Membrane Potential and Action Potential in Nervous Tissue

Nervous Tissue: Membrane Potential and Action Potential

Introduction to Cell Membrane Potential

The cell membrane (also known as the plasma membrane) is a critical barrier that separates the cytoplasm of the cell from its external environment. This separation establishes a distinction between "self" (inside) and "non-self" (outside). The inner surface of the plasma membrane typically has a slight negative charge compared to the outer surface, due to the distribution of ions and proteins.

• Membrane potential refers to the difference in electric charge across the cell membrane.

• Positive ions (e.g., Na+) are more concentrated outside the cell, while negatively charged proteins are more concentrated inside.

• This charge separation creates an electrochemical gradient, which is a form of potential energy used by neurons for information transmission.

• The unit of membrane potential is the volt (V), typically measured in millivolts (mV) in biological systems.

Example: Typical resting membrane potentials: red blood cells (~-40 mV), neurons (~-70 mV), skeletal muscle cells (~-85 mV), cardiac muscle cells (~-90 mV).

Establishment and Maintenance of Resting Membrane Potential

The resting membrane potential is the membrane potential of an undisturbed cell. It is established and maintained by the selective permeability of the plasma membrane and the activity of ion channels and pumps.

• Extracellular fluid (ECF) contains high concentrations of Na+ and Cl-.

• Intracellular fluid (cytosol) contains high concentrations of K+ and negatively charged proteins.

• Ions cannot freely cross the lipid bilayer; they require ion channels (membrane proteins) to move in and out.

• Types of ion channels: sodium (Na+), potassium (K+), calcium (Ca2+), chloride (Cl-).

• Membrane resistance determines how easily ions can cross; high resistance means low ion flow, and vice versa.

• Opening or closing ion channels changes membrane permeability and thus the membrane potential.

Equation:

$V_m = V_{inside} - V_{outside}$

where $V_m$ is the membrane potential.

Types of Membrane Ion Channels

Ion channels are essential for controlling the movement of ions across the membrane, thereby influencing membrane potential.

• Leak channels: Always open, allowing ions to move according to their electrochemical gradients.

• Gated channels: Usually closed, open in response to specific stimuli. Types include:

  • Chemically gated (ligand-gated): Open when a specific chemical (ligand) binds.

  • Voltage-gated: Open or close in response to changes in membrane potential.

  • Mechanically gated: Open or close in response to physical deformation (e.g., pressure, vibration).

Example: Voltage-gated Na+ channels have two gates (activation and inactivation), allowing three states: closed but capable of opening, open, and closed/inactivated.

Graded (Local) Potentials

Graded potentials are changes in membrane potential that occur locally and do not spread far from the site of stimulation. They are produced when a stimulus opens a gated channel, allowing ions to flow and locally depolarize the membrane.

• Depolarization: Membrane potential becomes less negative (moves toward zero).

• Hyperpolarization: Membrane potential becomes more negative than resting potential.

• Local currents: Movement of ions away from the site of stimulation, causing the graded potential to spread in all directions but diminish with distance.

Example: Opening of Na+ channels causes Na+ influx, leading to local depolarization.

Action Potentials

An action potential is a rapid, propagated change in membrane potential that travels along the entire excitable membrane (such as an axon). It is initiated when a graded potential depolarizes the membrane to a threshold value.

• All-or-none principle: An action potential is triggered only if the depolarization reaches threshold; otherwise, none occurs.

• Action potentials do not diminish as they propagate.

• Phases of action potential:

  1. Depolarization to threshold (e.g., -60 mV)

  2. Activation of voltage-gated Na+ channels and rapid depolarization (up to +30 mV)

  3. Inactivation of Na+ channels and activation of K+ channels (repolarization)

  4. Closing of K+ channels and brief hyperpolarization (e.g., -90 mV)

  5. Return to resting membrane potential (e.g., -70 mV)

Equation:

$I = \frac{V}{R}$

where $I$ is the current, $V$ is the voltage difference, and $R$ is the resistance.

Refractory Period

The refractory period is the time during which a neuron cannot initiate another action potential. This ensures unidirectional propagation of the action potential.

• Analogy: Like a toilet that cannot be flushed again until the tank refills.

• Prevents backward movement of the action potential.

Propagation of Action Potentials

Action potentials are propagated along the axon by a chain reaction of depolarization events.

• Continuous propagation: Occurs in unmyelinated axons; the action potential moves along every part of the membrane.

• Saltatory propagation: Occurs in myelinated axons; the action potential "jumps" from node to node (nodes of Ranvier), increasing speed and efficiency.

• Saltatory propagation is much faster (up to 100 m/sec) and uses less energy than continuous propagation (1 m/sec).

Types of Nerve Fibers

Nerve fibers are classified based on diameter, myelination, and conduction speed. These properties correlate with their functional roles in the nervous system.

Type	Diameter	Myelination	Conduction Speed	Function
Type A	Large (4-20 μm)	Myelinated	Up to 120 m/sec	Rapid sensory/motor signals (e.g., position, balance, touch, skeletal muscle control)
Type B	Medium (2-4 μm)	Lightly myelinated	~18 m/sec	Less urgent sensory/motor signals
Type C	Small (<2 μm)	Unmyelinated	~1 m/sec	Slow signals (e.g., temperature, pain, general touch)

Example: Type A fibers transmit urgent information quickly, while Type C fibers carry less urgent signals slowly.

Additional info: If all axons were myelinated and large, nerves and the spinal cord would be physically much larger, so the nervous system balances speed and space.

Summary Table: Key Concepts

Concept	Definition	Example/Application
Membrane Potential	Electrical potential difference across the cell membrane	Resting potential in neurons: ~-70 mV
Graded Potential	Local change in membrane potential	Depolarization from Na+ influx
Action Potential	Rapid, propagated change in membrane potential	Nerve impulse along axon
Refractory Period	Time when neuron cannot fire again	Ensures one-way propagation
Saltatory Propagation	Action potential jumps between nodes	Fast conduction in myelinated axons