Synaptic integration, Types of synapses, EPSP and IPSP

A brief overview of Synaptic integration, Types of Synapses and Excitatory and inhibitory post synaptic potentials.

1. SYNAPTIC INTEGRATION TYPES OF SYNAPSES EPSP & IPSP M.Rajagopalan V M.Sc Life sciences

2. SYNAPTIC INTEGRATION Neurons in the brain receive thousands of synaptic inputs from other neurons. Synaptic integration is the term used to describe how neurons ‘add up’ these inputs before the generation of a nerve impulse, or action potential. The ability of synaptic inputs to effect neuronal output is determined by a number of factors

3. Size , shape and relative timing of electrical potentials generated by synaptic inputs the geometric structure of the target neuron, the physical location of synaptic inputs within that structure expression of voltage‐gated channels in different regions of the neuronal membrane.

4. SYNAPTIC INTEGRATION AND ITS MECHANISM Neurons within a neural network receive information from, and send information to, many other cells, at specialised junctions called synapses . Synaptic integration is the computational process by which an individual neuron processes its synaptic inputs and converts them into an output signal. Neurons are specialised for electrical signalling , with the main neuronal input signal (synaptic potentials) and the main neuronal output signal (action potentials)

5. Synaptic potentials occur when neurotransmitter binds to and opens ligand ‐operated channels in the dendritic membrane, allowing ions to move into or out of the cell according to their electrochemical gradient. Synaptic potentials can be either excitatory or inhibitory depending on the direction and charge of ion movement. Action potentials occur if the summed synaptic inputs to a neuron reach a threshold level of depolarisation and trigger regenerative opening of voltage‐gated ion channels. Synaptic potentials are often brief and of small amplitude, therefore summation of inputs in time (temporal summation) or from multiple synaptic inputs (spatial summation) is usually required to reach action potential firing threshold.

6. TYPES OF SYNAPSES Types of synapses there are two types of synapses: electrical synapses chemical synapses

7. Electrical synapse electrical synapses are a  direct electrical coupling  between two cells mediated by  gap junctions , which are pores (as shown in the electron micrograph) constructed of  connexin  proteins essentially result in the passing of a gradient potential (may be depolarizing or hyperpolarizing) between two cells very rapid  (no synaptic delay) passive  process --> signal can degrade with distance-> may not produce a large enough depolarization to initiate an action potential in the postsynaptic cell bidirectional i.e., " post"synaptic cell can actually send messages to the " pre"synaptic cell

8. Chemical synapse Chemical synapses coupling between two cells through neuro -transmitters, ligand or voltage gated channels, receptors. Influenced by the concentration and types of ions on either side of the membrane. Glutamate, sodium, potassium, calcium are positively charged. GABA, chloride are negatively charged.

9. Chemical synapse Ionotropic receptors are single protein complexes that combine two functions. They have recognition sites on their surfaces extending into the extracellular fluid that allow them to interact with neurotransmitter molecules.  They also have the ability to open and close, allowing ions to move across the neural membrane. ionotropic receptors respond very quickly.

10. Chemical synapse Metabotropic receptors are made up of multiple protein complexes embedded in the neural membrane. One complex has the capacity to recognize neurotransmitter molecules but cannot open and close. Instead, the metabotropic receptor binds molecules of neurotransmitter It releases a G protein, or "second messenger," from its surface extending into the intracellular fluid of the neuron

11. Chemical synapse This G protein travels away from the receptor where it can interact with adjacent ion channels, which can then open and close like the ionotropic receptor. The eventual opening of ion channels is slower than it is with ionotropic receptors.

12. in contrast,  chemical synapses  are slow active  (require ligand -gated channels) pseudo-unidirectional

13. IPSP AND EPSP An electrical charge ( hyperpolarisation ) in the membrane of a postsynaptic neuron caused by the binding of an inhibitory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more difficult for a postsynaptic neuron to generate an action potential. An electrical change ( depolarisation ) in the membrane of a postsynaptic neurone caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more likely for a postsynaptic neurone to generate an action potential

14. EPSP Consider, for example, a neuronal synapse that uses glutamate as receptor. Receptors open ion channels that are non-selectively permeable to cations . When these glutamate receptors are activated, both Na +  and K +  flow across the postsynaptic membrane. The reversal potential ( E rev ) for the post - synaptic current is approximately 0 mV.

15. EPSP The resting potential of neurons is approximately -60 mV. The resulting EPSP will depolarize the post synaptic membrane potential, bringing it toward 0 mV.

16. IPSP As an example of inhibitory post synaptic s action, consider a neuronal synapse that uses GABA as its transmitter. At such synapses, the GABA receptors typically open channels that are selectively permeable to Cl - . When these channels open, negatively charged chloride ions can flow across the membrane.

17. IPSP Assume that the postsynaptic neuron has a resting potential of -60 mV and an action potential threshold of -40 mV. If  E Cl  is -70 mV, transmitter release at this synapse will inhibit the postsynaptic cell. Since E Cl  is more negative than the action potential threshold. It reduces the probability that the postsynaptic cell will fire an action potential.

18. Some types of neurotransmitters, such as glutamate, consistently result in EPSPs Others, such as GABA, consistently result in IPSPs. The action potential lasts about one msec , or 1/1000th of a second. In contrast, the EPSPs and IPSPs can last as long as 5 to 10 msec. This allows the effect of one postsynaptic potential to build upon the next and so on.

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