Troponin molecules are small globular units located at intervals along the tropomyosin molecules. Troponin T binds the other troponin components to tropomyosin. Troponin I inhibits the interaction of myosin with actin. Troponin C contains the binding sites for the Ca++ that initiates contraction.
Structure of Actin and Myosin 9-
Titin and Nebulin
Titin : biggest protein known (25,000 aa); elastic!
Stabilizes position of contractile filaments
Return to relaxed location
Nebulin : inelastic giant protein
Alignment of A & M
Made up of T-system & a sarcoplasmic reticulum. The T system are transverse tubules which are continuous with memb. of muscle fiber. The space between two layers of T-system is extension of ECS ( extra cellular space).
Sarcoplasmic reticulum has enlarged terminal cisterns. Central T sys. With a cistern of the sarcoplasmic retic.on either side called triads .
The function of T-sys. which is continuous with sarcolemma is rapid transmission of action potential from cell memb. to all fibrils in muscle. The sarcoplasmic retic. Is concerned with Ca++ movement & muscle metabolism .
Electrical phenomena & ionic fluxes :-
Resting memb. potential of sk.m. is − 90 mv . a.p. lasts 2- 4 ms. It conducted along m. fibers at about 5 m /s. absolute refractory period is 1-3 ms.
Ionic distribution & fluxes:-
It is similar to nerve cell memb. depol. Is a manifestation of Na+ influx. repol. manifestation of K+ efflux .
m. fiber memb. depol. Starts at motor end plate , the specialized structure under motor nerve ending . The a.p. transmitted along m.f. & initiates the contractile response.
Molecular basis of contraction:-
Sliding of thin filament over thick fil. Will lead to shortening & cont. of muscle. A band is constant but Z-lines move closer together until overlap.
The sliding occurs when myosin heads bond to actin, bend on rest of myosin molecule then detach . this cycle is repeated many times . Each myosin head has an actin-binding site & an ATP –binding site( an open cleft , when ATP enters it ,it hydrolyzed & cleft will close.). this will produce power stroke that moves actin on myosin .
Each thick fil. has 500 myosin head , each cycle about 5 times / s. during rapid cont.
ATP is catalyzed by ATPase activity in heads of myosin, in contact with actin.
Depol. Of m.f. which initiates cont. is called excitation –cont. coupling.
The a.p transmitted to all fibrils via T- system. releasing of Ca++ from terminal cisterns of sarcoplasmic retic. will initiate cont by binding to troponin C.
Cross-Bridge Movement 9-
In resting muscle , troponin I is tightly bound to actin & tropomyosin covers the sites where myosin heads bind to actin. Thus the troponin-tropomyosin complex constitutes a relaxing protein that inhibits the interaction between actin & myosin .
When Ca++ released by a.p , binds to troponin C, the binding of troponin I to actin is weakened , tropomyosin moves laterally, uncovers binding sites for myosin heads. ATP then split & contraction occur.
7 myosin binding sites are uncovered for each molecule of troponin that binds a Ca++ ion.
After releasing Ca++ , the sarcoplasmic retic. reaccumulate it by actively transporting it into longitudinal portion of reticulum by Ca++-Mg++ ATPase pump, then Ca++ diffuses to terminal cisterns ,where it is stored for next a.p.
Once Ca++ conc. has been lowered , muscle relaxes .ATP provides energy for both cont.& relax. if transport of Ca++ into retic. is inhibited ,relax. not occur , even though there are no a.p. leading to sustained cont. called a contracture .
Binding Site Tropomyosin Troponin
Skeletal muscle must be stimulated by a nerve or it will not contract (paralyzed)
Cell bodies of somatic motor neurons are in brainstem or spinal cord
Axons of somatic motor neurons are called somatic motor fibers
each branches, on average, into 200 terminal branches that supply one muscle fiber each
Each motor neuron and all the muscle fibers it innervates are called a motor unit
small motor units contain as few as 20 muscle fibers per nerve fiber
gastrocnemius muscle has 1000 fibers per nerve fiber
Synapse is region where nerve fiber makes a functional contact with its target cell (NMJ)
Neurotransmitter released from nerve fiber causes stimulation of muscle cell (acetylcholine)
Components of synapse
synaptic knob is swollen end of nerve fiber
contains vesicles filled with ACh
motor end plate is region of muscle cell surface
has ACh receptors which bind ACh released from nerve
acetylcholinesterase is enzyme that breaks down ACh & causes relaxation
schwann cell envelopes & isolates NMJ
Muscle Contraction & Relaxation
Four phases involved in this process
excitation where action potentials in the nerve lead to formation of action potentials in muscle fiber
excitation-contraction coupling refers to action potentials on the sarcolemma activate myofilaments
contraction is shortening of muscle fiber or at least formation of tension
relaxation is return of fiber to its resting length
Excitation (steps 1 & 2)
Nerve signal stimulates voltage-gated calcium channels that result in exocytosis of synaptic vesicles containing ACh
Excitation (steps 3 & 4)
Binding of ACh opens Na+ and K+ channels resulting in an end-plate potential (EPP)
Excitation (step 5)
Voltage change in end-plate region (EPP) opens nearby voltage-gated channels in plasma membrane producing an action potential
Excitation-Contraction Coupling(steps 6&7)
Action potential spreading over sarcolemma reaches T tubules -- voltage-gated channels open in T tubules causing calcium gates to open in SR
Excitation-Contraction Coupling (steps 8&9)
Calcium release causes binding of myosin to active sites on actin
Contraction (steps 10 & 11)
Myosin head with an ATP molecule bound to it can form a cross-bridge (myosin ATPase releases the energy allowing the head to move into position)
Contraction (steps 12 & 13)
Power stroke shows myosin head releasing the ADP & phosphate and flexing as it pulls thin filament along -- binding of more ATP releases head from the thin filament
Relaxation (steps 14 & 15)
Stimulation ceases and acetylcholinesterase removes ACh from receptors so stimulation of the muscle cell ceases
Relaxation (step 16)
Active transport pumps calcium back into SR where it binds to calsequestrin
ATP is needed for muscle relaxation as well as muscle contraction
Relaxation (steps 17 & 18)
Loss of calcium from sarcoplasm results in hiding of active sites and cessation of the production or maintenance of tension
Role of Calcium
Myosin cross-bridges can bind to actin only when binding sites are available – in resting muscle, myosin binding sites on actin thin filaments are covered by tropomyosin
Tropomyosin moves away from myosin binding sites when Ca2+ binds to troponin
Muscle Contraction Summary
Nerve impulse reaches myoneural junction
Acetylcholine is released from motor neuron
Ach binds with receptors in the muscle membrane to allow sodium to enter
Sodium influx will generate an action potential in the sarcolemma
Muscle Contraction Continued
Action potential travels down T tubule
Sarcoplamic reticulum releases calcium
Calcium binds with troponin to move the troponin, tropomyosin complex
Binding sites in the actin filament are exposed
Muscle Contraction Continued
Myosin head attach to binding sites and create a power stroke
ATP detaches myosin heads and energizes them for another contaction
When action potentials cease the muscle stop contracting
The muscle twitch:-
A simple a.p. causes a brief cont. followed by relaxation. this process called muscle twitch. Twitch starts 2 ms after a depol. Of memb. the duration varies with type of muscle. Fast m.f. ( those concerned with fine rapid movement) have short twitch duration as 7.5 ms. Slow m.f ( those involved in strong , sustained movements) have twitch durations up to 100 ms.
Threshold is minimum voltage necessary to produce contraction
a single brief stimulus at that voltage produces a quick cycle of contraction & relaxation called a twitch
Phases of a twitch contraction
latent period (2 msec) is delay between stimulus & onset of twitch
contraction phase is period during which tension develops and shortens
relaxation phase shows a loss of tension & return to resting length
refractory period is period when muscle will not respond to new stimulus
Summation of contractions:-
The fiber is electrically refractory only during rising & part of the falling phase of spike potential. at this time cont beginning by first stimulus, repeated stimulation before relaxation has occurred will produce additional cont. added to already present cont. this phenomena known as summation of contractions . The tension here is greater than single muscle twitch.
With repeated stimulation , continuous cont. occur called complete tetanus , when there is no relaxation .
Incomplete tetanus occur when there are periods of incomplete relaxation . The tension developed during complete tetanus is 4 times that of single muscle twitch.
When a series stimuli delivered to sk.m , at frequency just below tetanizing frequency , so there is an increase in tension until after several cont. a uniform tension / cont. will developed . this phenomenon known as treppe ( staircase).
Occurs in muscle rested for prolonged period
Each subsequent contraction is stronger than previous until all equal after few stimuli
As frequency of action potentials increase, frequency of contraction increases
Muscle fibers partially relax between contraction
No relaxation between contractions
Muscle tension increases as contraction frequencies increase
Relation between muscle length, tension & velocity of cont:
Passive tension is measured at a given distance ,then the muscle is stimulated electrically then total tension is measured. The difference between both is active tension .
The length of muscle which the active tension is max, is called resting length.
The reaction between length- tension in sk.m. is due to sliding filament ,and cross –linkage between actin & myosin molecules.
When muscle is stretched the overlap is reduced . when muscle is shorter than resting length , the thin filaments overlap & cross-linkage also reduces.
Muscle Length and Tension 9-
Types of Muscle Contractions
Isometric : No change in length but tension increases
Postural muscles of body
Isotonic : Change in length but tension constant
Concentric : Overcomes opposing resistance and muscle shortens
Eccentric : Tension maintained but muscle lengthens
Muscle tone : Constant tension by muscles for long periods of time
Isometric & Isotonic Contractions
Slow and Fast Fibers
Slow-twitch or high-oxidative
Contract more slowly, smaller in diameter, better blood supply, more mitochondria, more fatigue-resistant than fast-twitch
Fast-twitch or low-oxidative
Respond rapidly to nervous stimulation, contain myosin to break down ATP more rapidly, less blood supply, fewer and smaller mitochondria than slow-twitch
Distribution of fast-twitch and slow twitch
Most muscles have both but varies for each muscle
Muscle Fiber Classification Oxidative only Oxidative or glycolytic
Energy sources & metabolism:-
m. cont requires energy & muscle is called a “ machine” converting chemical energy into mechanical work . source of energy is energy – rich organic phosphate derivatives in muscle.
ATP is resynthesized from ADP by addition of a phosphate group . this reaction requires energy which supplied by break down of glucose to CO2 & H2O . but there is another compound called Phosphorylcreatine which is the source of energy of muscle cont .
Carbohydrate & lipid breakdown:-
At rest & light exercise muscle utilize free fatty acids for energy source but if intensity of exercise increases lipids alone cannot supply energy , so utilization of CHO will be predominant source for energy.
Glucose in bd stream enters cells, when after several chemical reactions become pyruvate . another source is glycogen which is present in liver & sk.m. when O2 present pyruvate enters citric acid cycle & end product is sufficient energy to form large quantities of ATP from ADP this process called aerobic glycolysis .
If O2 is insufficient pyruvate doesn’t enter citric acid cycle but reduced to lactate with net result of much small amount of ATP this process is called anaerobic glycolysis.
The oxygen debt mechanism:-
During m. exercise , the m. bd. Vessels dilate , blood flow is ↑ & O2 supply ↑ up to a point the ↑ in O2 consumption is proportionate to energy expended until it reaches a stage that aerobic pathway for production of ATP is not enough , so that anaerobic pathway will start by breakdown of glucose to lactate.
Use of anaerobic pathway is self-limiting because lactate accumulate in muscle leading to decline in PH.
After a period of exertion is over, extra O2 is consumed to remove lactate ,replenish ATP , Phosphorylcreatine stores & small amount of O2 that come from myoglobin. This extra O2 consumption called oxygen debt
Fatigue is progressive weakness & loss of contractility from prolonged use
ATP synthesis declines as glycogen is consumed
ATP shortage causes sodium-potassium pumps to fail to maintain membrane potential & excitability
lactic acid lowers pH of sarcoplasm inhibiting enzyme function
accumulation of extracellular K+ lowers the membrane potential & excitability
motor nerve fibers use up their acetylcholine
The striations in cardiac m. are similar to those in sk.m there are large no. of mitochondria .Z-lines are present .m.fibers branch & interdigitate , the end of one m.fiber abuts on another through an extensive series of folds. these areas occur at Z- line called intercalated disks . They provide a strong union between fibers ,that contractile unit can be transmitted along its axis to the next.
The cell membs of adjacent fibers fuse forming gap junctions . These junctions provide low-resistance bridges for:-
1) spread of excitation from one fiber to another .
2) they permit cardiac m. to function as if were a syncytium.
The T-system in cardiac m. is located at the Z-lines . like sk.m cardiac m. contains myosin, actin, tropomyosin & troponin. It also contains Dystrophin.
Resting memb. & action potentials:-
Resting memb. of cardiac.m. is about – 90mv. Stimulation produces a propagated a.p., then depol. Proceeds rapidly , an overshoot is present, but it is followed by a plateau before the memb. potential returns to the baseline. Depol. Lasts 2ms . But plateau & repol lasts 200ms . The extracellular recording include a spike & a later wave that resemble QRS complex & T wave of ECG.
Changes in external K+ conc. affect the resting memb. potential, whereas changes in external Na+ conc. affect the magnitude of a.p.
The initial rapid depol. & overshoot ( phase 0 ) are due to opening of voltage – gated Na+ channels.
The initial rapid repol. ( phase 1 ) is due to closure of Na+ channels .the prolonged plateau ( phase 2 ) is due to slower but prolonged opening of voltage – gated Ca++ channels. Final repol.( phase 3 ) is due to closure of Ca++ channels & K+ efflux. This restores the resting potential ( phase 4 ).
The fast Na+ channel in cardiac m. has two gates, an outer gate that opens at the start of depol. at a memb. potential of –70 to-80 mv .and an inner gate that then closes and precludes further influx until a.p is over(Na+ inactivation)
The the slow Ca++ channel is activated at a memb. potential of -30 to -40 mv . there are at least 8 kinds of K+ channel in the heart. in cardiac m. the repol. time decreases at rate of 75 b/mi.( a.p. is 0.25 sec), but at rate of 200 b/min. ( 0.15sec.)
It begins just after start of depol.& lasts 1.5 times as long as a.p.
The role of Ca++ is similar to sk.m. however it is the influx of extracellular Ca++ that is triggered by activation of dihydropyridine channels in the T-sys rather than depol by stored Ca++ from the sarcoplasmic reticulum.
During phase 0-2 and about half phase 3 until a.p. reaches approximately -50 mv during repol , cardiac m. cannot be excited again i.e. it is in its absolute refractory period .it remains refractory until phase 4. therefore , tetanus of sk.m cannot occur. Tetanization of cardiac m. is lethal
Cardiac muscle is slow and has low ATPase activity. Its fibers dependent on oxidative metabolism, needs continuous O2 supply.
The human heart contains α and β MHC both in atria, but α is more. Whereas only β isoforms is found in ventricles.
Correlation between m. fiber length & tension :-
It is similar to Sk.m. there is a resting length at which the tension developed is maximal.
In the body the initial length of the fibers is determined by degree of diastolic filling of heart & pressure in ventricle is proportionate to total tension develop
( starlings law of the heart ).
Force of cont. of cardiac m. are ↑ by catecholamines without ↑ in length and it is mediated through β1 adrenergic receptors & c- AMP . it is called positively inotropic effect of catecholamines
heart also contains β2 –adrenergic receptors which act through c- AMP and it is more in atria.
Digitalis glycosides ↑ cardiac cont. by inhibiting Na+ -K+ ATPase in cell memb. of m.fibers . the resultant ↑ in intracellular Na+ & ↓ Na+ gradient across cell memb.↓ Na+ influx & Ca++ efflux through Na+ -Ca++ exchange antiport in the cell memb. so ↑ intracellular Ca++ which ↑ strength of cont. of cardiac m.
Under basal conditions 35% of caloric needs of human heart are provided by CHO, 5% ketones & a.a & 60 % by fat.
Pace maker tissue:-
The heart continues to beat after all nerves to it are sectioned. It is due to presence of specialized pacemaker tissue that can initiate repetitive a.p. the pace maker tissue makes up the conduction system that spread impulses through out heart. They have unstable memb. potential that slowly decreases after each impulse until reaches firing level.
morphology:- Fusiform cells with one nucleus
30 to 200 microns long & 5 to 10 microns wide
1) S.m lacks cross striations.
2) Actin & myosin II are present & slide on
each other but not arranged in regular arrays.
3) Instead of Z- lines there are dense bodies in
cytoplasm & attached to cell memb. bound by
α- actinin to actin filaments.
4) Troponin is absent.
5) Sarcoplasmic reticulum is poorly developed.
6) Contain few mitochondria and depends on glycolysis for their metabolic needs.
Divided into :-
1- visceral s.m (single unit) :-
as in intestine , uterus , ureters .they has low –resistance bridges & functions in a syncytial fashion. The bridges like in cardiac m. form gap-junction.
2 - multi –unit s.m :-
made up of individual units without interconnecting bridges. It is found in iris of the eye , in which fine graded cont. occur. It is not under voluntary control, but has many functional similarities to sk. M.
Types of Smooth Muscle
Multiunit smooth muscle
in largest arteries, iris, pulmonary air passages, arrector pili muscles
terminal nerve branches synapse on individual myocytes in a motor unit
Single-unit smooth muscle
in most blood vessels & viscera as circular & longitudinal muscle layers
electrically coupled by gap junctions
large number of cells contract as a unit
Stimulation of Smooth Muscle
Involuntary & contracts without nerve stimulation
hormones, CO2, low pH, stretch, O2 deficiency
pacemaker cells in GI tract are autorhythmic
Autonomic nerve fibers have beadlike swellings called varicosities containing synaptic vesicles
stimulates multiple myocytes at diffuse junctions
Visceral s.m :-
electrical & mechanical activity:-
Visceral s.m has unstable memb. potential. it shows continuous irregular cont. independent of nerve supply. This maintains partial cont. called tonus or tone . The memb. potential is about -50 mv .
There are sin- wave like fluctuations, spikes duration 50 ms . the spikes may occur on rising or falling phases of the sine wave.
There are pacemaker potentials but generated in multiple foci .the excitation –cont. coupling is very slow the.m. starts to cont. about 200ms after start of spike & 150 ms after spike is over. Peak cont. reaches 500 ms after spike.
Molecular basis for cont.:-
Ca++ involved in initiation of cont. of s.m. like in sk.m but visceral s.m. has poorly developed sarcoplasmic reticulum, so intracellular Ca++ is due to Ca++ influx from ECF via voltage gated Ca++ channels.
Myosin must be phosphorylated for activation of myosin ATPase which is not necessary in sk.m.
In s.m Ca++ binds to calmodulin & resulting complex activates calmodulin- dependent myosin light chain kinase. this enzyme catalyzes phosphorylation of light chain & myosin ATPase will activate , actin slides on myosin & cont. occur, but in sk.m & cardiac m cont. is triggered by binding of Ca++ to troponin C.
Myosin dephosphorylated by phosphatase in the cell, but this will not lead to relaxation of s.m. instead s.m. has a latch bridge mechanism by which dephosphorylated myosin cross-bridges remain attached to actin for some time after the cytoplasmic Ca++ conc. falls .this produces sustained cont. which is important in vascular s.m. relaxation of s.m. occur when there is final dissociation of Ca++ -calmodulin complex.
Visceral s.m. contracts when stretched in the absence of extrinsic innervation, Stretch will lead to ↓ in memb. p , ↑ in frequencies of spikes & ↑ in tone.
Adding of E or NE to preparation of intestinal s.m . memb. p become larger, spike ↓ in frequency& muscle relaxes. NE exerts both β & α action on s.m. β action ↓ m. tension through c-AMP ( binding intracellular Ca++) . The α action is inhibition of cont. by ↑ Ca++ efflux from muscle cell.
Ach has opposite effect of E. Ach ↓ memb. p. , spikes become more frequent. Muscles become more active. The effect of Ach through phospholipase C& IP3 which ↑ intracellular Ca++ conc.
Function of the nerve supply to s.m
Mammals visceral m. has dual nerve supply from two divisions of autonomic nervous system.
Estrogen ↓the memb. p. of uterine s.m. progesterone ↑ memb. p. & inhibits the electrical & contractile activity of uterine m.
Relation of length to tension:-
If a piece of visceral s.m is stretched , first it ↑ tension however if s.m continue in stretching tension gradually ↓ .this is called plasticity of s.m .
Multi- unit s.m :-
Unlike visceral s.m multi unit s.m is
1) nonsyncytial & cont do not spread widely through it. because of this cont. multi- unit is more fine & localized than those of visceral s.m.
2) Like visceral multi-unit s.m is very sensitive to chemical mediators:- NE causes repeated firing of muscle after a single stimulus leading to an irregular tetanus rather than a single twitch.
The twitch cont of muscle-unit is like sk.m. except that its duration is 10 times as long.