Compare And Contrast Cardiac And Smooth Muscle
Compare and contrast cardiac and smooth muscle.
In cardiac muscle, each heartbeat is triggered by the hearts own pacemaker cells, which initiate electrical discharge and when this reaches the contractile muscle cells (the cardiomyocytes), they create an action potential which increases the concentration of calcium ions into the cell. Calcium ions play a key role in activating what is known as the contractile machinery – the actin and myosin filaments. The cardiac action potential, unlike other forms of muscular contraction, will last roughly as long contraction. This is shown below, depolarisation is followed by an unusual plateau, and the cardiac action potential can be split into five main phases.
Cardiomyocytes are also electrically coupled – so they all contract and relax in unison, enabling for a unified response in the heart (i.e. the atrial and ventral muscles contract in unison) In cardiac cells, the …show more content…
force created through contraction is controlled by the concentration of intracellular calcium ions in the cell and the stretch of the fibres during the diastolic period.
Ultrastructure of a cardiomyocyte
Adjacent myocytes in cardiac muscle are joined together via structures called intercalated discs. Such discs are made up of two different types of junctions:
• The gap junction, which provides electrical conduction in a coupled manner by allowing electrical current to flow through different cells in a low-resistant channel
• The desmosome, which provides mechanical and structural strength, physically joining the cells together
The gap junction is most important for the electrical activity of cardiomyocytes, they transmit ionic current and electrical excitation from one myocyte to the next and is made up of 6 sub-units of connexion proteins to create a channel called a connexon. This creates continuous channels that link myocytes together, allowing ions to flow freely between cells, this means that the whole group of myocytes joined together can act electrically coupled – ensuring that all the myocytes in cardiac muscle in the hear contract almost simultaneously. Clinically, improper formation can create serious problems, for example, in a myocardial ischaemia (where blood flow to heart muscle is decreased by a blockage of arteries), increases in intracellular acidity (due to lactate formation through anaerobic respiration) and increase in calcium ions can cause closure of some connexons, leading to poor electrical coupling of myocytes and eventually arrhythmia (irregular heartbeat)
Myocytes are packed with long, contractile bundles called myofibrils. Each myofibril is composed of numerous contractile units called sarcomeres, joined end-to-end – this is what gives cardiac muscle its striated appearance. A sarcomere consists of thick myosin filaments, thin actin filaments, and additional proteins tropomyosin, troponin and titin. At certain zones, the surface membrane is invaginated into, creating a set of T-tubules, which contain sodium ion and calcium ion channels, enabling them to transmit electrical excitation into the myocyte.
The sarcoplasmic reticulum (SR) is a second set of tubes that run through the myofibril and they run close to the T-tubules. The SR contains a store of calcium ions, and these calcium ions are partially released into the cytoplasm to create contraction. The SR has two different units, called:
- Junctional SR: This part of the SR approaches the t-tubules, forming a diad. The lumen contains a store of calcium ions loosely attached to a storage protein. Proteins extend from the JSR to the t-tubules called Calcium release channels, ryanodine receptors or CICR channels.
- Network SR: This part of the SR consists of tubes that run over the myofibrils and take up sarcoplasmic calcium ions through a calcium-ATPase pump. This pump is regulated through another protein called phospholamban.
Mechanism of contraction
Contraction is brought about by the actin filaments sliding over the thick myosin filaments in cycles. During excitation, the calcium concentration increases, which activates a fraction of the cross-bridges possible, meaning the contraction is weaker than it could possibly be. This means that the level/force of contraction can be controlled by increasing concentrations of calcium ions, providing the heart with an effective and efficient way to regulate the force of its contractions, as certain physiological conditions (i.e. during exercise) may require greater contractile force from the heart to increase cardiac output. The contractile force depends on the number of cross-bridges forming, which in turn depend on the calcium ion concentration. Hormones, such as adrenaline, can increase calcium ion concentrations in cells and hence increase contractile force.
Cardiomyocytes have three different cation channels – potassium, sodium and calcium ion channels, which change from open to closed conformations. The resting potential of a myocyte is determined by the high concentration of potassium ions in the cell and the high permeability to potassium of these channels.
The resting membrane potential is achieved through potassium ions leaking out, down their concentration gradient and being ‘pulled’ back in, through their electrochemical gradient. This carries on until it reaches a state where it is balanced and has no net influx or efflux of potassium ions. The membrane potential at which this equilibrium is reached can be calculated using the Nernst Equation. However, this equation doesn’t give us an accurate representation of the cardiomyocytes – as the permeability of the myocyte to sodium ions must also be taken into account. The presence of sodium ions flowing into the cell means that potassium ions move outside the cell. To account for this, we can produce the conductance equation to work out what the membrane potential would actually be. In some clinical conditions, extracellular potassium ions can greatly increase in concentration, such as renal failure or in a cardiac ischaemia.
As contraction depends on ionic concentrations, pumps and exchangers play a crucial role in maintaining normal function in the heart. As potassium ions slowly leak out of the cell, and sodium ions slowly leak into the cell, an imbalance could occur eventually. To rectify this, Na-K pumps, which are ATPase pumps, use ATP to pump sodium ions outside of the cell and pump potassium ions back into the cell. This works in conjunction with other important pumps, such as the Na-Ca pump and the Na-H pump, though their relevance will be explored later.
The resting membrane potential in a cardiomyocyte is roughly around -80mv, which, when stimulated, will produce an action potential, which will in turn create contraction. In cardiomyocytes however, action potentials will vary with the different cardiomyocytes, depending on which region you find them in. As in skeletal muscle and shown in figure 1, the action potential in cardiomyocytes consists of a depolarisation but crucially, unlike skeletal muscle, this is followed by a plateau before the membrane potential reaches its resting value again. The resting membrane potential of cardiomyocytes depend mostly on the potassium ion concentration gradient, while the magnitude of the action potential (i.e. the size of the initial peak in figure 1) depends mostly on the sodium ion concentration change.
Depolarisation in cardiomyocytes can be split into different phases:
Phase 0 – This is where depolarisation is caused by a rapid increase in membrane permeability to sodium ions – this rapid increase in permeability is caused by voltage-gated sodium channels opening, and sodium ions then travelling into the cell via their electrochemical gradient. Interestingly, the membrane potential never reaches the equilibrium potential for sodium ions, as there is a small outward current of potassium ions accompanying depolarisation. A period called the overshoot can also be seen. Eventually sodium channels inactivate.
Phase 1 – This is where the membrane undergoes a fast (but crucially, incomplete) repolarisation, due to an outward current, caused by the efflux of potassium ions through voltage-gated potassium channels. Alongside this, chloride ion influx into the cell also contributes to the partial repolarisation in phase 1. This partial repolarisation is important because it creates a strong electrochemical gradient for calcium ions (which are important in later phases) and potassium ion movements determine the exact length of an action potential. This can be important clinically because high levels of potassium ions in the heart can lead to a condition known as hyperkalaemia, where elevated levels of extracellular potassium ions can lead to the heart beating at a dangerously rapid rate (fibrillation) or stop beating altogether (cardiac arrest)
Phase 2 – Part A – The membrane potential at this phase is characterised by a plateau, the beginning of the plateau is caused by a small and long-lasting influx of calcium ions into the cell, and this influx of calcium ions prevents the cardiomyocytes from repolarising quickly. This can be proved experimentally – using tetrodotoxin.
This toxin blocks sodium ion channels and the inability of sodium ions to act causes the absence of the initial steep depolarisation, but crucially doesn’t remove the plateau towards the end. This influx of calcium ions is caused by the opening of L-type calcium ion channels. L-type channels are voltage-dependent calcium channels; the current peaks quickly and then decreases slowly, as the L-type channels close really slowly. The conductance of calcium channels in much smaller than that for sodium channels, meaning that the inward calcium current is much slower than the inward sodium current before it.
Phase 2 – Part B – The second part of the plateau is dependent upon sodium channels – they create an inward current of sodium ions is mediated through Na-Ca exchangers, three sodium ions are pumped into the cell, for every one calcium ion pumped out of the cell.
Phase 3 – As the plateau continues, the potassium conductance again begins to increase, due to the opening of delayed rectifier potassium channels – these act in a delayed or slow manner. As repolarisation occurs, the conductance of potassium ion channels decrease, causing final repolarisation. The size of the potassium ion current influx affects the duration of the plateau.
The refractory period is cardiomyocytes is during the depolarisation phase, where the cardiomyocyte cannot be electrically excited at all, this is called the absolute refractory period. As depolarisation occurs, when it reaches around -50mv, it enters its relative refractory period, where the cardiomyocyte can be excited, but only with a stimulus which is far stronger than the initial stimulus.
Excitation-contraction
coupling
Both intracellular and extracellular calcium ion concentrations are needed for contraction in cardiac muscle. Extracellular calcium is needed because the influx of calcium ions into the membrane triggers the release of calcium ions, which is stored in the sarcoplasmic reticulum, which mediate contraction. The release of calcium ions from the sarcoplasmic reticulum are needed for cross-bridge formation (between Actin and Myosin proteins) The increase in concentration of calcium in the cell come from two main sources:
- SR store
- Plateau calcium current
When an action potential is instigated in cardiomyocytes, there is a sharp increase in concentration of calcium ions in the cytoplasm – and some of these calcium ions will bind to troponin-C (a protein on Actin filaments) which will expose binding sites of Actin, enabling cross-bridge formation. There is also a correlation between the concentration of cytoplasmic calcium ions and the force of contraction produced by the muscle as a result – this was proved experimentally by using fluorescent dyes.
The junctional SR also contains a store of calcium ions, and contains RyR (ryanodine) receptors, which enable the release of these stored calcium ions into the cytoplasm. Calcium ions are release through T-tubules – these region contain both L-type calcium channels and normal calcium channels – the opening of the L-type channels causes the opening of the normal calcium release channels, hence the process is called calcium-induced-calcium release.
However, when calcium ions need to be up-taken for relaxation, the other unit of the SR, the network SR contains many Ca-ATPase pumps, reduce the concentration of calcium ions back to normal and pump the ions back into the SR interior. The calcium is also reduced through the Na-Ca exchanger. The protein phospholamban is an inhibitory protein, which regulates the SR Calcium ion pump. This produces a breaking effect on the pump and is reduced by the hormones adrenaline and noradrenaline. Adrenaline acts ionotropically (increases contractile force) and lusitropically (increases rate of relaxation)
There is a relation between resting fibre length and total tension produced from contraction in cardiac muscle – there is a resting length where the tension developed is maximal. The initial length of the fibres is determined by the degree of diastolic filling of the heart and as a result the pressure developed in the ventricle is proportionate to the volume of the ventricle at the end of the filling phase (this is called Starling’s law of the heart). The developed tension increases as the diastolic volume increases until it reaches a maximum, then starts to decrease. The decrease is due to the disruption of the myocardial fibres. The force of contraction of cardiac muscle can be also increased by catecholamines, which is mediated via innervated beta-1-adrenergic receptors and cyclic AMP.
Smooth Muscle
Ultrastructure of smooth muscle
Smooth muscle cells are relatively small, and are roughly cylindrical most of their length, but also thin at the end. The cells contains many areas where the membrane invaginates, creating structures called caveolae. Unlike cardiomyocytes, there is no T-tubule system and the SR is not as profound – instead, most of the cytoplasm consists of three myofilaments – thick, thin and intermediate. The thick filaments (myosin) are slightly different to the myosin filaments found in cardiomyocytes because the filaments are side-polar, with one cross-bridge on one side of the filament and another cross-bridge on the opposite site. Also unlike cardiomyocytes, the central region of the myosin filament is not empty. In the cytoplasm of cardiomyocytes dense, dark areas, called dense bodies, are present, which are due to the presence of intermediate and thin filaments. Dense bodies can act as anchors to transmit strong contractile forces between cells.
Unlike cardiomyocytes, smooth muscle cells don’t have a striated structure and don’t have the sarcomeric structure of cardiomyocytes.
Smooth muscle cells, like cardiomyocytes, are also coupled electrically through gap junctions, made up of 5 connexin proteins this time which form a connexon, again, this creates a low-resistant channel for electrical currents to travel to and from adjacent smooth muscle cells, allowing all smooth muscle cells in a tissue to be able to act in sync.
Mechanism of contraction
Like in cardiomyocytes, in order to mediate contraction, intracellular levels of calcium ions must be increased for smooth muscle contraction. This means that calcium influx and calcium efflux in the cell must be tightly controlled and regulated. Calcium entry can occur via various pathways, these include voltage-gated calcium channels, ligand-gated channels and stretch-activated calcium channels. The different ways in which calcium ions enter a smooth muscle cells enables us to create distinctions between different types of smooth muscle cells:
• Electrically excitable: This occurs mainly through nervous activity
• Electrically unexcitable: This occurs mainly through second messengers, such as cAMP
• Spontaneous electrically excitable, controlled by
When the membrane of smooth muscle cells become depolarised (because of an action potential), voltage-gated calcium ion channels open, which leads to the influx of calcium ions down their concentration gradient. Some of these calcium ions will directly contribute to contraction by interacting with the contractile machinery while some calcium ions from other sources will also contribute to contraction.
Although the sarcoplasmic reticulum is not as profound as in cardiac cells, they nonetheless make a significant contribution to smooth muscle contraction through calcium-induced-calcium release, just like the mechanism found in cardiomyocytes. Alongside this, calcium can also be released by another mechanism, which involves IP3. IP3 is a molecule formed by the activation of the enzyme PLC, by ligand-gated receptors. As a result of binding, IP3 and DAG form from PIP2. During contraction, smooth muscle cells will also have calcium channels that are activated by mechanical stress (i.e. stretching) During contraction of smooth muscle cells, the concentration of calcium ions in the cytoplasm is increased many orders of magnitude. However, during relaxation, calcium ions must be expelled from the cytoplasm – and this occurs in mainly through the SR (via Ca-ATPase carriers) and the rest is released from the cell by an ATP-transporter and the Na-Ca exchangers.
Control of smooth muscle contraction mainly relies on thick, myosin filaments – unlike the thin, actin filaments which contained troponin molecules that played a key role in modulating contraction in cardiomyocytes. Actin filaments in smooth muscle cells lack the protein troponin that calcium ions would normally bind to. When smooth muscle cells are at rest, there are no interactions between myosin and actin filaments because each myosin molecule contains four protein light chains – two essential light chains (needed for actin-myosin interactions) and two regulatory chains (needed for phosphorylation) The two regulatory chains contain locations where it can be phosphorylated, the enzyme that catalyses this process is called myosin light-chain kinase (MLCK)
When the regulatory chains are phosphorylated, the myosin heads can interact with actin filaments with the contractile process being similar to that found in cardiac cells. As the phosphorylation of MLCK plays a key role in modulating contraction, the protein is closely controlled. A protein called calmodulin (CaM) binds to calcium ions. When four calcium ions bind to CaM, the protein activates MLCK and phosphorylation can begin – this activation step is most sensitive to levels of calcium ions in the cytoplasm. This mechanism means that, similar to cardiomyocytes, the level of contraction can be controlled – the different level of calcium ion concentration in the cytoplasm will bring about different magnitudes of contractile forces.
This phosphorylation of myosin can be reversed by using the enzyme myosin light-chain phosphatase (MLCP). Although the enzyme is continually active (even during contraction), after contraction it is most active, hence encouraging relaxation.
Relaxation of smooth muscle is brought about through several mechanisms, though of all these mechanisms involve the reduction of cytoplasmic calcium ion concentration. Also, decreases MLCK activity and increases MLCP activity contribute to relaxation. This is because myosin, without phosphorylation, has a low affinity for actin, so the cross-bridge cycling cannot take place. The relaxation of smooth muscle is caused by a variety of mechanisms working together in tandem:
• Electrical repolarisation
• Ca pumps/exchangers
• Uptake of Ca back into the SR
Various second messengers also play a role in regulating smooth muscle relaxation – these predominantly include cAMP and cGMP
For example, binding of catecholamines to beta-adrenergic receptors in smooth muscle found in the lungs activates adenylyl cyclase, resulting in the formation cAMP. cAMP then goes onto trigger a cascade of events within the smooth muscle that eventually lead to relaxation. These include increased calcium uptake by the SR and decreases in myosin light-chain phosphorylation. Nitric oxide relax smooth muscle by activating guanylyl cyclase and producing the second messenger cGMP, which activates cGMP-dependent protein kinase (PKG), which in turn opens calcium-activated potassium ion channels in the membrane, leading to hyperpolarization and subsequent relaxation. PKG also activates MLCP and blocks the activity of PLC, hence reducing calcium ions release through IP3.
To conclude, regulation of cardiac muscle contraction is dependent on the number of cross-bridges forming, which in turn depends on calcium ion concentration in the cytoplasm. Calcium ions bind to troponin to modulate contraction. Regulation of smooth muscle contraction is also through calcium ions, calcium ions bind to Calmodulin on the MLCK which causes phosphorylation of the light chains on myosin. This enables the myosin filament to interact with actin and participate in the cross-bridge cycling. Both cardiac muscle and smooth muscle cells are electrically coupled. Relaxation in both smooth muscle and cardiac muscle is associated with lowering the level of calcium ions in the cytoplasm and (in smooth muscle cells) dephosphorylation of the myosin through a phosphatase enzyme, MLCP.
In cardiac muscle, each heartbeat is triggered by the hearts own pacemaker cells, which initiate electrical discharge and when this reaches the contractile muscle cells (the cardiomyocytes), they create an action potential which increases the concentration of calcium ions into the cell. Calcium ions play a key role in activating what is known as the contractile machinery – the actin and myosin filaments. The cardiac action potential, unlike other forms of muscular contraction, will last roughly as long contraction. This is shown below, depolarisation is followed by an unusual plateau, and the cardiac action potential can be split into five main phases.
Cardiomyocytes are also electrically coupled – so they all contract and relax in unison, enabling for a unified response in the heart (i.e. the atrial and ventral muscles contract in unison) In cardiac cells, the …show more content…
force created through contraction is controlled by the concentration of intracellular calcium ions in the cell and the stretch of the fibres during the diastolic period.
Ultrastructure of a cardiomyocyte
Adjacent myocytes in cardiac muscle are joined together via structures called intercalated discs. Such discs are made up of two different types of junctions:
• The gap junction, which provides electrical conduction in a coupled manner by allowing electrical current to flow through different cells in a low-resistant channel
• The desmosome, which provides mechanical and structural strength, physically joining the cells together
The gap junction is most important for the electrical activity of cardiomyocytes, they transmit ionic current and electrical excitation from one myocyte to the next and is made up of 6 sub-units of connexion proteins to create a channel called a connexon. This creates continuous channels that link myocytes together, allowing ions to flow freely between cells, this means that the whole group of myocytes joined together can act electrically coupled – ensuring that all the myocytes in cardiac muscle in the hear contract almost simultaneously. Clinically, improper formation can create serious problems, for example, in a myocardial ischaemia (where blood flow to heart muscle is decreased by a blockage of arteries), increases in intracellular acidity (due to lactate formation through anaerobic respiration) and increase in calcium ions can cause closure of some connexons, leading to poor electrical coupling of myocytes and eventually arrhythmia (irregular heartbeat)
Myocytes are packed with long, contractile bundles called myofibrils. Each myofibril is composed of numerous contractile units called sarcomeres, joined end-to-end – this is what gives cardiac muscle its striated appearance. A sarcomere consists of thick myosin filaments, thin actin filaments, and additional proteins tropomyosin, troponin and titin. At certain zones, the surface membrane is invaginated into, creating a set of T-tubules, which contain sodium ion and calcium ion channels, enabling them to transmit electrical excitation into the myocyte.
The sarcoplasmic reticulum (SR) is a second set of tubes that run through the myofibril and they run close to the T-tubules. The SR contains a store of calcium ions, and these calcium ions are partially released into the cytoplasm to create contraction. The SR has two different units, called:
- Junctional SR: This part of the SR approaches the t-tubules, forming a diad. The lumen contains a store of calcium ions loosely attached to a storage protein. Proteins extend from the JSR to the t-tubules called Calcium release channels, ryanodine receptors or CICR channels.
- Network SR: This part of the SR consists of tubes that run over the myofibrils and take up sarcoplasmic calcium ions through a calcium-ATPase pump. This pump is regulated through another protein called phospholamban.
Mechanism of contraction
Contraction is brought about by the actin filaments sliding over the thick myosin filaments in cycles. During excitation, the calcium concentration increases, which activates a fraction of the cross-bridges possible, meaning the contraction is weaker than it could possibly be. This means that the level/force of contraction can be controlled by increasing concentrations of calcium ions, providing the heart with an effective and efficient way to regulate the force of its contractions, as certain physiological conditions (i.e. during exercise) may require greater contractile force from the heart to increase cardiac output. The contractile force depends on the number of cross-bridges forming, which in turn depend on the calcium ion concentration. Hormones, such as adrenaline, can increase calcium ion concentrations in cells and hence increase contractile force.
Cardiomyocytes have three different cation channels – potassium, sodium and calcium ion channels, which change from open to closed conformations. The resting potential of a myocyte is determined by the high concentration of potassium ions in the cell and the high permeability to potassium of these channels.
The resting membrane potential is achieved through potassium ions leaking out, down their concentration gradient and being ‘pulled’ back in, through their electrochemical gradient. This carries on until it reaches a state where it is balanced and has no net influx or efflux of potassium ions. The membrane potential at which this equilibrium is reached can be calculated using the Nernst Equation. However, this equation doesn’t give us an accurate representation of the cardiomyocytes – as the permeability of the myocyte to sodium ions must also be taken into account. The presence of sodium ions flowing into the cell means that potassium ions move outside the cell. To account for this, we can produce the conductance equation to work out what the membrane potential would actually be. In some clinical conditions, extracellular potassium ions can greatly increase in concentration, such as renal failure or in a cardiac ischaemia.
As contraction depends on ionic concentrations, pumps and exchangers play a crucial role in maintaining normal function in the heart. As potassium ions slowly leak out of the cell, and sodium ions slowly leak into the cell, an imbalance could occur eventually. To rectify this, Na-K pumps, which are ATPase pumps, use ATP to pump sodium ions outside of the cell and pump potassium ions back into the cell. This works in conjunction with other important pumps, such as the Na-Ca pump and the Na-H pump, though their relevance will be explored later.
The resting membrane potential in a cardiomyocyte is roughly around -80mv, which, when stimulated, will produce an action potential, which will in turn create contraction. In cardiomyocytes however, action potentials will vary with the different cardiomyocytes, depending on which region you find them in. As in skeletal muscle and shown in figure 1, the action potential in cardiomyocytes consists of a depolarisation but crucially, unlike skeletal muscle, this is followed by a plateau before the membrane potential reaches its resting value again. The resting membrane potential of cardiomyocytes depend mostly on the potassium ion concentration gradient, while the magnitude of the action potential (i.e. the size of the initial peak in figure 1) depends mostly on the sodium ion concentration change.
Depolarisation in cardiomyocytes can be split into different phases:
Phase 0 – This is where depolarisation is caused by a rapid increase in membrane permeability to sodium ions – this rapid increase in permeability is caused by voltage-gated sodium channels opening, and sodium ions then travelling into the cell via their electrochemical gradient. Interestingly, the membrane potential never reaches the equilibrium potential for sodium ions, as there is a small outward current of potassium ions accompanying depolarisation. A period called the overshoot can also be seen. Eventually sodium channels inactivate.
Phase 1 – This is where the membrane undergoes a fast (but crucially, incomplete) repolarisation, due to an outward current, caused by the efflux of potassium ions through voltage-gated potassium channels. Alongside this, chloride ion influx into the cell also contributes to the partial repolarisation in phase 1. This partial repolarisation is important because it creates a strong electrochemical gradient for calcium ions (which are important in later phases) and potassium ion movements determine the exact length of an action potential. This can be important clinically because high levels of potassium ions in the heart can lead to a condition known as hyperkalaemia, where elevated levels of extracellular potassium ions can lead to the heart beating at a dangerously rapid rate (fibrillation) or stop beating altogether (cardiac arrest)
Phase 2 – Part A – The membrane potential at this phase is characterised by a plateau, the beginning of the plateau is caused by a small and long-lasting influx of calcium ions into the cell, and this influx of calcium ions prevents the cardiomyocytes from repolarising quickly. This can be proved experimentally – using tetrodotoxin.
This toxin blocks sodium ion channels and the inability of sodium ions to act causes the absence of the initial steep depolarisation, but crucially doesn’t remove the plateau towards the end. This influx of calcium ions is caused by the opening of L-type calcium ion channels. L-type channels are voltage-dependent calcium channels; the current peaks quickly and then decreases slowly, as the L-type channels close really slowly. The conductance of calcium channels in much smaller than that for sodium channels, meaning that the inward calcium current is much slower than the inward sodium current before it.
Phase 2 – Part B – The second part of the plateau is dependent upon sodium channels – they create an inward current of sodium ions is mediated through Na-Ca exchangers, three sodium ions are pumped into the cell, for every one calcium ion pumped out of the cell.
Phase 3 – As the plateau continues, the potassium conductance again begins to increase, due to the opening of delayed rectifier potassium channels – these act in a delayed or slow manner. As repolarisation occurs, the conductance of potassium ion channels decrease, causing final repolarisation. The size of the potassium ion current influx affects the duration of the plateau.
The refractory period is cardiomyocytes is during the depolarisation phase, where the cardiomyocyte cannot be electrically excited at all, this is called the absolute refractory period. As depolarisation occurs, when it reaches around -50mv, it enters its relative refractory period, where the cardiomyocyte can be excited, but only with a stimulus which is far stronger than the initial stimulus.
Excitation-contraction
coupling
Both intracellular and extracellular calcium ion concentrations are needed for contraction in cardiac muscle. Extracellular calcium is needed because the influx of calcium ions into the membrane triggers the release of calcium ions, which is stored in the sarcoplasmic reticulum, which mediate contraction. The release of calcium ions from the sarcoplasmic reticulum are needed for cross-bridge formation (between Actin and Myosin proteins) The increase in concentration of calcium in the cell come from two main sources:
- SR store
- Plateau calcium current
When an action potential is instigated in cardiomyocytes, there is a sharp increase in concentration of calcium ions in the cytoplasm – and some of these calcium ions will bind to troponin-C (a protein on Actin filaments) which will expose binding sites of Actin, enabling cross-bridge formation. There is also a correlation between the concentration of cytoplasmic calcium ions and the force of contraction produced by the muscle as a result – this was proved experimentally by using fluorescent dyes.
The junctional SR also contains a store of calcium ions, and contains RyR (ryanodine) receptors, which enable the release of these stored calcium ions into the cytoplasm. Calcium ions are release through T-tubules – these region contain both L-type calcium channels and normal calcium channels – the opening of the L-type channels causes the opening of the normal calcium release channels, hence the process is called calcium-induced-calcium release.
However, when calcium ions need to be up-taken for relaxation, the other unit of the SR, the network SR contains many Ca-ATPase pumps, reduce the concentration of calcium ions back to normal and pump the ions back into the SR interior. The calcium is also reduced through the Na-Ca exchanger. The protein phospholamban is an inhibitory protein, which regulates the SR Calcium ion pump. This produces a breaking effect on the pump and is reduced by the hormones adrenaline and noradrenaline. Adrenaline acts ionotropically (increases contractile force) and lusitropically (increases rate of relaxation)
There is a relation between resting fibre length and total tension produced from contraction in cardiac muscle – there is a resting length where the tension developed is maximal. The initial length of the fibres is determined by the degree of diastolic filling of the heart and as a result the pressure developed in the ventricle is proportionate to the volume of the ventricle at the end of the filling phase (this is called Starling’s law of the heart). The developed tension increases as the diastolic volume increases until it reaches a maximum, then starts to decrease. The decrease is due to the disruption of the myocardial fibres. The force of contraction of cardiac muscle can be also increased by catecholamines, which is mediated via innervated beta-1-adrenergic receptors and cyclic AMP.
Smooth Muscle
Ultrastructure of smooth muscle
Smooth muscle cells are relatively small, and are roughly cylindrical most of their length, but also thin at the end. The cells contains many areas where the membrane invaginates, creating structures called caveolae. Unlike cardiomyocytes, there is no T-tubule system and the SR is not as profound – instead, most of the cytoplasm consists of three myofilaments – thick, thin and intermediate. The thick filaments (myosin) are slightly different to the myosin filaments found in cardiomyocytes because the filaments are side-polar, with one cross-bridge on one side of the filament and another cross-bridge on the opposite site. Also unlike cardiomyocytes, the central region of the myosin filament is not empty. In the cytoplasm of cardiomyocytes dense, dark areas, called dense bodies, are present, which are due to the presence of intermediate and thin filaments. Dense bodies can act as anchors to transmit strong contractile forces between cells.
Unlike cardiomyocytes, smooth muscle cells don’t have a striated structure and don’t have the sarcomeric structure of cardiomyocytes.
Smooth muscle cells, like cardiomyocytes, are also coupled electrically through gap junctions, made up of 5 connexin proteins this time which form a connexon, again, this creates a low-resistant channel for electrical currents to travel to and from adjacent smooth muscle cells, allowing all smooth muscle cells in a tissue to be able to act in sync.
Mechanism of contraction
Like in cardiomyocytes, in order to mediate contraction, intracellular levels of calcium ions must be increased for smooth muscle contraction. This means that calcium influx and calcium efflux in the cell must be tightly controlled and regulated. Calcium entry can occur via various pathways, these include voltage-gated calcium channels, ligand-gated channels and stretch-activated calcium channels. The different ways in which calcium ions enter a smooth muscle cells enables us to create distinctions between different types of smooth muscle cells:
• Electrically excitable: This occurs mainly through nervous activity
• Electrically unexcitable: This occurs mainly through second messengers, such as cAMP
• Spontaneous electrically excitable, controlled by
When the membrane of smooth muscle cells become depolarised (because of an action potential), voltage-gated calcium ion channels open, which leads to the influx of calcium ions down their concentration gradient. Some of these calcium ions will directly contribute to contraction by interacting with the contractile machinery while some calcium ions from other sources will also contribute to contraction.
Although the sarcoplasmic reticulum is not as profound as in cardiac cells, they nonetheless make a significant contribution to smooth muscle contraction through calcium-induced-calcium release, just like the mechanism found in cardiomyocytes. Alongside this, calcium can also be released by another mechanism, which involves IP3. IP3 is a molecule formed by the activation of the enzyme PLC, by ligand-gated receptors. As a result of binding, IP3 and DAG form from PIP2. During contraction, smooth muscle cells will also have calcium channels that are activated by mechanical stress (i.e. stretching) During contraction of smooth muscle cells, the concentration of calcium ions in the cytoplasm is increased many orders of magnitude. However, during relaxation, calcium ions must be expelled from the cytoplasm – and this occurs in mainly through the SR (via Ca-ATPase carriers) and the rest is released from the cell by an ATP-transporter and the Na-Ca exchangers.
Control of smooth muscle contraction mainly relies on thick, myosin filaments – unlike the thin, actin filaments which contained troponin molecules that played a key role in modulating contraction in cardiomyocytes. Actin filaments in smooth muscle cells lack the protein troponin that calcium ions would normally bind to. When smooth muscle cells are at rest, there are no interactions between myosin and actin filaments because each myosin molecule contains four protein light chains – two essential light chains (needed for actin-myosin interactions) and two regulatory chains (needed for phosphorylation) The two regulatory chains contain locations where it can be phosphorylated, the enzyme that catalyses this process is called myosin light-chain kinase (MLCK)
When the regulatory chains are phosphorylated, the myosin heads can interact with actin filaments with the contractile process being similar to that found in cardiac cells. As the phosphorylation of MLCK plays a key role in modulating contraction, the protein is closely controlled. A protein called calmodulin (CaM) binds to calcium ions. When four calcium ions bind to CaM, the protein activates MLCK and phosphorylation can begin – this activation step is most sensitive to levels of calcium ions in the cytoplasm. This mechanism means that, similar to cardiomyocytes, the level of contraction can be controlled – the different level of calcium ion concentration in the cytoplasm will bring about different magnitudes of contractile forces.
This phosphorylation of myosin can be reversed by using the enzyme myosin light-chain phosphatase (MLCP). Although the enzyme is continually active (even during contraction), after contraction it is most active, hence encouraging relaxation.
Relaxation of smooth muscle is brought about through several mechanisms, though of all these mechanisms involve the reduction of cytoplasmic calcium ion concentration. Also, decreases MLCK activity and increases MLCP activity contribute to relaxation. This is because myosin, without phosphorylation, has a low affinity for actin, so the cross-bridge cycling cannot take place. The relaxation of smooth muscle is caused by a variety of mechanisms working together in tandem:
• Electrical repolarisation
• Ca pumps/exchangers
• Uptake of Ca back into the SR
Various second messengers also play a role in regulating smooth muscle relaxation – these predominantly include cAMP and cGMP
For example, binding of catecholamines to beta-adrenergic receptors in smooth muscle found in the lungs activates adenylyl cyclase, resulting in the formation cAMP. cAMP then goes onto trigger a cascade of events within the smooth muscle that eventually lead to relaxation. These include increased calcium uptake by the SR and decreases in myosin light-chain phosphorylation. Nitric oxide relax smooth muscle by activating guanylyl cyclase and producing the second messenger cGMP, which activates cGMP-dependent protein kinase (PKG), which in turn opens calcium-activated potassium ion channels in the membrane, leading to hyperpolarization and subsequent relaxation. PKG also activates MLCP and blocks the activity of PLC, hence reducing calcium ions release through IP3.
To conclude, regulation of cardiac muscle contraction is dependent on the number of cross-bridges forming, which in turn depends on calcium ion concentration in the cytoplasm. Calcium ions bind to troponin to modulate contraction. Regulation of smooth muscle contraction is also through calcium ions, calcium ions bind to Calmodulin on the MLCK which causes phosphorylation of the light chains on myosin. This enables the myosin filament to interact with actin and participate in the cross-bridge cycling. Both cardiac muscle and smooth muscle cells are electrically coupled. Relaxation in both smooth muscle and cardiac muscle is associated with lowering the level of calcium ions in the cytoplasm and (in smooth muscle cells) dephosphorylation of the myosin through a phosphatase enzyme, MLCP.