Autonomic Nervous System Neurotransmitters and Receptors

Autonomic Nervous System Neurotransmitters and Receptors

Overview

Greetings and introduction by Dr. Kaniside Hwadi, welcome to the fourth video in the autonomic nervous system series, focusing on neurotransmitters and receptors.

Learning Objectives

By the end of this video, students need to understand:

1. The definition of a neurotransmitter.

2. The two distinct types of neurotransmitters relevant to the autonomic nervous system.

3. The differentiation between neurotransmitters released by cholinergic and adrenergic receptors, including the specific nervous system fibers that release them.

4. A description of the two types of receptors in the autonomic nervous system.

Neurotransmitter Definition

• Neurotransmitters: Endogenous chemical messengers that enable communication between neurons throughout the body.

• Communication primarily occurs at synaptic terminals, where neurotransmitters are released.

Key Neurotransmitters in the Autonomic Nervous System

• Two significant neurotransmitters in the autonomic nervous system:
     - Acetylcholine (ACh)
     - Noradrenaline (norepinephrine)

Diagram Reference: The video references a diagram showing neurotransmitter release within the autonomic ganglia. This illustrates communication between two neurons (preganglionic and postganglionic) and between a neuron and an effector organ (muscle or gland cell).

Types of Autonomic Fibers

1. Cholinergic Fibers:
      - Release acetylcholine.
      - Include all preganglionic fibers in the autonomic nervous system and all parasympathetic postganglionic fibers.

2. Adrenergic Fibers:
      - Release noradrenaline (norepinephrine).
      - Typically consist of sympathetic postganglionic fibers, except for the acrine sweat glands which secrete acetylcholine.

Autonomic Nervous System Receptors

Ionotropic Receptors

• Made up of multiple protein subunits (4 to 5 proteins).

• Composed of an extracellular functional structure that binds ions and neurotransmitters, and a membrane-spanning channel that forms ion channels.

• When an ion binds to an ionotropic receptor, it activates the receptor, resulting in:
    1. Opening of ion channels allowing the flow of ions (e.g., calcium, sodium).
    2. Rapid postsynaptic effects manifesting as postsynaptic potentials arising within milliseconds of action potentials.
    3. Typically lead to depolarization of postganglionic neurons.

• Cholinergic receptors that bind to nicotinic receptors are classified as ionotropic.

Metabotropic Receptors

• Do not contain ion channels directly; affect other channels through the activation of intermediate molecules called G proteins.

• Also known as G protein-coupled receptors (GPCRs).

G Protein Composition

• Composed of three subunits:
     - Alpha subunit
     - Beta subunit
     - Gamma subunit

G Protein Activation Process

1. In the absence of an extracellular signal or neurotransmitter, the alpha subunit is bound to GDP (inactive state).

2. Upon neurotransmitter binding to metabotropic receptors:
      - Activation of the G protein occurs leading to the separation of the alpha subunit from the beta and gamma subunits.
      - The alpha subunit then binds to GTP (active state).

3. Activated G proteins partake in the activation of downstream effector molecules that mediate various intracellular responses, typically resulting in:
      - Activation of secondary messengers within the cell, resulting in slower responses compared to ionotropic receptors.

Functions of G Proteins

• Activated alpha subunit may regulate effector proteins or enzymes leading to subsequent generation and release of secondary messengers.

• Both alpha G protein bound to GTP and beta-gamma subunits can influence ion channels facilitating the opening and closing in response to neurotransmitter signaling.

Conclusion

Further exploration of second messengers will be addressed in subsequent videos, specifically their interactions with neurotransmitters and receptors.