Final answer:
Muscle contraction involves the sliding filament model where calcium release leads to cross-bridge formation between actin and myosin, powered by ATP. The strength of contraction is controlled by the number of motor units recruited. A muscle twitch consists of three phases: latent, contraction, and relaxation.
Step-by-step explanation:
Muscle Contraction at the Muscular Unit Level
The basis of muscular contraction at the level of the muscle unit involves several key components and mechanisms. In the sliding filament model, the neurotransmitter acetylcholine (ACh) binds at the neuromuscular junction triggering depolarization, which then causes an action potential to travel along the sarcolemma. This action potential triggers the release of calcium from the sarcoplasmic reticulum (SR). Calcium ions bind to the troponin-tropomyosin complex on the actin filaments, causing tropomyosin to move away from the myosin-binding sites on actin. This allows the myosin heads to form cross-bridges with actin by attaching to these exposed binding sites.
During the contraction phase, ATP is used to power the 'power stroke' where the myosin heads pull the actin filaments towards the center of the sarcomere, resulting in muscle fiber shortening and contraction. The process is reversed during relaxation where ATP helps detach myosin heads from actin, and the muscle fiber lengthens back. A motor unit consists of a motor neuron and the muscle fibers it innervates, and the number of activated motor units can vary the strength of contraction, a process known as recruitment. Notably, muscle tone refers to the constant, low-level contractions that maintain posture and stability.
There is a sequential phase to a single muscle contraction which is referred to as a twitch: a latent period, a contraction phase, and a relaxation phase. A graded muscle response allows variation in muscle tension, and successive stimuli can add up to produce a stronger contraction through summation. Continuous contraction, called tetanus, results from the fusion of multiple twitches.