8.2

• Motor units are the smallest units of motor activity.
• Acetylcholine, which is released by motor neurons at neuromuscular junctions, activates the motor end-plate on each muscle fiber and causes the fiber to contract.
• All of the motor neurons that innervate the fibers of a single muscle are called its motor pool.
• Flexors act to bend or flex a joint.
• Extensors act to straighten or extend a joint.
• Any two muscles whose contraction produces the same movement, be it flexion or extension, are said to be synergistic muscles
• Those muscles that act in opposition, like the biceps and the triceps, are said to be antagonistic muscles.
• Activation of a muscle can increase the tension that it exerts on two bones without shortening and pulling them together.

  isometric contraction
• Activation of a muscle can increase the tension that it exerts on two bones by shortening and pulling them together.

  dynamic contraction
• Golgi tendon organs are embedded in the tendons that respond to increases in muscle tension.
• Muscle spindles respond to changes in muscle length, but they do not respond to changes in muscle tension.
• Golgi tendon organs are embedded in the tendons, which connect each skeletal muscle to bone
• Muscle spindles are embedded in the muscle tissue itself.
• The resulting leg extension is called the patellar tendon reflex.
• A reflex elicited by a sudden external stretching force on a muscle.

  stretch reflex
• The sudden stretch of the thigh muscle stretches its muscle-spindle stretch receptors, which in turn initiate a volley of action potentials carried from the stretch receptors into the spinal cord by spindle afferent neurons via the dorsal root.
• We are sure that, at one time or another, you have touched something painful—a hot pot, for example—and suddenly pulled back your hand. This is a withdrawal reflex.
• Reciprocal innervation is an important principle of spinal cord circuitry. It refers to the fact that antagonistic muscles are innervated in a way that permits a smooth, unimpeded motor response: When one is contracted, the other relaxes.
• Movements produced by cocontraction are smooth, and they can be stopped with precision by a slight increase in the contraction of the antagonistic muscles.
• The inhibition produced by these local feedback circuits is called recurrent collateral inhibition 


  The small inhibitory interneurons that mediate recurrent collateral inhibition are called Renshaw cells.
• According to this view, all but the highest levels of the sensorimotor system have certain patterns of activity programmed into them, and complex movements are produced by activating the appropriate combinations of these programs.

  A hierarchy of central sensorimotor programs
• The fact that the same basic movement can be carried out in different ways involving different muscles is called motor equivalence.
• According to the response-chunking hypothesis, practice combines the central sensorimotor programs that control individual responses into programs that control sequences (chunks) of behavior.