· Web viewThe region of a myofibril between two successive Z discs Composed of myofilaments made...
Transcript of · Web viewThe region of a myofibril between two successive Z discs Composed of myofilaments made...
Chp 9 - Muscles and Muscle Tissue BIO 200
Muscle Tissue Overview
The three types of muscle tissue
1. Skeletal 2. Cardiac 3. Smooth
These types differ in structure, location, function, and means of activation
Muscles Similarities
Skeletal and smooth muscle cells are elongated and are called muscle fibers
Muscle contraction depends on two kinds of myofilaments – actin and myosin
Muscle terminology is similar
1. Sarcolemma – muscle plasma membrane
2. Sarcoplasm – cytoplasm of a muscle cell
3. Prefixes – myo, mys, and sarco all refer to muscle
Skeletal Muscle Tissue - Cover the bony skeleton; have obvious stripes called striations; are controlled voluntarily (conscious control) and contract rapidly but tire easily.
Skeletal Muscle Tissue -is responsible for overall body motility of the body; it is extremely adaptable; can exert forces ranging from just a fraction of an ounce to over 70 pounds
Cardiac Muscle Tissue - Found only in the heart; are Striated like skeletal muscle; Involuntary muscles movement and contract at a steady rate set by the heart’s pacemaker.
Cardiac Muscle Tissue – are Neural controls allow the heart to respond to changes in bodily needs
Smooth Muscle Tissue - are found in the walls of hollow visceral organs, such (stomach, urinary bladder, respiratory tract); forces food and other substances through internal body channels (intestinal wall) and are not striated muscle tissue. They are Involuntary muscle movement
Functional Characteristics of Muscle Tissue
1. Excitability - responsiveness or irritability; mean that it has the ability to receive and respond to stimuli
Contractility – the ability to shorten forcibly when stimulated.
Extensibility – the ability to stretched or extend
Elasticity – the ability to recoil and resume its original resting length
Muscle Function
Muscles maintain posture, stabilize joints, and generate heat and Provide strength. Skeletal muscles are responsible for locomotion
Cardiac muscle is responsible for pumping the blood through the body
Smooth muscles help maintain blood pressure, and squeezes or propels substances (food, feces) through organs
Skeletal Muscle
Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue. These tissues support each cell, reinforce the muscle and prevent bulging muscles from bursting. They includes three connective tissue sheaths:
1. Epimysium – outside the muscle - an overcoat of dense regular connective tissue that surrounds the entire muscle
2. Perimysium – around the muscle - fibrous connective tissue that surrounds groups of muscle fibers called fascicles
3. Endomysium – within the muscle - fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber
Skeletal Muscle –
Skeletal Muscle: Nerve and Blood Supply. Each muscle is served by one nerve, an artery, and one or more veins. Each skeletal muscle fiber is supplied with a nerve ending that controls contraction
Contracting fibers require continuous delivery of oxygen and nutrients via arteries. Wastes must be removed via veins
Skeletal Muscle: Attachments. Most skeletal muscles are attached to bone in at least two places
When muscles contract the movable bone, (muscle’s insertion) moves toward the immovable bone, the muscle’s origin.
Muscles attach:
Directly – epimysium of the muscle is fused to the periosteum of a bone
Indirectly – connective tissue wrappings extend beyond the muscle as a tendon
Microscopic Anatomy of a Skeletal Muscle Fiber
Each fiber is a long, cylindrical cell, multiple nuclei. The Fibers are 10 to 100 mm in diameter, and up to hundreds of centimeters long
Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin
Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules
Myofibrils are densely packed, rodlike contractile elements. They make up most of the muscle volume
The arrangement of myofibrils are perfectly aligned repeating series of dark A bands and light I bands
Sarcomeres - The smallest contractile unit of a muscle
The region of a myofibril between two successive Z discs
Composed of myofilaments made up of contractile proteins: two types – thick and thin
Myofilaments: Banding Pattern
Thick filaments – extend the entire length of an A band
Thin filaments – extend across the I band and partway into the A band. do not overlap thick filaments in the lighter H zone
Z-disc – sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another
Thin filaments M lines appear dark due to protein desmin
Ultrastructure of Myofilaments: Thick Filaments
Thick filaments are composed of protein myosin
Each myosin molecule has a rod-like tail and two globular heads
Tails – two interwoven polypeptide chains
Heads – two smaller, light polypeptide chains called cross bridges
Ultrastructure of Myofilaments: Thin Filaments
Thin filaments are composed of the protein actin
Each actin molecule is a helical polymer of globular subunits called G actin
The subunits contain active sites to which myosin heads attach during contraction
Tropomyosin and troponin are regulatory subunits bound to actin
Arrangement of the Filaments in a Sarcomere
Longitudinal section within one sarcomere
Sarcoplasmic Reticulum (SR)
SR is a smooth endoplasmic reticulum that runs longitudinally and surrounds each myofibril
Paired terminal cisternae
Functions in regulating intracellular calcium levels
T Tubules
T tubules are continuous with the sarcolemma
They conduct impulses to the deep regions of the muscle
These impulses signal Ca2+ release from terminal cisternae
T tubules and SR provide tightly linked signals for muscle contraction
T tubule proteins act as voltage sensors
Skeletal Muscle Contraction - In order to contract, a skeletal muscle must:
Be stimulated by a nerve ending
Propagate an electrical current, or action potential, along its sarcolemma
Have a rise in intracellular Ca2+ levels, the final trigger for contraction
Excitation-contraction coupling link the electrical signal to the contraction
Nerve Stimulus of Skeletal Muscle and the Skeletal muscles are stimulated by motor neurons from the nervous system
Axons of these neurons travel in nerves to muscle cells and branch out profusely as they enter muscles
Neuromuscular Junction:
Axon endings that have small membranous sacs (synaptic vesicles) contain the neurotransmitter acetylcholine (ACh)
The motor end plate of a muscle is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junction
When a nerve impulse reaches the end of an axon at the neuromuscular junction:
Voltage-regulated calcium channels open and allow Ca2+ to enter the axon
Ca2+ inside the axon terminal causes vesicles to fuse with the membrane
This fusion releases ACh into the synaptic cleft via exocytosis
ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma
Binding of ACh to its receptors initiates action potential in the muscle
Destruction of Acetylcholine
ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase
This destruction prevents continued muscle fiber contraction in the absence of additional stimuli
A transient depolarization event and polarity reversal of a sarcolemma (nerve cell membrane) that is conducted along the membrane of a muscle cell or nerve fiber
Role of Acetylcholine (Ach)
Depolarization – a local electrical event that loses polarity; loss or reduction of negative membrane potential
ACh binds its receptors at the motor end plate
Na+ and K+ diffuse out and the interior of the sarcolemma becomes less negative
Ignites the action potential that spreads throughout the neuromuscular junction across the sarcolemma
Repolarization – restores the sarcolemma to its initial polarized state
Action Potential: Polarized Sarcolemma
Depolarization: an electrical event called end plate potential
It ignites an action potential that spreads in all directions across the sarcolemma
The outside (extracellular) face is positive, while the inside face is negative
This difference in charge is the resting membrane potential
Action Potential: Polarized Sarcolemma
The predominant extracellular ion is Na+ and the predominant intracellular ion is K+
The sarcolemma is relatively impermeable to both ions
Action Potential: Depolarization
An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na+ (sodium channels open)
Na+ enters the cell, and the resting potential is decreased (depolarization occurs). If the stimulus is strong enough, an action potential is initiated
Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch
Voltage-regulated Na+ channels now open in the adjacent patch causing it to depolarize
The action potential travels rapidly along the sarcolemma
Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle
Action Potential: Repolarization
Immediately after the depolarization wave passes, the sarcolemma permeability changes
Na+ channels close and K+ channels open
K+ diffuses from the cell, restoring the electrical polarity of the sarcolemma
Action Potential: Repolarization
Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period)
The ionic concentration of the resting state is restored by the Na+-K+ pump
Action Potential: Repolarization
Excitation-Contraction (EC) Coupling
1. Action potential generated and propagated along sarcomere to T-tubules. Neurotransmitters are released and Diffuse across the synaptic cleft and Attaches to ACh receptors on the sarcolemma. (Sodium (Na +) initiates an action potential that propagates along the sarcomere down to the T-tubules
2. Action potential triggers Ca2+ release. Action potential triggers voltage-sensitive receptors that trigger Calcium (Ca2+) release from the SR.
Synapticcleft
Synapticvesicle
Axon terminal
AChAChAChNeurotransmitters are released andDiffuse across the synaptic cleft andAttaches to ACh receptors on the sarcolemma.
3. Ca++ bind to troponin; blocks release of tropomyosin. Ca+ ions bind totroponin; troponin changes shape and blocks release of tropomyosin. Actin active sites are exposed.
4. Contraction occurs via crossbridge formation and release of energy by ATP hyrdolysis
5. Removal of Ca+ by active transport occurs
6. Tropomyosin blockage is restorred and contraction ends and the muscle fiber relaxes
Net entry of Na+ Initiatesan action potential whichis propagated along thesarcolemma and downthe T tubules.
T tubuleSarcolemma
SR tubules (cut)
Synapticcleft
Synapticvesicle
Axon terminal
AChAChACh
Neurotransmitter released diffusesacross the synaptic cleft and attachesto ACh receptors on the sarcolemma.
Action potential inT tubule activatesvoltage-sensitive receptors,which in turn trigger Ca2+
release from terminalcisternae of SRinto cytosol.
SRCa2+Ca2+
Ca2+Ca2+
Ca2+Ca2+
Ca2+Ca2+
1
2
Net entry of Na+ Initiatesan action potential whichis propagated along thesarcolemma and downthe T tubules.
T tubuleSarcolemma
SR tubules (cut)
Synapticcleft
Synapticvesicle
Axon terminal
AChAChACh
Neurotransmitter released diffusesacross the synaptic cleft and attachesto ACh receptors on the sarcolemma.
Action potential inT tubule activatesvoltage-sensitive receptors,which in turn trigger Ca2+
release from terminalcisternae of SRinto cytosol.
Calcium ions bind to troponin;troponin changes shape, removingthe blocking action of tropomyosin;actin active sites exposed.
SR
Ca2+
Ca2+Ca2+
Ca2+Ca2+
Ca2+
Ca2+
Ca2+Ca2+
1
2
3
Net entry of Na+ Initiatesan action potential whichis propagated along thesarcolemma and downthe T tubules.
T tubuleSarcolemma
SR tubules (cut)
Synapticcleft
Synapticvesicle
Axon terminal
AChAChACh
Neurotransmitter released diffusesacross the synaptic cleft and attachesto ACh receptors on the sarcolemma.
Action potential inT tubule activatesvoltage-sensitive receptors,which in turn trigger Ca2+
release from terminalcisternae of SRinto cytosol.
Calcium ions bind to troponin;troponin changes shape, removingthe blocking action of tropomyosin;actin active sites exposed.Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments towardthe center of the sarcomere; release of energy byATP hydrolysis powers the cycling process.
SR
Ca2+
Ca2+Ca2+
Ca2+Ca2+
Ca2+Ca2+
Ca2+Ca2+
1
2
3
4
Net entry of Na+ Initiatesan action potential whichis propagated along thesarcolemma and downthe T tubules.
T tubuleSarcolemma
SR tubules (cut)
Synapticcleft
Synapticvesicle
Axon terminal
AChAChACh
Neurotransmitter released diffusesacross the synaptic cleft and attachesto ACh receptors on the sarcolemma.
Action potential inT tubule activatesvoltage-sensitive receptors,which in turn trigger Ca2+
release from terminalcisternae of SRinto cytosol.
Calcium ions bind to troponin;troponin changes shape, removingthe blocking action of tropomyosin;actin active sites exposed.Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments towardthe center of the sarcomere; release of energy byATP hydrolysis powers the cycling process.
Removal of Ca2+ by active transportinto the SR after the actionpotential ends.
SR
Ca2+
Ca2+
Ca2+Ca2+
Ca2+Ca2+
Ca2+Ca2+
Ca2+Ca2+
1
2
3
45
Contraction of Skeletal Muscle Fibers
Contraction – refers to the activation of myosin’s cross bridges
Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening
Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced
Contraction of Skeletal Muscle (Organ Level)
Contraction of muscle fibers (cells) and muscles (organs) is similar
The two types of muscle contractions are:
Isometric contraction – increasing muscle tension (muscle does not shorten during contraction)
Isotonic contraction – decreasing muscle length (muscle shortens during contraction)
The Nerve-Muscle Functional: Motor Unit
A motor unit is a motor neuron and all the muscle fibers it supplies
The number of muscle fibers per motor unit can vary from four to several hundred
Muscles that control fine movements (fingers, eyes) have small motor units
Large weight-bearing muscles have large motor units
Homeostasis imbalance
Rigor mortis
Muscles stiffen 3-4 hrs after death; peak at 2 hrs.
Gradual dissipation over the next 40-60 hrs.
ADPPi
Net entry of Na+ Initiatesan action potential whichis propagated along thesarcolemma and downthe T tubules.
T tubuleSarcolemma
SR tubules (cut)
Synapticcleft
Synapticvesicle
Axon terminal
AChAChACh
Neurotransmitter released diffusesacross the synaptic cleft and attachesto ACh receptors on the sarcolemma.
Action potential inT tubule activatesvoltage-sensitive receptors,which in turn trigger Ca2+
release from terminalcisternae of SRinto cytosol.
Calcium ions bind to troponin;troponin changes shape, removingthe blocking action of tropomyosin;actin active sites exposed.Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments towardthe center of the sarcomere; release of energy byATP hydrolysis powers the cycling process.
Removal of Ca2+ by active transportinto the SR after the actionpotential ends.
SR
Tropomyosin blockage restored,blocking myosin binding sites onactin; contraction ends andmuscle fiber relaxes.
Ca2+
Ca2+
Ca2+Ca2+
Ca2+Ca2+
Ca2+
Ca2+
Ca2+Ca2+
1
2
3
45
6
Dying cells cannot exlude calcium
Calcium in muscle cells promotes myosin cross bridges – detachment of cross bridge is unable to cease – actin & myosin cross-link become irreversible.
Rigor mortis disappears when muscle proteins break down.
BIO 200 - Muscle Tone Chp 9b
Muscle Twitch
Motor unit responds to a single action potential
Muscle fibers contract quickly & relax
3 distinct phases seen myogram
Latent period – muscle tension being to increase but not response is seen
Period of contraction – activation of cross-bridge
Period of relaxation – contraction is followed by relaxation because calcium re-enters into the SR
Graded muscle responses
Healthy muscle contractions are smooth; very in strength
Graded muscle contractions:
Change in the frequency of stimulation – wave summations: tetanus
Change in the strength of the stimulation – as stimulus strength increases, the contraction force increases
The larger the motor unit, the stronger the contractile force
Muscle Tone
A constant but slightly contracted state of all muscles - which do not produce active movements
Keeps the muscles firm, healthy, and ready to respond to stimulus
Spinal reflexes (CNS) account for muscle tone by:
Activating one motor unit and then another
Responding to activation of stretch receptors in muscles and tendons
The Nerve-Muscle Functional: Motor Unit
A motor unit is a motor neuron and all the muscle fibers it supplies
The number of muscle fibers per motor unit can vary from four to several hundred
Muscles that control fine movements (fingers, eyes) have small motor units
Large weight-bearing muscles have large motor units
Principles of muscle mechanics
1. Muscle fibers in skeltal muscle are huge in numbers and consistantly the same
2. Force exerted by a contracting muscle on an object - muscle tension
3. The opposing force exerted on the muscle by the weight of the object to be moved is – load
4. A contracting muscle does not always shorten – isometric
5. If the muscle tension overcomes the load, and the muscle shortens - isotonic
Skeletal muscle contracts with different forces and for different periods of time. However dependent on the frequency and intensity of the stimuli. Each muscle is served by a motor nerve (axon) and several hundreds of motor neurons. As the axon enters the muscle, it branches into terminals, each forming a neuromuscular junction with a single muscle fiber
Isotonic Contractions
In isotonic contractions, the muscle changes in length (decreasing the angle of the joint) and moves the load
The two types of isotonic contractions are concentric and eccentric
Concentric contractions – the muscle shortens and does work
Eccentric contractions – the muscle contracts as it lengthens
Isometric Contractions
Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens
Occurs if the load is greater than the tension the muscle is able to develop
Muscle Metabolism: Energy for Contraction
ATP is the only source used directly for contractile activity. As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are regenerated by:
1. The interaction of ADP with creatinine phosphate 2. Anaerobic glycolysis 3. Aerobic respiration
Muscle Metabolism: Anaerobic Glycolysis
When muscle contractile and activity reaches 70% of maximum:
Bulging muscles compress blood vessels
Oxygen delivery is impaired
Pyruvic acid is converted into lactic acid
The lactic acid:
Diffuses into the bloodstream and iIs picked up and used as fuel by the liver, kidneys, and heart. IIs converted back into pyruvic acid by the liver
Muscle fatigue – the muscle is in a state of physiological inability to contract. Muscle fatigue occurs when:
ATP production fails to keep pace with ATP use
There is a relative deficit of ATP, causing contractures (cramps)
Lactic acid accumulates in the muscle
Ionic imbalances are present
Intense exercise produces rapid muscle fatigue
Na+-K+ pumps cannot restore ionic balances quickly enough
SR is damaged and Ca2+ regulation is disrupted
Low-intensity exercise produces slow-developing fatigue
Oxygen Debt
Vigorous exercise causes dramatic changes in muscle chemistry. For a muscle to return to a resting state:
Oxygen reserves must be replenished
Lactic acid must be converted to pyruvic acid
Glycogen stores must be replaced
ATP and CP reserves must be resynthesized
Extra amount of O2 is needed for the above processes to be restored
Heat Production During Muscle Activity
Only 40% of the energy released in muscle activity is useful as work. The remaining 60% is given off as heat. Dangerous heat levels are prevented by radiation of heat from the skin and sweating
Force of Muscle Contraction - the force of contraction is affected by:
The number of muscle fibers contracting – the more motor fibers in a muscle, the stronger the contraction
The size of the muscle – the bulkier the muscle, the greater its strength
Degree of muscle stretch – muscles contract strongest when muscle fibers are 80-120% of their normal resting length
Muscle Fiber Type: Functional Characteristics
Speed of contraction – determined by speed in which ATPases split ATP
The two types of fibers are slow and fast
ATP-forming pathways
Oxidative fibers – use aerobic pathways
Glycolytic fibers – use anaerobic glycolysis
Muscle Fiber Type: Speed of Contraction
Slow oxidative fibers contract slowly, have slow acting myosin ATPases and are fatigue resistant
Fast oxidative fibers contract quickly, have fast myosin ATPases, and have moderate resistance to fatigue
Fast glycolytic fibers contract quickly, have fast myosin ATPases, and are easily fatigued
Effects of Aerobic Exercise - Aerobic exercise results in an increase of:
Muscle capillaries
Number of mitochondria
Myoglobin synthesis
Typically anaerobic exercises, results in:
Muscle hypertrophy
Increased mitochondria, myofilaments, and glycogen stores
The Overload Principle
Forcing a muscle to work promotes increased muscular strength but Muscles adapt to increased demands. Muscles must be overloaded to produce further gains
Smooth Muscle
Composed of spindle-shaped fibers, 2-10 mm in diameter and lengths = several hundred mm
Lack the coarse connective tissue sheaths of skeletal muscle, but have fine endomysium
Organized into two layers (longitudinal and circular) of closely apposed fibers
Found in walls of hollow organs (except the heart)
Essentially vave the same contractile mechanisms as skeletal muscle
Peristalsis - when the longitudinal layer contracts, the organ dilates and contracts and When the circular layer contracts, the organ elongates. Peristalsis is an alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs.
Innervation of Smooth Muscle
Smooth muscle lacks neuromuscular junctions
Innervating nerves have bulbous swellings called varicosities
Varicosities release neurotransmitters into wide synaptic clefts called diffuse junctions
Microscopic Anatomy of Smooth Muscle
SR is less developed than in skeletal muscle and lacks a specific pattern; T tubules are absent and the Plasma membranes have pouchlike infoldings called caveoli. There are no visible striations and no sarcomeres
Contraction of Smooth Muscle
Whole sheets of smooth muscle exhibit slow, synchronized contraction and they contract in unison. Action potentials are transmitted from cell to cell. Some smooth muscle cells:
Act as pacemakers and set the contractile pace for whole sheets of muscle
Are self-excitatory and depolarize without external stimuli
Some Special Features of Smooth Muscle Contraction are Unique and include:
Smooth muscle tone; Slow, prolonged contractile activity; they require low energy and response to stretch
Response to Stretch: smooth muscle exhibits a phenomenon called stress-relaxation response in which:
Responds to stretch only briefly, and then adapts to its new length
The new length retains its ability to contract
This enables organs such as the stomach and bladder to temporarily store contents
Hyperplasia
Certain smooth muscles can divide and increase their numbers by undergoing hyperplasia. This is shown by estrogen’s effect on the uterus
At puberty, estrogen stimulates the synthesis of more smooth muscle, causing the uterus to grow to adult size
During pregnancy, estrogen stimulates uterine growth to accommodate the increasing size of the growing fetus
Types of Smooth Muscle: Single Unit
The cells of single-unit smooth muscle visceral muscle:
Contract rhythmically as a unit; they are electrically coupled to one another via gap junctions; they often exhibit spontaneous action potentials; and they are arranged in opposing sheets and exhibit stress-relaxation response
Types of Smooth Muscle: Multiunit
Multiunit smooth muscles are found:
In large airways to the lungs; In large arteries; In arrector pili muscles’ Attached to hair follicles; and in the internal eye muscles
Their characteristics include: Rare gap junctions; Infrequent spontaneous depolarizations; Structurally independent muscle fibers ; A rich nerve supply, which, with a number of muscle fibers, forms motor units; and Graded contractions in response to neural stimuli
Muscular Dystrophy
Muscular dystrophy – group of inherited muscle-destroying diseases where muscles enlarge due to fat and connective tissue deposits, but muscle fibers atrophy
Duchenne muscular dystrophy (DMD)
Inherited, sex-linked disease carried by females and expressed in males (1/3500). Diagnoses occaurs between the ages of 2-10. Victims become clumsy and fall frequently as their muscles fail
Muscular Dystrophy is:
Progresses from the extremities upward, and victims die of respiratory failure in their 20s
Caused by a lack of the cytoplasmic protein dystrophin
There is no cure, but myoblast transfer therapy shows promise
Developmental Aspects
Muscle tissue develops from embryonic mesoderm called myoblasts
Male and Female differ:
greater strength in men than in women
Women’s skeletal muscle - 36% of body mass
Men’s skeletal muscle - 42% of body mass
hormone testosterone
Body strength per unit muscle mass is the same in both sexes
Developmental Aspects: Age Related
With age, connective tissue increases and muscle fibers decrease
Muscles become stringier and more sinewy
By age 80, 50% of muscle mass is lost (sarcopenia)
Regular exercise reverses sarcopenia
Aging of the cardiovascular system affects every organ in the body
Atherosclerosis may block distal arteries, leading to intermittent claudication and causing severe leg muscle pain