Synapses By Dr.D.Fisher
Anatomy of a Synapse Figure 8-25
Anatomy of a Synapse • Chemical Synapses • Components • Presynaptic terminal • Synaptic cleft • Postsynaptic membrane • Neurotransmitters released by action potentials in presynaptic terminal • Synaptic vesicles • Diffusion • Postsynaptic membrane • Neurotransmitter removal
Events at a Synapse • When an impulse reaches the presynaptic terminal the electrical changes in the membrane triggers events. • This leads to the fusion of synaptic vesicles with the presynaptic membrane. • The released neurotransmitters diffuse across the synaptic cleft. • attach to membrane receptors on the postsynaptic neuron.
Events at a synapse • The receptor-transmitter complex open ionic channels. • Ions then either enter or leave the postsynaptic neuron depending on their concentration gradient. • This results in a change in the postsynaptic membrane potential.
Pre-synaptic inhibition and facilitation • Release of transmitters onto synaptic terminals can alter synaptic transmission • Occurs at "piggy-back" synaptic contacts • Presynaptic inhibition is due to a transmitter-initiated decrease in synaptic release • One possible mechanism is a reduction in the depolarization of the terminal, thus decreasing Ca++ influx.
Pre-synaptic inhibition: Mechanisms • alternatively, presynaptic inhibition could be brought about by altering the state of the voltage-gated Ca++ channels • these channels may have to be phosphorylated in order to open in response to depolarization • if the transmitter could trigger dephosphorylation of the channels, then transmitter release would be decreased
Pre-synaptic inhibition: Mechanisms • increasing gK in the terminal is another means of producing presynaptic inhibition • this will decrease the duration of the presynaptic action potential • presynaptic facilitation entails a transmitter-initiated increase in synaptic release • one means of accomplishing this would be to decrease gK, thus prolonging the presynaptic action potential
Postsynaptic (Membrane) Potentials (PSP): • There are two types: • EPSP (excitatory Post synaptic potentials) • IPSP (inhibitory Post synaptic potentials) • EPSPs depolarize the PS membrane (< -) • IPSPs hyperpolarize the PS membrane (>-)
Post Synaptic Potentials • In other words the effects of a synapse can either excite or inhibit a PS neuron. • These effects on the PS neuron are brought about chemicals (neurotransmitters). • For example, acetylcholine (ACH) is an excitatory neurotransmitter, where GABA is an inhibitory neurotransmitter.
Postsynaptic Potentials • Excitatory postsynaptic potential (EPSP) • Depolarization occurs and response stimulatory • Depolarization might reach threshold producing an action potential and cell response • Inhibitory postsynaptic potential (IPSP) • Hyperpolarization and response inhibitory • Decrease action potentials by moving membrane potential farther from threshold
In Summary: • An EPSP is brought about by ionic events which bring the resting membrane potential closer to threshold potential (firing threshold). This makes the PS neuron closer to the potential whereby an impulse (action potential) will occur. • An IPSP is also brought about by ionic events which takes the resting membrane potential further from the threshold potential (firing threshold), thus making it more difficult for a simultaneous or subsequent postsynaptic potential to reach threshold potential.
Summation of postsynaptic potentials: • Potentials that occur simultaneously on the PS neuron can be added (summated). • EPSP summate towards the threshold potential • IPSP summate away from threshold potential. See diagrams that follow.
Summation Spatial Summation Temporal Summation
EPSP Summation – no AP IPSP
Spatial summation and temporal summation • Both types of summation can characterized as being graded (in other words they can differ in magnitude). • Spatial summation:many presynaptic neurons fire simultaneously onto the soma (cell body) of the post synaptic neuron resulting in threshold potential being reached and thus initiating an AP. • Temporal summation: a single presynaptic neuron fires repetitively onto the soma (cell body) of the post synaptic neuron the resulting EPSPs summating to reach firing threshold and initiate an AP.
Neuronal Pathways and Circuits • Organization of neurons in CNS varies • Convergent pathways: Many converge and synapse with smaller number of neurons • Divergent pathways: Small number of presynaptic neurons synapse with large number of postsynaptic neurons • Oscillating circuits: Arranged in circular fashion to allow action potentials to cause a neuron farther along circuit to produce an action potential more than once
Convergence Figure 8-24b
Divergence Figure 8-24a