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12.1 Brain and spinal cord of movement control

Hierarchy of control levels in central motor system

  • The highest level: areas of neocortex and basal ganglia of the forebrain
  • The middle level: represented by the motor cortex and cerebellum
  • The lowest levels: represented by the brain stem and spinal cord

Movement controlled by brain and spinal cord

  • Key components for arm movement
    • the joints
    • skeletal muscles
    • nerves
  • Each muscle fiber is innervated by a single motor axon
    • organised by neuromuscular junction
    • Each motor neuron may synapse with multiple muscle fibers

  • Two types of neurons in motor control

    • Lower motor neurons
      • the motor neurons whose cell bodies are located in the ==anterior horn of the spinal cord or brainstem cranial nerve nuclei==, and whose axons project to make connections with muscles at the neuromuscular junctions
    • Upper motor neurons
      • controlled by motor region of the cerebral cortex or in the brainstem and carry motor information down to the lower motor neurons

  • Motor unit and motor pool

    • Alpha motor neurons
      • Directly trigger the generation of force by muscles
      • One alpha motor neurons + all the muscle fibers --> motor unit, elementary component of motor control
      • ==Motor neuron pool==: The collection of alpha motor neurons that innervates a single muscle
    • Gamma motor neurons
      • Intrafusal muscle fibers (梭内肌纤维)
      • The tension in intrafusal can control the sensitivity of muscle spindles
      • independently targeted by descending pathways, thereby regulating muscle spindle sensitivity
    • Alpha-Gamma coactivation during voluntary movements
      • To ensure that the muscle spindles remain taut and can accurately detect changes in muscle length

  • Spindle can detect the changes in muscle length, which is a part of somatic sensory system and proprioception (how our body is positioned and moving in space)

Myotatic reflex (Stretch reflex)

  • the stretch reflex is a monosynaptic reflex
    • regulates muscle length, through neuronal stimulation at the muscle spindle
  • the knee-jerk reflex
    • tap the tendon beneath the kneecap --> stretches the quadriceps muscle of your thigh --> contracts and causes the leg to extend

Flexor and extensors

  • Flexor muscles:

    • Decrease the angle between bones at a joint
    • Biceps brachii(肱二头肌): Flexes the elbow

  • Extensor muscles:

    • increase the angle between bones at a joint (straightening the elbow or knee)
  • Open arm --> flexors relax and extensors contract
  • Close arm --> extensors relax and flexors contract

Reciprocal inhibition

  • When you move a joint, two groups of muscles work together: agonists (the prime movers) and antagonists (the opposing muscles). For smooth movement, when one contracts, the other must relax.
  • Inhibitory interneurons would prevent the contraction of antagonist muscle (flexor)

Withdrawal reflex (flexor reflex) and crossed-extensor reflex

  • The withdrawal reflex (flexor reflex) is your body's immediate response to pain:

    • 类似于条件反射
    • Step on tack --> pain fibers send signal to spinal cord
    • --> interneurons branch to different spinal cord segments
    • --> motor fibers in several segments are activaated
    • --> lift foot off to tack

  • The crossed-extensor reflex works alongside the withdrawal reflex to maintain balance

    • While one limb withdraws from pain, the opposite limb must support your body weight
    • The same apin signal would triggers withdrawal crosses to the opposite side of spinal cord
    • This prevents you from falling when you suddenly lift one foot

Brian mechanisms in controlling motor programs

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- These tracts are organized in two major functional systems: 1. Pyramidal System (Corticospinal tracts) - Direct pathways from cortex to spinal motor neurons - Responsible for voluntary, skilled movements - Phylogenetically newer system - Damage results in specific motor deficits 2. Extrapyramidal System (Rubrospinal and Tectospinal tracts, among others) - Indirect pathways involving subcortical nuclei - Controls posture, muscle tone, and automatic movements - Phylogenetically older system - Provides background support for pyramidal system