Synaptic Transmission Flashcards
(39 cards)
Charles Sherrington
- named the junctions between cells “synapses”
- English physiologist, 50 years of experiments.
- Studied (among other things) motor neurons and the motor cortex.
Synapse:
- functional contact between neurons
There are two types of synapses
- Electrical: direct transfer of ionic current from one cell to another
- Chemical: involves release of chemical neurotransmitters
Gap junctions:
- Electrical currents occur at gap junctions
- Electrical synapse
- Connexins form the connexon, and 2 connexons form a gap junction channel
- 3 nanometers
Electrical synapses at gap junctions:
- Ions pass directly from one cell to another.
- Ions can then change the membrane potential
cells are “electrically coupled”. - Positive charge can pass through gap junctions and depolarize the next cell.
Benefits of electrical synapses
1) Usually bidirectional
- if cell #2 is depolarized, current can pass to depolarize cell #1
2) no “synaptic delay”
- Examples: they can be used for rapid reflexes involving escape rxns.
- AP’s can be fired simultaneously
3) Can synchronize activity of neurons
- brainstem neurons generate rhythmic activity underlying breathing are synchronized by electrical synapses.
– hormone-secreting neurons in the hypothalamus burst of hormone secretion into the bloodstream
Otto Loewi and the discovery of chemical transmission
- Saline stops stimulus. The vagus nerve releases chemicals that contribute to slower heart rate when introduced to stimulus.
Synapses can occur anywhere on the neuron:
- Axo-dendritic
-> on dendrite - Axo-somatic
->cell body - Axo-axonic synapses
-> on the axon of another cell/neuron
One presynaptic terminal can
contact several…
- postsynaptic sites.
- Presynaptic side: “active zone”
-> has a clustering of presynaptic vesicles - Postsynaptic side: “postsynaptic density”
Steps of chemical synaptic transmission
1) Synthesize neurotransmitter
2) Pack the neurotransmitter into vesicles
-> there is also repackaging into vesicles
3) Release the neurotransmitter into the synaptic cleft in response to a
presynaptic action potential
4) Bind the neurotransmitter to a receptor
5) Produce a change in the postsynaptic cell
Major Neurotransmitters: Amino Acids
- Glutamate (Glu)
- Gamma-aminobutryic acid (GABA)
- Glycine (Gly)
Major NT’s: Amines
- Acetylcholine (ACh)
- Dopamine (DA)
- Epinephrine
- Norepinephrine (NE)
- Serotonin (5-HT)
Major NT’s: Peptides
- Substance P
- Neuropeptide Y
- Somatostatin
- Cholecystokinin (CCK)
- Vasopressin
- Oxytocin
Steps of chemical synaptic transmission
1) Synthesize neurotransmitter
2) Pack the neurotransmitter into vesicles
3) Release the neurotransmitter into the synaptic cleft in response to a presynaptic action potential
4) Bind the neurotransmitter to a receptor
5) Produce a change in the postsynaptic cell
Voltage-gated calcium channels
- Ion channels permeable to calcium
- Located in the active zones of the presynaptic membrane.
-> at the axon terminal (NT packaging) - Activated by depolarization of the presynaptic membrane.
- External concentration of Ca2+ is greater than internal Ca2+
Necessary vs. sufficient
- You need calcium in the presynaptic cell to produce an AP in the post-synaptic cell
- Calcium is sufficient to produce (release NT’s) an AP in the post-synaptic cell (depolarization)
Steps of neurotransmitter
release
Synapsin
- reversibly binds to synaptic vesicles
- binds vesicles to actin filaments in the
cytoskeleton - keeps vesicles tethered within the reserve pool
- when synapsin is phosphorylated by protein
kinases, it dissociates from the vesicles - phosphorylation allows vesicles to move to cytoplasmic membrane
SNAREs
- SNAp REceptors
- Family of proteins that controls specific binding
and fusion of membranes. - Responsible for fusing the vesicle to the
presynaptic membrane. - release contents into synapse.
- responsible for fusion
- some SNAREs on the vesicle itself and some are on the membrane.
More SNAREs (1)
- Each SNARE protein has a hydrophobic end that
embeds in the membrane and a longer tail that
projects into the cytosol. - One SNARE protein, synaptobrevin, is in the
membrane of vesicles (also called a v-SNARE) - Two other SNARE proteins (syntaxin and SNAP-
25) are in the plasma membrane (also called
t-SNAREs) - snares bind to each other and pull vesicle closer to the membrane
Toxins that affect SNAREs:
1) Tetanus toxin
2) Botulinum toxin
Both cleave the vSNARE synaptobrevin.
(Some forms of botulinum toxin cleave
syntaxin or SNAP-25).
More SNAREs (2)
-The SNARE proteins allow the vesicle to dock at
the presynaptic membrane.
- Synaptotagmin “senses” calcium increases and
triggers fusion of the vesicle and release of
neurotransmitter.
- Synaptotagmin inserts itself into the membrane
- The vesicle fuses with the membrane and
releases neurotransmitter into the synaptic cleft.
Two types of neurotransmitter receptors:
-Ionotropic receptors = ion channels
- Metabotropic receptors = G-protein coupled receptors
Three classes of ion channels:
- Voltage-gated channels (for example the voltage-gated sodium channel)
- Gap junctions
- Ligand-gated channels (the ionotropic receptors)
- Share a common architectural plan: segments that span the membrane
are arranged around a central axis to form a gated channel for ions
Voltage-gated channels
- Consist of one long peptide with four subunits.
- Each subunit has six membrane-spanning
helicies S1-S6 - S4 helix has charged residues: voltage sensor
- There is a pore loop between S5 and S6 which
dips into the membrane to become
the pore.
