Chapter 9 - Muscular System Flashcards
Connective Tissue around Muscles
- Fascia: thin covering of connective tissue around muscles
- Tendon: cord like mass of connective tissue which connects muscle to bone
- Aponeurosis: sheet like mass of connective tissue which connects a muscle to bone, skin or another muscle
Layers of CT surrounding muscle beneath the Fascia
- Epimysium: surrounds the whole muscle and lies beneath the fascia
- Perimysium: surrounds the fascicles within a muscle
- Endomysium: surrounds the muscle fibers(myofibrils) within a fascicle
- Perimysium: surrounds the fascicles within a muscle
Muscle Structure(least to most complex)
- Got thick and thin filaments; myosin and actin, which interact to cause muscle contractions
- Myofibrils: organelles which cause contractions and are in bundle form of Actin and myosin
- Muscle fibers / Myofibers: many myofibrils covered by the sarcolemma membrane which is then covered by a layer of connective tissue known as the endomysium;
○ which helps electrically insulate the muscle cells from each other and allow for more precise movement - Fasicle: many myofibers bundled together to form a muscle cell which is then covered by perimysium
- Fascicles are bundled together to form the entire muscle and is covered in epimysium,
- The fascia covers all of this
- The fascia, epimysium, endomysium and perimysium then extends past the muscle and connects to the bone to form a tendon
Skeletal Muscle Fiber Components/organelles
- Sarcolemma: cell membrane of muscle fiber
- Sarcoplasm: cytoplasm of muscle fiber
- Myofibrils: organelles in cells containing actin and myosin filaments
- Sarcoplasmic Reticulum: stores calcium and covers each myofibril
- Transverse tubule: relays electrical impulses to the SR
- Triad: unit consisting of 1 T tubule and 2 SR cisternae
Myofibril Structure
- composed of sarcomeres that are composed of Thick and Thin Filaments
Sarcomere Structure
○ I band: lIght band composed of only Actin filaments
○ A Band is the dArk band composed of myosin filaments
○ H Zone: center of A band which has only myosin filaments
○ Z line(disc): the sarcomere boundary where the center of I band anchors actin filaments in place
- M Line: the center of the A Band(H Zone) where myosin filaments are anchored
Thick and thin Filaments
Thick: composed of myosin proteins
- Composed of myosin protein
- Have heads which form cross bridges with thin filaments
Thin: composed of actin proteins
- Have an active binding site for myosin, which are covered when the muscle is not contracting by tropomyosin
○ Troponin removes tropomyosin when muscle contractions are wanted
How muscles Contract
- Muscles contract in response to neurostimulation from a motor neuron
This happens at a myoneural(neuromuscular) junction where the axon of a motor neuron and skeletal muscle fiber interacts
Parts of a Neuromuscular Junction
- Motor neuron
- Motor end plate: specialized folded portion of skeletal muscle fiber where the fiber binds to the neurotransmitter
- Synaptic cleft: space between neuron and muscle fiber which neurotransmitters travel
- Synaptic vessels: membrane bound sacs of neurotransmitters
- Neurotransmitters: chemical messengers (acetylcholine)
Steps in Neuromuscular Stimulation of a muscle
- Nerve impulses reaching the axon terminal open calcium channels, bringing them into the neuron and causing the release of ACh from synaptic vesicles
○ As ACh is released, the calcium is also released from the neuron again - ACh binds to the motor end plate and causes changes in the membrane permeability to sodium and potassium ions,
○ Moves sodium in the muscle, and potassium out, generating a muscle impulse(action potential) - Once the muscle impulse is generated, the ACh that was bound is broken down now by acetylchlolinesterase
- The impulse travels down the T tubule and causes the release of calcium ions from the terminal cisternae, into the cytosol (sarcoplasmic reticulum), causing muscular contractions
Excitation - Connection Coupling
connection between muscle fiber stimulation and muscle contraction
2 stages:
- muscle relaxation
- muscle stimulation
Muscle Relaxation Stage
- Calcium ions are stored in SR
- troponin, tropomyosin complex cover the binding sites on actin filaments
Muscle Stimulation Stage Steps(steps in muscular contractions)
- Muscle impulse travels across the sarcolemma and down the T tubules
- Calcium ions are released from the sarcoplasmic reticulum
- Calcium ions bind to troponin, causing a conformation change
- Tropomyosin is moved off the actin binding sites
- An energized myosin cross bridge binds to the actin molecule
- The power stroke of the myosin cross bridge causes the sliding of the thin filaments
- ATP binds to the cross bridge, causing it to disconnect from actin
- ATP is broken down into ADP which reenergizes and repositions the myosin cross bridge
- Calcium ions are moved back into the SR by active transport
Post Muscle Stimulation - Relaxation
- Acetylcholinesterase decomposes Ach remaining in the synapse, stopping the muscle impulse
- Calcium pump moves Calcium into the SR
- Troponin-tropomyosin complex again covers the actin binding site
Energy Stores for Contractions
ATP reserves:
- Muscle cells only store about 4-6 seconds
Creatine Phosphate:
- Initial source of energy regeneration by storing energy in the phosphate bonds with creatine
- Fuels the cell for approx 10 secs
Cellular Respiration:
- Fuels longer periods of contractions
- Breaks down glucose either stored in the muscle cells(glycogen) or coming from food to produce ATP
Anaerobic Cellular Respiration:
- Small amounts of ATP are produced from glucose when no oxygen is present
- Produces lactic acid as a by product
Muscle Fatigue
the inability to contract muscle due to troponin having less affinity fo calcium
- Causes are:
○ Decreased blood flow
○ Ion imbalances in the Sarcolemma
Accumulation of lactic acid
Muscle Cramps
sustained involuntary contractions due to changes in electrolyte concentration in extracellular fluids in the area
Threshold Stimulus
minimum strength of stimulation of a muscle fiber required to cause a contraction
Twitch: Single Muscle Contraction
Contractile response of a single muscle fiber to a single impulse
- Latent period: delay between stimulation and start of contraction - Period of contraction: fiber pulls at attachments - Period of relaxation: pulling force decreases ○ Takes more time than contraction as there is no force pushing the filaments away, its just the elastic recoil
Summation
What occurs when a muscle fiber is exposed to increasing stimuli frequency, resulting in it being unable to relax before being stimulated again, leading to the twitches combining and the contraction being sustained
- Classified as either partial or complete tetanic contractions depending on the stimulation frequency
Partial Tetany Summation Stimulation
occurs at higher frequencies of stimulation and involves partial relaxation
Complete Tetany Summation Stimulation
occurs at the highest frequencies of stimulation and involves full stimulation with no relaxation
- only achievable in the lab and not in the human body
