Nervous Control

• Pacemaker

  • The muscles in the heart are stimulated to contract by

  small electric currents produced by pacemaker cells and

  the conducting system of the heart.

  • Some people have a heart rate that is too fast, too slow,

  or irregular. They may have an artificial pacemaker fitted

  to control their heart beat.

  • The pacemaker also know as the Sino-atrial node (SAN) is

  located in the wall of the right atrium.

  • This SAN produces electrical impulses that make the

  cardiac cells contract at a certain rate.
• Conducting System

  • The wave of excitation originating in the SAN spreads through the wall of the two atria making them contract at the same time (systole).

  • The impulse is then picked up by the atrio-ventricular node (AVN) located at the base of the atria.

  • The impulse is then passed to a bundle of conducting

  fibres which divides into left and right branches.

  • Stimulation of these fibres that run up the side of the

  ventricle walls causes ventricular systole.
• Nervous Control

  • The pacemaker tissue alone causes the heartbeat, however, heart rate is not fixed as it is altered by nervous and hormonal control.

  • The medulla regulates the rate of the sino-atrial node thorough the antagonistic action of

  autonomic nervous system (ANS)
• Parasympathetic and sympathetic nerves

  • There are two pathways which are antagonistic to one another as they have opposite effects on the heart.

  • The parasympathetic nerve transmits nerve impulses which cause the heart rate to DECREASE.

  • Whereas the sympathetic nerve transmits impulses that cause the heart rate to INCREASE.
• Parasympathetic and sympathetic nerves

  • Neurotransmitter substances released by these two nerves have an effect on the SAN (pacemaker).

  • The sympathetic nerve releases nor-adrenaline which speeds up heart rate

  and the parasympathetic nerve releases acetylcholine which slows heart rate.