IMAT 2012 Q57 [Adrenaline]

Which of the following are increased when the level of adrenaline rises in a human?

  1. heart rate
  2. breathing rate
  3. impulse rate in a sensory neurone

A. 1 and 3 only
B. 1 only
C. 2 and 3 only
D. 1 and 2 only
E. 1, 2 and 3

The answer to this question is D.

What is adrenaline? Basically, adrenaline, also called epinephrine, is an important neurotransmitter and hormone that acts in our sympathetic nervous system which is also known as the fight-or-flight response that is activated especially in stressful or potentially dangerous situations.

Among the choices, what can adrenaline modulate when the sympathetic nervous system is activated?

  • The heart rate: As you know, the heart (especially the pacemakers, like the SA and AV nodes) contains β1 receptors that respond to catecholamines like epinephrine (adrenaline) by increasing the heart rate. Thus it would make sense, when the sympathetic nervous system is activated, there is a concomitant increase in epinephrine as well, which explains the increased heart rate.
  • The respiratory rate: the effects of the activation of the sympathetic nervous system is also related to autonomic receptors present in our airways. It is important to remember that our airways contain β2 receptors, that respond to adrenaline by relaxing airway smooth muscles, effectively mediating its dilation to allow more airflow. It would also be useful to correlate the increased breathing rate to the increase heart rate. Since there logically, would be more blood circulating within the respiratory system, the increased breathing rate allows the body to effectively oxygenate the increased blood that has passed through, secondary to the increased pumping function of the heart.

What then, increases the impulse rate of sensory neurons/nerve conduction, if it is not adrenaline?

  • The feature that mediates how fast action potential conduction is along the axon is myelination. The presence of myelin sheath increases the rate of nerve conduction due to the phenomenon called saltatory conduction. The reason this speeds up the rate of impulse is because the electrical impulse is forced to skip from node to node down the length of the axon. This explains why unmyelinated fibers are slow-conducting fibers, since the lack of the insulation of the myelin removes the ability of the impulse to skip from node to node.