Chap 10: Muscle Tissue

outstanding characteristic of muscle tissue

ability to shorten or contract

3 muscle tissue types

skeletal, cardiac, smooth

how are muscle tissue types classified?

structure, contractile props, location, and control mechanisms

skeletal muscle

• most attached to bones (some attached to skin or other skeletal muscles)

• contraction → skeleton movement

• controlled by somatic division of nervous system

• microscopic appearance: alternating transverse light and dark bands (striations)

• VOLUNTARY CONTROL

cardiac muscle

• heart wall

• contraction → propels blood through circulatory system

• control: autorhythmic - adjusted by autonomic NS and hormones

• INVOLUNTARY CONTROL

• microscope: striations, intercalated discs

smooth muscle

• surrounds hollow organs and tubes

• found as single Cs or in small groups

• contraction → propulsion of luminal contents or flow regulation

• controlled by autonomic NS, hormones, and intrinsic factors

• INVOLUNTARY CONTROL

• microscope = no striations (smooth)

Functions of muscular tissue

producing the body movements, stabilizing body, storing and moving substances within the body, and generating heat

Properties of muscular tissue

electrical excitability, contractility, extensibility, elasticity

electrical excitability

ability to recieve and respond to stimuli by action potential production

contractility

ability to shorten

extensibility

ability to stretch

elasticity

ability to recoil

What does skeletal muscle contain?

• muscle fibers: individual muscle

• CT: surrounds muscle fiber and whole muscle

• blood vessels and nerves

Connective tissue components

1) layers surround and protecting (3 layers)

• epimysium

• perimysium

• endomysium

2) fascia

• deep fascia

• superficial fascia

3) Skeletal Muscle Attachments

• tendons

• aponeurosis

Epimysium

CT that surrounds the ENTIRE muscle

Perimysium

CT that penetrates the muscle and separates and surrounds the muscle fibers into bundles of 10-100 fibers = “pascicles”

Endomysium

Thin CT extensions enveloping each muscle FIBER

Deep fascia

between neighboring muscles (carry nerves, blood vessels, etc.)

Superficial fascia

hypodermis, subcutaneous layer

• between muscle and skin (adipose)

Tendons

white, fibrous cords of dense, regular CT that attach muscle to bone

Aponeurosis

sheet like layer of CT joining a muscle to the part that it moves

Specific names for the attachments of both ends of a skeletal muscle

origin and insertion

origin

the more STATIONARY bone to which the muscle is attached (head) (usually proximal)

Insertion

the more MOBILE end (of bone) (usually distal)

Myoblasts

the immature contractions giving rise to muscle c (fibers)

• can divide

fuse

multinucleated mature muscle fiber (cannot divide)

satellite cs:

inactive myoblasts ass. with mature muscle fibers

• have the potential to divide

• increase # in young children

hypertrophy

enlargement of existing muscle fibers

• accounts for muscle growth after birth in response to hGH (note - testosterone promotes further muscle fiber enlargement)

fibrosis

replacement of muscle fibers by fibrous scar tissue following damage

sarcolemma

plasma membrane

sarcoplasm

cytoplasm

T (transverse) tubules

in folding of sarcolemma; carries electrical current (charge) from surface to cell interior → Ca++ release from terminal cisternae

What is muscle fiber made up of?

Myofibrils

Sarcoplasmic reticulum

membranous sacs encircle myofibril; stores Ca++

Terminal cisternae

dilated sacs of SR alongside T-tubules

Triad

T-tubule + 2 terminal cisternae (on either side)

Contractile Elements

• thick filaments

• thin filaments

• actin

• tropomyosin

• troponin

• A Band

• I Band

thick filaments

mainly made up of the contractile protein MYOSIN

• large protein molecule with a globular head attached to a long tail

• 300 myosin molecules

• tails: lie along long axis

• heads: extend outwards (forms crossbridges)

• myosin molecules in the 2 halves of each filament are oriented in opposite directions → all tails directed toward center (central area w no heads)

thin filaments

made up of the contractile protein ACTIN, plus proteins tropomyosin and troponin

actin

globular (G) actin subunits (contain myosin BINDING site)

• G actin subunits are helically intertwined into a filament = F actin

tropomyosin

thread like protein extending end to end along the actin surface (1 per 7 G-actin subunits)

• blocks myosin binding sites (active site) on actin

• regulatory protein

Troponin

small protein bound to tropomyosin; can bind Ca++

• regulatory protein

Striations

• occur due to actin and myosin organization in skeletal and cardiac muscle

• A bAnd = dArk band

  • thick filaments + overlapping think and thick filaments

• H zone

• M line

A band

extends entire length of thick filament

H zone

lighter region in middle of A band (o think fils)

M line

proteins at center of H zone (middle of sarcomere)

I Band

• lIghter band, thin filaments only

• Z-disc

Z-disc

narrow line bisecting I band

• protein to which thin filaments are anchored

Sarcomere

compartmental arrangement of the filaments

• each segment of myofibril from Z to Z

• functional contractile unit of muscle fiber

Dystrophin significance

skeletal muscles also contain structural proteins (example - see relationship to muscular dystrophy)

Main Steps of Contraction of a Skeletal Muscle Fiber

1. nervous system excites a muscle fiber

2. excitation-contraction coupling (cross-bridging)

3. muscle CONTRACTION

4. muscle RELAXATION

Neuromuscular Junction

Synapse between the motor neuron (mn) and the muscle fiber

• usually one per skeletal muscle

components of neuromuscular junction

motor neuron

motor end plate

cleft

motor neuron

(presynaptic membrane) somatic nerve cell supplying the neural stim for skeletal muscle fiber contraction

motor end plate (MEP)

region of fibers plasma membrane (sarcolemma), which lies directly under terminal portion of motor neuron axon - post synaptic membrane

cleft

separates the motor neuron and motor end plate

Excitation of a Skeletal Muscle Fibers Process

1. Release of Acetylcholine

2. Acetylcholine binds to acetylcholine receptors on fibers MEP

3. Production of Muscle Action Potential

4. Termination of Acetylcholine activity

What occurs at the release of acetylcholine stage of excitation?

• motor neuron transmits electrical impulses

• action potential reach the synaptic end bulb

• acetylcholine molecules released from motor neuron ending by exocytosis

• acetylcholine diffuse across gap (cleft)

What occurs at the Acetylcholine binds to Acetylcholine receptors on fibers MEP?

Na+ (sodium) channels open

What occurs during the production of the muscle action potential during the excitation process?

• depolarization of plasma membrane at MEP (EPP) due to NA+ channels opening leads to electrical impulse (AP)

  • AP propagates (travels) along the membrane

• AP initiates a series of intracellular events → the mechanical event of contraction

what occurs at the termination of ACh activity during the excitation of a skeletal muscle fiber?

breaking down acetylcholine

acetylcholinesterase

enzyme in cleft that breaks down Acetylcholine

slow acetylcholinesterase leads to an increase of acetylcholine in cleft which leads to increased muscle strength

Botulinum toxin

blocks exocytosis of acetylcholine from mn

curare

blocks acetylcholine receptors

Excitation Contraction Coupling

series of events by which a propagated action potential leads to thick and thin filament interaction

process of Excitation Contraction Coupling

1. propagated ap passes from the sarcolemma along the t-tubule

2. as the action potential passes along t-tubules it causes the opening of Calcium channels in the sarcoplasmic reticulum (SR) → Ca++ enters the cytosol

3. CA++ binds to troponin causing troponin to change shape

4. this conformation moves tropomyosin away from the myosin binding site on actin → cross-briding between thick and think filaments - > leads to muscle contraction

Contraction cycle

events that cause the filaments to slide

contraction cycle process

1. ATP hydrolysis

2. cross-bridges

3. power stroke

4. detachment of myosin from actin

ATP hydrolysis

energizes the myosin head

ATP → ADP + P

Cross-bridges

myson head attaches to actin

power stroke

cross-bridges rotate toward center of sarcomere

what causes detachment of myosin from actin

due to ATP binding

sliding filament theory

1. myosin cross bridges with actin 2. (ADP and P1 released) myosin power strokes (pulls towards the M line of sarcomere) 3. once new ATP attaches to myosin -> cross bridge detaches 4. atp breaks down into adp and p1 to allow the myosin to attach to an actin again

load

force exerted by the object (against tension)

Relaxation

why contraction doesn’t continue indefinitely

• Ca++ions free in the cytosol for only a short time

  • Ca++ actively pumped back into SR (see again how ATP is needed for relaxation) → troponin strengthens attachment w/ actin → tropomyosin moves back into blocking position until another AP arrives causing Ca++ release

muscle metabolism

• amount within fibers at the start of contractile activity is small

  • if a muscle fiber is to sustain contractile activity, more ATP must be produced

ATP

immediate energy source

3 ways a muscle fiber can form ATP

1. phosphorylation of ADP by creatine phosphate (CP) (unique to muscle fibers)

2. oxidation phosphorylation of ADP in the mitochondria (aerobic metabolism)

3. Substrate phosphorylation of ADP (anaerobic metabolism)

Creatine phosphate

• Immediate Energy

• phosphorylation of ADP by CP = rapid means of forming ATP

• CP + ADP ← → Creatine + ATP

• creatine kinase (CK) catalyzes reaction

• provides only enough ATP to support muscle contraction during strenuous exercise for few additional seconds

  • supports activities that require short bursts of intense muscle

Nutrients in aerobic respiration

glucose, glycogen, fatty acids

• breakdown within fibers provide ATP required to support continued activity

Aerobic Metabolism (respiration)

• Long Term Energy

• occurs if sufficient O2 available to muscle

• produces ATP by breaking down glycogen, glucose, and/or fatty acids

• A LOT OF INCREASE ATP but is slow and requires oxygen

• supports primarily light to moderate exercise

role of exercise in aerobic metabolism

exercise leads to an increase in breathing rate and depth and an increase in blood flow to skeletal muscle which also leads to increase oxygen delivery

pros and cons of aerobic metabolism

pros: increase in ATP

cons: slow

anaerobic metabolism (respiration)

• short term energy

• breaks down glycogen and glucose into lactic acid (non-O2 utilizing)

• occurs during periods of intense muscular activity when O2 cannot be supplied fast enough

• faster…can produce more ATP than aerobic metabolism over a limited time

• uses increased glucose

• generates H+

pros and cons of anaerobic metabolism

pros: no oxygen needed, fast

cons: less ATP, generates acid

Muscle fatigue

inability of a muscle to maintain a particular strength of contraction overtime

What might cause muscle fatigue

may be due to decreased nutrients, decrease calcium release from SR, and increased H+ (muscle acidity)

note - psychological fatigue or central fatigue is different = feeling tired occurs before muscle fatigue

oxygen debt

difference between the resting rate of O2 consumption and the increase rate following exercise = recovering oxygen uptake

contractile elements

those muscle structures actively involved in contraction (ex. thick and thin filaments)

series elastic elements

structures that resist stretching (but can be stretched)

• located between the contractile elements and load including CT (ex. tendon)

internal tension

the force generated by the contractile elements

external tension

force exerted on load

Tension

force exerted by contracting muscle on an object (opposite from load)

isotonic contraction

constant tension in muscle while length changes (ex. when muscle is moving a load)

concentric isotonic contraction

muscle shortens

eccentric isotonic contraction

muscle lengthens

isometric contraction

muscle develops tension, but no length change

example - when maintaining posture OR attempting to move a load that is greater than the tension developed

twitch contraction

the mechanical response of a muscle fiber or motor unit to a single AP

• muscle contracts rapidly, then relaxes

• last longer than AP

  • 3 phases (latent, contraction, relaxation)

Latent period

delay immediately following stimulus arrival; short (a few msecs); associated with excitation - contraction coupling processes

contraction period

tension develops, cross bridges form