Chapter 7: Movement

• Why do we have a brain? Plants survive just fine without one. So do sponges, which are animals, even if they don't act like them. But plants don't move, and neither do sponges.
• A sea squirt (a marine invertebrate) has a brain during its infant stage, when it swims, but when it transforms into an adult, it attaches to a surface, becomes a stationary filter feeder, and digests its own brain, as if to say, "Now that I've stopped traveling, I won't need this brain thing anymore."
• Ultimately, the purpose of a brain is to control behaviors, and behaviors are movements.
• Vertebrate muscles

  • Smooth muscles that control the digestive system and other organs
  • Skeletal or striated muscles that control movement of the body in relation to the environment
  • Cardiac muscles that control the heart
• Neuromuscular junction


  A synapse between a motor neuron axon and a muscle fiber
• In skeletal muscles, every axon releases acetylcholine at the neuromuscular junction, and acetylcholine always excites the muscle to contract.
• A deficit of acetylcholine or its receptors impairs movement.
• Antagonistic muscles


  Opposing sets of muscles that move a limb back and forth, e.g. flexors and extensors at the elbow
• Fish muscles

  • Red muscles produce the slowest movements, but they do not fatigue
  • White muscles produce the fastest movements, but they fatigue rapidly
  • Pink muscles are intermediate in speed and rate of fatigue
• At cold temperatures, fish rely more on white muscles, maintaining their speed but fatiguing faster.
• Human and mammalian muscle fibers

  • Fast-twitch fibers with fast contractions and rapid fatigue
  • Slow-twitch fibers with less vigorous contractions and no fatigue
• Slow-twitch fibers are aerobic, using oxygen during their movements, while fast-twitch fibers are anaerobic, building up an oxygen debt.
• People vary in their percentages of fast-twitch and slow-twitch fibers, based on both genetics and training.
• Competitive sprinters have more fast-twitch fibers and other adaptations for speed instead of endurance.
• Proprioceptors


  Receptors that detect the position or movement of a part of the body
• Stretch reflex

  1. Sensory nerves of muscle spindles send action potentials to motor neuron in spinal cord

  2. Motor neuron sends action potentials to extensor muscle

  3. Contracting extensor muscle straightens leg, adjusting for a bump
• Golgi tendon organs


  Proprioceptors that respond to increases in muscle tension, acting as a brake against excessive contraction
• Proprioceptors not only control important reflexes but also provide the brain with information.
• Stretch reflex

  1. Muscle spindle detects stretch

  2. Spinal cord sends signal to contract muscle reflexively
• Muscle spindle

  • Receptor parallel to the muscle that responds to a stretch
• Golgi tendon organ


  • Acts as a brake or shock absorber to prevent a contraction that is too quick or extreme
• Muscle is stretched

  Muscle spindle sends message to motor neuron in spinal cord, which sends message back to muscle causing contraction
• Proprioceptors not only control important reflexes but also provide the brain with information
• Illusion to demonstrate proprioceptors

  • Drop a small, dense object and a larger, less dense object that weighs the same onto someone's hand
  • Smaller object will feel heavier
• The brain reacts to sensations that differ from its expectations or predictions
• Ballistic movement


  Executed as a whole, cannot be altered once initiated
• Corrected movement

  Subject to feedback correction, can be readjusted
• Central pattern generators

  1. Neural mechanisms in spinal cord that generate rhythmic patterns of motor output

  2. Examples: wing flapping in birds, fin movements in fish, "wet dog shake"
• Motor program


  Fixed sequence of movements
• Motor programs

  • Mouse grooming sequence
  • Wing extension and flapping in chickens
• Ostriches, emus, and rheas have lost the genetic programming for flight movements and do not flap their wings when dropped
• Yawning and certain facial expressions are examples of built-in motor programs in humans
• Primary motor cortex

  • Electrical stimulation elicits movements
  • Axons extend to brainstem and spinal cord, which generate impulses that control muscles
  • In humans and other primates, some axons go directly from cerebral cortex to motor neurons, giving greater dexterity
• Cerebral cortex


  Important for complex actions like talking or writing<|>Has much less control over coughing, sneezing, gagging, laughing, or crying
• Primary motor cortex is active when imagining movements, remembering movements, or understanding verbs related to movements
• Somatosensory cortex and motor cortex

  • Aligned, with motor area responsible for moving a body part aligned with somatosensory area responsible for feeling that body part
  • Communication between sensing and moving is essential
• Planning a movement

  1. Posterior parietal cortex monitors body position relative to world

  2. Prefrontal cortex and supplementary motor cortex important for planning and organizing rapid movement sequences

  3. Premotor cortex active immediately before movement, receives information about target and body position/posture

  4. Prefrontal cortex stores sensory information relevant to movement and considers probable outcomes
• Stimulation of motor cortex can elicit complex movement patterns, not just muscle twitches
• Motor cortex orders an outcome and leaves it to spinal cord and other areas to find the right combination of muscles
• Antisaccade task

  Requires looking the opposite direction from a moving stimulus, tests ability to inhibit unwanted action and substitute different one<|>Ability improves with prefrontal cortex maturation, deteriorates in old age