Biology 20 AP: Unit F - Muscles

Muscle cells

highly specialized to convert chemical energy (ATP) into kinetic energy

3 main types of muscles

1. Smooth

2. Cardiac

3. Skeletal

Smooth muscle cells

non-striated (no lines)

single nucleus

contract involuntarily

walls of internal organs

sustain prolonged contraction without fatigue

locations: walls of blood vessels, digestive tract, internal organs, iris

Cardiac muscle cells

striated, tubular, & branched

single nucleus

contract involuntarily

walls of heart (only)

Skeletal muscle cells

striated & tubular

many nuclei

contract voluntarily

attached to bones of skeleton and eyelids

Skeletal muscle function

support: contraction of muscles opposes force of gravity

movement: allows for movement of bones (arms & legs) as well as eyes and face

maintain body temp: ATP breakdown releases heat then spreads throughout body

protection: pads bones and cushions organs

stabilize joints: tendons help hold bones to joints

Cooperation of skeletal muscles

when muscles contract, they shorten (PULL, not push)

contraction = work

relaxation = no work

muscles are found in pairs (one action always has an opposing action)

Hierarchy of muscle structure

muscles: largest unit; attached to bone by tendons

muscle fibres: organized into larger bundles; up to 20cm long

myofibrils: thousands of cylindrical subunits

myofilaments: protein structured responsible for muscle contraction

• actin

• myosin

Parts of skeletal muscle fibre

myogoblin

sarcolemma

sarcoplasm

sarcoplasmic reticulum

Myoglobin

oxygen-binding pigment (like hemoglobin)

stores oxygen for muscle contractions

Sarcolemma

membrane surrounding muscle fibre

regulates entry and exit of materials

Sarcoplasm

cytoplasm of muscle fibre

site of metallic processes

contains myoglobin and glycogen

Sarcoplasmic reticulum

smooth ER in muscle fibre

stores calcium ions for muscle contractions

intersection: Z lines

Actin

thin myofilament(s)

composed of globular actin proteins and ion receptor proteins called troponin and tropomyosin

strand of pearls

Myosin

thick myofilament(s)

composed of myosin proteins → two polypeptide chains wrapped around each other

ends have globular heads

look like golf clubs

Sarcomere

contractile unit of muscle cell

contains actin and myosin

Sliding filament model

sliding of actin past myosin during muscle contraction

Major steps of sliding filament model

1. Presence of calcium ions allow myosin head to attach to actin

2. Myosin head flexes; pulling on the actin filament

3. Myosin head releases and unflexes via ATP

4. Myosin reattaches to actin further down fibre

Role of calcium (specific details of step 1 of sliding filament model)

muscle relaxed: tropomyosin blocks myosin binding sites along the actin molecule

muscle contracted: Ca²⁺ bind to troponin on actin causing tropomyosin to reposition and expose myosin binding sites

Energy for muscle contraction

ATP produced before exercise gets used up very quickly → muscle cells must acquire new ATP

3 mechanisms to make ATP

What are the 3 mechanisms to make ATP?

1. Breakdown of creatine phosphate (no oxygen)

2. Aerobic cellular respiration

3. Fermentation (no oxygen)

Ways to use energy

1. ATP

2. Creatine phosphate

3. Glucose

4. Glycogen (animal carb storage in muscles & liver) → glucose

5. Lipids → fatty acids → glucose (skip if starving)

6. Protein → glucose

Creatin phosphate breakdown

4-6s

creatine phosphate: high energy molecule that is built up during rest → CANNOT directly generate muscle contraction

regenerates ATP by contributing a phosphate in the midst of sliding filaments

Aerobic cellular respiration

long term

provides majority of energy for muscles (up to 95%)

oxygen present → glucose (from glycogen) or fatty acids (from fat) converted to ATP

myoglobin: pigment that has higher affinity for oxygen than hemoglobin

• → provides oxygen to mitochondria at beginning of cellular respiration

Fermentation for lactic acid

up to a minute

accumulation of lactate in muscle fibre makes sarcoplasm more acidic → enzymes stop functioning

fermentation longer than 2-3 minutes → cramping, fatigue (lack of ATP therefore muscles cannot relax)

Brain vs. muscles

muscles lack oxygen → fermentation provides some ATP

brain cells lack oxygen for some amount of time → brain damage

muscles can use glucose OR fatty acids to make ATP

brain can ONLY use glucose

Effect of athletic training

increases muscle mitochondria

allows fatty acids to be used my muscles and saving blood glucose for brain

Rigor mortis

animals die → no more ATP → no release of myosin from actin myofilament (lasts ~36 hours)

Tendons

connect muscle to bones

Ligaments

connect bones to bones (within joints)

Sprain

stretching/tearing of ligaments

Atrophy

reduction of size, tone, and power of muscle; often caused by disuse

Hypertrophy

exercise induced increase in muscle mass

Myograms

force of a muscle contraction in a skeletal muscle with time

simple muscle twitch: three periods (latent, contraction, and relaxation)

muscle not allowed to relax completely between stimuli: contraction gradually increases in intensity until it reaches a maximum (tetanus) which is sustained until the muscle fatigues

Slow-twitch

type 1 of muscle fibres

dark

contain myoglobin and have many surrounding capillaries

contract slowly but resist fatigue

produce most ATP aerobically (tire only when fuel supply is gone)

glycogen and fat allow the abundant mitochondria to maintain steady, prolonged ATP production

best for endurance

Fast-twitch

type 2

light

little or no myoglobin and fewer blood vessels

adapted for rapid generation of power but fatigue quickly

rich in glycogen; large number of sarcomeres

depend on anaerobic energy production, putting them at risk of lactate accumulation

best for short-term

Intermediate fibres

fast twitch but high oxidative capacity (fatigues slowly)

increased by endurance training and genetics

Force & response times

skeletal muscles have different proportions of fast-twitch and slow-twitch fibres

force and response times of their contractions differ

Latent period of sliding filament theory

time it takes for the stimulus to reach the sarcoplasmic reticulum

Ca²⁺ diffuses into the spaces between actin and myosin, reposition tropomyosin

Contraction period of sliding filament theory

myosin heads attach to the actin molecules in a series of ratchet-like movements and pull the Z lines together

Relaxation period of sliding filament theory

ATP used to detach myosin heads from the actin

Ca²⁺ ions are pumped back into the sarcoplasmic reticulum

tropomyosin returns to block the binding of myosin heads to the actin

Z lines move apart and the myofilaments slide into resting positions