Synaptic Transmission
• Classical
– Mediated by
Neurotransmitter Gated
Ion Channel aka
ionotropic receptors
• Neuromodulatory
– Mediated by
Metabotropic
Receptors
Both cause a post-synaptic potential, ie a change in the
Membrane potential of the post-synaptic plasma membrane
The psp can be depolarizing or hyperpolarizing
Synaptic Potentials and Their
Integration
• EPSP: excitatory post-synaptic
potential
• IPSP: inhibitory post-synaptic
potential
• Temporal Summation
• Spatial Summation
Classical Neurotransmission
• Effects due to direct gating of ion channel
• Direct postsynaptic effects last for tens of
milliseconds
• No secondary effects
• Postsynaptic electrical effects are fast and
strong
Neuromuscular Junction
• is always excitatory
• is one for one
• 1 AP in presynaptic MN= 1 AP in post-synaptic
muscle NMJ caused by release of 200 synaptic
vesicles
• In the rest of the NS, it is not 1 for 1, the psp is so
small that an AP is not always triggered at the
hillock
• AP can cause release of 1 synaptic vesicle
Excitatory Transmission
• Synaptic transmission that causes
depolarization of the postsynaptic neuron
• Increases the probability that the post
synaptic neuron will fire an action potential
• Increases amount of neurotransmitter
released from post synaptic neuron by
presynaptic facilitation
Excitatory Post-synaptic Potential
= EPSP
• Depolarization of the post-synaptic
membrane caused by the neurotransmitter
brings the membrane potential close to the
threshold for firing an action potential
• Can increase sodium or calcium
permeability or can be caused by decreasing
potassium permeability
Inhibitory Transmission
• Synaptic transmission that causes transient
hyperpolarization of the postsynaptic
neuron
• Decreases the probability that the post
synaptic neuron will fire an action potential
• This is called an inhibitory post-synaptic
potential ipsp
I.P.S.P.
• Caused by increase in potassium
permeability similar to the undershoot of
the action potential
• Increase in chloride permeability
• If ECl=Vr then no change in Vr will be
observed, however an epsp would be
smaller if the Cl permeability is still high
Neuronal Integration
• Summing of all ipsp and epsp to determine
if threshold has been met for AP generation
• Based on temporal summation
– Time constant
• Based on spatial summation
– Space constant
Temporal Summation
• Rapid firing from a single presynaptic input
leads to repeated post-synaptic potentials in
a short period of time
• Causes repeated depolarization of
membrane without time to go back to
resting state
• Allows a weak presynaptic input to generate
an action potential in post synaptic neuron
Time Constant
• The amount of time that a psp will last at a
given membrane location= tau
• tau=membrane resistance x membrane
capacitance
• Time it takes for constant applied voltage to
build up to 63% of its final value
Temporal Summation
• Neurons with membranes that have long
time constants show more temporal
summation for conduction of psp
• Typical values are 10 msec
• Membrane resistance is reflected by number
of open channels and channel density
Membrane Capacitance
Spatial Summation
• The simultaneous firing of multiple
individual presynaptic neurons to one postsynaptic neuron.
• The post-synaptic effects sum and can bring
the post synaptic membrane closer or
further away from threshold.
Length constant
• Distance that a psp can spread along the
membrane= lambda
• Lambda= resistance of membrane/resistance of
cytoplasm
• Distance along a neurite at which a constant
applied voltage will decay to 37% of its original
value. Common value is 100-300 um to mm.
• The greater the membrane resistance, ie no
channels the longer the psp travels
Synaptic Integration
• Look at Geometry of Inputs and the
liklihood that any synapse will lead to an
action potential in the axon of the postsynaptic neuron
PreSynaptic Inhibition and
Facilitation
• Requires 3 synapses
• The middle synapse can be active or
inactive
Types of CNS Synapses
•
•
•
•
Axodendritic
Axosomatic
Axoaxonic
Dendrodendritic
Functional/Structural Synapse
Classification
• Gray’s Type I
– Post-synaptic membrane is thicker than pre-synaptic
– Asymmetrical
– Excitatory
• Gray’s Type II
– Symmetrical synapse, pre & post-synaptic densities are
similar thickness
– Inhibitory