Download Synapses and Neurotransmitters

Synapses and
Neurotransmitters
Action Potentials
We have been talking about action potentials
and how they allow an electrical impulse to
travel from the dendrites to the end plates of a
neuron.
These action potentials do not just move down
a single neuron and then stop.
Action Potentials
Your brain is a network of millions of neurons
that are, in essence, talking to one another.
Action potentials are the message and the
synapse is how the message is transferred
from one neuron to another.
Synaptic Transmission
The tiny spaces between neurons and effectors
are called synapses or synaptic clefts.
Synapses are formed between the end plates of
one neuron and the dendrites, axon, or cell body
of another neuron. They can also be formed
between the endplates of one neuron and an
effector (muscle)
Synaptic Transmission
The terminal branches
of a single neuron allow
it to join with many
different neurons
allowing the message
from one to be multiplied
very quickly by sending
it to many other
neurons.
Synaptic Transmission
Small vesicles in the end plates of neurons contain
chemical messengers called neurotransmitters.
As an impulse moves along a neuron, it causes the
release of these neurotransmitters from the end
plates.
Neurotransmitters are released from the
presynaptic neuron into the synaptic cleft.
Synaptic Transmission
Once neurotransmitters are in the synapse, they
diffuse across it until they attach to receptors on
the dendrites, axon, or cell body of the
postsynaptic neuron.
This binding of neurotransmitters creates a
depolarization of the postsynaptic neuron
stimulating an action potential and allowing the
message to move on.
Synaptic Transmission
Stages:
1. Action potential moves toward end plates
stimulating calcium channels to open
stimulating movement of vesicles.
2. Vesicles with neurotransmitter move towards
endplate of presynaptic neuron.
3. Neurotransmitters are released into synapse
through exocytosis.
4. Neurotransmitters diffuse across synaptic cleft.
Synaptic Transmission
Stages:
5. Neurotransmitters bind to receptors on
postsynaptic neuron.
6. Bound neurotransmitter stimulates response.
7. Neurotransmitter fragments released after use.
8. Fragments move back to presynaptic neuron
and re-enter cell through endocytosis for
recycling.
Synaptic Transmission
A synaptic cleft is extremely small (about 20nm
wide). Even with this small space, diffusion is a
slow process.
So when neurotransmitters are diffusing across the
synapse, the transmission of the message slows
down a bit.
Synaptic Transmission
Areas of the body with fewer synapses will result
in quicker transmission of an impulse.
What are examples of areas where we need to
react quickly?
Synaptic Transmission
Areas of the body with many synapses will result
in slower transmission of an impulse.
What are examples of areas where we react
slower?
Neurotransmitters
Neurotransmitters are used by neurons to change
the membrane potential of postsynaptic neurons.
They can either stimulate an action potential or
inhibit one.
Neurotransmitters that cause action potentials to
occur are said to be excitatory while those that
stop them from happening are called inhibitory.
Neurotransmitters
Acetylcholine (a-see-tyl-kol-een) is an excitatory
neurotransmitter found in the end plates of most
neurons.
When it attaches to the receptors on a
postsynaptic neuron it causes sodium channels
to open. Once this occurs, sodium ions rush in
causing depolarization to occur in this neuron.
Neurotransmitters
This stimulation of an action potential means that
the message is being passed on, which is a good
thing.
But, remembering action potentials, we know that
the cell needs to reach a repolarization stage
which means that sodium channels need to close.
Neurotransmitters
If acetylcholine remains attached, the postsynaptic
neuron is stuck in the depolarization stage.
An enzyme called cholinesterase (colon-esteraze) is
released by the presynaptic neuron.
This enzyme destroys acetylcholine allowing the
postsynaptic neuron to begin the recovery stages of
action potential.
Neurotransmitters
Scientists have used this knowledge of acetylcholine
and cholinesterase in the development of many
insecticides.
Cholinesterase is blocked which causes the heart of
the insect (which is completely controlled by nerves) to
contract but never relax therefore killing it.
Neurotransmitters
In humans, low levels of acetylcholine has been
related to deterioration of memory and mental capacity
giving evidence to this depletion being a cause of
Alzheimer’s disease.
Neurotransmitters
Although acetylcholine is considered an excitatory
neurotransmitter, there are some cases where it can
also be inhibitory.
Inhibitory neurotransmitters cause the membrane of
the postsynaptic neuron to become more permeable to
potassium ions. This leads to a hyperpolarization of
the membrane which means that an action potential
cannot occur.
Neurotransmitters
An example of an inhibitory neurotransmitter is
serotonin.
Action potentials are blocked to allow your brain to
enter a state of rest and allows you to sleep.
People with low levels of serotonin generally have a
hard time falling asleep or staying asleep.
Neurotransmitters
Another inhibitory neurotransmitter is gamma
aminobutyric acid (GABA)
GABA is the most abundant neurotransmitter in the
brain and is used to calm action potentials in the brain.
Having GABA in the brain allows you to prioritize
information and to focus on many different things at
once.
Neurotransmitters
People with low levels of GABA neurotransmitters can
suffer from certain anxiety disorders, panic disorders,
and Parkinson’s disease.
Certain drugs, like caffeine, inhibits the release of
GABA causing your brain to become ‘more alert.’ AKA
removing the inhibiting effect on action potentials.
Summation
It needs to be understood that in many cases, the
neurotransmitters released from a single neuron are
not enough to reach the threshold level in the
postsynaptic neuron which means an action potential
will NOT occur.
The effect produced by the accumulation of
neurotransmitters released from two or more neurons
is called summation.
Summation
This graph shows that
Neuron A and Neuron B
are both excitatory
because they are
stimulating a change
while Neuron C is
inhibitory.
Summation
It also shows that alone,
Neurons A and B cannot
reach the threshold level,
but if you combine their
reactions (summation),
the threshold is met and
an action potential will
occur.
Questions
Page 425
#3, 4, 5, 6