Flashcards in Physiology - Exam 3, Deck #1 - Muscles Deck (109):
What are Smooth Muscles?
-Slower, more sustained contractions;
-No clear filament organizations (NO sarcomeres);
-Stomach, arteries, etc.
What are Cardiac Muscles?
-Short, branches muscle fibers connected by INTERCALATED DISCS;
-Forms a “mesh”;
-When one-cell contracts they all do because the action potential is propagated from one cell t the next through intercalated discs
What are Skeletal Muscles?
**Voluntary and Striated;
-Comprise 90% of our total muscles mass;
-Voluntary muscles arranged in ANTAGONISTIC pairs;
-Muscles are attached to tendons
How are skeletal muscles attached to bones?
-By TENDONS on each end of the bone;
-Contracting muscles cause tension on tendons which move bones at a joint;
1. INSERTION = the more movable attachment;
2. Insertion is pulled toward the ORIGIN = less moveable attachment
Flexors vs. Extensors
-Flexors DECREASE angle of joint;
-Extensors INCREASE angle of joint
What is Agonist Muscle?
-Prime mover of any skeletal movement;
-ANTAGONISTIC muscles are flexors & extensors that act on the same joint to produce OPPOSITE actions
What is the Epimysium of muscles?
Sheaths of fibrous connective tissue from tendons that extend around and into skeletal muscle
What are Fascicles of muscles?
Inside the muscle this connective tissue divides muscle into columns called FASCICLES
What is the Perimysium of muscles?
Connective tissue around fascicles
What are skeletal muscles composed of?
-Numerous muscles fibers (cells) ranging from 10 to 80 microns in diameter;
-In most muscles the fibers extend the entire length of the muscle;
-All but about 2% of the fibers are innervated by on ONE nerve ending located near the middle of the fiber;
-Muscle fibers are similar to other cells except are MULTINUCLEATE & STRIATED
What is the Sarcolemma?
The plasma membrane surrounding muscle fibers;
-Consistes of a cell membrane and an outer coat that fuses with a tendon fiber
What are Tendon Fibers
Tendon fibers are collected into bundles to form muscle tendons that are inserted into bones
What are Myofibrils?
-Found with muscle fibers and contain numbers FILAMENTS;
-Myofibrils are 1 moron in diameter & extend length of fiber;
-Composed of Actin and Myosin Filaments
What are Myofilaments?
Myofilaments are composed of thick & thin filaments lying side-by-side that give rise to bands which underlie STRIATIONS;
1. ~ 1500 Myosin filaments = THICK;
2. ~ 3,000 Actin filametns - THIN
What is the function of Myofilaments?
Filaments are large polymerized protein molecules that are responsible for MUSCLE CONTRACTIONS;
-Filaments partially interdigitate and cause the myofibrils to have alternate light and dark bands
What are “A Bands”?
"A band" is DARK;
-Contains thick filaments = mostly MYOSIN
What is the “H band” or “H zone”?
-Light/clear area at center of A band is "H band” = area where actin & myosin don’t overlap
What are “M Lines”?
M lines are structural proteins in the middle of A bonds;
-Join thick myosin filaments together and anchor them during contraction
What are “I Bands”?
I band is LIGHT;
-Contains thin filaments = mostly ACTIN;;
-At center of I band is Z line/disc where actions attach
What is a “Z Disc"
Z-discs run perpendicular to actin filaments and attaches to the enter of each molecule;
-Serve as the boundaries for Sarcomeres
What are Sarcomeres?
Are contractile units of skeletal muscle consisting of components between 2 Z discs ;
-Sarcomeres are joined to together by ACTIN molecules that are located in 2 adjacent sarcomeres
What is Titin?
-ELASTIC large, protein attaching MYOSIN to Z disc that contributes to elastic recoil of muscle;
-Begin at the M-Lines and end at the Z discs;
-Stabilize the position of Myosin (thick) in each sarcomere and elasticity helps muscle return to resting length
What are Cross-Bridges?
-Small projections (heavy meromyosin) from the sides of myosin filaments;
-Are formed by heads of myosin molecules that extend toward & interact with actin
What is produced by the movement of Cross-Bridges?
**Interactions between cross bridges on myosin heads and actin that yield CONTRACTIONS;
-Sliding of filaments is produced by actions of cross bridges
What happens to H-Zones when a sarcomere contracts?
-Relaxed muscle = myosin only;
-Contracted muscle = disappears
What is the Sarcoplasma?
-It is the matrix for the myofibrils in the muscle fibers;
-Includes a rich supply of mitochondria and an extensive SARCOPLASMIC RETICULUM (endoplasmic reticulum)
What is the function of the Sarcoplasmic Reticulum?
SR has a special organization that is very important in the control of muscle contraction;
-The special organization of the SR forms the Transverse (T-tubule)-Sarcoplasmic Reticulum System;
-Also serves as a Ca2+ reservoir in muscle!
What is the Sliding Filament Theory of Muscle Fiber Contraction?
Muscle contraction invokes the sliding of ACTIN and MYOSIN myofilaments past each other to SHORTEN the length of each sarcomere (Z disc to Z disc);
-Results from making/breaking chemical bonds between cross bridges of myosin filaments and G-actin molecules of the actin strand
What happens during a muscle contraction?
**Occurs because thin filaments slide over & between thick filaments towards center =
1. A bands (containing actin) move closer together, DON’T shorten;
2. I bands shorten because they define distance between A bands of successive sarcomeres;
3. H bands (containing myosin) shorten
What is ATPase?
-Alass of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion;
-Each myosin head contains an ATP-binding site which functions as an ATPase
What is the role of ATPase in initiating a muscle contraction?
-Myosin CAN’T bind to actin unless it is “cocked” by ATP;
-After binding, myosin undergoes conformational change (POWER STROKE) which exerts force on actin;
-After power stroke myosin detaches
What happens as a result of a Power Stroke?
Myosin will be released and the energy from the PO4 bond will be released and pull the Actin thread through the myosin yield a contraction
What is the Cross-Bridge Cycle?
1. Resting fiber = cross bridge not attache to actin;
2. Cross-bridge attaches to actin;
3. PO4- is released, causing conformation change in myosin;
4. Power stroke causes filaments to slide — ADP is released;
5. New ATP binds to myosin head releasing it from actin;
6. ATP is hydrolyzed and cross-bridge returns to resting state
What controls Cross-Bridge to Actin attachment?
-Control of cross bridge attachment to actin is via TROPONIN-TROPOMYOSIN SYSTEM;
-Serves as a switch for muscle contraction & relaxation
What is Tropomyosin?
The filament tropomyosin lies in grove between double row of G-actins (that make up actin thin filament)
What is Troponin?
Troponin complex is 3 globular proteins that are attached to tropomyosin at intervals of every 7 actins
What is the state of the troponin-tropomyosin system in a RELAXED muscle?
-In relaxed muscle, tropomyosin BLOCKS binding sites on actin so crossbridges can’t occur;
-This occurs when Ca++ levels are low (<10-6 M);
-Contraction can occur only when binding sites are exposed
How does Troponin initiate a contraction?
The strong affinity of TROPONIN for calcium ion initiates contraction of each sarcomere by altering the blocking of actin binding sites by the myosin cross-bridges
What happens to start a contraction?
-When Ca2+ levels rise (>10-6 M), Ca2+ binds to TROPONIN causing conformational change which moves tropomyosin & exposes binding sites;
-Allowing crossbridges & contraction to occur;
-Cross-bridge cycles stop when Ca2+ levels decrease (<10-6 M)
What happens as Ca2+ levels decrease?
-Ca2+ levels decrease because it is continually pumped BACK INOT the sarcoplasmic reticulum (SR - a calcium reservoir in muscle);
-Most Ca2+ in SR is in TERMINAL CISTERN;
-Running along terminal cisternae are T TUBULES
What is Excitation-Contraction Coupling?
-Skeletal muscle sarcolemma is excitable;
-Conducts APs just like axons;
-Release of ACh at NMJ causes large depolarizing end-plate potentials & APs in muscle;
-APs race over sarcolemma & down into muscle via T tubules
How do the T-Tubules transit the APs?
-T tubules are extensions of sarcolemma;
-T-tubules have very high concentration of Ca2+;
-Ca2+ channels in SR are mechanically linked to channels in T tubules;
-APs in T tubules cause release of Ca2+ from cisternae via V-gated Ca2+ release channels = ELECTROMECHANICAL RELEASE
What causes the initiation of a muscle contraction?
Begins with action potentials from the neuromuscular junction that spread over the muscle fibers;
-APs generate electrical currents that spread to the interior of the muscle fiber by the T-Tubulue-Sarcoplasmic Reticulum System;
-Currents then cause the release of Ca2+ from SR to sarcoplasm
What happens at the Cross-Bridge during excitation-contraction coupling?
-ATPase binds ATP to TROPONIN;
-A POWER STROKE is produced
What is the purpose of ATP?
New ATP has to be present for the cross-bridge to DETACH;
-A lack of ATP produces rigor mortis
How long do muscle contractions continue?
-Contraction wil continue as long as the Ca2+ remains HIGH in the sarcoplasmic fluid;
-BUT, a continually active calcium pump in the walls of the SR pumps Ca2+ back into the SR
What causes muscles to relax?
-When APs cease, muscle relaxes = Because Ca2+ channels close & Ca2+ is pumped back into SR
What is a Motor End Plate?
Place on sarcolemma where the neuromuscular junction occurs
What is a Motor Unit?
-A motor unit includes each motor neuron & all fibers it innervates;
-Each motor neuron branches to innervate a variable number of muscle fibers;
-The average number of muscle fibers to a motor unit it 150
What happens when a motor unit is activated?
-When a motor neuron is activated, all muscle fibers in its motor unit contract;
-Number of muscle fibers in motor unit varies according to degree of fine control capability of the muscle
What is an Innervation Ratio?
Number of motor neuron to muscle fibers
What is Fine Control?
Fine control occurs when motor units are SMALL
— 1 motor neuron innervates small number of fibers
Since individual motor units fire "all-or-none," how do skeletal muscles perform smooth movements?
RECRUITMENT is used:
1. Brain estimates number of motor units required & stimulates them to contract
2. It keeps recruiting more units until desired movement is accomplished in smooth fashion
3. More & larger motor units are activated to produce greater strength
What is a Muscle Twitch?
-A single rapid contraction & relaxation of muscle fibers;
-Accomplished by instantaneously exciting the nerve to a muscle with an electrical stimulus through the muscle itself, giving rise to a single, sudden contraction for a fraction of a second
What is Summation?
-Means the adding together of individual muscle twitches to make strong and concert muscle movements;
-If 2nd stimulus occurs before muscle relaxes from 1st, the 2nd twitch will be greater
- Types =
1. Multiple Motor Unit or Spatial Summation;
2. Wave or Temporal Summation
What are Graded Contractions?
-Contractions of carrying strengths;
-Obtained by stimulation of varying numbers of fibers;
-Increasing frequency increase strength of subsequent contractions!
What is a Multiple Motor Unit or Spatial Summation?
Summation due to increasing the number of motor units contracting SIMULTANEOUSLY
What is a Wave Summation or Temporal Summation?
-Summation due to increasing the frequency of contraction of individual motor units
What is Treppe?
AKA: Staircase Effect;
-If muscle is repeatedly stimulated with maximum voltage to produce individual twitches, successive twitches get larger;
-Caused by accumulation of intracellular Ca2+
What is Tetanization?
-At even higher frequencies of stimulation (> 35 stimuli per second) the successive contractions fuse together and cannot be distinguished from one another
What is Critical Frequency?
The lowest frequency at which tetanization occurs
What is an Incomplete Tetanus?
If muscle is stimulated by an increasing frequency of ELECTRICAL SHOCKS, its tension will increase to a maximum
What is a Complete Tetanus?
If frequency is so fast NO relaxation occurs, a smooth sustained contraction results
What is an ISOTONIC Contraction?
= Same strength;
-Force remains CONSTANT throughout shortening process;
-The muscle shortens while the tension of the muscle remains constant
What are ISOMETRIC Contractions?
= Same length of muscle;
-Exerted force does not cause load to move & length of fibers remains constant;
-The muscle does NOT shorten during contraction but the tension varies;
-Isometric contraction in different skeletal muscles can last for different periods of time
What are ECCENTRIC Contractions?
-Load is greater than exerted force & fibers lengthen
What is the Series-Elastic Component of muscles?
-Tendons & connective tissue are elastic & absorb tension as muscle contracts;
-They recoil as muscle relaxes & spring back to resting length
What is the respiration of SLOWER fibers?
-Slow fibers are AEROBIC because of many capillaries, more myoglobin, and mitochondria
What is the respiration of FAST fibers?
Fast fibers are ANAEROBIC
How are skeletal muscle fibers be divided?
-On the basis of contraction and resistance to fatigue;
— SLOW twitch fibers fatigue slowly = Type 1 Fibers;
— FAST twitch fibers fatigue rapidly = Type IIA and IIX Fibers
What causes the differences in twitch speed?
Twitch speed due to different myosin ATPases that are slow or fast
What determines muscle fiber type?
-Genetic inheritance determines relative percentages of fast twitch to slow twitch fibers in a muscle;
-This in turn dictates to an extent the sport in which an athlete best performs
What are the differences in fast and slow twitch fibers?
1. FAST are about twice the diameter of slow twitch fibers;
2. Enzymes that promote the rapid release of energy of the phosphagen and glycogen-lactic acid energy systems are 2-3x as active in fast twitch fibers — maximal power than can be achieved by fast twitch fibers as great as 2x that of slow;
3. SLOW twitch fibers are mainly organized for ENDURANCE — aerobic energy = more mitochondria, myoglobin and aerobic enzymes;
4. Number of capillaries per mass of fibers is greater in SLOW than fast fibers
Fast vs. Slow Fibers
-FAST can deliver extreme amounts of power for a few seconds to a minute or so;
-SLOW can produce ENDURANCE, delivering prolonged strength or contraction over many minutes to hours
What are Type 1 Fibers?
-Also called RED SLOW OXIDATIVE;
-Adapted to contract SLOWLY without fatiguing;
-Uses mostly AEROBIC respiration;
-Rich capillary supply, many mitochondria, & aerobic enzymes;
-Has lots of myoglobin (O2 storage molecule) = red color;
-Have small motor neurons with small motor units
What are Type IIX Fibers?
-Type IIX fibers also called WHITE FAST GLYCOLYTIC;
-Adapted to contract fast using anaerobic metabolism;
-Has large stores of glycogen, few capillaries & mitochondria, little myoglobin
What are Type IIA Fibers?
-Type II A fibers also called WHITE FAST OXIDATIVE;
-Adapted to contract fast using aerobic metabolism
Intermediate to Type I & Type IIX;
-Have large motor neurons with large motor units
What is the respiration of skeletal muscles?
-Skeletal muscles respire anaerobically 1st 45-90 sec of moderate-to-heavy exercise ;
-Cardiopulmonary system requires this time to increase 02 supply to exercising muscles;
-If exercise is moderate, aerobic respiration contributes majority of muscle requirements after 1st 2 min
What is Maximum oxygen uptake (aerobic capacity)?
-Maximum rate of oxygen consumption (V02 max);
-Determined by age, gender, & size
What is the Lactate (anaerobic) threshold?
-Percentage of max 02 uptake at which there is significant rise in blood lactate levels;
-In healthy individuals this is at 50– 70% V02 max
What do muscles use for energy during LIGHT exercise?
During light exercise, most energy is derived from AEROBIC respiration of plasma free fatty acids
What do muscles use for energy during MODERATE exercise?
During moderate exercise, energy derived equally from fatty acids & glucose
What do muscles use for energy during HEAVY exercise?
-During heavy exercise, glycogen supplies 2/3 of energy;
-Liver increases glycogenolysis;
-GLUT-4 carrier is moved to muscle cell’s plasma membrane
What is Oxygen Debt?
-When exercise stops, rate of oxygen uptake does not immediately return to pre-exercise levels;
-Oxygen debt accumulates during exercise;
-When oxygen is withdrawn from hemoglobin & myoglobin;
-& because of 02 needed for metabolism of lactic acid produced by anaerobic respiration
What is the Phosphocreatine System?
-During exercise ATP can be used faster than can generated by respiration;
-Phosphocreatine (creatine phosphate) is source of high energy Ps to regenerate ATP from ADP;
-Phosphocreatine levels are 3X ATP = refuels ATP pool
What is Muscle Fatigue?
-Exercise-induced reduction in ability of muscle to generate force;
-Sustained muscle contraction fatigue is due to accumulation of extracellular K+ = From K+ efflux during AP
When does Muscle Fatigue occur?
-Occurs in moderate exercise as slow-twitch fibers deplete glycogen stores;
-Fast twitch fibers are then recruited, converting GLUCOSE TO LACTIC ACID which interferes with Ca2+ transport
What is Central Fatigue?
Central fatigue occurs as BRAIN is less able to activate muscles even when muscle is not fatigued
How does the muscle adapt to exercise?
-AEROBIC training improves aerobic capacity (by 20%) & lactate threshold (by 30%);
-Weight training increases muscle size by increasing NUMBER of MYOFIBRILS/FIBER
& in some cases fibers can split
What are the effects of endurance training on skeletal muscles?
1. Imprpoved ability to obtain ATP;
2. Increased size/number of mitochondria;
3. Less lactic acid produced;
4. Increased myoglobin content;
5. Increased intramuscular triglyceride;
6. Increased lipoprotein lipase’s (enzyme needed to use lipids in blood);
7. Increased proportion of energy derived from fat; less from CHO;
8. Lower rate of glycogen depletion;
9. Improved efficiency if extracting O2 from blood;
10. Decreased number of type IIX (fast glycolytic) fibers; Increased type IIA (fast oxidative fibers)
Where are motor neurons of skeletal muscle located?
-Lower Motor Neurons — a neuron cell bodies are in VENTRAL HORN OF spinal cord;
-Axons leave in VENTRAL ROOT = Lower Common Pathway
What influences the activity of the lower motor neurons?
-Activity influenced by sensory feedback from muscles & tendons;
-& facilitory & inhibitory activity from upper motor neurons
Where do the neurons receive sensory feedback form?
-To control skeletal muscle movements, neurons must receive continuous sensory feedback;
-Info comes from tension from GOLGI TENDON ORGANS;
-& on length of muscle from MUSCLE SPINDLE APPARATUS
What makes up Muscle Spindle Apparatus?
-Spindles are arranged in parallel with extrafusals inserting into tendons at each end of muscle
What are Intrafusal Fibers?
-Modified thin muscle cells;
-Have nuclei in central region instead of contractile filaments
What are the different arrangements of nuclei in the muscle spindle apparatus?
1. NUCLEAR BAG FIBERS have nuclei arranged in loose aggregate;
2. NUCLEAR CHAIN FIBERS have nuclei arranged in rows
What are Extrafusal Fibers?
-Regular muscle fibers
What is the Golgi Tendon Organ Reflex?
-Involves 2 synapse in the CNS = DISYNAPTIC REFLEX;
-Sensory axons from Golgi tendon organ synapse on interneurons
What is the function of the Golgi Tendon Organ Reflex?
-Sensory axons synapsing on interneurons makes INHIBITORY synapses on motor neurons;
-Prevents excessive muscle contraction or passive muscle stretching
What is the cellular structure of Cardiac Muscle?
1. Contractile apparatus similar to skeletal;
2. STRIATED like skeletal;
3. INVOLUNTARY like smooth;
4. Forms two functional sancta = ATRIA and VENTRICLES;
5. Branched muscle fibers are electrically coupled and joined by INTERCALATED DISKS (gap junctions) = Allow APs to spread throughout cardiac muscle
What is the cellular structure Smooth Muscle?
-Has NO sarcomeres
-Has gap junctions;
-Contains 16X more actin than myosin = Allows greater stretching & contracting;
-Actin filaments are much longer and anchored to DENSE BODIES
What controls Smooth Muscle Contractions?
-Controlled by Ca2+ but different from striated muscle;
-Has little sarcoplasmic reticulum & NO troponin/tropomyosin
What initiates a Smooth Muscle Contraction?
1. Ca2+ enters thru channels in plasma membrane;
2. Binds with CALMODULIN;
3. Ca2+-calmodulin complex activates MYOSIN LIGHT CHAIN KINASE (MLCK) = Which phosphorylates & activates myosin;
4. Myosin forms crossbridges with actin
What causes Smooth Muscle to Relax?
1. Relaxation occurs when Ca2+ concentration DECREASES;
2. Myosin is dephosphorylated by MYOSIN PHOSPHATASE;
3. Myosin can no longer form cross bridges
Why are Smooth Muscle Contractions slower than striated muscles?
-Smooth muscle has slower contractions than striated;
-Can form a state of prolonged binding of myosin to actin (latch state);
-Maintains force using little energy
What is the mechanism for a Smooth Muscle Contraction?
1 APs caused depolarization of the plasma membrane;
2. Voltage-gate Ca2+ channels in the membrane open;
3. Ca2+ enter the membrane and binds with CALMODULIN to create the Ca2+-Calmodulin complex;
4. Activates MLCK;
5. MLCK activates Myosin Light Chain by phosphorylation;
6. Cross-bridge forms and contraction occurs;
7. Dephosphorylation of myosin by MYOSIN PHOSPHATASE breaks the cross-bridge;
8. Muscle relaxes
What is a Single Unit Smooth Muscle?
-Single unit is SPONTANEOUSLY active = MYOGENIC;
-Some cells are pacemakers;
-Has gap junctions to spread electrical activity
What is a Multiunit Smooth Muscle?
-Multiunit requires NERVE STIMULATION by ANS;
-Neurotransmitters are released along a series of synapses called VARICOSITIES