Muscle System PP’s(4) A&P1

Types of Muscle Tissues

Cardiac (Heart)

Skeletal

(Muscles attached to bone)

Smooth

(Organs, lining of blood vessels)

Muscle contraction depends on?

Myofilaments (2)

Actin & Myosin

Sarcolemma

cell membrane of a muscle cell

Sarcoplasm

cytoplasm of a muscle cel!

Skeletal Muscle Characteristics

Long, thin, multinucleate fibers

Striations (sarcomeres)

Voluntary control

attach to and cover bony skeleton

Contracts rapidly & vigorously, tires

easily - May exert great force

Cardiac Muscle Characteristics

1 - 2 nuclei

Network of fibers - intercalated disks

Only in heart

Striations (sarcomeres)

Involuntary control

rhythmic, steady rate (autorhythmic)

Smooth Muscle Characteristics

Spindle-shaped, one centrally-located

nucleus (uninucleate)

No striations (sarcomeres)

Involuntary control

Walls of hollow organs & blood vessels

Slow & sustained contractions

Functions of Skeletal Muscle Tissue

Produce Skeletal Movement

(contractions pull on tendons)

Maintain Posture + Position (Tension)

Support Soft Tissues

(Abdominal & pelvic cavity visceral organs)

Guard Entrances and Exits (open digestive & urinary tract)

Maintain Body Temperature (Thermogenesis)

Store Nutrient Reserves

amino acids (Intense exercise, insufficient diet, stress) & Glucose storage

Functional Properties of Muscle

Tissue

Excitability

(receive and respond to stimuli)

Contractility

(shorten forcibly when stimulated)

Extensibility

(be stretched or extended)

Elasticity

(return to the original length)

Skeletal Muscle Anatomy CT components

Fascia (sheet of fibrous CT)

Superficial - separates muscle trom skin Deep - lines body wall and limbs; holds muscle with similar functions together

Tendon - dense regular connective tissue that attaches skeletal muscle to bone

Aponeurosis - a sheet of connective tissue

Skeletal Muscle Anatomy CT Wrappings

Three layers of CT are part of each skeletal muscle

Epimysium (outermost)

Perimysium

Endomysium (innermost)

Epimysium

Surrounds entire skeletal muscle

- Collagen fibers

- Separates muscles from tissues/organs

- Connected to deep fascia

Perimysium

Surrounds fascicles

(bundles of muscle fibers)

- Collagen and elastic

fibers

- Contains blood vessels and nerves to fascicles

Endomysium

Delicate, elastic CT

- Surrounds individual skeletal muscle cells (fibers)

- Contains: Capillary networks, Satellite cells, & Nerve fibers

Why is every muscle fiber (cell) supplied with an extensive supply of blood vessels and nerve fibers?

facilitate high-energy contraction and precise, voluntary control

blood vessels remove metabolic waste (like lactic acid) to sustain metabolism

Microscopic Anatomy of Skeletal Muscle Fibers

Multinucleate

T (transverse) tubules

Sarcoplasmic reticulum (SR)

Multinucleate

(fusion of myoblasts in embryo)

- Have lost ability to undergo mitosis

T (transverse) tubules

- membranous channels extend into sarcoplasm as invaginations continuous with sarcolemma

Filled with extracellular fluid and extend deep into cell

Sarcoplasmic reticulum (SR)

- Network of membranous channels that surround each myofibril and run parallel

- Same as endoplasmic reticulum in other cells

- Has a high concentration of calcium ions compared to sarcoplasm

- Membrane becomes more permeable to calcium ions when stimulated

Triads

Terminal cisternae - dilated ends of sarcoplasmic reticulum

- Terminal cisternae butt against T tubule on both sides of T tubule

Triad = TC - T - TC

Muscle Fibers

Each muscle fiber is one long, thin cell

• Myofibrils make up muscle fibers

• Myotilaments make up myofibrils

• Two types of myofilaments

- Thick filaments (myosin)

- Thin filaments (actin)

Skeletal muscle cells (fibers) are

multinucleate. How is this important to the

function of skeletal muscle cells?

to manage their extreme length (up to 30 cm) and high metabolic demands

Striations

The striped appearance (striations) of skeletal muscle due to the arrangements of thick & thin myofilaments within the myofibrils

- A band = Dark area overlap of thick and thin

- I band = Light area thin filaments alone

Sarcomeres

Length of myofibril divided into sarcomeres

- Sarcomere - functional unit of myofibril

- Z disc - separate one sarcomere from the

next sarcomere

- Z line - dense material between sarcomeres

- A band - dark band (length of thick)/ thick &

thin filaments

- I band - thin, but no thick mun

- H zone - center of A (heavy) -m

- M line - middle line or sarcomere; supporting proteins that hold thick filaments together; center of H zone

Levels of Functional Organization in a Skeletal Muscle

Skeletal Muscle (Surrounded by epimysium & Contains muscle fascicles)

Muscle Fascicle (Surrounded by perimysium & Contains muscle fibers)

Muscle Fiber (Surrounded by endomysium & Contains myofibrils)

Myofibril (Surrounded by sarcoplasmic reticulum & Consists of sarcomere)

Sarcomere (Contains thick (Myosin) &

thin (actin) filament)

Skeletal Muscle Proteins

Contractile Proteins

1. Myosin

2. Actin

Regulatory Proteins

1. Tropomyosin

2. Troponin

Structural proteins

1. Titin

2. Dystrophin

3. Myomesin

Contractile Proteins

Myosin (thick filaments) - Rod-like tail that ends in two globular heads (cross bridges)

- Cross bridges (head) interacts with active sites on thin filaments

Actin (thin filaments) - Coiled helical structure

- Myosin-binding site on each 'bead' of actin

Regulatory Proteins

Part of thin filament along with actin

Tropomyosin

- Rod-shaped protein

- Covers myosin-binding site on actin

Troponin

- Complex of 3 globular proteins

- One binds actin, one binds tropomyosin, one binds calcium

Structural Proteins

Titin - Holds the thick filaments in place; maintaining the organization of the A band

- Assists muscle cell back into shape after being stretched

- Helps muscle resist excessive stretching

Dystrophin - A cytoplasmic protein that links cytoskeleton to extracellular matrix; stabilizes the sarcolemma

Myomesin

- M line of the sarcomere; anchors myosin in A band

Skeletal Muscle Contraction

1. the H zones and I bands get smaller

2. The zones of overlap get larger

3. The Z lines move closer together

4. The width of the A band remains constant

Sliding filament theory

thin filaments sliding toward the center of each sarcomere, alongside the thick filaments (Hugh Huxley 1954)

- Muscle contraction involves the sliding movement of the thin filaments (actin) past the thick filaments (myosin)

- Sliding continues until the overlapping between the thin & thick filaments is complete

- (in a relaxed muscle cell, overlapping of thick & thin is only slight

Myofibrils shortening cause what?

The muscle fiber to shorten

Control of Skeletal Muscle Activity

Skeletal fibers contract only under control of nervous system

- Communication occurs at Neuromuscular Junction (NMJ)

- Each skeletal muscle fiber controlled by neuron at single NMJ

- Single axon braches within perimysium • End of branch = synaptic terminal (contains acetylcholine - ACh)

Synaptic cleft

- Motor end plate - region on sarcolemma that contains membrane receptors for ACh

- Junctional fold - increases surface area

Behavior of Skeletal Muscle Fibers (4)

Four major phases of contraction and relaxation

- Excitation (nerve action potentials lead to muscle action potentials)

- Excitation contraction coupling (event action potentials on sarcolemma to activation of myofilaments, preparing them to contract)

- Contraction (muscle fiber develops tension and may shorten)

- Relaxation (When stimulation ends, muscle fiber relaxes and returns to its resting length)

Contraction Process

ATP is already attached

• ATPase in myosin head hydrolyzes an ATP molecule

• Activates the head "cocking" it in an extended position - ADP + P; remain attached

• Head binds to actin active site forming a myosin-actin cross-bridge

Myosin releases ADP and Pi, and flexes pulling thin filament with it-power

stroke

• Upon binding more ATP, myosin releases actin & process can be repeated

- Recovery stroke recocks head

• Each head performs five power strokes per second

- Each stroke utilizes one molecule of ATP

Relaxation Process

Nerve stimulation and ACh release stop

• AChE breaks down ACh and fragments are reabsorbed into knob

• Stimulation by ACh stops

Ca+2 pumped back into SR by active transport

• Ca+2 binds to calsequestrin while in storage in SR

Ca+2 removed from troponin is pumped back into SR

•Tropomyosin reblocks the active sites of actin

Muscle fiber ceases to produce or maintain tension

Muscle fiber returns to its resting length - Due to recoil of elastic components and contraction of antagonistic muscles

Rigor Mortis

At death

- Circulation ceases

- No nutrients or oxygen to skeletal muscles

- Within hours skeletal muscles deprived of ATP

- SR unable to pump Ca++ out of SP

- Ca++ leaks into SP from ECF

- Triggers sustained contraction

- Without ATP, cross-bridges cannot detach

- Body becomes 'stiff as a board' - Rigor mortis lasts until lysosomal enzymes break down Z lines and titin

- Last about 15-25 hours

Botulism

Clostridium botulinum produces botulism toxin

• Botulism toxin is one of the deadliest

• Botulism toxin blocks exocytosis of synaptic vesicle at NMJ

• ACh is not released and the muscle contraction does not

occur

• Flaccid paralysis of skeletal muscle results

• Death usually results from paralysis of diaphragm and other respiratory muscles

Botox

Botox injections used cosmetically

• Medical use in treatment of

- Strabismus (crossed eyes)

- Blepharospasm (uncontrollable blinking)

- Chronic back spasms

Tetanus

Clostridium tetani

• Tetanus toxin blocks AChE

• Sustained powerful contractions

• Spastic paralysis

• 60% mortality rate

• 500,000 cases worldwide/year

• Only 100 cases/year in U.S

Tension Production

Tension is the force generated by the shortening of a sarcomere.

- Specific, quantifiable measure of the muscle's output

• Strength of the tension produced by muscle

fibers depends on:

- Number of pivoting cross bridges - Fiber's resting length at time of stimulation and

degree of overlap

- Frequency of stimulation

Length-Tension Relationship

Max. tension during contraction occurs when

resting sarcomere length is 2.0 - 2.4 um

• Too short: myosin crumples

• Overstretched: little to no crossbridges

• Resting muscle held to optimum length by: - Neural control & reflexes (e.g, stretch receptors)

- Joint positioning and bone structure

- Titin (elastic protein within sarcomeres)

Muscular Responses Recording a Muscle Contraction

Myogram - recording of a muscle contraction

• Twitch - a single contraction that lasts a fraction of a second (followed by relaxation) - Latent period - delay between

stimulation and contraction

- Contraction period

- Relaxation period

• Refractory period - time where muscle must return to its resting state (-90 mV) before it can be stimulated again

Contraction Strength of Twitches

Low frequency stimuli produce identical twitches

• Higher frequency stimuli (eg., 20 stimuli/s) produce temporal (wave) summation

- Each new twitch rides on the previous one generating higher tension

- Only partial relaxation between stimuli resulting in fluttering incomplete tetanus

• Unnaturally high stimulus frequencies (in lab experiments) cause a steady, contraction called complete (fused) tetanus

Muscular Responses

• Threshold Stimulus - The minimal strength of stimulation required to cause contraction

- A skeletal muscle fiber's resting membrane potential is~ -90 mV

- A skeletal muscle fiber must be depolarized from -90 mV to -55 mV before an impulse begins

- Therefore, the threshold stimulus is +35 mv

All-or-Nothing Response

If a muscle fiber is brought to threshold or above, it responds with a complete twitch

• If the stimulus is sub-threshold, the muscle fiber will not respond

Factors Affecting Tension at the Whole Muscle

Even if the same voltage is delivered, different stimuli cause twitches varying in tension, because:

- Fatigue - Muscles tire after continual use, reducing force output.

- Temperature - Warmer muscles' enzymes work more quickly, enhancing contraction speed

- Hydration - Affects cross-bridge formation and sarcomere function

Muscular Responses Motor Units

A motor unit is a motor neuron and the many skeletal muscle fibers it stimulates

• Because the motor neuron branches into several motor nerve endings, it can stimulate many skeletal muscle fibers simultaneously (causing simultaneous contraction)

• The number of muscle fibers in a motor unit varies from ~10 - over 1,000

Motor Unit Size

Small motor units-fine degree of control

- Three to six muscle tibers per neuron

- Eye and hand muscles

• Large motor units—more strength than control

- Powerful contractions supplied by large motor units with hundreds of fibers

- Gastrocnemius of calf has 1,000 muscle fibers per neuron

Muscular Responses Recruitment of Motor Units

Muscle is composed of many motor units, controlled by many different motor neurons, simultaneous contraction of all units does not necessarily occur

• As the intensity of stimulation increases, recruitment of motor units increases, until all contract simultaneously

• Muscles that produce precise movements are made up of small motor units

• Large motor units are active where greater contraction strength is needed, and precision is less important

Muscular Responses Muscle Tone

A certain amount of sustained contraction (tension) occurs in muscle fibers, even when a skeletal muscle is at rest

• Important in maintaining posture

• Small groups of motor units are alternately active and inactive

• Keeps skeletal muscles firm, but does not result in force strong enough to produce movement

Muscular Responses Types of Contractions - Isotonic

ISO = equal Ton = tension

• Muscle shortens and its

attachments move(s)

• Used for body movements and moving objects

- Concentric

- Eccentric

Concentric Isotonic Contraction

A muscle shortens and pulls on another structure (like a tendon) to produce movement and reduce the angle at joint

Eccentric Isotonic Contractions

The tension exerted by the myosin cross-bridges resists movement of a load and slows the lengthening process

Muscular Responses Types of Contraction - Isometric

• Iso - equal/same

• Metric - length

• The muscle becomes taut, but the attachments) do not move (no contraction or extension)
  Myosin crossbridges generate tension but the muscle does not shorten because the force of the load equals the muscle tension

• Pushing against a brick wall

• Important for...

• Posture

• Joint stability

Energy Use and Muscular Activity

A single muscle fiber contains ~ 15 billion

thick filaments • When contracting, each thick filament

breaks down ~2500 ATP/second • Even small muscle contains thousands of muscle fibers

Muscle Metabolism Energy for Muscle Contraction

The energy used to power the interaction between actin and myosin comes from ATP

• ATP stored in skeletal muscles only lasts about six seconds

• АТР must be regenerated continuously if contraction is to continue

Production of ATP in Muscle Fibers

Three pathways in which ATP is

regenerated

1. Coupled reaction with Creatine Phosphate (CP)

2. Anaerobic cellular respiration

3. Aerobic cellular respiration

(ATP) Coupled Reaction with Creatine Phosphate (CP)

Relaxed muscles have excess ATP

• Use ATP to make CP

• ATP + C → ADP + CP

• CP stored in sarcoplasm

• Active muscle: CP gives P to ADP → ATP

• 15 sec burst of energy

• 100 m Dash

(ATP) Anaerobic Respiration

No O2 need for ATP synthesis

• Glycolysis: glucose

→ 2 pyruvic acid (PA) • If no O2 PA → lactic acid in cytosol

• Only 2 ATP/glucose

• Energy for 30 sec

• 400 m dash

(ATP) Aerobic Respiration

Muscle work > 30 sec requires O2

• If O2 PA to mitochondria

• Krebs cycle and Electron Transport

• 36 ATP/glucose • ~100 ATP/fatty acid

• O2 from blood and myoglobin

Glycogen

Glucose in active skeletal muscle obtained from glycogen

• Glycogen is polymer of glucose stored in

liver & skeletal muscles

• ~1.5% of total muscle weight is glycogen

• When muscle fiber begins to run short of ATP and CP, enzymes split glycogen into glucose subunits

• Glucose used to generate more ATP

Resting skeletal muscle Energy Use

- ATP Demand low

- Ample oxygen available

- Mitochondria produces surplus of ATF

- Extra ATP used to build up reserves of CP and glycogen

Moderate activity Energy Use

- Demand for ATP increases

- ATP production rises (oxygen

consumption increases)

- All ATP produced is used, no surplus available

Peak levels of activity Energy Use

- Enormous ATP demands; oxygen

limited

- Lactic acid formation (anaerobic)

Muscle Fatigue

A state of physiological inability to contract

• If no oxygen is available in muscle cells to complete aerobic respiration pyruvic acid (PA) is converted to lactic acid (LA) [lactate]

• Lactic acid causes muscle fatigue and soreness

Fatigue results from?

A relative deficit of ATP and/or accumulation of lactic acid (decreases pH; increases acidity)

What enables a skeletal muscle to continue contracting when there’s insufficient oxygen?

Glycolysis

What is more efficient than glycolysis?

Aerobic metabolism

Oxygen Debt (Postexercise)

• Elevated oxygen use after exercise

• Caused by change in muscle's chemistry during vigorous exercise

• Extra oxygen is used for restoring muscle processes

Use of extra oxygen (function)

• To convert lactic acid to glycogen (liver)

• to resynthesize creatine phosphate and ATP in muscle fibers

• To replace oxygen in myoglobin

Heat Production

Almost half of the energy released during muscle contraction is lost to heat, helps maintain body temperature at 37°C

• Excessive heat lost through negative feedback; sweating, dilation of superficial blood vessels, increased breathing rate and increased heart rate.

Graded Responses

• Muscle contractions are smooth and vary in strength based on demands

• Smoothness achieved by graded responses

• Variations in degrees of muscle contraction

• Changing the frequency of stimulation

• Changing the strength of the stimulus

Force of Contraction

• more motor units recruited = stronger contraction

• bulkier muscle = greater strength

(exercise causes hypertrophy)

Series-elastic elements are:

• Noncontractile structures of the body

• Tension must be exerted on tendons & coverings

Degree of muscle stretch = length-tension relationship

Velocity and Duration of Contraction

• Slow fibers

• Fast fibers

Major pathways for forming ATP

• Oxidative fibers - rely on aerobic pathways for ATP generation

• Glycolytic fibers - rely on anaerobic glycolysis

Muscle Fiber Characteristics

Color = Myoglobin content

• Red muscle fibers - high myoglobin content

• White muscle fibers - low myoglobin

• Fiber diameter varies

• Allocation of mitochondria varies

• Contraction velocity and resistance to fatigue differ between fibers

Slow Oxidative (SO) Muscle Fiber Type

Least powerful

• Fatigue resistant, red, myoglobin, many mitochondria, small fiber diameter

• Suited for endurance-type activity

• Marathon or triathalon