Tema 16 Flashcards
what is a chemical synapse
A presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (NT release).
Then the postsynaptic neuron converts this chemical signal back into an electrical signal (postsynaptic potential).
2 processes:
TRANSMISSION: release of the chemical messenger
RECEPTION: binding of the transmitter to the receptor molecules of
the postsynaptic cell.
Differences between chemical and electrical synapse
There is no structural continuity between pre and postsynaptic neurons
The synaptic cleft is wider (20-40 nm)
The synaptic vesicles release their neurotransmitter in response to the flow of Ca2+
General overview of chemical synapse
- The transmitter is synthesized and stored in the synaptic vesicles
- The action potential reaches the presynaptic terminal
- Depolarization causes the opening of the voltage- dependent Ca2+ channels
- Flow of Ca2+ through the voltage-dependent calcium channels
- Ca2+ produces fusion between the presynaptic membrane and the vesicles
- The transmitter is released into the synaptic cleft by exocytosis
- The transmitter binds to the receptor molecule of the postsynaptic membrane
- Postsynaptic channels open or close
- The postsynaptic current causes an inhibitory or excitatory postsynaptic potential
- Re-captation of the vesicular membrane by the plasma membrane
Signal amplification
Chemical synapses can amplify signals. With the release of a single synaptic vesicle thousands of
molecules of NT are poured.
Only two NT molecules are required to bind to
the postsynaptic ion channel and open it.
Therefore, the action of a vesicle produces the opening of thousands of ion channels in the postsynaptic cell.
Role of Ca2+ in chemical synapses
The voltage-dependent Ca2+ channels in the presynaptic membrane provide Ca2+ to initiate neurotransmitter release.
If the extracellular Ca2+ is removed or the Ca2+ entry is blocked, there will be no release.
The intracellular injection of Ca2+ in the presynaptic terminal will stimulate the release of neurotransmitter.
Intracellular injection of a Ca2+ chelator will inhibit the release.
Important proteins in the release of vesicles
SNARES
-Syntaxin 1
-SNAP-25
-Synaptobrevin (VAMP)
Synaptotagmin
Ca2+ channel
*SNAP-25 regulates the assembly of.
Synaptobrevin in the vesicle
Syntaxin in plasma memb
*c2+ binds to synaptotagmin and it catalyzes the fusion of the membrane (exocytosis)
electrical vs chemical synapse
ELECTRIC
3.5 nm pre-post distance Cytoplasmic continuity Bidirectional conduction possible Independent of Ca2+
Very fast
Ion currents
CHEMICAL
20-40 nm pre-post distance
Without cytoplasmic continuity
Unidirectional conduction Dependent of Ca2+
Synaptic delay (0.2-0.5 ms) Chemical messenger
Synaptic delay and fatigue
A synaptic delay of 0.2-0.5 ms occurs at the synapses
between:
1. The arrival of the action potential to the synaptic termination.
2. The effect on the postsynaptic membrane.
- A smaller number of synapses means a faster response.
- Some reflexes are mediated only by a single synapse.
Fatigue of the synaptic transmission:
* Due to NT depletion
* Mechanism of defense against increases in excitability (epilepsy)
what is a neurotransmitter
It is a chemical mediator released into the synaptic space when the
presynaptic terminal is stimulated
It is able to bind to receptors located in postsynaptic cells
It can generate a physiological response
-They are present in the presynaptic neurons and released in response to depolarization, in a Ca2+ -dependent process.
-It has specific receptors in the postsynaptic cell.
-There is a specific mechanism to eliminate the neurotransmitter
NT CLASSIFICATION
1)Small molecule:
-amines
-amino acids
-purinergic
-Ach
2)Neuropeptides
-hypothalamic and pituitary
3)Others; NO
Peptide cotransmitters vs small molecule NT:
VESICLES
Peptide cotransmitters:
– large synaptic vesicles
– dense center
– away from the plasma
membrane
Small molecule
neurotransmitters:
– small vesicles
– clear center
– close to the membrane
Peptide cotransmitters vs small molecule NT:
SYNTHESIS
Small molecule NT:
The precursor molecules are recaptured by active transport.
The NT is synthesized and packaged in the synaptic
terminal.
Neuropeptides:
They are synthesized in the soma of the neuron.
They are transported through the microtubule tracks
Elimination of NT
Diffusion outside the synaptic cleft
Enzymatic degradation
Cellular reputare by neurons that release them o to glial cells
Vesicle recycling
Protein-mediated endocytosis:
Clatrin
Dinamin
Caveoli
The “Kiss and run” model
Neuromodulators
They have long-term effects which appear slowly.
The answers involve multiple steps and intermediate
compounds.
They affect the presynaptic, postsynaptic membrane or both.
They are released alone or together with the neurotransmitter.
