The myosin crossbridges move, shortening the muscle fiber. Myofibrils extract energy provided by the mitochondria. The Golgi tendon organ detects muscle length. The monosynaptic stretch reflex initiates limb withdrawal in response to pain. A polysynaptic reflex can be demonstrated by tapping the patellar tendon.
Muscle spindles are sensitive to changes in blood levels of calcium.
The principal cortical input to the primary motor cortex is temporal cortex. The fact that monkeys with bilateral pyramidal tract lesions had no trouble opening their hands when climbing but had difficulty opening their hands when eating indicates that these monkeys can still successfully forage for food. The reticulospinal tracts controls movements of the fingers and hands. Apraxia is the inability to properly execute a learned skilled movement. Limb apraxia is assessed by asking a patient to perform a movement.