Synaptic Transmission Classical –Mediated by Neurotransmitter Gated Ion Channel aka ionotropic...

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