Synaptic strength

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Synaptic strength and importance

What is synaptic strength?

It is the potential instigated in the presynaptic neuron relative to action potential threshold.

2 situations could occur:

1. EPSP (excitatory post synaptic potential) >>> threshold

Strong transmission

Occurs in auditory pathways such as Calyx of Held

When the output at a presynaptic neuron needs to be converted into a different chemical e.g. excitatory to inhibitory

2. EPSP < threshold

Summation required

Why is it important?

The strength of synaptic transmission allows a neuron to integrate EPSPs (summation) and process the information of whether to fire an AP/frequency.

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Types of summation and their role

1. Temporal summation

When a neuron receives EPSPs from the same axon over time.

The axon has to be firing at a high frequency for an AP to be generated.

High frequency input ---> Low frequency output; frequency divider.

2. Spatial summation

EPSPs generated from various axons in the same time frame.

APs can only be triggered given the firing of A and B. This way neuronal inputs can be associated with each other. It plays a key role in how memory works as the strength of different inputs can be altered to affect the outputs.

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Synaptic inhibition

Occurs when negative potentials negate positive ones.

Strong inhibitory synapse does not only equate to how large of a hyperpolarization it causes.

Inhibitory shunting

Changes the excitatory current which makes it harder for depolarization to occur.

Reduces synaptic strength of excitatory inputs.

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Factors affecting synaptic strength

1. Number of synapses surrounding the target

Number of NT release sites in presynaptic neuron. Some neurons have many release sites and are treated as seperate synapses in that one structure.

Number of seperate terminals

Number of dendritic spines on post synaptic neuron (each with their post synaptic densities)

2. Location of synapses

Synapses on dendrites more distal to cell body are less effective

Synapses on dendrites closer to cell body are more effective

3. Probability of NT release

A vesicle has to be in the right position at the right time (docked at release site)

Depends on Ca2+ concentration

Ca2+ concentration is determined by prior activity of axon - Facilitation where the summation of Ca2+ affects NT release, Post tetanic potentiation where a neuron is stimulated actively, resulting in a an increased frequency of EPSPs for the same stimulation, Depression where a release site is inactivated or NT's are depleted in nverve terminal.

4. Types of receptors found on post synaptic membrane

Each type of receptor has each characteristics e.g.

AMPA and NMDA

AMPA has higher conductance of Na+ than NMDA, however closes quickly.

NMDA has lower conductance of Na+ than AMPA, however it conducts Ca2+ too. It is voltage dependent and has longer latency.

This results in NMDA providing a greater likelihood for summation (?).

Also since both receptors bind to PSD95, they are localised in the same area.

Activation of AMPA could result in NMDA being activated at the next depolarization/NT release.

5. Number of receptors on post synaptic membrane

Transmitter > receptor - All or nothing response

Receptor < NT - Response varies