Neurotransmitters – Study Notes

Definition

• Neurotransmitters: chemicals produced and released by neurons.

• Effects: Excitatory (stimulate signal) or Inhibitory (block signal).

• Released into the synapse (gap between neurons).

• Can bind to neurons, muscles, glands, or other effectors.

• Over 100 types exist; main ones covered: Acetylcholine, Noradrenaline/Adrenaline, Dopamine, Serotonin, GABA, Glutamate, Substance P.

Acetylcholine (ACh)

• Receptors: Nicotinic & Muscarinic.

• Peripheral Nervous System (PNS):

  • Crucial for skeletal muscle contraction (via nicotinic receptors).

• Autonomic Nervous System (ANS):

  • Parasympathetic (“rest & digest”): uses ACh at every neuron.

  • Sympathetic (“fight or flight”): uses ACh only from the first neuron.

• Target effectors: digestive system, heart, pupils, salivary glands.

• Central Nervous System (CNS):

  • Roles in memory & cognition.

  • Implicated in Parkinson’s disease (movement disorder) & Alzheimer’s disease (neurodegenerative).

Catecholamines (from Tyrosine → Dopamine → Noradrenaline → Adrenaline)

• Derived from tyrosine (from diet/phenylalanine).

• Receptors: Alpha (α1, α2) and Beta (β1, β2, β3).

  • α1: blood vessels → vasoconstriction → ↑ BP (excitatory).

  • β1: heart → ↑ HR & contractility (excitatory).

  • α2: presynaptic neuron → negative feedback → ↓ sympathetic firing (inhibitory).

  • β2: lungs → bronchodilation (inhibitory).

  • β3: fat cells (limited role, no major drugs).

• CNS: involved in endogenous opioid release (pain, anxiety control).

• PNS: primary NTs of the sympathetic nervous system.

Dopamine

• Receptors: D1 (excitatory), D2 (inhibitory).

• CNS Functions:

  • Reward & motivation (reward pathway).

  • Motor control (basal ganglia → initiation & smoothing of movement).

  • Parkinson’s disease: dopamine neurons degenerate → tremor, instability, shuffling gait.

• PNS Functions:

  • Regulates blood vessel diameter, GI motility, and renal sodium excretion.

Serotonin (5-HT)

• Receptors:

  • 1 & 5 → inhibitory.

  • 2, 3, 4, 6, 7 → excitatory.

• CNS Roles: mood regulation, sleep.

• PNS Roles:

  • Gut (enteric nervous system): stimulates GI contractions.

    • Excess → diarrhea, rapid GI transit.

  • Bone remodeling: may strengthen or weaken bone depending on context.

GABA (Gamma-Aminobutyric Acid)

• Main inhibitory neurotransmitter.

• Receptors: GABA-A & GABA-B.

• Mechanism: increases Cl⁻ influx → hyperpolarization → neurons less likely to fire.

• Clinical relevance: low GABA activity linked to epilepsy/seizures. Drugs enhance GABA action to prevent firing.

Glutamate

• Most abundant excitatory neurotransmitter.

• Receptors: NMDA, AMPA.

• Function: stimulates neurons to fire.

• Toxicity: excess glutamate → neuronal death (“glutamate toxicity”), implicated in Alzheimer’s & dementia.

Substance P

• Receptor: NK-1.

• Function: mediates pain & inflammation in both CNS & PNS.

• Clinical relevance: drugs targeting Substance P may reduce pain perception.

✅ Summary:

• Excitatory NTs: Glutamate, Dopamine (D1), Serotonin (most subtypes), Noradrenaline/Adrenaline (α1, β1).

• Inhibitory NTs: GABA, Dopamine (D2), Serotonin (5-HT1, 5-HT5), Noradrenaline (α2, β2).

• Disorders linked to NTs:

  • ACh → Alzheimer’s, Parkinson’s.

  • Dopamine → Parkinson’s, addiction.

  • Glutamate → dementia, neurotoxicity.

  • GABA → epilepsy, seizures.

  • Serotonin → mood disorders, GI dysfunction.