Actions

Action

What is the Motor Hierarchy?

Literally just the traveling path of signals to create action in the body.

Top Part of the Motor Hierachy

Cortical Associations, Premotor, and Supplementary Motor Areas in the brain.

Translates intentions and goals into action plans and movement patterns.

Middle Part of the Motor Hierarchy

Primary Motor Cortex, Brainstem, Basal Ganglia, and Cerebellum Areas in the brain.

Converts action plans and movement patterns into commands for the muscles.

Bottom Part of the Motor Hierarchy

Spinal Motor Neurons and Spinal Sensory Neurons in the Spinal Cord.

Innervate muscles and combine to produce simple reflexes.

Motor Hierarchy Diagram

What is Muscle Innervation?

The process by which nerves supply muscles, allowing them to contract and relax through signals sent from the nervous system.

Antagonist Pairs

Flexor and Extensors - If one muscle is flexed/contracted, the other is extended/relaxed.

Neurotransmitter for Muscle Contraction

Acetylcholine released by firing of alpha motor neurons

Force of Muscles - Determiner

Firing frequency of alpha motor neurons and number of muscle fibres.

Alpha Motor Neurons

Originating in the spinal cord, specifically the ventral horn, and exits through the ventral root.

Muscle Spindles

Detect the stretch/contraction of the muscle.

Process of Postural Stability & Stretch Reflex (Hammer Example)

Hammer hits knee → Causes relaxation in knee.

Muscle spindle detects relaxation → Sends signal to counterbalance this relaxation.

Sensory neurons in muscles go up dorsal root to the dorsal horn, then through interneuron to the alpha motor neurons.

Alpha motor neuron exits ventral horn, then ventral root, to muscle.

Releases acetylcholine to contract the muscle and results in the stretch reflex and thus postural stability.

Gamma Motor Neurons

The use of these is to keep the muscle spindle taunt to be able to detect any changes.

Without this the muscle spindle would be slack and unable to detect changes like overstretching.

Flexor/Extensor Inhibitory vs Excitatory

Inhibitory signals relax the muscle, excitatory signals contract the muscle.

Spinal Patterns of Movements

There are certain patterns in the spinal cord that the brain simply activates such as walking - doesn’t require the brain to fully monitor it.

12 Cranial Nerve are Important For:

Breathing, eating, eye movements, facial expressions, etc. Critical Reflexes!

Key Structure in the Brainstem

Substantia Niagra, Vestibular Nuclei, Reticular Formation Nuclei.

Extrapyramidal Tracts

Projections to the spinal cord responsible for functions such as controlling posture, muscle tone, and movement speed.

Cerebellum Information

Contains 75% of all neurons in the CNS.

Important for error correction (forward models).

Damage results in ataxia: deficit in controlling coordinated movements and maintaining balance.

Basal Ganglia Information

Contains 5 nuclei with outputs to the cortex via the thalamus. 

Key role in movement selection and initiation (gating function).

Damage can result in Parkinson’s or Huntington’s disease.

Primary Motor Cortex (M1/BA4)

Key area for motor initiation and activation of lower levels.

Main output: pyramidal (corticospinal) tract.

Secondary Motor Areas

Key areas for movement planning and control.

Premotor cortex (lateral part of BA6)

Supplementary Motor Area (medial part of BA6)

Additional Key Cortical Areas

Broca’s Area (BA 44/45)

Inferior Parietal Lobule (BA 39/40)

Superior Parietal Lobule (BA 5/7)

Frontal Eye Fields (BA8)

Primary Motor Cortex (M1)

Key area for motor initiation. Controls the contralateral side via the corticospinal tract. Injury leads to Hemiplegia.

Hemiplegia

Loss of control of movement on contralateral side.

Somatotopic Organization of M1

Different regions of the primary motor cortex represent different parts of the body.

This representation is coarser than in the somatosensory cortex (S1).

This representation can be mapped with MRIs and TMS.

Secondary Motor Areas

Premotor Cortex & Supplementary Motor Area.

Both involved in planning and control of movements.

Premotor Cortex

It has connections with the parietal cortex (spatial location) and important for sensory guided movement sequences.

Supplementary Motor Areas

It has connections with the medial frontal cortex (preferences and goals), important in deciding which object to choose and for memory-guided sequences.

Two Dorsal Streams to Premotor Cortex

Dorso-Dorsal & Ventro-Dorsal!

Dorso-Dorsal Stream

All about reaching!!! Optic Ataxia (inability to reach through vision) if damaged.

From SPL (Superior Parietal Lobule).

Ventro-Dorsal Stream

All about transitive (manipulation of objects) & intransitive (signifying intentions) gestures! Apraxia (inability to use objects and link gestures to meaningful actions) if damaged.

From IPL (Inferior Parietal Lobule).

Parietal Links To…

Motor Intention

Premotor Links To…

Motor Execution

SMA Lesions Result In…

Impairment of actions with both hands required.

Also results in Alien Hand Syndrome

Alien Hand Syndrome

A limb does something, but the individual denies responsibility to it.

If corpus callosum is lesioned, the hands can still work together.

Mirror Neurons

Activated by both doing and seeing expected, goal orientated actions.

Mirror Neurons Debate:

Activation for understanding actions? Or only from the priming effect (reflecting automatic action planning when a familiar stimuli is present).

Basal Ganglia

The gatekeeper! Key in movement initiation.

Basal Ganglia Direct Pathway

Cortex activates Striatum which sends inhibitory signal to the Internal Globus Pallidus (GPi) which sends an inhibitory signal to the thalamus which then sends an excitatory signal to the cortex.

Activates Cortex & Starts Movement

Basal Ganglia Indirect Pathway

Cortex activates Striatum which sends an inhibitory signal to the External Globus Pallidus (GPe) which either sends a less inhibitory signal to the GPi OR goes to the Subthalamic Nucleus (STN) which sends an excitatory signal to GPi.

Both paths result in a stronger inhibition of the thalamus, leading to…

Inhibits Cortex & Stops Movement

Basal Ganglia Pathways Diagram

Basal Ganglia Disorders

Huntington’s Disease & Parkinson’s Disease

Huntington’s Disease

Reduced inhibition of the indirect pathway, meaning the GPe becomes very strong, which highly inhibits the GPi, meaning the signal sent to inhibit and stop movement to the thalamus is weak, so the thalamus then becomes strong because nothing inhibits it, so there is hyperkinesia!

Huntington’s Disease: A neurodegenerative and genetic disorder that results in clumsiness, hyperkinesia, chorea, and abnormalities in normal mental attitude.

Parkinson’s Disease

Opposite of Huntington’s disease! The signal from the cortex to the GPe is extremely strong, meaning the signal from GPe to the GPi is weak, so the GPi is extra strong and highly inhibits the thalamus, meaning that there is not very much activation of the cortex, so very little movement.

Parkinson’s Disease: Involved with hypokinesia and bradykinesia.

Akinesia

A = Not, Kinesia = Movement, thus Akinesia = Inability to voluntarily move.

Hyperkinesia

Hyper = Excessive, thus Hyperkinesia = Excessive movements.

Hypokinesia

Hypo = Under, thus Hypokinesia = reduced ability to initiate movements.

Bradykinesia

Brady = Slow, thus Bradykinesia = slow movements.

Forward Model

About how our motor system makes predictions of the anticipated sensory consequences of the movement.

Efference Copy

A copy of the movement is sent from the cortex to the muscles which is used to make predictions.

Sensory Prediction Error

When the predicted and actual sensory feedback do not match, there is learning!

Key Structure for Forward Models

Cerebellum - Involved in the prediction that are temporarily precise (one debate).

Habitual Learning is Controlled by What?

The subcortex! With enough learning motor controls shift from the cortex to the subcortex (such as the cerebellum and basal ganglia).

Population Vector

The sum of preferred directions of neurons.