Outline the control of the heart beat in terms of myogenic muscle contraction, the role of the pacemaker, nerves, the medulla of the brain and adrenalin (epinephrine).

Outline the control of the heart beat in terms of myogenic muscle contraction, the role of the pacemaker, nerves, the medulla of the brain and adrenalin (epinephrine).

++The heart is is myogenic

The cardiac muscle in the heart can contract and relax without stimulation from the nervous system+Myogenic muscle contraction describes the way the heart generate its own impulses to contract. It does not require external nerve input 3

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+Three stages of cardiac cycle Atrial Systole (contraction) The sinoatrial node (SAN) contracts and transmits electrical impulses throughout the atria, which both contract, pumping blood into the ventricles. Ventricular Systole (contraction). The electrical impulse passes to the ventricles via the atrioventricular node (AVN)Diastole (relaxed) The atria and the ventricles relax, while the atria fill with blood. The semilunar valves in the arteries close as the arterial blood falls back against them. +

How the heart functions+Cells of the sino-atrial node generate an impulse that spreads across the muscle cells of the atria This causes the atria to contract (both of them) The impulse cannot spread to the muscle cells of the ventricles The impulse is picked up by a sensory ending calledthe AVN

The atria have already contracted sending blood down into the ventricles The ventricles are stretched and full of blood

The impulse to contract generated in the SAN and is picked up by the AVN Impulse to contract travels down through the septum of the heart The impulse emerges from the apex of the heart causing it to contract The impulse moves high and higher causing the rest of the regiion to contract

The effect is to spread the contraction fron the apex upwards, pushing blood towards the semi lunar valve 7

+Key terms Sinoatrial node (SAN) Atrio-ventricular node (AVN) Atrial systoleVentricular systole Diastole

+Pacemaker

A pacemaker replaces the sinus node when your heart beats to fast, having a heart failure or a heart block which is a delay in the electrical conduction.

+ Pacemaker inside the heart.

+Possible reason for implanting a pacemakerAlmost all pacemakers are implanted to treat slow heart beating, which is called bradycardia, possible reason could be :sick sinus syndrom a medical procedure to treat an arrhythmia called atrial fibrillation.heart medicines, such as beta blockers. faint or have other symptoms of a slow heartbeatheart muscle problems that cause electrical signals to travel too slowly through your heart muscle. the heart usually beats about 50 to 70 times each minute, and the heart rate may increase 2- to 3-fold during stress or exercise. If the heart beats too slowly, the brain and body do not get enough blood flow and a variety of symptoms may result.+Nerves and hormones can transmit messages to the pacemakerSympathetic nerve secretes adrenaline (also known as epinephrine)The sympathetic nervous system belongs to autonomic nervous system along with parasympathetic nervous system. which carries messages from the brain to the pacemaker telling the pacemaker to speed up the beating of the heart. vagus nerve tells it to slow it down, and Adrenalin, carried to the pacemaker by the bloodstream tells it to speed up the beating of the heart.Nerves and Hormones

Medulla and heart beat modification

parasympathetic sympathetic+Changes to our heart rate are not under our conscious control but result from impulses sent from a control center in the part of the brain stem known as the medulla.

Decelerator nerve is known as parasympathetic and decreases the rate of polarization at the SAN. The synapses releases acetyl choline which causes the heart rate to slow down.

Accelerator nerve is known as sympathetic nerves and accelerates the rate of depolarization at the SAN

The medulla monitors blood pressure and carbon dioxide levels using information it receives from receptors in arteries.

14Adrenaline and heart rate

+The hormone epinephrine is produced in the adrenal glands(an endocrine gland) The hormone travels through the blood to its target tissue the SAN

15Adrenaline

+The epinephrine increases the rate of depolarization of the SAN This accelerates heart heart. This reaction is associated with the fight or flight response

Activated to give the body increased strength and speed in anticipation of fighting

i.e. pupils dilating Blood clotting to prevent excessive blood loss

Heart pumps faster to deliver more oxygen to the rest of the body 16