15.5. Regulation of Cardiac Activity

Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue), hence the heart is called myogenic.

The heart has the unique ability to regulate its own activities without relying on external signals. This is due to specialized muscle cells known as nodal tissue, which are primarily responsible for initiating and controlling the heartbeat. Unlike skeletal muscles that require nerve signals for contraction, the heart muscles can contract automatically, a quality termed myogenic activity.

Think of the heart as a drummer in a band who sets the beat for music. Even if the other band members are late to tune in, the drummer keeps the rhythm steady, allowing the rest to join in. Similarly, the heart’s nodal tissue ensures that the heart beats consistently, setting the rhythm for the entire circulatory system.

A special neural centre in the medulla oblangata can moderate the cardiac function through autonomic nervous system (ANS).

The autonomic nervous system (ANS) consists of two branches: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic system can increase heart rate and the strength of heart contractions, which enhances cardiac output during activities like exercise. In contrast, the parasympathetic system slows down the heart rate and reduces contractions, which helps in recovery and calming the body after stressful situations.

Imagine the accelerator and brake in a car. When you want to speed up, you press the accelerator (sympathetic nervous system), making the car go faster. When you're parking or moving slowly, you apply the brake (parasympathetic nervous system), slowing down the car. This balance maintains the optimal functioning of the heart based on the body’s needs.

Neural signals through the sympathetic nerves (part of ANS) can increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output. On the other hand, parasympathetic neural signals (another component of ANS) decrease the rate of heart beat, speed of conduction of action potential and thereby the cardiac output. Adrenal medullary hormones can also increase the cardiac output.

Hormones released by the adrenal glands, such as adrenaline, can increase heart rate and the force of heart contractions, similar to sympathetic nervous stimulation. This is particularly important during situations requiring a quick response, such as in an emergency ('fight or flight' response). In contrast, hormones can also help slow down the heart when your body is at rest.

Think of hormones as fuel for a fire. In a campfire, adding more wood (like adrenaline) makes the fire burn brighter and hotter, similar to how the heart speeds up during stress or excitement. Conversely, removing some wood reduces the fire's intensity, just like how the heart slows during relaxation.

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