Motor Control Notes

Motor Control

Introduction

Motor control is the study of how the nervous system controls movement. Movements can range from simple reflexes to complex actions. The primary purpose of the brain is to guide movement.

Movement Terms

• Reflexes: Simple, brief muscle activations (e.g., eyeblink, hiccup, finger twitch).

• Acts/Action Patterns: Complex, sequential movements (e.g., honking a car horn, writing your name, playing guitar).

• Motor Plan/Motor Program: A set of muscle commands established before an action occurs.

Control Mechanisms

Two control mechanisms optimize accuracy and speed:

Open-Loop Control

Maximizes speed by using pre-programmed instructions without external feedback. Ballistic movements are rapid and completed regardless of sensory feedback. No guiding external feedback.

Closed-Loop Control

Maximizes accuracy by using feedback from the controlled action to guide movements. Ramp movements are smooth, slower, and sustained, and are guided by feedback.

Hierarchy of Motor Control

1. Nonprimary Motor Cortex: Initiates cortical processing.

2. Primary Motor Cortex: Initiates commands for action.

3. Brainstem: Integrates motor commands.

4. Cerebellum and Basal Ganglia: Tweak these systems.

5. Spinal Cord: Controls skeletal muscles.

6. Skeletal Muscles: Power movement.

   

Muscle Mechanics

• Antagonists: Biceps and triceps are antagonists.

• Balance:At rest, flexor and extensor muscles are balanced.

• Tremor: Alternation of flexor-extensor (antagonist)/bicep-tricep contraction. Normally present, but debilitating if poorly regulated.

Motoneurons

Motoneurons send their axons to innervate muscles. Action potentials travel along the motoneuron from the spinal cord to the muscle.

Neuromuscular Junction

• Neuromuscular Junction: Point where a motor neuron terminal and muscle fiber meet.

• Acetylcholine: Neurotransmitter released at the neuromuscular junction.

  

Motor Units

• Motor Unit: One motor neuron's axon and all its target fibers.

• Fine Movements: Muscles that make fine, precise movements have only a few muscle fibers per axon.

• Innervation Ratio: Examples include 1/3 and 1/13, which describe the ratio of motor neurons to muscle fibers.

• 1 axon and 3 fibers attached will produce fine movements

• 1 axon and 13 movements will NOT

• 

Sensory Feedback

Action of muscles is guided by sensory feedback:

• Muscle Spindles: Responsive to muscle stretch.

• Golgi Tendon Organs: Respond to muscle contraction, less to stretch.

• Proprioception: Information about body movements and position.

Muscle Spindles

Muscle spindles provide feedback on muscle length and stretch.

α-γ Coactivation

alpha-gamma Coactivation mechanism involving sensory neuron, \alpha motor neuron, and \gamma motor neuron.

Muscle Spindle Stretch Reflex

1. Muscle is stretched.

2. Excitation of muscle spindle afferents. muscle spindles stretching and excite.

3. Excitation of \alpha -motoneurons. Beta and alpha neurons are excited.

4. Agonist muscle (biceps and triceps) stimulated to oppose stretch.

5. Antagonist muscle is inhibited. Muscle goes to relaxed state neither stretching or contractions.

This reflex prevents you from falling over when you extend your arm.

Spasticity

Impaired control of the stretch reflex. Motor cortex normally inhibits reflex behavior. When cortical input is cut off, spinal cord is released from inhibition, exaggerating reflexes. Hyperreflexia, Clonus.

Pyramidal vs. Extrapyramidal Motor Systems

• Pyramidal System: Includes the primary motor cortex, and is responsible for voluntary movement.

• Extrapyramidal System: Includes the basal ganglia and cerebellum, modulates and tweaks movements.

Pyramidal System Pathway

1. Upper Motor Neuron: From primary motor cortex to the medulla, where it crosses over (decussation), then down the spinal cord to the anterior horn cell (lower motoneuron).

2. Lower Motor Neuron: From spinal cord to muscle.

   

Primary Motor Cortex (M1)

Primary motor cortex changes as a result of learning. Early music training results in expansion of motor cortex.

Nonprimary Motor Cortex

Learns and plans movement:

• Supplementary Motor Area (SMA): Encodes sequences of movements during skill acquisition.

• Premotor Cortex: Neurons fire just before performing an activity.

• 

Extrapyramidal System - Basal Ganglia/Cerebellum

Basal Ganglia

Includes the caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra.

Cerebellum

Guides movement through inhibition via Purkinje cells, which send inhibitory messages.

Cerebellum and Basal Ganglia

Primary motor cortex and basal ganglia are more involved in early phase of a movement than SMA and cerebellum.

Postural Adjustment

Basal ganglia and cerebellum (extrapyramidal system) predict postural consequences of planned (pyramidal) movement and act to prevent loss of balance.

Gastronemius is activated before he lifts bell? Why? Because of extrapyrimidal movements.

Central Pattern Generators

Neural circuits generate rhythmic behaviors (e.g., walking) in the spinal cord.

Input to Output

A visual task that requires a simple choice and a button push involves a complex cascade of processes.

Movement Control Cascade

1. What is the current state of the body?

2. Should I move?

3. Select the motor plan.

4. Load the motor plan.

5. Execute the plan.

6. Feedback on how the plan is going. Change it?

   

Clinical Neurology of Movement

Strength vs. Tone

• Strength: Largely a pyramidal function (power).

• Tone: Largely an extrapyramidal function (posture).

These are not independent.

Pyramidal vs. Extrapyramidal Damage

• Pyramidal damage causes weakness.

• Extrapyramidal damage impairs movement control.

  

What Goes Wrong with the Motor System?

Myopathy

Primary disorder of muscle.

Muscular Dystrophy

Dystrophin is a protein needed for normal muscle function, produced by the X chromosome. Patients make no dystrophin or an abnormal dystrophin molecule, leading to progressive degeneration of muscle.

Myasthenia Gravis

Autoimmune disorder where patients develop antibodies to their own ACh receptors. This causes weakness of skeletal muscles that develops over the day and resolves with rest/sleep.

Polio

Poliovirus destroys spinal motoneurons and sometimes cranial motoneurons. There is no treatment for polio.

Amyotrophic Lateral Sclerosis (ALS)

Lou Gehrig disease, degeneration of motoneurons and consequent loss of their target muscles. Fasciculations.

Spina Bifida and Spinal Cord Injuries

Spinal cord injuries result in paralysis; reflexes, sensation, and strength below the level of the injury are lost.

Apraxia

Higher-level (motor processing) disorder: inability to sequence movements, though no muscle paralysis exists. New acts are ramped (feedback-controlled) - slow, variable - frontal and parietal cortex. Well-learned acts are ballistic - fast, consistent - cortex and basal ganglia.

Parkinson Disease

• Tremor

• Bradykinesia

• Shuffling gait

• Postural instability

Degeneration of dopamine cells in the substantia nigra, which project to the basal ganglia. L-dopa, a precursor to dopamine, improves symptoms. Environmental exposures, especially pesticides, contribute.

Huntington Chorea

Progressive destruction of the caudate nucleus and putamen. Cerebral cortex also is impaired. The gene responsible (HTT) has a trinucleotide repeat (CAG). If CAG repeats too many times, the disease develops.

Ataxia

Cerebellar damage impairs motor control. Purkinje cells die. Causes include childhood tumors of the cerebellar vermis, alcoholism, and inherited degeneration of the cerebellum.

Dystonia

Abnormal sustained posture due to basal ganglia dysfunction.

Tourette Syndrome

Tics and Obsessive-Compulsive Disorder. Basal ganglia and cortex disorder. Clinical features: more common in boys, tics usually end by adulthood, tics affect face and shoulders more than hands and legs, coprolalia is rare in children.

Hemiparetic Gait

Motor cortex damage, such as stroke, causes motor impairment. Weakness (paresis) of voluntary movements.