SIGNAL TRANSMISSION AT SYNAPSES

By: Mary Claire ButayaBSPSYCH-3

What is a SYNAPSE?

• It is the site of communication of neurons which consists of:

A. Presynaptic neuron (membrane)

B. Postsynaptic neuron (membrane)

Sends the signal

Receives the

message

Types of Synapse

A. Electrical Synapse

B. Chemical Synapse

Action potentials(impulses)

conduct directly between adjacent cells through GAP

JUNCTIONS

Membranes are close but do not touch with

the presence of SYNAPTIC CLEFT

A. Electrical Synapse

- Transmission is DIRECT because of gap junctions.

- GAP JUNCTIONS: contain a hundred or so tubular connexons.

- Gap junctions are common in visceral smooth muscle, cardiac muscle, and the developing embryo.

B. Chemical Synapse

- Transmission is INDIRECT because of synaptic cleft.

- SYNAPTIC CLEFT: a space of 20-50nm that is filled with interstitial fluid; separates the neurons.

Types of Chemical Synapses

• Axodendritic – axon to dendrite

• Axosomatic – axon to soma

• Axoaxonic – axon to axon

The PRESYNAPTIC NEURON converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter).

The POSTSYNAPTIC NEURON receives the chemical signal and, in turn, generates an electrical signal (postsynaptic potential).

How a chemical synapse transmits a signal

1. A nerve impulse (action potential) arrives at a synaptic end bulb of a presynaptic neuron.

2. the depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels; Ca2+ flows inward through the opened channels.

3. An increase in the concentration of Ca2+ inside the presynaptic neuron is the signal that triggers exocytosis of some of the synaptic vesicles.

4. The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron’s plasma membrane.

5. Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels (inflow of ions).

6. POSTSYNAPTIC POTENTIAL: change in membrane voltage. (as ions flow through the opened channels, the voltage across the membrane changes.

7. When a depolarizing postsynaptic potential reaches threshold, it triggers one or more nerve impulses.

Excitatory and Inhibitory Postsynaptic Potentials

• Excitatory: if the neurotransmitter depolarizes the postsynaptic membrane. (EPSP)

• Inhibitory: if the neurotransmitter causes hyperpolarization of the postsynaptic membrane. (IPSP)

Removal of Neurotransmitter

1. Diffusion

2. Enzymatic degradation

3. Uptake by cells

Spatial and Temporal Summation

• Spatial Summation

Results from buildup of neurotransmitter released

simultaneously by SEVERAL presynaptic end

bulbs.

• Temporal Summation

Results from builup of neurotransmitter released by a SINGLE presynaptic

end bulb two or more times in rapid succession.