Untitled Quiz

Motor Cortex: Initiating Movement

• Betz cells, located in layer 5 of the primary motor cortex (M1), are pyramidal cells that play a critical role in movement initiation.
• Two primary sources of input to Betz cells:
  • Cortical areas
  • Thalamus
• Many neurons in M1 encode force and direction of movement, although their tuning is broad.
• A single neuron's activity reflects a "vote" toward a specific direction; the collective activity of many neurons determines the actual direction of movement.
• Population vectors represent the collective activity of neurons, providing a comprehensive representation of movement direction.

Malleable Motor Maps

• Microstimulation of the M1 cortex typically triggers whisker movement.
• However, if the nerve supplying whisker muscles is severed, microstimulation will instead lead to forelimb or eye movements, highlighting the plasticity of motor maps.

Cerebellum: Fine-Tuning Movement

• Cerebellum is crucial for the coordinated execution of voluntary movements.
• Plays a significant role in motor learning, creating and refining motor programs to ensure smooth movement.

The Cerebellar Loop

• The corticopontocerebellar projection is a massive pathway between the motor cortex and the cerebellum, twenty times larger than the pyramidal tract.
• Axons from layer V pyramidal cells in the sensorimotor cortex form the majority of this projection.

Spinal Cord Anatomy

• Resembles a butterfly or the letter "H" in cross-section.
• Three areas of gray matter on each side of the cord, mirror images:
  • Dorsal horns: interneurons receiving somatic and visceral sensory input.
  • Ventral horns: some interneurons and somatic motor neurons.
  • Lateral horns (thoracic and superior lumbar regions only): sympathetic neurons.
• Gray commissure connects the two halves of the gray matter, enclosing the central canal.
• Ventral roots: carry motor neuron axons exiting the spinal cord.
• Dorsal roots: carry sensory input to the spinal cord.
• Dorsal root (spinal) ganglia: contain cell bodies of sensory neurons.
• Spinal nerves: formed by the fusion of dorsal and ventral roots.

The Stretch Reflex

• Known as the knee-jerk or myotatic reflex.
• Usually monosynaptic.
• The brain sets a muscle's length, and the stretch reflex ensures it stays at that length, contributing to muscle tone and posture.

The Flexor Reflex

• Activated by painful stimuli, for example, stepping on a nail.
• Also known as the flexion, withdrawal, or "ouch" reflex.
• Always polysynaptic.

The Golgi Tendon Reflex

• Also known as the autogenic inhibition or inverse myotatic reflex.
• Always polysynaptic.
• Protects muscles from excessive tension.

Brain Control of Movement

• Hierarchical control of movement:
  • Highest level: strategy
  • Middle level: tactics
  • Lowest level: execution
• The sensorimotor system utilizes sensory information at all levels of motor control.

Descending Motor Pathways

• Deliver efferent impulses from the brain to the spinal cord.
• Two main groups:
  • Direct pathways: pyramidal tracts
  • Indirect pathways: all other pathways
• Two types of motor neurons involved:
  • Upper motor neurons: pyramidal cells in the primary motor cortex.
  • Lower motor neurons: ventral horn motor neurons, innervate skeletal muscles.

Corticospinal Pathway (Direct)

• Impulses from pyramidal neurons in the precentral gyri travel through the pyramidal (lateral and ventral corticospinal) tracts.
• They descend directly to the spinal cord, synapsing at the end of the tract.
• In the spinal cord, axons synapse with either interneurons (lateral tract) or ventral horn motor neurons (ventral tract).
• Regulates fast and fine movements.

Indirect Pathways

• Also known as multineuronal pathways, complex and multisynaptic.
• Includes brain stem motor nuclei and all motor pathways besides the pyramidal pathways.
• Regulate:
  • Axial muscles, maintaining balance and posture.
  • Muscles controlling coarse limb movements.
  • Head, neck, and eye movements tracking objects.
• Major indirect pathways:
  • Reticulospinal and vestibulospinal tracts: regulate balance by adjusting postural muscle tone.
  • Rubrospinal tracts: control flexor muscles.
  • Tectospinal tracts: originate in the superior colliculi, mediating head movements in response to visual stimuli.

Planning of Movement by the Cerebral Cortex

• Motor cortex: areas 4 and 6 of the frontal lobe.
  • Area 4: primary motor cortex (M1).
  • Area 6: "higher" motor area, referred to by Penfield.
    • Lateral region: premotor area (PMA).
    • Medial region: supplementary motor area (SMA).
    • Motor maps in PMA and SMA have similar functions but innervate different muscle groups.

Posterior Parietal and Prefrontal Cortex Contributions to Motor Control

• These areas represent the highest levels of motor control.
• They contribute to:
  • Decisions about actions and their consequences.
  • Area 5: receives input from areas 3, 1, and 2.
  • Area 7: receives input from higher-order visual cortical areas, such as MT.

Higher-Level Motor Control

• Anterior frontal lobes: abstract thought, decision-making, and anticipating consequences of actions.
• Area 6: converts actions into signals specifying how they will be performed.