The synapse is the connection between two neurons where information is transferred and interpreted.
It is a major site for the toxic actions by many insecticidal chemicals.
To understand how an insecticide can disrupt the nervous system,
it is essential to understand how a synapse functions.
Here we see a group of neurons that illustrate
a simple network for the interchange of information within the nervous system.
Information comes into the network and is distributed between the neurons at their synapse.
A synapse consists of a presynaptic neuron that brings in the information signal,
a synaptic junction where the information is transferred
from the presynaptic axon to the dendrite of a postsynaptic neuron,
and the postsynaptic axon that carries the information away to other neurons.
Information is encoded and transferred in the nervous system as action potentials.
Action potentials are discrete regions of negative depolarization
that are propagated actively along the axon membrane.
Let us look closely at the organization of the synapse.
The synapse consists of a button-like ending at the tip of the branches at the end of the presynaptic axon.
This presynaptic button is separated by a gap from the dendrite of the postsynaptic neuron.
The synaptic membrane of the postsynaptic dendrite
has numerous sodium channels that are associated with proteins called receptors.
receptors can detect special chemicals released by the presynaptic axon terminal
When an action potential enters the presynaptic axon terminal,
the terminal releases a special chemical called a neurotransmitter.
The message encoded in the frequency of the presynaptic action potentials
is converted by the presynaptic
axon into neurotransmitter chemicals
that are released and carry the message across the synaptic gap
to the postsynaptic dendrite.
The neurotransmitter chemicals bind to the receptor proteins
associated with the sodium channels of the postsynaptic membrane.
Receptor binding of the neurotransmitter chemical stimulates
the sodium channels to open and allows positive sodium ions
to flow into the postsynaptic dendrite.
Transfer of the sodium ions to the inside of the postsynaptic membrane
results in the outside of the postsynaptic membrane
becoming negatively charged in the region of the open sodium channels.
The negative depolarization of the postsynaptic area
spreads over the sensitive postsynaptic region
and generates new action potentials that are carried away by the postsynaptic axon.
To stop the depolarization of the post-synaptic region by the neurotransmitter chemicals,
proteins called enzymes are present that destroy the neurotransmitter chemicals.
When the neurotransmitter chemicals are eliminated, the postsynaptic sodium channels close,
the sodium is transported back to the outside of the postsynaptic membrane
by the reactivated sodium pumps,
and the outside of the post synaptic membrane returns to its normal positive charge.
It is relevant to note that the synapse illustrated here uses acetylcholine as its neurotransmitter chemical,
and the enzyme that destroys acetylcholine is called acetylcholinesterase.
Acetylcholine and acetylcholinesterase are important for their relationships
with the toxic actions of some insecticides.
In the next scene, we shall demonstrate how disrupting nervous functions
causes the twitching symptoms that are characteristic of poisoning
by many of the commonly used insecticides.