3.6.1.3 Control of heart rate (A-level only)

Question 1

Define myogenic.

Answer 1

= Can contract and relax without receiving signals from nerves.

Question 2

How does cardiac muscle control the regular beating of the heart?

Answer 2

1. SAN in wall of right atrium sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls —> left and right atria contract at the same time.
2. Band of non-conducting collagen tissue prevents waves of electrical activity from being passed directly from the atria to the ventricles.
3. Waves of electrical activity are transferred from the SAN to the AVN instead.
4. AVN responsible for passing waves of electrical activity onto the Bundle of His. Slight delay before AVN reacts to make sure atria have emptied before ventricles contract.
5. Bundle of His = group of muscle fibres responsible for conducting the waves of electrical activity between ventricles to apex (bottom) of the heart.
   - –> bundle splits into finer muscle fibres in the right and left ventricle walls, called the Purkyne tissue.
6. Purkyne tissue carries waves of electrical activity into muscular walls of the right and left ventricles, causing them to contract simultaneously, from the bottom up.

Question 3

How do the brain and the autonomic nervous system control heart rate?

Answer 3

• Rate at which SAN fires (i.e. heart rate) is unconsciously controlled by the medulla oblongata in brain.
• Animals need to alter heart rate to respond to internal stimuli —> prevent fainting due to low blood pressure or ensuring enough O2 in blood.
  1. Baroreceptors (aorta and carotid arteries - stimulated by high / low blood pressure)
  and
  Chemoreceptors (aorta, carotid arteries and medulla - monitor [O2/ CO2] / pH) send electrical impulses to the medulla oblongata along sensory neurones.
  2. Medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones (part of the autonomic nervous system).
  How do high blood pressure / high blood O2 / high pH / low CO2 affect heart rate?

Question 4

How is this change brought about?

Answer 4

1. Baroreceptors detect high blood pressure and Chemoreceptors detect high blood O2 / low CO2 / high pH.
2. Impulses sent to medulla, which sends impulses along parasympathetic neurones - secrete acetylcholine which binds to receptors on the SAN.
3. Heart rate slows down to reduce blood pressure and put other levels back to normal etc.
   NB => cardiac muscles are the effectors.

Question 5

How do low blood pressure / low blood O2 / low pH / high CO2 affect heart rate? How is this change brought about?

Answer 5

1. Baroreceptors detect low blood pressure and Chemoreceptors detect low blood O2 / high CO2 / low pH.
2. Impulses sent to medulla, which sends impulses along sympathetic neurones - secrete noradrenaline which binds to receptors on the SAN.
3. Heart rate increases to increase blood pressure and put other levels back to normal etc.
   NB => cardiac muscles are the effectors.