Muscle Contraction: Membrane, Potential & Synapse

Key Membrane Structures

• The sarcoplasmic reticulum is a specialized endoplasmic reticulum that is not contiguous with the plasma membrane.
• Myofibrils, plasma membrane, and mitochondria are also key membrane structures.
• Transverse tubules (T tubules) are extensions of the plasma membrane that extend into the muscle fiber.

Action Potential Propagation

• Transverse tubules (T tubules) facilitate the rapid propagation of an action potential from the muscle fiber surface to its interior.

Motor Neuron Action Potentials and Skeletal Muscle Contractions

• Motor neurons release Acetylcholine (Ach).
• Ach binds to nicotinic acetylcholine receptors (nAchR), which are ionotropic receptors permeable to Na+ and K+ ions, with an Echannel around 0mV.

Synaptic Transmission at the Neuromuscular Junction

• The steps are similar to those in an excitatory synapse.
• Acetylcholine (Ach) is removed from the synaptic cleft, similar to typical chemical neurotransmitters.
• A motor neuron action potential releases Ach, causing enough depolarization to initiate an action potential in skeletal muscle.
• This action potential leads to muscle contraction.

Muscle Fiber Membrane and T-Tubules

• Action potential propagates along the muscle fiber membrane and into T-tubules.
• The DHP receptor is a voltage-gated Ca2+ channel that physically interacts with a Ca2+ channel in the sarcoplasmic reticulum, known as the ryanodine receptor.
• Action potential arrival causes the DHP receptor to sense a voltage change, leading to a conformational change that opens the ryanodine receptor.
• Ca2+ enters the cytoplasm from the sarcoplasmic reticulum.
• Ca2+ binds to troponin, causing tropomyosin to shift and expose myosin binding sites on actin.
• Myosin cross-bridges bind actin, causing contraction.
• Ca2+ ATPase pumps Ca2+ back into the sarcoplasmic reticulum, reducing free Ca2+ available to bind troponin.
• Tropomyosin blocks myosin binding sites on actin again.

Muscle Contraction Facilitation

• DHP receptors are ion channels that do not need ions to flow through them to play their role.

Muscle Contraction

• At low cytosolic Ca2+ levels in a relaxed muscle, energized cross-bridges cannot bind to actin.
• During high cytosolic Ca2+ levels in an activated muscle, cross-bridge binding sites are exposed, allowing cross-bridges to bind to actin and generate force.

Myosin-Actin Interaction in Muscle Contraction

• Myosin interaction with actin causes muscle contraction.

Myosin-Actin Interaction and Force Generation Cycle

• The cycle involves the role of Ca2+, the interaction of cross-bridges with actin, movement of cross-bridges, and the roles of ADP, Pi, and ATP.
• Myosin has ATPase activity, hydrolyzing ATP to energize cross-bridges.
• ATP binds to myosin, causing cross-bridges to detach.

Muscle Fiber Tension

• Muscle tension is maintained with a disruption of the physical interaction between the DHP receptor and the ryanodine receptor.

Generating Contractions

• One must understand how a signal travels from a motor neuron to the contractile units within muscle fibers.
• Myosin-actin interactions are crucial for driving contraction.
• The regulation of force and considerations of energy are key aspects.
• Factors such as motor neuron frequency
• And different fiber/motor unit types play a role.

Functional Comparison of Muscle Types

• Focuses on structure and function with less molecular detail.

Regulation of Force

• Force is exerted by skeletal muscle fibers; increasing AP frequency in motor neurons increases force.

Muscle Contraction Force

• Force is increases by recruiting more motor units simultaneously.
• Motor units/muscle fibers vary structurally and functionally.

Muscle Fiber

• Types of muscle fibers and motor units include fast glycolytic fibers (Type 2X), fast oxidative glycolytic fibers (Type 2A), and slow oxidative fibers (Type 1).
• A single motor unit contains the same type of fibers; thus, "motor unit" can be substituted for "fiber."

Force Generation

• A fast motor unit generates force more rapidly with one contraction cycle.

Fiber Diameter

• Slow oxidative fibers are the smallest in diameter.
• Muscle exhibits a mix of fiber types interspersed.

Fiber Recruitment

• Fibers are recruited slowest to fastest.

Motor Unit Fatigue

• The slower the motor unit, the less easily it fatigues.

Muscle Fiber Type Comparison

• Slow oxidative fibers (Type 1) compared to fast oxidative glycolytic (Type 2A) fibers and fast glycolytic fibers (Type 2X).

Dense Capillaries

• It is important for slow oxidative fibers to have more dense capillaries than fast glycolytic fibers.

Smooth Muscle

• Smooth muscles surround hollow areas, regulating movement in spaces, such as blood vessels or the GI tract.

Cardiac Muscle

• Cardiac muscle is found only in the heart, generating blood flow.
• Coordinated contractions occur via electrical synapses (gap junctions) between cells.

Muscle Types Comparison

• Skeletal, smooth, and cardiac muscles differ in voluntary control, striated appearance, actin-myosin-based contraction, the presence of electrical synapses, and the role of neural input.