NEUR2020 Neuroscience for Psychologists: Motor Control

NEUR2020 Neuroscience for Psychologists: Motor Control Notes

Acknowledgement of Country

The University of Queensland (UQ) acknowledges the Traditional Owners and their custodianship of the lands on which they meet. Respect is paid to Ancestors and descendants who maintain cultural and spiritual connections to Country, recognizing their valuable contributions to Australian and global society.

Introduction: The Essence of Action

Charles Sherrington's quote, "Life’s aim is an act, not a thought," encapsulates the fundamental role of motor control. The brain primarily functions as a manager of muscles, with the ultimate goal of all cognition being action. Actions were crucial for the survival and reproduction of our ancestors, highlighting their centrality in brain function.

Overview of Motor Control Systems

Motor control involves a complex system characterized by several key organizational principles:

  • Hierarchical Organization: Control flows from higher cortical areas down to spinal circuits.
  • Functional Segregation: Different brain regions specialize in different aspects of motor control.
  • Parallel Processing: Multiple pathways operate simultaneously.
  • Feedback Loops: Sensory information continuously feeds back into the system to adjust and refine movements.

The overall hierarchical arrangement includes:

  1. Association Cortex
  2. Secondary Motor Cortex
  3. Primary Motor Cortex
  4. Brain Stem Motor Nuclei
  5. Spinal Motor Circuits
  6. Muscle (the effector)

Descending motor circuits carry commands from the brain to the spinal cord, while feedback circuits provide sensory information from the muscles and periphery back to the brain.

Spinal Circuits and Motor Units

Motor Units

The motor unit is the smallest functional unit of motor control, comprising a single motor neuron and all the skeletal muscle fibers it innervates.

  • Neuromuscular Junction: This is the specialized synapse between a motor neuron and a muscle fiber. The release of acetylcholine by the motor neuron activates the motor end plate (post-synaptic membrane of the muscle fiber), leading to muscle fiber contraction.
  • Innervation Ratio: Each motor neuron can innervate multiple muscle fibers, but each muscle fiber is innervated by only one motor neuron. The number of fibers reflects the fineness of control:
    • For eye muscles, a motor neuron may innervate as few as 5 fibers (out of 22,000 total fibers).
    • For large leg muscles, a motor neuron may innervate up to 1,800 fibers (out of 1 million total fibers).
    • This ratio can vary widely even within a single muscle.
  • Motor Pool: This refers to the collection of all motor neurons that supply a single muscle. A typical muscle is controlled by a pool of a few hundred motor neurons.
  • Properties of Motor Units: Motor units exhibit three main properties:
    1. Contraction speed
    2. Maximal force
    3. Fatiguability

Lower Motor Neurons

Lower motor neurons are key components of the motor system:

  • Location: They are located in the ventral horn of the spinal cord and brain stem.
  • Projection: They project directly out to muscles via the ventral root.
  • Inputs: They receive diverse inputs from:
    • The brain (descending pathways).
    • Muscle spindles (sensory receptors within muscles).
    • Spinal interneurons (which can be excitatory or inhibitory).

Spinal Motor Circuits

Spinal cord motor circuits exhibit significant complexity, organizing various motor behaviors:

  • Reflexes:
    • Stretch Reflex (e.g., Patellar Reflex): A direct (monosynaptic) reflex where a muscle spindle afferent neuron synapses directly onto a lower motor neuron, causing the stretched muscle to contract.
    • Withdrawal Reflex: A polysynaptic reflex involving interneurons. It simultaneously excites flexor muscles and inhibits extensor muscles to withdraw a limb from a painful stimulus.
  • Recurrent Collateral Inhibition: A mechanism where a motor neuron's axon branches to an inhibitory interneuron, which then projects back to the original motor neuron. This causes a brief self-inhibition after firing, allowing for a short break and distributing responsibility to other motor neurons in the muscle's motor pool.
  • Reciprocal Innervation: Essential for smooth and precise movements, this mechanism ensures that when an agonist muscle contracts, its antagonistic counterpart is reciprocally inhibited. This prevents constant contraction of most muscles and allows for coordinated adjustments.
  • Locomotion (Spinal Walking Circuits): Studies in cats with severed spinal cords demonstrate that basic motor patterns for stepping can be generated by spinal circuits when provided with appropriate sensory feedback (e.g., on a treadmill). However, the initiation and fine control of locomotion require input from the brain.

Descending Motor Pathways

Lower motor neurons receive numerous inputs, with major contributions from the brain. These brain signals can synapse directly onto lower motor neurons or, more commonly, indirectly via spinal interneurons.

The primary motor cortex sends signals to muscles via four main pathways, divided into two groups based on their location in the spinal cord:

1. Dorsolateral Tracts

These two tracts primarily control limbs, especially the independent movement of digits and fine motor skills. They mostly terminate in the contralateral half of one spinal cord segment, sometimes directly on a motor neuron.

  • Dorsolateral Corticospinal Tract (Direct Pathway):
    • Course: Descends contralaterally.
    • Synapse: Primarily synapses on small interneurons in the spinal gray matter, which then synapse on lower motor neurons innervating distal muscles (wrist, hands, fingers, toes). In animals capable of independent digit movement, some axons synapse directly onto motor neurons.
    • Function: Crucial for fine, independent movements of the limbs, particularly dexterity.
  • Dorsolateral Corticorubrospinal Tract (Indirect Pathway):
    • Course: Descends contralaterally, after synapsing in the red nucleus.
    • Function: Ultimately controls distal muscles of the arms and legs.

2. Ventromedial Tracts

These two tracts are more diffuse and control body movements, posture, and whole-body movements, including limb movements involved in these larger actions. Their axons innervate interneurons in several segments of the spinal cord.

  • Ventromedial Corticospinal Tract (Direct Pathway):
    • Course: Descends ipsilaterally, with axons branching diffusely to innervate interneurons on both sides of the spinal cord at several levels.
    • Function: Controls proximal muscles of the trunk and limbs.
  • Ventromedial Cortico-brainstem-spinal Tract (Indirect Pathway):
    • Course: Upper motor neurons feed a complex network of brainstem structures, including the tectum, vestibular nuclei, and motor programs in the reticular formation. Outputs descend bilaterally (each side carrying information from both hemispheres).
    • Synapse: Each neuron synapses on interneurons over several spinal segments.
    • Function: Innervates proximal muscles of the trunk and limbs (e.g., shoulder, hip), contributing significantly to posture and whole-body movements.

Experimental Evidence (Lawrence & Kuypers, 1968)

Transection studies in monkeys provided crucial insights into the functions of these pathways:

  • Dorsolateral Tracts Transected: Monkeys could still stand, walk, and climb, but lost the ability to use limbs for precise activities (e.g., reaching for objects) and could not move fingers independently.
  • Ventromedial Tracts Transected: Monkeys exhibited postural abnormalities and impaired walking and sitting abilities.

These experiments confirm the specialization of dorsolateral tracts for fine, independent limb movements and ventromedial tracts for posture and whole-body movements.

Motor Neuron Disease (MND)

MND is a group of neurodegenerative diseases characterized by the progressive loss of motor neurons (upper, lower, or both).

  • Amyotrophic Lateral Sclerosis (ALS): The most common form of MND. It causes progressive muscle weakness and wasting, typically without cognitive impairment.
  • Variability: The pattern of weakness, rate of progression, and survival time vary significantly among individuals.
  • Prognosis: There is currently no cure or effective treatment, with an average survival of 2-5 years from onset.
  • Cause: The cause is uncertain, potentially involving environmental factors, lifestyle, and subtle genetic predispositions (5-10% of cases have a family history).
  • Pathology: Characterized by the presence of cytoplasmic protein aggregates known as inclusion bodies.
  • Diagnosis: Early signs are subtle, making diagnosis challenging (often taking 10-18 months). There can also be confusion with conditions like myasthenia gravis.

Motor Cortex

The motor cortex is essential for voluntary movement, defined as purposeful interactions with the environment that differ from reflexes and basic locomotor patterns. Voluntary movements are:

  • Intentional: Originate from an internal decision to act.
  • Goal-Oriented: Organized to achieve a specific goal.
  • Context-Dependent: Associations with sensory input guide actions.
  • Learned: The ability to acquire new skills and improve with experience.

Primary Motor Cortex (M1)

M1 is a critical hub in the motor system:

  • Outgoing Point: Serves as a major (though not exclusive) outgoing point for signals descending from the cortex.
  • Convergence Point: Receives major sensorimotor signals from areas like the Posterior Parietal Cortex (PPC), Supplementary Motor Area (SMA), frontal regions, basal ganglia, and cerebellum.
  • Somatotopic Organization: Early research by Penfield, using electrical stimulation, revealed a motor homunculus – a somatotopic representation of the body, where stimulation of specific M1 regions elicited simple movements of contralateral muscles. This also demonstrated cortical magnification, meaning areas requiring fine control (e.g., hands, face) have larger cortical representations.
  • Subdivisions: M1 in primates has two subdivisions: an older rostral and a newer caudal part. The caudal M1 contains neurons that can synapse directly onto lower motor neurons for upper limbs, contributing significantly to dexterity via the dorsolateral corticospinal (direct) tracts.
  • Neuronal Firing: While previously thought to encode the direction of a single muscle's movement, newer research suggests M1 neurons can:
    • Elicit complex, species-typical natural response sequences (e.g., feeding) when stimulated with longer bursts resembling natural motor responses.
    • Show activity related to the end-point of a movement rather than just its direction. For example, a 90-degree elbow bend might elicit different neuronal responses depending on the initial joint configuration (e.g., starting from 180 degrees vs. 45 degrees).
  • Lesions: Damage to M1 can lead to:
    • Hemiplegia: Paralysis on one side of the body.
    • Disrupted movement of particular body parts independently.
    • Reduced movement speed, accuracy, and force.
    • Crucially, it does not entirely eliminate voluntary movement, as descending pathways from secondary motor areas and subcortical structures can still contribute.
    • Distal extremities are typically much more affected than proximal limb and trunk movements.

Secondary Motor Cortex

This region plays a crucial role in planning and programming movements.

  • Inputs: Receives input from association cortex (posterior parietal and dorsolateral prefrontal cortex - dlPFC).
  • Outputs: Projects to the primary motor cortex (M1).
  • Subdivisions: Originally conceived as two main areas (Premotor Cortex (PMC) and Supplementary Motor Area (SMA)), there are now recognized to be at least 8 distinct areas in each hemisphere, including:
    • SMA (Supplementary Motor Area)
    • Pre-SMA
    • Supplementary Eye Fields
    • Dorsal and Ventral Premotor Cortex (PMC)
    • Three small Cingulate Motor Areas (at least two identified in humans).
  • Activity Timing: Becomes active just before the initiation of voluntary movement and remains active during the movement itself.
  • Electrical Stimulation: Results in more complex, typically bilateral movements, unlike the simpler contralateral movements elicited by M1 stimulation.
  • Function: Involved in the programming of specific patterns of movement, receiving crucial input from the dorsolateral prefrontal cortex for higher-level planning.
  • Functional Distinction (PMC vs. SMA):
    • PMC: Strongly connected with the posterior parietal cortex, involved in sensory-guided actions (e.g., catching a ball).
    • SMA: Strongly connected with the medial frontal cortex, involved in internally guided goals (e.g., playing a piano from memory).

Mirror Neurons

Mirror neurons are a fascinating class of neurons involved in action understanding and imitation.

  • Discovery (Rizzolatti et al., 1990s): Discovered in the monkey premotor cortex during single-cell recordings. Researchers observed neurons that fired preferentially when the monkey performed complex hand (and mouth) actions like reaching for food or a toy. Crucially, some of these neurons also responded identically when the monkey observed a human performing the same action.
  • Definition: Mirror neurons fire both when an individual performs a particular goal-directed hand movement and when observing the same movement performed by another.
  • Goal-Directed Action: Mirror neurons respond to the goal of an action, not just the movement kinematics. For instance:
    • They respond only to goal-directed actions (e.g., grasping an object), not similar mimed actions without an object.
    • They respond to the goal even if performed with different tools requiring inverse movement sequences (e.g., grasping with normal pliers vs. reverse pliers, which require opposite finger movements).
  • Understanding Action: The system can infer observed actions even when the key action is partially obscured, provided there are enough clues to create a mental representation. However, if it's clear no object is present (e.g., behind a screen), there is no mirror neuron response.
  • Purpose of the Action: Some mirror neurons, particularly in the inferior posterior parietal cortex (PPC), respond to the purpose of an action rather than just the action itself. For example, they might fire more strongly when food is grasped to be eaten compared to when it's repeatedly grasped to be placed in a bowl.
  • Implications for Social Cognition: Mirror neurons are hypothesized to play a significant role in:
    • Action Understanding: Enabling comprehension of others' intentions and actions.
    • Social Learning: Facilitating cooperation, teaching, and imitation.
    • Language Acquisition: Potentially linking observed actions to linguistic representations.
    • Emotional Understanding/Empathy: By simulating the observed actions and associated states in one's own motor system.
  • Mirror Neurons in Humans: While direct single-cell confirmation is limited, fMRI and EEG studies suggest the existence of similar mirror networks (large-scale) in humans.
    • Motor Imagery: Imagining performing an action activates PMC, PPC, and M1. Observing an action produces weaker motor activation, primarily visual.
    • Expertise-Specific Recruitment: Viewing ballet recruits premotor and parietal mirror areas more strongly in expert ballet dancers than in non-dancers or martial arts teachers, suggesting motor expertise is more correlated with this recruitment than visual expertise.
    • Learning and Judgment: Practicing an unusual action (even blindfolded) can improve the ability to judge its goal, indicating the system's adaptability and experience-specific nature.

Association Cortex

The association cortex is crucial for preparing and deciding on movements.

Requirements for Movement:

  • Spatial Awareness: Knowing where external objects are and where parts of one's own body are located.
  • Decision-Making: Making an internal decision to initiate voluntary movement.

Posterior Parietal Cortex (PPC)

  • Inputs: Receives convergent input from multiple sensory systems (visual, auditory, somatosensory).
  • Function: Integrates these sensory inputs for:
    • Localization: Establishing the location of the body and external objects in space.
    • Attention: Directing spatial attention.
    • Dorsal Pathway: Is a key component of the dorsal stream (where/how pathway) for spatial processing.
  • Outputs: Projects to secondary motor areas, frontal eye fields (FEF), and the dorsolateral prefrontal cortex (dlPFC).
  • Subregions: Contains subregions associated with specific movements (eye, hand, or arm movements).
  • Damage: Lesions to the PPC can result in severe movement and spatial disorders:
    • Apraxia: A disorder of voluntary movement where individuals have difficulty making movements on request, but can perform the same movements fluently under natural conditions (e.g., can hammer a nail but cannot demonstrate hammering when asked). It reflects a deficit in the conscious planning of complex coordinated actions, typically presents with bilateral symptoms despite usually being caused by unilateral damage (often left PPC).
    • Contralateral Neglect: A disturbance in the ability to respond to stimuli on the side opposite to the lesion (e.g., right lesions causing neglect of the left visual field and left side of the body). Patients with contralateral neglect often exhibit anosognosia, failing to appreciate that they have a problem. The neglect is often egocentric left (relative to the body's midline), meaning a head tilt does not change the field of neglect. It also affects representation of the left side of the body.
    • Body Representation Damage (Case PJ): A 50-year-old female (PJ) with a head injury experienced loss of knowledge of her right arm and leg position unless she could see them. She perceived her limbs to drift and fade and had issues with them drifting into aisles in public transport. An MRI revealed a cyst encroaching on the cortex and subcortical white matter of the left superior parietal lobe, illustrating PPC's role in intrinsic spatial coding and body awareness, particularly when a body part is obscured from vision during planning or execution.

Dorsolateral Prefrontal Cortex (dlPFC)

  • Inputs: Receives input from the PPC.
  • Outputs: Projects to M1, secondary motor areas, and FEF.
  • Function: Critical for higher-level cognitive control of movement:
    • Evaluation: Evaluates external stimuli.
    • Decision-Making: Decides whether and how to act based on goals.
    • Initiation: Plays a key role in the decision to initiate voluntary movement.

Subcortical Control

Two major subcortical structures are vital for motor control, each with distinct roles:

  • Basal Ganglia: Primarily involved in the selection and initiation of action.
  • Cerebellum: Primarily involved in the fine-tuning and learning of action.
    Both interact with the cortex via relay through the thalamus.

Basal Ganglia

The basal ganglia are a complex, heterogeneous collection of interconnected nuclei that exert a modulatory influence on motor control.

  • Components: Key nuclei include the caudate nucleus, putamen (together forming the striatum), globus pallidus (external (GPe) and internal (GPi)), subthalamic nucleus, and substantia nigra (SN).
  • Circuitry: They form loops receiving cortical input and projecting back to the cortex via the thalamus, in addition to non-motor functions.
  • Inputs: Primarily from various cortical areas (sensory, motor, association) to the striatum.
  • Outputs: Primarily from the GPi and SN, projecting to motor and frontal cortex via the thalamus.
  • Role: Critical for the selection and initiation of appropriate actions while suppressing unwanted movements.
Basal Ganglia Circuit Model (Simplified)

The basal ganglia function through two main pathways that modulate thalamic output to the cortex:

  1. Direct Pathway: Cortex $\rightarrow$ Striatum (excitatory) $\rightarrow$ GPi (inhibitory) $\rightarrow$ Thalamus (excitatory) $\rightarrow$ Motor Cortex.
    • This pathway enhances thalamic output, thereby increasing cortical activity and promoting movement.
    • Striatum directly inhibits GPi. Less GPi inhibition of thalamus means more excitatory output from thalamus to cortex.
  2. Indirect Pathway: Cortex $\rightarrow$ Striatum (excitatory) $\rightarrow$ GPe (inhibitory) $\rightarrow$ Subthalamic Nucleus (STN) (excitatory) $\rightarrow$ GPi (excitatory) $\rightarrow$ Thalamus (excitatory) $\rightarrow$ Motor Cortex.
    • This pathway inhibits thalamic output, thereby decreasing cortical activity and suppressing movement.
    • Striatum inhibits GPe. Less GPe inhibition of STN means more excitatory output from STN to GPi. More STN excitation of GPi means more GPi inhibition of thalamus. More GPi inhibition of thalamus means less excitatory output from thalamus to cortex.

Substantia Nigra (SN) Input: Dopaminergic neurons from the SN project to the striatum and have a complex modulatory effect:

  • They turn up the Direct Pathway (enhancing thalamic output).
  • They turn down the Indirect Pathway (reducing thalamic inhibition).
  • The net effect is to enhance the overall process of movement initiation.

Temporal Dynamics: The indirect pathway is slightly delayed, leading to a brief enhancement of thalamic output followed by a balance between enhancement and inhibition.

Limitations of the Model: This circuit model is highly speculative and simplified. The actual system is far more complicated, involving numerous other inputs, outputs, connections, and complexities. Factors beyond simple discharge rates, such as neuronal firing patterns and the synchrony of higher-level activity, are likely important. Understanding of basal ganglia function largely comes from studying related disorders.

Basal Ganglia Disorders

Disorders of the basal ganglia result in characteristic motor control impairments:

  1. Parkinson's Disease (PD):
    • Pathology: Characterized by the degenerative loss of dopaminergic neurons in the substantia nigra. These are the neurons that normally enhance the direct pathway and suppress the indirect pathway.
    • Circuit Effect: Loss of dopamine leads to an increase in the indirect pathway's inhibitory effect and a decrease in the direct pathway's excitatory effect. The net result is a significant decrease in thalamic output to the motor cortex, leading to reduced cortical activity. Recent research also points to changes in neuronal firing patterns and synchrony rather than just reduced discharge rates.
    • Cardinal Features: Akinesia (slowness/difficulty initiating movement), bradykinesia (slowed movement speed), muscle rigidity, and resting tremor.
    • Other Symptoms: Reduction in spontaneous movement (hypokinesia), slow gait (often with freezing and small steps, poor arm swing), progressive slowing or freezing during movement, reduced range and scale of movement, postural instability (leading to falls), dull/weak voice (hypophonia) and slow speech, and a mask-like, unemotional facial expression.
    • Treatment:
      • Dopamine Agonists (e.g., L-Dopa): A precursor to dopamine that increases general dopamine levels in the brain. Efficacy tends to decrease over time, and it has numerous side effects (e.g., dyskinesias).
      • Chronic High-Frequency Deep Brain Stimulation (DBS): Involves implanting a device to stimulate brain regions, usually the subthalamic nucleus (STN). The high-frequency stimulation disrupts abnormal activity patterns. The exact