Motor Systems and Motor Control Notes

Motor Systems & Motor Control

Motor Control

Cortical Areas
  • BA4, BA6 (Brodmann areas 4 and 6) are involved in motor control.
Subcortical Areas
  • Cerebellum
  • Subthalamic Nucleus
  • Globus Pallidus
  • Substantia Nigra
  • Striatum

Initiating a Motor Sequence

  • Posterior Sensory Cortex: Sends goals for movement.
  • Prefrontal Cortex: Plans the motor sequence.
  • Premotor Cortex: Sequences the motor sequence.
  • Motor Cortex: Executes the motor sequence.

Functions of the Motor System During a Hand Movement

(This section is a general heading indicating the following content pertains to motor system functions during hand movements.)

Organization of the Motor System

  • The term "motor system" typically refers to:
    • Parts most directly involved in producing movement.
    • Spinal-cord neural circuits that send commands to muscles through peripheral nerves.

Penfield’s Motor Homunculus

  • Precentral Gyrus: Also known as the primary motor cortex or M1.
    • Stimulating different areas of the precentral gyrus produces movement in different body areas.
  • Motor Homunculus: "Little Human"
    • Represents a disproportion in the relative sizes of body parts compared to their actual sizes.
    • Larger areas in the homunculus correspond to increased motor control.

What Do Motor Maps Represent?

  • Possible representations:
    • Map of musculature controlling individual muscles.
    • Map of movements controlling multiple muscle groups to generate specific movements.
    • Map of intentions coding for the goal of movements.

Penfield's Theory

  • Motor Cortex = Map of Muscles: Each part of the homunculus controls muscles in that part of the body.
  • Information from other cortical regions is sent to the motor homunculus, which activates body muscles.
  • Improved Electrical Stimulation (ES) and imaging techniques provide a more detailed view than Penfield's initial proposal.
    • ES of different sites can initiate specific movements, such as finger or hand/arm movements.

MI Neurons and Movement Direction

  • Apostolos Georgopoulos: Discovered that M1 neurons are tuned to directions of movements.

Population Code

  • Direction is coded by the coordinated activity of populations of cells.

Coordinated Movements vs. Muscle Activity

  • Microstimulation of M1 sites near the middle of the precentral gyrus results in movement.
  • Movement and muscle activity depend on the start and end point of the movement.

Topography of Behaviorally Relevant Movements

  • Experiments by Graziano and colleagues (2005) in macaque monkeys revealed the topography of behaviorally relevant movements in M1.

Representations of Movements in Motor Cortex

  • Early Idea: Each part of the homunculus controls muscles in that part of the body, and information from other cortical regions is sent to the motor homunculus to activate muscles.
  • More Recent Experiments: Suggest that the motor cortex represents a repertoire of fundamental movement categories (Michael Graziano, 2006) rather than individual muscles.

Motor Cortex Specialization

  • The motor cortex is specialized for the control of movements rather than the control of individual muscles.
  • Three important aspects for motor cortex organization:
    • Part of the body to be moved.
    • Spatial location to which the movement is directed.
    • Movement’s function.
  • Each representation of the body is responsible for somewhat different movements.
    • Whole-body movements are represented in the premotor cortex.
    • More discrete movements are represented in the motor cortex.

Organization of Motor and Premotor Cortex

  • MI (Primary Motor Cortex): Involved in more elementary movements, including hand and mouth movements.
  • PMA/SMA (Premotor/Supplementary Motor Area): Contains a 'lexicon' or repertoire of movement categories.

Basic vs. Complex Movements

  • Basic movements encoded in MI can be extended to more complex ones in the Premotor Cortex (PMC).

Organization of the Monkey Premotor Cortex

  • Organization includes:
    • Emotional somatic behavior
    • Selecting and organizing purposeful movements of limb and face
    • Voluntary gaze shifts
    • Broca's area
    • Mirror neuron system

Mirror Neurons

  • Mirror neurons fire when we see others make a movement, but don’t respond well to pictures or videos of movements
    • Encode a complete action and are broadly tuned.
    • The target of the action is more important than the details of the action.
    • Can "fill in the blanks" when part of a movement is absent.
    • Actions can be used for imitating and for understanding others’ actions.
    • Related to symptoms of some disorders, such as the absence of empathy in autism.

MI Neurons and Planning & Execution

  • Edvard Evarts: MI neurons are involved in both planning and execution of movements.

Evidence for a Human Movement Lexicon

  • When lesions occur to the thumb region, weakness appears in all the fingers.
  • The movement lexicon is not entirely learned but is part of a pre-wired movement lexicon.
  • Encoded as basic movement patterns modified based on situational demands.

The Movement Lexicon in Humans

  • Suggests that the repertoire available to the premotor cortex is more complex than that available to the motor cortex.
  • Premotor cortex plays a greater role in organizing whole-body movements, while the motor cortex controls specific acts.

Learning Complex Motor Skills

  • Entails learning how to use pre-organized movement patterns.
  • Example: Coordination of the whole body to throw a ball to a target involves:
    • Stepping movements of the leg.
    • Constant adjustment of the trunk for balance.
    • Throwing movement with the arm, which is an elaboration of walking movements.

Connections of the Premotor Cortex

  • Intensive reciprocal connections with MI.
  • Direct projections to the brainstem and spinal cord (30%!).
  • Extensive inputs from multisensory inferior and superior parietal areas, as well as from the rostral Prefrontal Cortex (PFC) (intentions, motivation).

Visual-Parietal-Motor Connections

  • The motor cortex is not the only region from which movements can be evoked.
  • Similar movements can be elicited by electrically stimulating the parietal cortex.
  • Parietal topography mirrors the motor homunculus.
  • Topographical regions of the motor cortex have dense anatomical connections with matching parietal regions.

Dual Channels for Reaching and Grasping

  • Dorsal channel for reaching and ventral channel for grasping.

The Mirror Neuron System

  • Includes frontal and parietal areas.

Brain Stem Centers for Motor Control

  • Besides direct corticospinal pathways, there are about 26 pathways from the brainstem to the spinal cord.
  • These pathways mostly pertain to:
    • Posture and balance.
    • Eating and drinking.
    • Sexual behaviors.
    • Control of the autonomous system.

Stimulation of Brain Stem

  • Walter R. Hess (Suisse physiologist), 1957: Stimulation of the brainstem can elicit nearly any innate behavior in a freely moving animal.
    • Movements start abruptly with stimulation onset and end as abruptly when turned off.
    • The vigor of movement is dependent on the stimulation amplitude.
    • Induced a resting cat to suddenly leap up with arched back and erect hair, as if frightened by an approaching dog.
    • Also induced head turning, walking, or other movements, as well as complex movement sequences like grooming behavior.

Upper Motor Neuron (MN) Centers in the Brain Stem

  • Example: Stabilize lower body muscles (reflexive or anticipative) when performing or preparing voluntary arm movements.
  • Posture and gait.
  • Visual gaze.
  • Support reflexive motor activities and work in coordination with motor cortical control of voluntary actions.

Maintenance of Body Posture

  • Maintained by the Reticular Formation (RF) and Vestibular Nuclei (VN).

The Descending Motor System

  • Corticobulbar Tracts: Neocortex sends major projections to the brainstem.
    • Controls facial muscles and takes part in controlling facial movements.
  • Corticospinal Tracts (Pyramidal Tracts): Neocortex sends major projections to the spinal cord.
    • Controls movement of the limbs and body.

M1 Layer 5 Pyramidal Cells

  • Have especially large cell bodies of upper motor neurons (UMN) to serve extremely long axons.

Afferent and Efferent Pathways

  • *Afferent pathway = sensory pathway
  • *Efferent pathway = motor pathway

The Corticospinal Tract

  • Connects the cortex with the spinal cord through the pyramidal protrusion.
  • Lateral Corticospinal Tract: Crosses the midline in the brainstem for movements of limbs and digits (~95% of motor axons).
  • Ventral (Anterior) Corticospinal Tract: Controls muscles of the midline of the body (no crossing).
  • Axons mainly from MI, but also from PMC, terminate on interneurons and lower motor neurons.

Relations Among Interneurons, Motor Neurons, and Muscles in the Spinal Cord

  • Lower motor neurons (LMN) also called alpha motor neurons.

Muscular Coordination by the Spinal Cord

  • Limb muscles are arranged in pairs.
    • The extensor muscle moves the limb away from the trunk of the body.
    • The flexor muscle moves the limb toward the trunk.

Motor Unit Pools

  • No simple 1:1 mapping of upper motor neurons to spinal lower motor neurons!
  • LMNs are clustered in columnar spinal nuclei ('motor neuron pools' or 'motor nuclei'), all together innervating one single muscle.
  • Each individual muscle fiber is innervated by only one MN, but any MN can innervate several muscle fibers (= motor unit).
  • Difference between fine and course movements is due to the number of fibers innervated.
  • LMN axons are organized in columns, originating in many different spinal cord segments.
  • UMN axons synapse with LMN in those columns and thus also different segments.

Control of Muscle Force

I. MN Firing Rate
  • The rate at which the motor neurons are firing determines the force of the muscles.
II. Size Principle
  • Recruitment of additional Motor Units (MUs).
  • Size of MU is proportional to the number of fibers innervated.
  • Small MU is recruited first, and large MU is recruited last; allows fine-tuning under low loads.
    *MU = Motor Neuron + Muscle Fiber

Voluntary Force

  • Both the number of active MUs and the firing rate increase with voluntary force.

Different Kinds of Motor Units

Small (S) Motor Units
  • Comprise small ('red') muscle fibers (rich in myoglobin, mitochondria, and capillary beds).
  • Contract slowly, with relatively small forces, and are resistant to fatigue.
  • Important for activities requiring sustained muscular contractions.
Fast Fatigable (FF) Motor Units
  • Larger MN and larger muscle fibers, generate more force, but have sparse mitochondria.
  • Important for brief actions requiring large forces.
Fast Fatigue-Resistant (FR) Motor Units
  • Intermediate size and not as fast as FF.
  • Generate twice the force of S and are resistant to fatigue.

Recruitment in Cat Gastrocnemius Muscle

Recruitment in the cat gastrocnemius muscle occurs under different behavioral conditions, demonstrating the flexibility of the motor system.

Signs and Symptoms of Upper and Lower Motor Neuron Lesions

Upper motor neuron lesions result in different symptoms than lower motor neuron lesions, helping localize the site of damage.

Neuroprosthetics

  • Juliano Pinot, a paraplegic, walked onto the soccer pitch and performed the honorary kickoff for the World Cup 2014.
  • Aided by the Walk Again Project, led by Miguel Nicolelis (2012).
  • Highlights the success of research in the field of Neuroprosthetics: developing computer-assisted devices to replace lost body function.