SF018-021_Muscle_Physiology.pdf

Voluntary muscle contraction is initiated inside the central nervous system. The signal eventually reach and trigger an AP in the alpha motor neuron which propagates to the end of its axon(s) and opens the voltage‐ gated Ca2+ channels which allow Ca2+ to enter the presynaptic nerve terminal (also ca...

Voluntary muscle contraction is initiated inside the central nervous system. The signal eventually reach and trigger an AP in the alpha motor neuron which propagates to the end of its axon(s) and opens the voltage‐ gated Ca2+ channels which allow Ca2+ to enter the presynaptic nerve terminal (also called bouton). The increase in [Ca2+]i triggers exocytosis of stored vesicles containing acetylcholine (ACh) and release ACh into the synaptic cleft (the gap between the nerve and the muscle). The ACh diffuses across the synaptic cleft and binds to a receptor on the postsynaptic membrane (also called motor end plate). The receptor, nicotinic ACh receptor, acts as a ligand‐gated nonselective cation channel that allows Na+ and K+ to pass through and depolarizes the postsynaptic membrane. The resulting local depolarization, end‐plate potential (EPP), spreads to the nearby plasma membrane and triggers a muscle AP which propagates down the T‐tubules and causes contraction. ACh is then rapidly hydrolyzed to acetate and choline by the enzyme acetylcholinesterase (AChE) in the basal lamina. The choline is transported into the presynaptic terminal and used as a substrate for new ACh synthesis (using enzyme choline acetyltransferase). The amount of ACh released is much higher that what is need to trigger the muscle AP. This serves as a safety factor to ensure that the muscle AP will always be triggered when the nerve AP reaches the presynaptic terminal. The signal that triggers muscle contraction is an action potential (AP) from the motor neuron innervating that particular muscle cell. The nerve AP starts a chain of events (synaptic transmission) that transmits the signal from the neuron to the muscle cell and results in the generation of a muscle AP on the sarcolemma at the motor end plate (neuromuscular junction). The muscle AP propagates along the cell surface and reaches deeper inside the myofiber via invaginations of sarcolemma called T‐tubules (transverse‐tubules). Each T‐tubule is flanked on both sides by terminal cisternae of sarcoplasmic reticulum (SR). The T‐ tubule and its neighboring terminal cisternae are together called a triad. When the T‐tubule is depolarized, voltage‐gated L‐type Ca2+ channels, called DHPRs (dihydropyridine receptors), are activated. Opening of DHPRs allows extracellular calcium to enter the sarcoplasm (which is the most important mechanism in cardiac muscle contraction). Opening of DHPRs also mechanically trigger the opening of another type of Ca2+ channel on the SR membrane called ryanodine receptor (RYR) which allows a large amount of Ca2+ stored in the SR to enter the sarcoplasm, increases [Ca2+]i, and initiates muscle contraction. This latter mechanism is unique to skeletal muscles and allows skeletal muscles to contract without relying on the extracellular Ca2+ levels. The graph illustrates the order of events in a single muscle contraction (twitch). AP precedes the rise in intracellular Ca2+ and the increase in [Ca2+]i precedes the actual shortening of the muscle (contraction). Muscle contains protein complexes (myofilaments) that arrange into contractile units (sarcomeres) which can be shortened by the sliding of the filaments past one another. Skeletal muscle has its sarcomeres arranged along the longitudinal axis of the muscle. The orderly arranged sarcomeres give the skeletal muscle its characteristic banding pattern (striation) seen under light and electron microscopy. Thin myofilaments composed of three proteins: actin, tropomyosin, and troponin. Filamentous‐ or F‐actin acts as the main structure of the thin filament, is formed by two helical strands of polymerized actin molecules, and provides binding sites for myosin. Tropomyosin runs along the groove formed by F‐actin strands and, at rest, blocks myosin binding sites. Troponin complex consists of three subunits (troponin T, troponin I, and troponin C). Troponin T binds to the tropomyosin. Troponin I binds to actin and inhibits contraction. Troponin C binds to calcium and acts as the regulatory element of the troponin complex. Thick myofilaments composed primarily of a large molecular weight protein called myosin. Each myosin molecule has 6 protein subunits. Two myosin heavy chains form a long helical ‘tail’ and two globular ‘heads’. Myosin is arranged inside the sarcomere so that the tail points toward the center (M line) while the heads point to the end (Z disk). The heads stick out of the myofilament on hinges and has three distinct regions: actin‐binding site, ATP‐binding site, and light chain‐binding site. The actin‐ and ATP‐binding sites are important to the contractile mechanism of the skeletal muscle. Myosin light chains bind to the light chain‐binding sites but their role in skeletal muscle contraction is unknown. Titin is a very large protein that links the thick myofilament to the Z disks. It also contributes to the strength and elasticity of the sarcomere. Actin filaments insert into the Z‐disk via binding to ⍺‐actinin. Z‐disks in all myofilaments are aligned and linked by intermediate filament proteins between themselves and to the sarcolemma at specialized plasma membrane regions called costameres. Costameres also contain other membrane proteins (e.g., dystroglycans, integrins) that bind to extracellular matrix proteins (e.g., laminin, fibronectin). These proteins mechanically link the contractile elements in myofibrils to the surrounding structures while also protecting the sarcolemma from injuries. Mutation of these proteins could result in muscular diseases. When sarcomere shortens, the A band (thick filament) does not change in width throughout the contractile cycle while the I band (non‐overlapping region of thin filament) and H zone (non‐overlapping region of thick filament) narrow. This led to the proposal of sliding filament theory/model which states that the thick and thin filaments slide past each other when sarcomere shortens pulling the two adjacent Z‐lines closer. The contractile cycle or crossbridge cycle is a fundamental biochemical cycle of muscle contraction. (1) In the presence of Ca2+, myosin (M) binds to actin (A). (2) A conformational change in the myosin head which pulls the actin filament towards the center of the sarcomere (called power stroke) occurs. The ADP is then released. (3) In the presence of ATP, a new ATP molecule binds to the myosin and causes its release from actin. (4) Myosin ATPase hydrolyzes the new ATP and cocks the head, readying it for the next cycle. The cycle will keep repeating as long as there are Ca2+ and ATP present. In a physiologic state, the intracellular Ca2+ level drops when muscle is not stimulated (see next slide), causing the cycle to stop in a relaxed state. When ATPs are depleted, the cycle will stop in a contracted state which is seen in rigor mortis. Each power stroke moves the actin filament by only a very short distance (~0.01 µm) and generates very little force. However, as the cycle continues, the shortening and tension increase. The shortening and tension are multiply by the number of sarcomeres inside a myofibril. Additionally, the tension is also further multiply by the number of myofibrils inside a myofiber, and the number of myofibers inside a particular muscle generating a substantial amount of force. As previously stated, Ca2+ serves as the regulator molecule for the crossbridge cycle. When a muscle cell is stimulated (see next slide, excitation‐contraction coupling), the intracellular Ca2+ ([Ca2+]i) concentration is increased. Ca2+ binds to troponin C subunit and causes a conformational change of the troponin molecule which shifts tropomyosin and uncovers myosin‐binding sites on the actin filament. Myosin head can then bind to actin and crossbridge cycling continue. The relaxation of skeletal muscle occur due to a reduction of [Ca2+]i by two mechanisms. The main mechanism relies on a pump called sarcoplasmic and endoplasmic reticulum calcium ATPase (SERCA). SERCA uses one molecule of ATP to transport two Ca2+ molecules into the SR lumen and is distributed throughout the SR membrane. Ca2+‐binding proteins (e.g., calsequestrin) inside the SR helps buffer Ca2+ which allows a large amount of Ca2+ to be stored inside the SR. Another mechanism used to reduce [Ca2+]i is via transporters on the sarcolemma. Two transporters involved are Na/Ca exchanger and plasma membrane Ca ATPase. The total tension observed during a muscle contraction consists of two components. The active tension is a result of sarcomere shortening as discussed earlier. The passive tension is caused by stretching of various structural elements of the muscle (both in side the muscle cells and outside e.g., tendons, ligaments). There are two types of muscle contraction. Isometric contraction occurs when the muscle length is fixed, the shortening of sarcomeres results in an increase in tension of the muscle. On the other hand, isotonic contraction causes the length of the whole muscle to change while the tension of the muscle stays the same. When a muscle is stimulated to contract isometrically at different lengths, the force of contraction varies. Plotting a graph between the muscle initial length (sarcomere length) and the force (tension) generated, we get a curve that show an increase in contractile force as sarcomere length increases up to a certain point (~2.0 µm). At this length, the optimal length, the maximum force is generated. As the sarcomere length is increased further, the force decreases until the contraction does not occur anymore. This phenomenon demonstrate the length‐tension relationship (or preload) in skeletal muscles and can be explained by the sliding filament model. At low sarcomere lengths, the actin filaments overlap in the middle and interfere with crossbridge formation, decreasing the force. As sarcomere lengthens to the optimal length, the perfect alignment of the thick and thin filaments allow them to interact and form crossbridges properly which results in the maximum force being generated. When sarcomere length gets too long, the thick and thin filaments are no longer overlapped, preventing crossbridge formation to occur. For most skeletal muscles, the resting length of a muscle is usually close to its optimal length making this not as important as in cardiac muscle (preload) and some smooth muscles. The velocity that a muscle shortens is inversely proportional to the amount of load it needs to overcome. So at zero load, the muscle shortens with a maximum velocity (y‐intercept) which depends on the rate of crossbridge cycling which, in turn, depends on the composition of that muscle (fast‐twitch : slow‐ twitch). As the load increases, the shortening velocity decreases until the load is at maximum load (x‐ intercept) which is high enough to prevent any shortening at all (isometric contraction). The maximum load depends on total amount of crossbridge formation that can occur. So, changing the initial length would affect the x‐intercept while y‐intercept stays the same. At loads higher than the maximum load, the muscle lengthens and the contraction becomes eccentric contraction (as opposed to concentric contraction when muscles shorten). ‐‐‐The material below will not be on the test‐‐‐ Power can be used to determined the work output of the muscle and can be calculated by multiplying force (load) and velocity. As shown in the graph, the maximum power is at around a third of maximum load. Efficiency is defined as work output per unit of energy input. Smooth muscles have the best energy efficiency while skeletal muscles have the worst energy efficiency for their contraction (but they are the best in term of shortening velocity). The force of skeletal muscle contraction can be further modulated by two extrinsic mechanisms. The nerve impulse to each muscle fiber via an alpha motor neuron can be increased in frequency so that the AP generated in the muscle is increased in frequency. The more frequent APs stimulate the more Ca2+ is released from the SR. The increase in [Ca2+]i allows more crossbridges to form and more force to develop. The term tetanus is used to describe a contraction which starts before the muscle fully relaxes from previous contraction. Incomplete tetanus occurs when the muscle has a chance to relax before being stimulated to contract further while in a complete tetanus, the muscle never relaxes until the stimulation ends. Since this type of modulation of skeletal muscle contraction is achieved by changing the frequency of AP, it is also called frequency summation or temporal summation or just summation. The other method of changing the force of skeletal muscle contraction is called recruitment. When more force is needed, more motor neurons are stimulated to participate in the contraction by sending APs to the muscle fibers they innervated. By recruiting more motor units, the total strength of contraction is increased. The order of motor unit recruitment follows the size principle which states that the smaller motor units will be recruited first before larger motor units are recruited when more force is needed. This method of modulation is also called spatial summation. Skeletal muscles can change physically as a response to growth or training. Growing muscles lengthen by adding more sarcomere in series, which increases their length and also allow them to shorten more and with a faster velocity. Hypertrophy increases the number of myofibrils in each muscle cell. Hyperplasia increases the number of myofiber in each muscle. Both hypertrophy and hyperplasia add more sarcomeres in parallel, which increases the maximum force that the muscle can achieve but does not affect their shortening length or velocity. Muscle spindles (also called intrafusal fibers/muscle stretch receptors) are mechanoreceptors located inside the muscle. They run parallel to the regular muscle fibers (extrafusal fibers) and sense the length and the rate of stretching of the muscle. Stretching the spindles triggers AP. Their length is continuously adjusted to ensure they are able to respond to stretching at any muscle lengths. They are involved in the stretch reflex (e.g., knee‐jerk reflex) which is a monosynaptic reflex causing a contraction of the muscle when it is suddenly stretched. The muscle spindles also play a role in maintaining a basal level of contractile activity in muscles (muscle tone). Golgi tendon organs (GTOs) located in the tendons of muscles. GTO senses the tension of the muscle and sends the information to the brain. GTO also is involved in a bisynaptic reflex arc (inverse myotatic reflex) that prevents muscle injury by inhibiting the contraction of that muscle. This can also be seen in a reflex seen in some patients called clasp‐knife reflex. Apart from transmitting signals, nerve endings also have trophic (growth‐ and differentiation‐promoting) effects on muscle fibers. When a nerve innervating a skeletal muscle is damaged, the denervated muscle becomes flaccid (loses its tone) and paralyzed. ACh from presynaptic terminals of the damaged nerve is released causing small, random contractions of the muscle fibers called fasciculation. After several days, fibrillation develops as a result of hypersensitivity to ACh (denervation hypersensitivity) due to the spreading of AChR from motor end plate throughout the membrane and manifests as spontaneous, repetitive contractions of the muscle, occasionally are visible as ripples on the overlying skin. Denervation also causes atrophy of the affect muscle. These changes can be reversed if reinnervation occurs within a few months. On the other hand, if reinnervation does not occur, the muscle will be replaced by adipose and fibrous tissues after a period of time resulting in permanent shortening and deformity of the muscle (contracture). Electromyography (EMG) is the study of bioelectricity of the skeletal muscle. The equipment uses at least two electrodes to detect electrical potential differences which can pick up compound muscle action potential from the muscle(s) located between the recording electrodes. The signal is then amplified, recorded, and analyzed to detect any abnormality in the electrical activity of the muscle. EMGs in different group of diseases show different patterns but they are not pathognomonic (uniquely characteristic of a particular disease/condition) so they are only used as a supportive investigation to confirm or rule out but never directly diagnose diseases. Oftentimes EMG is used in conjunction with other electrophysiology study such as a nerve conduction study because diseases affecting muscle could be caused by pathologies in either nerve, neuromuscular junction, or the muscle itself. Examples of diseases which EMG can assist in their diagnosis are multiple sclerosis, neuropathies, myasthenias, myotonias, myopathies, and muscular dystrophies. The energy for muscle contraction derives from ATP (adenosine triphosphate). However, the amount of ATPs inside a myofiber is limited and can only sustain a few seconds of maximum muscle contraction, so the cell need to continuously synthesize ATP from other high‐energy molecules in order to maintain its contractile function. There are three major pathways for ATP synthesis in muscle cells. The first and most immediate fuel source is the phosphagen system. This system uses creatine phosphate (CP) [also called phosphocreatine (PCr) or phosphoryl creatine] to produce ATP. The conversion between CP and ATP is reversible and is catalyzed by enzyme creatine kinase. This system would also be depleted within a short period (approximately 10 seconds) of maximum muscle contraction. The second system that provides energy for ATP synthesis is the glycolysis pathway. Glycolysis is a fast but inefficient system that uses one molecule of glucose to produce two net ATPs. The end product of glycolysis is pyruvate which can be used as a substrate for the oxidative phosphorylation in the presence of oxygen (see below) or converted to lactate when O2 is low. Without O2, glycolysis can only sustain maximum muscle contraction for 1‐2 minutes (until lactate and other metabolites accumulate and slow it down). The last system, the oxidative phosphorylation, is an aerobic metabolism of fats and carbohydrate that occurs in mitochondria and produces ATPs, along with CO2 and water as its byproducts. The yield of ATPs from this process is much higher than the other systems and it can continues indefinitely as long as the substrates (fat/carbohydrate AND O2) are available; but it is slower than the others systems. Muscle fatigue is a decline in muscle contraction strength (force) and speed (shortening velocity). Can be categorized by causes into: Central fatigue – due to changes in the central nervous system that related to motor control Neuromuscular fatigue – abnormal transmission of motor command from the motor neuron to the muscle could be a defect in the motor nerve or the neuromuscular junction. Muscular fatigue – caused by something inside the muscle itself such as accumulation of metabolites, depleted SR Ca2+ store, diminished energy substrates, etc. Skeletal muscle fibers can be classified by the speed of contraction in to two groups: slow‐twitch and fast‐ twitch muscle fibers. The difference in myosin ATPase activity correlates with the rate of crossbridge cycling and the speed of contraction. Slow‐twitch (type I, red) muscle fibers contain a slow type I myosin ATPase which hydrolyzes ATP at a lower speed. Slow‐twitch fibers also have higher oxidative capacity and more resistant to fatigue than the fast‐twitch fibers. Furthermore, the slow‐twitch fibers are innervated by smaller motor neurons forming smaller motor units (a motor unit is a single motor neuron and all muscle fibers innervated by it). Fast‐twitch (type II, white) muscle fibers contain several isoforms of fast type II myosin ATPases and can be further classified into type IIa (fast oxidative) and IIb (fast glycolytic) based on their oxidative capacity. Both types of type II fibers are more susceptible to fatigue but can contract with a faster speed and stronger contraction than type I fibers. They are innervated by larger motor neurons forming larger motor units. While a motor neuron always innervate only one type of muscle fibers, most skeletal muscles consist of a combination of all types of muscle fibers but the composition varies from one muscle to another. Muscles involved in postural stability have a higher proportion of slow‐twitch fibers while those required a quick burst of power rely on a larger percentage of fast‐twitch fibers. The composition can be altered by external factor such as athletic training, and diseases. Smooth muscles are histologically and functionally different from the skeletal and cardiac muscles. They contain thick and thin filaments similar to other muscle types but the arrangement and regulation are different. Thick and thin filaments in smooth muscle are arranged in different directions. Instead of z‐discs, actins are anchored to dense bodies which are located in both the cytoplasm and the plasma membrane. Small invaginations of the plasma membrane called caveolae serve as the equivalent of T‐tubules. Most smooth muscles are innervated by neurons from autonomic nervous systems. The neurons synapse onto smooth muscle cells at multiple sites by a series of swelling called varicosities. Smooth muscles can be categorized into two types by their innervation patterns. Each cell of multiunit smooth muscle may contract independently of its neighbors due to the lack of electrical connection via gap junction. Each multiunit smooth muscle myocyte is then required to be innervated by a neuron which make them capable of finer control. Examples of multiunit smooth muscles are the iris, the ciliary muscles, the vas deferens and the piloerector muscles. On the other hand, unitary (single‐unit) smooth muscle cells are electrically coupled via gap junctions. All the cells in unitary smooth muscle contracts together as one unit (functional syncytium) by the diffusion of ions (and propagation of depolarization) though gap junctions. This allow for coordinated, but less specific, contraction of the muscle. Examples of this unitary smooth muscles are the urinary bladder and the gastrointestinal tract. Smooth muscles in different organs display different patterns of activity. Smooth muscles in some organs maintain a level of contractile force (tone or tonus) at all times and are called tonic smooth muscles such as the smooth muscles of sphincters, airways, and blood vessels. On the other hand, in organs such as gastrointestinal tract, urinary bladder, and uterine wall, phasic smooth muscles contract rhythmically or transiently upon stimulation. Smooth muscles can be stimulated by a variety of stimuli including nervous, humoral, and mechanical. Some of these stimuli cause a change in the membrane potential which can trigger AP(s) if the membrane potential is above the threshold while the others can cause a contraction without a significant change in membrane potential. Smooth muscle APs can be in many forms: a simple spike, a spike followed by a plateau, or a slow wave with a series of spikes on top. One important difference from skeletal muscle AP is that the depolarization phase relies on Ca2+ current, instead of Na+, which results in a slower depolarization. Smooth muscles in several hollow organs are sensitive to mechanical force. In autoregulation of blood vessels, stretching a vessel (due to an increase in blood pressure) causes vasoconstriction while reducing its tension causes vasodilation. While in other organs, usually those with storage function, stretching causes an initial contraction of the organ followed by a gradual relaxation, a phenomenon known as stress relaxation. Unlike skeletal muscle, the smooth muscle is more dependent on extracellular Ca2+ for its contraction. Ca2+ enters a smooth muscle cells through multiple pathways. 1. Extracellular Ca2+ enters through voltage‐gated channels on the membrane. 2. Sarcoplasmic Ca2+ release by binding of Ca2+ to RYR (ryanodine receptor); Ca2+‐induced Ca2+ release. 3. Second messenger‐mediated Ca2+ release from SR by inositol triphosphate (IP3) (which explains why some smooth muscle cells can contract without a change in membrane potential). Reduction in Ca2+ is achieved by similar mechanisms to the skeletal muscle. SERCA pumps Ca2+ into the SR. Membrane transporters (NCX, Ca2+‐ATPase) extrude Ca2+ into the ECF. In contrast to the skeletal muscle, the regulatory site for smooth muscle contraction is on the myosin molecule. Instead of troponin, smooth muscle cells contain a calcium binding molecule closely related to troponin C called calmodulin in its cytoplasm. When [Ca2+]i increases, four Ca2+ bind to calmodulin and form a Ca2+‐calmodulin (CaCM) complex. The CaCM complex then binds to and activates an enzyme called myosin light‐chain kinase (MLCK). MLCK phosphorylates the regulatory light chain on the neck of myosin which causes conformational change of the myosin molecule and increases its ATPase activity, allowing it to form crossbridge and the contraction to occur. The contraction speed in smooth muscle is much slower that other muscle types because the MLCK phosphorylation and activation of myosin ATPase is relatively slow in addition to slower rate of myosin ATPase activity (due to the slower isoforms expressed in smooth muscles). Relaxation of smooth muscles requires another enzyme, myosin light‐chain phosphatase (MLCP), which dephosphorylates the regulatory light chain of myosin. Tonic smooth muscles can sustain a high level of tension while keeping the energy expenditure low in what is called the latch state, the mechanism of which is poorly understood.