Properties of Cardiac Muscle :-
Introduction :
Properties of Cardiac Muscle – The heart muscle is similar to other muscles in its properties. The features of cardiac muscle are derived from the interlocking cardiac muscle cells, or filaments, that comprise heart muscle tissue. There is only one core in each striated cardiac muscle filament. Beneath the magnifying glass, we can see both the bright and dark groups. The thick protein fibers shown in the dull groups are made of myosin protein, whereas the lean protein fibers are composed of actin protein. Myosin in cardiac muscle is a hexamer composed of two myosin light chains (MLC) connected to two myosin overwhelming chains (MHC). There are several isoforms of each subunit in cardiac muscles, and these isoforms alter the components of the cross-bridge during compression, hence affecting the drive that cardiac myosin can provide.
Properties of Cardiac Muscle :
1. Rhythmicity
2. Excitability
3. Conductivity
4. Contractility
5. All or none response
6. Staircase phenomenon
7. Refractory period
8. Tonicity and functional syncytium.
1. Properties of cardiac muscle- RHYTHMICITY
The rhythmicity of the heart muscle is extraordinary. The heart’s muscles can fortify themselves.
The capacity of the heart muscle to musically start its claim motivation is one of its essential characteristics. They contract in reaction to excitability.
The SA hub, AV hub, atrial muscle, Purkinje fiber, and harmed ventricular muscle fiber are illustrations of the cardiac muscle’s inherent rhythmical quality. The rhythmicity rate is 70 to 80 beats per minute in the SA node, 40 to 60 beats per minute in the AV node, 60 beats per minute in the atrium, and 20 to 40 beats per minute in the ventricle. The SA node regulates the remaining cardiac muscle and causes the heart to beat in time with its higher rhythmical quality. The AV node assumes responsibility for maintaining heartbeat when the SA node fails, and the atrium and ventricle thereafter take over if the AV node fails.
SINOATRIAL NODE :- The Sinoatrial Node is a small fix of changed cardiac muscle found specifically underneath the prevalent vena cava opening in the upper portion of the right atrium’s sidelong divider. This node’s strands need contractile components. Since these filaments are nonstop with atrial muscle strands, the SA node’s motivation diffused rapidly into the atria.
The ventricle, atria, and atrioventricular node of the heart can too produce impulses and act as pacemakers. Since the SA node produces motivations at a higher rate than other nodes, it is still alluded to be the pacemaker. It moves between 70 and 80 m/min.
2. Properties of cardiac muscle- EXCITABILITY
This capacity of a living thing to react to a stimuli is known as excitability. An action potential is the first electrical activity that occurs in all tissues in response to a stimulus. Mechanical activity, like as contraction and secretion, comes next.
Cardiac Muscle Action Potential :
Action potential – Heart muscle’s action potential differs compared to that of other muscle types, including smooth muscle, skeletal muscle, and nerve tissue. In cardiac muscle, the action potential lasts between 250 and 350 msec, or 0.25 and 0.35 seconds.
Two distinctive sorts of activity possibilities are seen in the cardiac muscles: postponed reaction activity possibilities are appeared in the atrioventricular (AV) gesture and sino-atrial hub, whereas quick activity possibilities are seen in the ventricular, atrial, and Purkinje fiber muscles. The Purkinje strands, atrial, and ventricular muscles’ cardiac activity possibilities have three particular stages: A extended level stage, a quick depolarization stage, and a genuine resting potential. The taking after ionic conductance are in charge of diverse phases:
Phase 0- Extreme depolarization and overshoot :- The influx of Na+ ions is increased when voltage-gated sodium channels open hundreds of times. And at a membrane potential of -60 mV, this happens. Through a mechanism known as auto-activation, the opened sodium channel further stimulates the opening of the reverse voltage-gated sodium channels. This is transient, and at -30 to -40 mV, there is a noticeable rise in Ca++ permeability. Within the cell, the membrane potential peaks between +20 and +30 m V when it is positive.
Phase 1- Initial fast repolarization:- K+ conductance increases and Na+ conductance decreases. The fast Na+ channels are inactivated. The inflow of sodium stops. Potassium ions exit when the external transitory rectifying K+ channels open.
Phase 2- The plateau phase is brought on by an elevated Ca++ conductance brought on by the calcium channel’s sluggish and prolonged opening. Potassium ions are efluxing through the K+ channel of the slow delayed rectifier. The action potential graph records a plateau when the influx of calcium inside the cell is equal to the outflow of potassium to the cell’s outside.
Phase 3- Repolarization: A decrease in Ca++ conductance and an increase in K+ cause this phase. Calcium inflow is stopped by L-type Ca2+ channel closure. More potassium goes outside when the external rectifying K+ channels open, increasing potassium permeability.
Phase 4- The film potential of a cell at rest is known as the resting potential. A level line in the non-nodal tissue activity potential speaks to this stage. Decreased Na+ and Ca++ conductance and a rise in K+ are what cause this phase.
The resting layer potential is reestablished when the internal evaluating K+ channels open, reestablishing the penetrability of the film to potassium particles.
Resting membrane potential :-
1. Purkinje fibers: -90 to -100 mV
2. Only one cardiac muscle fiber : -85 to -95 mV
3. Sinoatrial node : -55 to -60 mV
3. Properties of cardiac muscle- CONDUCTIVITY
across the internodal fibers, the impulse that started at the SA node travels across the atria and ends up at the AV node. The electrical signal is sent to the both ventricles by the AV node via the tangled network of His and its branches. The impulse travels from the heart’s apex to its base via the Purkinje fibers. One meter per second is the rate of conduction in the Purkinje and His bundle of fibers, 0.4 m/s in the ventricular muscles, 0.05 m/s in the SA node, and 0.1 m/s in the AV node
Conductive system impulses velocity-
1. Purkinje fibers: 4.0 m/sec.
2. Internodal fibers: 1.0 m/sec.
3. Ventricular muscle fibers: 0.5 m/sec.
4. Atrial muscle fibers: 0.3 m/sec.
5. Bundle of His: 0.12 m/sec.
6. AV node: 0.05 m/sec.
4. Properties of cardiac muscle- CONTRACTILITY
Similar to other muscle tissue, the cardiac muscle contracts in response to suitable stimuli because it is excitable. Myofibril, which is made up of the protein units actin and myosin, is the basic contractile unit of heart muscle. These two units are linked together during contraction while ATP is present, shortening the fiber; but, during rest, they separate apart once more as ATP is synthesized anew. Myosin is a “ATPase” enzyme that can dephosphorylate ATP on its own. The activation of ATPase activity by the Ca++ ion promotes the fast interaction of actin-myosin and ADP complex. Because of the connection of more contractile units, excess calcium always maintains the muscle block in the contracting state (calcium rigidity). Actin and myosin interaction is not encouraged by K+ ions. Therefore, the cardiac muscle gradually ceases in diastole if too much K+ is supplied to the extracellular space.
5. Properties of cardiac muscle- ALL Or NONE RESPONSE
When electrical shocks with varying strengths are applied widely apart to a quiescent heart muscle, the muscle only contracts in its entirety once the threshold strength is reached. However, there was no corresponding increase in contraction amplitude when stimulus intensity increased. This is the behavior of a single skeletal muscle fiber, however graded responses occur when the muscle as a whole is activated by stimuli of varying intensity.
In summary, the muscle is unresponsive to stimuli that are below its threshold and contracts to its maximum when the power of the stimulus reaches it.
Heart muscle in its whole is subject to the all-or-none law. It results from the heart muscle’s syncytial configuration. Only one muscle fiber is affected by the all-or-none law when it comes to skeletal muscle.
6. Properties of cardiac muscle- STAIRCASE PHENOMENON
If the heart muscle is stimulated with an induced current during a Stannius planning, the ventricular muscle will first contract with a progressive increase in size before becoming stable. The term “treppe” or “staircase phenomenon” describes this.
The control of withdrawal rises dynamically for the introductory few withdrawals when the ventricular muscle of a frog’s tranquil heart is fortified at interims of as it were two seconds, without modifying its quality. After that, it remains consistent. Staircase wonder is the term utilized to describe a dynamic increment in withdrawal constrain.
Benefits that increase the power of subsequent contraction are what generate the staircase phenomenon. Thus, the force of contraction increases gradually.
7. Properties of cardiac muscle- REFRACTORY PERIOD
The term “refractory period” refers to the time when a muscle does not respond to a stimulation. The heart’s refractory period is lengthy and can be split into two sections.
1. Absolute refractory period- This time frame encompasses the entirety of the contraction. If a stimulus occurs during this time, no matter how powerful, it will not cause a reaction. Heart muscle cannot be tetanized as a result. This extended refractory period provides adequate time for the heart muscle to heal. The heart muscle cannot become exhausted for this reason. This time frame corresponds to the depolarization phase’s beginning to the repolarization phase ending at the threshold potential.
2. Relative refractory period- This involves the initial phase of relaxation and begins right after the absolute refractory period. A really powerful stimulus is the only one that will work. This phase lasts from immediately before the repolarization phase is stopped until the transmembrane voltage during the phase has barely achieved the lowest possible potential (-60 mV).
The AV node has the greatest refractory period, taken after by the ventricles’ intermediate period and the atria’s least.
Quinidine and digitalis are two medicines that protract the supreme refractive time. By diminishing the systole, excitation of the vagus nerve abbreviates the refractory period.
The heart’s diastole keeps going contrarily longer than its systole, and bad habit versa for the length of the refractory stage. It will hence depend on the heart rate. The outright refractory time for speeds up to 100 m/min is generally 0.2 s.
8. Properties of cardiac muscle- TONICITY
Heart muscle tone is achieved. This tone is flexible and insensitive to anxiety. In this way, it can maintain a fairly constant strain on its varying contents. It has been observed that not all heart tissues acquire the different properties of the cardiac muscle in the same order. Some tissues have evolved special characteristics, whereas others have not. It’s also clear that these functions are related to the size and molecular composition of the muscle cells.
Heart muscle can subsequently be categorized into four groups:
1. The most minor strands at the hubs with the slightest sum of glycogen.
2. The more extensive strands in the ventricles that contain more glycogen.
3. The indeed more extensive filaments in the atria that contain more glycogen.
4. The bundle of His and its branches, the largest filaments with an plenitude of glycogen in the Purkinje fibers.
Both the rate of conduction and the sum of glycogen increment with the length of the strands. In any case, the hard-headed period, rhythmicity, and systole length develop in the inverse heading.