Chapter 10

step one of contraction cycle

ATP hydrolysis. Begins with the arrival of calcium ions within the zone of overlap.myosin head includes an ATP‐binding site that functions as an ATPase—an enzyme that hydrolyzes ATP into ADP (adenosine diphosphate) and a phosphate group. The energy generated from this hydrolysis reaction is stored in the myosin head for later use during the contraction cycle. The myosin head is said to be energized when it contains stored en ergy. The energized myosin head assumes a "cocked" position, like a stretched spring. In this position, the myosin head is perpendicular (at a 90° angle) relative to the thick and thin filaments and has the proper orientation to bind to an actin molecule. Notice that the products of ATP hydrolysis—ADP and a phosphate group—are still attached to the myosin head.

Step 2 of nerve impulse (nerve action potential)

Activation of ACh receptors. Binding of two molecules of ACh to the receptor on the motor end plate opens an ion channel in the ACh receptor. Once the channel is open, small cations, most importantly Na+, can flow across the membrane.

Myomesin

-found in the M line -binds to titin and thick filaments to connect them together at the M line

What is a motor unit?

A motor neuron and all of the muscle fibers it innervates

Twitch contraction

A twitch contraction is the brief contraction of all muscle fibers in a motor unit in response to a single action potential in its motor neuron

third source of ATP

Aerobic respiration is the breakdown of glucose or other nutrients in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria. The inputs for aerobic respiration include glucose circulating in the bloodstream, pyruvic acid, and fatty acids. Aerobic respiration is much more efficient than anaerobic glycolysis, producing approximately 36 ATPs per molecule of glucose versus four from glycolysis. However, aerobic respiration cannot be sustained without a steady supply of O2 to the skeletal muscle and is much slower. To compensate, muscles store small amount of excess oxygen in proteins call myoglobin, allowing for more efficient muscle contractions and less fatigue. Aerobic training also increases the efficiency of the circulatory system so that O2 can be supplied to the muscles for longer periods of time.

Beginning of contraction cycle

At the onset of contraction, the sarcoplasmic reticulum releases calcium ions (Ca2+) into the sarcoplasm. There, they bind to troponin. Troponin then moves tropomyosin away from the myosin‐binding sites on actin. Once the binding sites are "free," the contraction cycle—the repeating sequence of events that causes the filaments to slide—begins. The contraction cycle consists of four steps

step two of contraction cycle

Attachment of myosin to actin. The energized myosin head attaches to the myosin‐binding site on actin and releases the previously hydrolyzed phosphate group. When a myosin head attaches to actin during the contraction cycle, the myosin head is referred to as a cross‐bridge. Although a single myosin molecule has a double head, only one head binds to actin at a time.

troponin and tropomyosin(both in thin filament)

Contraction Inhibiting Proteins (off) within each sarcomere that are attached to actin. Tropomyosin strands in turn are held in place by troponin molecules. You will soon learn that when calcium ions (Ca2+) bind to troponin, troponin undergoes a conformational change (change in shape); this change moves tropomyosin away from myosin‐binding sites on actin, and muscle contraction subsequently begins as myosin binds to actin.

Step four of the contraction cycle

Detachment of myosin from actin. At the end of the power stroke, the cross‐bridge remains firmly attached to actin until it binds another molecule of ATP. As ATP binds to the ATP‐binding site on the myosin head, the myosin head detaches from actin

AChE function

Enzyme that breaks down ACh into acetate and choline, products that cannot activate the ACh receptor

(See image) Epimysium, Perimysium, endomysium

Epimysium- outer layer, encircling the entire muscle. It consists of dense irregular connective tissue. Perimysium- also a layer of dense irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibers, separating into bundles called fascicles. Many large enough to be seen with eye. They give a cut of meat its characteristic "grain"; if you tear a piece of meat, it rips apart along the fascicles. Endomysium- penetrates the interior of each fascicle and separates individual muscle fibers from one another. The endomysium is mostly reticular fibers. All three may extend beyond the muscle fibers to form the tendon that attaches to the periosteum of bone.

Dystrophin

Links thin filaments to proteins of sarcolemma which are attached in turn to proteins in the connective tissue extracellular matrix that surrounds muscle fibers. Dystrophin and its associated proteins are thought to reinforce the sarcolemma and help transmit the tension generated by the sarcomeres to the tendons

Nebulin

Nebulin (NEB‐ū‐lin) is a long, nonelastic protein wrapped around the entire length of each thin filament. It helps anchor the thin filaments to the Z discs and regulates the length of thin filaments during development. Holds F-actin strands together

Step three of the contraction cycle

Power stroke. After a cross‐bridges form, the myosin head pivots, changing its position from a 90° angle to a 45° angle relative to the thick and thin filaments. As the myosin head changes to its new position, it pulls the thin filament past the thick filament toward the center of the sarcomere, generating tension (force) in the process. This event is known as the power stroke. The energy required for the power stroke is derived from the energy stored in the myosin head from the hydrolysis of ATP (see step 1). Once the power stroke occurs, ADP is released from the myosin head.

Functions of Muscular Tissue

Producing body movements, stabilizing body positions, storing and moving substances within the body, and generating heat.

Step 3 of nerve impulse (nerve action potential)

Production of muscle action potential. The inflow of Na+ (down its electrochemical gradient) makes the inside of the muscle fiber more positively charged. This change in the membrane potential triggers a muscle action potential. Each nerve impulse normally elicits one muscle action potential. The muscle action potential then propagates along the sarcolemma into the system of T tubules. This causes the sarcoplasmic reticulum to release its stored Ca2+ into the sarcoplasm, and the muscle fiber subsequently contracts.

Step 1 of nerve impulse (nerve action potential)

Release of acetylcholine. Arrival of the nerve impulse at the synaptic end bulbs stimulates voltage‐gated channels to open. Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the open channels. The entering Ca2+ in turn stimulates the synaptic vesicles to undergo exocytosis. During exocytosis, the synaptic vesicles fuse with the motor neuron's plasma membrane, liberating ACh into the synaptic cleft. The ACh then diffuses across the synaptic cleft between the motor neuron and the motor end plate.

Oxygen debt

This extra oxygen (after workout) is used to "pay back" or restore metabolic conditions to the resting level in three ways: (1) to convert lactic acid back into glycogen stores in the liver, (2) to resynthesize creatine phosphate and ATP in muscle fibers, and (3) to replace the oxygen removed from myoglobin.

Three main types of muscle in our body

Skeletal muscle tissue-most move the bones of the skeleton. Facial moves skin. striated: Alternating light and dark protein bands. Mostly involuntary, some voluntary. Neurons- somatic (voluntary) Cardiac muscle tissue- only in heart. Striated, involuntary. Neurons- autonomic. Built-in heart beat- autorhythmicity. Smooth muscle tissue- in the walls of hollow internal structures, such as blood vessels, airways, and most organs in the abdominopelvic cavity. also found in the skin, attached to hair follicles. lacks the striations of skeletal and cardiac muscle tissue, termed smooth because it looks unstriated. Usually involuntary, some have autorhythmicity (muscles that propel food through tract) regulated by neurons that are a part of the autonomic (involuntary) division of nervous system, and by hormones

Size of motor unit/function?

Whole muscles that control precise movements consist of many small motor units. For instance, muscles of the larynx (voice box) that control voice production have as few as two or three muscle fibers per motor unit, and muscles controlling eye movements may have 10 to 20 muscle fibers per motor unit. In contrast, skeletal muscles responsible for large‐scale and powerful movements, such as the biceps brachii muscle in the arm and the gastrocnemius muscle in the calf of the leg, have as many as 2000 to 3000 muscle fibers in some motor units. Because all of the muscle fibers of a motor unit contract and relax together, the total strength of a contraction depends, in part, on the size of the motor units and the number that are activated at a given time.

Step 4 of nerve impulse (nerve action potential)

Termination of ACh activity. The effect of ACh binding lasts only briefly because ACh is rapidly broken down by an enzyme called acetylcholinesterase (AChE) (as′‐ē‐til‐kō′‐lin‐ES‐ter‐ās). This enzyme is located on the extracellular side of the motor end plate membrane. AChE breaks down ACh into acetate and choline, products that cannot activate the ACh receptor.

Components of a sarcomere- bands and zones

The components of a sarcomere are organized into a variety of bands and zones. The darker middle part of the sarcomere is the A band, which extends the entire length of the thick filaments. Toward each end of the A band is a zone of overlap, where the thick and thin filaments lie side by side. The I band is a lighter, less dense area that contains the rest of the thin filaments but no thick filaments, and a Z disc passes through the center of each I band. The alternating dark A bands ands light I bands create the striations that can be seen in both myofibrils and in whole skeletal and cardiac muscle fibers. A narrow H zone in the center of each A band contains thick but not thin filaments. M line in center of sarcomere

a-actinin location

The dense material of the Z discs contains molecules of α‐actinin, which bind to actin molecules of the thin filament and to titin.

Sarcomere

a structural unit of a myofibril in striated muscle, consisting of a dark band and the nearer half of each adjacent pale band. Within myofibrils are smaller protein structures called filaments or myofilaments (Figure 10.2c). Thin filaments are 8 nm in diameter and 1-2 μm long and composed mostly of the protein actin, while thick filaments are 16 nm in diameter and 1-2 μm long and composed of the protein myosin. Both thin and thick filaments are directly involved in the contractile process. Overall, there are two thin filaments for every thick filament in the regions of filament overlap. The filaments inside a myofibril do not extend the entire length of a muscle fiber. Instead, they are arranged in compartments called sarcomeres. A sarcomere extends from one Z disc to the next Z disc.

Wave summation

addition of successive neural stimuli to produce greater contraction

What can't muscle fibers and neurons do?

cannot divide by mitosis or regenerate

myosin and actin(in thin filament)

contractile proteins that generate force. In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin‐binding sites on actin

Properties of Muscular Tissue

electrical excitability, contractility, extensibility, elasticity

ACh function

enables muscle action, learning, and memory (neurotransmitter)

AChE located?

extracellular side of the motor end plate membrane.

Second source of ATP

muscles turn to glycolysis as an ATP source. Glycolysis is an anaerobic (non-oxygen-dependent) process that breaks down glucose (sugar) to produce ATP; however, glycolysis cannot generate ATP as quickly as creatine phosphate. Thus, the switch to glycolysis results in a slower rate of ATP availability to the muscle. The sugar used in glycolysis can be provided by blood glucose or by metabolizing glycogen that is stored in the muscle. The breakdown of one glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or when oxygen levels are low, converted to lactic acid. If oxygen is available, pyruvic acid is used in aerobic respiration. However, if oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. Can fuel for about 1 minute.

How muscle contraction occurs. because...

myosin heads attach to and "walk" along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line. As a result, the thin filaments slide inward and meet at the center of a sarcomere. They may even move so far inward that their ends overlap. As the thin filaments slide inward, the I band and H zone narrow and eventually disappear altogether when the muscle is maximally contracted. However, the width of the A band and the individual lengths of the thick and thin filaments remain unchanged. Since the thin filaments on each side of the sarcomere are attached to Z discs, when the thin filaments slide inward, the Z discs come closer together, and the sarcomere shortens. Shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle.

Motor end plate

region of the sarcolemma opposite the synaptic end bulbs.

Aponeurosis

strong sheet of tissue that acts as a tendon to attach muscles to bone. Flat

Several key structural proteins are titin, α‐actinin, myomesin, nebulin, and dystrophin

structural proteins which contribute to the alignment, stability, elasticity, and extensibility of myofibrils. Titin-third most plentiful protein in skeletal muscle (after actin and myosin). This molecule's name reflects its huge size. Molecule spans half a sarcomere, from a Z disc to an M line

Where is ACh stored?

synaptic vesicles (end of the motor neuron called the axon terminal, divides into clusters of synaptic end bulbs)

The two isotonic contractions

tension developed in the muscle remains almost constant while the muscle changes its length. Used for body movements and for moving objects. Concentric is great enough to overcome the resistance of the object to be moved, the muscle shortens and pulls on another structure, such as a tendon, to produce movement and to reduce the angle at a joint. Eccentric is the tension exerted by the myosin cross‐bridges resists movement of a load and slows the lengthening process. For reasons that are not well understood, repeated eccentric isotonic contractions (for example, walking downhill) produce more muscle damage and more delayed‐onset muscle soreness than do concentric isotonic contractions.

Muscle tone

the state of balanced muscle tension that makes normal posture, coordination, and movement possible. Maintained by prolonged presence of Ca^++ in cytosol

unfused tetanus

type of wave summation with partial relaxation observed between twitches

fused tetanus

when stimulus frequency is so high that no muscle relaxation takes place between stimuli

First source of ATP

Direct phosphorylation of ATP by creatine phosphate

satellite cells

During embryonic development, many myoblasts fuse to form one skeletal muscle fiber. Once fusion has occurred, a skeletal muscle fiber loses the ability to undergo cell division, but satellite cells retain this ability