Neurons: Cellular and Network Properties

Electrical Signals in Neurons

Neurons communicate via electrical signals generated by ion movement across their membranes. The membrane potential is determined by the concentration gradients and permeability of ions such as potassium (K+), sodium (Na+), and chloride (Cl–).

• Nernst Equation: Predicts the equilibrium potential for a single ion if the membrane is permeable only to that ion. Key Point: The equilibrium potential is influenced by the ion's concentration gradient and charge.

• Goldman-Hodgkin-Katz (GHK) Equation: Calculates the membrane potential considering multiple ions. Key Point: Membrane potential is a composite result of all permeable ions.

Ion Concentrations and Equilibrium Potentials

Ion concentrations inside and outside the cell determine the equilibrium potentials for each ion.

Example: K+ tends to move out of the cell, Na+ and Cl– tend to move in.

Ion Movement and Membrane Potential

Resting membrane potential is primarily determined by the K+ concentration gradient and the cell's permeability to K+, Na+, and Cl–. Changes in permeability result in ion movement, creating electrical signals.

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

Gated Ion Channels

Ion channels are named for the primary ion they conduct. Gated channels control ion permeability and are classified as mechanically gated, chemically gated, or voltage-gated.

• Threshold voltage: The voltage at which a channel opens, varies by channel type.

• Conductance: The ease with which ions pass through a channel.

Ohm’s Law in Neuronal Current

Current flow in neurons obeys Ohm’s Law, relating voltage (V), current (I), and resistance (R):

• High resistance = low current; low resistance = high current.

• Resistance can be from the cell membrane (Rm) or cytoplasm (Ri).

Types of Electrical Signals: Graded vs. Action Potentials

Neurons use two main types of electrical signals:

• Graded Potentials: Variable strength, used for short-distance communication.

• Action Potentials: Brief, large depolarizations, used for rapid signaling over long distances.

Comparison Table: Graded vs. Action Potentials

Graded Potentials: Properties and Excitability

Graded potentials reflect stimulus strength and lose strength as they move through the cell due to current leak and cytoplasmic resistance. If strong enough, they reach the trigger zone and initiate an action potential.

• Excitatory: Depolarizing, increases likelihood of action potential.

• Inhibitory: Hyperpolarizing, decreases likelihood of action potential.

Action Potentials: Conduction and Phases

Action potentials travel long distances via conduction, maintaining constant amplitude. They are all-or-none events.

• Rising phase: Voltage-gated Na+ channels open, Na+ enters, depolarizing the cell.

• Falling phase: Na+ channels close, K+ channels open, K+ exits, repolarizing and hyperpolarizing the cell.

• Return to resting potential: K+ channels close, cell returns to -70 mV.

Voltage-Gated Na+ Channels: Gates and Refractory Periods

Axonal Na+ channels have two gates: activation (opens rapidly) and inactivation (closes slowly). The refractory period prevents backward conduction and limits the rate of action potential transmission.

• Absolute refractory period: No action potential can be fired.

• Relative refractory period: Only a stronger stimulus can fire an action potential.

Local Current Flow and Positive Feedback

Depolarization spreads by local current flow, causing adjacent sections of the membrane to depolarize. Na+ entry during an action potential creates a positive feedback loop, which stops when inactivation gates close.

Conduction of Action Potentials

Continuous entry of Na+ along the axon as Na+ channels open creates an electrical signal whose strength remains constant over distance. The refractory period prevents backward conduction.

Speed of Action Potential: Diameter and Myelination

Larger neurons and myelinated neurons conduct action potentials faster. Myelin increases resistance to ion leakage, enabling saltatory conduction between nodes of Ranvier. Demyelinating diseases (e.g., multiple sclerosis) slow or block conduction.

• A-Delta fibers: Large, myelinated, fast pain.

• C fibers: Small, unmyelinated, slow pain.

Chemical Factors Affecting Electrical Activity

Chemicals and ion concentrations in the extracellular fluid (ECF) can alter neuronal excitability. Hyperkalemia (high K+) brings neurons closer to threshold, while hypokalemia (low K+) moves them further from threshold.

Cell-to-Cell Communication: Synaptic Transmission

Neurons communicate at synapses via neurotransmitter release from vesicles. The classical pathway involves action potential arrival, Ca2+ influx, exocytosis, neurotransmitter diffusion, and receptor binding. Termination occurs by diffusion, enzymatic breakdown, or uptake.

• Acetylcholinesterase (AChE): Enzyme that breaks down acetylcholine.

• Recycling: Neurotransmitters can be taken up by presynaptic terminals or glial cells.

Strength of Stimulus and Neurotransmitter Release

The frequency of action potentials codes the strength and duration of a stimulus. Stronger stimuli produce more action potentials and release more neurotransmitter.

Summary of Key Concepts

• Membrane potential is determined by ion gradients and permeability.

• Graded potentials are local, variable, and can be summed; action potentials are all-or-none and propagate long distances.

• Voltage-gated Na+ channels have two gates and create refractory periods.

• Myelination and axon diameter affect conduction speed.

• Synaptic transmission involves neurotransmitter release, receptor binding, and termination mechanisms.

• Stimulus strength is coded by action potential frequency.