Membrane Potentials Notes

Review of Neurons

  • Definition: Neurons are specialized cells that transmit electrical impulses throughout the body.

  • Structural Components:

    • Soma (Cell Body): Contains the nucleus and organelles, responsible for metabolic activities.

    • Dendrites: Receive signals from other neurons, increasing surface area for connections.

    • Axon: Conducts impulses away from the cell body; can be myelinated (faster transmission) or unmyelinated.

    • Axon Terminals: Release neurotransmitters to communicate with other neurons.

  • Nerve vs. Tract:

    • Nerve: Bundle of axons in the peripheral nervous system (PNS).

    • Tract: Bundle of axons in the central nervous system (CNS).

  • Nucleus vs. Ganglion:

    • Nucleus: Collection of neuron cell bodies in the CNS.

    • Ganglion: Collection of neuron cell bodies in the PNS.

  • Classification of Neurons:

    • By Structure:

      • Unipolar: Single process from cell body.

      • Bipolar: Two processes from cell body.

      • Multipolar: Multiple processes from cell body.

    • By Function:

      • Sensory Neurons: Transmit sensory information to the CNS.

      • Motor Neurons: Carry impulses from CNS to effectors (muscles/glands).

      • Interneurons: Connect sensory and motor neurons within the CNS.

Membrane Potential

  • Resting Membrane Potential: Depends on differences in ion concentration (sodium and potassium) and membrane permeability.

  • Voltage: Measure of electrical potential difference across a membrane.

  • Current: Flow of electrical charge (ions) dependent on voltage and resistance.

Types of Ion Channels

  • Ion Channels: Selectively allow certain ions to pass through the membrane.

    • Leakage Channels: Non-gated, always open, facilitating continuous ion flow.

    • Chemically-Gated Channels: Open in response to specific chemicals binding (ligand-gated).

    • Mechanically-Gated Channels: Open when physically deformed (bent or stretched).

    • Voltage-Gated Channels: Open in response to changes in membrane potential.

Resting Membrane Potential (RMP)

  • Exists only across the membrane, fluids inside and outside are electrically neutral.

  • Caused by:

    • Differences in ion concentration (K+ higher inside, Na+ higher outside).

    • Differential permeability to ions due to specific ion channels.

  • Leakage: More K+ leakage channels than Na+, resulting in negative charge inside the membrane.

  • Concentrations:

    • Higher Na+ outside and K+ inside the cell; due to ion pumps, ATP is spent moving Na+ out and K+ in.

Changing Membrane Potential

  • Neurons use changes in membrane potential as signals for communication.

  • Types of Potentials:

    • Graded Potentials: Local changes in potential, can be depolarizations (less negative) or hyperpolarizations (more negative).

    • Action Potentials: Long-distance signals that propagate along the axon after reaching a threshold potential.

  • Graded potentials are accumulative and can lead to action potentials if the summation exceeds threshold intensity.

Action Potentials

  • Phases:

    1. Resting State: All ion channels are closed, maintaining resting membrane potential.

    2. Depolarization Phase: Na+ channels open, Na+ enters cell, causing depolarization.

    3. Repolarization Phase: Na+ channels close, K+ channels open, K+ leaves, restoring negative charge inside.

    4. Restoration: K+ outflow continues until resting potential is achieved again.

  • All-or-None Principle: Action potentials occur fully or not at all; higher stimulus intensity is interpreted as increased frequency of action potentials, not larger action potential amplitudes.

Understanding the Relationship Between Current, Voltage, and Resistance

  • Current (I): Flow of electrical charge; can be influenced by the voltage across a membrane and the resistance to that flow.

  • Voltage (V): The potential difference that drives current; measured in volts (V).

  • Resistance (R): Opposition to the flow of charge; can be influenced by the properties of the membrane (e.g., ion channel availability).

  • Ohm’s Law: I = V/R; establishes the relationship between current, voltage, and resistance in biological systems.

Identifying Different Types of Membrane Ion Channels

  1. Ion Channels: Specialized proteins that allow ions to pass through the cell membrane.

    • Leakage Channels: Non-gated channels that remain open, allowing continual ion flow (e.g., K+ channels).

    • Chemically-Gated Channels (Ligand-Gated): Open in response to specific chemicals; crucial for synaptic transmission.

    • Mechanically-Gated Channels: Open due to physical deformation of the membrane (e.g., pressure/force).

    • Voltage-Gated Channels: Open in response to changes in membrane potential, essential for action potentials.

Defining Resting Membrane Potential and its Electrochemical Basis

  • Resting Membrane Potential (RMP): The voltage difference across the plasma membrane when a neuron is not transmitting signals, typically around -70mV.

  • **Factors Contributing to RMP:

    • Ion Concentration Gradients: Na+ higher outside; K+ higher inside.

    • Selective Permeability: Membrane is more permeable to K+, leading to the inside of the cell being more negative compared to the outside.

    • Ion Pumps (e.g., Na+/K+ pump): Actively transports Na+ out and K+ into the cell, contributing to the concentration gradient.

Comparing and Contrasting Graded Potentials and Action Potentials

  • Graded Potentials:

    • Changes in membrane potential that are localized and variable in size (amplitude).

    • Can be de/hyperpolarizing; the amplitude depends on the strength of the stimulus.

    • Are accumulative; summation can lead to threshold which triggers an action potential.

  • Action Potentials:

    • Rapid, large changes in membrane potential that travel along the axon once the threshold is reached.

    • Characterized by a depolarization phase followed by repolarization, and then a return to resting potential.

    • All-or-None Principle: Once initiated, action potentials always have the same amplitude; stronger stimuli increase the frequency of potentials rather than their amplitude.

Explaining the Generation of Action Potentials and Relevant Ion Movements

  1. Resting State: All ion channels are closed; resting membrane potential is maintained.

  2. Depolarization Phase:

    • Threshold potential is reached, causing voltage-gated Na+ channels to open. Na+ floods into the cell, leading to an increase in membrane potential (becomes positive).

  3. Repolarization Phase:

    • Na+ channels close and K+ channels open, allowing K+ to exit the cell, restoring the negative charge inside.

  4. Restoration: K+ channels remain open until resting potential is achieved, after which they close.

    • Refractory Period: Following an action potential, there is a brief period when the neuron cannot fire another action potential.

Key Takeaways

  • Understanding the interplay between current, resistance, and voltage is crucial for grasping how neurons and their ion channels function.

  • Recognizing the differences between graded and action potentials is essential for understanding neuronal signaling.