Nerve Cells, Membrane Potentials, and Action Potentials: Study Notes for GOB Chemistry

Nerve Cells and Nervous System Structure

Specialized Cells and Nerve Impulses

The nervous system is composed of specialized cells called neurons that transmit information using electrical signals known as action potentials (also called nerve impulses). These signals allow rapid communication throughout the body.

• Neurons: Cells that send and receive electrical impulses.

• Nerve fibers: Bundles of axons wrapped in connective tissue, forming cable-like structures for signal transmission.

• Glial cells: Support, protect, and nourish neurons.

Neuron Structure

Neurons have distinct structural components that enable their function.

• Cell body (soma): Contains the nucleus and organelles; integrates incoming signals.

• Dendrites: Receive incoming messages and direct them toward the cell body.

• Axon: Conducts electrical signals (action potentials) away from the cell body.

• Axon hillock: Region where action potentials are initiated.

• Axon terminals: Relay messages to other neurons or effector cells.

• Myelin sheath: Insulating layer formed by glial cells (e.g., Schwann cells) that speeds up signal transmission.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials "hop" during propagation.

Cell Projections (Extensions)

Dendrites and Axons

Dendrites and axons are specialized extensions of the neuron that facilitate communication.

• Dendrites: Receive and transmit incoming signals toward the cell body.

• Axons: Conduct action potentials away from the cell body to other cells.

Myelin Sheath and Signal Transmission

Function and Structure

The myelin sheath insulates axons, allowing faster conduction of action potentials. Schwann cells wrap around axons, forming multiple layers of phospholipid insulation.

• Insulation: Prevents loss of electrical signal and increases speed.

• Nodes of Ranvier: Small gaps between myelin segments where action potentials are regenerated.

• Multiple Sclerosis (MS): Disease that damages the myelin sheath, slowing or blocking nerve impulses.

Membrane Potential

Definition and Importance

The membrane potential is the voltage difference across a cell membrane due to the distribution of ions. It drives the movement of ions and is essential for nerve signal transmission.

• [ ]: Concentration of positive ions.

• Outside the cell: Relatively positive.

• Inside the cell: Relatively negative.

• At rest, the membrane potential is negative.

Ion Channels and Pumps

Ion channels and pumps regulate the movement of ions across the membrane, changing the membrane potential.

• Ion channels: Allow specific ions to move down their concentration gradient at certain voltages.

• Ion pumps: Actively move ions against their concentration gradient using energy (ATP).

• Sodium-potassium pump (Na+/K+ pump): Maintains the resting potential by pumping 3 Na+ out and 2 K+ in per ATP molecule.

Key equation:

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

Action Potentials

Definition and Function

An action potential is a rapid change in membrane potential that travels along a neuron, allowing communication over long distances.

• Stimulus: Any factor that changes the membrane potential (e.g., light, heat, chemicals).

• Action potential starts at the axon hillock if the stimulus is strong enough.

Phases of the Action Potential

The action potential consists of several distinct phases:

1. Stimulation: Increase in Na+ ions inside the cell; Na+ channels open.

2. Rising phase (Depolarization): Na+ ions flow in, making the membrane potential more positive.

3. Peak: Na+ channels close; K+ channels open.

4. Falling phase (Repolarization): K+ ions flow out, restoring negative membrane potential.

5. Recovery (Refractory phase): Na+/K+ pump restores ion gradients.

Summary Table: Events in an Action Potential

Phase	Na+ Channels	K+ Channels	Membrane Potential
Stimulation	Open	Closed	Becomes less negative
Rising phase (Depolarization)	Open	Closed	Rapidly positive
Peak	Closed	Open	Maximum positive
Falling phase (Repolarization)	Closed	Open	Returns negative
Recovery (Refractory phase)	Closed	Closed	Restored to resting

Action Potential Propagation

Mechanism

Action potentials propagate along the axon by causing voltage-gated Na+ channels in adjacent regions to open, creating a "domino effect" that rapidly spreads the signal.

• Propagation is necessary for signals to travel the full length of the neuron.

• Myelin sheath speeds up propagation by allowing action potentials to "hop" between nodes of Ranvier (saltatory conduction).

Summary Table: Action Potential Propagation Phases

Propagation Phase	Section A	Section B	Section C
Action potential initiated	Active	Resting	Resting
Propagation begins	Refractory	Active	Resting
Propagation continues	Resting	Refractory	Active

Muscle Fatigue and Ion Pumps

Role of Sodium-Potassium Pump

During muscle fatigue, sodium-potassium pumps may stop working, causing ions to accumulate in the wrong places and impairing nerve and muscle function.

• ATP is required for pump function.

• Failure of pumps disrupts ion gradients and action potential propagation.

Key Terms and Definitions

• Neuron: A nerve cell specialized for transmitting electrical signals.

• Action potential: A rapid, temporary change in membrane potential that travels along a neuron.

• Membrane potential: The voltage difference across a cell membrane due to ion distribution.

• Depolarization: The process of making the membrane potential less negative.

• Repolarization: The process of returning the membrane potential to a negative value.

• Refractory phase: Period during which a neuron cannot fire another action potential.

• Myelin sheath: Insulating layer around axons formed by glial cells.

• Node of Ranvier: Gap in the myelin sheath where action potentials are regenerated.

• Sodium-potassium pump: Membrane protein that maintains ion gradients by pumping Na+ out and K+ in.

Example: Action Potential in a Neuron

When a stimulus (such as a touch or chemical signal) reaches a neuron, it causes Na+ channels to open at the axon hillock. If the membrane potential rises above the threshold, an action potential is triggered and propagates down the axon, allowing the neuron to communicate with other cells.

Additional info: These notes expand on the original content by providing definitions, context, and structured tables for clarity. The material is highly relevant to GOB Chemistry, especially in the context of biological chemistry and the role of ions and membrane potentials in physiology.