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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 membraneThe 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 post-synaptic 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 post-synaptic 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 SynapseClassification
• 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