Plasma Membrane, Graded Potential, and Action Potential

The plasma membrane's characteristics, specifically the uneven distribution of ions between the intracellular and extracellular environments, and its selective permeability, are crucial for maintaining membrane potential.
Controlling the opening and closing of specific ion channels regulates the influx or efflux of electrolytes, which in turn maintains cell viability by managing intracellular and extracellular polarity, leading to varied membrane potentials.
At rest, this is referred to as the resting membrane potential.

The concentration gradient is the primary driving force for ion movement.
Membrane permeability is a characteristic feature of the plasma membrane.
The combination of concentration gradient and membrane permeability gives rise to excitable cells, which can generate electrical signals (e.g., in neurons).
These electrical signals include graded potentials and action potentials.

The permeability of ion channels within membranes is a function of the specific channel, meaning different channels activate under different circumstances.
Changes in permeability (opening and closing of channels) are highly dependent on environmental conditions.
- Chemical Stimuli: Ligands or substrates binding to receptors (e.g., neurotransmitters).
- Physical Stimuli: Changes in tension, temperature, detected by sensory neurons.
Environmental conditions alter channel permeability allowing ion influx or efflux, which changes polarity and generates membrane potential.
Most channels involved in membrane potential changes due to environmental stimuli are gated channels.

Three distinct ways to change membrane potential involve altering the permeability of specific membrane channels to allow charged particles (ions) to pass through.
The movement of charged particles through channels generates an electrical current.
Channel Conductance: Refers to the flow of current through these channels.
- Activation: Opening of a channel.
- Inactivation: Closing of a channel.
While activation implies a cellular response, it doesn't always guarantee an action potential. The opening and closing (activation/inactivation) mechanism generates signals that can propagate throughout the body.

Leaky Channels: These channels are open most of the time.
- Example: Leaky potassium channels are primarily responsible for establishing the resting membrane potential, aided by the $\text{Na}^+/\text{K}^+\text{ ATPase}$ pump.
- Note: These channels are not always simple and can be influenced (e.g., in sibafish, hypoxic conditions can close leaky potassium channels, leading to increased excitability).
Ligand-Gated Channels: Open when a specific ligand (chemical substance) binds to a receptor on the channel.
- Example: Acetylcholine (ACh) binding to its receptor causes such a channel to open.
Voltage-Gated Channels: Open or close in response to changes in membrane potential.
- Example: If the resting membrane potential of $-70 \text{ mV}$ changes to a threshold potential of, for instance, $-50 \text{ mV}$ , these channels open.
Mechanical Channels: Open in response to physical deformation or pressure.
- Example: Poking the skin activates sensory neurons, leading to the opening of these channels, generating an action potential if strong enough.

Most channels influencing membrane potential changes cause an influx (ions entering the cell) of $Na^+$ and $Ca^{2+}$ into the cell, leading to depolarization (excitation).
An efflux (ions leaving the cell) of $K^+$ from the cell is a well-known mechanism that typically leads to hyperpolarization (inhibition).
Excitatory events are generally associated with positive charges moving from the extracellular space into the intracellular space.
Inhibitory events are generally associated with positive charges moving from inside the cell to outside the cell.

Location: Occur only at the dendrites and cell body of a neuron.
Depolarization: Caused by the influx of $Na^+$ (and sometimes $Ca^{2+}$ ), leading to excitation.
Hyperpolarization: Caused by the efflux of $K^+$ , leading to inhibition.
Terminology for Membrane Potential Changes:
- Depolarization: Membrane potential becomes less negative (e.g., from $-70 \text{ mV}$ to $-40 \text{ mV}$ or $-30 \text{ mV}$ ). This is a decrease in polarity (less polarized) but an increase in the numerical value of the membrane potential (VM becomes more positive).
- Hyperpolarization: Membrane potential becomes more negative (e.g., below resting potential). This is an increase in polarity (more polarized) and a decrease in the numerical value of the membrane potential (VM becomes more negative).
Nature: Graded potentials are inhibitory when they cause hyperpolarization and excitatory when they cause depolarization.
Neural Integration: Dendrites receive multiple signals (excitatory and inhibitory) from various sources. These signals are integrated, akin to a