cortex issues descending command
  4. Brain-stem & corticospinal tracts transmit signal; decussation results in contralateral control
  5. α-MN in ventral horn fires → neuromuscular junction → muscle contraction
  6. Sensory feedback (skin, muscle spindles, vision, vestibular) ascends for real-time comparison & adjustment

Sensory Contributions to Motor Control

  • Sensory information provides
    • Pre-movement feed-forward (planning & anticipation)
    • Online feedback (error detection/correction)
    • Post-movement evaluation (outcome success)
  • Exteroception (vision, audition) + interoception (proprioception, vestibular, tactile) merge into perceptual-motor processes

Vision — Dominant Modality

  • Structural optics: cornea, pupil, lens (accommodation)
  • Retina: rods (peripheral, night, low acuity) & cones (foveal, colour, high acuity)
  • Optic nerve conveys signals to visual cortex; two cortical streams
    • Dorsal \"where\" (ambient/peripheral): spatial localisation, action guidance
    • Ventral \"what\" (focal/central): object identification
  • Central (foveal) vision: 2-5^{\circ} field, conscious, high acuity — \"what is it?\"
  • Peripheral vision: up to \approx200^{\circ} horizontally, subconscious, locates \"where is it?\"
  • Binocular cues → depth perception; monocular cues → shape, size, texture
  • Perception-action coupling: gaze generally leads movement; movement alters visual input; cyclical relationship
  • Practical example (cricket batting)
    • Ball flight \sim500\text{ ms}; experts saccade to predicted bounce point, track 100{-}200\text{ ms} post-bounce; can still hit even when vision occluded pre-bounce (use early kinematic cues)
Research Methods
  • Eye-tracking: fewer, longer fixations in experts → efficient information extraction
  • Occlusion paradigms
    • Temporal occlusion: video stopped at defined times; liquid-crystal goggles
    • Spatial occlusion: hide specific body parts or objects (e.g., thrower’s arm)

Tactile (Touch) Information

  • Skin mechanoreceptors (highest density in fingertips) detect pressure, pain, temperature
  • Anesthetising digits degrades accuracy, consistency & force modulation in precision tasks → tactile input critical when manipulating objects

Proprioception

  • Internal sense of limb/head position & movement (kinaesthesis)
  • Primary receptors
  1. Muscle spindles: in parallel with fibres; detect length & velocity changes; inform joint angle & movement direction
  2. Golgi tendon organs: at muscle–tendon junction; detect tension/force; poor length detection
  3. Joint receptors: in capsules/ligaments; detect extreme angles, rotation & force at joint surfaces
  • Loss or degradation (e.g., neuropathy) → clumsy, poorly coordinated movement despite intact musculature

Vestibular System

  • Otolith organs & three semicircular canals in inner ear
    • Detect linear & angular acceleration, gravity; informs balance & gaze stability (vestibulo-ocular reflex)
  • Works synergistically with vision & proprioception to orient the body in space

Practical, Clinical & Ethical Implications

  • Training & Rehabilitation
    • Eye-tracking and occlusion drills enhance anticipatory skills in sport
    • Sensory substitution (e.g., vibrotactile feedback) for individuals with proprioceptive loss
  • Neurodegenerative Conditions
    • Basal ganglia disorders (Parkinson’s, Huntington’s) affect initiation & scaling → informs therapy/drug targets
  • Safety & Ergonomics
    • Understanding sensory dominance helps design cockpits, dashboards, VR to avoid sensory conflict illusions (e.g., car-moving illusion at traffic lights)
  • Ethical / Philosophical Considerations
    • Neuro-enhancement technologies (exoskeletons, neural implants) must respect autonomy & equitable access
    • Privacy of eye-tracking data in performance analytics

Review & Reflection Questions

  • Differentiate CNS vs PNS roles in motor control.
  • Name & describe the four cortical lobes and relate each to movement.
  • Outline neuron anatomy and the sequence of neural transmission.
  • Compare α-MN and γ-MN functions.
  • Explain how the size principle governs smooth force production.
  • List specialised receptors for vision (rods, cones), proprioception (spindles, GTOs, joint receptors) & vestibular sense (otoliths, semicircular canals).
  • Describe a practical scenario where tactile feedback is crucial.
  • How would loss of binocular vision affect depth-dependent skills?