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

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.

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…

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).

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.

The Contraction Cycle

The Contraction Cycle- Filament Level 1 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

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 2000-3000! A muscle twitch is when a nerve signals a motor unit without your permission. For shame neuron. For shame.

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

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

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 24-72 hrs later… minute tears are. Its totes ok. Making you stronger. Stretch!

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

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)