Resting Membrane Potential

Definition and Generation

The resting membrane potential is the electrical potential difference across the plasma membrane of a resting neuron, typically ranging from -70 mV to -90 mV. The inside of the cell is negatively charged relative to the outside.

• Generated by:

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

  • Differences in plasma membrane permeability

• Key ions involved: Potassium (K+) and Sodium (Na+)

• ICF has higher K+ and negatively charged proteins; ECF has higher Na+ and Cl-

• Membrane is more permeable to K+ than Na+

• Sodium-potassium pump maintains the resting potential by pumping 3 Na+ out and 2 K+ in

Example: In a typical neuron, the resting membrane potential is about -70 mV, with the inside of the cell being negative compared to the outside.

Differences in Plasma Membrane Permeability

• Membrane is 25x more permeable to K+ than Na+ (more K+ leakage channels)

• Na+ diffuses in, but less than K+ diffuses out

• Cl- has high concentration outside the cell

• Sodium-potassium pump stabilizes the resting membrane potential

Changing the Resting Membrane Potential

Changes occur when concentrations of ions across the membrane change or membrane permeability to ions changes.

• Signals are produced by changes in membrane potential

• Two types of signals:

  • Graded potentials

  • Action potentials

Membrane Potential Changes and Signal Transmission

Depolarization and Hyperpolarization

• Depolarization: Decrease in membrane potential (inside becomes less negative)

• Hyperpolarization: Increase in membrane potential (inside becomes more negative)

Graded Potentials

• Short-lived, localized changes in membrane potential

• Triggered by a stimulus that opens gated ion channels

• Strength decreases with distance

Action Potentials (AP)

• Principal way neurons send signals over long distances

• Brief reversal of membrane potential: change of about 100 mV (from -70 mV to +30 mV)

• Do not decay over distance

• Also called a nerve impulse

Phases of an Action Potential

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

2. Depolarization: Na+ channels open, Na+ enters cell, membrane potential becomes less negative

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

4. Hyperpolarization: Some K+ channels remain open, Na+ channels reset

Threshold: Minimum depolarization needed to trigger an AP, typically 15-20 mV above resting potential.

All-or-None Principle: An AP either happens completely or not at all.

Propagation of Action Potentials

• APs are transmitted from the origin down the entire axon length toward the axon terminals

• Leads to depolarization in each adjacent membrane area

• APs only travel in a forward direction due to inactivation of Na+ channels behind the AP

Coding for Stimulus Intensity

• Frequency of APs encodes stimulus intensity

• Stronger stimuli generate higher frequency of APs

Refractory Periods

Time during which a neuron cannot trigger another AP.

• Absolute refractory period: No AP possible, Na+ channels are open or inactivated

• Relative refractory period: AP possible only with exceptionally strong stimulus, K+ channels are open

Conduction Velocity

• AP conduction velocity varies with axon diameter and degree of myelination

• Larger diameter fibers conduct faster

• Myelinated fibers conduct much faster due to saltatory conduction (AP jumps from node to node)

Myelin Sheath and Multiple Sclerosis

• Myelin in CNS destroyed in multiple sclerosis (MS), slowing or blocking AP conduction

• Symptoms: visual disturbance, weakness, loss of muscle control, speech disturbance

• Treatments: immune system modulation, vitamin D

Nerve Fibers

• Classified by diameter, myelination, and speed

Additional info: Groups B and C include autonomic and sensory fibers.

Synaptic Transmission

Synapses

• Junctions that mediate information transfer from one neuron to another or to an effector cell

• Presynaptic neuron: Conducts impulses toward the synapse

• Postsynaptic neuron: Transmits impulses away from the synapse

Types of Synapses

• Chemical synapse: Most common, uses neurotransmitters

• Electrical synapse: Less common, direct electrical connection

Chemical Synapse Structure and Function

• Composed of:

  • Axon terminal of presynaptic neuron (contains synaptic vesicles with neurotransmitter)

  • Receptor region on postsynaptic neuron (receives neurotransmitter)

• Separated by a synaptic cleft (fluid-filled space)

• Transmission is chemical, not electrical

• Ensures unidirectional communication

Transmission Across Synaptic Cleft

• Neurotransmitter released from presynaptic neuron

• Diffuses across cleft and binds to receptors on postsynaptic neuron

• Binding opens ion channels, causing graded potentials

Example: Acetylcholine is released at neuromuscular junctions to stimulate muscle contraction.