Cardiac Muscle Physiology PDF

 Cardiac muscle The heart muscle is located in the middle layer of the heart, the myocardium. It is located between the endocardium (inner layer) and the epicardium (outer layer). Cell Structure: Heart muscle cells are called cardiomyocytes. Cardiomyocytes are short, branched, and interconne...

 Cardiac muscle The heart muscle is located in the middle layer of the heart, the myocardium. It is located between the endocardium (inner layer) and the epicardium (outer layer). Cell Structure: Heart muscle cells are called cardiomyocytes. Cardiomyocytes are short, branched, and interconnected cells. Multiple cells are connected to each other via intercalated discs, providing electrical communication and mechanical connection. cardiac muscle layers Epicardium: The outermost layer of the heart. Functions: Protects the heart and acts as a buffer against external impacts. Coronary arteries and nerves pass through this layer. Structure: Consists of thin connective tissue and mesothelium.  Cardiac muscle layers Myocardial Layer: This is the most critical layer, the pumping power of the heart comes from here. Myocardium: The middle and thickest layer of the heart. Functions: The main muscle layer that pumps blood. The contraction function is performed by cardiomyocytes. Structure: Striated muscle cells (cardiomyocytes) are connected by intercalary discs. cardiac muscle layers Endocardium: The innermost layer of the heart. Functions: Lining the heart chambers and valves. Provides a smooth surface that directs blood flow properly. Structure: Consists of thin connective tissue and endothelial cells.  Intercalated Discs: Gap junctions: Provide rapid passage of electrical impulses. Desmosomes: Provide mechanical strength, help cells stay together during contraction. Striations: Heart muscle, like skeletal muscle, has a striated structure. This is due to the regular arrangement of actin and myosin filaments.  DIAD Cardiac muscle cells (cardiomyocytes) have a special cellular organization that is critical for the rhythmic contraction and relaxation function of the heart. The DIAD structure found in cardiac muscle cells plays an important role in the contraction mechanism. The DIAD is a cellular structure that integrates the electrical and mechanical events of the muscle cell.  DIAD is a structure formed by the T tubule (Transverse tubule) and the terminal cisternae of the sarcoplasmic reticulum. Unlike the triad structure found in skeletal muscle, DIAD (dyadic structure) is found in cardiac muscle. T tubule: Tubular structures derived from the cell membrane (sarcolemma) and extending deep into the cell. Terminal cisterna: The calcium-storing part of the sarcoplasmic reticulum. DIAD is formed by the union of these two structures and has a critical role in regulating calcium ions that trigger contraction within the cell.  DIAD ROLES Regulation of Calcium Signal: Contraction in cardiac muscle begins with Ca²⁺ ions entering the cell (calcium-induced calcium release). DIAD transmits the action potential from the T tubule to the sarcoplasmic reticulum and provides calcium release from there. Relationship between Electrical Stimulus and Contraction: The T tubule allows the action potential to spread rapidly into the depths of the cell. Calcium released from the terminal cisterna activates actin-myosin filaments to initiate contraction. Difference in Contraction Mechanism: While skeletal muscle has a triad structure, cardiac muscle has DIAD. This difference causes cardiac muscle to exhibit less regular and slower contraction dynamics.  Cardiac anatomy The human heart is located in the middle mediastinum, at the level of thoracic vertebrae T5-T8. The heart has 4 chambers, with the atria positioned above and the ventricles below, they forming a right and left heart. The right and left sides of the heart are separated by interatrial and interventricular septa. The interventricular septum is thicker than the interatrial septum because it needs to generate more pressure during ventricular contraction. The right atrium receives blood from the superior and inferior vena cava. After leaving the heart, the aorta gives rise to three branches in sequence from right to left: 1. brachiocephalic, 2. left carotid artery, 3. left subclavian artery. The brachiocephalic artery further divides into; 1. right carotid artery 2. right subclavian artery.  Valves Heart valves are one-way valves that normally allow blood to flow in only one direction from the heart. There are two main types of valves in the heart: atrioventricular valves and semilunar valves, Totaling four valves – two AV valves and two semilunar valves. On the right side, tricuspid valve, three leaflets and on the left side, there is the bicuspid valve (mitral valve), two leaflets Semilunar valves are located at the exit points of the aorta and pulmonary trunk. The one at the aortic exit is called the aortic valve, three leaflets and the one at the pulmonary trunk is called the pulmonary valve , three leaflets The opening and closing of the valves depend passively on the pressure gradient.  The heart wall consists of three layers: the inner-- endocardium, the middle--- myocardium, and the outer ----epicardium. The middle layer of the heart wall is the myocardium, which is the heart muscle. It has a striated muscle tissue appearance but functions involuntarily like smooth muscle. Within the myocardial layer, there are cells called cardiomyocytes that are responsible for the actual contraction of the heart muscle. Intercalated discs are present among cardiomyocytes, serving as bridges that allow the initiation of a signal in one cell to be rapidly transmitted to the entire heart. This rapid spread of a signal from one cell without damping in a synchronized manner across the entire heart is known as syncytium.  There are two functional syncytia in the heart: the atrial syncytium and the ventricular syncytium. These two syncytia are separated by a non- conductive fibrous tissue called the annulus fibrosus Having these two distinct syncytia allows the atria to contract before the ventricles. Pacemaker cells Pacemaker cells constitute the conduction system of the heart, including the SA (sinoatrial) and AV (atrioventricular) node cells. These cells generate spontaneous electrical impulses. The SA node is located near the entrance of the superior vena cava in the right atrium. It serves as the primary pacemaker of the heart, producing 60-80 impulses per minute. The heart rhythm generated by the impulses from the SA node is called the sinus rhythm.  The AV node is located in the interatrial septum, just behind the tricuspid valve. It has a conduction speed of 0.05 m/s and a firing frequency of 40-60 beats per minute.   The heart's electrical and conduction system consists of the following components: - It begins at the SA (sinoatrial) node, spreading to the atria. - Interatrial and internodal pathways transmit signals to all atria, and internodal pathways lead to the AV (atrioventricular) node. - The AV node receives the signal. - The His bundle transmits the signal from the atria to the ventricles. - Purkinje fibers distribute the signal to all parts of the ventricles.   AV Semilunar Stage Status of ventricles and atria; and blood flow valves* valves† Semilunar (pulmonary and aortic) valves close at end of 1 Isovolumic relaxation Closed Closed ejection stage; blood flow stops. Ventricles and atria together relax and expand; blood flows 2a Inflow: (Ventricular filling) Open Closed Diastole to the heart during ventricular and atrial diastole. Ventricles relaxed and expanded; atrial contraction (systole) Inflow: (Ventricular filling 2b Open Closed forces blood under pressure into ventricles with Atrial systole#) during ventricular diastole–late. AV valves close at end of ventricular diastole; blood flow 3 Isovolumic contraction Closed Closed stops; ventricles begin to contract. Systole Ventricles contract (ventricular systole); blood flows from 4 Ejection: Ventricular ejection Closed Open the heart—to the lungs and to rest of body during ventricular ejection Heart rate Heart rate is the frequency of the heartbeat measured by the number of contractions of the heart per minute (beats per minute, or bpm). The heart rate at which it can vary according to the body's physical needs, including the need to absorb oxygen and excrete carbon dioxide normal resting adult human heart rate is 60–100 bpm.  Tachycardia is a high heart rate, defined as above 100 bpm at rest. Bradycardia is a low heart rate, defined as below 60 bpm at rest. When a human sleeps, a heartbeat with rates around 40–50 bpm is common and is considered normal. When the heart is not beating in a regular pattern, this is referred to as an arrhythmia. When the heart rate increases, both systolic and diastolic durations shorten. ❖If the heart rate increases: ❖Diastolic duration shortens. ❖Ventricular filling decreases. ❖Stroke volume (the amount of blood pumped per beat) decreases. ❖Cardiac output (the volume of blood pumped by the heart per minute) decreases.  Normally, Cardiac Output (CO) is calculated as; Stroke Volume (SV) and Heart Rate (HR) CO=SV×HR For example, if the stroke volume is 70 ml and the heart rate is 80 beats per minute: CO=70 ml×80 bpm=5600 ml/min This represents the normal amount pumped by the heart in a minute.  Heart sounds Heart sounds S1 and S2 are the primary sounds heard during the cardiac cycle: 1.S1 (First Heart Sound): 1. Occurs at the beginning of ventricular systole. 2. Associated with the closure of the atrioventricular (AV) valves (tricuspid and mitral valves). 3. "Lub" sound is often described as the sound of the heart "beating." 2.S2 (Second Heart Sound): 1. Occurs at the beginning of ventricular diastole. 2. Associated with the closure of the semilunar valves (aortic and pulmonary valves). 3. "Dub" sound is often likened to the closing of doors. In a normal heartbeat, the sequence is S1 followed by S2, creating the characteristic "lub-dub" sound. These sounds provide essential information about the functioning of the heart valves and the phases of the cardiac cycle.  Coronary circulation Coronary circulation refers to the network of blood vessels that supply the myocardium with oxygen and nutrients. Coronary Arteries: 1.Left Coronary Artery (LCA): 1. Branches off the aorta and divides into two main branches: 1. Left Anterior Descending (LAD) Artery: Supplies the front and a large part of the septum of the left ventricle. 2. Circumflex Artery: Wraps around the left side of the heart, supplying the left atrium and a portion of the left ventricle. 2.Right Coronary Artery (RCA): 1. Arises from the aorta and travels along the right side of the heart. 2. Supplies the right atrium, right ventricle, and part of the septum. Cardiac action potential Cardiac action potential (ventricular cardiac muscle cell (cardiomyocytes)-myocardial)  Cardiomyocyte action potential phases Phase 4 (resting phase): Only potassium (K+) channels are open during the resting phase and efflux of potassium establishes a negative resting membrane potential (approximately –90 mV). Phase 0 (depolarization): Upon stimulation, rapid depolarization occurs via influx (inward flow) of sodium (Na+) and the cell becomes positively charged (approximately 20 mV). Phase 1 (early repolarization): During this phase, another type of potassium (K+) channels opens and a brief efflux of potassium repolarizes the cell slightly. Phase 2 (plateau phase): Almost simultaneous with the opening of potassium channels in phase 1, persistent calcium (Ca2+) channels open whereby calcium flows into the cell. Plateau phase; It prevents additional impulses from spreading through the heart prematurely! Allows the cardiac muscle sufficient time to contract and pump blood effectively ! Gives time for the nearby cardiac muscle cells to depolarize ! Ca2+ current responsible for maintaining the plateau phase of the action potential. Phase 3 (repolarization): Calcium (Ca2+) channels close and potassium (K+) channels open again and the efflux of potassium repolarizes the cell. SA Node Pacemaker Potential SA node cells are specialized cardiomyocyte cells. The membrane resting potential of SA node cells is closer to 0 than a normal cardiomyocyte cell. Normal cardiomyocyte cell resting membrane potential: -90 mV SA node cell resting membrane potential: -60 mV However, if you remember, each cell had a threshold potential value that it had to exceed in order to generate an action potential. In general; cells with membrane resting potential close to this threshold value can be stimulated much more easily.  The biggest difference between the cardiomyocyte cell and the SA node cell is that; the resting membrane potential of the SA node is close to the cell threshold value. That's why the SA node is called the primary pacemaker node or leading focus. Funny Channels (Na+ leak channels) Due to the funny leak Na+ channels found in the SA node cell membrane, the resting membrane potential here is less negative. So spontaneous warnings depend on funny leak Na+ channels 😊 This Na+ leak inputs serve to bring the SA node resting membrane potential closer to the threshold value. Therefore, SA node cells generate stimulation on their own without the need for any other nerve innervation. VENOUS CIRCULATION: The ability of veins to contract and relax is low. But they can store blood, their other name is capacitance vessels. Blood from all systemic veins is collected in the right atrium. Vein valves and venous pump: The valves in the veins are positioned so that the direction of blood flow is towards the heart. The pressure difference between the head and feet is 0-90 mmHg. This occurs due to gravity. Foot vein pressure of a person who stands for a long time may increase to 90 mmHg.  Venous pump: When a person sits, the leg muscles tighten, the veins in between also compress and push the blood towards the heart, this is the venous pump. Thanks to the venous pump, the pressure in the foot veins of the walker is kept below +20 mmHg. The function of the valves is to prevent blood from returning. It moves in one direction, towards the heart. In a person standing motionless, the pressure in the foot veins increases to +90 mmHg in 30 seconds. Due to the increase in pressure in the capillaries, fluid leaks into the interstitial space, the legs swell, and blood volume decreases. Varicose vein Varicose veins are caused by increased blood pressure in the veins. Varicose veins happen in the veins near the surface of the skin (superficial). The blood moves towards the heart by one-way valves in the veins. When the valves become weakened or damaged, blood can collect in the veins. Autonomic nervous system Sympatic system Fight or flight T1-L5 Toraco-lumbal outflow Parasympatic system Rest and digest CN(3-7-9-10)-S1-S5 Cranio-Sacral outflow blood pressure regulation Blood pressure is constantly kept under control to maintain cardiac homeostasis. Blood pressure may be affected due to some external factors or pathological factors. Therefore, it is important to know the mechanisms of blood pressure regulation. Kan basıncı kısa-uzun olmak üzere 2 şekilde düzenlenir: Short-term blood pressure regulation (neural) Long-term blood pressure regulation (hormonal) Short-term regulation Long-term regulation (Renin-angiotensin-aldosteron system)  Thank you ☺