Chapter 10 - Muscular Tissue

What are intercalated discs?

- unique to cardiac muscle fibers - they are irregular transverse thickenings of the sarcolemma that connects the ends of cardiac muscle fibers to one another - discs contain desmosomes which hold the fibers together and gap junctions which allow muscle action potentials to spread from one cardiac muscle fiber to another

What is a synapse?

A synapse is a region where communication occurs between two neurons, or between a neuron and a target cell, i.e. muscle fiber.

What is the role of actin?

Actin is the main component of thin filaments, which are anchored to Z discs. Actin filaments join to form an actin filament that is twisted into a helix. On each actin molecule is a myosin-binding site, where a myosin head can attach.

What is the second phase of contraction?

Contraction period. Ca2+ binds to troponin; binding sites are exposed; crossbridges form; peak tension develops in muscle.

What are the three layers of connective tissue that surround skeletal muscle?

Epimysium: outermost layer, encircling the entire muscle; Perimysium: surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called fascicles; Endomysium: separates individual muscle fibers from one another; All three layers may extend beyond the muscle fibers to form a tendon.

What is the name for the brief delay between application of the stimulus and the beginning of the contraction?

Latent period. During this period, the muscle action potential sweeps over the sarcolemma and Ca2+ is released from the SR.

How does aerobic cellular respiration work?

Produces ATP in the mitochondria. With O2 present, pyruvic acid formed by glycolysis is completely oxidized in reactions that generate ATP, CO2, water, and heat. Slower than glycolysis but yields more ATP (36). Two sources of O2: - O2 that diffuses from the blood - O2 released by myoglobin within muscle fibers

What is the third phase of contraction?

Relaxation period. Ca2+ is actively transported back into the SR; binding sites are covered by tropomyosin; myosin heads detach from actin; tension decreases.

What is the I band?

The lighter, less dense area that contains the rest of the thin filaments but no thick filaments. Z disc in center.

What is the motor end plate?

The region of the sarcolemma opposite the synaptic end bulbs--the muscle fiber part of the NMJ. Within the motor end plate are 30-40 million ACh receptors, integral transmembrane proteins that bind specifically to ACh.

What is the SR?

The sarcoplasmic reticulum is a fluid-filled system of membranous sacs that encircle the myofibril. Similar to smooth ER in non-muscular cells. The SR contains large stores of calcium, which it sequesters and then releases when the muscle cell is stimulated.

What is meant by excitation-contraction coupling?

The steps that connect excitation (a muscle action potential propagating along the sarcolemma and into the T tubules) to contraction (sliding of the filaments) make up the excitation-contraction coupling.

How does anaerobic cellular respiration work?

aka Glycolysis When creatine phosphate is depleted, glucose is catabolized to generate ATP. Series of 10 reactions breaks down each glucose molecule into two molecules of pyruvic acid. Provides energy for 30-40 seconds.

What is muscular atrophy and hypertrophy?

atrophy = wasting away of muscles; individual muscle fibers decrease in size because of progressive loss of myofibrils hypertrophy = an increase in the diameter of muscle fibers due to increased production of myofibrils, mitochondria, SR, and other organelles

What are the four special properties of muscular tissue that enable it to function and contribute to homeostasis?

- electrical excitability: the ability to respond to certain stimuli by producing electrical signals called action potentials - contractility: the ability of muscular tissue to contract forcefully when stimulated by an action potential - extensibility: the ability to stretch without being damaged - elasticity: the ability to return to its original length and shape after contraction or extension

What are some reasons for muscle fatigue?

- inadequate release of Ca2+ from SR - depletion of creatine phosphate - insufficient O2, depletion of glycogen, and other nutrients

Briefly describe the Sliding Filament Mechanism.

- muscle contraction occurs because myosin heads attach to and 'walk' along the thin filament at both ends of a sarcomere, progressively pulling the thin filaments toward the M line - the thin filaments slide inward and meet at the center of a sarcomere, however the lengths of thin and thick do not change - shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle

What are the four key functions of muscle?

- producing body movements - stabilizing body positions - storing and moving substances within the body (i.e. through use of sphincters) - generating heat

What are the 3 ways to produce ATP for use by muscle fibers?

1) From creatine phosphate 2) By anaerobic cellular respiration 3) By aerobic cellular respiration

What is a tendon?

A cord of dense, regular connective tissue composed of parallel bundles of collagen fibers that attach to the periosteum of a bone.

What is a motor unit?

A motor unit consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates.

What stimulates the release of Ca2+?

A muscle potential that propagates along the sarcolemma and into the T tubules causes the Ca2+ release channels in the SR membrane to open and Ca2+ to flow out into the cytosol.

What is myoglobin?

A red-coloured protein found in the sarcoplasm that binds oxygen molecules that diffuse into muscle fibers from interstitial fluid. Only found in muscle.

What is the neurotransmitter released at the NMJ?

Acetylcholine (ACh)

What are the two contractile proteins in muscle fibers?

Actin and myosin

What is the M line?

Center of the H zone, so named because it is at the middle of the sarcomere.

What is fascia?

Fascia is a dense sheet or broad band of dense irregular connective tissue that lines the body wall and limbs and supports and surrounds muscles and other organs. It holds muscles with similar functions together.

What are sarcomeres?

Filaments inside a myofibril do not extend the entire length of a muscle fiber; instead, they are arranged in compartments called sarcomeres, which are basic fundamental units of a myofibril.

Within myofibrils are smaller structures called_________.

Filaments: - thin filaments (8nm) - thick filaments (16nm) The thick and thin filaments overlap one another to a greater or lesser extent, depending on whether the muscle is contracted, relaxed, or stretched. The pattern of their overlap, consisting of a variety of zones and bands, creates the striation that can be seen both in single myofibrils and in whole muscle fibers.

What is the refractory period?

If two stimuli are applied, one after another, the muscle will respond only to the first because it temporarily loses its excitability and cannot respond.

What is the Length-Tension relationship?

Indicates how the forcefulness of muscle contraction depends on the length of the sarcomeres within a muscle before contraction begins.

What does oxygen debt refer to?

The added oxygen, over and above the resting oxygen consumption, that is taken into the body after exercise to restore metabolic conditions to resting levels (i.e. resynthesizing creatine phosphate and ATP, convert lactic acid to glycogen, replace oxygen to myoglobin).

What is a twitch contraction?

The brief contraction of all the muscle fibers in a motor unit in response to a single action potential in its motor neuron.

What is the sarcoplasm?

The cytoplasm of a muscle fiber.

What is the A band?

The darker, middle part, which extends the entire length of the thick filaments.

Which neurons stimulate skeletal muscles?

somatic motor neurons

What is an aponeurosis?

When the connective tissue elements extend as a broad, flat layer, the tendon is called an aponeurosis.

What are myofibrils?

Within the sarcoplasm are small threads called myofibrils, the contractile organelle of skeletal muscle. Their prominent striations make the entire skeletal muscle fiber appear striated.

What is the name of part of a myofibril that separates one sarcomere from the next?

Z disc

What is hypertrophy?

Dramatic muscle growth that occurs after birth-->an enlargement of existing muscle fibers.

What is the NMJ?

Muscle action potentials arise at the neuromuscular junction--the synapse between a somatic motor neuron and a skeletal muscle fiber.

What is the plasma membrane of a muscle fiber called?

Sarcolemma

What are the three types of muscular tissue?

Skeletal, cardiac, and smooth

Briefly compare the three types of muscular tissue.

Skeletal: - most move bones of the skeleton - striated --> alternating light and dark protein bands (striations) - works mainly in a voluntary manner Cardiac: - forms most of the heart wall - also striated - action is involuntary (heart beats because it has a pacemaker that initiates each contraction --> autorhythmicity) Smooth: - located in the walls of hollow internal structures, such as blood vessels, airways, and most organs in the abdominopelvic cavity - appears nonstriated - action is usually involuntary and some has autorhythmicity, i.e. esophagus

What are T tubules?

Transverse (T) tubules are tiny invaginations of the sarcolemma that tunnel in from the surface toward the center of each muscle fiber. T tubules are open to the outside of the fiber and thus are filled with interstitial fluid. Muscle action potentials travel along the sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber.

What are the two regulatory proteins that also make up the thin filaments?

Tropomyosin and troponin. Tropomyosin covers the myosin-binding sites on actin and blocks myosin from binding in relaxed muscles. The tropomyosin strands in turn are held in place by troponin molecules.

What is the H zone?

In the center of each A band contains thick but no thin filaments.

How does the action potential terminate?

The effect of ACh binding lasts only briefly because ACh is rapidly broken down by an enzyme called acetylcholinesterase (AChE).

What are the four steps of the contraction cycle?

1) ATP Hydrolysis - ATPase hydrolyzes ATP into ADP and a phosphate group, which reorients and energizes myosin head 2) Attachment of myosin to actin to form crossbridges - energized myosin head attaches to the myosin-binding site on actin and releases PO43- group; when myosin heads attach to actin, they are referred to as crossbridges 3) Power Stroke - site on crossbridge where ADP is still bound opens; crossbridge rotates and releases ADP - crossbridge generates force as it rotates sliding the thin filament past the thick filament toward the M line 4) Detachment of myosin from actin - at the end of the power stroke, the crossbridge remains firmly attached to actin until it binds another molecule of ATP - as ATP binds, myosin head detaches from actin Contraction cycle continues as long as ATP is available and the Ca2+ level near the thin filament is sufficiently high.

How does creatine phosphate work?

An energy-rich molecule found only in muscle fibers, it is synthesized when there is extra ATP (i.e. periods of relaxation). When contraction begins and ADP levels start to rise, creatine kinase (CK) catalyzes the transfer of a high-energy phosphate group from creatine phosphate back to ADP, which quickly regenerates new ATP molecules. Allow for contractions of ~15 seconds.

What is a synaptic cleft?

At most synapses a small gap, called the synaptic cleft, separates the two cells. The first cell communicates with the second by releasing a chemical called a neurotransmitter.

What is the role of calcium in the contraction cycle?

At the onset of contraction, the SR releases Ca2+ into the cytosol. There, they bind to troponin, which then moves tropomyosin away from the myosin-binding sites on actin. Once binding sites are "free", the contraction cycle begins.

What is the role of myosin?

Main component of thick filaments. Functions as a motor protein in all three types of muscle tissue. Motor proteins push or pull various cellular structures to achieve movement by converting chemical energy in ATP to the mechanical energy of motion or the production of force. Each myosin molecule is shaped like two golf clubs twisted together. The tails (handles) point towards the M line. The two projections are called myosin heads and they project outward from the shaft in a spiralling fashion, each extending toward one of the six thin filaments that surround each thick filament.