Membrane Potentials in Neurons and Muscle Cells

Distribution of Charges Across the Plasma Membrane

The plasma membrane of neurons and muscle cells exhibits an unequal distribution of positive and negative charges, resulting in electrical polarization.

• Polarization: One side of the membrane is more negative, while the other is more positive.

• Membrane Potential: The difference in charge across the membrane, measured in millivolts (mV). In resting neurons and muscle fibers, this is typically between -70 mV and -85 mV (the inside is more negative).

Selective Permeability of Plasma Membranes

Plasma membranes are selectively permeable, allowing certain ions to pass while restricting others.

• Negative Charge Contributors: Negatively charged proteins inside the cell membrane cannot cross the membrane, contributing to the negative internal environment.

• Positive Charge Contributors: Sodium ions (Na+) are more concentrated outside the cell, while potassium ions (K+) are more concentrated inside.

Ion Movement and Membrane Potential Maintenance

Specific ion channels facilitate the movement of Na+ and K+ across the membrane.

• K+ moves more easily through its channels than Na+.

• The Na+/K+ pump actively transports 3 Na+ out and 2 K+ in to maintain concentration gradients.

Equation:

Electrically Excitable Membranes

Voltage-Gated Channels

Neurons and skeletal muscle cells possess electrically excitable membranes containing voltage-gated channels.

• Voltage-Gated Channels: Proteins that open or close in response to changes in membrane potential.

• Small increases in Na+ permeability change the membrane potential, leading to the threshold for action potential initiation.

• When threshold is reached, voltage-gated Na+ channels open, allowing Na+ ions to rush in.

• Depolarization: The cell becomes less negative as Na+ enters.

• Subsequently, K+ channels open and Na+ channels close, leading to repolarization as K+ exits the cell.

Equation (Action Potential):

Action Potential Propagation

Action potentials travel along the membrane in a unidirectional manner, enabling rapid communication within the cell.

• Unidirectional Propagation: Ensures signals move in one direction.

• Refractory Period: Time during which the membrane cannot respond to another stimulus (~2 ms).

Neuromuscular Junction and Excitation-Contraction Coupling

Structure and Function of the Neuromuscular Junction (NMJ)

The NMJ is a specialized synapse where a motor neuron communicates with a skeletal muscle fiber.

• Motor Neuron: Nerve cell that transmits signals from the nervous system to muscle fibers.

• Axon Terminal: End of the motor neuron that releases neurotransmitters.

• Synaptic Cleft: Space between the neuron and muscle fiber.

• Motor End Plate: Region of the muscle fiber membrane that receives the signal.

Action Potential Transmission and Muscle Fiber Activation

Action potentials travel along the neuron, reaching the axon terminal and triggering the release of acetylcholine (ACh).

• ACh binds to receptors on the motor end plate, increasing Na+ permeability and causing Na+ influx.

• This generates an action potential in the sarcolemma (muscle cell membrane).

• The action potential moves down the T tubule, affecting the sarcoplasmic reticulum's (SR) permeability and causing Ca2+ ions to flood into the sarcomere.

• This process is known as excitation-contraction coupling.

Role of Calcium and Cross-Bridge Cycling

Calcium ions play a critical role in muscle contraction by binding to troponin and exposing active sites on actin filaments.

• Calcium Binding: Ca2+ binds to troponin, causing a conformational change that exposes actin's active sites.

• Cross-Bridge Formation: Myosin (thick filaments) binds to actin (thin filaments), forming cross-bridges.

• Myosin heads pivot, pulling actin filaments; the bond is then broken as myosin binds ATP and resets.

• This cycle repeats rapidly until excitation ceases, a process called cross-bridge cycling.

Muscle Fiber Contraction and Relaxation

Steps of Muscle Fiber Contraction and Relaxation

The following table summarizes the main steps involved in muscle fiber contraction and relaxation:

Additional info: Steps inferred and grouped for clarity; original table was incomplete.

Summary of Excitation-Contraction Coupling

• Action potential travels from neuron to muscle fiber via NMJ.

• ACh release and binding initiates muscle fiber depolarization.

• Ca2+ release from SR enables cross-bridge formation and cycling.

• ATP is required for myosin head detachment and resetting.

• Relaxation occurs when Ca2+ is reabsorbed and cross-bridge cycling ceases.