Myogenic contraction
Heart muscle is myogenic. This means contraction can be initiated by the heart itself rather than requiring nervous stimulation to start each heartbeat.
The heart therefore has a baseline rhythm. Nervous input can alter this rate when the body's requirements change, but it does not normally initiate each contraction.
The pacemaker
A region of specialised cells called the sinoatrial node (SAN) lies in the wall of the right atrium. It acts as the heart's pacemaker.
The SAN initiates a wave of electrical activity that spreads through the atrial walls, causing the atria to contract.
Heart muscle
Can initiate contraction without nervous stimulation.
SAN
Initiates the wave of electrical activity that begins each heartbeat.
Exam Tip: Keep these roles separate: the SAN initiates the heartbeat, while autonomic stimulation can change the rate at which the SAN initiates beats.