lecture 32 - skeletal, smooth (and cardiac) muscle 1 - PoNF

functions of muscle

generate force and movement

maintain posture

stand against gravity

what are muscles

bundles of muscle fibres held together by connective tissue sheaths

how are muscles attached to bone

tendons

how are muscle cells replaced after injury

satellite cells

- differentiate to form new muscle cells

- type of myoblasts

- limited supply

when a muscle is injured, how do other muscles compensate

hypertrophy

do muscles ever completely recovery

no

3 main types of muscle

skeletal, smooth, cardiac

structure of skeletal muscle

striated

multinucleate

contractile proteins

peripheral nuclei

striated muscle

skeletal

cardiac

Formation of Skeletal Muscle Fibers

formed in utero from mononucleate MYOBLASTS

what are striations

bundles of myofibrils

stripes and line markers of skeletal muscle

thicker lines - myosin

thinner lines - actin

Z lines - denser protein deposits which help link together all the actin filaments

What is a sarcomere?

contractile unit of a muscle fiber

how are myosin filaments arranged? and why?

3D space in triangular patterns and the actin filaments filaments surround in a hexagonal pattern

- gives strength and durability

- helps sliding filament theory

- safety mechanism

what happens to a muscle when it contracts

shortens and thickens

what causes shortening of the muscle in contraction (sliding filament theory) (5)

actin and myosin filaments slide past each other in an active process causing a contraction

myosin has lots of cross-bridges

Z discs of sarcomere move closer towards each other and towards M line

I band reduced (only actin)

H zone reduced (only mysosin and M line)

what remains constant despite shortening of sarcomere?

A band

Function of myosin heads

Stores ADP until contracting which then stores ATP

myosin cross bridges

attach to thin filament and force thin filament toward center of sarcomere

binding sites of myosin cross bridge

2 actin binding sites

2 ATP binding sites

when do muscles fail to relax

ATP levels fall way below normal - actin and myosin do not detach from each other

power stroke of skeletal muscle

movement of myosin head that releases ADP

cross bridge cycle (after Ca2+ removes troponin from tropomyosin) (4)

1 myosin head binds to bidning site on actin - cross bridge formed

2 myosin head bends and pulls actin filaments together (power stroke, ADP release)

3 ATP binds to myosin head so it can detach

4 myosin head hydolyses ATP to ADP and Pi and uses energy released to return to original position

increased calcium levels effect on contraction

more contraction

function of troponin, tropomyosin and Ca2+ during contraction

Ca2+ regulates contraction

tropomyosin partially covers myosin binding site on actin

held in position by troponin

Ca2+ binds to troponin

troponin alters shape - pulls tropomyosin away allowing myosin heads to bind to actin

removal of calcium - blocks site again

describe the process of excitation-contraction coupling (4)

action potential in transverse tubule depolarises DHP

DHP (special Ca2+ channel) opens RyR in tubule

Ca2+ released from sarcoplasmic reticulum lateral sac into cytoplasm

Ca2+ binds to troponin allowing actin-myosin binding

describe the process of excitation-contraction relaxation (4)

sarcoplasmic Ca2+ATPase pumps Ca2+ back into SR

decrease free cytosolic Ca2+

Ca2+ unbinds from troponin

tropomyosin recovers myosin binding site on actin

why does blood surround muscles

full access to ATP and O2 in muscles

removal of waste products such as CO2 and ADP + Pi

what is the motor unit

motor neurones + muscle fibres

small motor unit innervation

1 motor neurone innervates FEW muscle fibres (fine motor control)

large motor unit innervation

1 motor neurone innervates MANY muscle fibres (simple movements like squat jumps)

force exerted by muscle

tension

force exerted ON a muscle

load

contraction with constant length

isometric contraction

contraction with shortening of length

isotonic/concentric contraction

twitch contraction

quick, jerky response to a stimulus - single action potential

latent period of muscle twitch

period after stimulus before contraction begins

3 muscle twitch phases

latent phase

contraction phase

relaxation phase

as muscle load increases

contraction velocity and distance shortened decreases

tetanus

small action potentials - summation

multiple frequent stimuli - max tension

Why is tetanic tension greater than twitch tension?

[Ca2+] never gets low enough to allow troponin/tropomyosin too re block myosin binding sites

unfused tetanus (most common)

some relaxation occurs between contractions

fused tetanus

No evidence of relaxation before the following contractions

The result is a sustained muscle contraction

less overlap of filaments

less tension

too much overlap of filaments

too much tension that filaments interfere with each other

Muscles work in

antagonistic pairs