Question

Describe temporal and spatial synaptic summations, and presynaptic inhibition. Describe, how these processes which express synaptic plasticity combined with different types of neuronal connections can contribute to short and long term memory

Answer 1

Temporal Summation: In this an excitatory postsynaptic potential(EPSP) is produced by an additive effect of many EPSPs that have been generated at the same synapse by a series of high-frequency action potentials on the presynaptic neuron.

Spatial summation: In this an EPSP is produced by the additive effect of by many EPSPs that have been generated at many different synapses on the same postsynaptic neuron at the same time.

Types of Summation 3. Postsynaptic neuron fires EPSPs spread from several synapses to axon hillock 1. Simultaneous stimulation by several presynaptic neurons Spatial summation 3. Postsynaptic neuron fires 2. EPSPs spread from one synapse to axon hillock 1. High Frequency stimulation by one presynaptic neuror Temporal summation

Presynaptic inhibition: It is mostly found in spinal cords and it helps in transmission of information in higher brain centers. An excitatory postsynaptic potential has been produced in the postsynaptic cell because of the action potential of presynaptic cell. The modulatory cell makes an axoaxonic synapse(one neuron onto the synapse of another neuron) with the presynaptic cell. After firing cell M1, the EPSP in the postsynaptic cell is smaller. This phenomenon is called presynaptic inhibition,

Short term plasticity: Short-term synaptic plasticity acts on for tens of milliseconds to a few minutes. It can strengthen as well as can weaken a synapse.Short-term synaptic can be increased by producing more synaptic terminals releasing transmitters in response to pre-synaptic action potentials. Depending on the time scales synaptic enhancement may occur because of neural facilitation, synaptic augmentation or post-tetanic potentiation.

It can also result in synaptic fatigue or depression if there is depletion of the readily releasable vesicles.

Long term plasticity: This plasticity lasts from minutes to hours. It can have both homosynaptic and heterosynaptic components. If the pathway is repeatedly stimulated, the amplitude of EPSP is constant. A tetanus produces post-tetanic potentiation (PTP) that dies away after several minutes leaving a very enduring enhancement of the EPSP. It is the kind of mechanism necessary to store memory.