Muscle Contraction Notes

MyoFibrol & Sarcomere

• The sarcomere is the smallest functional unit of muscle tissue.
• It gives muscle its striated appearance due to actin and myosin filaments.
  • Myosin: Thicker filament.
  • Actin: Thinner filament.

Striations in Skeletal Muscle

• Dark band: Where myosin is located.
• Lighter bands: Areas with no myosin.
• Alternating bands of dark and light create striations.
• Myosin blocks light, while actin allows more light to pass through.

Sarcomere Function

• Myosin heads grab onto actin and pull it to contract the sarcomere.
• Contracted sarcomere: Less space.
• Relaxed sarcomere: More space.

Sarcomere Definition

• Each packet of myofilaments and regulatory proteins (troponin and tropomyosin) is a sarcomere.
• Sarcomere: Smallest functional unit of a skeletal muscle fiber.
• Each myofibril contains nearly 10,000 sarcomeres end to end.

Myofibril Composition

• Sarcomere: From Z line to Z line (also called Z discs).
• I band: Light band.
• A band: Dark band.

Bands Explanation

• Dark band (A band): Where myosin is located.
• Light band (I band): Where actin is located alone.
• Red fiber: Thick myosin filament.
• Blue fiber: Actin (thin filament).

Structures Surrounding Myofilaments

• Sarcoplasmic reticulum surrounds each myofilament.
• T tubule surrounds it with an opening to the sarcolemma (plasma membrane of the muscle cell).

T Tubules

• Projections found deep in skeletal and cardiac muscles.
• Membranes contain many ion channels, pumps, and transporters.
• Permit rapid transmission of an action potential into the cell.
• Regulate cellular calcium concentration.

Sarcolemma

• Plasma membrane of the skeletal muscle fiber.
• Acts as a barrier between extracellular and intracellular compartments.
• Defines the individual muscle fiber from its surroundings.

Sarcoplasmic Reticulum

• Membrane-bound structure within muscle.
• Stores and releases calcium ions.
• Calcium facilitates the interaction between actin and myosin during contractions.

Sarcomere Structure Details

• M line: Holds myosin fibers together in the middle of the sarcomere (center).
• Z line: Contains actin fibers or actin protein filaments.
• I band: Contains only thin filaments (actin).
  • Mnemonic: second letter in lIght band is I
• A band: Dark band made up of myosin.
  • Mnemonic: second letter in dArk band is A
• Z discs: Define the boundaries of each sarcomere and become smaller during contraction.
• H zone: Area between the M line and where actin filaments end; contains only thick filaments.

Muscle Contraction Steps Overview

• Three categories:
  • Steps to initiate contraction.
  • Contraction cycle.
  • Steps to end contraction.
• Begins with a message from the nervous system.
  • Action potential travels down the axon.
  • Triggers vesicles filled with acetylcholine to bind with the cell membrane and release contents into the synaptic cleft.
  • Acetylcholine binds with receptors on the motor end plate, triggering a change in membrane permeability.
  • Sodium ions rush in and cause an action potential.
• Action potential travels from the motor end plate along the sarcolemma down the T tubule to the sarcoplasmic reticulum.
• Sarcoplasmic reticulum releases calcium.
• Calcium binds to troponin on the actin filament, moving tropomyosin away from active sites on actin.
• Myosin head binds to actin and performs the power stroke (ATP dependent).
• Acetylcholinesterase enzyme in the synaptic cleft breaks down acetylcholine.
• Stops the release of calcium, covering active sites on actin, preventing myosin binding.

Steps to Initiate Contraction

• Acetylcholine is released and binds to acetylcholine receptors on the sarcolemma.
• Action potential reaches the T tubule.
• Sarcoplasmic reticulum releases calcium.

Contraction Cycle

• Begins with the release of calcium.
• When a sarcomere is resting, each of the myosin heads are energized with $ATP$.
• The breakdown products of $ATP$, $ADP$, and phosphate remain bound to the myosin head

Active Site Exposure

• Calcium binds to troponin, weakening the bond between actin and the troponin-tropomyosin complex.
• This leads to the exposure of active sites on the actin molecules.

Myosin Head Binding

• Myosin head binds to the exposed active site on actin, forming a cross-bridge.

Myosin Head Pivoting

• The energy stored in the myosin head is used to pivot the myosin heads towards the M line of the sarcomere.
• $ADP$ and phosphate are released.

Cross-Bridge Detachment

• Another $ATP$ molecule binds to the myosin head.
• Breaks the link between the active site on the actin molecule and the myosin head.
• The active site is exposed and can form another cross-bridge.

Myosin Reactivation

• Myosin reactivates when the free myosin head splits $ATP$ into $ADP$ and releases energy.
• Energy is used to recock the myosin head, putting it back into position for the next contraction cycle.
• This entire cycle repeats many times as long as calcium and $ATP$ reserves are sufficient.

Ending the Contraction

• If the stimulus is removed, the action potential ends.
• Calcium ion pumps pull calcium from the cytosol and store it within the terminal cisternae.
  • Terminal cisternae: Parts of the sarcoplasmic reticulum next to the T tubule.
• Troponin shifts positions, causing tropomyosin molecules to cover the active sites.
• Prevents further cross-bridge formation and muscle contraction.

End Steps

• Acetylcholine is broken down at the neuromuscular junction, ending the action potential.
• Sarcoplasmic reticulum reabsorbs calcium to reduce the amount available near the actin.
• Active sites become covered: tropomyosin rolls back over because troponin is not bound to calcium.
• Prevents further cross-bridge formation.
• Contraction ends, and the muscle relaxes.
• With no further exposure sites, the muscle contraction ends, and the muscle returns to its resting length.