Animal Body Systems - Neuron Potentials

Animal Body Systems - Electrochemical Potentials in Neurons

• Electrochemical potentials in neurons are crucial for communication and function.
• All living cells are electrically polarized, meaning they have a membrane potential (MP).
• The inside of the membrane is negative relative to the outside.
• The size of the MP ranges from -10 to -90 mV.
• Neurons and muscle cells are excitable cells, having large membrane potentials and specialized mechanisms to regulate.
• Three significant types of membrane potentials include resting membrane potential, electrotonic potentials, and action potentials.
• Membrane potentials and currents are reliant on inorganic ions.
• All cells possess resting membrane potential, measured when inactive.
• Unequal distribution of positive and negative charges across the membrane creates a potential difference (resting potential).
• Key ions involved are sodium (Na⁺) and potassium (K⁺).

Ion Concentrations in Cells

• Extracellular fluid generally has high sodium (Na⁺) and low potassium (K⁺) concentrations.
• Intracellular fluid usually has high potassium (K⁺) and low sodium (Na⁺) concentrations.
• Specific concentrations, in millimoles (mM) vary:
  • Intracellular Na⁺: ~15 mM
  • Extracellular Na⁺: ~150 mM
  • Intracellular K⁺: ~150 mM
  • Extracellular K⁺: ~5 mM

Ion Gradients in Cells

• Ion gradients are maintained by active transport using the Na⁺/K⁺ ATPase pump.
• This pump moves 3 sodium ions out of the cell and 2 potassium ions into the cell.
• The pump is electrogenic, meaning it generates a small electrical potential.
• A -10 mV potential is generated from ion gradients.
• Anionic proteins, within the cell, contribute to a -5 mV potential.

The Additional Potential in Neurons

• The resting potential of neurons (-55 mV) arises primarily from passive diffusion of K+ through open K+ channels.
• There's a chemical gradient for K+, but no electrical gradient.
• Na+/K+ ATPase and leak channels together create an electrochemical gradient.

Resting Potential of Neurons

• Na+/K+ active transport pumps establish concentration gradients of Na+ and K+ across the membrane.
• Open channels (leak channels) allow K+ ions to flow freely outward.
• Negatively charged molecules (proteins) inside cannot cross the membrane.
• Results in a resting membrane potential of -70 mV.

Membrane Ion Channels

• Channels are specific to each ion.
• Leak channels are always open, e.g., K⁺ channel at rest.
• Other channels are regulated (gated).
• Voltage-gated channels are common in neurons.
• Ion movement depends on the concentration gradient.

Graded Potentials

• Changes in membrane potential due to ion permeability are graded potentials.
• Electrotonic potentials are a type of graded potential common in dendrites and cell bodies.

Electrotonic Potentials

• Current (ions) travels along the membrane surface, generating a small (mV) potential change.
• Can depolarize or hyperpolarize.
• Short distance travel.
• Critical for initiating action potentials in the axon hillock.

Action Potentials

• Action potentials are rapid and large changes in membrane potential.
• Transient and have an all-or-nothing characteristic.
• Depolarizes the membrane (from -70 to +35 mV).
• Initiated at the axon hillock and travel along the entire axon.
• Rely on ion currents through voltage-gated ion channels.

Generation of an Action Potential

• Triggered when a stimulus pushes the resting potential to threshold.
• Voltage-gated Na⁺ and K⁺ channels open and close in response to voltage changes.
• Inward flow of Na⁺ changes the membrane potential to positive, and outward flow of K⁺ returns it to negative values.

Depolarization

• Action potentials depend on voltage-gated ion channels (Na⁺ and K⁺).
• Na⁺, voltage-gated sodium channels open, causing rapid Na⁺ influx.
• K⁺, voltage-gated potassium channels open, causing K⁺ efflux.

The Rest of the Action Potential (Falling Phase)

• K⁺ channels open more, leading to K⁺ efflux and repolarization (return to negative potential).
• Na⁺ channels become inactivated; K⁺ channels close eventually.

Propagation of Action Potentials

• Action potentials move along axons as depolarization occurs in successive segments.

AP Propagation Along Axon

• Initiated at the axon hillock.
• Movements are unchanged along the axon membrane towards terminals.
• Dendrites and cell bodies have a concentration of K⁺ channels that reduce backward propagation.

AP Conduction in Unmyelinated Axons

• Threshold reduction occurs at the axon hillock.
• Na⁺ concentration influences channel locations.
• Currents spread along the membrane to create new action potentials.

AP Conduction in Myelinated Axons - Saltatory Conduction

• Myelin prevents ions from crossing the membrane (reduced current loss).
• Na⁺ and K⁺ concentrations are high at nodes.
• Currents leap between nodes resulting in faster conduction.

The Hodgkin-Huxley Cycle

• The rise phase of the action potential is positive feedback.
• Further depolarization → opening of Na⁺ channels → increased Na⁺ flow → further depolarization.