Lecture on Neurotransmitters

Introduction to Neurotransmitters

• Neurotransmitters are the chemical language of the nervous system.
• Over 50 different neurotransmitters identified.
• Most neurons produce more than one neurotransmitter.
• Release influenced by excitation frequency and other neurotransmitters.
• Classified by chemical structure and function.

Acetylcholine (ACh)

• First identified and best understood neurotransmitter.
• Found in neuromuscular junctions, peripheral, and central nervous systems.
• Synthesized from acetic acid and choline by choline acetyltransferase.
• Broken down by acetylcholinesterase to prevent excessive excitation.

Biogenic Amines

• Include catecholamines (norepinephrine, epinephrine, dopamine) and indolamines (serotonin, histamine).
• Catecholamines made from tyrosine; involved in movement and fight/flight response.
• Indolamines made from different amino acids (serotonin from tryptophan, histamine from histidine).
• Associated with brain functions like movement, emotional behavior, and biological clocks.
• Imbalances linked to mental illnesses.

Amino Acid Neurotransmitters

• Directly used as neurotransmitters: glutamate, aspartate (excitatory), glycine, GABA (inhibitory).

Neuropeptides

• Strings of amino acids with diverse functions.
• Include substance P (pain mediator) and endorphins (natural painkillers).
• Gut-brain peptides like somatostatin regulate digestion.

Purines

• Include ATP, used as neurotransmitter in taste cells and other areas.
• Adenosine acts as an inhibitory neurotransmitter; caffeine blocks its receptors.

Gases and Lipids

• Gasotransmitters: nitric oxide, carbon monoxide, hydrogen sulfide.
• Involved in learning, memory, and other processes.
• Endocannabinoids act on THC receptors, involved in appetite control and nausea.

Neurotransmitter Functions

Effects

• Excitatory (depolarizing) or inhibitory (hyperpolarizing).
• Dependent on receptor binding (e.g., GABA and glycine are inhibitory).

Actions

• Direct: Neurotransmitters bind to ion channels (e.g., acetylcholine binds to nicotinic receptors at neuromuscular junctions).
• Indirect: Involve G protein-coupled receptors; broader, long-lasting effects.

Neuromodulators

• Chemical messengers influencing synaptic transmission strength.
• Affect synthesis, release, degradation, or reuptake of neurotransmitters.
• Typically act locally.

Receptors

Channel-Linked Receptors

• Ligand-gated ion channels, immediate and brief action.
• Excitatory (e.g., sodium influx) or inhibitory (e.g., chloride or potassium influx).

G Protein-Coupled Receptors

• Indirect, complex, slow, but prolonged response.
• Initiate intracellular signaling cascades.
• Examples include muscarinic receptors and adrenergic receptors.
• Activate secondary messengers like cyclic AMP and influence various cellular processes.