What does stimulation of the heart by autonomic nerve fibres travelling within the vagus nerve lead to?

Stimulation of the heart by autonomic nerve fibers traveling within the vagus nerve leads to a decrease in heart rate. Here's a step-by-step explanation:

1. The vagus nerve is a part of the parasympathetic nervous system, which is responsible for the body's 'rest and digest' functions.

2. When the vagus nerve is stimulated, it releases a neurotransmitter called acetylcholine.

3. Acetylcholine binds to receptors on the heart's pacemaker cells, which are responsible for controlling the heart rate.

4. This binding causes the pacemaker cells to slow down the rate at which they fire, which in turn slows down the heart rate.

5. This is a protective mechanism for the heart, as it prevents the heart from beating too fast and wearing out over time.

So, in summary, stimulation of the heart by autonomic nerve fibers traveling within the vagus nerve leads to a slower heart rate.