Nervous System Physiology: Membrane Potentials and Action Potentials

The Nervous System: Membrane Potentials and Action Potentials

Membrane Potentials

The membrane potential is a fundamental property of excitable cells, such as neurons and muscle cells. It refers to the voltage difference across the cell membrane, which is essential for the transmission of electrical signals.

• Resting membrane potential: The baseline electrical charge difference across the plasma membrane when the cell is not actively sending a signal.

• Unlike most other cells, neurons are highly excitable and can rapidly change their membrane potential.

Basic Principles of Electricity in Neurons

Neuronal function relies on the movement of ions across the membrane, which generates electrical currents and potentials.

• Role of membrane ion channels: Proteins that allow specific ions to pass through the membrane.

• Types of ion channels:

  • Leakage channels: Always open, allowing ions to diffuse according to their concentration gradients.

  • Gated channels: Open or close in response to specific stimuli.

    • Chemically gated (ligand-gated) channels: Open in response to binding of a chemical messenger (e.g., neurotransmitter).

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

    • Mechanically gated channels: Open in response to physical deformation of the receptor (e.g., touch).

• When gated channels are open, ions diffuse along their electrochemical gradients.

Generating the Resting Membrane Potential

The resting membrane potential is established by differences in ionic composition and membrane permeability.

• Measuring the charge difference: The cytoplasmic side of the membrane is negatively charged relative to the extracellular side.

• Potential generated by:

  • Differences in ionic composition of intracellular fluid (ICF) and extracellular fluid (ECF).

  • Differences in plasma membrane permeability.

• Key ions: Sodium (Na+), Potassium (K+), and Chloride (Cl-).

• K+ plays the most important role in establishing the resting membrane potential.

Differences in Ionic Composition

• ECF has higher concentration of Na+ than ICF.

• ICF has higher concentration of K+ than ECF.

Differences in Plasma Membrane Permeability

• Impermeable to large anionic proteins.

• Slightly permeable to Na+ (through leakage channels).

• Quite permeable to Cl-.

• 25 times more permeable to K+ than Na+ (more K+ leakage channels).

• More potassium diffuses out than sodium diffuses in.

• Sodium-potassium pump (Na+/K+ ATPase): Maintains concentration gradients by pumping 3 Na+ out and 2 K+ in.

$\text{Na}^+/\text{K}^+ \text{ ATPase:} \quad 3\ \text{Na}^+ \text{ out},\ 2\ \text{K}^+ \text{ in}$

Changing the Resting Membrane Potential

Membrane potential changes occur when ion concentrations or membrane permeability change, producing electrical signals.

• Changes produce two types of signals:

  • Graded potentials: Short-lived, localized changes in membrane potential.

  • Action potentials: Long-lasting, propagated changes in membrane potential.

• Changes in membrane potential are used as signals for communication in neurons.

Key Terms

• Depolarization: Inside of membrane becomes less negative (more positive) than resting potential; increases probability of producing a nerve impulse.

• Hyperpolarization: Inside of membrane becomes more negative than resting potential; decreases probability of producing a nerve impulse.

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential, triggered by stimuli that open gated ion channels.

• Signal strength is key; graded potentials decay with distance.

• Named according to location and function:

  • Receptor potential: Sensory neurons.

  • Postsynaptic potential: Neuron graded potential.

Action Potentials (APs)

Action potentials are the principal means of long-distance neural communication. They are brief, rapid, and propagate along axons.

• Only certain cells (neurons and muscle cells) can generate APs.

• APs are long-lasting and do not decay with distance.

• Specific channels must be open for APs to occur.

Mechanisms of Channel Activation

• Sodium channels: Have two gates (activation and inactivation) and three states (closed, open, inactivated).

• Potassium channels: Have one voltage-activated gate (closed, then open).

Steps in Generating an Action Potential

1. Resting state: All gated Na+ and K+ channels are closed.

2. Depolarization: Na+ channels open, Na+ enters the cell, membrane potential becomes more positive.

3. Repolarization: Na+ channels inactivate, K+ channels open, K+ exits the cell, membrane potential returns to negative.

4. Hyperpolarization: Some K+ channels remain open, membrane potential becomes more negative than resting.

Threshold and the All-or-None Phenomenon

• Not all depolarization events produce APs.

• For an AP to fire, depolarization must reach threshold.

• At threshold:

  • Membrane is depolarized.

  • Na+ permeability increases.

  • Na+ influx exceeds K+ efflux.

  • Positive feedback cycle begins.

  • AP is all-or-none: it either happens completely or not at all.

Propagation of an Action Potential

Once initiated, an action potential is propagated along the axon, moving in one direction due to the sequential opening of voltage-gated sodium channels.

• Propagation in myelinated axons differs from unmyelinated axons.

• Sodium channels are voltage sensitive; AP occurs only in a forward direction.

Conduction Velocity

The speed at which an action potential travels along an axon depends on two main factors.

• Axon diameter: Larger diameter = faster conduction.

• Degree of myelination: Myelinated axons conduct faster than unmyelinated axons.

Types of Conduction

• Continuous conduction: Occurs in unmyelinated axons; AP travels along every part of the membrane.

• Saltatory conduction: Occurs in myelinated axons; AP jumps from one node of Ranvier to the next, greatly increasing speed.

Clinical Connection: Homeostatic Imbalance 11.2 (Multiple Sclerosis)

Multiple sclerosis (MS) is an autoimmune disease affecting the central nervous system, primarily in young adults.

• Immune system attacks myelin sheaths in CNS.

• Turns myelin into hardened lesions called scleroses.

• Impulse conduction slows and eventually ceases.

• Symptoms: Visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence.

• Treatment: Drugs that modify immune system activity.

• High blood levels of vitamin D may reduce risk of development.

Summary Table: Types of Ion Channels

Channel Type	Stimulus	Location	Function
Leakage Channel	None (always open)	Throughout neuron membrane	Maintains resting membrane potential
Chemically Gated Channel	Ligand (e.g., neurotransmitter)	Dendrites, cell body	Initiates graded potentials
Voltage-Gated Channel	Change in membrane potential	Axon hillock, axon	Initiates and propagates action potentials
Mechanically Gated Channel	Physical deformation	Sensory receptors	Initiates receptor potentials

Summary Table: Steps of Action Potential

Step	Channel State	Membrane Potential Change
Resting State	All gated Na+ and K+ channels closed	-70 mV (resting)
Depolarization	Na+ channels open	Membrane potential rises toward +30 mV
Repolarization	Na+ channels inactivate, K+ channels open	Membrane potential returns toward -70 mV
Hyperpolarization	Some K+ channels remain open	Membrane potential drops below -70 mV

Example: In a typical neuron, the resting membrane potential is approximately -70 mV. When a stimulus depolarizes the membrane to threshold (about -55 mV), an action potential is triggered, rapidly rising to +30 mV before repolarizing and briefly hyperpolarizing.

Additional info: The notes have been expanded to include definitions, mechanisms, and clinical relevance for a comprehensive understanding of membrane potentials and action potentials in neurons.