Synaptic transmission in CNS

1. Central Nervous
system
Synaptic transmission in CNS
Dr. S. Parasuraman, M.Pharm., Ph.D.,
Associate Professor,
Faculty of Pharmacy, AIMST University.
Transmission of nerve impulse

2. Cellular Organization of the Brain
• The CNS is made up of several types of specialized
cells that are physiologically integrated to form
complex functional brain tissue. The primary
communicating cell is the neuron.
• Neurons are the highly polarized signaling cells of the
brain.
• Neurons are electrically active cells that express a
variety of ion channels and ion transport proteins.
• The sites of interneuronal communication in the
CNS are termed synapses.
• Support cells in in the CNS: Neuroglia, vascular
elements, CSF-forming cells and meninges
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3. Synapse
• Synapse: The point at which the nerve impulse passes
from one to another is the synapse. Synapses allow for
chemical communication between neurons and other
cells through the use of neurotransmitters.
• Synapses are junctional complexes between presynaptic
membranes (synaptic knobs) and postsynaptic
membranes (receptor surfaces of recipient neurons or
effectors)
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4. Synapse
Fact: Synapses usually form between axon terminals
and dendritic spines, but this is not universally true.
There are also axon-to-axon, dendrite-to-dendrite,
and axon-to-cell body synapses.
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5. Synapse
• Synaptic transmission: is the process whereby one
neuron (nerve cell) communicates with other
neurons or effectors , such as a muscle cell, at a
synapse.
• Types of synapse
– Electrical synapse: Electrical synapses are a direct electrical
coupling between two cells.
– Chemical synapse: These synapses communicate using
chemical messengers.
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6. Synapse
• Types of synapse
– Electrical synapse Vs Chemical synapse
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Ref: Pereda AE. Electrical synapses and their functional interactions with chemical synapses. Nature Reviews Neuroscience.
2014 ;15(4):250.

7. Synapse
• Types of synapse
– Electrical synapse:
– electrical synapses are fewer in number than chemical
synapses.
– Some electrical synapses are bidirectional.
– There is a direct physical connection between the
presynaptic neuron and the postsynaptic neuron. This
connection takes the form of a channel called a gap
junction, which allows current—ions—to flow directly
from one cell into another.
– E.g.: Electrical synapses in the thalamus are thought to
regulate slow-wave sleep, and disruption of these
synapses can cause seizures.
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8. Synapse
• Types of synapse
– Chemical synapse:
– Almost every synapse in a human being are chemical.
– Chemical synapses are specialized junctions through which
cells of the nervous system signal to one another and to
non-neuronal cells such as muscles or glands. A chemical
synapse between a motor neuron and a muscle cell is
called a neuromuscular junction.
– Synaptic cleft/ synaptic gap: he space between neurons at
a nerve synapse
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9. Synapse
• Types of synapse
– Chemical synapse:
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10. Synapse
• Types of synapse
– Chemical synapse:
– Process of chemical synaptic transmission:
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Ref: http://www.interactive-biology.com/3950/the-chemical-synaptic-transmission-how-it-happens/

11. Synapse
• Types of synapse
– Chemical synapse:
– Process of chemical synaptic transmission:
1. Action potential reaches the axon terminal (depolarizes
the membrane and opens voltage-gated Na+ channels).
Na+ ions enter the cell, further depolarizing the
presynaptic membrane.
2. This depolarization causes voltage-gated Ca2+ channels to
open. Calcium ions entering the cell initiate a signaling
cascade that causes small membrane-bound vesicles
(synaptic vesicles).
3. Fusion of a vesicle with the presynaptic membrane causes
neurotransmitter to be released into the synaptic cleft.
4. The neurotransmitter diffuses across the synaptic cleft and
binds to receptor proteins on the postsynaptic membrane.
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12. Synapse
• Types of synapse
– Chemical synapse:
– Process of chemical synaptic transmission:
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Examples of well known neurotransmitters
Neurotransmitter Function Location
Acetylcholine muscle control, memory CNS and/or PNS
Serotonin
intestinal movement, mood regulation,
sleep
gut, CNS
Dopamine
voluntary muscle movements, cognition,
reward pathways
hypothalamus
Norepinephrine fight or flight response adrenal medulla
GABA inhibits CNS brain
Glutamate
generally an excitatory neurotransmitter,
memory
CNS, PNS

13. Synapse
• Types of synapse
– Chemical synapse:
– Process of chemical synaptic transmission:
– Excitatory postsynaptic potential (EPSP): Na+ enters the
postsynaptic cell and causes the postsynaptic membrane
to depolarize. This depolarization is called an EPSP.
– Inhibitory postsynaptic potentials (IPSPs): Release of
neurotransmitter at inhibitory synapses causes IPSPs, a
hyperpolarization of the presynaptic membrane.
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14. Synapse
• Properties of synaptic transmission
1. One-way conduction: Impulses are transmitted only
in one direction in synapse (from pre- to
postsynaptic neuron) and not vice versa.
2. Synaptic delay: During the transmission of impulses
via the synapse there is short delay in the
transmission. Is due to the time taken for
a) release of neurotransmitter
b) movement of the neurotransmitter
c) action of the neurotransmitter.
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15. Synapse
• Properties of synaptic transmission
3. Fatigability: When synapses are continuously
stimulated, after some time, due to exhaustion of
neurotransmitter at presynaptic terminals, impulses
fail to get conducted. This results in fatigue
occurring at level of synapse. Fatigue is a temporary
phenomenon.
4. Convergence and divergence: Convergence allows a
neuron to receive input from many neurons in a
network. Divergence allows one neuron to
communicate with many other neurons in a
network.
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16. Synapse
• Properties of synaptic transmission
5. Summation: The additive effect of several electrical
impulses on a neuromuscular junction. Individually the
stimuli cannot evoke a response, but collectively they
can generate a response. Spatial summation and
temporal summation are the two types of summation.
6. Excitation or inhibition: The impulse conduction across
a synapse may either stimulate or inhibit activity of
postsynaptic region.
7. Synaptic facilitation: Synaptic facilitation is a form of
short-term plasticity that enhances synaptic
transmission for less than a second.
16Ref: http://www.biologydiscussion.com/nervous-system/synapse-definition-mechanism-and-properties-with-
diagram/62838