Resting Membrane Potential and Action Potentials: Key Concepts for General Chemistry

Resting Membrane Potential and Action Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell when it is not actively sending a signal. This potential is crucial for the function of excitable cells such as neurons and muscle cells.

• Key Contributors: The sodium-potassium pump, ion channels, and selective permeability of the cell membrane maintain the resting potential.

• Typical Value: For most neurons, the resting membrane potential is approximately -70 mV.

• Equation: The Nernst equation can be used to calculate the equilibrium potential for a particular ion: $E = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

• Example: The resting potential is essential for the transmission of nerve impulses.

Action Potentials

An action potential is a rapid change in membrane potential that travels along the membrane of excitable cells, such as neurons and muscle fibers.

• All-or-None Principle: Action potentials are triggered only if the membrane potential reaches a certain threshold; otherwise, no action potential occurs.

• Graded Potentials: These are changes in membrane potential that vary in magnitude and do not follow the all-or-none principle. They occur in response to stimuli of varying strength.

• Hyperpolarization and Action Potentials: Prolonged opening of chloride channels or potassium channels can cause hyperpolarization, making the neuron less likely to fire an action potential.

• Relative Refractory Period: After an action potential, a stronger stimulus is required to generate another action potential due to the efflux of potassium ions ($K^+$).

• Equation: The Hodgkin-Huxley model describes the ionic basis of action potentials: $I = C_m \frac{dV}{dt} + I_{\text{ion}}$

• Example: Action potentials are responsible for the transmission of signals in nerves and muscles.

Specialized Cells and Structures

• Pacemaker Cells: Cardiac muscle cells that spontaneously generate action potentials without external input, regulating the heartbeat.

• Endothelial Cells: These cells line blood vessels and interact with other cell types to maintain vascular health.

• Purkinje Fibers: Specialized cardiac fibers that conduct electrical signals rapidly to coordinate heart contractions.

Sensory Receptors and Neural Pathways

• Purely Sensory Cranial Nerve: Some cranial nerves are purely sensory, transmitting information such as vision or smell.

• Referred Pain: Pain felt in a location other than its source, such as pain from a heart attack felt in the jaw.

• Sensory Cortex Representation: The brain allocates more space to body regions with higher sensory input, such as the hands and face.

• Free Nerve Endings: Responsible for detecting changes in temperature and pain.

• Tonic Receptors: These receptors adapt slowly to stimuli and are active during prolonged stimulation, such as sensitivity to light after entering a dark room.

Comparison of Receptor Types

Receptor Type	Function	Adaptation Rate
Tonic Receptors	Detect prolonged or continuous stimuli	Slow
Phasic Receptors	Detect changes in stimulus intensity	Fast

Additional info: The above notes expand on the brief points in the original material, providing definitions, examples, and equations relevant to General Chemistry and introductory biochemistry/physiology.