Control of heart rate

• The nervous system is split into 2 different systems - the central nervous system (CNS) and the peripheral nervous system:

  • the CNS is made up of the brain and spinal cord
  • the peripheral nervous system is made up of the neurones that connect the CNS to the rest of the body
• The peripheral nervous system also has 2 different systems: the somatic and autonomic

  • somatic - controls conscious activities e.g. running and playing video games
  • autonomic - controls unconscious activities e.g. digestion
• The autonomic nervous system is split into the sympathetic and parasympathetic nervous systems:

  • have opposite effects on the body
  • the sympathetic nervous system is the 'fight or flight' system that gets the body ready for action
  • parasympathetic is the 'rest and digest' system that calms the body down
  • the autonomic system is involved in the control of heart rate
• Control of heart beat:

  • cardiac muscle is 'myogenic' - this means it can contract and relax without receiving signals from nerves
  • this pattern of contractions controls the regular heartbeat
• • process starts at the sinoatrial node - small mass of tissue in the wall of the right atrium
  • SAN is like a pacemaker - sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls
  • this causes the right and left atria to contract at the same time
  • band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles
  • instead these waves of electrical activity are transferred from the SAN to the atrioventricular node - AVN
• • the AVN is responsible for passing the waves of electrical activity on to the bundle of His
  • but there is a slight delay before the AVN reacts - to make sure that the atria have emptied before the ventricles contract
• • the bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex (bottom of the heart)
  • the bundle of His splits into finer muscle fibres in the right and left ventricle walls - purkyne tissue/ fibres
  • the purkyne tissue carries the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously from the bottom up
• delay is before the AVN reacts
• Communication between the heart and brain:

  • the SAN generates electrical impulses that cause the cardiac muscles to contract
  • the rate at which the SAN fires (i.e. heart rate) is unconsciously controlled by a part of the brain called the medulla
• • animals need to alter their heart rate to respond to internal stimuli, e.g. to prevent fainting due to low blood pressure or to make sure the heart rate is high enough to supply the body with enough oxygen
• internal stimuli are detected by pressure receptors and chemical receptors:

  • there are pressure receptors called baroreceptors in the aorta and carotid arteries - stimulated by high and low blood pressure
  • there are chemical receptors called chemoreceptors in the aorta, carotid arteries and medulla - monitor the oxygen level in the blood and also carbon dioxide and pH - indicators of O2 level
• electrical impulses from receptors are sent to the medulla along sensory neurones. the medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones
• High blood pressure:

  • baroreceptors detect high blood pressure and send impulses along sensory neurones to the medulla
  • which sends impulses along parasympathetic neurones
  • these secrete acetylcholine which binds to the receptors on the SAN
  • this causes the heart rate to slow down
  • to reduce blood pressure back to normal
• Low blood pressure:

  • baroreceptors detect low blood pressure and send impulses along sensory neurones to the medulla
  • which sends impulses along sympathetic neurones
  • these secrete noradrenaline, which binds the the receptors on the SAN
  • this causes the heart rate to increase
  • to increase blood pressure back to normal
• medulla oblongata
• the carotid arteries are major arteries in the neck
• acetylcholine and noradrenaline are types of neurotransmitter
• High blood O2, low CO2, high blood pH:

  • chemoreceptors detect chemical changes in the blood and send impulses along sensory neurones to the medulla
  • which sends impulses along parasympathetic neurones
  • these secrete acetylcholine which binds to receptors on the SAN
  • this causes the heart rate to decrease in order to return oxygen, CO2 and pH levels back to normal
• Low blood O2, CO2 or low blood pH:

  • chemoreceptors detect chemical changes in the blood and send impulses along a sensory neurone to the medulla
  • which sends impulses along sympathetic neurones
  • these secrete noradrenaline, which binds to receptors on the SAN
  • this causes the heart rate to increase in order to return oxygen, CO2 and pH levels back to normal
• low blood O2, high Co2 or low blood pH are a result of increased respiration