NEUROTRANSMITTERS Dr Fawzia ALRoug, MBBS, Master, Ph.D Assistant Professor, Department of Physiology, College of Medicine, King Khalid University Hospital, Riyadh, Saudi Arabia

NEUROTRANSMITTERS DEFINITION: Are chemical transducers which are released by electrical impulse into the synaptic cleft from presynaptic membrane from synaptic vesicles. It then diffuse to the postsynaptic membrane and react and activate the receptors present leading to initiation of new electrical signals. 2

Discovery of neurotransmitters Loewi, 1921 frog hearts in saline solution Stimulation of vagus nerve results in lower heart rate – gave long vagal nerve stimulation Heart #2: – Exposed to saline solution from heart #1 – Slowed heart rate Conclusion: Neurotransmission is chemical – nerve releases chemical that can influence other cells 3 Fig 8.1, Zigmond “Fundamental Neuroscience”

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Fate of neurotransmitters Are as, 1.It is consumed ( broken down or used up) at postsynaptic membrane leading to action potential generation. 2.Degraded by enzymes present in synaptic cleft. 3.Reuptake mechanism( reutilization) this is the most common fate. 6

Types of responses on postsynaptic membrane Excitatory postsynaptic potential (EPSPs) It is caused by depolarization. Inhibitory Postsynaptic potential (IPSPs) It is caused by hyperpolarization. 7

8 Neurotransmitter Postsynaptic effect Functions 1.Acetyl choline (Ach) ExcitatoryCognitive functions e.g. memory Peripheral action e.g. cardiovascular system 2. Catecholamines i. Epinephrine (adrenaline) Excitatory in some but inhibitory in other For details refer ANS. e.g. fight or flight, on heart, BP, gastrointestinal activity etc. Norepinehrine controls attention & arousal. ii.NorepinephrineExcitatory iii. DopamineExcitatoryDecreased dopamine in parkinson’s disease. Increased dopamine concentration causes schizophrenia

9 Neurotransmitter Postsynaptic effect Functions 3. serotonin (5HT) ExcitatoryMood control, sleep, pain feeling, temperature, BP, & hormonal activity 4. HistamineExcitatoryArousal, pain threshold, blood pressure, blood flow control, gut secretion, allergic reaction (involved in sensation of itch) 5. GlutamateExcitatory 75% of excitatory transmission in the brain Long term potentiation involved in memory and learning by causing Ca ++ influx.

10 Neurotransmitter Postsynaptic effect FateFunctions 6. AspartateExcitatory Aspartate & Glycine form an excitatory / inhibitory pair in the ventral spinal cord 7. Gama amino butyric acid(GABA) Major inhibitory mediator GABA – A causes hyperpolarization (inhibition) Anxiolytic drugs like benzodiazepine cause increase in Cl - entry into the cell & cause soothing effects. GABA – B cause increase conductance of K + into the cell. 8. GlycineInhibitory Glycine is inhibitory transmitted found in the ventral spinal cord. It is inhibitory transmitter to Renshaw cells.

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