Muscle Tissue

Functions of Muscle Tissue

MovementHeat productionMaintenance of postureMuscle contraction produces 85% of

body heat

Characteristics of Muscle Tissue

Excitability - able to be stimulatedContractibility - able to shorten and

thickenExtensibility - stretchable and

extendableElasticity - the ability to return to its

original shape

Muscle Tissue Types

Skeletal - found attached to bone, striated, voluntary

Cardiac - forms the walls of heart, striated, involuntary

Smooth - found in viscera, non-striated, involuntary

Gross Anatomy of Skeletal Muscle

Each muscle is an organ, containing muscle, blood vessels, nerves, & connective tissue

Connective Tissue Components

Fascia - found under the skin, covering organs, and muscles

Epimysium – surrounds entire musclePerimysium – surrounds groups of muscle

fibers called “fascicles”Endomysium – surrounds individual

muscle fibersTendons; Aponeurosis – connect muscle to

bones

Cell Structure

Muscle Cell = Muscle Fiber Elongated, multi-nucleate, striated cells

containing parallel bundles of myofibrils Sarcolemma – plasma membrane Sarcoplasm – cytoplasm containing:

Myoglobin – stores oxygenGlycosomes – store starch

Peripheral nuclei Sarcoplasmic reticulum – smooth E.R. –

maintains calcium levels Transverse tubules – network of membranes

connected to sarcolemma; penetrates deep into each contractile unit.

Specialized contractile organelles

Myofibrils - thread-like structures 100’s to 1000’s in each muscle fiber

(cell) Actin - thin filaments containing actin

protein, 2 strands of tropomyosin, & troponin

Myosin - thick protein filaments composed of myosin molecules

Myofibrils

Actin

Myosin

SARCOMERE – the basic contractile unit

Z discs (lines) - separate sarcomeres; anchor thin filaments

A band - overlapping area of thick and thin filaments

I band - contains only actinH zone - part of A band containing

only myosinM line - center of H zone; anchors

myosin

Sliding Filament Mechanism

Skeletal Muscle Contraction

Muscle contraction occurs when actin and myosin are allowed to interact with each other and form crossbridges

The binding sites on actin are blocked by the troponin/tropomyosin complex

Calcium ions in the sarcoplasm will bind to troponin

Muscle Contraction

This binding will cause the troponin / tropomyosin complex to pull away from the active binding site on actin, thus allowing myosin to bind

The myosin head pivots, pulling the thin filaments toward the center of the sarcomere thus shortening the sarcomere

Repeated cycles of attachment, pivoting, detach and release occurs

Muscle Contraction

Successive interaction causes “sliding” of the filament, shortening of the sarcomere, thus shortening of the entire muscle

Calcium is removed from the troponin molecule and returned to the S.R.

Relaxation occurs

What Role Does Calcium Play?

What triggers the release of calcium?

ACTION POTENTIALS

The sudden change in the transmembrane potential

Action Potentials

Resting Membrane PotentialPolarized - positive charge outside,

negative charge insideDepolarized - positive charge inside,

negative charge outside Repolarized - positive charge

reestablished outside, negative charge inside

Resting Potential/ Polarized

When muscle is relaxed, the sarcolemma is polarized having a charge difference between the inside /outside of the cell

When a stimulus is received opening a channel gate , Na+ ions will flow into the cell changing the polarity of the cell

Depolarized

The net charge of the sarcolemma becomes negative in regards to the inside of the cell which is now positive.

The cell is said to be depolarized and the muscle contracted

Repolarized

Membrane pumps quickly restore the original status or condition

The positive charge outside is reestablished once again and resting membrane potential is restored

Neuromuscular Junction

Each fiber is controlled by a motor neuron at a neuromuscular junction

Motor neurons stimulate muscle fibersAcetylcholine (ACh ) is released into the

synaptic cleft with the arrival of an action potential

ACh diffuses across the cleft, binding to receptors on the motor end plate, initiating a muscle action potential

Once initiated, the action potential is unstoppable and self-propagating

RELAXATION

Resting membrane potential is restored by: Acetylcholinesterase active transport pumps that pump Ca+2

ions back into the sarcoplasmic reticulum

Calsequestrin – binds calcium

Role of ATP

Used to activate the myosin head in order to bind to actin

After power stroke, ATP used to break the bond between actin and myosin

ATP used to pump calcium back into the SR

Production of ATP for muscles

Direct phosphorylation Creatine phosphate couples with ADP to form ATP Provides about 15 sec of energy

Glycolysis Glucose broken down anaerobically Produces lactic acid as waste product Provides about 30-60 sec of energy

Aerobic Respiration Glucose broken down with oxygen Hours of energy

Muscle Fatigue

Insufficient oxygenBuild-up of lactic acidDepletion of glycogen

RECOVERY OXYGEN CONSUMPTION OXYGEN DEBT

ALL - or -None Principle

Muscle fibers will contract fully OR not at all once they are stimulated

Threshold stimulus minimal level of stimulation needed to

cause the muscle to contract

Motor Units

Motor units - motor neuron and all the muscle fibers it controls

Number of muscle fibers in motor unit will vary

The fewer the number of fibers per motor unit, the more precise the contraction

The number of motor units being stimulated will determine the strength of contraction of the entire muscle

Muscle Contraction

Twitch contraction rapid, jerky contraction to a single stimuli phases: latent, contraction,

relaxation,refractoryWave summation

increase in the strength of muscle contraction due to rapid successive stimulation

Tetany continuous, smooth, sustained contraction

Muscle Contraction

Treppe repeated stimulation following stimulation

causing a staircase effectIsotonic

tone or tension remains constant-muscle shortens

Isometric tension increases - muscle length remains

same

Muscle Fiber TypesRed oxidative fibers

more myoglobin more capillaries more mitochondria long, slow

contraction

sustained energy aerobic respiration non-fatiguable fibers

White glycolytic fibers less myoglobin less capillaries fewer mitochondria rapid,powerful

contraction quick energy anaerobic respiration fatigue easily

Benefits of Exercise

Increase the size of size and strength of each fiber

Increase muscle toneIncreases the blood supply, thus

increasing the number of red blood cellsIncreased respiratory and

cardiovascular functionLowers blood pressure

Cardiac Muscle

InvoluntaryIntercalated discsForms syncytiumLong refractory periodLong contraction rateMore mitochondria than skeletal

muscle

Smooth Muscle

Involuntary - neural & hormonal stimulation

No sarcomeres - no striations

Very, very long contraction rate

Calmodulin - regulatory protein

No tendons or aponeuroses

Muscle / Bone Interaction

Origin – attachment of a muscle to a stationary bone

Insertion – attachment of a muscle to a movable bone

Prime mover – provides major force for specific movement

Antagonist – opposes prime moverSynergist – assists the prime mover

(secondary muscle)

Muscle / Bone Interactions

Levers – rigid bar (bones) moving on fixed point

Fulcrum = fixed point (joints)Effort = applied forceResistance = load

Levers

First class Fulcrum in center = seesaw Lifting head off chest

Second class Load (resistance) in center = wheelbarrow Least common Standing on tiptoes

Third class Effort in center = tweezers Biceps brachii Most common