Medical What controls heartbeat?

  1. I thought that the brain, specifically the medulla in the lower brain stem, controls heatbeat. But according to what I am reading, heartbeat is caused by electrical impulses in the heart which cause the heart muscles to contract. What is the source flow of this electricity? Does it come from the medulla, or does the medulla only regulate the pulse when there is already a pulse?

    In the case of sudden cardiac arrest, defillibration is applied to the heart directly. Is there then a feedback from the heart to the medulla causing it to adjust its heatbeat commands? What exactly causes the electrical activity of the heart to become irregular?

  2. jcsd
  3. berkeman

    Staff: Mentor

    In a healthy heart, it is the Sinus Node at the top of the heart that sets the pace:

    When the Sinus Node malfunctions, the AV Node will generally set the pace, albeit slower than the normal sinus rhythm.
  4. Thanks, a very good animation!

    Meantime I made some progress concerning the function of the Medulla. It seems that the Medulla can release adrenaline into the bloodstream which increases overall sympathetic activity, including being received by adrenoceptors at the Sinus Node. Thus the Sinus Node is influenced by the brain to increase heartbeat, but the brain does not initiate heartbeat. In this sense, the heart is autonomous.

    If this is correct, the only piece of my question which I am now missing is the mechanism which causes the Sinus Node to malfunction.

  5. berkeman

    Staff: Mentor

    There are a number of things that can cause the SA node to stop firing regularly. A Google search on Sinus Node Malfunction gives a number of helpful links. Here is one of them:

  6. Thanks. It seems that the answer to my question "What controls heartbeat?" is as follows:

    The Wiki entry on the Autonomic Nervous System (ANS), which is located in the Medulla Oblongata in the brain stem, explains that the Sympathetic and Parasympathetic nervous systems have excitory and dampening effects respectively on various organs including the heart. There is a neuron pathway via ganglions from the Medulla to the heart (and to other organs). Sympathetic ganglionic neurons release noradrenalin and other transmitters such as ATP at the end synapses, which act on adrenergic receptors at the SA Node, AV Node and heart muscles. This increases the heart rate when necessary for fight or flight actions or for other movements which would otherwise result in a fall in blood pressure.

    Thus it seems to be confirmed that, although the heart is an autonomous organ, the rate of heartbeat is determined by the brain, provided the Medulla Oblongata is working. If this part of the brain is out of action, the heart may continue to function at a normal frequency, assuming that the lungs are also working. The purpose of CPR and defillibration is to maintain minimum heart and lung action, in the case that they have been disturbed, in order to delay tissue death, brain damage and other problems, until the ANS can resume its normal heartbeat control function.

  7. berkeman

    Staff: Mentor

    Mostly right, but the heart rate is usually set by the SA node, and *modified* by the sympathetic and parasympathetic systems.

    The purpose of CPR is as you described, to provide pumping action to keep blood circulating and perfusing the body. The purpose of defibrillation is to temporarily stop the heart when it is in certain (bad) rhythms, in the hope that the heart will return to a better rhythm after the shock.
  8. Its a membrane channel of the SA node cells that causes a current called If (channel is called the funny channel or HCN)- that spontaneously depolarizes after hyperpolarization that sets heart rate - the CNS then only acts as a modifier of the intrinsic rate of the SA node by modifying the rate of depolarization caused by If by changing the activity of this channel. So heart rate is set by a type of membrane channel that is quite unusual in its activation!
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