Detailed University Study Notes on Neural Movement and Motor Control

Introduction to Movement and the Nervous System

• The primary purpose of the nervous system is to regulate movement.

• Reference: Fargie veld O

• Philosophy and Embodiment:

  • Alan Turing proposed that for a machine to think like a human, it might require a human-like body.

  • Embodied Cognitive Science: Focuses on the role the actual body plays in cognitive processes.

Muscle Anatomy and Physiology

• Muscles are composed of many individual fibers.

• Innervation Principles:

  • Each muscle fiber receives information from only one axon.

  • Conversely, a single axon may innervate many muscle fibers.

• The Neuromuscular Junction (NMJ):

  • This is a synapse between a motor neuron axon and a muscle fiber.

  • Contraction is triggered by the release of chemicals (neurotransmitters) at this junction.

Proprioception and Fluidity of Movement

• Fluidity of movement depends heavily on proprioceptors, which are sensory receptors that detect the position or movement of a part of the body.

• Two primary types of proprioceptors regulate muscle contractions:

  • Muscle Spindles: These are receptors parallel to the muscle that respond to a stretch. They trigger the stretch reflex.

  • Stretch Reflex: Occurs when muscle spindles transmit impulses that lead to the contraction of the muscle to oppose the stretch.

  • Golgi Tendon Organ: Located at opposite ends of the muscle; they respond to increases in muscle tension.

  • Function of Golgi Tendon Organ: Acts as a brake or shock absorber to prevent contractions that are too quick or extreme.

• Figure 8.6 Example: The Knee-jerk reflex is a classic example of a stretch reflex.

Units of Movement: Reflexes and Patterns

• Reflexes: Defined as involuntary, consistent, and automatic responses to stimuli.

• Infant-Specific Reflexes (ordinarily not seen in adults):

  • Grasp Reflex: The infant grasps objects placed firmly in the hand.

  • Babinski Reflex: The infant extends the big toe and fans the others when the sole of the foot is stroked.

  • Rooting Reflex: The infant turns the head toward the cheek that is stimulated and begins sucking.

• Sequences of Movement:

  • Many behaviors consist of rapid sequences of individual movements.

  • Central Pattern Generators: Neural circuits located in the spinal cord that generate rhythmic patterns of motor output.

  • Example: Wing flapping in birds.

  • Central pattern generators occupy a middle ground between involuntary reflexes and voluntary movement.

The Primary Motor Cortex (M1)

• Function: Referred to as the "final common pathway."

• Electrical stimulation of M1 produces movement.

• Input: M1 receives input from all cortical and subcortical areas involved in movement.

• Clinical Implications: Damage to M1 causes Hemiplegia.

  • Hemiplegia Characteristics: Loss of voluntary movement on the contralateral (opposite) side of the body, decreased muscle tone, and spastic reflexes.

• Neural Coding of Movement:

  • Movement is coded by the activity of single cells in M1.

  • Each cell has a "preferred direction" of movement.

• Population Vector:

  • Moving beyond single cells, a population vector uses the combined activity of many neurons to predict the direction of movement very accurately.

  • Note: Not all cells in M1 have strong direction preferences, and preferences can change, suggesting neurons are not static representational devices.

Brain-Machine Interface (BMI)

• Clinical Application: Used by patients like "M.N." who have tetraplegia.

• Research: Hochberg et al. (2006). "Neuronal ensemble control of prosthetic devices by a human with tetraplegia."

• Technical Details:

  • Sensors (e.g., $1.0\,\text{mm}$ array) detect spikes in neuronal activity.

  • Onset of cue prompts the patient to imagine movement, which is translated by the BMI into action.

Brain Activity and Awareness of Action

• The Libet Experiment Process:

  1. Observe a clock.

  2. Note the clock position at the time of conscious intention (the urge to act).

  3. Perform the action.

  4. Report the clock position at the moment of conscious intention (e.g., "The clock hand was at position 4").

• Findings:

  • Readiness Potential ($\mu\text{V}$): Brain activity begins before the conscious urge to move is reported.

  • There is a time gap between the onset of brain activity (Readiness potential) and the actual movement onset.

Secondary and Association Motor Areas

• Supplementary Motor Cortex (SMA):

  • Characterized by extensive connections with the frontal lobes.

  • Governing internally guided movement sequences.

  • Specialized for complex sequences of action that do not require interaction with the environment.

• Premotor Cortex (PMC):

  • Characterized by extensive connections with the posterior parietal lobe.

  • Governing externally guided movement sequences (sensory-guided).

• Damage to SMA/PMC:

  • Does not result in hemiplegia or changes in muscle tone.

  • Results in Apraxia: The loss of skilled movement.

Posterior Parietal Cortex (PPC) and Spatial Processing

• Function: Critical for the representation of spatial information and associating sensory input to coordinate motor responses.

• Types of Apraxia associated with damage:

  • Ideational Apraxia: The patient no longer comprehends the uses of an object (e.g., attempting to brush teeth with a comb).

  • Ideomotor Apraxia: The patient understands the desired action but has trouble executing it (more common with SMA/PMC damage).

• Optic Ataxia:

  • Characterized by a spared "what" (object recognition) but impaired "where"/"how" (reaching).

  • Results in poor visual guidance of reaching.

Ventral Stream and Visual Control

• Damage to the Ventral Stream results in Visual Agnosia.

  • Characteristics: Impaired "what" but spared "where"/"how".

  • Case Study: Patient DF (Milner et al., 1991; Goodale et al., 1994).

  • Implications: Suggests a dissociation between visual perception (knowing what an object is) and the visual control of action (knowing how to interact with it).

The Basal Ganglia: The Gatekeeper of Movement

• Anatomical Components:

  • Input: Primarily to the Striatum (Caudate + Putamen).

  • Output: Limited to the Globus Pallidus (GP) and Substantia Nigra (SN).

• Circuitry:

  • All areas modulate each other.

  • Output travels to the Thalamus and then back to the Cortex.

• Role: Selection and initiation of action. It acts as a gatekeeper to determine the most active and appropriate response.

• Pathways:

  • Direct Pathway.

  • Indirect Pathway (involving the Subthalamic Nucleus - STN and Globus Pallidus external - GPe).

  • Substantia Nigra pars compacta ($SNC$) provides dopaminergic input.

Disorders of the Basal Ganglia

• Huntington’s Disease:

  • Cause: Autosomal dominant gene mutation affecting output from the striatum to the globus pallidus.

  • Symptoms: Excessive spontaneous movements (Chorea); irregular, random, brief, and abrupt; non-repetitive; distal predominance; facial grimacing; and abnormal respiratory sounds.

• Parkinson’s Disease:

  • Cause: Cell death in the substantia nigra ($SN$).

  • Symptoms: Akinesia/Bradykinesia (lack of or slow movement), rigidity, and tremor.

• Hemiballismus:

  • Characterized by wild, involuntary movements.

• Dystonia:

  • Characterized by sustained muscle contractions causing abnormal posture. Can range from a single muscle to the whole body.

Hierarchical Control and Motor Pathways

• General Structure:

  • Control is hierarchical.

  • All output goes via spinal neurons ($SC$).

  • Motor areas are influenced by the Cerebellum ($C$) and Basal Ganglia ($BG$).

• Key Regions:

  • Motor Cortex ($MC$).

  • Premotor Cortex ($PM$).

  • Supplementary Motor Cortex ($SMA$).

  • Brainstem ($BS$).

Disorders Affecting Lower Motor Neurons

• Myasthenia Gravis:

  • Type: Autoimmune disorder.

  • Mechanism: Antibodies are directed toward acetylcholine ($ACH$) receptors at the neuromuscular junction ($NM$ junction).

  • Symptoms: Muscle weakness, fatigue on exertion, and Ptosis (drooping eyelid) present in $90\%$ of cases.

• Guillain-Barr syndrome:

  • Type: Autoimmune demyelinating disorder of the peripheral nervous system ($PNS$).

  • Onset: Acute onset, sometimes following surgery or infection.

  • Symptoms: Ascending paralysis moving from lower limbs toward the head.

  • Severity: Eventually requires hospitalization and ventilatory assistance.

Mirror Neurons

• Location: Ventral pre-motor cortex.

• Function: Distributed network involved in both action production and action comprehension.

• Importance:

  • May be critical for understanding, identifying, and imitating others.

  • Likely involved in complex social behaviors.

Global Brain Involvement in Action

• Frontal Lobes: Planning actions, maintaining goals, executing actions.

• Parietal Lobes: Locating objects in space, sensory-motor transformation.

• Temporal Lobes: Object recognition, object knowledge.

• Occipital Lobes: Visual analysis of the scene.

• Subcortex (Basal Ganglia): Initiating action, modulating force and the likelihood of action.

• Subcortex (Cerebellum): Monitoring action online.

Computational Challenges of Action

• Degrees of Freedom Problem: The body has too many moving parts to control individually. Generalized motor programs (stored routines) minimize this problem.

• Sensory-motor Transformation: The difficulty of linking the position of an object in retinal space (eyes) with the position of limbs in bodily space (proprioception).