CH10: Muscular Tissue Flashcards
Functions and structure of the muscle tissue type
Skeletal muscle tissue: Named as such because most skeletal muscles move the bones of the skeleton. Multinucleated and striated, and is controlled voluntarily, controlled by neurons that are part of the voluntary system of the nervous system.
Cardiac muscle tissue: Found in heart, pumps blood around cardiovascular system. One nucleus, striated and has intercalated discs. Involuntarily controlled, as its heartbeat is the contraction and relaxation of the heart, which occur due to the hearts natural pacemaker. Several hormones and neurotransmitters can adjust heart rate by speeding or slowing the pacemaker
Smooth (Visceral) muscle tissue: Functions for several things, like peristalsis. Has one nucleus and no striations. Involuntary control
Functions of muscle tissue in general
Producing body movements: Movements of the whole body including running and localized movements like grabbing a pen or nodding the head rely on integrated functioning of skeletal muscles, bones and joints
Stabilizing body positions: Skeletal muscle contractions stabilize joints and help maintain body positions, such as standing or sitting. Postural muscles contract continuously when you are awake.
Storing and mobilizing substances within the body: Smooth muscle sphincters control storage by preventing outflow in hollow organs like the stomach and urinary bladder. Cardiac muscle contractions pump blood through blood vessels, while smooth muscle in blood vessel walls regulates blood flow. Smooth muscle contractions assist in moving food, bile, and substances through the digestive system, as well as in propelling gametes and urine through genital and urinary systems. Skeletal muscle contractions help with lymph plasma flow and the return of blood in veins to the heart.
Generating heat: As muscular tissue contracts, it produces heat in a process known as thermogenesis. Involuntary contractions of skeletal muscles, known as shivering, can increase the rate of heat production
Properties of muscle tissue in general
Electrical excitability: Recall form CH.4 that this is the ability to respond to certain stimuli by producing electrical signals called action potentials (called muscle action potentials when they are in muscle). The two types of stimuli that trigger action potentials are electrical signals arising in muscular tissue itself, such as the hearts actual pacemaker, and chemical stimuli, such as neurotransmitters released by neurons hormones distributed by the blood, or even local changes in pH
Contractility: The ability of muscular tissue to contract forcefully when stimulated by a nerve impulse. When a skeletal muscle contracts, it generates tension (force of contraction) while pulling on its attachment points. If the tension generated is great enough to overcome the resistance of the object being moved, the muscle shortens and movement occurs
Extensibility: The ability of muscular tissue to stretch, within limits, without being damaged. The connective tissue within the muscle limits the range of extensibility and keeps it within the contractile range of the muscle cells
Elasticity: The ability of muscular tissue to return to its original length and shape after contraction or extension.
Define the structure of a skeletal muscle
A skeleltal muscle consists of bundles of 10-100 fibers called fascicles. On the exterior of these bundles is perimysium, whlie sperating the bundles is endomysium. The epimysium coats the entire muscle. Within each fiber are myofibrils, bundles of interrelated proteins, containing various bands filaments to generate contractile force. The cytoplasm where this takes place is called the sacroplasm, and the membrane is called the sarcolemma
- Z band seperates the repeating units of myofibrils, the sarcomere
- A band is a long unit containing the entire length of a thick filament and a zone of overlap with some thin filament.
- I band covers the remaining think filaments. Z band passes through the middle.
- H band is a narrow region in the middle of the A band, annd therefore contains some thick but no thin filament
- M line is a center in the middle of the H band that hold thick filaments together at the center of a sarcomere.
- Sarcoplasimc reticulum is on outside of, containing calcium ions which diffuse through the end part of sarcoplasmic reticulum, the terminal cistern. Two terminal cisterns of two different sr’s diffuse into a t-tubule, an invagination, to bring calcium ions into the cell.
3 types of proteins in skeletal muscle
Contracile proteins: Those that generate force during contraction
Regulatory: Those that switch the contractile process on and off
Structural: Those that keep thick and thin filaments in proper alignment.
Name the two types of contractile proteins, their structure and function
Myosin is the main component of thick filaments and functions as a motor protein in all 3 types of muscle tissue, responsible for pulling various cellular structures to achieve movement by converting energy in ATP to mechanical energy of motion
In the skeletal muscle, about 300 molecules of myosin form a single thick filament. They are shaped like two golf clubs twisted together.
The myosin tail (golf club handle), points toward the M line of the sarcomere. Tails of neighboring myosin molecules lie parallel to one another to form the shaft of the thick filament.
The two projections of the myosin molecule (club heads) are called the myosin head, and each has two binding sites: an actin binding site and an ATP binding
The ATP-binding site also functions as an ATPase—an enzyme that hydrolyzes ATP to generate energy for muscle contraction
The main component of the thin filament is the protein actin. Individual actin molecules combine 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
Two regulatory proteins
The two regulatory proteins are tropomyosin and troponin, which make up a small part of the thin filament.
Tropomyosin: When a muscle is relaxed, strands of tropomyosin cover myosin binding sites to prevent actin to bind to it
Troponin: Regulatory protein that is a component of thin filament; when calcium ions (Ca2+) bind to troponin, it changes shape; this conformational change moves tropomyosin away from myosin binding
sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin.
Describe the contraction cycle
At the onset of contraction, the sarcoplasmic reticulum releases calcium ions into the sarcoplasm, where they then bind to troponin, the regulatory protein found in thin filament.
Troponin then moves tropomyosin away from the myosin binding sites on actin, and once the binding sites are free, the contraction cycle begins, the repeating sequence of events that causes filaments to slide. This consists of 4 steps
ATP hydrolysis: Recall the ATP-binding site on the myosin that functions as an ATPase, an enzyme that hydrolyzes ATP into ADP and a phosphate group. This releases energy, and the myosin head is said to be energized, assuming a “cocked” position, assuming a 90o angle relative to the thick and thin filaments and has the proper orientation to bind an actin molecule.
Attachment of myosin to actin: Energized myosin head now 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, it is called a cross-bridge. Only one myosin head binds at a time
Power stroke: After a crossbridge forms, the myosin head changes its angle from 90o to 45o, relative to the thick and thin filaments, which pulls the thick filament toward the center of the sarcomere, generating tension (force) in the process. This is known as the powerstroke. The energy required for this is derived from the energy stored in the myosin head from step 1. ADP is released from the myosin head
Detachment of myosin form 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, the myosin head detaches from actin
Describe excitation-contraction coupling
Excitation-contraction coupling describes the period between when muscle action travels thorugh the sarcolemma and T-tubule to contraction, detailing the release of calcium ions which go on to start the contraction cycle.
At a given triad, within the membranes of the t-tubules are integral proteins called voltage-gated Ca2+ channels. These are arranged in groups of 4 known as tetrads. These are responsible for detecting voltage differences that occur through action potential, which trigger the opening of another key protein, Ca2+ release channels.
Ca2+ release channels are blocked in a resting muscle by voltage gated calcium ion channels. They are located in the terminal cistern membranes. When a skeletal muscle fiber is excited and an action potential travels along the T tubule, the voltage-gated Ca2+ channels detect the change in voltage and undergo a conformational change that ultimately causes the Ca2+ release channels to open
The terminal cisternal membrane of the sarcoplasmic reticulum also contains Ca2+-ATPase pumps that use ATP to constantly transport calcium ions from the sarcoplasm into the SR. As long as muscle action potentials continue to propagate along the T tubules, the Ca2+ release channels remain open and calcium ions flow into the the sarcoplasm faster than it is transported back into the SR by Ca2+-ATPase pumps
Describe the length-tension relationship
The force of a muscle contraction depends on the length of the sarcomeres in a muscle prior to contraction
At a sarcomere length of about 2.0–2.4 µm (which is very close to the resting length in most muscles), the zone of overlap in each sarcomere is optimal, and the muscle fiber can develop maximum tension.
Maximum tension (100%) occurs when the zone of overlap between a thick and thin filament extends from the edge of the H zone to one end of a thick filament.
As the sarcomeres of a muscle fiber are stretched to a longer length, the zone of overlap shortens, and fewer myosin heads can make contact with thin filaments
When a skeletal muscle fiber is stretched to 170% of its optimal length, there is no overlap between the thick and thin filaments. Because none of the myosin heads can bind to thin filaments, the muscle fiber cannot contract, and tension is zero
As sarcomere lengths become increasingly shorter than the optimum, the tension that can develop again decreases. This is because thick filaments crumple as they are compressed by the Z discs, resulting in fewer myosin heads making contact with thin filaments3 4
Describe how a muscle action potential is generated and how it ends
Release of acetylcholine: Once the nerve impulse reaches the synaptic end bulbs, voltage-gated channels are opened, and through concentration difference, Ca2+ flows inward through the open channels, which stimulates synaptic vesicles to undergo exocytosis, releasing ACh into the synaptic cleft
Activation of ACh receptors: Binding two molecules of acetylcholine to the receptor on the motor end plate opens an ion channel in the ACh receptor, and once the channel is open, small cations, mainly sodium ions, can flow across the membrane
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.
Termination of ACh activity: The effect of ACh binding lasts only briefly because ACh is rapidly broken down by an enzyme called acetylcholinesterase (AChE). This enzyme is located on the extracellular side of the motor end plate membrane. AChE breaks down ACh into acetyl and choline, products that cannot activate the ACh receptor
Describe the 3 ways muscle cells produce ATP
Creatine Phosphate: Creatine kinase catalyzes the transfer of a phosphate group from CP to ADP to rapidly yield ATP
Anaerobic Glycolysis: When CP stores are depleted, glucose is converted into pyruvic acid to generate ATP
Aerobic Respiration: Under aerobic conditions, pyruvic acid can enter the mitochondria and undergo a series of oxygen-requiring reactions to generate large amounts of ATP
What is muscle and central fatigue and how do they arrive
Muscle fatigue is the inability to continue contraction after prolonged activity it is caused by
- Inadequate release of calcium ions
- ADP and lactic acid build up
- Depletion of CP, oxygen levels and nutrition
- Lack of ACh release at NMJ
Central fatigue is the feeling of tiredness and desire to cease activities, due to changes in the nervous system?
What is a motor unit and define motor unit recruitment
A motor unit consists of one somatic motor neuron and the muscle fibers to which it forms nms with.
Motor unit recruitment is the addition of active muscle fibers to icnrease strength during a contraction, starting from weaker motor units to stronger motor units.
4 periods of twich contraction?
A twitch contraction is the brief contraction of all muscle fibers in a motor unit in response to a single action potential. The record of a muscle contraction is called a myogram,
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Latent period: The delay in response to stimulus. Action potential sweeps over sarcolemma and Ca2+ released
Contraction period: Ca2+ binds to troponin, myosin-binding sites on actin are exposed, and cross-bridges form. Peak tension develops in the muscle fiber (10-100 ms).
Relaxation period: Ca2+ actively transported back into the sarcoplasmic reticulum, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases. The actual duration of these periods depends on the type of skeletal muscle fiber (10-100ms)
Refractory period: If two stimuli are applied, one immediately after the other, the muscle will respond to the first stimulus but not to the second, as the muscle tem[orarily loses its excitability and cannot respond for a time
