Electrical Signals in Neurons: Graded and Action Potentials N4

Electrical Signals in Neurons

Introduction

Neurons communicate through electrical signals that result from changes in membrane potential. These signals are essential for the transmission of information throughout the nervous system. The two main types of electrical signals in neurons are graded potentials and action potentials.

Types of Electrical Signals

• Graded Potentials: Variable-strength signals that travel short distances and decrease in strength as they spread. They can be depolarizing (making the membrane potential less negative) or hyperpolarizing (making it more negative). If a graded potential is strong enough, it can trigger an action potential.

• Action Potentials: Brief, large depolarizations that travel long distances without losing strength. These are all-or-none signals, meaning they either occur fully or not at all.

Graded Potentials

Definition and Properties

Graded potentials are changes in membrane potential that vary in amplitude depending on the strength of the stimulus. They occur in the dendrites and cell body of neurons and are crucial for initiating action potentials.

• Amplitude: Directly proportional to the strength of the stimulus.

• Spread: Graded potentials lose strength as they move through the cell due to current leak (ions leaking out through open channels) and cytoplasmic resistance (resistance to ion flow within the cell).

• Types:

  • Excitatory Postsynaptic Potential (EPSP): Depolarization that increases the likelihood of an action potential.

  • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarization that decreases the likelihood of an action potential.

• Trigger Zone: The axon hillock and initial segment (AIS) contain a high concentration of voltage-gated Na+ channels. If the membrane potential reaches approximately -55 mV, an action potential is generated.

Mechanisms of Graded Potentials

• Generated by the opening or closing of chemically-gated, mechanically-gated, or thermally-gated ion channels.

• Can be caused by neurotransmitters, sensory stimuli, or other factors.

Action Potentials

Definition and Properties

Action potentials are rapid, uniform electrical signals that travel from the trigger zone to the axon terminals. They are essential for long-distance communication within the nervous system.

• All-or-None Principle: Action potentials occur fully if the threshold is reached; otherwise, they do not occur.

• Propagation: Action potentials are regenerated at each segment of the axon, allowing them to travel without losing strength.

• Key Ions: Na+ and K+ are the primary ions involved in generating action potentials.

Phases of the Action Potential

1. Rising Phase (Depolarization):

   • Voltage-gated Na+ channels open when the membrane potential reaches threshold (~-55 mV).

   • Na+ enters the cell, causing rapid depolarization up to ~+30 mV.

   • At peak depolarization, Na+ channels inactivate.

2. Falling Phase (Repolarization):

   • Voltage-gated K+ channels open more slowly in response to depolarization.

   • K+ exits the cell, causing the membrane potential to return toward -70 mV.

3. After-Hyperpolarization (Undershoot):

   • K+ channels remain open briefly after reaching resting potential, causing the membrane potential to dip below -70 mV.

   • Leak channels and the Na+/K+ ATPase restore the resting membrane potential.

Na+ Channel Gating

• Voltage-gated Na+ channels have two gates: activation gate (opens with depolarization) and inactivation gate (closes at peak depolarization).

• The sequential opening and closing of these gates create the refractory period.

Refractory Periods

Definition and Purpose

The refractory period is the time during which a neuron cannot fire another action potential or requires a stronger stimulus to do so. It ensures unidirectional propagation of the action potential and limits the frequency of neuronal firing.

• Absolute Refractory Period: No action potential can be initiated, regardless of stimulus strength. Lasts about 1-2 msec.

• Relative Refractory Period: A second action potential can be initiated, but only by a larger-than-normal stimulus. Lasts about 2-5 msec.

• Functions:

  • Ensures one-way travel of action potentials.

  • Limits the rate of signal transmission.

  • Prevents excitotoxicity (damage from excessive firing).

  • Information can be encoded in the frequency of action potentials.

Movement of Ions During Action Potentials

Key Points

• Na+ influx causes depolarization.

• K+ efflux causes repolarization and after-hyperpolarization.

• Leak channels and Na+/K+ ATPase maintain resting membrane potential.

Relevant Equations

• Nernst Equation: Used to calculate the equilibrium potential for a particular ion. $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• Ohm's Law (for current flow): $I = V / R$

Table: Comparison of Graded Potentials and Action Potentials

Feature	Graded Potentials	Action Potentials
Amplitude	Variable, depends on stimulus	All-or-none, uniform
Distance Traveled	Short, loses strength	Long, no loss of strength
Location	Dendrites, cell body	Axon
Types	Depolarizing or hyperpolarizing	Always depolarizing
Threshold	No threshold	Requires threshold (-55 mV)
Summation	Can summate	Cannot summate

Example: Sensory Neuron Signal Transmission

In sensory neurons, graded potentials are generated at sensory receptors in response to stimuli. If the graded potential is strong enough, it triggers an action potential at the trigger zone, which then propagates along the axon to the central nervous system.

Additional info:

• The first image ("Neuro 4") likely shows a fluorescent micrograph of neurons, with axons highlighted in green and cell bodies in red, illustrating the structural basis for electrical signaling.

• References to "USASK STUDENTS" and QR codes are not relevant to Anatomy & Physiology content and are omitted from the study notes.