1. The Sliding Filament Mechanism
Let’s take a “walk”.

2. Muscle Contraction- Summary
The Sliding Filament Mechanism
Myosin heads attach to actin and walk along the actin on towards both ends of the sarcomere.
Actin, therefore, moves toward the center of the sarcomere.
Z discs, therefore, come closer together and the sarcomere shortens.
When done in mass, the entire muscle shortens.

3. Give me a Sign! The Neuromuscular Junction
Muscles don’t twitch unless they receive a nerve impulse- they only do as they’re told. Very polite.
All or nothing
An entire cell will contract if given the sensory input. Strong contraction = most/all cells have been told to contract. Weak contraction = fewer cells have been told to contract.
Synaptic cleft
The nerve and muscle cells must make contact, but they never touch.
Neurotransmitters to the rescue!
Acetylcholine (ACh for short)
Ready to go!
Muscle might be relaxed, but not lounging around on the couch relaxed.
Myosin head is cocked and ready to go with ATP, it is just awaiting the stimulus.
When enough ACh floods the muscle cell, the muscle cells releases sodium ions…

4. The Contraction Cycle
Contraction begins with the influx of Na+ (triggered by…?)
The Na+ triggers the sarcoplasmic reticulum (SR) to release Ca2+ into the muscle cell
Ca2+ bind to actin to open the myosin binding sites (moves tropomyosin)
Formation of cross-bridges
Remember that myosin head already has ATP and is ready for this signal
A cross-bridge is what the formation is called when a myosin head attaches to the myosin binding site (on the actin).

5. The Contraction Cycle Cont…
Power stroke
The myosin tugs each side of the actin towards the center of the sarcomere causing the z disks to come closer together.
Detachment of myosin from actin
As the next ATP binds to the myosin head, the myosin head detaches from the actin
Repeat?
The contraction cycle repeats as long as ATP is available and the Ca2+ level is sufficiently high
The strength of the continuing contraction will be a combination of the strength of the contraction at the point of the second stimulus (it didn’t completely relax), as well as the new contraction.
This staircase effect is called treppe.

6. The Contraction Cycle

7. The Contraction Cycle- Filament Level1
Myosin heads
hydrolyze ATP and
become reoriented
and energized
Myosin heads bind to actin,
forming
crossbridges
Myosin crossbridges
rotate toward center of the
sarcomere (power stroke)
As myosin heads
bind ATP, the
crossbridges detach
from actin
Contraction cycle continues if
ATP is available and Ca2+ level in
the sarcoplasm is high
ADP
ATP P
= Ca2+
Key: 2 3 4

8. Control of Muscle Tension
The tension (force) of muscle contraction varies
Maximum tension is dependent on
The rate at which the nerve impulses arrive
The amount of stretch before contraction
Nutrient and oxygen availability
The size of the motor unit
Motor Units
Motor neuron and the muscle fibers it stimulates
The total muscle tension depends on the size of the motor units and the number that are activated.
Voice muscles have 2-3 muscle fibers per motor unit
Eyes have 10-23
Arms and legs have !
A muscle twitch is when a nerve signals a motor unit without your permission. For shame neuron. For shame.

9. Relaxation
ACh effects last only a brief amount of time b/c it is rapidly broken down.
The sarcolemma contains Ca2+ pumps to return Ca2+ back to the SR quickly
As the Ca2+ level drops, tropomyosin covers the myosin-binding site, causing the muscle to relax

10. ACh diffuses across
synaptic cleft, binds
to its receptors in the
motor end plate, and
triggers a muscle
action potential (AP).
Nerve impulse arrives at
axon terminal of motor
neuron and triggers release
of acetylcholine (ACh).
Synaptic vesicle
filled with ACh
ACh receptor
Acetylcholinesterase in
synaptic cleft destroys
ACh so another muscle
action potential does not
arise unless more ACh is
released from motor neuron.
Ca2+
Muscle action
potential
Nerve
impulse SR
Contraction: power strokes
use ATP; myosin heads bind
to actin, swivel, and release;
thin filaments are pulled toward
center of sarcomere.
Troponin–tropomyosin
complex slides back
into position where it
blocks the myosin
binding sites on actin.
Muscle relaxes.
Ca2+ active
transport pumps
Ca2+ release channels in
SR close and Ca2+ active
transport pumps use ATP
to restore low level of
Ca2+ in sarcoplasm.
Ca2+ binds to troponin on
the thin filament, exposing
the binding sites for myosin.
Muscle AP travelling along
transverse tubule opens Ca2+
release channels in the
sarcoplasmic reticulum (SR)
membrane, which allows
calcium ions to flood into the
sarcoplasm.
Elevated Ca2+ 1 2 3 4 9 5 6 7 8
Transverse tubule

11. Out of Breath? C6H12O6 + 6H2O + 6O2 12H2O + 6CO2 + ATP
All the mitochondria
Muscle cells have a lot of mitochondria in order to keep up with ATP demand.
62% is given off as heat = you’re sweaty
Still can’t keep up
Even with all the mitochondria, your muscle cells cant keep up.
Oxygen demands not glucose
Glycolysis still proceeds as normal, but Krebs cannot
Ouchie ouchie
Pyruvic acid is then converted to lactic acid, which doesn’t feel good in the moment.
Lactic acid is not the problem hrs later… minute tears are.
Its totes ok. Making you stronger. Stretch!

12. Rigor Mortis You think you’ll stop being hungry when you die?
Nope! Sorta.
Your body will start breaking down your own tissues to get ATP.
Longer in the big muscles.
Eventually you run out of ATP, and then…
Active transport stops, so…
Na+ and Ca2+ flood into sarcoplasm
Causing the tropomyosin to move, and cross-bridges to form
Without any more ATP the cross-bridges cannot be broken, but the muscle cannot contract

13. Botox Botulinum toxin Blocks release of ACh from nerve
May be found in improperly canned foods
A tiny amount can cause death by paralyzing respiratory muscles
Used as a wrinkle fix among other medical uses
Relaxes muscles that cause facial wrinkles, but can also be used to…
Help alleviate chronic back pain
Spasms of the vocal chords
Blepharospasm (uncontrollable blinking) and
Strabismus (crossed eyes)