Control of the Cardiac Cycle

The basic control is myogenic/self-exciting but the heart beat can also be altered by two nerves initiated in the cardiovascular control centre of the medulla.

1. Accelerator or Sympathetic nerve: this increases the heart rate and the stroke volume. 2.Vagus or Parasympathetic nerve: this decreases the heart rate and the stroke volume.

The accelerator nerve comes from the cardiovascular centre in the medulla oblongata. It passes down the spinal cord to the heart where it:

1.Stimulates the SAN to send more frequent waves of excitation and so more frequent waves of contraction over the heart muscle so the heart rate increases.

2.Increases the force of the muscle contraction thus this will increase the stroke volume.

The neurotransmitter used to stimulate the heart muscle is nor-adrenaline.

The vagus nerve also comes from the cardiovascular centre in the medulla oblongata. It leaves the brain and passes to the heart where it:

1. Makes the heart pump slower by inhibiting the SAN (sino-atrial node) less frequent waves of excitation and contractions pass over the heart muscle for heart rate to decrease.

2. It decreases the force of heart muscle contraction so it will decreases the stroke volume.

The neurotransmitter used to inhibit the heart beat is acetylcholine.

The two nerves are antagonistic (opposing actions).

1. During vigourous activity the cells use more oxygen for muscle contraction.

2. The cells produce more CO2 (Carbon Dioxide).

3. The increase in CO2 will lower the pH. (because the CO2 dissolves in water forming carbonic acid).

4. This low pH is detected by the chemoreceptors in the medulla oblongata and chemoreceptors in the aorta and the carrotid arteries, which are connected to the medulla oblongata by a sensory nerve.

5.This causes more impulses per unit time to pass along the sympathetic nerve to the cardiac muscle wall.

6. Stimulates the SAN to send more frequent waves of excitation and so more frequent waves of contraction over the heart muscle so this increases the heart rate.

7. Increases the force of the muscle contraction and thus increases the stroke volume.

Stress, excitement or anticipation generates more nervous impulses per unit time that pass down the accelerator nerve to the adrenal glands that produce adrenaline. Adrenaline mimics the sympathetic system.

What neurotransmitter is used by the sympathetic nerve?: nor-adrenaline

Thus adrenaline:

• increases the heart rate.
• increases the stroke volume.

When stress or excitement reduces then less adrenaline is produced so

• less stimulation of the SAN to send more frequent waves of excitation and so more frequent waves of contraction over the heart muscle so this heart rate decreases.
• decreases the force of the cardiac muscle contraction and so this decreases the stroke volume.

Stretch receptors can be found in most muscles, from the muscles surrounding the bronchioles to the large muscles of the legs.

They respond to the stretch of a muscle, usually because an antagonistic muscle has contracted. Impulses are sent along sensory nerves to the medulla and the brain anticipates a need for extra oxygen. The cardiovascular centre sends impulses along the vagus nerve to increase the heart rate and the stroke volume.

Blood Pressure - When heart rate and storke volume are increasing

As the heart rate and the stroke volume increase this will cause a greater volume of blood to leave the heart and enter the aorta. This will increase the BP. This greater force on the walls is monitored by pressure (baroreceptors) or stretch receptors in the walls of the carrotid sinus, a small swelling in the carrotid artery. The stretch receptors send signals to the cardiovascular centre in the medulla oblongata. Signals are passed down the parasympathetic nerve to reduce the heart rate and the stroke volume.

As the heart gets older there may be some damage to the heart.

• The heart rate may slow down
• The atria may continue to beat normally but the ventricles, which no longer recieve impulses via the bundle of His contract much more slowly than normal and so the heart rhythm is disturbed.

The person may have:

• Short of breath
• Suffer dizziness or blackouts

Pacemakers can:

• Generate impulses and stimulate the cardiac muscle to contract regularly e.g. 70 times per minute. The pacemaker mimics the SAN.
• The pacemaker can detect the person's heartbeat and alter the rate depending on the physiological demand e.g. the rate will increase during physical exercise.