C5 - Skeletal, Smooth and Cardiac Muscle (PDF)

Physio 5 - Membrane excitability: Skeletal, Smooth and cardiac muscle I.​ Skeletal muscle 1.​ Reminder on the contraction of the skeletal muscle The sarcolemma is a thin membrane enclosing a...

Physio 5 - Membrane excitability: Skeletal, Smooth and cardiac muscle I.​ Skeletal muscle 1.​ Reminder on the contraction of the skeletal muscle The sarcolemma is a thin membrane enclosing a skeletal muscle fiber. Myofibrils are composed of actin and myosin filaments. Titin filamentous molecules keep the myosin and actin filaments in place. Each titin molecule extends from the Z disk to the M line. Part of the titin molecule is closely associated with the myosin thick filament, whereas the rest of the molecule is springy and changes length as the sarcomere contracts and relaxes. The sarcoplasm is the IC fluid between myofibrils. The sarcoplasmic reticulum is a specific endoplasmic reticulum of the skeletal muscle. The steps for muscle contraction are: (1)​ An action potential travels along a motor nerve to its endings on muscle fiber. (2)​ At each ending, the nerve secretes a small amount of the neurotransmitter acetylcholine. (3)​ Ach acts on a local area of the muscle fiber membrane to open acetylcholine-gated cation channels through protein molecules floating in the membrane. (4)​ The opening of acetylcholine-gated channels allows large quantities of Na+ ions to diffuse to the interior of the muscle fiber membrane. This action causes a local depolarization that in turn leads to the opening of voltage-gated sodium channels, which initiates an action potential at the membrane. (5)​ The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes. (6)​ The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic  reticulum to release large quantities of calcium ions that have been stored within this reticulum. (7)​ The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process. (8)​ After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca2+ membrane pump and remain stored in the re- ticulum until a new muscle action potential comes along; this removal of calcium ions from the myofi- brils causes the muscle contraction to cease. 2.​ Neuromuscular Transmission and Excitation-Contraction Coupling, the neuromuscular junction (NMJ) Skeletal muscle fibers are innervated by large myelinated nerve fibers that originate from large motoneurons in the anterior horns of the spinal cord, lamina IX. The synapse between the motor neuron and the striated muscle cell is the first chemical synapse studied and called the neuro-muscular junction (NMJ). One axon will drive a group of muscle cells. The functional unit for muscle contraction is the motor unit and it consists of the motor neuron and the innervated muscle cell. 1 muscle fiber receive terminations from 1 axon of a motor neuron and each motor neuron is responsible for 1 muscle group. The current to frequency coding matters because we need to induce an action potential that will become a mechanical event. The muscular portion has involutions to increase the surface of absorption (increasing the number of channels expressed), that are called cristae. The high number of channels allows more conductance, which leads to a higher current, therefore allowing the cell to reach its threshold. a.​ Definition A neuromuscular junction from a large myelinated nerve fiber to a skeletal muscle fiber is formed of a nerve fiber that forms a complex of branching nerve terminals that invaginate into the surface of the muscle fiber but lie outside the muscle fiber plasma membrane. The entire structure is called the motor end plate. It is covered by one or more Schwann cells that insulate it from the surrounding fluids. The invaginated membrane is called the synaptic gutter and the space between the terminal and the fiber membrane is called the synaptic space or synaptic cleft, which is 20 to 30 nanometers wide. At the bottom of the gutter are numerous smaller folds of the muscle membrane called subneural clefts, which greatly increase the surface area at which the synaptic transmitter can act. 3.​ Events The events at the level of the NMJ are: (1)​ Calcium enters and NTs are released (acetylcholine) (2)​ acetylcholine binds to the receptors (nicotinic, ionotropic) (3)​ sodium and potassium channels open initiating a current: depolarization is called end plate potential and it’s a graded potential. The sodium and potassium permeability is nearly equal, however the driving force of sodium is larger. (4)​ action potential in the muscle: it is always above the threshold and will generate an action potential, once it arrives at this point it is not possible to stop it. There is no need of summation of graded potentials since the number of channels in the junctions is very high. (5)​ neurotransmitters clearance: acetylcholinesterase will cleave acetylcholine into acetate and choline and together with the potassium current going we have a repolarization step. The muscle goes back to the reversal potential. 4.​ Ach receptor Nicotinic (Ionotropic) receptors is are present in the subneural clefts of the NMJ. The open conformation of the channel allows Na ions to enter the muscle fiber and excite contraction. 5.​ End plate potential (graded) The sudden insurgence of sodium ions into the muscle fiber when the acetylcholine-gated chan- nels open causes the electrical potential inside the fiber at the local area of the end plate to increase in the positive direction as much as 50 to 75 millivolts, creating a local potential called the end plate potential. REMINDER: A sudden increase in nerve membrane potential of more than 20 to 30 millivolts is normally sufficient to initiate more and more sodium channel opening, thus ini- tiating an action potential at the muscle fiber membrane. This figure shows three separate end plate potentials. End plate potentials A and C are too weak to elicit an action potential, but they do produce weak local end plate voltage changes, as recorded in the figure. The weakness of the end plate potential at point A was caused by poisoning of the muscle fiber with curare, a drug that blocks the gating action of acetylcholine on the acetylcholine channels by competing for the acetylcholine receptor sites. The weakness of the end plate potential at point C resulted from the effect of botulinum toxin, a bacterial poison that decreases the quantity of acetylcholine release by the nerve terminals. By contrast, end plate potential B is much stronger and causes enough sodium channels to open so that the self-regenerative effect of more and more sodium ions flowing to the interior of the fiber initiates an action potential. End plate potential is a graded potential (λ). There is a decremental conduction along the membrane. EPP is larger at the end plate, and further away the response is smaller and more slowly rising. The attenuation with distance confirms that EPP is generated by channels opening under the nerve terminal, then spreading out and eventually disappearing along the fiber. End plate potentials are the summation of miniature end plate potentials. Each MEPP is the depolarization resulting from the release of one Ach vesicle. A MEPP can therefore be considered as a quantum of EPP and an EPP is therefore characterised by a specific amount of Ach vesicle release. 6.​ Action potential The action potentials in nerve fibers applies equally to skeletal muscle fibers, except for quantitative differences. Some of the quantitative aspects of muscle potentials are as follows: -​ The resting membrane potential is about −80 to −90 mV in skeletal fibers, about 10 to 20 mV more negative than in neurons. -​ The duration of the action potential is 1 to 5 milliseconds in skeletal muscle, about five times as long as in large myelinated nerves.  -​ The velocity of conduction is 3 to 5 m/sec, about 1/13 the velocity of conduction in the large myelinated nerve fibers that excite skeletal muscle. 7.​ Magnesium and Ca2+ The amount of the NT released depends on the amount of Ca entering the nerve terminal. Thus, the Extracellular Calcium Concentration should be kept constant in order to avoid an EPP deficit. Magnesium is a competitor of Ca2+ for the voltage dependant channel, therefore the relationship between the EPP amplitude and the Calcium Concentration is a function of the Magnesium Concentration 8.​ Inactivation and removal The clearance id Ach is done by acetylcholine esterase, an enzyme that can be found in the Post SM of the NMJ, able to hydrolyze ACh into acetate and choline. similarities differences Both consist of two excitable cells separated by A synapse is between two neurons while NMJ is a narrow cleft for chemical transmission between a motor neuron and a skeletal muscle fiber Axon terminal stores NTs in vesicles that are One-to-one transmission of AP in the NMJ, while released by Ca induced exocytosis when an AP one AP in a presynaptic neuron cannot bring to reaches the terminal AP in a postsynaptic neuron (need of summation) Binding of NT with receptor channels in the A NMJ is always excitatory (EPP) while a synapse membrane of the postsynaptic cell/muscle cell, may be excitatory (EPSP) or inhibitory (IPSP) permitting ionic movement that alters the and NMJ is not plastic and cannot be modulated. membrane potential The resultant change in membrane potential is a Inhibition of skeletal muscles cannot occur at graded potential the NMJ level but only in the CNS through IPSP II.​ Smooth muscle Smooth muscle is found in the walls of the hollow organs and tubes. A correct controlled contraction of smooth muscle regulates movement through BV, GIT, respiratory tract, urinary system,… Smooth muscle cells do not posses a motor end-plate region but have instead varicosities. Varicosities are areas where the axon is expanded, allowing the release of NT is a vast region for a unified contraction of the entire organ.  1.​ Types of SMC Smooth muscle can be single unit or multi-unit a.​ Multi-unit Definition Multi-unit smooth muscle is composed of discrete, separate, smooth muscle fibers. Each fiber operates independently of the others and often is innervated by a single nerve ending, as occurs for skeletal muscle fibers. Furthermore, the outer surfaces of these fibers, like those of skeletal muscle fibers, are covered by a thin layer of basement membrane–like substance, a mixture of fine collagen and glycoprotein that helps insulate the separate fibers from one another. Important characteristics of multi-unit smooth muscle fibers are that each fiber can contract indepen- dently of the others, and their control is exerted mainly by nerve signals. In contrast, a major share of control of unitary smooth muscle is exerted by non-nervous stimuli. Some examples of multi-unit smooth muscle are the ciliary muscle of the eye, the iris muscle of the eye, and the piloerector muscles that cause erection of the hairs when stimulated by the sympathetic nervous system. Contraction The multi-unit smooth muscle contraction is neurogenic. The cells are excited by the by release of NT by the varicosities. A given NT can produce opposite effects in ≠ SM tissues. For example, Nor (norepinephrine) enhances contraction of most vascular smooth muscles by acting on α-adrenergic receptors, but produce relaxation of airway-bronchiolar smooth muscles by acting on β-2-adrenergic receptors. The type of response (excitatory or inhibitory) depends not on the chemical messenger per se, but on the receptors the chemical messenger binds to in the membrane and on the intracellular signaling mechanisms those receptors activate. Multiunit smooth muscles are never myogenic and need an outer signal to induce AP. Their properties are partway between skeletal and single unit smooth muscles. b.​ Unitary (single unit) Definition Unitary smooth muscle is also called syncytial smooth muscle or visceral smooth muscle. The term unitary does not mean single muscle fibers. Instead, it means a mass of hundreds to thousands of smooth muscle fibers that contract together as a single unit. The fibers usually are arranged in sheets or bundles, and their cell membranes are adherent to one another at multiple points so that force generated in one muscle fiber can be transmitted to the next. In addition, the cell membranes are joined by many GAP junctions through which ions can flow freely from one muscle cell to the next so that action potentials, or ion flow without action potentials, can travel from one fiber to the next and cause the muscle fibers to contract together. This type of smooth muscle is also known as syncytial smooth muscle because of its syncytial interconnections among fibers. It is also called visceral smooth muscle because it is found in the walls of most viscera of the body, including the gastrointestinal tract, bile ducts, ureters, uterus, and many blood vessels. Contraction The muscle fibers are exited and contract as a single unit. The muscle fibers in a single unit smooth muscle fiber are electrically linked by a GAP junctions. When an AP occurs anywhere within a sheet if single unit smooth muscle, it is propagated throughout the whole unit via the points of electrical contact, the GAP junctions. They function as a syntitium. The single unit smooth muscle is myogenic. The single unit smooth muscle is self-excitable. It does not require nervous stimulation for contraction. Clusters of specialized cells within a functional syntitium display a spontaneous electrical activity. The self excitable cells of a single unit smooth muscle do not maintain a constant resting potential. The major types of spontaneous depolarizations displayed by self-excitable cells are: -​ Pacemaker potentials: a rhythmic action potentials -​ Slow-wave potentials: a continuous change in membrane potential (and when reaches threshold, fires). The amplitude of slow-wave potential can be modulated by NTs. Pace-maker potentials Self-exitable smooth muscle pacemaker cells are specialized to initiate action potentials, but are not equipped to contract. Only a few of all cells in a functional syncytium are non-contractile pacemaker cells. Most smooth muscle cells are specialized to contract, but cannot self-initiate APs Once an AP is initiated by a self-excitable pacemaker cell, it is conducted to the remaining contractile cells of the syncytium by gap junctions, so the entire groups of connected cells contract as a unit without a nervous input. Nerve-independent contractile activity: myogenic activity (in contrast to the neurogenic activity of skeletal muscle and multi-unit smooth muscles). Slow wave potentials Slow-wave potential are spontaneous, gradually alternating depolarizing and hyperpolarizing swings in potential, brought by un-known mechanisms. They occur only in smooth muscle of the digestive tract Slow-wave potentials are initiated by specialized clusters of non-muscle pacemaker cells within the digestive tract wall and spread to the adjacent smooth muscle cells via gap junctions. The potential is moved farther from threshold during each hyperpolarizing swing and closer to the threshold during each depolarizing swing. -​ If threshold is reached, a burst of APs brings about myogenically induced contraction. -​ Threshold is not always reached, the oscillating slow-wave potentials can continue without generating APs and contractive activity Whether threshold is reached depends on the starting point of the membrane potential at the onset of its depolarizing swing. The starting point, in turn, is influenced by neural and local factors (associated with meals). They can be influenced: -​ Acetylcholine increases the amplitude of the slow wave -​ Norepinephrine decreases the amplitude of the slow wave 2.​ Factors influencing activity in SM The factors influencing activity in the SM are: -​ spontaneous electrical activity in the plasma membrane of the muscle cell -​ neurotransmitters released by the ANS -​ hormones -​ locally induced changes in the chemical composition (in the extracellular fluid): paracrine agents, acidity, oxygen, osmolarity and ion concentration, stretch. SUMMARY OF THE VARIOUS MEANS OF EXCITING ACTION POTENTIALS IN EXCITABLE TISSUES Method of depolarizing Type of tissue involved Description of the triggering event the membrane to the threshold potential Summation of EPSPs Efferent neurons and interneurons Temporal or spatial summation of slight depolarizations (EPSPs) of the dendrite and cell body end of the neuron brought about by changes in channel permeability in response to binding of an excitatory neurotransmitter with surface membrane receptors Receptor Potential Afferent neurons Typically a depolarization of the afferent neuron’s receptor initiated by changes in channel permeability in response to the neuron’s adequate stimulus End-Plate Potential Skeletal muscle Depolarization of the motor end plate brought about by changes in channel permeability in response to binding of the neurotransmitter ACh with receptors on the end-plate membrane Pacemaker Potental Smooth muscle and cardiac muscle Gradual depolarization of the membrane on its own because of shifts in passive ionic fluxes accompanying automatic changes in channel permeability Slow-Wave Potential Smooth muscle in GIT only Gradual alternating depolarizing and hyperpolarizing swings in potential initiated by associated nonmuscle self-excitable cells; the depolarizing swing may or may not reach threshold 5.​ Clinical point – myasthenia gravis Myasthenia gravis inactivates acetylcholine receptor-channels. It is a disease involving the NMJ, characterized by extreme muscle weakness. It is an autoimmune condition in which the body erroneously produces antibodies against its own motor end plate acetylcholine receptor channels. So, when acetylcholine is released, not all find a functioning receptor to bind to. As a result, acetylcholinesterase destroys much of the acetylcholine before it ever has a chance to interact with a receptor and contribute to the EPP. Signs: muscle weakness, ptosis (drooping eyelid)  III.​ Cardiac muscle ​ The heart is actually two separate pumps, -​ a right heart that pumps blood through the lungs and a -​ left heart that pumps blood through the systemic circulation that provides blood flow to the other organs and tissues of the body. Each of these is a pulsatile, two-chamber pump composed of an atrium and a ventricle. Each atrium is a weak primer pump for the ventricle, helping to move blood into the ventricle. The ventricles then supply the main pumping force that propels the blood either (1) through the pulmonary circulation by the right ventricle (2) through the systemic circulation by the left ventricle. The heart is surrounded by a two-layer sac called the pericardium, which protects the heart and holds it in place. Special mechanisms in the heart cause a continuing succession of contractions called cardiac rhythmicity, transmitting action potentials throughout the cardiac muscle to cause the heart’s rhythmical beat. 1.​ Types of cardiac muscle The heart is composed of three major types of cardiac muscle—atrial muscle, ventricular muscle, and special- ized excitatory and conductive muscle fibers. The atrial and ventricular types of muscle contract in much the same way as skeletal muscle, except that the duration of contraction is much longer. The specialized excitatory and conductive fibers of the heart, however, contract feebly because they contain few contractile fibrils. Instead, they exhibit automatic rhythmical electrical discharge in the form of action potentials or conduction of the action potentials through the heart, providing an excitatory system that controls the rhythmical beating of the heart 2.​ A syncytium The cardiac muscle is a syncytium. Cardiac cells contain 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. At each intercalated disc, the cell membranes fuse with one another to form permeable communicating junctions (gap junctions) that allow rapid diffusion of ions. The heart actually is composed of two syncytia; the atrial syncytium, which constitutes the walls of the two atria; and the ventricular syncytium, which constitutes the walls of the two ventricles. The atria are separated from the ventricles by fibrous tissue that surrounds the atrioventricular (A-V) valvular openings between the atria and ventricles. Normally, potentials are not con- ducted from the atrial syncytium into the ventricular syncytium directly through this fibrous tissue. Instead, they are only conducted by way of a specialized conductive system called the A-V bundle, a bundle of conductive fibers.  3.​ Heart action potential The ECG is the external recording of the global activation of the heart (detects the perturbation of the electrical field in the surroundings outside the heart) and a sequence of waves can be observed. The ECG doesn’t have anything to do with the mechanical activity of the heart, only the electrical activity of the heart, but you can deduce the mechanical activity from the electrical one The heart has pacemaker cells located in specific regions. They are modified myocardia cells with intrinsic properties (of their membrane) capable to generate rhythmic potentials on their own. In the picture the simultaneous action potentials at different levels of the heart conduction system is recorded as an electrocardiogram.. BLUE: ​ ​ SA node GREEN: ​ Atrial muscle ORANGE: ​AV node Bright RED: ​ AV bundle Dark RED: ​ bundle brances PURPLE: ​Purkinje fibers GRAY: ​ ​ Ventricular muscle The excitation starts in the sino-atrial node (SA node) and it is transmitted progressively to the other structures: the atrial chamber, the atrio-ventricular node (AV node), the bundle, branches, Purkinje cells and finally to the cardiac muscle. This sequence is very precise. The last signal detected in the image represents the ECG (electrocardiogram): the recording of the depolarization of the heart from the SA node to the ventricular muscle and then its repolarization. The recording is done not from across the membrane, but from outside the body, placing electrodes on the skin. In this way it is possible to record the perturbation of the interstitial fluid outside the myocardial cells which is spread towards the skin, amplified and reported from the skin. This recording is basically a sequence of waves, rhythmically repeating over and over again. If the ECG is recorded simultaneously to all the other recordings, it is possible to establish a temporal correspondence between the waves and the membrane potential. In order to interpret correctly the ECG is therefore important to establish exactly the relationship between the membrane potential variation, the electrical field potential variation and the ECG potential recording: -​ P wave: the first wave, called P wave, is recorded when the SA node and atrial chamber are depolarized. After that, there is an isoelectric recording, meaning that there is no change in potential in this interval. -​ QAS complex: It has a much larger amplitude with respect to the P wave. It corresponds to the depolarization of the ventricular chamber. The duration of the complex is of about 100 ms and, if compared to the one of this potential, it seems that the complex ends way before the final repolarization of the membrane. In reality, the isoelectric line of the complex indicates that the membrane is not changing its depolarisation, indicating that the membrane is maintaining its depolarization. The depolarisation is therefore maintained until the T wave. -​ T wave: The T wave corresponds to the repolarization of the membranes of the ventricles. Therefore, it is possible to infer from an electrical recording outside the body (therefore non-invasive) the activity of the heart, or more precisely the conduction of the action potential from the origin to the end and its repolarization. 4.​ Heart action potentials: fast and slow response In the image are depicted the shape of the action potential recorded in the SA and AV node (slow response) and the shape recorded in the conduction system of the ventricle and the cardiac muscle (atrial and ventricular myocardial cells - fast response). Pacemaker (slow response) have no resting membrane potential. Fast and slow doesn’t refer to the average duration of the depolarization but rather to the depolarisation. Looking at the time axis, it is possible to note that the spike lasts about 200 ms which is a very long potential compared for example to the one characterizing the neurons. Action potential of the sinoatrial node is the natural leader. The cells have a common property along with the ones of the AV node and the Purkinje fibers: they spontaneously depolarize without any aid, rhytmically generating the action potential for our whole life. Neurons indeed need a stimulus (synapses) to generate the action potential. 5.​ Pacemaker potential The pacemaker (or auto-rhythmic) cells and some of the smooth muscle cells are able to generate the potential without any other intervention because they have membrane properties that will allow a rhythmic continuous generation of action potentials. They are called pacemakers because their action potentials are not autonomously generated randomly but with a very systematic rhythmicity. In the image below two action potentials recorded at the level of the SA node can be seen. -​ The graph indicates the currents -​ The graphs going down indicate the inwards currents (CaT, CaL, Na) -​ The graphs going up indicate the outwards currents (K solw delayed rectiflier and fast delayed rectifier) Given that the membrane potential and the currents are simultaneously recorded, it’s possible to establish a time relationship between the current and the phase of the action potential. (1)​ In the first graph we can notice how at -60 mV the channels for a specific ion open allowing an inward current (negative in the graph) and causing the depolarization. The ion channels for this phase are the F (funny current), they are the Na channels (If) (2)​ After -5mV is overcome, another type of channel causes another paired inward current: the calcium ion channels (CaT= Ca2+ transient). The equilibrium potential for CaT is +133mV, the driving force is very big. This channel causes the spike! The current goes on until the threshold is reached. (3)​ At the threshold, the steep phase is mediated by L type calcium channels, CaL (black current: huge). In this phase the membrane potential switches signs and becomes positive. (4)​ After the spike the K+ channels (two channels, the potassium slow delayed rectifier and the potassium rapid delayed rectifier) open causing an outward current (repolarization phase). (5)​ When -60mV values are reached the If current causes a second depolarization The If (funny current) is the first current to be involved. It is called funny because it was funny to see the Na entering in the cell when the cell was going towards the resting membrane potential (usually the Na is not free to enter during a repolarized state). If is not the only current involved in the first depolarization. The ICaT channels (Calcium transient current) opens as the membrane depolarises. This can be considered as the last kick to reach the threshold. Therefore, first If and then If and ICaT bring the depolarisation from -60mV to the threshold potential. When the threshold is reached, there is another influx of Ca via the so-called Ca long-lasting channels, ICaL. ICaL is a huge current, which is in fact responsible for the overshoot (the polarity of the membrane is reversed). Then there is a repolarization which is due to a potassium current Ik. The pacemaker potential is due to the fact that these special cells located in the SA and AV node (and occasionally in other cells of the conduction system) are equipped with voltage-gated channels that, when reaching -60 mV, open the gate for Na, then for Ca, reach the threshold, open the gate for Ca again and then for K. This sequence of conductance variations leads into a sequence of currents which leads to this long-time course (200/250 ms). 6.​ Modulation of the heart rhythm In the image is depicted what happens in the SA node. The sinusal rhythm is the physiologic rhythm followed by the heart and originating at the level of the SA node (the P wave is generated at the level of the SA node, which is then conducted at the level of the atrial chamber, AV node, conductance system and myocardial cells). The cells in the SA node are able to excite themselves without any additional help. However, the frequency of the pacemaker cells through the autonomic (vegetative) nervous system is mainly done in the SA node. Therefore, the autonomic NS can affect this potential by increasing/decreasing the frequencies of discharge, which is usually 70 beats/min in an adult healthy at rest person. In stressful situations the frequencies will increase leading to tachycardia and increased cardiac output. The frequency of the heart can also decrease, for example during restoring sleep. Parasympathetic and ortosymphatetic branches of the vegetative system can affect the duration of the action potential and at the end the frequency of discharge. This effect is obtained by means of synapses acting on SA node cells and affecting the state of the channels. ​ the orthosympathetic nervous system increases the conductance of Na (opens more channels), playing with the If funny current. The gray line is the spontaneous frequency. In red (sympathetic stimulation), it is possible to see that the velocity to reach the threshold is faster. There is a deperpolarization, the membrane potential won’t reach -60mV in the hyperpolarization phase ​ In the blue graph, we can notice that the the repolarization phase is altered in fact ending in hyperpolarization, so the parasympathetic nervous system plays with K+ channels. The beat is slower because it takes more time to reach -40 mV if the starting point is below -60mV. At rest, the parasympathetic system is always in the effect. If the frequency needs to be increased from 70 to 100bpm, the parasympathetic action is reduced. If the frequency needs to be increased further than that, the sympathetic system is stimutlated. The vegetative system is just modulating the potential, not determining it; in absence of the two, pacemaker cells in AV and SA nodes will be able to autonomously generate action potentials. 7.​ Myocardial cell potential The time course of the myocardial cell potential is different from the one observed at the level of the SA and AV nodes. This shape is characterized by a very fast depolarization phase, followed by the most positive potential and then by a repolarization phase which is interrupted because the cell keeps going on a depolarized state for a while (plateau potential). In conclusion, the sequence of events is fast depolarization, plateau potential and finally repolarization. The cell potential is divided in 5 phases: Phase 0 (Depolarization): Fast Sodium Channels Open: When the cardiac cell is stimulated and depoizes, the membrane potential becomes more positive. Voltage-gated Na channels (fast Na channels) open and permit Na to rapidly flow into the cell and depolarize the membrane. The membrane potential reaches about +30 millivolts (green dashed line) before the sodium channels clos Phase 1 (Initial Repolarization): Fast Sodium Channels Close. The Na channels close, the cell begins to repolarize, and the conductance for K+ and Cl- ions increases. Phase 2 (Plateau): Ca Channels Open and Fast K Channels Close. A brief initial repolarization occurs and the action potential then plateaus as a result of increased calcium ion permeability and decreased K ion permeability. The voltage-gated Ca ion channels open slowly during phases 1 and 0, and Ca enters the cell. K channels then close, and the combination of decreased K ion efflux and increased Ca ion influx causes the action potential to plateau. Phase 3 (Rapid Repolarization): Ca Channels Close and Slow K Channels Open. The closure of Ca ion channels and increased K ion permeability, permitting K ions to exit the cell rapidly, ends the plateau and returns the cell membrane potential to its resting level. Phase 4 (Resting Membrane Potential):. This averages about−80 to −90 millivolts. Na dominates phase 0; Ca and K balancing each other in phase 1; Ca reduced in phase 2, whereas the K dominates bringing down the potential; during phase 3 there is just the K; in phase 4 the channels which are responsible for the resting potential are open, allowing the passage of K out and Na+Ca in. Looking at the image, it is evident that the equilibrium potential for Na and Ca are never reached because the K enters. 8.​ Different time course of action potentials in cardiac cells The plateau phase is much shorter in the atrium and much longer in the ventricle (image on the left). Considering the different positions of the ventricle (epi/mid/endocardium) it is possible to see the different length of the action potentials in the atrium and ventricular cells (image on the right). 9.​ The heart as a functional syncytium When an action potential occurs in pacemaker cells, it is quickly propagated via gap junctions throughout the entire group of interconnected cells, which then contracts as a coordinated unit. Such a group of interconnected muscle cells that function electrically and mechanically as a unit is known as functional syncytium. The signal generated at the level of the nodes is conducted through the myocardial cells, which are short, striated cells having attributes both of the smooth and the skeletal muscle cells. They are striated as they have the typical organization of the contractile proteins inside but, on the other hand, they are short, paired and provided with gap junctions, similarly to the unitary smooth muscle. This electrical synapse makes the heart an electrical syncytium: from a very tiny spot generating current, the current itself is spread all over around the spot via the gap junctions. This transmission is very fast because it is only a matter of diffusion from one cell to the other leading to a progressive excitation. Summarizing, the rhythmic potential is then transferred to the neighbour myocardial potential until threshold is reached, they fire and then the current enters and spreads all over the atrial chamber. Red = depolarization. Blue = resting state. The SA node is generating the potential, which spreads all over the atrial chambers via the gap junctions (very fast depolarization); in fact, it is possible to observe the red wave dominating the atrial chambers until the complete depolarization of the atrial chambers is reached. This excitation doesn’t go directly down to the ventricles because there are no electrical excitable cells (muscle) in the separation between the atrial and ventricular chambers, it is a wall. Therefore, to have the excitation of the ventricles, it is necessary to excite the AV node first. AV node is excited by gap junctions as the stimulus converges here and leads to depolarization. The excitation is in turn transmitted to the branches conveying the depolarization to the Purkinje cells and finally to the myocardial ventricular cells. The wave of depolarization starts from the tip of the heart and goes backward towards the base and the upper portion of the ventricles. The depolarization reaches the whole ventricular chambers, whereas the atrial chambers are now in a resting state. Looking at this sequence of events in time, it is evident that when the atrial chambers are excited the ventricular chambers are not and vice versa. This opposition of phase in excitation can be translated into a mechanical event leading to the definition of systole and diastole. -​ Diastole is when the heart is not in contraction, -​ Systole is when the chamber is contracted. In yellow, the vector of depolarization indicating the direction of the depolarization. At the beginning it goes from upper to lower portion in the midline, from right to left. After entering the conduction system, it seems that the vector goes towards the right but it actually goes down towards the tip of the heart, starts depolarization and then depolarizes the whole ventricles. However, it can be noticed that the vector points towards the left: this can be explained considering the thickness of the ventricles; in fact, the thickness of the wall of the left ventricle is way higher with respect to the one of right one and this is because they have to face different pressures in order to pump blood outside into the vessels. Therefore, the vector goes towards the left because there are way more cells that need to be depolarized. After depolarization, the ventricles are repolarized. In this very brief window of time, both the atrial and ventricular chambers are relaxed: however, the ventricular chambers have just finished their repolarization whereas the atrial chambers are just about to start a new excitation. The repolarization is the opposite phenomenon with respect to the depolarization but with the same direction. It is possible to infer the direction of the depolarization (the direction of the conduction of the action potential towards the heart) with calculations made on the ECG; by measuring the ECG signal it is possible to infer which is the main vector (not the instantaneous one) which is supposed to be average from right to left, from up to down. Pathologies affecting the depolarization of the heart can change the amplitude and the direction of the vector therefore it is important to know very well the relationship between what happens at the level of the membranes, what happens in the electrical fields and what happens at the level of the electrodes where the recording is taking place. It is very important to always refer the recording to the electrodes which are recording, as there will be different shapes and amplitudes of waves depending on which is the position of the electrodes. 10.​ Pacemaker Potential and Latent Pacemakers Looking at the SA and AV nodes frequency of discharge: -​ SA node fires spontaneously up to 100 impulses per minute which means that the cardiac frequency will be 100 impulses/min. -​ AV node is slower and spontaneously goes up to 60 impulses/min (considering the spontaneous frequency not the frequency affected by the parasympathetic or orthosympathetic). -​ Considering the Bundle of His (the common bundle), even though the shape of the potential resembles the potential recorded in the myocardial cells, they are able to generate spontaneous potentials in absence of the others. o​ Frequency is 40 for a Bundle of His o​ 15-20 for Purkinje fibres. This is an advantage because if there is a problem with the SA node, the AV node can take over the lead. The frequency of discharge in the AV node is fast enough to sustain the cardiac output, of course intervention is needed, but there is a safe mechanism which takes over. When the AV node is impaired, there will be a problem. Because even if the common bundle takes over the lead (this is called idioventricular rhythm because it is generated in the ventricles), 40 and 15-20 are very low frequencies, so it is not possible to maintain the cardiac output with these frequencies of impulses. Normally the frequency of SA node is up to 100 impulses/min but if it is measured, the frequency of our impulses per minute is 65-75 at rest. This means that the parasympathetic nervous system always slows down the frequency of discharge of SA node. It is not needed to have 100 impulses per minute at rest. Normally as a default state parasympathetic slows it down. This is advantageous since the cardiac performance can be increased by removing the action of the parasympathetic and only when more than 100 is needed, the orthosympathetic is activated to increase the frequency of discharge. Normally in quiet, there is the modulation in between 70-100, but when more is needed (e.g. running) the orthosympathetic is activated. When the AV node takes the lead, given that it is fast enough, through the gap junction the potential will spread down to the ventricle but also up to the atrial chambers: the atrial chambers are excited together with the ventricles but there will be no atrial contraction before the ventricular contraction, which could be a problem in terms of feeding the ventricle. However, the vast majority of blood goes from the atrial chambers to the ventricular chambers thanks to the passive diffusion so there are no big troubles. In this condition, there will be an overlap and the lack of the P where it is expected, because there is no sinusoidal excitation, but there is the excitation of the atrial chambers together with the ventricular. The signal from the ventricle is much higher than the atrial which is covered completely. So, there is an absence but in functional terms there is an overlap, it cannot be seen and it is hidden because QRS is so big and completely overlaps. 11.​ Pacemaker Dominance The AV node can generate the action potential, the atrial chamber follows the SA node and the ventricular follows the AV node. In order for 2 chambers to go perfectly in a sequence, the faster chamber takes the lead because they are paired with electrical synapses. Looking at the graph, there is a system with 2 separate cells: A and B. -​ A is a pacemaker cell which fires at 40-50 impulse per minute, -​ B is another pacemaker cell which fires at 10 impulse per minute. These 2 cells are separated, so none controls the other: the 2 cells fire and each one follows its own rhythm. But when these cells are connected with an electrical synapse, cell A conditions completely the pattern of discharge of the cell B. Cell B discharges at a lower frequency because it takes more time to reach the threshold from the starting potential due to its condunctance. The same happens for the SA and AV nodes. When 2 cells are connected, cell A arrives at the threshold and fires, so the wave of charges arrives in cell B. When cell B is slowly approaching the threshold, a huge wave of charges enters bringing the membrane immediately to the threshold. So, the 2nd cell doesn't have the time to follow its own rhythm, because the wave of depolarization enters before and brings the 2nd cell to the threshold. There would be a disconnected random discharge if the SA and AV node were separated by a wall of non-excitable cells, but this is not the case, because the SA node spreads the excitation, the excitation reaches the AV fast, the AV is completely overwhelmed and responds immediately, so the rhythm of the SA becomes the rhythm of the AV. Only if SA is lacking, AV takes the lead with its own rhythm. Note: When you hear the heart with the stethoscope you hear different sounds, not only one and these are the turbulences of different valves. So, we don’t hear the beat generated, but we hear more than one phenomenon related to the blood flow turbulence, so the consequences of the contraction on the turbulence of the blood when valves close, we can say that we hear all parts but mainly the ventricular one.