Muscles and Muscle Tissue: Part A

Three Types of Muscle Tissue

•

•Attached to bones and skin

• •Voluntary (i.e., conscious control)

• •Primary topic of this chapter

Three Types of Muscle Tissue

•

•Striated

Three Types of Muscle Tissue

•:

•In the walls of hollow organs, e.g., stomach, urinary bladder, and airways

•Not striated

• •More details later in this chapter

Special Characteristics of Muscle Tissue

•______(responsiveness or irritability): ability to receive and respond to stimuli

•______: ability to shorten when stimulated

•______: ability to be stretched

•______: ability to recoil to resting length

Muscle Functions

•

•______(especially skeletal muscle)

Skeletal Muscle

•Each muscle is served by one ______

•All enter or exit near the central part of muscle

•

•Give off large amounts of waste

Skeletal Muscle

•Connective tissue sheaths of skeletal muscle:

•______: dense regular connective tissue surrounding entire muscle

•______: fibrous connective tissue surrounding fascicles (groups of muscle fibers)

•______: fine areolar connective tissue surrounding each muscle fiber

Skeletal Muscle: Attachments

•Muscles attach:

•Directly—epimysium of muscle is fused to the ______

•Indirectly—connective tissue wrappings extend beyond the muscle as a ______

•Connect to other muscles

Microscopic Anatomy of a Skeletal Muscle Fiber

•Cylindrical cell 10 to 100 m in diameter, up to 30 cm long

•Multiple peripheral nuclei

•

•Glycosomes for glycogen storage, myoglobin for O2 storage

•Also contain ______

Myofibrils

•Densely packed, rodlike elements

•

•Exhibit striations: perfectly aligned repeating series of dark ______

•Contain the contractile elements of skeletal muscle

Sarcomere

•Smallest contractile unit (functional unit) of a muscle fiber

•The region of a myofibril between two successive Z discs

•Composed of thick and thin myofilaments made of contractile proteins

Features of a Sarcomere

•Thick filaments (myosin): run the entire length of an A band

•Thin filaments (actin): run the length of the I band and partway into the A band

•Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another

•H zone: lighter midregion where filaments do not overlap

•M line: line of protein myomesin that holds adjacent thick filaments together

Skeletal Muscle Fiber

Ultrastructure of Thick Filament

•Composed of the protein myosin

•Myosin tails contain:

•2 interwoven chains

•Myosin heads contain:

•2 smaller chains that act as cross bridges during contraction

•Link the thick and thin filaments together

•Binding sites for ATP

•ATPase enzymes-split ATP to generate energy

Ultrastructure of Thin Filament

•Twisted double strand of fibrous protein F actin

•F actin consists of G (globular) actin subunits

•G actin bears active sites for myosin head attachment during contraction

•Tropomyosin and troponin: regulatory proteins bound to actin

•Both help control the myosin-actin interactions involved in contractions

Thick (myosin) and Thin (actin) Filaments

Sarcoplasmic Reticulum (SR)

•Network of smooth endoplasmic reticulum surrounding each myofibril

•Pairs of terminal cisternae form perpendicular cross channels

•Functions in the regulation of intracellular Ca2+ levels

•Release Ca2+ when muscle contracts

T Tubules

•Continuous with the sarcolemma

•Penetrate the cell’s interior at each A band–I band junction

•Associate with the paired terminal cisternae to form triads that encircle each sarcomere

Triad Relationships

•T tubules conduct impulses deep into muscle fiber

•T tubules run between paired terminal cisternae

•Forms triads

•Terminal cisternae – t tubule – terminal cisternae

Contraction

•The generation of force

•Does not necessarily cause shortening of the fiber

•Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening

Sliding Filament Model of Contraction

•In the relaxed state, thin and thick filaments overlap only slightly

•During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line

•As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

Requirements for Skeletal Muscle Contraction

•Activation: neural stimulation at aneuromuscular junction

•Excitation-contraction coupling:

•Generation and propagation of an action potential along the sarcolemma

•Final trigger: a brief rise in intracellular Ca2+ levels

Events at the Neuromuscular Junction

•Skeletal muscles are stimulated by somatic motor neurons

•Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles

•Each axon forms several branches as it enters a muscle

•Each axon ending forms a neuromuscular junction with a single muscle fiber

Neuromuscular Junction

•Situated midway along the length of a muscle fiber

•Muscle fiber and axon terminal (nerve ending) seperated by space – synaptic cleft

•Synaptic vesicles of axon terminal contain the neurotransmitter acetylcholine (ACh)

•Junctional folds of the sarcolemma contain ACh receptors

Events at the Neuromuscular Junction

•Nerve impulse arrives at axon terminal

•ACh is released and binds with receptors on the sarcolemma

•Electrical events lead to the generation of an action potential

Destruction of Acetylcholine

•ACh effects are quickly terminated by the enzyme acetylcholinesterase

•Prevents continued muscle fiber contraction in the absence of additional stimulation

•Myasthenia gravis – shortage of Ach receptors; autoimmune disease

Events in Generation of an Action Potential

•Local depolarization (end plate potential):

•ACh binding opens chemically (ligand) gated ion channels

•Simultaneous diffusion of Na+ (inward) and K+ (outward)

•More Na+ diffuses, so the interior of the sarcolemma becomes less negative

•Local depolarization – end plate potential

Events in Generation of an Action Potential

•Generation and propagation of an action potential:

•End plate potential spreads to adjacent membrane areas

•Voltage-gated Na+ channels open

•Na+ influx decreases the membrane voltage toward a critical threshold

•If threshold is reached, an action potential is generated (propogated)

Events in Generation of an Action Potential

•Repolarization:

•Na+ channels close and voltage-gated K+ channels open

•K+ efflux rapidly restores the resting polarity

•Fiber cannot be stimulated and is in a refractory period until repolarization is complete

•Ionic conditions of the resting state are restored by the Na+-K+ pump

Excitation-Contraction (E-C) Coupling

•Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments

•Latent period:

•Time when E-C coupling events occur

•Time between AP initiation and the beginning of contraction

Events of Excitation-Contraction (E-C) Coupling

•AP is propagated along sarcomere to T tubules

•Voltage-sensitive proteins stimulate Ca2+ release from SR

•Ca2+ is necessary for contraction

Role of Calcium (Ca2+) in Contraction

•At low intracellular Ca2+ concentration:

•Tropomyosin blocks the active sites on actin

•Myosin heads cannot attach to actin

•Muscle fiber relaxes

Role of Calcium (Ca2+) in Contraction

•At higher intracellular Ca2+ concentrations:

•Ca2+ binds to troponin

•Troponin changes shape and moves tropomyosin away from active sites

•Events of the cross bridge cycle occur

•When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends

Cross Bridge Cycle

•Continues as long as the Ca2+ signal and adequate ATP are present

•Cross bridge formation—high-energy myosin head attaches to thin filament

•Working (power) stroke—myosin head pivots and pulls thin filament toward M line

Cross Bridge Cycle

•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

•Rigor mortis

•Dying cells can not remove calcium

•This promotes myosin cross bridging

•After breathing ATP synthesis stops but it is still used

•Cross bridging detachment is impossible

•Only thing that stops it is muscle protein breakdown

Review Principles of Muscle Mechanics

•Same principles apply to contraction of a single fiber and a whole muscle

•Contraction produces tension, the force exerted on the load or object to be moved

Review Principles of Muscle Mechanics

•Contraction does not always shorten a muscle:

•Isometric contraction: no shortening; muscle tension increases but does not exceed the load

•Isotonic contraction: muscle shortens because muscle tension exceeds the load

•Force and duration of contraction vary in response to stimuli of different frequencies and intensities

Motor Unit: The Nerve-Muscle Functional Unit

•Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies

Motor Unit

•Small motor units in muscles that control fine movements (fingers, eyes)

•Large motor units in large weight-bearing muscles (thighs, hips)

Motor Unit

•Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle

•Motor units in a muscle usually contract asynchronously; helps prevent fatigue

Muscle Twitch

•Response of a muscle to a single, brief threshold stimulus

•Simplest contraction observable in the lab (recorded as a myogram)

Muscle Twitch

•Three phases of a twitch:

•Latent period: events of excitation-contraction coupling

•Period of contraction: cross bridge formation; tension increases

•Period of relaxation: Ca2+ reentry into the SR; tension declines to zero

Muscle Twitch Comparisons

Different strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles

Graded Muscle Responses

•Variations in the degree of muscle contraction

•Required for proper control of skeletal movement

Responses are graded by:

•Changing the frequency of stimulation

•Changing the strength of the stimulus

Response to Change in Stimulus Frequency

•If stimuli are given quickly enough, fused (complete) tetany results

Response to Change in Stimulus Strength

•Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs

•Muscle contracts more vigorously as stimulus strength is increased above threshold

•Motor unit summation – the more motor units recruited, the stronger the contraction

Response to Change in Stimulus Strength

•Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases

Muscle Tone

•Constant, slightly contracted state of all muscles

•Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles

•Keeps muscles firm, healthy, and ready to respond

Isotonic Contractions

•Muscle changes in length and moves the load

•Isotonic contractions are either concentric or eccentric:

•Concentric contractions—the muscle shortens and does work

•Eccentriccontractions—the muscle contracts as it lengthens

Isometric Contractions

•The load is greater than the tension the muscle is able to develop

•Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens

Muscle Metabolism: Energy for Contraction

•ATP is the only source used directly for contractile activities

•Supplies the energy needed cross bridge movement

•Also operates the calcium pump

•Available stores of ATP are depleted in 4–6 seconds

Muscle Metabolism: Energy for Contraction

•ATP is regenerated by:

•Direct phosphorylation of ADP by creatine phosphate (CP)

•Anaerobic pathway (glycolysis)

•Aerobic respiration

Anaerobic Pathway

•At 70% of maximum contractile activity:

•Bulging muscles compress blood vessels

•Oxygen delivery is impaired

•Pyruvic acid is converted into lactic acid

•Lactic acid:

•Diffuses into the bloodstream

•Used as fuel by the liver, kidneys, and heart

•Converted back into pyruvic acid by the liver

Aerobic Pathway

•Produces 95% of ATP during rest and light to moderate exercise

•Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids

Muscle Fatigue

•Physiological inability to contract

•Occurs when:

•Ionic imbalances (K+, Ca2+, Pi) interfere with E-C coupling

•Prolonged exercise damages the SR and interferes with Ca2+ regulation and release

•Total depletion of ATP rarely occurs

•If it does, contractures occur and cross bridging detachment can not occur

Oxygen Deficit

Extra O2 needed after exercise for:

•Replenishment of

•Oxygen reserves

•Glycogen stores must be replenished

•ATP and CP reserves must be resynthesized

• Conversion of lactic acid to pyruvic acid, glucose, and glycogen

Heat Production During Muscle Activity

•~ 40% of the energy released in muscle activity is useful as work

•Remaining energy (60%) 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:

•Number of muscle fibers stimulated (recruitment)

•Relative size of the fibers—hypertrophy of cells increases strength

•Frequency of stimulation

•Length-tension relationship

Velocity and Duration of Contraction

Influenced by:

•Muscle fiber type

•Load

•Recruitment

Muscle Fiber Type

Classified according to two characteristics:

•Speed of contraction: slow twitch or fast twitch, according to:

•Speed at which myosin ATPases split ATP

•Pattern of electrical activity of the motor neurons

Muscle Fiber Type

•Metabolic pathways for ATP synthesis:

•Oxidative (slow) fibers—use aerobic pathways

•Glycolytic (fast) fibers—use anaerobic glycolysis

Muscle Fiber Type

Three types:

•Slow oxidative fibers

•Fast oxidative fibers

•Fast glycolytic fibers

Influence of Load and Recruitment

load  latent period,  contraction, and  duration of contraction

Recruitment  faster contraction and  duration of contraction

Effects of Exercise

Aerobic (endurance) exercise:

•Leads to increased:

•Muscle capillaries

•Number of mitochondria

•Myoglobin synthesis

•Results in greater endurance, strength, and resistance to fatigue

• May convert fast glycolytic fibers into fast oxidative fibers

Effects of Resistance Exercise

•Resistance exercise (typically anaerobic) results in:

•Muscle hypertrophy (due to increase in fiber size)

•Increased mitochondria, myofilaments, glycogen stores, and connective tissue

The Overload Principle

•Forcing a muscle to work hard promotes increased muscle strength and endurance

•Muscles adapt to increased demands

•Muscles must be overloaded to produce further gains