Muscle Excitation and Relaxation

tetrodotoxin

1) Blockade of voltage-gated Na+ channels in both the nerve and muscle membranes by ____________ prevents the action potential.

asynchronous activity

1. Synchronous motor unit summation (recruitment or spatial summation). Achieved by increasing the number of motor units activated simultaneously and thus increase the number of contracting muscle fibers. Different motor units have different activation thresholds. As the stimulus intensity increases, more and more motor units are recruited. The motor neurons with the fewest number of fibers are excited at the lowest stimulation intensity. The largest motor units are the last to be activated because they require the highest degree of stimulation. Only in rare circumstances are these motor units activated. Their complete activation may account for unusual feats of strength. 2. Asynchronous motor unit summation Muscle tension can be sustained for long periods by coordinating the activation of many different motor units. At any given time, a constant number of muscle fibers are contracting to sustain tension while other fibers are relaxed. The maintenance of a nearly constant tension in an entire muscle is a result of the ______________ of individual motor units of the muscle.

glycogen; acetylcholine in the nerve terminals;

1.Muscle fatigue usually occurs after strenuous and/or long term muscle activity. Fatigue may relate to diminished muscle energy stores such as __________. It may also result from diminished _______________________.

dendrotoxin

2) Blockade of voltage-gated K+ channels with ______________ prevents repolarization of the presynaptic membrane and prolongs the duration of the action potential and facilitates the release of ACh.

TnC; tropomyosin

2) When you decide to contract a muscle, excitatory neurons in the ventral horn release excitatory transmitters to depolarize the alpha motor neuron cell body, ultimately initiating an action potential. The action potential travels down the neuron, depolarizing the terminal boutons. Then via the sequence of events discussed previously, Ca++ ions are released from the sarcoplasmic reticulum. The Ca++ binds to the ________, causing conformational change. The change in the conformation of troponin moves the ____________ filament with respect to the actin filament, exposing the actin "active" site. Now the myosin head can bind to the actin active site.

w-Conotoxin

3) Blockade of voltage-gated Ca++ channels in the nerve with _______________ prevents exocytosis of ACh.

tension

3. Temporal summation - Increasing the frequency of muscle stimulation will increase the ________ developed by the muscle. The short refractory period of the motor nerve allows for multiple action potentials delivered to the muscle during a contraction and relaxation cycle.

Botulinum toxin

4) _____________ inhibits synaptic vesicle exocytosis.

Succinylcholine

5) ______________ is an AChR agonist that is not metabolized by AChE. It causes a prolonged depolarization and produces a flaccid paralysis as Na+ channels near the end-plate become inactive.

d-turbocurarine (curare); a-bungarotoxin

6) ______________ (curare) and _______________ are AChR antagonists that cause flaccid paralysis by blocking the AChR of the post-synaptic membrane.

Physostigmine

7) ______________ is an anti-AChE that prolongs the postsynaptic response to ACh.

isometric; isotonic ; isotonic

A muscle will generate a greater maximal tension during ___________ contraction than ____________ because, with ___________ contractions, the muscle will generate only slightly more force than is necessary to move or lift its imposed load.

ryanodine receptors; DHPR; DHPR; Ryanodine channel

Another type of calcium release does require the influx of Ca++ from the extracellular fluid. This is called "Calcium-induced calcium release" or CICR. In this case, depolarization of the t-tubule stimulates the L-type voltage-gated calcium channels to open, and Ca++ moves down its concentration gradient into the cell. This Ca++ directly stimulates the _____________ to release Ca++ from the sarcoplasmic reticulum stores. CICR is not an essential process in skeletal muscle, but we will see it is essential for contraction to occur in cardiac muscle. Ca++ - induced Ca++ release occurs when the ______ changes conformation during depolarization by an action potential. Extracellular Ca++ enters through the _______ and binds to its receptor on the _______________. As free intracellular Ca++ increases more RYRs open and the Ca++ transient increases.

total amount of tension developed

As sarcomeres are stretched beyond optimal cross-bridge overlap, the developed active tension decreases. However, the series elastic elements in the muscle and tendon begin to stretch increasing the______________. Since the intact skeleton maintains the sarcomere length near optimal, passive tension is not a major contributor to tension under physiologic conditions.

dihydropyridine receptors (DHPR); ryanodine receptor (RYR)

As the action potential spreads down the T-tubules, voltage-gated calcium channels in the sarcolemma are activated. These are called "L-type" calcium channels, due to the "long" time during which they remain open once stimulated. The channels are also called _________________, These serve as both voltage sensors, and voltage-gated calcium channels. In skeletal muscle, each one is linked to a calcium release channel, called a ______________ in the membrane of the sarcoplasmic reticulum.

DHPR; ryanodine calcium release channels; Calcium channel-induced calcium release

As the wave of depolarization (reverse polarity) spreads along the sarcolemma and down the T-tubules, L-type calcium channels are stimulated. Through an unknown process, the ______ stimulates the ____________ in the SR to open. Once the RYR opens, Ca++ flows down its concentration gradient from the SR and into the cytosol (sarcoplasm). The sarcoplasmic reticulum contains a very high concentration of Ca++. Opening of the RYR quickly increases the sarcoplasmic Ca++ levels from the nanomolar to micromolar range. Note that in this case, there is no influx of calcium from the extracellular fluid. Consequently, we call this particular form of calcium release "_______________"

isometric; isotonic

As weight is added to subsequent twitches, the ___________ phase becomes longer and the _________ phase shortens until the point is reached where the muscle is no longer able to move the load for a given Ca++ transient.

cAMP; myosin light chain phosphatase

Because smooth muscle requires both an increase in intracellular [Ca++]i and phosphorylation of the myosin light chain, the removal of sarcoplasmic Ca++ (by sarcolemmal pumps) may not cause relaxation by itself. For relaxation to occur 1) Ca++ must be sequestered or removed to the ECF, 2) the MLCK inactivated by ________ and 3) the MLC dephophorylated by _____________. The "Latch state" is unique to smooth muscle. It is a state where the myosin is dephosphorylated but ATP is not readily bound to myosin so cross-bridge detachment is slowed. This results in a high tension-low energy consumption state.

troponin complex; tropomyosin

Before muscle contraction can be initiated, certain biochemical changes alter the thick and the thin myofilaments. 1) When the muscle is at rest (no excitation of a MN), the intracellular Ca++ concentration is low. In the absence of high Ca++, the _____________ is positioned so that the ____________ strands cover the actin "active" site, preventing the myosin head from binding to actin.

2.0 - 2.2 microns

Both skeletal and cardiac muscle exhibit a length-tension relationship. This means that the amount of stretch of the muscle prior to contraction (preload) alters the amount of tension developed. As sarcomeres are stretched, the overlap between the myosin heads and the actin filaments changes. The degree of overlap determines the amount of tension that can be developed and the amount of shortening that can occur. In skeletal muscle, the skeleton fixes the muscle at the optimal length for contraction. The maximum tension developed during an isometric contraction is developed when the sarcomere length is ___________. At this length there is optimum overlap of actin and myosin myofilaments. At lengths less than or greater than the optimum resting sarcomere length, there is too much or too little overlap of myofilaments so that the developed tension is less than maximum.

calsequestrin

Ca++ is pumped into the SR, and is subsequently bound to ____________ which maintains a low free concentration of Ca++ in the SR, thus providing for a lower concentration gradient for the SERCA channel to operate against. This protein also localizes the Ca++ in the junctional SR, for easy release during CICR or CCICR.

angiotensin II; norepinephrine; IP3

During smooth muscle excitation, intracellular Ca++ concentration can be increased in many different ways: 3. Ligand-gated calcium channels-hormones and neurotransmitters in the ECF can allow the influx of Ca++. Examples of ligands include ____________ or _____________. These channels operate independently of the membrane potential 4. Extracellular ligands can also activate G-protein coupled receptors, resulting in the release of intracellular _____ leading to the release of Ca++ from the SR.

regulatory light chain

Each thick myofilament is composed of 300-400 myosin molecules, bundled together. Each myosin molecule consists of two heavy chains that form a double helix and are divided into 3 regions: a tail, an arm (hinge), and a double globular head. The head portion of each myosin forms a complex with 2 light chains. The alkali light chain stablizes the head and the _________________ regulates the ATPase activity of myosin.

TnC

Each troponin complex is bound to tropomyosin at an interval of 7 actin monomers. The tropomyosin-troponin complex plays a key role in regulating the actin-myosin interaction. The binding of Ca++ to _______ causes the troponin-tropomyosin complex to change position, revealing the actin "active" sites (7 sites per tropomyosin molecule) and allowing myosin to bind to actin.

tropomyosin

FOUR STEPS OF ACTIN & MYOSIN INTERACTIONS Step #1. The myosin head has an ATP binding site near the ATPase site. Since ATP is in large supply in the cytoplasm of a healthy muscle cell, the binding site is occupied by an ATP molecule forming a Myosin-ATP complex. The Myosin-ATP complex has a low affinity for actin and will not bind to it. Step #2. Immediately, the myosin ATPase cleaves the ATP to ADP + Pi. The ADP and Pi molecules remain on the myosin head. The energy derived from the hydrolysis of ATP is "stored" and will be expended later. The myosin-ADP complex has a high affinity for actin and will bind to the active site of a G-actin subunit as soon as an actin "active site" becomes available. As long as the intracellular Ca++ level remains low, there will be no interaction between the "high affinity" myosin and the actin because the actin "active sites" are covered by ___________.

phospholamban; relaxation

In cardiac muscle, Ca++ is both resequestered into the SR by a Ca-ATPase pump (SERCA) and extruded from the cytoplasm by a Na-Ca exchanger and a sarcolemmal Ca-ATPase. The removal of Ca++ from the cytoplasm is in approximately the same ratio as when Ca++ enters the cardiomyocyte; that is 80% goes back into the SR and 20% is removed from the cell. However, on some occasions more Ca++ is moved by the SERCA pump into the SR and subsequent contractions are slightly more forceful. Additionally, the amount of reuptake of Ca++ into the SR is regulated by the SR protein ____________. Phosphorylation of this protein increases the activity of the SERCA pump promoting __________.

sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) pump; phospholamban

In order to terminate muscle contraction, the cell must resequester Ca++ into the sarcoplasmic reticulum. This occurs largely through use of a ____________. This moves Ca++ from the sarcoplasm into the sarcoplasmic reticulum against its concentration gradient using primary active transport. In some types of skeletal muscle (as well as cardiac and smooth muscle), SERCA channels are also associated with a regulatory protein called ______________. When phosphorylated, it produces an increase in the activity of the pump, thus increasing the resequestration of Ca++ in the SR.

maximal amount of tension a muscle will develop

Isometric contraction - both ends of the muscle are held at fixed points so that the total length of the anatomical muscle cannot change during contraction. The muscle develops tension but the muscle length does not change. Though the anatomical muscle does not shorten during contraction, it does develop tension because the actin and myosin myofilaments interact and sarcomeres shorten. The elastic components of the muscle and tendons are stretched as the sarcomeres shorten, but the total anatomical muscle length does not change. The isometric contraction curve represents the ______________ to a given Ca++ transient.

only contract isometrically.

Isotonic Contraction - occurs when one end of the muscle is free to move, provided the muscle is able to generate a force greater than the load imposed on it. At first, the muscle will contract isometrically until it generates more tension than the load imposed on the muscle. Once the muscle has generated more tension than the load, it will shorten. During shortening, the tension developed by the muscle remains constant. If the load imposed on the muscle exceeds the capacity of the muscle to generate tension, the muscle will _____________

latent period; contraction period; relaxation period; isometric ; isotonic

MUSCLE TWITCH - a single, brief contraction (and the subsequent relaxation) of a muscle in response to a single stimulus applied directly to the muscle or applied to the nerve which innervates the muscle. It is an all or none response to a single Ca++ transient. Phases of a muscle twitch: a. _____________ b. __________________ c. _______________ A single muscle twitch may be _________ or __________.

myosin phosphatase;

Nitrovasodilators like nitroglycerin and sodium nitroprusside donate nitric oxide which promotes the activity of ______________ which causes dephosphorylation of the regulatory light chain & relaxation of the vascular smooth muscle.

T-tubules; terminal cisterns; troponin (TnC); tropomyosin

SUMMARY 1. An action potential travels along the sarcolemma bi-directionally from the motor end-plate and is directed into the cell via the ____________. 2. The impulse triggers the release of Ca++ from the ______________ of the sarcoplasmic reticulum. 3. The Ca++ bind with ____________. 4. The conformational change in the troponin complex causes the __________ filament to move and expose actin "active" sites on the thin filaments. 5. The high affinity myosin heads (myosin-ADP) bind with the actin "active" sites. 6. Energy stored in the myosin heads is discharged and the myosin heads ratchet and pull the thin filaments toward the center of the sarcomere. 7. With the discharge of ADP from the myosin head, a new ATP molecule binds to the myosin site, decreasing the affinity of the myosin for the actin "active site". The myosin-ATP complex dissociates from the actin. 8. The new bound ATP is hydrolyzed by the ATPase recreating a high affinity, high energy myosin-ADP head. If actin "active sites" are still exposed (because the Ca++ is still high), the myosin head will bind to another actin and repeat the ratcheting cycle 9. If the Ca++ has fallen below a critical point (because nerve activity to the muscle has ceased and there is no more release of calcium from the terminal cisterns) the tropomyosin molecules will again cover over the actin "active" sites and the muscle will relax.

calmodulin; myosin light chain kinase (MLCK); MLCK; calmodulin

Smooth muscle differs from skeletal and cardiac muscle in that regulation of contractile force occurs at the thick filament. The free intracellular Ca++ complexes with _________. This converts inactive ___________ to active ______ which then phosporylates the regulatory light chain of the myosin head (myosin-P) , increases its ATPase activity and allows the actin myosin interaction. Therefore, ___________ and not TnC is the Ca++ binding protein responsible for regulating contraction. Smooth muscle is "thick filament regulated" instead of "thin filament regulated" like striated muscle.

actinomyosin; tilt or ratchet

Step #3. When the alpha motor neuron becomes excited and the nerve terminal depolarizes, the excitation-contraction process is initiated, resulting in an increase in intracellular Ca++ within the muscle fiber. Ca++ bind to troponin (TnC), causing a change in the conformation of this protein. The change in troponin moves the tropomyosin, uncovering an active site on an adjacent actin monomer. As soon as the active sites of actin are exposed, the high affinity, myosin-ADP binds to actin, forming a new complex called ___________. Step #4. When the actinomyosin complex forms, the ADP + Pi molecules are released from the myosin head. The energy produced when the ATP was hydrolyzed is now expended to cause the myosin head to ____________. This movement of the myosin head pulls the actin filament toward the center of the sarcomere. As a result, the sarcomere shortens (contracts), causing tension to develop in the anatomical muscle.

many actin sites

Step #5. Immediately after the ADP molecule is released from the myosin head, a new ATP molecule will occupy the binding site. Since there is plenty of ATP in the cytoplasm of a normal cell, ATP is readily available. Step #6. When ATP binds to myosin, the affinity of myosin for actin decreases so the myosin releases from actin. The process, beginning at step #2, is repeated over and over, as long as intracellular Ca++ remains high. During a normal muscle contraction, a single myosin will interact with ___________. The cycle ends as soon as the Ca++ level falls below a critical level.

actin filament

The contraction of skeletal muscle is said to be regulated by the _______________ because unless the "active" sites are uncovered, contraction will not occur. The myosin is normally ready to interact with the actin filament as soon as the active sites are exposed.

intracellular Ca++ ; fast sodium channels

The depolarization of smooth muscle is due to an increase in ____________, rather than the _____________ found in skeletal and cardiac muscle.

A band width; H zone and I band

The lengths of thin and thick filaments remain unchanged when the sarcomere shortens. During shortening the distance between Z lines decreases - The ________ remains constant throughout contraction. - The ___________ widths decrease during contraction.

a) actin filaments are anchored to Z discs. b) Tropomyosin - lies within the groove formed by the intertwining actin filaments. Each molecule of tropomyosin extends over 7 G-actin molecules. c) Troponin - a globular protein composed of 3 subunits. TNC - attaches to free intracellular Ca++ TNT - attaches to tropomyosin TNI - binds to actin and holds the troponin-tropomyosin in a position where the tropomyosin covers the active actin sites and inhibits actin-myosin interactions.

Thin filaments consist of: 1) __________ which is anchored to ________ 2) _________ 3) ____________ composed of three subunits: _____ _____ _____

cellular respiration; glycolytic processes

Type 1, slow (red) muscles rely heavily on ___________ for energy production. These muscles are able to generate contractions for extended periods of time, as would be necessary in long-term endurance events such as marathon running. Type 2, or fast (white) muscles, on the other hand, rely more heavily on _____________ for energy production. These muscles are capable of very rapid contraction (fast SERCA), but are unable to endure long periods of sustained contraction. These muscles would be important for activities such as sprinting. All of the muscle fibers in one motor unit are of the same type. However, a given muscle may contain a mixture of both slow and fast fibers. Furthermore, the motor unit types can be modified over time.

•Smooth muscle does not have troponin! •Regulation of contraction is at the thick filament •Calcium binds to the regulatory protein calmodulin in the sarcoplasm •The calcium/calmodulin complex also forms in cardiac muscle to regulate the force of contraction

What is so special about smooth muscle contraction?

troponin C

When the Ca++ is released from the junctional sarcoplasmic reticulum, the intracellular Ca++ concentration increases about 100X. The free intracellular Ca++ is available to bind with any of several Ca++ binding proteins including ___________ which then permits actin-myosin crossbridge cycling.

calsequestrin, phospholamban

________ and _________ proteins are important for Ca++ sequestration into the SR.

Troponin C

__________ binds calcium to the thin filaments in both skeletal and cardiac muscle.

Parvalbumin

__________ concentration is related to the speed of the muscle.

Troponin C, Calmodulin, Parvalbumin

__________, ________, ________proteins compete for free intracellular Ca++. Therefore when Ca++ is released from the SR or enters the cell during an action potential, not all the Ca++ causes contraction. As Ca++ is resequestered or extruded, the Ca++ dissociates from these proteins.

Temporal summation

____________ can be explained by two mechanisms. First, as the frequency of stimulation increases there is less time for Ca++ sequestration back into the SR before more Ca++ is released. Thus, the intracellular Ca++ level remains high. Second, with high frequencies of stimulation, the muscle does not have sufficient time to relax between stimulations, so the elastic elements are more fully stretched. Slight sarcomere shortening in response to the next stimulus translates into larger increases in tension because the elastic elements are already stretched.

Calmodulin; Calmodulin

____________ complexes with Ca++ and activates MLCK to increase the force of contraction in heart cells. Ca++ - _____________ is the regulatory protein in smooth muscle.

Phospholamban

_____________ is a SR protein that, when phosphorylated, increases SR Ca++ ATPase (SERCA) activity in cardiac muscle.

Calsequestrin

______________ binds Ca++ and localizes them in the JSR.

DHPRs

in cardiac muscle, some _________ are present in the sarcolemma allowing greater influx of extracellular Ca2+ required for contraction.

unitary - slow waves (spontanous) multi-unit - autonomic nervous system

unitary vs multi-unit smooth muscle