SYNAPTIC INTEGRATIONTYPES OF SYNAPSES

EPSP & IPSP

M.Rajagopalan

V M.Sc Life sciences

SYNAPTIC INTEGRATION• Neurons in the brain receive thousands of

synaptic inputs from other neurons.

• Synaptic integration is the term used todescribe how neurons ‘add up’ these inputsbefore the generation of a nerve impulse, oraction potential.

• The ability of synaptic inputs to effectneuronal output is determined by a number offactors

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.

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 anindividual neuron processes its synaptic inputs and convertsthem into an output signal.

Neurons are specialised for electrical signalling, with the mainneuronal input signal (synaptic potentials) and the mainneuronal output signal (action potentials)

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 theirelectrochemical gradient.

Synaptic potentials can be either excitatory or inhibitorydepending on the direction and charge of ion movement.

Action potentials occur if the summed synaptic inputs to aneuron reach a threshold level of depolarisation and triggerregenerative 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 usuallyrequired to reach action potential firing threshold.

TYPES OF SYNAPSES

• Types of synapses

• there are two types of synapses:

– electrical synapses

– chemical synapses

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

Chemical synapse

• Chemical synapses coupling between two cellsthrough neuro-transmitters, ligand or voltagegated channels, receptors.

• Influenced by the concentration and types ofions on either side of the membrane.

• Glutamate, sodium, potassium, calcium arepositively charged.

• GABA, chloride are negatively charged.

Chemical synapse

• Ionotropic receptors are single proteincomplexes that combine two functions.

• They have recognition sites on their surfacesextending into the extracellular fluid thatallow them to interact with neurotransmittermolecules.

• They also have the ability to open and close,allowing ions to move across the neuralmembrane.

• ionotropic receptors respond very quickly.

Chemical synapse• Metabotropic receptors are made up of

multiple protein complexes embedded in theneural membrane.

• One complex has the capacity to recognizeneurotransmitter molecules but cannot openand close.

• Instead, the metabotropic receptor bindsmolecules of neurotransmitter

• It releases a G protein, or "secondmessenger," from its surface extending intothe intracellular fluid of the neuron

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.

• in contrast, chemical synapses are

• slow

• active (require ligand-gated channels)

• pseudo-unidirectional

IPSP AND EPSP• An electrical charge (hyperpolarisation) in the membrane of a

postsynaptic neuron caused by the binding of an inhibitoryneurotransmitter from a presynaptic cell to a postsynapticreceptor; makes it more difficult for a postsynaptic neuron togenerate an action potential.

• An electrical change (depolarisation) in the membrane of apostsynaptic neurone caused by the binding of an excitatoryneurotransmitter from a presynaptic cell to a postsynapticreceptor; makes it more likely for a postsynaptic neurone togenerate an action potential

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 areactivated, both Na+ and K+ flow acrossthe postsynaptic membrane.

• The reversal potential (Erev) for the post -synaptic current is approximately 0 mV.

EPSP• The resting potential of neurons is

approximately -60 mV.

• The resulting EPSP will depolarize the postsynaptic membrane potential, bringing ittoward 0 mV.

IPSP

• As an example of inhibitory post synaptics action, consider a neuronal synapse thatuses GABA as its transmitter.

• At such synapses, the GABA receptors typicallyopen channels that are selectively permeableto Cl-.

• When these channels open, negativelycharged chloride ions can flow across themembrane.

IPSP

• Assume that the postsynaptic neuron hasa resting potential of -60 mV and an actionpotential threshold of -40 mV.

• If ECl is -70 mV, transmitter release at thissynapse will inhibit the postsynaptic cell.

• Since ECl is more negative than the actionpotential threshold.

• It reduces the probability that thepostsynaptic cell will fire an action potential.

• Some types of neurotransmitters, such asglutamate, consistently result in EPSPs

• Others, such as GABA, consistently result inIPSPs.

• The action potential lasts about one msec, or1/1000th of a second.

• In contrast, the EPSPs and IPSPs can last aslong as 5 to 10 msec. This allows the effect ofone postsynaptic potential to build upon thenext and so on.

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