Chapter 9 Muscles and Muscle Tissues Flashcards
Muscle Overview
There are three types of muscle tissue which are skeletal, cardiac and smooth
These times differ in structure, location, function and means of activation
Muscles Similarities
Skeletal and smooth muscles cells are elongated and are called muscle fibers
Muscle contraction depends on two kinds of myofilaments-actin and myosin
Muscle Terminology is similar:
-Sacrolemma; muscle plasma membrane
-Sarcoplasm;cytoplasm of a muscle cell
-Prefixes; myo, mys, and sarco all refer to muscle.
Skeletal Muscle Tissue
Packaged in skeletal muscles that attach to and cover the bony skeleton
Has obvious stripes called striation.
Is controlled voluntarily (i.e.,by conscious control)
Contracts rapidly but tires easily
Is responsible for overall body motility
Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds
Cardiac Muscle Tissue
Occurs only in the heart
Is striated like skeletal muscle but is not voluntary (involuntary)
Contracts at a fairly steady rate set by the heart’s pacemaker
Neural controls allow the heart to respond to changes in bodily needs
Smooth Muscle Tissue
Found in the walls of hollow visceral organs, such as the stomach, urinary bladder and respiratory passages
Forces food and other substances through internal body channels
It is not striated and is involuntary
Functional Characteristics of Muscle Tissue
Excitability, or irritability- the ability to receive and respond to stimuli
Contractility- the ability to shorten forcibly
Extensibility- the ability to be stretched or extended
Elasticity- the ability to recoil and resume the original resting length
Muscle Function
Skeletal muscles are responsible for all locomotion
Cardiac muscle is responsible for coursing the blood through the body
Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs
Muscles also maintain posture, stabilize joints and generate heat
Skeletal Muscle
The three connective tissue sheaths are:
1. Endomysium-fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber
2. Perimysium- fibrous connective tissue that surrounds groups of muscle fibers called fascicles
3. Epimysium- an overcoat of dense regular connective tissue that surrounds the entire 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 span joints and are attached to bone in at least two places.
When muscles contract the insertion on the moveable bone moves towards the origin on the immoveable bone.
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 or aponeurosis
Microscopic Anatomy of a Skeletal Muscle Fiber
Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma
Fibers are 10 to 100 m 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
Dark-A
Light-I
Myofibrils are densely packed, rod-like contractile elements
They make up most of the muscle volume
The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident
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
-Myofilaments are 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
Z-disc-coin-shaped sheet of proteins (connects) that anchors the thin filaments and connects myofibrils to one another
Thin filaments do not overlap thick filaments in the lighter H zone
M lines appear darker due to the presence of the protein desmin
Ultrastructure of Myofilaments: Thick Filaments
Thick filaments are composed of the protein myosin
Each myosin molecule has a rod-like tail and two globular heads
-Tails; two interwoven, heavy polypeptide chains
-Heads; two smaller, light polypeptide chains called cross bridge
Ultrastructure of Myofilaments: Thin Filaments
Thin filaments are chiefly composed of the protein actin
Each actin molecule is a helical polymer of globular subunits called G actin
The subunits contain the 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 an elaborate, smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril
Paired terminal cisternae form perpendicular cross channels
Functions in the regulation of intracellular calcium levels
Elongated tubes called T tubules penetrate into the cell’s interior at each A band-I band junction
T tubules associate with the paired terminal cisternae to form triads
T Tubules
T tubules are continuous with the sarcolemma
They conduct impulses to the deepest regions of the muscle
These impulses signal for the release of Ca2+ from adjacent terminal cisternae
Triad Relationships
T tubules and SR provide tightly linked signals for muscle contraction
A double zipper of integral membrane proteins protrudes into the intermembrane space
T tubules proteins act as voltage sensors
SR foot proteins are receptors that regulate Ca2+ release from the SR cisternae
Sliding Filament Model of Contraction
Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree
In the relaxed state, thin and thick filaments overlap only slightly
Upon stimulation, myosin heads bind to actin and sliding begins
Sliding Filament Model of Contraction
Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere
As this event occurs throughout the sarcomeres, the muscle shortens
Skeletal Muscle Contractions
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
Linking the electrical signal to the contraction is excitation-contraction coupling
Nerve Stimulus of Skeletal Muscles
Skeletal muscles are stimulated by motor neurons of the somatic nervous system
Axons of these neurons travel in nerves to muscle cells
Axons of motor neurons branch profusely as they enter muscles
Each axonal branch forms a neuromuscular junction with a single muscle fiber
Neuromuscular Junction
The neuromuscular junction is formed from:
-Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)
-The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junctions
Through exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft
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 axonal vesicles to fuse with the axonal membrane
-This fuse 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 an 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
Action Potential
A transient depolarization even that includes polarity reversal of a sarcolemma (or nerve cell membrane) and the propagation of an action potential along the membrane
Role of Acetylcholine (ACh)
ACh binds its receptors at the motor end plate
Binding opens chemically y(ligand) gated channels
Na+ and K+ diffuse out and the interior of the sarcolemma becomes less negative
This event is called depolarization
Depolarization
Initially, this is a local electrical event called end plate potential
Later, it ignites an action potential that spreads in all directions across the sarcolemma
Excitation- Contraction Coupling
Once generated, the action potential:
-Is propagated along the sarcolemma
-Travels down the T tubules
-Triggers Ca2+ release from terminal cisternae
Ca2+ binds to troponin and cause:
-The blocking action of tropomyosin to cease
-Actin active binding sites to be exposed
Excitation-Contraction Coupling
Myosin cross bridges alternately attach and detach
Thin filaments move toward the center of the sarcomere
Hydrolysis of ATP powers this cycling process
Ca2+ is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes
Excitation-Contraction (EC) Coupling
- Action potential generated and propagated along sarcomere to T-tubules
- Action potential triggers Ca2+ release
- Ca2++ bind to troponin; blocking action of tropomyosin released
- contraction via crossbridge formation ATP hydrolysis
- Removal of Ca+2 by active transport
- tropomyosin blockage restored; contraction ends
Sequential Events of Contraction
Cross bridge formation- myosin cross bridge attaches to actin filament
Working (power) stroke- myosin head pivots and pulls actin filament toward M line
Cross bridge detachment- ATP attaches to myosin head and the cross bridge detaches
Cocking of the myosin head- energy from hydrolysis of ATP cocks the myosin head into the high-energy state
Contraction of Skeletal Muscle Fibers
Contraction- refers to the activation of myosin’s cross bridges (force-generating sites)
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 Fibers (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)
Motor Unit: The Nerve-Muscle Functional 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 unit
Motor Unit: The Nerve-Muscle Functional Unit
Large weight-bearing muscles( thighs, hips) have large motor units
Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle
Muscle Twitch
A muscle twitch is the response of a muscle to a single, brief threshold stimulus
There are three phases to a muscle twitch:
-Latent period
-Period of contraction
-Period of relaxation
Phases of a Muscle Twitch
Latent period-first few msec after stimulus; EC coupling taking place.
Period of contraction
-cross bridges from; muscle shortens
Period of relaxation- Ca2+ reabsorbed; muscle tension goes to zero
Graded Muscle Responses
Graded muscle responses are:
-Variations in the degree of muscle contractions
-Required for proper control of skeletal movement
Responses are graded by:
-Changing the frequency of stimulus
-Changing the strength of the stimulus
Muscle Response to Varying Stimuli
A single stimulus results in a single contractile response- a muscle twitch
Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force-wave summation
Muscle Response to Varying Stimuli
More rapidly delivered stimuli result in incomplete tetanus
If stimuli are given quickly enough, complete tetanus results
Muscle Response: Stimulation Strength
Threshold stimulus-the stimulus strength at which the first observable muscle contraction occurs
Beyond threshold, muscle contracts more vigorously as stimulus strength is increased.
Force of contraction is precisely controlled by multiple motor unit summation
This phenomenon, called recruitment, brings more and more muscle fibers into play
Treppe: The Staircase Effect
Staircase-increased contraction in response to multiple stimuli of the same strength
Contractions increase because:
-There is increasing availability of Ca2+ in the sarcoplasm
-Muscle enzyme systems become more efficient because heat is increased as muscle contracts
Muscle Tone
Muscle Tone:
-Is the constant, slightly contracted state of all muscles, which does not produce active movements
-Keeps the muscle firm, healthy, and ready to respond to stimulus
Spinal reflexes account for muscle tone by:
-Activating one motor unit and then another
-Responding to activation of stretch receptors in muscles and tendons
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:
-The interaction of ADP with creatine phosphate (CP)
-Anaerobic glycolysis
-Aerobic respiration
Muscle Fatigue
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 relative deficit of ATP, causing contractures
-Lactic acid accumulates in the muscle
-Ionic imbalances are present
Muscle Fatigue
Intense exercise produces rapid muscle fatigue (with rapid recovery)
Na+-K+ pumps cannot restore ionic balances quickly enough
Low-intensity exercise produces slow-developing fatigue
SR is damaged and Ca2+ regulation is disrupted
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
Oxygen debt- the extra amount of O2 needed for the above restorative processes
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 relative size of the muscle- the bulkier the muscle, the greater 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
These two criteria define three categories-slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers
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
Effects of Resistance Exercise
Resistance exercise (typically anaerobic) results in:
-Muscle hypertrophy
-Increased mitochondria, myofilaments, and glycogen stores
The Overload Principle
Forcing a muscle to work promotes increased muscular strength
Muscles adapt to increased demands
Muscles must be overloaded to produce further gains
Muscular Dystrophy
Muscular dystrophy- group of inherited muscle-destroying diseases where muscles enlarge due to fat and connective tissue deposits, but muscle fibers atrophy
Muscular Dystrophy
Duchenne muscular dystrophy (DMD)
-inherited, sex-linked disease carried by females and expressed in males (1/3500)
-Diagnosed between the ages of 2-10
-Victims become clumsy and fall frequently as their muscles fail
-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: Male and Female
There is a biological basis for greater strength in men than women
Women’s skeletal muscle makes up 36% of their body mass
Men’s skeletal muscle makes up 42% of their body mass
These differences are due primarily to the male sex hormone testosterone
With more muscle mass, men are generally stronger than women
Body strength per unit muscle mass, however, 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
