Question

Fred and Mary were working in the library and were asked to shift textbooks from one shelf to another. Fred asked Mary to hold out her hands so he could stack several books on her arms to carry multiple books at once. Fred noticed when he placed a textbook on Mary’s arms they would extend downward slightly for a moment and then return quickly to the original position, this happened with each additional textbook he added. Mary was surprised by this as she didn’t feel as if she was voluntarily restoring her arm position. When Fred placed a 5th very heavy textbook onto the stack in Mary’s arms her arms relaxed and all the text books fell to the floor, Mary insisted she didn’t drop the books purposefully but did feel the load was becoming too heavy for her to maintain her arm position.

1a) Which reflex is first being elicited as Fred adds each book to Mary’s arms to enable Mary to maintain her arm position? Provide a description of this neural reflex pathway, including the stimulus, receptor, neural pathway, effector and response. You may use a diagram if you wish.

1b) As Fred added the 5th textbook the increased load caused Mary’s arms to relax. Which sensory receptor was activated here and what is this detecting in the muscle? How does this result in Mary dropping the books?

Answer 1

1a) stretch reflex is initiated first when Fred added each book to Mary's arms. This reflexes works when an unexpected increased load is applied to muscles, to maintain the joint position. Stretch reflex is also called myotatic reflex. When Fred put textbook on Mary hand, each time, the increased weight of book activated her biceps muscle, which activated muscle spindle Ia afferent fibers . It will cause monosynaptic activation and lead to alpha motor neuron activation and cause biceps muscle to contract.

1b) When Fred added the fifth text book Mary arms can’t hold the increased weight and it leaded to formation of autogenic inhibition reflex, this is a spinal reflex caused by the activation of Golgi tendon organ of biceps muscle .this leads to formation of inhibitory signal and leads to cease of muscle contraction and result in relaxation of agonist muscle and caused dropping of book from Mary’s arms.