Transcript با کلیک‌کردن بر روی این نوشته فایلِ مربوط به جلسه‌های سوّم و چهارم درس

‫سیناپس‬
‫الف) سیناپس الکتریکی‬
‫ب) سیناپس شیمیایی‬
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Types of cell-cell junctions:
Tight junctions
– membranes fused
Gap junctions
– close juxtaposition (2-4 nm)
– electrical synapse
Chemical synapses
– synaptic cleft (20-30 nm)
– polarized
Electrical Synapse (rare)
Transmission of an Action Potential across an Electrical
Synapse and a chemical synapse
Most of the synaptic delay (1-2 mSec) is due to the time it takes to organize
the presynaptic processes
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Release
of
synaptic
vesicles
‫تاخیر‬
‫سیناپسی‪ :‬زمان مورد نیاز برای عبور پیام از‬
‫شکاف سیناپسی ‪ /‬نیم تا یک میلی ثانیه‬
‫تاخیر هسته ای‪ :‬زمان مورد نیاز برای جمع پذیری فضایی‬
‫و زمانی تحریکات وارده بردندریتها و جسم سلولی نورون‬
‫است که طی آن پتانسیل غشای نورون پس سیناپسی ممکن‬
‫است تا آستانه تحریک دپالریزه شود‪ /.‬حدود دو میلی ثانیه‬
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‫انواع سیناپس‬
‫شیمیایی‬
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CNS Synapses – types of connections (1)
Axodendritic
Axosomatic
Axoaxonic
CNS Synapses – types of connections (2)
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‫محل سیناپسها در نورون پس سیناپسی‬
‫‪ -1‬قشر‬
‫مغز‪ 98 :‬درصد بر روی دندریتها ‪/‬‬
‫‪ 2‬درصد بر روی جسم سلولی‬
‫•‬
‫‪ -2‬نخاع‪:‬‬
‫•‬
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‫‪ 75‬درصد بر روی دندریتها ‪/‬‬
‫‪ 25‬درصد بر روی جسم سلولی‬
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Receptors
• neurotransmitter specific postsynaptic
receptors
• open to allow ions to flow into the
postsynaptic neuron
• two main types
• ionotropic
• metabotropic
An example of a ligand gated ion channel
The nicotinic Ach
receptor. It needs
two molecules of
Ach in order to
open. It is
permeable to
cations. It is found
in the NMJ,
autonomic
ganglia,
hippocampus,
thalamus, etc.
Ionotropic Ach receptor
‫انواع كانال‬
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Receptors
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Action Potential
and
EPSP & IPSP
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Postsynaptic Potential
IPSP
EPSP
Resting
Potential
Time
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Summation
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Convergence
III. Neural Integration
Convergence of input as a factor in Summation
Summation (spatial & temporal)
Summation
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Temporal Summation From the Same Synapse
Occur when AP arriving from a presynaptic axon terminal of neuron A occur
close enough together in time such that EPSPs produced in response to the
binding of neurotransmitter overlap and sum.
EPSPs
• Graded in magnitude
• Have no threshold
• Cause depolarization
• Summate
• Have no refractory
period
Spatial Summation
SIMULTANEOUS
INPUT FROM TWO
SYNAPSES: ABOVE
THRESHOLD
Threshold
Time
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EPSP-IPSP Cancellation
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‫مهار و تسهیل پیش سیناپسی‬
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‫میانجیهای عصبی‬
Neurotransmitters
Neurotransmitters
• Neurotransmitters are released into the synapse and bind
with receptors on the postsynaptic cell membrane, which
cause ion channels to open in the new cell.
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, Aminoacids, Peptides, …
– Functionally:
• Excitatory or inhibitory
• Direct / Ionotropic (open ion channels) or Indirect /
metabotropic (activate G-proteins) that create a
metabolic change in cell
Neurotransmitter Recovery and Degradation
• Neurotransmitters must be cleared from the synapse to
permit another round of synaptic transmission.
• Methods:
– Diffusion
– Enzymatic degradation in the synapse.
– Presynaptic reuptake followed by degradation or
recycling.
– Uptake by glia
– Uptake by the postsynaptic neuron and desensitization.
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‫انواع میانجیها‬
‫• ‪ -1‬کوچک ملکولها‬
‫• ‪ -2‬نوروپپتیدها‬
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Neurotransmitters
• Electrochemical messengers:
• 1) Small Molecules
• Acetylcholine
• Biogenic Amines (Catecholamines, consisting of
Epinephrine and Norepinephrine ,Dopamine &
Serotonin & Histamine)
• Amino acids (Glycine, GABA / Aspartate, Glutamate)
• And also:
• Adenosine & ATP
• Gases (NO & CO)
• Others
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Neurotransmitters
2) Neuropeptides
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Acetylcholine – Example Neurotransmitter
Acetylcholine
Found in PNS and CNS
Most abundant neurotransmitter in PNS
Synthesis
Acetyl CoA + choline  acetylcholine + CoA
 Synthesized in cytosol of axon terminal
 Enzyme for synthesis = choline acetyl transferase (CAT)
Breakdown of Acetylcholine
Acetylcholine  acetate + choline
 Degradation occurs in synaptic cleft
 Enzyme of degradation = acetylcholinesterase (AChE)
Acetylcholinesterase (AChE)
• Inactivates ACh, terminating its action; located in
cleft
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‫بازجذب میانجی‬
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‫ورود میانجی به داخل وزیکول‬
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)1( ‫درمان افسردگي‬
Antidepressants
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