Neurotransmitters – Functions & Mechanism Of Action In Cns

Neurotransmitters – Functions & Mechanism Of Action In Cns


NEUROTRANSMITTERS

  • Chemicals allowing transmission either between target cells/neurons across the synapse.
  • Stored in secretory vesicles of pre-synaptic neurons

Types:

Classification criteria:

1. Based on structure & size of synaptic vesicles:

Small clear vesicles:

  • Acetylcholine, glycine, GABA, and glutamate

Small vesicles with dense core:

  • Catecholamine

Large vesicles with dense core:

  • Neuropeptide 

2. Based on chemical nature:

Class I – Acetylcholine

Class II – Amines:

  • Norepinephrine
  • Epinephrine
  • Dopamine
  • Histamine
  • Serotonin

Class III – Amino Acids

  • Glutamate
  • Glycine
  • GABA
  • Aspartate
  • Class IV – Nitric oxide

3. Based on molecular size of neurotransmitter:

a. Small molecular transmitters:

  • Acetylcholine
  • Norepinephrine
  • Epinephrine
  • Dopamine
  • Serotonin
  • Glutamate
  • Glycine
  • GABA
  • Aspartate
  • Nitric oxide

b. Large molecule transmitters: 

Neuropeptides:

  • Substance P
  • Encephalin
  • Beta-endorphins
  • Hypothalamic releasing hormones

4. Classification based on mode of action:

Excitatory:

  • Glutamate – Chief excitatory neurotransmitter in brain & spinal cord
  • Aspartate – Chief excitatory neurotransmitter in cortical pyramidal cells

Inhibitory:

  • Gamma-AminoButyric Acid (GABA)
  • Major inhibitory neurotransmitter of CNS & exerts indirect inhibition.
  • Glycine – Direct inhibition.

5. Classification based on receptors:

Inotropic:

  • Glutamate: →  AMPA, Kainate, & NMDA
  • GABA: → GABA a type
  • Acetylcholine:  → Nicotinic (N m and N n)
  • Serotonin: →  5HT3 

Functions:

  • Transmembrane ion channels
  • Facilitates transport of different ions in & out of cell
  • Binding of neurotransmitter either opens or closes ion channel.
  • Hence, “Ligand-gated transmembrane ion channels”

Metabotropic:

  • Glutamate: → mGluR (Seven types – mGluR (1 – 7)
  • GABA: →  GABA b type 
  • Acetylcholine: → Muscarinic – M1 to M5 (five types)
  • Norepinephrine: → Alpha & Beta receptors
  • Serotonin: → 5HT 1A, 5HT 1B, 5HT 1D, 5HT 2A, 5HT 2C, 5HT 4

Functions:

  • G-protein coupled receptors
  • Acting through second messenger

Characteristics of neurotransmitters:

Acetylcholine:

Origin: 

  • Secreted by neurons in,
  • Terminals of large pyramidal cells of motor cortex
  • Neurons of basal ganglia
  • Motor nerves innervating the skeletal muscles

Mode of action: 

  • Excitatory action(Predominately)
  • Combination of excitatory & inhibitory actions.

Nor-epinephrine:

Origin:

  • Secreted by terminals of neurons
  • With cell bodies in brainstem & hypothalamus.  
  • Specific location: Locus cerulus

Mode of action:

  • Combination of excitatory & inhibitory actions.

Dopamine:

  • Origin: Substania nigra
  • Mode of action: Inhibitory action

Glycine:

  • Origin: Synapses in Spinal cord
  • Mode of action: Inhibitory action

GABA:

  • Origin: Nerve ending of spinal cord, basal ganglia, cerebellum, & areas of cortex.
  • Mode of action: Inhibitory action
Glutamate:
  • Origin:Mode of action: Excitatory action
  • Secreted by presynaptic terminals of sensory pathways entering CNS
  • Cerebral cortex

Serotonin:

Origin:

  • Nuclei from median raphe of brainstem
  • Projecting to dorsal horn of spinal cord & to hypothalamus.

Mode of action:

  • Inhibitory of pain pathways
  • Actions on higher regions of nervous system
  • Controls mood of a person and induce sleep.

Nitric oxide:

Characteristic feature:

  • Neither produced from the neuronal calls nor stored in vesicles

Mode of action:

  • Direct and instant action on target areas by simple diffusion

Neuropeptide transmitters:

Origin:

  • Synthesized in ribosome-golgi apparatus in neuronal cell body 
  • Stored in nerve terminals

Function: 

  • Most act as a hormone eg:- ADH, Oxytocin 

Substance P:

  • Released by primary pain nerve endings
  • Dorsal horn of spinal cord, Hypothalamus & from nigrostrial pathway of basal ganglia.

Opioid peptides:

  • Encephalin, endorphins & dynorphin

Function:

  • Important role as central inhibitor of pain signals
Exam Question
 

NEUROTRANSMITTERS

Classification:

1. Based on chemical nature:

Class I – Acetylcholine

Class II – Amines:

  • Norepinephrine
  • Epinephrine

Class III – Amino Acids

  • Glutamate
  • Glycine
  • GABA
  • Aspartate

2. Based on molecular size of neurotransmitter:

a. Small molecular transmitters:

  • Acetylcholine
  • Serotonin
  • Glutamate
  • Glycine
  • GABA
  • Aspartate
  • Nitric oxide

b. Large molecule transmitters: 

Neuropeptides:

  • Substance P
  • Encephalin
  • Beta-endorphins

4. Classification based on mode of action:

Excitatory:

  • Glutamate – Cheif excitatory neurotransmitter in brain & spinal cord
  • Aspartate – Cheif excitatory neurotransmitter in cortical pyramidal cells

Inhibitory:

  • Gamma-AminoButyric Acid (GABA)
  • Major inhibitory neurotransmitter of CNS
  • Exerts indirect inhibition
  • Glycine – Direct inhibition.

5. Classification based on receptors:

Inotropic:

  • Glutamate: → AMPA, Kainate, & NMDA
  • GABA: →  GABA a type
  • Acetylcholine: → Nicotinic (N m and N n)
  • Serotonin:        5HT3 

Functions:

  • G-protein coupled receptors
  • Acting through second messenger

Characteristics of neurotransmitters:

1. Acetylcholine: Excitatory action

2. Nor-epinephrine:

Origin:

  • Secreted by terminals of neurons
  • With cell bodies in brainstem & hypothalamus.  
  • Specific location: Locus cerulus

3. Dopamine, Glycine, & GABA: Inhibitory action

4. Glutamate: Excitatory action

5. Serotonin: 

  • Inhibitory of pain pathways
  • Actions on higher regions of nervous system
  • Controls mood of a person and induce sleep.

6. Nitric oxide:

  • Neither produced from neuronal calls nor stored in vesicles.

7. Neuropeptide transmitters:

  • Synthesized in ribosome-golgi apparatus in neuronal cell body 
  • Stored in nerve terminals
  • Mostly act as hormone (eg:- ADH, Oxytocin)

Substance P:

  • Dorsal horn of spinal cord, hypothalamus & from nigrostrial pathway of basal ganglia.

Opioid peptides:

  • Encephalin, endorphins & dynorphin
  • Important role as central inhibitor of pain signals.
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