A 42-year-old weightlifter is preparing for an upcoming strength competition. After lifting a series of progressively heavier weights, he decides to increase the weight on the exercise bar to 175 kg (385 lbs), the heaviest lift he has ever attempted. He is able to hold the weight over his head for several seconds. However, his arms suddenly and involuntarily give way and he drops the weight to the ground. Which of the following structures was most likely responsible for the sudden muscle relaxation?

Question

A 42-year-old weightlifter is preparing for an upcoming strength competition.  After lifting a series of progressively heavier weights, he decides to increase the weight on the exercise bar to 175 kg (385 lbs), the heaviest lift he has ever attempted.  He is able to hold the weight over his head for several seconds.  However, his arms suddenly and involuntarily give way and he drops the weight to the ground.  Which of the following structures was most likely responsible for the sudden muscle relaxation?

Quizplus · Accepted Answer

11ec160b_3d03_2454_9249_af27548f5783_MD0001_00 The Golgi tendon organs (GTOs) are sensory receptors located at the junction of the muscle and tendon that are innervated by group Ib sensory axons. GTOs are connected in series with the contracting extrafusal skeletal muscle fibers. When a muscle actively contracts against resistance, the increase in tension is transmitted through the tendon activating the GTOs in the process. In contrast, GTOs are relatively insensitive to changes in muscle length because the lengthening that occurs when a muscle is passively stretched takes place primarily in the muscle fibers and not in the tendon. The Ib sensory axons from the GTOs contact inhibitory interneurons in the spinal cord, which in turn synapse with the α motor neurons that innervate the same muscle. Thus, the Golgi tendon circuit is a negative feedback system that regulates and maintains muscle tension. When a muscle exerts too much force, the GTOs inhibit contraction of the muscle, causing sudden muscle relaxation. This prevents damage to the musculoskeletal system. (Choice A) A-delta fibers are thin, myelinated nerve fibers whose free nerve endings detect temperature and nociceptive stimuli. They are associated with acute (sharp) pain and constitute the afferent portion of the reflex arc that mediates withdrawal from noxious stimuli (e.g., retracting the hand away from a hot stove). (Choice C) In contrast to the in-series arrangement of GTOs, muscle spindles (intrafusal muscle fibers) are connected in parallel with extrafusal fibers. They are innervated by group Ia and group II sensory axons and are sensitive to changes in muscle length. Muscle spindles mediate the stretch reflex (myotatic reflex), which is commonly tested (deep tendon reflexes) during the neurologic examination. When a muscle is stretched, there is monosynaptic reflex activation of the α motor neuron (of the same muscle), causing contraction that resists the stretch. (Choices D and E) Pacinian corpuscles are rapidly adapting mechanoreceptors located in the subcutaneous tissue of the skin as well as the mesentery, peritoneum, and joint capsules. Ruffini's end organs are slowly adapting mechanoreceptors that exist in the skin, subcutaneous tissue, and joint capsule. Both help to mediate touch, proprioception, and vibratory sensation and are innervated by myelinated A-beta fibers. Educational objective: The muscle spindle system is a feedback system that monitors and maintains muscle length, while the Golgi tendon system is a feedback system that monitors and maintains muscle force. GTOs are exquisitely sensitive to increases in muscle tension but are relatively insensitive to passive stretch. __________ References: The Golgi tendon organ: a review and update. (http://www.ncbi.nlm.nih.gov/pubmed/6375383)

A 42-Year-Old Weightlifter Is Preparing for an Upcoming Strength Competition