Muscles Flashcards
(40 cards)
List Functions of Skeletal Muscles (6)
- Produce skeletal movement
- Maintain posture
- Support soft tissue (think abdominal muscles)
- Guard openings (sphincter muscles / mouth)
- Maintain body temperature (waste heat)
- Store nutrient reserves (proteins -> ketone bodies)
List Properties of Skeletal Muscles (4)
- Electrical excitable - respond to electrical signals
- Contractile
- Extensibility: can be stretched without being damaged
- Elasticity: return to original shape.
List tissue types of skeletal muscles
- Connective Tissues
- Epimysium: Dense irregular connective tissue that surrounds the entire muscle.
- Perimysium: Dense irregular connective tissue that surrounds bundles of muscle fibers (fascicles).
- Endomysium: Loose areolar connective tissue that surrounds individual muscle fibers.
- Deep fascia: Dense irregular connective tissue located external to the epimysium, separating muscles and groups of muscles from one another. - nerves - skeletal muscles are voluntary, controlled by central nervous system.
- Blood vessels - supply oxygen, nutrients, and carry away waste
- muscle tissue
Describe skeletal muscle formation and shape
Skeletal muscle is:
- striated
- multi-nucleated (allows for efficient control through length of muscle)
- very long and cylindrical
Skeletal muscle fibers are formed:
- through the fusion of mesodermal stem cells, called myoblasts
Describe Muscle Organization
- Muscles are surround by a dense irregular connective tissue layer called epimysium
- Within muscles there are many muscle fascicles, bundles of muscle fibers (muscle cells) surrounded by perimysium.
- Each muscle cell is surrounded by endomysium and contains many myofibrils, nucleuses, and mitochondria.
The endomysium, perimysium, and epimysium come together at ends of muscles to form tendons or aponeurosis, connecting muscle to bone.
aponeurosis -> CT sheet
Describe Components of Muscle Fiber / Muscle Cell
- sarcolemma: Muscle cell plasma membrane
- sarcoplasm: Muscle cell cytoplasm
- sarcoplasmic reticulum: specialized smooth endoplasmic reticulum. Stores calcium and regulates calcium release. Sarcoplasmic reticulum surrounds each myofibril.
- terminal cisterna: Enlarged areas of the SR with large amounts of calcium. Are on either side of t-tubules. Release of calcium from cisternae triggers muscle contractions.
- T-tubules: invaginations of the sarcolemma which penetrate into the interior of the muscle fiber surrounding each sarcoplasmic reticulum. Transmit action potential through the cell allowing simultaneous contraction of entire cell. are filled with extracellular fluid, essentially
- Triad: Two terminal cisterna surrounding a t-tubule.
- myofibril: Contractile component, composed of bundles of myofilaments. There are thin and thick filaments.
- mitochondria: energy producers.
- multiple nuclei
Draw / Describe structure of Sarcomeres
- contractile unit of muscle, structural unit of myofibrils.
Thick filaments: Composed primarily of myosin protein, these filaments are responsible for muscle contraction through their interaction with actin.
Thin filaments: Made mainly of actin, along with troponin and tropomyosin, these filaments slide past thick filaments during contraction.
A band: The region of the sarcomere that contains thick filaments, including areas where they overlap with thin filaments.
I band: The lighter region that contains only thin filaments and is divided by the Z disc
Zone of overlap: The area within the A band where thick and thin filaments overlap, crucial for muscle contraction.
H band: The central part of the A band where only thick filaments are present, without any overlapping thin filaments.
M line: The middle line of the sarcomere that anchors and aligns the central part of the thick filaments.
Z disc / z line: The boundary structure of the sarcomere that anchors the thin filaments and connects adjacent sarcomeres.
dArk = A bands
lIght = I bands
Describe structure and function of thin filaments
- F-actin: A polymerized string of G-actin. Active sites on G-actin bind to myosin
- Tropomyosin: A regulatory protein that wraps around actin filaments, blocking myosin binding sites on G-actin when the muscle is relaxed.
- Troponin: A complex of three proteins that binds to tropomyosin and associates with actin. When calcium binds to troponin, it causes a conformational change, shifting tropomyosin, exposing myosin-binding sites on G-actin and enabling muscle contraction
Describe the structure of thick filaments
- primarily composed of myosin (roughly 300) which has a long tail and two globular heads. The tails intertwine to form the filament’s backbone.
- Each myosin head has binding sites for actin (on thin filaments) and ATP. The heads use ATP hydrolysis to pivot and pull thin filaments during contraction.
- Titin, an elastic protein, anchors the thick filaments to the Z-disc, contributing to muscle elasticity and alignment
Describe changes that occur in sarcomere during muscle contraction
- Calcium Ions Released: Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm.
- Troponin Binds Calcium: Calcium ions bind to troponin, causing a conformational change.
- Tropomyosin Moves: The conformational change in troponin shifts tropomyosin away from the myosin-binding sites on actin filaments.
- Myosin Binding Sites Exposed: With tropomyosin moved, the myosin-binding sites on actin are exposed.
- Cross-Bridge Formation: Myosin heads bind to the exposed sites on actin, forming cross-bridges.
- Power Stroke: Using energy from ATP hydrolysis, the myosin heads pivot, pulling the actin filaments toward the center of the sarcomere.
- Z Discs Move Closer: The Z discs at each end of the sarcomere move closer together.
- I Bands Shorten: The I bands, which contain only thin filaments, shorten.
- H Zone Narrows: The H zone, where only thick filaments are present, becomes narrower.
- A Band Stays the Same: The A band, the length of the thick filaments, remains unchanged.
- Zone of Overlap Increases: The overlap between thick and thin filaments
Define action potential. Describe action potential in muscles
A rapid depolarization and repolarization of the membrane potential that propagates along the axon of a neuron and along a muscle fiber.
- resting membrane potential is between -70 to -90 volts. The sodium potassium ATPase maintains this potential by moving two potassium (K+) ions into the cell and and three sodium (Na+) ions out. It is more negative directly bellow cell membrane surface. There is a higher concentration of Na+ out of the cell (wants to come in) and a higher concentration of K+ in the cell (wants to leave)
- AP reaches synaptic terminal, triggering release of ACh.
- ACh binds nicotinic acetylcholine receptors, a ligand gated sodium channel.
- sodium enters the cell, causing initial depolarization.
- initial depolarization opens voltage gated sodium channels (around -55mV). More sodium enters the cell, membrane, membrane potential increases to around +30 V.
- AP propagates along sarcolemma (diffusion), and deep into the muscles via T-tubules .
- Depolarization along T-tubules opens voltage gated calcium ion channels in the cisternae of the SR, triggering the release of calcium into the sarcoplasm.
- Ca+ release causes contraction. Allows myosin to bind to action and the muscle to contract.
- Maximum depolarization (+30mV) causes the opening of voltage gated potassium channels, potassium exits the cell, depolarizing the membrane (often overshoots a bit). Around the same time time-dependent inactivation gates close the sodium channels (gates or “reset” when membrane reaches resting potential).
excitation-contraction coupling: Link between electrical signals and muscle contraction
With rapid or continuous stimulation (such as during tetanus), the action potentials occur in quick succession, but the basic sequence of Na⁺ channel activation, inactivation, and K⁺ channel opening still occurs. The channels cycle through their states more frequently, but they do not bypass the inactivation and opening processes. This is why there’s a limit to how rapidly action potentials can occur, defined by the refractory period.
Define synaptic terminal and cleft and motor end plate
synaptic terminal: The end of a neuron where neurotransmitters are stored and released into the synaptic cleft.
synaptic cleft: The space between the synaptic terminal of a neuron and the target cell where neurotransmitters are released.
motor end plate: The region of the muscle fiber membrane directly opposite the synaptic terminal of a motor neuron, contains acetylcholine receptors.
Define Acetylcholine, nicotinic acetylcholine receptors, and acetylcholine esterase
acetylcholine: A neurotransmitter. In muscle contractions it is released at the the synaptic terminal and binds nicotinic ACh receptors in the motor end plate.
nicotinic acetylcholine receptors: ligand gated sodium channels in motor end plate. Open when bound to ACh causing initial depolarization of membrane.
acetylcholine esterase: An enzyme located in the synaptic cleft that breaks down acetylcholine into acetate and choline, terminating the signal at the neuromuscular junction and allowing the muscle to relax.
Describe the contraction cycle
- At resting, myosin heads are in their cocked position and bound to ADP and P. The tropomyosin / troponin complex is blocking the myosin binding sites on G-actin.
- Calcium binds troponin. Causing a conformational change which moves the tropomyosin, exposing G-actin binding sites.
- The myosin head binds the G-actin binding site, creating a cross bridge.
- Myosin releases the bound ADP + P. This causes the myosin head to pivot, pulling the actin filament towards the center of the sarcomere (power stroke)
- Myosin then binds ATP causing detachment from the G-actin myosin binding site.
- Myosin cleaves ATP to ADP + P and returns to the resting cocked position.
What factors influence contraction duration
- Duration of the neural stimulus
- Number of free calcium ions in the sarcoplasm
- ATP availability
What is the all-or-none principle of muscle fiber contraction
When a muscle fiber is stimulated to the threshold level, it will contract fully (engage in contraction). If that threshold in not reached, no contraction will occur.
Of note the muscle tension depends on how many cross-bridge cycles occur.
What factors influence the amount of tension in a single muscle fiber?
The number of piviting cross-bridges formed.
- The fibers’ resting length at time of stimulation
- The frequency of the stimulus
With a single contraction (twitch) ~20-40% of myosin heads form cross-bridges. At tetanus 70-80%.
What is the length-tension relationship?
- Maximum tension is can be generated at optimal sarcomere length. This allows for maximal overlap between actin and myosin -> the maximum number of active cross-bridges at a given instant can be formed. If the sarcomere is too short, overlap is excessive, reducing cross-bridge formation and tension potential. If the sarcomere is too stretched, there is insufficient overlap for cross-bridge formation.
- Normal sarcomere length is between 75-130% optimal.
- Tension directly proportional to the number of active cross-bridges at a given time.
For example the optimal position for the bicep is around 90 degrees, when the fore-arm is closer to the shoulder there is excessive overlap and when the forearm is extended passed 90, there is insufficient overlap. In theory we can produce the maximum tension in our biceps at 90 degrees
What is a twitch, what are the phases?
twitch: a single muscle contraction produced by a single neural stimulation (only happens in a lab)
Phases:
1. Latent period: Action potential moves through sarcolemma, triggers Ca2+ release
2. Contraction phase: Calcium ions bind, muscle tension builds
3. relaxation phase: Ca2+ is taken back up by SR, contraction ends, tension falls to resting levels.
Describe four types of muscle tension
- Treppe: occurs when a muscle is stimulated repeatedly with identical stimuli, and each successive contraction produces slightly more tension than the previous one, even when the muscle is allowed to fully relax between contractions. This increase in tension continues until it reaches a plateau. The mechanisms behind treppe include a gradual accumulation of calcium ions in the muscle fibers, which allows more cross-bridges to form in subsequent contractions
- Wave summation: occurs when successive action potentials arrive before the muscle has fully relaxed from the previous contraction. Each subsequent action potential causes more calcium to be released into the cytoplasm, increasing the number of cross-bridges that can form and thereby increasing tension. As a result, the tension produced by each successive contraction is greater than the tension produced by the previous one, leading to a progressively stronger contraction if the stimuli continue.
- Incomplete tetanus: muscle stimulated by a series of action potentials at a frequency high enough that individual twitches begin to sum together, increasing tension, but not so high that the muscle has no time to relax between stimuli. In this state, the muscle fibers partially relax between stimuli, leading to a wavering or oscillating tension that is not maximal but higher than what is produced by individual twitches
- Complete tetanus: occurs when the frequency of stimulation is so high that the muscle does not have time to relax at all between stimuli. As a result, the twitches fully merge, producing a smooth, sustained contraction with maximal tension. In this state, the muscle remains in a constant state of contraction, with no observable relaxation phase, leading to the highest possible tension that the muscle fiber can produce
How do you increase whole skeletal muscle tension
- The more motor units recruited the greater the tension (motor unit = all muscle fibers innervated by a neuron).
- The greater the frequency of stimulation the more cross-bridges formed. As the frequency increases, individual twitches begin to summate, leading to incomplete tetanus. If the frequency is high enough, the muscle reaches complete tetanus, where cross-bridge formation is maximized, and the muscle generates peak tension with no relaxation phases in between.
Describe Maximum Tension vs Sustained Tension and Muscle Tone
Maximum tension Achieved when all motor units are recruited, are at complete tetanus, and optimal length. This level of contraction can only be maintained for a very short period of time.
Sustained Tension: A whole skeletal muscle achieves sustained tension by cycling the activation of different motor units in an asynchronous manner. While some motor units contract, others rest and recover. This rotation allows the muscle to maintain a steady level of tension over time without leading to rapid fatigue, as not all motor units are active simultaneously
Muscle Tone: Muscle tone is the continuous and passive partial contraction of muscles,which maintains the normal tension and firmness of a muscle at rest. It is maintained by the nervous system’s constant, low-level activation of a small number of motor units, which keeps muscles firm and ready for action while also stabilizing joints and maintaining posture.
What are the three primary sources of ATP for skeletal muscles
- Aerobic respiration: Complete oxidation of glucose in the presence of oxygen in the ETC of the mitochondria. Primary source of ATP at rest (oxidation of fatty acids) and during moderate exercise (oxidation of glycogen). Produces 34 ATP per glucose
- Anaerobic glycolysis: Break down of glucose (normally from glycogen) into pyruvate without oxygen. Primary energy source for peak muscular activity. Only produces 2 ATP per glucose, lactic acid produced as byproduct.
- Creatine phosphate: Provides a rapid source of ATP. Directly donates a phosphate to ADP. Only lasts for about the first 15 seconds of activity.
What are the four different types of muscle contraction
- Isotonic: When skeletal muscle changes in length, it results in motion.
- Concentric: When the muscle shortens.
- Eccentric: When the muscle lengthens.
- Isometric: When skeletal muscle develops tension but length does not change.