simple motor control

Question 1

what are 3 types of muscle

Answer 1

• smooth
• cardiac
• striate / skeletal — allows fast, voluntary movements

Question 2

muscle fibre structure

Answer 2

• skeletal muscles are made of muscle fibers
  
  • each muscle fiber receives input from a single spinal motor neuron
• spinal motor neuron cell bodies live in the ventral horn of the spinal cord
  
  • one spinal neuron may contact several fibers.
• all muscle fibers supplied by one motor neuron are called a motor unit
• each time the motor neuron fires an action potential, the fibers perform a short contraction (twitch)

Question 3

muscle fiber organization & function

Answer 3

• skeletal muscle is called striate because under the microscope it can be seen to contain alternating “stripes” or actin and myosin filaments.
• nerve fiber activation causes action potentials along the muscle fiber membrane, which triggers Ca++ release that, in turn, allows myosin proteins to latch on to actin and to “ratchet” themselves along the actin fibers, leading to contraction
• if there are no further inputs, Ca++ is reabsorbed, myosin lets go of the actin and the muscle relaxes

Question 4

the myosin cycle

Answer 4

1. myosin binds to actin
2. myosin head bends backward, releasing ADP and pulling itself forward
3. ATP binds to myosin, causing release from actin
4. myosin uses energy from ATP hydrolysis to stretch itself, ready to undergo new binding and pulling cycle

Question 5

feedback from muscle and reflexes

Answer 5

• in addition to normal muscle fibers, skeletal muscles contain specialized fibers called spindles which incorporate stretch receptors
• stretch receptors send signals through fast-conducting A-beta fibers back to the spinal cord.
• muscle spindles can also contract
  
  • their contractions are controlled through so-called “gamma” motor neurons. in contrast, the fibers controlling the contraction of ordinary muscle fibers are called “alpha” motor neurons.
• in the spinal cord, muscle spindle afferents can connect straight through to alpha motor neurons to form monosynaptic reflex arcs.