A note on the nerve impulse conduction through a synapse.

1. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 1
Synaptic Transmission
The synapse (Greek. synapsis, connection) is the junction between the axon and the
dendrite of another neuron or effector cell.
After an action potential travels along an axon, it reaches its end called the end bulb.
The neuron carrying the action potential towards the synapse is called a presynaptic
(before the synapse) neuron. It causes a response in the receptive segment of a
postsynaptic (after the synapse) cell, leading away from the synapse. The
presynaptic cell is always a neuron, whereas the postsynaptic cell may be a neuron
or an effector cell (muscle cell or gland cell).
TYPES OF SYNAPSES:
There are two main types of synapses: (1) chemical synapse and (2) electrical
synapse.
In a chemical synapse, the presynaptic neuron releases a chemical called a
neurotransmitter. This neurotransmitter in turn acts on receptive proteins in the
membrane of the postsynaptic cell. It changes the resting potential in the plasma
membrane of the receptive segment of the postsynaptic cell and creates an action
potential in it, which continues the transmission of the impulse. Some of the best-
known neurotransmitters are acetylcholine, epinephrine, norepinephrine, histamine,
glycine, serotonin, etc.
In an electrical synapse, direct open fluid channels conduct electricity from one cell
to the next. The positive ions (cations) move from one cell to the next. These ions
depolarize (cause action potential) the postsynaptic membrane, as though the two
cells were electrically coupled.
The chemical synapse is better than the electrical synapse in one aspect, that is that
it conducts the nerve impulse in only one direction, i.e: from a presynaptic cell to a

2. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 2
postsynaptic cell. Whereas, the electrical synapse can conduct the impulse in either
direction.
PHYSIOLOGICAL ANATOMY OF THE SYNAPSE:
The presynaptic terminals are usually in the form of small round or oval knobs and
are hence called synaptic knobs. The presynaptic terminal is separated from the
postsynaptic cell by a synaptic cleft. Inside the terminal, are two important structures:
transmitter vesicles and mitochondria. The transmitter vesicles contain the
neurotransmitters, that are released into the synaptic cleft and it either excites or
inhibits the postsynaptic cell. The mitochondria provide ATP (adenosine
triphosphate) molecules which provide energy for synthesizing neurotransmitters.
When an action potential spreads over a presynaptic terminal, depolarization of its
membrane causes a small number of transmitter vesicles to empty into the cleft. The
released neurotransmitter causes excitation or inhibition of the postsynaptic cell.
Figure: Physiological anatomy of the synapse
MECHANISM OF RELEASE OF NEUROTRANSMITTER:
The membrane of the presynaptic terminal, also known as the presynaptic
membrane contains a large number of voltage-gated calcium channels. When an

3. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 3
action potential depolarizes the presynaptic membrane, these calcium channels
open and allow a large number of calcium ions to flow into the terminal. The quantity
of neurotransmitters to be released is directly proportional to the quantity of calcium
ions released.
RECEPTOR PROTEINS OF THE POSTSYNAPTIC CELL:
The membrane of the postsynaptic neuron contains various receptor proteins. These
molecules have two important components: (1) a binding component which binds
with the neurotransmitter and a (2) an ionophore component which passes through
the postsynaptic membrane to the interior of the postsynaptic neuron. The ionophore
component is of two types: (1) ion channel and (2) second messenger activator.
Ion Channels:
The ion channel allows the passage of specific ions through the membrane. These
are of two types : (1) cation channels and (2) anion channels.
The cation channels allow sodium, potassium, and/or calcium ions to pass through it.
The positive charge that enters the postsynaptic cell, through the cation channel,
excites it. Hence, if a neurotransmitter opens a cation channel, it is called an
excitatory transmitter. The anion channel mainly allows chloride ions to pass through
it and some other anions as well. The negative charge that enters the postsynaptic
cell, through the anion channel, inhibits it. Hence, the neurotransmitter that opens
the anion channels, is called an inhibitory transmitter. The effect of ion channels is
not long-lasting, it only lasts for a fraction of a millisecond.
Second Messenger Activator:
For long-lasting postsynaptic neuronal excitation or inhibition, there is a second
messenger chemical system in the neuron itself. The effect of the second messenger
is long-lasting. One of the most common types of second messenger systems is the
system involving G proteins.

4. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 4
The G protein consists of three components: (1) alpha component (α) which is the
activator portion, (2) beta component (β), and (3) gamma component (γ). When it is
activated by a nerve impulse, the alpha portion of the G protein separates and
performs one or more of the following functions:
i. Opening specific ion channels through the postsynaptic cell membrane
(which stays open for a long time).
ii. Activation of cyclic adenosine monophosphate (cAMP) or cyclic guanosine
monophosphate (cGMP) in the cell (which alters the metabolism of the
cell).
iii. Activation of one or more intracellular enzymes.
iv. Activation of gene transcription.
Figure: Second Messenger system in which one neuron can activate a second
neuron by causing the release of G protein in the second neuron's cytoplasm. The G
protein can (1) open an ion channel, (2) activate an enzyme system [cAMP and
cGMP], (3) activate an intracellular enzyme system, and/or (4) cause gene
transcription.

5. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 5
CHEMICAL SUBSTANCES THAT ACT AS NEUROTRANSMITTERS:
More than 50 chemical substances act as neurotransmitters. They are divided into
two categories: small molecule – rapidly acting transmitters and neuropeptides.
Small Molecule – Rapidly Acting Transmitters:
Small molecule transmitters are various types of small molecules, some of them are
as follows:
 Amino acid neurotransmitters such as glutamate, GABA (γ-aminobutyric acid),
and glycine.
 Biogenic amines such as dopamine, norepinephrine, epinephrine, serotonin,
and histamine, which are made from amino acid precursors.
 Purinergic (made from purines) neurotransmitters such as ATP and
adenosine, which are nucleotides and nucleosides.
 Acetylcholine, which does not fit into any of the other structural categories, but
is a key neurotransmitter at neuromuscular junctions (where nerves connect
to muscles), as well as certain other synapses.
Neuropeptides:
The neuropeptides are each made up of three or more amino acids and are larger
than the small molecule transmitters. Some examples of neuropeptides include
the endorphins and enkephalins, which inhibit pain; Substance P, which carries pain
signals; and Neuropeptide Y, which stimulates eating and may act to prevent
seizures.