Skeletal, Smooth, and Cardiac Muscle Physiology: Action Potentials, Contraction, and Neuromuscular Junctions

What is the resting membrane potential (RMP)?

The voltage difference across the plasma membrane when the cell is at rest, typically between -70 to -90 mV.

How is the resting membrane potential generated?

It is generated by the uneven distribution of ions, primarily K⁺ and Na⁺, across the plasma membrane, maintained by the Na⁺/K⁺ pump.

What role do leaky channels play in resting membrane potential?

Leaky K⁺ channels allow K⁺ to diffuse out of the cell, making the inside more negative.

What is the all-or-none principle in relation to action potentials?

All action potentials in a cell are identical and propagate without diminishing, similar to a domino effect.

What are the phases of an action potential?

Depolarization (inside becomes less negative/more positive) and repolarization (returns to resting membrane potential).

What triggers the depolarization phase of an action potential?

A neural signal causes gated Na⁺ channels to open, allowing Na⁺ to enter the cell.

What occurs during the repolarization phase of an action potential?

Gated K⁺ channels open, allowing K⁺ to leave the cell, returning the inside to a more negative charge.

What is the role of the neuromuscular junction (NMJ)?

It is the site where an action potential causes muscle fiber contraction through neurotransmitter release.

What neurotransmitter is released at the NMJ?

Acetylcholine.

What is excitation-contraction coupling?

The process by which an action potential leads to muscle contraction, involving Ca²⁺ release from the sarcoplasmic reticulum.

What structures are involved in excitation-contraction coupling?

T-tubules, terminal cisternae of the sarcoplasmic reticulum, and troponin.

How does the action potential propagate along the muscle fiber?

It travels along the sarcolemma and into the T-tubules, triggering Ca²⁺ release.

What is the function of the Na⁺/K⁺ pump?

It maintains the uneven distribution of Na⁺ and K⁺ across the plasma membrane by pumping 3 Na⁺ out and 2 K⁺ in.

What is the significance of voltage-gated ion channels?

They open and close in response to voltage changes, allowing specific ions to flow and contribute to action potentials.

What happens to the membrane potential during depolarization?

The inside of the membrane becomes more positive as Na⁺ enters the cell.

What is the role of troponin in muscle contraction?

Troponin binds Ca²⁺, causing a conformational change that allows cross-bridge cycling to occur.

What is the role of ligands in gated ion channels?

Ligands are signal molecules that bind to receptors, causing gated channels to open.

What is the concentration gradient of K⁺ and Na⁺ at resting membrane potential?

Higher [K⁺] inside the cell and higher [Na⁺] outside the cell.

What is the effect of stronger stimuli on action potentials?

Stronger stimuli produce more frequent action potentials, leading to greater neurotransmitter release.

What occurs at the synaptic cleft during neurotransmitter release?

Neurotransmitters diffuse across the cleft and bind to receptors on the post-synaptic cell.

What is the role of Ca²⁺ in muscle contraction?

Ca²⁺ released from the sarcoplasmic reticulum initiates the contraction process by enabling cross-bridge cycling.

What neurotransmitter is released from presynaptic vesicles to initiate muscle contraction?

Acetylcholine

What happens when acetylcholine stimulates Na⁺ channels on the postsynaptic membrane?

Na⁺ diffuses into the muscle fiber, initiating an action potential.

What is the role of action potentials in the T tubules?

They cause the sarcoplasmic reticulum to release Ca²⁺.

What does Ca²⁺ bind to on the actin during muscle contraction?

Troponin

What is the effect of Ca²⁺ binding to troponin?

It moves tropomyosin and exposes myosin head attachment sites.

What is released when ATP molecules on myosin heads are broken down?

Energy, ADP, and inorganic phosphate (P)

What is the first step in the cross-bridge movement cycle?

Myosin head is in a high-energy position from ATP breakdown.

What triggers the power stroke in the cross-bridge cycle?

The binding of myosin to actin after Ca²⁺ binds to troponin.

What happens during the power stroke?

Myosin heads bend and pull actin toward the H zone.

What occurs after ATP binds to the myosin head?

Detachment from actin.

What is the final step of the cross-bridge cycle?

ATP is hydrolyzed to ADP and P, returning the myosin head to a high-energy position.

How long does the cross-bridge cycle continue during muscle contraction?

As long as Ca²⁺ is present.

What process pumps Ca²⁺ back into the sarcoplasmic reticulum?

Active transport

What happens to troponin when Ca²⁺ unbinds from it?

The troponin-tropomyosin complex re-establishes its position, blocking active sites.

What is spastic paralysis?

A condition where muscles contract and cannot relax, caused by poisons that inhibit acetylcholinesterase.

What causes flaccid paralysis?

It occurs when ACh cannot bind to receptors, rendering the muscle incapable of contracting.

What is Myasthenia Gravis?

An autoimmune disorder where antibodies bind to and destroy ACh receptors, leading to flaccid paralysis.

What are the three phases of a muscle twitch?

Lag (latent), Contraction, and Relaxation.

What occurs during the lag phase of a muscle twitch?

It is the gap between stimulus and contraction.

What is the contraction phase of a muscle twitch characterized by?

Cross-bridge cycling occurs during this phase.

What happens during the relaxation phase of a muscle twitch?

Ca²⁺ is actively transported into the sarcoplasmic reticulum, taking longer than contraction.

What defines a motor unit?

A single somatic motor neuron and all the skeletal muscle fibers it innervates.

How does the size of a motor unit relate to muscle movement?

Small muscles have fewer fibers for delicate movements, while large muscles have many fibers for coarse movements.

What is the relationship between individual muscle fibers and force?

The force depends on the number of cross-bridges formed.

What is treppe in muscle physiology?

A graded response where each contraction is stronger than the previous due to increased Ca²⁺.

What is the difference between incomplete tetanus and complete tetanus?

Incomplete tetanus has partial relaxation, while complete tetanus has no relaxation between contractions.

What are the two types of muscle contractions?

Isometric (no length change, tension increases) and Isotonic (length changes, tension constant).

What are the characteristics of slow-twitch muscle fibers?

They have slow ATPase, smaller diameter, are aerobic, fatigue resistant, and are found in postural muscles.

What distinguishes fast-twitch muscle fibers?

They have fast ATPase, larger diameter, less blood and mitochondria, and fatigue faster, typically used for sprinting.

What is hypertrophy in muscle physiology?

The increase in muscle size.

What causes muscle fatigue?

It can be due to psychological factors, ATP depletion, acidosis, oxidative stress, or local inflammation.

What is rigor mortis?

A post-mortem condition where Ca²⁺ leaks in and, without ATP, cross-bridges remain formed until tissue deteriorates.

What are the defining features of smooth muscle?

It is not striated, has smaller fibers, is spindle-shaped with a single nucleus, and contains more actin than myosin.

What is the role of calmodulin in smooth muscle contraction?

Ca²⁺ binds to calmodulin, activating myosin kinase to form cross-bridges.

What are the two types of smooth muscle?

Single Unit (Visceral) and Multi Unit.

How is cardiac muscle similar to skeletal muscle?

Both are striated and have sarcomeres.

What unique feature does cardiac muscle share with smooth muscle?

It has gap junctions and is autorhythmic.