Cyclic AMP opens potassium channels. Sodium channels open. Calcium enters the cytoplasm. The movement of actin causes muscle fiber to shorten. 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.