Synaptic Transmission Flashcards
Function of a synapse:
enables a neuron to transmit a signal/impulse to another cell
Two types of synapses:
- electrical
- chemical
Advantage of electrical synapse and advantage of chemical synapse:
- electrical synapses transmit the signal faster but chemical synapses allow for more biological flexibility (different types of neurotransmitters that can be used)
The synapse
Electrical synapses:
- occur at?
- channels are made of?
- process?
- motion of ions is
- occur at gap junctions
- **gap junction channel = 2 connexon
- ***connexon is formed from six connexin subunits (12 connexin total), which spans both the pre and post synaptic membranes
- the action potential from the pre-synaptic cell generates a postsynaptic potential in the postsynaptic cell
- a postsynaptic potential has a low amplitude and may not generate an action potential; may require neural integration of many signals
- gap junctions are bidirectional and hence ions move in both directions
Synaptic Integration:
- neurons summate all the signals received before generating an action potential; most important integration occurs as dendritic integration
Synaptic Integration:
Connexon
Electrical Synapse
Chemical Synapse:
- process
- locations
- not directly via gap junctions, but propagated via neurotransmitters (small molecules)
- over synaptic cleft = 30nm wide
- active zones = sites of neurotransmitter release
- from an axon to a dendrite, to closer to the soma, to another axon which both travel to a dendrite
Chemical Synapse:
Location of chemical synapses:
Neurotransmitters are
small molecules that diffuse across the synaptic cleft and bind to receptor targets on the post-synaptic membrane affecting the movement of ions through the semi-permeable membrane
Neurotransmitters Chharacteristics:
- synthesised in a neuron
- concentrated in the presynaptic termini
- acts on specific targets on the postsynaptic cell
- if administered exogenously then mimics the action of endogenously released ligand
- is removed from the synaptic cleft in recycling
Events at the Synapse Order:
- axonal transport
- axonal action potential
- neurotransmitter release
- neurotransmitter clearance
Events at the Synapse: Axonal Transport:
- neurotransmitter is synthesised in
axonal cytosol - stored in vesicles
- action potential reaches axon
terminal
Events at the Synapse: Arrival of Action Potential:
- action potential arrives
- Ca2+ voltage gated ion channels
open - influx of Ca2+
- changes proteins on vesicles and
presynaptic membrane - vesicle fusion is initiated
Events at the Synapse: Neurotransmitter Release:
- docked vesicles are held in position
at the active zone by specialised
proteins called v-snares and t-snares - vescicles fuse with the membrane
- an increase in Ca2+ causes their
conformation to change and
exocytosis to commence - exocytosis is very rapid (less than
0.2ms) because Ca2+ enters the
active zone and the local
concentration rises rapidly
Events at the Synapse: Neurotransmitter Clearance:
1) Enzymatic degradation in the
synaptic cleft
2) Uptake into glial cells with
subsequent enzymatic degradation
3) Absorption into the presynaptic cell
for repackaging
4) Absorption into the presynaptic cell
for degradation
Neurotransmitters must be cleared from the synaptic cleft to prevent over activation of ion channel targets on the postsynaptic membrane
Neuromuscular Junction: Motor End Plate:
- motor nerve axons branch near their
termini to contact skeletal muscle
fibres - myelination halts before the synapse
in a structure called the motor end
plate
Motor End Plate:
NMJ: Label the Diagram:
Purpose of Subneural Clefts in NMJs:
subneural clefts increase the surface area for which the neurotransmitter can act
NMJ Process:
- there are 300,000 synaptic vesicles
with acetylcholine - as with other chemical synapses
Ca2+ influx initiates exocytosis - from an action potential 125 vesicles
of ach are released
Acetylcholine receptor at NMJ is composed of
5 homologous subunits (2alpha,beta,gamma,delta)
ach binds to the alpha subunit
the receptor can allow the passage of K+ and Ca2+ but mainly only Na+
What in a neuromuscular junction results in local depolarisation?
binding of ach to ach receptors allowing influx of Na+
***enough influx of Na+ results in local depolarisation
the remaining Na+ channels open and an action potential is initiated resulting fiber contraction
NMJ action potential summary:
- action potential at axon terminal
- no myelination at motor end plate
- Ca2+ influx initiates exocytosis of ach
- vescicle docks and then fuses to
membrane - ach released into synaptic cledt and
diffuses into subneural clefts which
provide a larger SA for action and
hence more binding site - ach binds to alpha subunits on
receptor - influx of Na+ some passage of K+
and Ca2+ - local depolarisation (end plate
potential) - remaining Na+ channels open,
depolarisation reaches threshold
initiating action potential - results in fibre contraction
Subneural Clefts:
