Chapter 5: Communication Between Neurons
o Tested with frog’s heart.
Found that the vagus nerve exerts a chemical that decreases rate of beating
Accelerator nerve was found to increase the rate of beating
Loewi identified that:
o acetylcholine (ACh) slows/inhibits frogs heart rate
o Epinephrine (EP or adrenaline) speeds up the frogs heart rate
Groups of neurons that release a chemical neurotransmitter of a certain type are named after
o E.g neurons that release Ach are called acetylcholine neurons or cholinergic neurons.
Walter Cannon demonstrated that cholinergic neurons and noradrenergic neurons play
complementary roles in controlling many bodily functions in the ANS
o ‘rest and digest’ to summarize the actions of acetylcholine neurons in the
o ‘fight or flight’ to summarize the collective excitatory actions of norepinephrine neurons
in the sympathetic ANS
The structure of Synapses
The 3 mains parts of a synapse are:
o Postsynaptic membrane
o Synaptic cleft
o Presynaptic membrane
Step in Neurotransmission
o 1. During synthesis, either the transmitter is created by the cell’s DNA or its building
blocks are imported and stored in the axon terminal.
o 2. During release, the transmitter is transported to the presynaptic membrane and
released in response to an action potential.
o 3. During receptor action, the transmitter traverses the synaptic cleft and interacts with
receptors on the membrane of the target cell
o 4. During inactivation, the transmitter either is drawn back into the axon of the
presynaptic cell or breaks down in the synaptic cleft. Otherwise, it would continue to
work indefinitely. Types of Synapses
-Nervous system features many different kinds of synapse, specialized in regard to location,
structure and function
-synapse is very versatile.
-Axodendritiv synapse : the axon terminal of a neuron meets a dendrite or dendritic spine of
-Somatic synapse: a case where axon terminal ends on a cell body
- Axoaxonic synapse: a case where axon terminal ends on another axon
-Axosynaptic synapse: case where axon terminal ends at another terminal.
In cases like axosecretory axon terminal synapses with capillary and seretes its transmitter
directly into the blood.
This explains that synapses need not include even a single axon terminal.
Excitatory and Inhibitory Messages
-Despite the variability in synapses, they convey only two types of message:
1) Excitatory- have round synaptic vesicles
2)inhibitory-flattened synaptic vesicles
also called Type 1 and type 2 synapses respectively and the different locations of the two divides the
neuron into two zones: Excitatory dendritic tree and inhibitory cell body.
Varieties of Neurotransmitters
four experimental criteria is used to identify neurotransmitter
1)The chemical must be synthesized in the neuron or otherwise be present in it
2). When the neuron is active, the chemical must be released and produce a response in some
3).The same response(receptor action) must be obtained when the chemical is experimentally
placed on the target
4)A mechanism must exist for deactivating or removing the chemical from its site of action after its
work is done.
-However, for many neurotramistter in the CNS, this criteria is only partly met.
-A chemical that is suspected ov being a neurotransmitter but has not yet met all the criteria for
proof is called a putative transmitter(supposed).
*Acetylcholin was the first substance identified as a neurotransmitter in the CNS
Neurotramistter may carry a message from one neuron to another by influencing the voltage
on the postsynaptic membrane. It may also induce effects such as changing