Motor Systems and Motor Control

Motor Systems and Motor Control

Based on Chapter 11 in Behavioral Neuroscience by Breedlove SM, Watson NV (Rosenzweig MR), 8th ed., Sinauer Assoc. /Oxford University Press, 2017.

Overview

  • Reflexes vs. Plans: The Behavioral View.

  • Accuracy vs. Speed: The Control Systems View.

  • Hierarchical Systems: The Neuroscience View.

  • Spinal Cord: Crucial link in controlling body movement.

  • Pathways from the Brain: Control different aspects of movements.

  • Extrapyramidal Systems: Mediate Motor Commands.

  • Brain Disorders: Can disrupt movement.

Behavioral View

  • Charles Sherrington (1857-1957):

    • Decerebration - spinal animals.

    • Motor integration - reflexes.

  • Critics:

    • Motor plan (motor program).

  • Analysis of movements:

    • Trajectories.

    • Electromiography (EMG).

  • Reflex: Simple, unvarying, and unlearned responses to sensory stimuli (touch, pressure, pain).

  • Motor Plan/Program: Set of muscle commands established before the action occurs.

Behavioral View: Recording Trajectories

  • (A) Visually Guided Reaching Task

    • Subject places a cursor (glowing light) on the center point to start.

    • When a target light goes on (yellow), the subject rapidly moves the cursor to reach it.

    • Cameras monitor and record movements.

  • (B) Examples of Cursor Movements After 200 Practice Trials

    • A normal control subject reaches accurately, with successive trials coinciding closely.

    • A patient with Huntington's disease shows greater trial-to-trial variability, and correction movements near the target are much larger than in control subjects.

    • Irregularities of reaching, especially in correction movements, can be seen in a person with the gene for Huntington's disease, even years before the disease becomes apparent.

Control System View

  • Open Loop: Maximizes speed, no feedback possible.

  • Closed Loop: Maximizes accuracy, with feedback signal.

Skeletal System

  • Bones and Joints: Ellipsoidal (wrist), ball-and-socket (hip), hinge (knee).

    • Flexion

    • Extension

  • Muscle Organisation: Antagonists, synergists

    • Biceps / Triceps

    • Tendons

Composition of Muscles

  • Types of muscle tissue:

    • Striated muscle

    • Smooth muscle

    • Cardiac muscle

  • Types of striated muscles:

    • Slow-twitch (red) fibres

    • Fast-twitch (white) fibres

  • Molecular mechanisms of muscle contraction:

    • Actin/myosin complexes

    • Cross bridges (sliding filaments)

Neuromuscular Junction (NMJ)

  • Action potentials travel down the motor neuron and branch into many terminals, where acetylcholine (ACh) is released.

  • Targets where the neuron terminates on the muscle fiber are called neuromuscular junctions (NMJ).

  • The NMJ is an effective synapse; almost every action potential elicits a contraction.

Neuromuscular Junction Process

  1. Nerve action (spike) potential approaches junction.

  2. Spike reaches junction and triggers release of ACh.

  3. ACh acts on muscle fiber to produce muscle action potential.

  4. Muscle contracts.

Muscle Contraction as a Function of Stimulation Frequency

  • Twitch

  • Wave Summation

  • Unfused (incomplete) tetanus

  • Fused (complete) tetanus

  • Muscle Response to Stronger Stimuli: Multiple Motor Unit Summation (Recruitment)

Measuring Muscle Electric Activity: Electromyography (EMG)

  • EMG signal measures the electrical activity of muscles.

  • Electrode Placement:

    • White Lead (-)

    • Red Lead (+)

    • Black Lead (Ground)

  • Compound motor unit action potentials (MUAP)

  • Detection = (m{1}+n) - (m{2}+n) = m{1} - m{2}

Electromyography (EMG) in Clinical Practice

  • Examination of nerve-muscle connections

  • Raw EMG signal, full-wave rectified EMG signal, full-wave rectified and smoothed EMG signal, full-wave rectified and integrated EMG signal, and EMG power spectrum.

EMG Recording Methods in Psychology

  • Surface EMG

EMG Signal Analysis

  • Raw waveforms

  • Half-wave rectified waveforms

  • Full-wave rectified waveforms

  • Smoothed waveforms

  • True integrated waveforms

Simultaneous EMG and Force Measurement

  • Measurement of human joint force, surface electromyograms, and functional MRI-measured brain activation.

    • Liu et al., Journal of Neuroscience Methods 101 (2000) 49–57. doi:10.1016/S0165-0270(00)00252-1

EMG in Psychology - Emotions

  • Facial muscles and electrode placements for surface EMG recording.

  • Most popular muscle (regions) for EMG

EMG and Facial Expressions

  • Corrugator EMG correlates with frowning.

  • Zygomatic EMG correlates with smiling.

Behavioral/ (phycho)physiological View: Electromyography (EMG)

  • Fine needle electrodes placed in a muscle, or electrodes placed on the skin over a muscle, can detect electrical indications of muscle activity.

  • Muscle tone at rest, or timing and strength of contraction of the muscles (involved in a movement) can be recorded.

  • EMGs can show motor planning, such as leg adjustments before arm movement.

When the Motor Endplate Is Poisoned: The Story of C. Botulinum Toxin (Botox)

  • Botulinum toxin (Botox) is a neurotoxic protein produced by the bacterium Clostridium botulinum.

  • The toxin prevents the release of acetylcholine from axon terminals at the neuromuscular junction, causing flaccid paralysis (botulism).

  • Also used commercially for medical and cosmetic purposes to reduce muscle contraction and remove persistent wrinkles on the face.

Neuroscience View

  • Skeletal system

  • Spinal cord

  • Brainstem

  • Primary motor cortex

  • Nonprimary motor cortices

  • Cerebellum

  • Basal ganglia

  • Final common pathway (Sherrington)

Sensory Feedback: Proprioception

  • Primary afferent endings are maximally sensitive early in a stretch; communicate velocity.

  • Secondary afferent endings are slow to change rate, maximally sensitive to maintained length; communicate muscle length.

  • Golgi tendon organs are responsive to muscle contraction but not to stretch

  • Gamma motoneurons, or gamma efferents, alter tension within the spindle and control receptor sensitivity.

  • Alpha motoneurons go to the extrafusal muscle fibers.

Sensory Feedback and Muscle State

  • Muscle relaxed: Low level of Spindle and Golgi tendon organ activity

  • Muscle stretched: excitation of both receptors

  • Muscle contracted: Tendon organ excited; spindle not excited

The Spinal Cord: The Crucial Link

  • Controls most body movements.

  • Spinal reflexes.

  • Spinal pattern generators.

Spinal Reflex Circuits

  • Monosynaptic reflex circuit.

Pattern Generators

  • Central pattern generator.

Pathways From the Brain

  • Influence on spinal motor neurons.

  • Corticospinal tracts.

Corticospinal Tracts

  • Lateral.

  • Anterior (Ventral).

Motor Homunculus

  • Map of the body represented on the cortex.

  • Disproportionate representation (cortical magnification).

Control of Different Aspects of Movements

  • Primary motor cortex (M1).

  • Nonprimary motor cortex.

Primary Motor Cortex (M1)

  • Encoding specific movements.

  • Plasticity in motor maps.

Nonprimary Motor Cortex

  • Premotor cortex.

  • Supplementary motor area (SMA).

Mirror Neurons

  • Active both when an individual makes a movement and when an individual sees another individual make the same movement.

  • Lie within the premotor cortex.

Extrapyramidal Systems

  • Basal Ganglia.

  • Cerebellum.

Basal Ganglia

  • Receive inputs from throughout the cortex.

  • Project back to the cortex, as well as to the brainstem.

The Cerebellum

  • Receives information from the spinal cord, sensory systems, and the cortex.

  • Projects back to the cortex and the spinal cord.

Brain Disorders Can Disrupt Movement

  • Damage to the spinal cord.

  • Polio.

  • Amyotrophic Lateral Sclerosis (ALS).

  • Damage to the brain.

  • Parkinson's Disease.

  • Huntington's Disease.

  • Cerebellar damage.

Damage to the Spinal Cord

  • Transection of the spinal cord.

  • Spinal shock.

  • Paraplegia.

  • Quadriplegia.

Polio

  • Caused by poliovirus that destroys spinal motoneurons and sometimes cranial motoneurons.

  • Only ~ 1% of polio-infections result in paralysis.

Amyotrophic Lateral Sclerosis (ALS)

  • Progressive degeneration of motoneurons and consequent loss of their target muscles.

Damage to the Brain

  • Stroke (cerebrovascular accident).

  • Apraxia.

Parkinson's Disease

  • Progressive loss of dopaminergic cells in the substantia nigra.

  • Rigidity, slow movements, and tremors.

  • Treatment: L-DOPA, Dopamine agonists, Deep Brain Stimulation

Huntington's Disease

  • Progressive damage to the basal ganglia, especially the caudate and putamen.

  • Involuntary, jerky movements (chorea).

  • Cognitive and psychiatric disturbances.

Cerebellar Damage

  • Ataxia.