Final answer:
A sudden stretch to the tendon of a muscle results in a muscle contraction response known as the stretch reflex, which is a protective mechanism to maintain muscle tone and prevent overstretching.
Step-by-step explanation:
A sudden stretch to the tendon of a muscle results in a muscle contraction response known as the stretch reflex.Muscle contraction can either result in an increase in tension or a decrease in the length of a muscle. The process of contraction is detailed by the sliding filament model. When a muscle fiber receives a nervous stimulus, notably the neurotransmitter acetylcholine (ACh) at the neuromuscular junction (NMJ), it triggers depolarization and an action potential travels along the sarcolemma. This then results in calcium release from the sarcoplasmic reticulum (SR). In response, the calcium ions bind to troponin, causing a position shift in the protein tropomyosin, which consequently exposes binding sites on the actin filaments. Myosin heads then attach to these sites, performing a 'power stroke' by sliding the thin filaments past the thick ones, leading to muscle fiber shortening and hence, contraction. The amount of tension produced by a muscle is determined by the number of formed cross-bridges between actin and myosin.
When a muscle is stretched suddenly, sensory receptors within it, known as muscle spindles, are stimulated. This stimulation sends a signal to the spinal cord, which instantaneously triggers the motor neurons of the same muscle to elicit a contraction, referred to as the stretch reflex. This reflex helps maintain muscle tone and posture and is a protective response to prevent muscle overstretching and injury.