Motor Control and Movement

Motor Control

• Motor control involves coordinating and executing movement
• Current understanding of motor control is limited
• Prosthetics and robotics have limitations in replicating human movement precision and are expensive
• Advances in prosthetics and robotics are being made
• Prosthetics often struggle with mimicking the fine motor control of natural limbs

Motor Equivalence

• Motor equivalence refers to achieving the same movement goal using different muscle groups
• An example is using the foot to write after losing hand function
• It allows adaptation and use of alternative muscle groups to accomplish the same task
• It shows the brain's flexibility in controlling movements

Speed-Accuracy Tradeoff

• Faster movements are generally less accurate
• This is due to a reliance on visual feedback
• Typing quickly can result in more typos
• Understanding this tradeoff helps optimize performance in tasks that require either speed or accuracy

Types of Movements

• Reflexes are fast, automatic movements, such as the knee-jerk reflex
• Rhythmic motor patterns combine voluntary and reflexive movements, like walking
• Voluntary movements are purposeful and learned, like driving a car
• Phasic control involves discrete movements requiring brief muscle activation
• Tonic control involves steady muscle activation to stabilize joints

Levels of Motor Control

• The spinal cord is responsible for reflexes and basic motor patterns
• The brainstem (descending system) coordinates movement and posture
• The motor cortex handles higher-level planning and execution
• Lower levels (spinal cord) handle reflexes
• Higher levels (brain and cortex) handle more complex motor patterns
• Hierarchical organization means that lower levels handle simple tasks (e.g., reflexes)
• Higher levels manage more complex movements (e.g., planning and coordination)

Motor Neuron Location & Final Common Path

• Motor neuron cell bodies are located in the spinal cord and brainstem
• The final common path is the converging pathway where all motor control signals meet to activate motor neurons
• It is the final point where motor control signals converge and are passed on to the muscles

Motor Cortex and Tracts

• The motor cortex acts directly on motor neurons via the corticospinal tract
• Indirect pathways include the basal ganglia and cerebellum, which modulate movement
• A tract is a bundle of nerve fibers (axons) that transmit signals

Somatotopic Organization

• The body is mapped in the motor cortex
• Areas like the hands and face have larger representations due to the need for fine motor control
• These areas require fine motor control, hence they have a larger proportion of motor cortex dedicated to them

Basal Ganglia Role

• The basal ganglia regulate movement initiation and smooth execution
• Dysfunction can lead to movement disorders like Parkinson's disease
• Parkinson’s Disease causes issues like tremors and rigidity

Muscle Control

• Agonists are prime movers
• Antagonists counteract movements

Descending Motor Pathways

• The medial pathway is for posture and balance and originates in the brainstem
• The lateral pathway is for precise distal limb movements; originating in the cortex and brainstem

Corticospinal Tract

• The corticospinal tract is the largest descending fiber tract
• It controls fine motor movements in the hands and fingers
• It crosses at the pyramidal decussation (medulla-spinal cord junction)

Cerebellum & Basal Ganglia

• The cerebellum compares motor commands to actual movements for coordination
• The basal ganglia are involved in motor planning
• Dysfunction there causes involuntary movements

Motor Patterns

• Motor patterns depend on the mechanical properties of the muscle, bone, and joint
• Each movement attempts to compensate for inertia

Cerebral Cortex

• Primary cortex, lateral premotor area, and supplementary motor area each project to the spinal cord
• The supplementary motor area coordinates and plans complex movement sequences from the parietal and prefrontal association cortices

Spinal Cord Organization

• The spinal cord consists of gray matter (cell bodies) and white matter (axons)
• Motor neuron cell bodies are in the ventral horns of the gray matter
• Motor neurons controlling the same muscle form motor nuclei (motor neuron pools)

Motor Nuclei Organization - Rules

• Proximal-distal rule: Neurons for proximal (axial) muscles are medial; neurons for distal muscles are lateral
• Flexor-extensor rule: Extensor muscle neurons are ventral to flexor muscle neurons

Functional Organization

• Axial and proximal muscles (e.g., leg extensors) help with posture and balance
• Distal muscles (e.g., hands) are used for fine motor control
• Medial and lateral motor neurons have separate interneuron and descending controls
• Motor nuclei in the spinal cord are organized into medial and lateral groups
  • The medial group controls axial muscles (neck & back)
  • The lateral group controls limb muscles (distal & proximal)

Brain Stem Control of Motor Neurons

• The brain stem influences spinal motor neurons through two pathways:
  • Medial pathways (posture control) project to the ventromedial spinal gray matter and affect axial & proximal muscles
  • Lateral pathways (goal-directed movement) project to the dorsolateral spinal gray matter and affect distal limb muscles (arms & hands)

Medial Descending Pathways (Posture & Balance Control)

• Three main tracts descend in the ventral spinal cord to medial motor neurons:
  • Vestibulospinal tract: From vestibular nuclei; controls balance & posture via the vestibular labyrinth
  • Reticulospinal tract: From the reticular formation (pons & medulla); maintains posture with excitatory & inhibitory signals
  • Tectospinal tract: From the superior colliculus (midbrain); coordinates head & eye movements
• Medial pathways distribute signals broadly, ensuring stability & coordination

Lateral Pathways & Cortical Control of Movement

• Lateral pathways control distal muscles
• The rubrospinal tract (main lateral descending pathway) originates in the red nucleus (midbrain)
• Fibers descend through the medulla and terminate in the dorsolateral spinal cord
• They control distal limb muscles; essential for fine motor skills (e.g., grasping, manipulating objects)

Motor Cortex Influence on Spinal Motor Neurons

• The motor cortex controls complex movements & precision via:
  • The Corticobulbar tract controls cranial motor nerves (facial muscles)
  • The Corticospinal tract controls trunk & limb muscles directly and via brain stem pathways

Corticospinal Tract (Largest Descending Pathway)

• The corticospinal tract contains ~1 million axons
• Half originate in the primary motor cortex (area 4, precentral gyrus)
• Additional origins:
  • Premotor area (area 6) plans movements
  • Somatosensory cortex (areas 3, 2, 1) mediates sensory input
• Pathway: Internal capsule → midbrain → pons → medullary pyramid → spinal cord
• Pyramidal decussation (crossing):
  • ~75% cross to form the lateral corticospinal tract which controls distal muscles
  • Uncrossed fibers form the ventral corticospinal tract, which controls axial muscles bilaterally

Evolution of Cortical Control

• Corticospinal & corticobulbar tracts evolved in mammals to regulate sensory-motor integration
• Higher mammals develop direct corticospinal connections for finer control of distal limb muscles
• Primates show progressive refinement, with more direct cortical control over spinal motor neurons, peaking in humans

Anatomy of the Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord
• Brain
  • Cerebrum: The largest part, divided into two cerebral hemispheres
  • Brainstem: Consists of (from top to bottom): Midbrain, Pons, Medulla Oblongata (technically part of the brainstem)
  • Cerebellum: Connected to the brainstem, involved in coordination and balance
• Spinal Cord: Extends from the brainstem, transmitting signals between the brain and body

Embryonic Development

• The forebrain develops into the cerebrum
• The midbrain remains as the midbrain
• The hindbrain develops into the pons and medulla oblongata

Peripheral Nervous System (PNS)

• Nerves extend throughout the body, carrying neuron axons
• Ganglia are clusters of neuron cell bodies attached to nerves
• Spinal Nerves: 31 pairs exit through the spine, part of the PNS
• Cranial Nerves: 13 pairs, exit directly from the brain (through the cranium)
• Spinal Nerve Roots:
  • Anterior (Ventral) Roots contain motor (efferent) fibers
  • Posterior (Dorsal) Roots contain sensory (afferent) fibers
• Mixed nerves carry both afferent (sensory) and efferent (motor) signals

Physiology of the Nervous System

• Afferent pathways carry sensory information to the CNS
• Efferent pathways carry motor commands from the CNS to muscles and glands