Heart- Myogenic heart . Mechanism of heart beat

Heart tissue and its mechanism

1. MYOGENIC HEART & SPECIALIZED TISSUES HEMA T MSc BIOCHEMISTRY BHARATHIAR UNIVERSITY 2018-2020

2. HEART

3. HEART- A MUSCULAR PUMP The normal heart is a strong, muscular pump a little larger than a fist. It pumps blood continuously through the circulatory system. Each day the average heart "beats" (expands and contracts) 100,000 times and pumps about 2,000 gallons of blood. In a 70-year lifetime, an average human heart beats more than 2.5  billion  times.

4. SPECIALIZED TISSUES Cardiac muscle tissue  is an extremely specialized form of  muscle tissue  that has evolved to pump  blood  throughout the body. In fact,  cardiac muscle  is only found in the heart and makes up the bulk of the heart's mass . The heart is composed of three major types of cardiac muscle: atrial muscle, ventricular muscle , and specialized excitatory and conductive muscle fibers . Cardiac Muscle as a Syncytium. The dark areas crossing the cardiac muscle fibers are called intercalated discs ; they are actually cell membranes that separate individual cardiac muscle cells from one another. That is, cardiac muscle fibers are made up of many individual cells connected in series and in parallel with one another.

5. CARDIAC SYNCYTIUM

6. INTERCALATED DISCS At each intercalated disc the cell membranes fuse with one another in such a way that they form permeable “communicating ” junctions (gap junctions) that allow almost totally free diffusion of ions. Therefore, from a functional point of view, ions move with ease in the intracellular fluid along the longitudinal axes of the cardiac muscle fibers, so that action potentials travel easily from one cardiac muscle cell to the next, past the intercalated discs. Thus, cardiac muscle is a syncytium .

7. HOW DOES THE HEART PUMP BLOOD? A heart's four chambers must beat in an organized manner. This is governed by an electrical impulse . A chamber of the heart contracts when an electrical impulse moves across it. Such a signal starts in a small bundle of highly specialized cells in the right atrium — the sinoatrial node (SA node), also called the sinus node. A discharge from this natural "pacemaker" causes the heart to beat. This pacemaker generates electrical impulses at a given rate, but emotional reactions and hormonal factors can affect its rate of discharge. This lets the heart rate respond to varying demands.

8. MYOGENIC MUSCLE CELLS SPONTANE- OUSLY DEPOLARIZE Cardiomyocytes within the vertebrate heart are considered to be myogenic muscle because they undergo spontaneous sarcolemmal depolarization. The cells that depolarize fastest are called pacemaker cells . Pacemaker cells exhibit an unstable resting membrane potential . They depolarize slowly until a threshold potential triggers a large spontaneous depolarization . Myogenic cells can control excitation by regulating ion channels to alter their susceptibility to s pontaneous depolarization.

9. NEUROGENIC MUSCLE IS EXCITED BY NEUROTRANS- M ITTERS In contrast to myogenic muscles, neurogenic muscle excitation is initiated by neural impulses. Vertebrate skeletal muscle is an example of a neurogenic muscle. Neurotransmitter acetylcholine plays a major role.

10. CONTROL OF CONTRACTION Vertebrate hearts are myogenic; their cardiomyocytes can produce spontaneous rhythmic depolar izations that initiate contraction. But in order for the heart to contract in a coordinated way, cardiomyocytes must be electrically coupled via gap junctions so that the depolarization in one cell can spread to adjacent cells, triggering co ordinated contractions. I n other verte brates pace maker cells are located in an area of the right atrium called the sinoatrial (SA) node , close to the point where the superior vena cava enters the right ‘ atrium .

11. PACEMAKER CELLS INITIATE THE HEARTBEAT Although derived from muscle cells, pacemaker cells are small with few myofibrils, mitochondria, or other organelles, and they do not contract. These cells have an unstable resting membrane potential (called the pacemaker potential) that slowly drifts upward from the starting potential of about -60 mV until it reaches threshold (about 4-40 mV) and initiates an action potential . As in all other cells, there is a continuous leak of potassium ions at the resting membrane potential. In pacemaker cells, however, this potassium permeability decreases over time. The slow decrease in potassium movement contributes to the slow depolarization of the cell. In addition, pacemaker cells experience a slow inward movement of sodium, which is called the “funny” current (If) because of its unusual behavior.

12. THE PACEMAKER POTENTIAL In myogenic hearts, the pacemaker cells have an unstable resting membrane potential (the pacemaker potential ).Funny channels open, increasing the permeability (P) of the membrane to Na+, which causes the membrane potential to increase gradually. As the membrane approaches threshold, T-type Ca2+ channels open, triggering an action potential. After about 200 msec these channels close and K+ channels open, repolarizing the cell, and the cycle begins again.

13. CONDUCTING PATHWAYS SPREAD THE DEPOLARIZA-TION ACROSS THE HEART T he pacemaker cells in the sinoatrial (SA) node initiate an action potential, the depolarization spreads rapidly via the inter nodal pathway through the walls of the atria. At the same time, the depolarization spreads more slowly through the contractile cells of the atrium via gap junctions. causing the atrium to contract. After traveling through the inter nodal pathway , the depolarization reaches the atrio ~ ventricular (AV) node, which communicates the electrical signal to the ventricle . The contractile cells of the atrium and ventricle do not form gap junctions with each other, and thus are not electrically coupled, so the depolarization cannot spread directly from the atrium to the ventricle, but instead can only pass through the AV node .

14. AV NODE TRANSMITS SIGNAL The AV node transmits signals a little more slowly than the other cells of the conducting pathways, so the signal gets delayed slightly. This signal delay allows the atrium to finish contracting before the ventricle starts to contract. The signal travels from the AV node through the bundle of His , which splits into the left and right bundle branches that conduct electrical signals to the ventricles. The electrical signal then spreads into a network of conducting pathways called the Purkinje fibers . From the Purkinje fibers, the signal spreads from cell to cell in the ventricular myocardium via gap junctions. The contraction of the ventricle begins at the bottom (or apex) of the heart and Spreads up through the myocardium, pushing blood upward toward the arteries.

15. CARDIAC CONDUCTION

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