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 can summate to trigger an action potential.

• 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: These muscle cells can spontaneously generate action potentials without external input, crucial for maintaining the heartbeat.

• Endothelial Cells: These cells line blood vessels and interact with other cell types to regulate blood flow and maintain the blood-brain barrier.

• Purkinje System: Specialized cardiac fibers that conduct electrical impulses rapidly through the heart.

Sensory Receptors and Cranial Nerves

• Purely Sensory Cranial Nerve: The optic nerve is an example of a cranial nerve that is purely sensory, responsible for vision.

• Referred Pain in Myocardial Infarction: Pain from the heart attack can be felt in areas such as the jaw, due to shared neural pathways.

• Somatosensory Cortex Representation: The brain allocates more space to the upper lip, eyes, and hands, reflecting their sensitivity.

• Free Nerve Endings for Temperature and Pain: These are responsible for detecting changes in temperature and pain.

• Tonic Receptors: These receptors are activated during prolonged or continuous stimuli, such as heightened sensitivity to light after entering a dark room.

Table: Types of Sensory Receptors

Receptor Type	Stimulus	Function
Tonic Receptors	Prolonged/Continuous	Adapt slowly, maintain response
Phasic Receptors	Rapidly changing	Adapt quickly, respond to changes
Free Nerve Endings	Temperature, Pain	Detect harmful stimuli
Pacemaker Cells	Spontaneous depolarization	Initiate rhythmic activity

Additional info: The above notes expand on the brief points in the source material to provide a comprehensive overview suitable for General Chemistry students, including definitions, examples, and equations relevant to membrane potentials and action potentials.