MovmENt

• Sensorimotor system


  • Hierarchically organized
  • Parallel, functionally segregated, hierarchical system
• Levels of sensorimotor hierarchy

  • Association cortex/company president (highest level)
  • Primary motor cortex/workers (lowest level)
• Higher levels of hierarchy


  Left free to perform more complex functions
• Sensory systems vs sensorimotor system

  • Sensory systems - information mainly flows up
  • Sensorimotor system - information mainly flows down
• Motor output is guided by sensory input
• Efficient companies


  Monitor the effects of their own activities and use this information to fine-tune their activities
• Sensory feedback mechanisms


  • Eyes
  • Organs of balance
  • Receptors in skin, muscles, and joints
• Ballistic movements


  Brief, all or none, high speed movements not normally influenced by sensory feedback
• Many adjustments in motor output are controlled unconsciously by lower levels of sensorimotor hierarchy without involvement of higher levels
• Motor learning


  1. Initial stages - individual responses performed under conscious control

  2. After practice - individual responses organized into continuous integrated sequences adjusted by sensory feedback without conscious regulation
• Sensorimotor association cortex


  • Integrates spatial information about body and external objects
  • Damage can result in deficits in perception, memory, reaching, grasping, eye movement, attention, apraxia, contralateral neglect
• Dorsolateral prefrontal association cortex


  • Receives input from posterior parietal cortex, sends output to secondary motor cortex, primary motor cortex, frontal eye field
  • Involved in decision to initiate voluntary movements
• Areas of secondary motor cortex

  • Supplementary motor area
  • Premotor cortex
  • Cingulate motor areas
• Mirror neurons


  Neurons that fire when an individual performs a particular goal-directed hand movement or observes the same movement performed by another
• Primary motor cortex


  • Located in precentral gyrus
  • Most dedicated to controlling hands and mouth
  • Receives sensory feedback from muscles and joints
  • Plays main role in initiating body movements
• Cerebellum


  • Contains more than half of brain neurons
  • Receives information from primary and secondary motor cortex
  • Plays major role in motor learning of movement sequences
  • Damage results in deficits in movement control, balance, gait, speech, eye movement, and difficulty learning new motor sequences
• Basal ganglia


  • Less neurons than cerebellum, more complex
  • Perform modulatory function, involved in cognitive functions, participate in habit learning
• Motor pathways from primary motor cortex

  • Dorsolateral region of spinal cord
  • Ventromedial motor pathways
• Sensorimotor system requires good communication from cortex to spinal motor circuits to muscles
• Motor units


  Smallest units of motor activity, comprising a single motor neuron and all the muscle fibers it innervates
• Types of skeletal muscle fibers

  • Fast muscle fibers - relax and contract quickly, generate great force, fatigue quickly
  • Slow muscle fibers - slower and weaker, capable of more sustained contraction
• Types of muscle contractions

  • Isometric - increase tension without shortening
  • Dynamic - shorten and pull bones together
• Golgi tendon organs


  Respond to increase in muscle tension, provide CNS with information about muscle tension, serve protective function
• Muscle spindles


  Respond to changes in muscle length, provide CNS with information about muscle length, innervated by intrafusal motor neurons to adjust intrafusal muscle length
• Stretch reflex


  1. Sudden stretch of muscle activates spindle receptors

  2. Signals carried to spinal cord

  3. Initiates reflex muscle contraction to counteract stretch
• Withdrawal reflex


  Reflex response to painful stimulus, first recorded in motor neurons of arm
• Reciprocal innervation


  Simultaneous excitation of agonist muscles and inhibition of antagonist muscles for quickest movements
• Cocontraction


  Simultaneous contraction of agonist and antagonist muscles, produces smooth movements that can be precisely stopped
• Recurrent collateral inhibition


  Inhibition produced by local feedback circuits
• Sensorimotor programs must integrate visual, somatosensory, and balance information
• The sensorimotor system exhibits motor equivalence - the same basic movement can be carried out in different ways
• Hierarchical organization of sensorimotor system

  • Higher levels (e.g. association cortex) initiate and set goals, lower levels (e.g. brainstem, spinal cord) execute detailed motor patterns
  • Lower levels have preprogrammed patterns that can operate autonomously based on sensory feedback
  • Allows efficient motor control with higher levels focused on planning and lower levels on execution
• Importance of hierarchical arrangement

  • Enables efficient motor control, flexibility and adaptability, speed and precision, learning and plasticity