Q.Answer the following briefly (any three) : 2×3=6

When you clench and release your fist about seventy times a minute, you have to keep deciding to do it. The heart does something similar all day and all night, yet you never think about it — and, remarkably, it does not wait for the brain to tell it when to beat. A heart that generates its own beat from within its own muscle is called myogenic, and that self-driven contraction is what this idea is about.

The key lies in a special kind of cardiac muscle called nodal tissue, spread within the walls of the heart. This tissue is autoexcitable: it can generate electrical impulses (action potentials) entirely on its own, with no outside stimulus. Because the beat originates in the heart's own muscle rather than being started by a nerve, we say the heart is myogenic and that its activity is auto-regulated.

One patch of nodal tissue, the sino-atrial node (SAN) in the right upper corner of the right atrium, fires impulses faster than any other part — about seventy to seventy-five times per minute. Because it sets the fastest pace, it leads the whole heart and is called the pacemaker. This is why a resting heart settles at roughly seventy-two beats a minute on its own.