Excitable NT

Introduction to Excitatory Neurotransmitters

• Definition: An excitatory neurotransmitter is a chemical messenger that, upon release from nerve cells, binds to receptors on target effector cells, such as neurons or muscle cells, leading to a change in the effector cell's function.

Mechanism of Action

• Release of Neurotransmitter:

  • Neurotransmitter molecules are released from nerve endings.

  • Upon release, these molecules diffuse toward the effector cell they are intended to influence.

• Target Effector Cell:

  • The effector cell could either be another neuron or a muscle cell.

• Binding to Receptors:

  • Once close to the effector cell, neurotransmitter molecules search for specific receptors on the cell's membrane.

  • Receptors referred to are large protein molecules embedded in the cell membrane that interact with neurotransmitters or hormones.

  • A variety of receptors exist on a single cell's membrane.

Neurotransmitter Receptors

• Structure of Receptors:

  • The receptors discussed activate ion channels.

  • In the provided image reference, receptors for the specific neurotransmitter were highlighted, showing a closer view of the receptor structure.

• Function of Ion Channels:

  • The ion channel gate initially remains closed.

  • When the neurotransmitter binds to its receptor, the gate of the ion channel opens.

  • Such channels are classified as ligand-gated channels since their activation depends on the binding of a molecule (the ligand).

Role of Ion Movement in Voltage Changes

• Membrane Voltage:

  • Neurons and muscle cells possess measurable voltages across their membranes, typically expressed in millivolts (mV).

  • The cell's voltage at rest refers to its resting membrane potential.

  • The resting voltage indicates the state of the cell before neurotransmitter action.

• Effect of Neurotransmitter Binding:

  • When the neurotransmitter binds to its receptor and the ion channel opens, ions begin to move across the cell membrane.

  • Specifically, sodium ions (Na+) typically play a critical role in depolarization.

  • There exists a higher concentration of sodium ions in the interstitial fluid outside the cell compared to inside, creating a concentration gradient.

• Ionic Movement:

  • Sodium ions diffuse into the cell due to this gradient, leading to an influx of positive charges.

  • This influx results in a significant change in the cell's voltage, moving it closer to a threshold voltage.

Depolarization and Excitation

• Threshold and Depolarization:

  • As sodium ions move into the cell, the membrane potential shifts towards the threshold level, indicating depolarization.

  • Excitation of the cell occurs as its voltage rises.

  • When depolarization surpasses the threshold, an action potential may be triggered in neurons.

• Why the Term Excitatory?

  • This change in voltage, resulting from the opening of sodium ion channels and the subsequent influx of sodium ions, is the reason the neurotransmitter is classified as an excitatory neurotransmitter.

  • In summary, excitatory neurotransmitters promote depolarization of the cell by facilitating the entry of sodium ions, contributing to the cell's overall excitability and readiness to transmit signals.