Question 1
Multiple choiceMedium1 markA sharp object touches a hand, which is then withdrawn. The table gives the first time after contact at which activity is detected at each site. Which pathway and response description is consistent with the data?
| Site | First activity after contact / ms |
|---|---|
| Sensory neuron entering spinal cord | 2 |
| Skeletal muscle of arm | 9 |
| One interneuron in grey matter of spinal cord | 4 |
| Free sensory nerve ending in hand | 0 |
| Motor neuron leaving spinal cord | 6 |
Show answer and explanation
Answer: D. The activity occurs in sequence at the free sensory nerve ending, sensory neuron, spinal interneuron, motor neuron and skeletal muscle. This is an involuntary pain reflex pathway, with the skeletal muscle as the effector.
A: reverses the order of the interneuron and motor neuron, contrary to their recorded activity times.
B: gives the correct pathway but misclassifies the reflex response as voluntary.
C: substitutes smooth muscle for the skeletal muscle recorded as active.
Remember: a pain reflex passes from a sensory neuron through an interneuron in spinal grey matter to a motor neuron and skeletal muscle.