Impulse Transmission I

Overview

This study guide covers the fundamental mechanisms of neuronal communication, focusing on graded potentials, action potentials, the refractory period, and synaptic transmission. These processes are essential for understanding how neurons transmit signals within the nervous system.

Neuronal Communication

Introduction to Neuronal Communication

• Neurons are specialized cells that transmit electrical and chemical signals throughout the nervous system.

• Communication occurs via changes in membrane potential and the release of neurotransmitters at synapses.

• Key processes include graded potentials, action potentials, and synaptic transmission.

Graded Potentials

Definition and Properties

• Graded potentials are small, localized changes in membrane potential that occur in the dendrites and cell body of a neuron.

• They are usually initiated by the binding of neurotransmitters to receptors (input via neurotransmitter receptors).

• Receptors involved can be ionotropic (ligand-gated ion channels) or metabotropic (coupled to ion channels via signaling pathways).

• Graded potentials can be depolarizing (making the inside of the cell less negative, possibly triggering an action potential) or hyperpolarizing (making the inside more negative, inhibiting action potential generation).

• The amplitude of a graded potential decreases with distance from the site of origin (decremental conduction).

• Graded potentials can be summed (added together) to reach the threshold for action potential initiation.

Example: Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are types of graded potentials.

Location of Channels

• Ligand-gated channels (for graded potentials) are found on dendrites and the cell body.

• Voltage-gated channels (for action potentials) are concentrated at the axon hillock, axon, and axon terminals.

• K+ leak channels are distributed throughout the neuron and help maintain resting membrane potential.

Action Potentials

Definition and Phases

• An action potential (AP) is a rapid, large change in membrane potential that propagates along the axon.

• Action potentials are all-or-none: they occur fully if threshold is reached, or not at all.

• Phases of the action potential:

  • Depolarization (Rising Phase): Voltage-gated Na+ channels open, Na+ enters the cell, causing rapid depolarization.

  • Repolarization (Falling Phase): Na+ channels inactivate, voltage-gated K+ channels open, K+ exits the cell, restoring negativity.

  • Hyperpolarization (Undershoot): K+ channels remain open longer, membrane potential becomes more negative than resting potential before returning to rest.

• Action potentials are self-propagating and travel in one direction along the axon.

Threshold: The minimum depolarization required to trigger an action potential is typically about 15 mV above the resting membrane potential (e.g., from -70 mV to -55 mV).

Key Ion Channels Involved

• Voltage-gated Na+ channels: Open rapidly during depolarization, then inactivate.

• Voltage-gated K+ channels: Open more slowly, responsible for repolarization and hyperpolarization.

• K+ leak channels: Always open, help maintain resting membrane potential.

Action Potential Propagation

• Depolarization at one segment of the axon triggers depolarization in the next segment, like falling dominoes.

• Ensures rapid and efficient signal transmission over long distances.

Refractory Period of Action Potentials

Absolute Refractory Period

• During this period, a neuron cannot generate another action potential, regardless of stimulus strength.

• Occurs when Na+ channels are either open or inactivated.

• Ensures each action potential is a discrete event and enforces one-way transmission.

Relative Refractory Period

• Follows the absolute refractory period.

• Some Na+ channels have returned to resting state, but K+ channels remain open.

• A stronger-than-normal stimulus is required to trigger another action potential.

• Membrane potential is more negative than the resting potential (hyperpolarized).

Encoding Stimulus Intensity

• All action potentials have the same amplitude and duration (all-or-none law).

• Stimulus intensity is encoded by the frequency of action potentials, not their size.

• Stronger stimuli produce higher frequencies of action potentials.

Synaptic Transmission

Structure of a Synapse

• A synapse is the junction between two neurons or between a neuron and another cell (e.g., muscle).

• Consists of:

  • Presynaptic neuron: Sends the signal, contains synaptic vesicles with neurotransmitter.

  • Synaptic cleft: Fluid-filled gap between pre- and postsynaptic membranes.

  • Postsynaptic neuron: Receives the signal, contains neurotransmitter receptors.

Steps of Synaptic Transmission

1. An action potential arrives at the axon terminal of the presynaptic neuron.

2. Depolarization opens voltage-gated Ca2+ channels; Ca2+ enters the terminal.

3. Ca2+ triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft (exocytosis).

4. Neurotransmitter binds to receptors on the postsynaptic membrane, causing ion channels to open and generating a graded potential in the postsynaptic cell.

Termination of Neurotransmitter Signal

• Neurotransmitter action is terminated by:

  • Diffusion away from the synaptic cleft

  • Enzymatic degradation in the synaptic cleft

  • Reuptake by the presynaptic neuron or surrounding glial cells (astrocytes)

• Ca2+ is removed from the presynaptic terminal by Ca2+ ATPase and Na+/Ca2+ exchangers.

Restoration of Resting Membrane Potential

• Na+/K+ ATPase pumps restore ion gradients after action potentials.

• K+ leak channels help maintain the negative resting membrane potential.

Summary Table: Key Differences Between Graded and Action Potentials

Additional info: This guide expands on the provided lecture notes by clarifying definitions, mechanisms, and the sequence of events in neuronal signaling, as well as including a summary comparison table for exam preparation.