Chapter 9 Muscle and Muscle Tissue Flashcards
Skeletal Muscle: Span Joints and Attachment
Span joints and attach to bone (directly or indirectly)
* direct attachment – epimysium of the muscle is fused to the periosteum of a bone (or perichondrium of cartilage) * indirect attachment = tendon (rope-like) or aponeuorsis (sheet-like)
Skeletal Muscle: Origen
Origin = attachment site of muscle on bone that is *less moveable
Skeletal Muscle: Insertion
Attachment site of muscle on bone that when muscle contracts *moves bone toward the muscle’s origin
Skeletal Muscle: Organ
Each skeletal muscle is a discrete organ – skeletal muscle fibers (cells), blood vessels, nerves and connective tissue
* generally one nerve, one artery and one or two veins enter (or exit) near the center of each muscle * *every muscle cell is supplied with a nerve ending
Skeletal Muscle: Connective Tissue Sheaths (revestimento)
Several different connective tissue sheaths associated with each muscle
Superficial to deep:
**epimysium –dense irregular connective tissue that surrounds whole muscle
**perimysium – dense irregular connective tissue that wraps fascicles = groups of muscle cells
**endomysium – areolar connective tissue that surround each muscle cell
Skeletal Muscle- Connective Tissue Sheaths: Epimysium
Dense irregular connective tissue that surrounds whole muscle
Skeletal Muscle- Connective Tissue Sheaths: Perimysium
Dense irregular connective tissue that wraps fascicles = groups of muscle cells
Skeletal Muscle- Connective Tissue Sheaths: Endomysium
Areolar connective tissue that surround each muscle cell
Skeletal Myofiber: Composition
Skeletal myofiber = skeletal muscle cell
* long cylindrical with multiple nuclei just beneath the sarcolemma (plasma membrane) * sarcoplasm (cytoplasm) * myofibrils * sarcomere – about 2 um long = functional unit of skeletal muscle * sarcoplasmic reticulum (SR) = elaborate smooth endoplasmic reticulum * T-tubules = deep invaginations of the sarcolemma
Skeletal Myofiber: Sarcoplasm
Sarcoplasm (cytoplasm) – contains large amounts of:
* glycosomes = granules of glycogen (complex carbohydrate – repeating units of glucose) * myoglobin = pigment that stores oxygen (similar to haemoglobin)
Skeletal Myofiber: Myofibrils
Myofibrils (100-1000) = tightly packed protein bundles that run the entire length of the cell
* mitochondria and other organelles squeezed between them * made up of **repeating units of sarcomeres
Skeletal Myofiber:
Sarcomere
Sarcomere – about 2 um long = functional unit of skeletal muscle
= region of myofibril between two successive Z discs
-contains myofilaments = contractile fibers and their arrangement produces the striations
THIN FILAMENTS = filamentous (F) actin
* anchored to the Z discs (alpha actin) * produce the light coloured (I) band * subunit for actin (actin G) has binding site for myosin
REGULATORY PROTEINS – intertwined along the actin filament
* tropomyosin – rod-shaped protein that spiral around actin core * troponin – globular three polypeptide complex - TnI - inhibitory unit that binds to actin - TnT – binds to tropomyosin - **TnC – binds calcium
THICK FILAMENTS = myosin
* forms the dark (A) band * attached to an elastic protein (titin) that attaches to the Z-line * each myosin molecule= 2 heavy chain and 4 light chain polypeptides attached to globular **myosin heads - during contraction myosin heads bind to actin forming crossbridges
Skeletal Myofiber:
Sarcoplasmic Reticulum
Sarcoplasmic reticulum (SR) = elaborate smooth endoplasmic reticulum
* *stores calcium and has gated channels to allow its release * surrounds each myofibril like a sleeve with enlarged terminal cisterns that flank each side of a T-tubule forming a triad
Skeletal Myofiber:
T-tubules
T-tubules = deep invaginations of the sarcolemma
* increases the surface area of the myofiber and T-tubular contains extracellular fluid * conduct nerve impulses to the deepest regions of the muscle cell * contain voltage sensor that trigger the opening of gated calcium channels on the SR
Skeletal Myofiber:
Sarcomere
THIN FILAMENTS = filamentous (F) actin
* anchored to the Z discs (alpha actin) * produce the light coloured (I) band * subunit for actin (actin G) has binding site for myosin
Skeletal Myofiber:
Sarcomere
REGULATORY PROTEINS – intertwined along the actin filament
* tropomyosin – rod-shaped protein that spiral around actin core * troponin – globular three polypeptide complex - TnI - inhibitory unit that binds to actin - TnT – binds to tropomyosin - **TnC – binds calcium
Skeletal Myofiber:
Sarcomere
THICK FILAMENTS = myosin
* forms the dark (A) band * attached to an elastic protein (titin) that attaches to the Z-line * each myosin molecule= 2 heavy chain and 4 light chain polypeptides attached to globular **myosin heads - during contraction myosin heads bind to actin forming crossbridges
Sliding Filament Model of Contraction
*in relaxed muscle fibers – actin and myosin overlap only and the ends
*contraction = formation of crossbridges between myosin head and actin filament
-shortening occurs when crossbridges generate enough tension to exceed the forces opposing shortening
-ratchet-like formation, release and reattachment of crossbridges cause thin filament slide past the thick ones
actin and myosin filaments overlap to a greater degree
-Z lines of sarcomere get closer together and overall – myofibril shortens
Excitation/Contraction Coupling
- sequence of electrical and mechanical events that lead to contraction
- electrical events requires nerve stimulus and interaction with the skeletal muscle cell at the neuromuscular junction
Excitation/Contraction Coupling:
Somatic Motor Neurons
SOMATIC MOTOR NEURONS (part of somatic -voluntary- nervous system)
*cell bodies reside in brain or spinal cord and axons extend to muscle cells
*branched endings (terminals) at the end of each axon
*neuromuscular junction (or motor end plate) = specific connection between:
#one **axon terminal
contains synaptic vesicles filled with the neurotransmitter, acetylcholine (ACh)
#and **sarcolemma of one myofiber (skeletal muscle cell) contains millions of **ACh receptors
#separated by the synaptic cleft – very small physical space (50-80 nm) filled with extracellular fluid contains **acetylcholinesterase = enzyme that breaks down ACh
#**myasthenia gravis = autoimmune disease that destroys ACh receptors
Excitation/Contraction Coupling
- Somatic Motor Neurons diseases:
Myasthenia gravis
#**myasthenia gravis = autoimmune disease that destroys ACh receptors *symptoms include: drooping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness
Excitation/Contraction Coupling:
Generation of action potential across the sarcolemma
Generation of action potential across the sarcolemma
* myofiber has negative resting membrane potential (very polarized -90 mV) * action potential (AP) = all-or-none series of predictable and repeatable electrical changes lasting only a few miliseconds #generation of an end plate potential #depolarization – generation and propagation of action potential #repolarization – restoring sarcolemma to initial polarized state #refractory period = time during which cell cannot be stimulated again *propagation of AP along sarcolemma and down T-tubule
Generation of an end plate potential
Excitation/Contraction Coupling: Generation of action potential across the sarcolemma
Generation of an end plate potential
* binding of ACh to its receptor open Na+ channel (ligand-gated channel) * Na+ enters and produces local depolarization = end plate potential (interior less negative than before
Depolarization
Excitation/Contraction Coupling: Generation of action potential across the sarcolemma
Depolarization – generation and propagation of action potential
*spread of end plate potential to neighbouring areas triggers opening of voltage-gated Na+ channels
*THRESHOLD = voltage at which AP generated (positive feedback mechanism)
AP initiated in one spot – propagates – moves along length of sarcolemma
depolarization wave triggers AP at new location
Repolarization
Excitation/Contraction Coupling: Generation of action potential across the sarcolemma
Repolarization – restoring sarcolemma to initial polarized state
* voltage-gated Na+ channels close and voltage-gated K+ channels open * Na+ no longer enters, and K+ leaves returning cell to negative charged conditions * Na+ - K+ ATPase – restores appropriate ionic conditions
Propagation of AP along sarcolemma and down T-tubule
Excitation/Contraction Coupling: Generation of action potential across the sarcolemma
Propagation of AP along sarcolemma and down T-tubule
* changes the shape of the **voltage-sensitive (DHP receptors) proteins in T-tubule * interaction of T-tubule with SR at triads triggers the **opening of calcium channels on SR
Excitation/Contraction Coupling:
Increased intracellular calcium
Increased intracellular calcium
* release of Ca++ from SR and entry of into the sarcoplasm = intracellular event that couples the AP with muscle contraction * *calcium binds to troponin – and inhibitory proteins on actin moved out of the way - shape change allows troponin to roll off tropomyosin the binding sites for the myosin heads on the actin filament * *myosin head binds to actin and crossbridge cycling begins - requires maintained elevated intracellular Ca++ levels and ATP
Excitation/Contraction Coupling:
Calcium removal terminates contraction
Calcium removal terminates contraction
*calcium pumped back into SR and inhibitory proteins block myosin binding site on actin
Rigor Mortis
Excitation/Contraction Coupling:
Calcium removal terminates contraction
- *rigor mortis – muscle stiffness following death
- cross bridge detachment is ATP driven
- as cells begin to die, cannot pump out calcium so crossbridges form
- ATP synthesis ceases so crossbridges cannot release
- muscles begin to stiffen 3-4 hours after death, peak rigidity at 12 hours then dissipates over next 48-60 hours as muscle proteins breakdown
Factors affecting muscle contraction
- motor unit
- muscle twitch
- graded muscles responses
- frequency of the stimulation = wave or temporal summation
- strength of the stimulus = motor unit recruitment
- muscle tension = force exerted on an object by contracting muscle
- muscle tone
- load = opposing force exerted on muscle by the weight of the object to be moved
- force of muscle contraction
- isometric contraction
- isotonic contraction
- energy supply
- direct phosphorylation- anaerobic glycolysis
- aerobic respiration
- anaerobic glycolysis
- exercise
Factors affecting muscle contraction # Motor unit
Motor unit = one motor neuron and all the muscle fibers it innervates (supplies)
* number of myofibers per motor unit may be as few as four – for muscles that exert fine control (muscles of fingers or eyes) or up to several hundred – for weight-bearing (postural) muscles * muscle fibers in a single motor unit are not clustered but are spread throughout muscle - stimulation of a single motor unit causes a weak contraction of the entire muscle
Factors affecting muscle contraction #muscle twitch
Motor unit’s response to a single action potential
Factors affecting muscle contraction #graded muscles responses
Not isolated twitches but smooth muscle contractions that vary in strength as different demands are placed on them and are altered by changing the:
FREQUENCY OF THE STIMULATION = wave or temporal summation
*single AP produces a twitch
*if multiple AP’s arrive one after the other at the neuromuscular junction, sequential twitch contractions can begin before the tension from the previous contraction has subsided
-Due to release of new calcium from SR before the calcium level from the previous release has returned to normal – i.e. the level of intracellular calcium increases
-Contractions add together and as frequency of delivery closer and closer together – get quivering contraction = unfused or incomplete tetanus
-(Complete or fused) tetanus = sustained fused contraction – reaches maximum tension with no sign of relaxation
Prolonged tetanus leads to muscle fatigue = physiological inability to contract even though muscle still receiving stimuli - tension drops to zero
Attributed to ionic imbalances and calcium regulation and release
STRENGTH OF THE STIMULUS = motor unit recruitment
* subthreshold stimuli - no observable contractions * threshold stimulus - observable contractions appear * maximal stimulus – strongest stimulus that increases contractile force * size principle – motor units with the smallest muscle fibers activated first as controlled by most excitable motor neurons; largest motor units with large course muscle fibers controlled by largest and least excitable (highest threshold) neurons
Factors affecting muscle contraction
#graded muscles responses: FREQUENCY OF THE STIMULATION
FREQUENCY OF THE STIMULATION = wave or temporal summation
*single AP produces a twitch
*if multiple AP’s arrive one after the other at the neuromuscular junction, sequential twitch contractions can begin before the tension from the previous contraction has subsided
-Due to release of new calcium from SR before the calcium level from the previous release has returned to normal – i.e. the level of intracellular calcium increases
-Contractions add together and as frequency of delivery closer and closer together – get quivering contraction = unfused or incomplete tetanus
-(Complete or fused) tetanus = sustained fused contraction – reaches maximum tension with no sign of relaxation
Prolonged tetanus leads to muscle fatigue = physiological inability to contract even though muscle still receiving stimuli - tension drops to zero
Attributed to ionic imbalances and calcium regulation and release
Factors affecting muscle contraction #graded muscles responses: STRENGTH OF THE STIMULUS
STRENGTH OF THE STIMULUS = motor unit recruitment
* subthreshold stimuli - no observable contractions * threshold stimulus - observable contractions appear * maximal stimulus – strongest stimulus that increases contractile force * size principle – motor units with the smallest muscle fibers activated first as controlled by most excitable motor neurons; largest motor units with large course muscle fibers controlled by largest and least excitable (highest threshold) neurons
Factors affecting muscle contraction #Muscle tension
Muscle tension = force exerted on an object by contracting muscle
* muscle tone = presence of slight contraction in muscle even when relaxed - helps stabilize joints and maintain posture
Factors affecting muscle contraction #Load
Load = opposing force exerted on muscle by the weight of the object to be moved
Factors affecting muscle contraction #Force of muscle contraction
Force of muscle contraction influenced by:
* number of motor units recruited * muscle fiber size * frequency of stimulation * degree of stretch on muscle = length-tension relationship
Factors affecting muscle contraction #Isometric contraction
Isometric contraction = contraction in which muscle tension develops but load is not moved – i.e. muscle does not shorten
*increasing muscle tension = measure for isometric contractions
Factors affecting muscle contraction #Isotonic contraction
Isotonic contraction = contraction where muscle tension overcomes load and muscle shortens
* concentric contraction – muscle shortens and does work * eccentric contraction – muscle generates force as it lengthens - important in deceleration activities or counteracting gravity effects (climbing steps) * amount of muscle shortening = measure of isotonic contractions
Factors affecting muscle contraction #Energy supply
Energy supply
- source and availability of ATP * *direct phosphorylation – transfer of phosphate from creatine phosphate(CP) to ADP using creatine kinase * *anaerobic glycolysis – formation of ATP via catabolism of glucose in the cytoplasm of the cell and in the absence of oxygen – forms lactic acid as waste product - poor efficiency but ATP produce 2.5 x faster than aerobic respiration * *aerobic respiration – catabolism of glucose, glycogen, fatty acids (and amino acids) to ATP in (cytoplasm and) mitochondria in the presence of oxygen - 95% of ATP used comes from aerobic respiration
Factors affecting muscle contraction #Energy supply: direct phosphorylation
**direct phosphorylation – transfer of phosphate from creatine phosphate(CP) to ADP using creatine kinase
Factors affecting muscle contraction #Energy supply: anaerobic glycolysis
- *anaerobic glycolysis – formation of ATP via catabolism of glucose in the cytoplasm of the cell and in the absence of oxygen – forms lactic acid as waste product
- poor efficiency but ATP produce 2.5 x faster than aerobic respiration
Factors affecting muscle contraction #Energy supply: aerobic respiration
- *aerobic respiration – catabolism of glucose, glycogen, fatty acids (and amino acids) to ATP in (cytoplasm and) mitochondria in the presence of oxygen
- 95% of ATP used comes from aerobic respiration
Factors affecting muscle contraction #Exercise
Exercise
*activities that require a surge of power but last only a few seconds rely entirely on ATP and CP stores
* aerobic endurance = length of time a muscle can continue to contract using aerobic pathways
* anaerobic threshold = point at which muscle metabolism converts to anaerobic glycolysis
Factors affecting muscle contraction #Exercise: aerobic endurance
Aerobic endurance = length of time a muscle can continue to contract using aerobic pathways
*prolonged activities where endurance rather than power is the goal – jogging, marathon runs, swimming, biking
*aerobic activity over time increases:
• number of capillaries surrounding muscle fibers
• number of mitochondria
• cellular content of myoglobin
Factors affecting muscle contraction #Exercise: anaerobic threshold
Anaerobic threshold = point at which muscle metabolism converts to anaerobic glycolysis
* longer bursts of activity – tennis, soccer, 100-meter swim mostly anaerobic glycolysis * resistance training (e.g. weight lifting) – increases muscle bulk by increasing - number of contractile proteins (myofilaments and myofibrils) within myofibers - some increases in number of mitochondria - storage of glycogen - connective tissue between cells
Factors affecting muscle contraction #Exercise: disuse atrophy
Disuse atrophy = degeneration and loss of muscle mass resulting from immobilization of muscle
* e.g. cast, enforced bed rest, loss of neural stimulation * paralyzed muscle may atrophy to ¼ of initial size and lost muscle tissue replaced by fibrous connective tissue
Smooth muscle #Characteristic
- small, spindle-shaped cells with centrally located nucleus
- has small amount of endomysium but no perimysium or epimysium
- muscle present in walls of hollow organs (except heart)
- organized in sheets usually:
- longitudinal layer
- circular layer
- chemical and mechanical events of contractions are the same for cardiac and skeletal muscle and similar for smooth muscle
Smooth muscle #Longitudinal layer #circular layer
Organized in sheets usually:
* longitudinal layer – runs parallel to long axis of organ and contraction shortens length of organ * circular layer – runs around circumference and contraction constricts (narrow) lumen
Smooth muscle #chemical and mechanical events of contractions
• chemical and mechanical events of contractions are the same for cardiac and skeletal muscle and similar for smooth muscle
* actin and myosin interact by sliding filament model * final trigger for contraction = rise in intercellular calcium * ATP energizes sliding process
Smooth muscle #differences with smooth muscle
Differences with smooth muscle
* lacks highly structured neuromuscular junctions * nerve fibers of autonomic (involuntary) motor system have **varicosities = numerous terminal bulbous swellings * has no T-tubules but sarcolemma has multiple **caveolae = pouchlike infoldings containing large number of calcium channels * *calcium enters from extracellular space (only small amount from SR)
Smooth muscle
#differences with smooth muscle:
No striations and no sarcomeres
No striations and no sarcomeres:
* Contain thick and thin filaments but myosin filaments are shorter and organization is different - thick and thin filaments arranged diagonally - have intermediate filaments (non-contractile) and dense bodies = points of attachment for thin filaments and tethered to sarcolemma * Have fewer thick filaments but myosin heads along entire length * *no troponin complex in thin filaments * *calmodulin = protein in cytoplasm that binds calcium - interacts with **myosin light chain kinase - **phosphorylates myosin –activating it
Smooth muscle #differences with smooth muscle
- *gap junctions = electrical coupling between cells that allow slow synchronized contractions
- takes 30 times longer to contract and relax than skeletal muscle
- can maintain same contractile tension for prolonged periods of time with less than 1% the energy cost of skeletal muscle
Smooth muscle
#differences with smooth muscle:
Can be regulated by nerves, hormones or local chemicals, and stretch
Can be regulated by nerves, hormones or local chemicals, and stretch
* nerves are from the autonomic nervous system and a variety of neurotransmitters are associated with the system - some are considered excitatory, others inhibitory and some can be both depending on what receptors are present for that neurotransmitter * hormone signalling is often associated with G-protein mediated intracellular events * direct response of smooth muscle to certain hormones or chemicals is probably responsible for smooth muscle tone * stretch provokes contraction which automatically moves substances along a tract - increased tension persists briefly and then muscle adapts to new length and relaxes while still retaining it ability to contract (stretch-relaxation response) - stretches much more and generates more tension than skeletal muscle stretched to a comparable extent
