Test Your Numeracy ...

This shows part of an MSK examination question for Medicine Year 1. A substantial number of students failed to get the calculation right, either because of arithmetical errors, or because of a confusion about units (milliseconds vs seconds, etc)

The diagram to the right illustrates a simple reflex loop involved in controlling the length of a skeletal muscle. Stretch of the muscle is detected by stretch receptors embedded in the muscle and this triggers an action potential in the sensory nerve. This nerve enters the spinal cord via the posterior (dorsal) root and synapses with an alpha motor neuron. Release of the neurotransmitter from the sensory neuron terminals triggers an action potential in the motor neuron, leading to contraction and thus shortening of the muscle.

The approximate length of the nerves is 500 mm each. Assuming that:

• a) conduction velocity in the sensory nerve axons is 100 m.sec-1
• b) conduction velocity in the motor axons is 70 m.sec-1,
• c) each synapse introduces a delay of 1 msec