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 textbooks 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.

a) 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. Include or draw a diagram. (6 points worth)

b) 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? (4 points worth)

Answer 1

A. Our skin has numerous sensory receptors to transmit different senses through the spinal cord up to the brain. Pressure and position senses are transmitted through the dorsal column pathway----> to the somasensory cortex. Then the motor cortex activates the corticospinal pathway to send impulses to the flexors and extensors of the hands.

B. Increased load causes increased tension in the muscle. Every voluntary muscle tendon contains Golgi tendon organs, which senses tension in the muscle. It acts to reduce the tension in the muscle and relaxes muscle by:

1. Increased tension in the muscle is recognized by the 1 beta fiber of Golgi tendon organ
2. Information transmitted via dorsal root ganglion
3. 'Neuron synapses at inhibitory interneuron
4. Interneuron inhibits motor neuron to the agonist muscle and reduces tension with in the muscle.