EXCITATION CONTRACTION COUPLING

Acetylcholine release at the neuromuscular junction triggers the Na+-dependent action potential in the muscle fiber.

1)The NEURONAL action potential travels down the motor neuron axon in a saltatory (leaping) fashion between nodes of Ranvier in myelin sheath. The action potential propagation is depicted with the blue lines. The NEURONAL action potential travels to the terminal button (or bouton) which can also be called the presynaptic terminal. 2)The NEURONAL action potential depolarizes the terminal bouton and this causes the voltage-gated calcium channels in the plasma membrane of the bouton to open. This allows calcium ions to rush into the bouton. 3) Elevated calcium within the terminal bouton causes the vesicles of acetylcholine to fuse with the membrane and dump their contents (a neurotransmitter called acetylcholine) into the synaptic cleft. The cleft is the space between the terminal bouton and the motor end plate. 4)Acetylcholine binds the Acetylcholine receptors embedded in the MUSCLE plasma membrane (at the motor endplate) 5)The Acetylcholine receptors then open, allowing sodium (Na+) to rush into the muscle cell and depolarize the motor endplate. This produces a change in the endplate potential. Some potassium (K+) can also come out of the cells through the open Acetylcholine receptor. 6)The depolarized endplate potential causes nearby voltage-gated sodium channels in the MUSCLE plasma membrane to open leading to the initiation of a MUSCLE action potential that propagates down the length of the muscle fiber. 7)Acetylcholinesterase quickly inactivates the released acetylcholine.

PAIN

Acute Muscle Pain: "the burn" occurs during or just after heavy muscle use and is due to lactic acid buildup Delayed Onset Muscle Sorenes (DOMS): occurs days after use and is thought to be due to muscle damage and possibly an inflammatory response. Muscle pain (burn) is produced by lactic acid build up when heavy muscle activity and low oxygen lead cause the muscle to rely on anaerobic glycolysis to produce ATP. Anaerobic glycolysis produces 2 lactic acid molecules from 1 glucose. Lactic acid then accumulate in the muscle fiber and produce the burning sensation. In heart muscle this is called angina. There is now some evidence that lactic acid is not always bad and may have a role in regulating muscle excitability during extreme exertion. Delayed Onset Muscle Soreness (DOMS): Is not correlated with lactic acid levels in muscle. Instead this form of soreness that occurs in the days following intense muscle use is associated with muscle damage, and possibly and inflammatory response.

Some Causes of Muscle Pain and Fatigue

C6H12O6 --> 2C3H6O3 (lactic acid) + energy (2 ATP)

The Na+-dependent action potential triggers Ca2+ release from the sarcoplasmic reticulum.

Calcium movements during contraction and relaxation in skeletal muscle. The muscle action potential (AP) (1) depolarizes the sarcolemma (the muscle fibers plasma membrane), including the transverse t-tubules. The skeletal muscle action potential is sodium (Na+) dependent and brief (~2 msec). Depolarization of the T-tubules (2) opens calcium channels (ryanodine receptors) in the sarcoplasmic reticulum (SR), releasing Ca++ i into the sarcoplasm (3) and raising sarcoplasmic (cytosolic ) calcium [Ca++] from 10-7 M to 10-5 M. Thus the sodium dependent action potential triggered by Acetylcholine release at the neuromuscular junction triggers the release of calcium from an internal store (the SR) inside the muscle cell (muscle fiber).This means that skeletal muscle can contract in the absence of extracellular calcium.Calcium allows actin to interact with myosin cross-bridges, thus resulting in the development of tension.After a brief time interval, and if there are no other action potentials, calcium is actively transported back into the SR (4) by a calcium ATPase ion pump, resulting in relaxation.Some calcium is bound to calsequestrin in the SR.Note that the cycling of calcium is intracellular in skeletal muscle

Myasthenia gravis is a disorder of EC coupling caused by an autoimmune response to acetylcholine receptors.

Clinical Manifestations of Myasthenia Gravis In myasthenia gravis, an autoimmune disease, antibodies block or reduce the number of nicotinic acetylcholine receptors at the neuromuscular junction, resulting in muscle fatigability. Diagnostic tests include the edrophonium test. Edrophonium chloride is a cholinesterase inhibitor and thus increases acetylcholine at the neuromuscular junction. Administered intravenously, it will temporarily relieve symptoms of muscle weakness, including diplopia (double vision), in myasthenia gravis

Review

MU 10. List the energy sources of muscle contraction and rank the sources with respect to their relative speed and capacity to supply ATP for contraction and how they are different in the three muscle types. MU 13. List the steps in excitation contraction coupling in skeletal muscle, and describe the roles of the sarcolemma, transverse tubules, sarcoplasmic reticulum, thin filaments, and calcium ions.MU 14. Describe the roles of ATP in skeletal muscle contraction and relaxation.MU 15. Draw the structure of the neuromuscular junction.MU 16. List in sequence the steps involved in neuromuscular transmission in skeletal muscle and point out the location of each step on a diagram of the neuromuscular junction.MU 17. Distinguish between an endplate potential and an action potential in skeletal muscle.MU 18. List the possible sites for blocking neuromuscular transmission in skeletal muscle. MU 19. Distinguish between a twitch and tetanus in skeletal muscle and explain why a twitch is smaller in amplitude than tetanus and the continuum of force development between a twitch and tetanus including the intracellular events.

Large motor units are recruited after small motor units as more force is required.

Motor units are recruited according to the Size Principle. Smaller motor units (few muscle fibers) are recruited first. As more force is required, larger motor units are recruited. When requirements for force are low, but control demands are high (e.g. writing, performing surgery) the ability to recruit only a few muscle fibers gives the possibility of fine control. As more force is needed the impact of each new motor unit on total force production becomes greater. Generally, smaller motor units are composed slow twitch fibers, while the larger motor units are composed of fast twitch fibers.

FATIGUE

Muscle Fatigue: Decline in muscle tension as a result of muscle use. 1) Less maximal tension. 2) Decreased shortening velocity (Vmax). 3) Slower rate of relaxation. Probable Causes: 1) Gradual depletion of SR calcium store with intense muscle use. 2) Buildup of Pi (inorganic phosphate) that inhibits Pi release from myosin. 3) Lactic acid buildup leading to lower pH (higher acidity). a) TnC shows lower affinity for calcium at low pH b) Myosin release of ADP is slower in low pH 4) NOT caused by ATP depletion Recovery slow -May protect muscle from damage - High frequency fatigue from high intensity short duration exercise. Caused by failure of action potential conduction in the T-tubule. Fast recovery. Muscle Fatigue is defined as a decline in muscle tension as a result of previous contractile activity. A fatigued muscle also has decreased shortening velocity and slower rate of relaxation. It is caused by the buildup of lactic acid and of inorganic phosphates (from phosphocreatine breakdown). Fatigue is NOT due to low ATP since a fatigued muscle still has quite high concentration of ATP. Fatigue may in fact be an adaptation to prevent rigor that will result from very low ATP level. Note: low ATP favors stable actin myosin complexes, high calcium levels in the sarcoplasm, and therefore rigor. High frequency fatigue accompanying high intensity, short duration exercise is due to failure in the conduction of action potential in the T tubule. Recovery from such fatigue is rapid. Low frequency fatigue seen with low intensity, long duration exercise is due to the build up of lactic acid and phosphates which may change the conformation of muscle proteins. Recovery from such fatigue is slow.

Ca2+ release at triad involves the physical coupling of L-type calcium channels in the T-tubule with ryanodine receptors in the SR

Negative charges represent the voltage sensor of the dihydropyridine (DHP) receptor (L type calcium channel) in the T-tubule membrane. Purple channels represents the ryanodine receptor (RyR) in the SR membrane. Depolarization of the t-tubule membrane induces a change in the structure of the DHP receptor (an L-type calcium channel) , which in turn opens a calcium channel gate in the ryanodine receptor in the sarcoplasmic reticulum membrane causing calcium release into the sarcoplasm (cytosoplasm) and triggering sarcomere contraction. Ryanodine is a plant alkaloid that binds to and opens SR calcium release channels (ryanodine receptors) at nanamolar concentration. Higher concentration of ryanodine (micromolar) closes ryanodine receptors. Calcium Induced Calcium Release (CICR). The sarcoplasmic reticulum releases its calcium store rapidly through the process of calcium induced calcium release. In addition to being opened by DHP receptors in the triad, the ryanodine receptor is stimulated to open by the presence of cytoplasmic calcium. This means that the small amount of calcium released into the cytoplasm (sarcoplasm) by ryanodine receptors at the triad, triggers adjacent ryanodine receptors (away from the triad) to open and release calcium, which in turn triggers the opening of additional ryanodine receptors. In this way, calcium release occurs rapidly along the entire length of the sarcoplasmic reticulum, rather than just at the triad.

A single action potential produces a muscle twitch, high frequency action potentials produce unfused and then fused tetanus

Regulation of muscle tension by rate coding and summation. Increase in muscle tension from successive action potentials is called summation and a maintained contraction in response to repetitive stimulation is called tetanus. If a tetanus oscillates, it is called unfused tetanus while a tetanus without oscillations is called fused tetanus. A single action potential produces a twitch, where tension rises and then returns to baseline. However, the functional refractory period (FRP) of the AP is much shorter than the contraction time. This means that high frequency APs cause contractions can summate (add together). At very high frequency APs this leads to fused tetanus. Thus the strength of contraction of skeletal muscle is graded by: (1)rate coding or frequency of stimulation as shown here and (2) as well as by recruitment of additional motor units dependent which is discussed next.

One motor neuron and multiple myofibers = a motor unit

Regulation of muscle tension by recruitment. A motor unit is a somatic motor neuron and all the muscle fibers (myofibers) it innervates. A single neuron will innervate multiple muscle fibers BUT a single skeletal muscle fiber is innervated by only one neuron. Large motor units comprise a single neuron and 2000 or more muscle fibers( e.g. gastronecmius muscle). The smallest motor unit comprises a single neuron and as few a 3 muscle fibers (e.g., extraocular muscle). Generally large motor units are found in fast twitch muscle (Type II) while small motor units are found in slow twitch (Type I) muscle. When a motor unit is activated, all of the innervated muscle fibers are simultaneously stimulated to contract with all-or- none twitches. Rate coding and summation can then occur if the motor neuron fires action potentials repetitively.

Treppe is the steady increase in tension in successive twitches. This is not summation.

Repeated stimulation of the muscle at low frequency can produce "treppe". THIS IS NOT SUMMATION because each twitch relaxes to zero tension before the next twitch initiates. However, in treppe successive twitches show greater peak tension. This occurs in a step-wise fashion hence the phenomenon is called treppe (german for stairs or steps). Treppe may occur because Ca++ released from previous twitches exceeds Ca++ reuptake and this results in an increase in Ca++ concentration. This in turn increases the number of cross-bridges that form in the following contractions. Another possibility is that frequent stimulation "warms up" the muscle and thereby increases the enzymatic rate.

The skeletal muscle triad is composed of 1 T-tubule and 2 sarcoplasmic reticulum cisternae

Schematic of skeletal muscle shows the triad junction consisting of the transverse T-tubule adjacent to the terminal cisternae. The T-tubule is an invagination of the muscle cells plasma membrane (the sarcolemma) and therefore Na+-dependent action potentials traveling on the surface of the muscle also travel down into the T-tubule. The terminal cisternae are physically attached to the T-tubule through the direct coupling of L-type calcium channels embedded in the T-tubule membrane to the Ryanodine receptors embedded in the membrane of the sarcoplasmic reticulum

Skeletal muscle relaxation occurs when calcium is removed from the sarcoplasm by calcium exchanger and calcium pumps

Skeletal muscle contraction is terminated by the removal of calcium from the sarcoplasm (cytoplasm). Sarcoplasmic calcium levels go from 10-5 M during contraction to about 10 -7 M during rest. The major mechanism for calcium removal involves the sarcoplasmic reticulum calcium ATPase (SERCA). SERCA uses the energy of ATP hydrolysis to pump calcium back into the sarcoplasmic reticulum where it can be bound by calcium binding proteins Calreticulin and Calsequestrin. Because the calcium needed for contraction is released from, and then returned to the sarcoplasmic reticulum, skeletal muscle does not require calcium influx from outside the cell (myofiber) to contract. However, there are two other mechanisms of calcium removal from the sarcoplasm that do extrude calcium into the extracellular fluid (outside the cell). The first is called the plasma membrane calcium ATPase or PMCA that uses the energy of ATP hydrolysis to remove calcium from the sarcoplasm. PCMA pumps 1 calcium ion out of the cell at the expense of 1 ATP molecule. The second mechanism is the Sodium Calcium eXchanger (NCX). NCX lets 3 sodium ions into the cell (down the Na+ gradient) to remove 1 calcium ion from the cell.

Summary of the energy sources available to skeletal muscle

Sources of ATP to fuel muscle contraction. ATP is used to fuel the crossbridge cycle and to maintain ionic gradients. A muscle fiber has only a few second supply of ATP. Phosphocreatine can rapidly transfer a high energy phosphate to ADP regenerating ATP. However the phosphocreatine supply will last for 5-8 seconds of muscle activity. Glycogen stored in muscle is rapidly broken down to glucose, that is either metabolized anaerobically (in the absence of oxygen) to produce ATP. Anaerobic metabolism produces ATP rapidly, but is an inefficient process and produces lactic acid. In contrast aerobic metabolism of glucose is slow, but is comparatively efficient and can provide sustained ATP. Similarly oxidative metabolism of lipids (fatty acids) and proteins (amino acids) can provide extended supplies of ATP for muscle contraction.

Outline of excitation contraction coupling

The overall scheme of excitation-contraction coupling in skeletal muscle involves the following sequential steps: 1. A motor action potential travels along a motoneuron to the motor endplate at the neuromuscular junction. 2. The nerve endings secrete acetylcholine which acts on a local area of the sarcolemma to open numerous acetylcholine-gated ion channels. 3. Opening of these channels permits sodium ions to flow into the muscle, thus depolarizing the muscle membrane potential, and initiating an action potential which propagates along the muscle fiber membrane in the same way that action potentials propagate along nerve axons. 4. The muscle action potential propagates down the T-tubule (transverse tubule) membranes into the interior of the muscle fiber to the triad junction, where it causes release of calcium ions that have been sequestered in the longitudinal sarcoplasmic reticulum. 5. The increased concentration of calcium ions in the sarcoplasm causes the actin and myosin filaments to interact with each other, resulting in a sliding motion that shortens the length of the sarcomere. 6. The calcium ions are then pumped back into the sarcoplasmic reticulum by the Ca-ATPase ion pump located in the sarcoplasmic reticulum membrane, thus reducing the concentration of calcium in the sarcoplasm, and allowing the muscle fiber to relax. 7. Lengthening of the muscle is achieved by contraction of an antagonistic muscle, e.g. contraction of the triceps lengthens the biceps.

Temporal relation between skeletal action potential, calcium and tension during a twitch

The timing of contraction events. A schematic shows the relationship between the action potential (AP), calcium transient (Ca++) and tension development in a generic fast twitch fiber. The sodium-dependent action potential IN RED (<5 msec) triggers a transient increase in sarcoplasmic calcium concentration (Ca++) IN GRAY leading to the slower development of tension IN BLUE during a skeletal muscle fiber twitch. Note: in some cases tension duration can be from 10 msec, as during the blink of an eye, or many seconds for some types of slow twitch muscle.

Twitch duration is directly related to calcium transient duration in fast and slow twitch muscle

Twitch tension (upper) and calcium transients (lower) of three fiber types from toadfish. In each case, the force and the calcium records have been normalized to their maximum value. The twitch and calcium transient become briefer, going from the slow-twitch red fiber (r), to the fast-twitch white fiber (w), to the superfast-twitch fiber (s).

Summary of mechanisms that allow force variation in skeletal muscle

Voluntary control of skeletal muscle force generation. The force generated when skeletal muscle is stimulated is related to the size of the motor units stimulated (A), the number of motor units activated and the frequency of stimulation of the muscle fibers (B). A. Small motor units are involved in fine motor control, for example in fingers and eyes, while larger motor units are involved in coarser movements. Skeletal muscles produce more force when more (and larger) motor units are recruited. B. With repeated stimulation of a muscle fiber, summation occurs, in which individual twitches occur without complete relaxation between twitches.