· 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


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


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


T tubuleSarcolemma

SR tubules (cut)

Synapticcleft

Synapticvesicle

Axon terminal

AChAChACh




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


T tubuleSarcolemma

SR tubules (cut)

Synapticcleft

Synapticvesicle

Axon terminal

AChAChACh




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


T tubuleSarcolemma

SR tubules (cut)

Synapticcleft

Synapticvesicle

Axon terminal

AChAChACh






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


T tubuleSarcolemma

SR tubules (cut)

Synapticcleft

Synapticvesicle

Axon terminal

AChAChACh






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

· web viewthe region of a myofibril between two successive z discs composed of myofilaments made...

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