The Autonomic Nervous System
- There are several circumstances that can cause an individual's heart rate to increase, such as during exercise
- The brain is involved in this response of the heart however it does not require any thinking
- There is a specific region of the brain that plays a vital role in controlling the heart rate
- This cardioregulatory centre in the brain is called the medulla
- The medulla is found at the base of the brain near the top of the spinal cord
- The medulla is made up of two distinct parts:
- The acceleratory centre, which causes the heart to speed up
- The inhibitory centre, which causes the heart to slow down
- Both centres are connected to the sinoatrial node (SAN) by nerves
- These specific nerves are different from the nerves that control conscious activities. They make up the autonomic nervous system
- The autonomic nervous system is self-controlling
The acceleratory centre
- Once the acceleratory centre has been activated impulses are sent along the sympathetic neurones to the SAN
- Noradrenaline is secreted at the synapse with the SAN
- Noradrenaline causes the SAN to increase the frequency of the electrical waves that it produces
- This results in an increased heart rate
The inhibitory centre
- Once the inhibitory centre has been activated impulses are sent along the parasympathetic neurones to the SAN
- Acetylcholine is secreted at the synapse with the SAN
- This neurotransmitter causes the SAN to reduce the frequency of the electrical waves that it produces
- This reduces the elevated heart rate towards the resting rate
The location of the medulla helps to keep it protected from harm. It has an essential function as a cardioregulatory centre.
Activation of the acceleratory and inhibitory centres
- Exercise causes several internal conditions to change, creating internal stimuli:
- Carbon dioxide concentration in the blood increases
- There is an initial fall in blood pressure caused by the dilation of muscle arterioles
- These internal stimuli can be detected by chemoreceptors and pressure receptors located in the aorta (close to the heart) and in the carotid arteries (they supply the head with oxygenated blood)
- These receptors release nerve impulses that are sent to the acceleratory and inhibitory centres (coordinators)
- The frequency of the nerve impulses increases or decreases depending on how stimulated the receptors are:
- Lower frequency impulses activate the inhibitory centre to slow down the heart rate
- Higher frequency impulses activate the acceleratory centre to speed up the heart rate
The processes involved in the control of the heart rate.The internal stimuli are detected by chemoreceptors and pressure receptors that send impulses to coordinators (accelerator centre or inhibitory centre). The coordinators send signals to the effector (SAN) which produces a specific response.
Adrenaline
- Adrenaline is a hormone produced by the adrenal glands
- It is chemically similar to noradrenaline
- It is released during a fight-or-flight response
- For example, when a zebra sees a hunting lion
- It causes the heart rate to increase
- The increase in heart rate is beneficial as it allows for a rapid increase in blood supply to respiring muscles
- This means the muscles will have more oxygen and glucose for respiration
- It enables high-intensity activities like running away from a predator to be an immediate response
