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Presentation on theme: "SYNAPSE AND NEUROTRANSMITTER"— Presentation transcript:


2 Synapses A junction that mediates information transfer from one neuron: To another neuron To an effector cell Presynaptic neuron – conducts impulses toward the synapse Postsynaptic neuron – transmits impulses away from the synapse

3 The Synapse Junction between two cells
Site where action potentials in one cell cause action potentials in another cell Types of cells in synapse Presynaptic Postsynaptic

4 Synapses Axodendritic synapse Axosomatic synapse Axoaxonic synapse
Figure 11.17

5 Electrical Synapses Gap junctions that allow local current to flow between adjacent cells. Connexons: protein tubes in cell membrane. Found in cardiac muscle and many types of smooth muscle. Action potential of one cell causes action potential in next cell, almost as if the tissue were one cell. Important where contractile activity among a group of cells important.

6 Chemical Synapses Components
Presynaptic terminal Synaptic cleft Postsynaptic membrane Neurotransmitters released by action potentials in presynaptic terminal Synaptic vesicles: action potential causes Ca 2+ to enter cell that causes neurotransmitter to be released from vesicles Diffusion of neurotransmitter across synapse Postsynaptic membrane: when ACh binds to receptor, ligand-gated Na+ channels open. If enough Na+ diffuses into postsynaptic cell, it fires.

7 Chemical Synapse Events at a chemical synapse
1. Arrival of nerve impulse opens volage-gated calcium channels. 2. Ca++ influx into presynaptic term. 3. Ca++ acts as intracellular messenger stimulating synaptic vesicles to fuse with membrane and release NT via exocytosis. 4. Ca++ removed from terminal by mitochondria or calcium-pumps. 5. NT diffuses across synaptic cleft and binds to receptor on postsynaptic memb 6. Receptor changes shape of ion channel opening it and changing membrane potential 7. NT is quickly destroyed by enzymes or taken back up by astrocytes or presynaptic membrane. Note: For each nerve impulse reaching the presynaptic terminal, about 300 vesicles are emptied into the cleft.

8 Neurotransmitter Removal
Method depends on neurotransmitter/synapse. ACh: acetylcholinesterase splits ACh into acetic acid and choline. Choline recycled within presynaptic neuron. Norepinephrine: recycled within presynaptic neuron or diffuses away from synapse. Enzyme monoamine oxidase (MAO). Absorbed into circulation, broken down in liver.

9 Removal of Neurotransmitter from Synaptic Cleft

10 Receptor Molecules and Neurotransmitters
Neurotransmitter only "fits" in one receptor. Not all cells have receptors. Neurotransmitters are excitatory in some cells and inhibitory in others. Some neurotransmitters (norepinephrine) attach to the presynaptic terminal as well as postsynaptic and then inhibit the release of more neurotransmitter.

11 Neurotransmitters found in the nervous system
EXCITATORY Acetylcholine Aspartate Dopamine Histamine Norepinephrine Epinephrine Glutamate Serotonin INHIBITORY GABA Glycine

12 Neurotransmitters Chemicals used for neuronal communication with the body and the brain 50 different neurotransmitters have been identified Classified chemically and functionally Chemically: ACh, Biogenic amines, Peptides Functionally: Excitatory or inhibitory Direct/Ionotropic (open ion channels) or Indirect/metabotropic (activate G-proteins) that create a metabolic change in cell

13 Chemical Neurotransmitters
Acetylcholine (ACh) Biogenic amines Amino acids Peptides Novel messengers: ATP and dissolved gases NO and CO

14 Neurotransmitters: Acetylcholine
First neurotransmitter identified, and best understood Released at the neuromuscular junction Synthesized and enclosed in synaptic vesicles Degraded by the enzyme acetylcholinesterase (AChE) Released by: All neurons that stimulate skeletal muscle Some neurons in the autonomic nervous system Binds to cholinergic receptors known as nicotinic or muscarinic receptors Nicotinic receptors Neuromuscular junction of skeletal muscles

15 Acetylcholine synthesis:
In the cholinergic neurons acetylcholine is synthesized from choline. This reaction is activated by cholineacetyltransferase As soon as acetylcholine is synthesized, it is stored within synaptic vesicles.

16 Structure of AchE Acetylcholinesterase (AchE) is an enzyme, which hydrolyses the neurotransmitter acetylcholine. The active site of AChE is made up of two subsites, both of which are critical to the breakdown of ACh. The anionic site serves to bind a molecule of ACh to the enzyme. Once the ACh is bound, the hydrolytic reaction occurs at a second region of the active site called the esteratic subsite. Here, the ester bond of ACh is broken, releasing acetate and choline. Choline is then immediately taken up again by the high affinity choline uptake system on the presynaptic membrane.



19 Cholinergic Receptors
Nicotinic receptors - On neuromuscular junction of skeletal muscle - On all ganglionic neurons of autonomic nervous system - Excitatory Muscarinic receptors - All parasympathetic target organs (cardiac and smooth muscle) - Exciatory in most cases

20 Acetylcholine Effects prolonged (leading to tetanic muscle spasms and neural “frying”) by nerve gas and organophosphate insecticides (Malathion). ACH receptors destroyed in myasthenia gravis Binding to receptors inhibited by curare (a muscle paralytic agent-blowdarts in south American tribes) and some snake venoms.

21 FUNCTIONS OF ACh Acetylcholine is involved in a variety of functions
including pain, recent memory, nicotine addiction, salivation, locomotion, regulation of circadian rhythm and thermoregulation. 2. It has also been demonstrated that brain cholinergic neurons play a critical role in Alzheimer’s disease, Huntington’s chorea and in the generation of epileptic seizures.

22 Neurotransmitters: Biogenic Amines
Include: Catecholamines – dopamine, norepinephrine (NE), and epinephrine (EP) Indolamines – serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviors and our biological clock

23 Synthesis of Catecholamines
AA tyrosine parent cpd Enzymes present in the cell determine length of biosynthetic pathway Norepinephrine and dopamine are synthesized in axonal terminals Epinephrine is released by the adrenal medulla as a hormone Figure 11.22

24 BIOGENIC AMINES Norepinephrine (aka Noradrenaline) Dopamine
Main NT of the sympathetic branch of autonomic nervous system Binds to adrenergic receptors ( or  -many subtypes, 1, 2, etc) Excitatory or inhibitory depending on receptor type bound “Feeling good” NT Release enhanced by amphetamines Removal from synapse blocked by antidepressants and cocaine Dopamine Binds to dopaminergic receptors of substantia nigra of midbrain and hypothalamus Reuptake block by cocaine Deficient in Parkinson’s disease May be involved in pathogenesis of schizophrenia

25 Synthesized from a.a. tryptophan
Serotonin (5-HT) Synthesized from a.a. tryptophan The synthesis of serotonin involve two reactions: 1) Hydroxylation: Tryptophan 5- Hydroxytryptophan The enzyme catalyzes this reaction is Tryptophan Hydroxylase. The Co- factor is Tetrahydrobiopterin, which converted in this reaction to Dihydrobiopterin

26 2) Decarboxylation: 5- hydroxytryptophan Serotonin
The enzyme is hydroxytryptophan decarboxylase. Serotonin is synthesized in CNS, & Chromaffin cells.

27 Break down of serotonin:
Serotonin is degraded in two recations 1) Oxidation: 5-hydroxytryptoamine + O2 + H2O 5- Hydroxyinodole-3- acetaldehyde 2) Dehydrogenation 5- Hydroxyinodole-3-acetaldehyde hydroxindole-3-acetate (Anion of 5-hydroxyindoleacetic acid) Monoamine oxidase Aldehyde dehydrogenase 

28 May play a role in sleep, appetite, and
regulation of moods Drugs that block its uptake relieve anxiety and depression SSRI’s = selective serotonin reuptake inhibitors Include drugs such as Prozac, Celexa, Lexapro, Zoloft

29 Neurotransmitters: Amino Acids
Include: GABA – Gamma ()-aminobutyric acid Glycine Aspartate Glutamate Found only in the CNS

30 Amino Acids GABA Main inhibitory neurotransmitter in the brain
Inhibitory effects augmented by alcohol and antianxiety drugs like Valium Increases influx of Cl- in postsynaptic neuron, hyperpolarising it and thus inhibiting it! GLUTAMATE * Widespread in brain where it represents the major excitatory neurotransmitter Important in learning and memory “Stroke NT” -excessive release produces excitotoxicity: neurons literally stimulated to death; most commonly caused by ischemia due to stroke (Ouch!) Aids tumor advance when released by gliomas (ouch!)




1. Glutamate acts as the major excitatory transmitter in the brain 2. Excess glutamate causes neuronal damage and death, principally by elevating cellular Ca+2. This phenomenon has significance for a number of pathologies such as Alzheimer’s disease, ALS, Ischemia and Hypoxia, Epilepsy and Schizophrenia. 3. Glutamate receptors are involved in a physiological phenomenon called long-term potentiation (LTP) - a cellular model of learning and memory. The NMDA receptor activation is an absolute requirement for LTP induction, however, AMPA and metabotropic glutamate receptors also play important roles.


36 GABA Gambar 46. Metabolisme γ amino butirat Catatan: PLP = piridoksal fosfat.


38 GABA acts as the major inhibitory transmitter in the brain
FUNCTIONS OF GABA GABA acts as the major inhibitory transmitter in the brain 2. GABA has been implicated in several neurological and psychiatricdisorders of humans including Huntington’s chorea, epilepsy, alcoholism, Parkinson’s disease and anxiety disorders. 3. Antiepileptic and anxiolytic properties of benzodiazepine and phenobarbital suggest an important role of GABA in epilepsy as well as anxiety disorders.

39 Neurotransmitters: Peptides
Include: Substance P – mediator of pain signals Beta endorphin, dynorphin, and enkephalins Act as natural opiates, reducing our perception of pain Found in higher concentrations in marathoners and women who have just delivered Bind to the same receptors as opiates and morphine

40 Neurotransmitters: Novel Messengers
Nitric oxide (NO) A short-lived toxic gas; diffuses through post-synaptic membrane to bind with intracellular receptor (guanynyl cyclase) Is involved in learning and memory Some types of male impotence treated by stimulating NO release (Viagra) Viagra  NO release  cGMP  smooth muscle relaxation  increased blood flow  erection Can’t be taken when other pills to dilate coronary b.v. taken Carbon monoxide (CO) is a main regulator of cGMP in the brain

41 Summary: Neurotransmitter Molecule Derived From Site of Synthesis
Acetylcholine Choline CNS, parasympathetic nerves Serotonin 5-Hydroxytryptamine (5-HT) Tryptophan CNS, chromaffin cells of the gut, enteric cells GABA Glutamate CNS Histamine Histidine hypothalamus Epinephrine synthesis pathway Tyrosine adrenal medulla, some CNS cells Norpinephrine CNS, sympathetic nerves Dopamine Nitric oxide, NO Arginine CNS, gastrointestinal tract


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