The following links outline the structure of the thick and thin
Note the structure of the myosin (thick) filament with the heads
(crossbridges) for grabbing and pulling the actin(thin) filament.
The actin (thin) filaments consisting of a helix of spherical
molecules with tropomyosin wrapped around and troponin
sitting on top.
• The signal from the brain travels through the axon until it reaches the
axon terminal at the motor end plate
• This signals the release of a neurotransmitter (acetylcholine) into
the synaptic junction
• The binding of acetylcholine allows sodium ions to flow through the
sarcolemma, causing depolarization of the membrane
• This depolarization spreads across the sarcolemma (action potential)
and down the transverse tubules (to the interior of the muscle
The following links will guide you through the activity that takes place at the
Neuromuscular junction 1 Neuromuscular junction 2
Ca2+ Ca2+ ATP
STRONG BINDING SITES
POWER STROKE ADP
ATP + Pi
7 ATP 3
Ca2+ Ca2+ 4 ATP SARCOPLASMIC RETICULUM
(THIN FILAMENT) MUSCLE
STRONG BINDING SITES
ACTI N 6
MYOSIN CROSSBRIDGE ATP ADP + Pi
MYOSIN (THICK FILAMENT) RELAXES
Sliding Filament Theory
- muscle cells or fibres contract by shortening of the myofibrils due to
actin(thin filaments) sliding over the myosin (thick filaments)
- the sarcomere shortens as the z-lines move closer together
- the I-band gets closer smaller (dist. b/w sucessive thick filaments)
- the the H-band disappears (dist. b/w the ends of the thin filaments)
- the the A-band doesn’t change (spans the thick filament)
I, A band
I, A bands
NO H zone
The energy for muscular contractions comes from the
breakdown of ATP by the enzyme myosin ATPase.
This enzyme is located on the head of the myosin cross-
The breakdown of ATP to ADP and Pi releases energy
which allows the heads of the cross-bridges to pull the
actin molecules over them, shortening the muscle.
- myosin cross-bridges are always attached to actin (unless moving
from site to site)
- cross-bridges move from “weak binding sites” to “strong binding sites”
- proteins tropomyosin and troponin restrict access of cross-bridges
to “strong binding sites”
- proteins tropomyosin and troponin restrict(inhibit) access of cross-
bridges to “strong binding sites”
- calcium released from the sarcoplasmic reticulum combines with
troponin which then causes the tropomyosin to move off the
“strong binding sites”
- myosin then binds with “strong site” to pull actin across itself
- one cycle shortens muscle ~1% of its resting length, cycle will
repeat until muscle is fully contracted (up to 60% of resting length)
Muscle contractions require a stimulus to get them started.
Depending on the nerve impulse, the magnitude or threshold of the
stimulus can vary.
The greater the threshold stimuli, the greater number of motor units
that will be recruited.
e.g. Greater weight → More motor units recruited
As long as the stimuli is of threshold strength, the muscle fibre will
contract to its fullest extent.
If the stimuli is not of threshold strength, the fibre will not contract. (no
partial contractions of muscle fibres)
Muscles consist of many motor units. Each of which reponds in an
all-or-none fashion. A whole muscle however can partially contract
depending on the number of motor units recruited.