Transcript 02 Physiology of synapses, interneuronal connections

Physiology of synapses,
interneuronal connections
What is a synapse?
A synapse
is the junction between 2
neurones.
A specialized junction that transfers
nerve impulse information between
neurons

Synapses
A junction that mediates information transfer from one
neuron:
– To another neuron
 Called neuro-synapses or just synapse
– To an effector cell
 Neuromuscular synapse if muscle involved
 Neuroglandular synapse if gland involve
Presynaptic neuron – conducts impulses toward the
synapse
 Postsynaptic neuron – transmits impulses away from
the synapse
 Two major types:

– Electrical synapses
– Chemical synapses
Anatomical Types of Synapses
Axo-dendritic – synapses between the axon of
one neuron and the dendrite of another
 Axo-somatic – synapses between the axon of one
neuron and the soma of another
 Other types of synapses include:

– Axo-axonic (axon to axon)
– Dendro-dendritic (dendrite to dendrite)
– Dendro-somatic (dendrites to soma)
Functional classification
or Types of comnication

A.Chemical synapse
 Almost all synapses used for signal transmission
in the CNS of human being are chemical
synapses.
 First neuron secretes a chemical substance
called neurotransmitter at the synapse to act on
receptor on the next neuron to excite it, inhibit or
modify its sensitivity.
The chemical synapse is a specialized junction that
transfers nerve impulse information from a presynaptic
membrane to a postsynaptic membrane using
neurotransmitters.
Axo-dendritic synapse
Axo-somatic synanpse
Axo-axonic synapse
The Chemical Synapse
R.E.B, 4MedStudents.com, 2003
Neurotransmitters

Properties of neurotransmitters:
1) synthesized in the presynaptic neuron
2) Localized to vesicles in the presynaptic neuron
3) Released from the presynaptic neuron under
physiological conditions
4) Rabidly removed from the synaptic cleft by uptake or
degradation
5) Presence of receptor on the post-synaptic neuron.
6) Binding to the receptor elicits a biological response
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Neurotransmitters found in the
nervous system

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EXCITATORY
Acetylcholine
Dopamine
Histamine
Nonepinephrine
Epinephrine
Glutamate
Serotonin
INHIBITORY

GABA

Glycine
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The Synapse
Structure of a synapse
Excitatory postsynaptic potential
 Single
stimuli applied to the sensory nerves
in the experimental situation described
above characteristically do not lead to the
formation of a propagated action potential
in the postsynaptic neuron. Instead, the
stimulation produces either a transient,
partial depolarization or a transient
hyperpolarization.

If positive ion gates open (which allow more
Na+ and Ca2+ to enter than K+ to exit), the
membrane becomes depolarized, which results in
an excitatory postsynaptic potential (EPSP). If
the threshold potential is exceeded, an action
potential is generated.

If K+ or chlorine ion (Cl−) gates open (allowing
K+ to exit or Cl− to enter), the membrane
becomes more depolarized (hyperpolarized),
which results in an inhibitory postsynaptic
potential (IPSP). As a result, it becomes more
difficult to generate an action potential on this
membrane.
Summary of
Synaptic
Transmission
Chemical Synapse
Events at a chemical synapse
1. Arrival of action potential on presynaptic
neuron opens volage-gated Ca++ channels.
2. Ca++ influx into presynaptic term.
3. Ca++ acts as intracellular messenger
stimulating synaptic vesicles to fuse with
membrane and release NT via exocytosis.
4. Ca++ removed from synaptic knob by
mitochondria or calcium-pumps.
5. NT diffuses across synaptic cleft and
binds to receptor on postsynaptic membran
6. Receptor changes shape of ion channel
opening it and changing membrane
potential
7. NT is quickly destroyed by enzymes or
taken back up by astrocytes or presynaptic
membrane.
Note: For each nerve impulse reaching the
presynaptic terminal, about 300 vesicles
are emptied into the cleft. Each vesicle
contains about 3000 molecules.
Postsynaptic Potentials

NT affects the postsynaptic membrane potential
 Effect depends on:
– The amount of neurotransmitter released
– The amount of time the neurotransmitter is
bound to receptors
 The two types of postsynaptic potentials are:
– EPSP – excitatory postsynaptic potentials
– IPSP – inhibitory postsynaptic potentials
Inhibitory Synapses

Neurotransmitter binding to a receptor at
inhibitory synapses:
– Causes the membrane to become more permeable to
potassium and chloride ions
– Leaves the charge on the inner surface more negative
(flow of K+ out of the cytosol makes the interior more
negative relative to the exterior of the membrane
– Reduces the postsynaptic neuron’s ability to produce
an action potential
Electrical Synapses

Pre- and postsynaptic neurons
joined by gap junctions
– allow local current to flow
between adjacent cells.
Connexons: protein tubes in
cell membrane.

Rare in CNS or PNS
 Found in cardiac muscle and
many types of smooth
muscle. Action potential of
one cell causes action
potential in next cell, almost
as if the tissue were one cell.
 Important where contractile
activity among a group of
cells important.
The Synapse
The junction between two
neurons is termed a synapse
(synapsis = point of contact)
The narrow gap between the
two neurons at the synapse
is the synaptic cleft; the cleft
is filled with extracellular
fluid and spans an area of
= synapse
approximately 20 nm
A neuron that conducts impulses toward a synapse is
called a pre-synaptic neuron
A neuron that conducts impulses away from a
synapse is called a post-synaptic neuron
The Synapse
axon terminal
synaptic
knob
A single neuron may
have many thousands of
synaptic junctions on its
dendrites and cell body
mitochondrion
synaptic vesicle
(contains neurotransmitter)
pre-synaptic synaptic post-synaptic membrane
(with receptors for
membrane
cleft
neurotransmitter)
Events at the synapse
Voltage-gated calcium ion
channels open in the
pre-synaptic membrane
An action potential travels down
the axon of the neuron to the
synaptic knob and depolarises
the pre-synaptic membrane
Calcium ions diffuse
into the synaptic knob
post-synaptic
membrane
calcium ions
in the
extra-cellular
fluid
Events at the synapse
Voltage-gated calcium
channels open in the
pre-synaptic membrane
Calcium ions diffuse
into the synaptic knob
post-synaptic
membrane
An action potential travels down
the axon of the neuron to the
synaptic knob and depolarises
the pre-synaptic membrane
The uptake of calcium ions
triggers the fusion of
the synaptic vesicles with the
pre-synaptic membrane
Events at the synapse
Voltage-gated calcium
channels open in the
pre-synaptic membrane
Calcium ions diffuse
into the synaptic knob
The uptake of calcium ions triggers
the fusion of the synaptic vesicles
with the pre-synaptic membrane
Neurotransmitter is released
into the synaptic cleft
by EXOCYTOSIS
Neurotransmitter
diffuses across the cleft
and binds to specific
protein receptors
embedded in the
post-synaptic
membrane
post-synaptic
membrane
receptors in the
post-synaptic
membrane
Events at the synapse
Binding of neurotransmitter opens
Na+ gates in the membrane and
there is an influx of Na+ into the
post-synaptic neuron
post-synaptic
membrane
Neurotransmitter diffuses across
the cleft and binds to specific
protein receptors
embedded in the
post-synaptic membrane
An excitatory post-synaptic
potential (EPSP) builds
up across the membrane
and if this reaches
threshold, an action
potential is triggered in
the post-synaptic neuron
sodium
ions
Depolarisation of the post-synaptic membrane
Events at the synapse
Following activation of the
post-synaptic membrane,
neurotransmitter is removed from
the synaptic cleft to enable
further stimulation to occur
post-synaptic
membrane
The neurotransmitter, acetylcholine,
is hydrolysed by the enzyme
acetylcholinesterase, which is
located at the surface of the
post-synaptic membrane
The neurotransmitter,
noradrenaline, is actively
transported back into
the axon terminals
sodium
ions
Depolarisation of the post-synaptic membrane
Unidirectionality
Unidirectionality describes the one-way
transmission of nerve impulses between neurons
Neurotransmitter is stored and
released only on the pre-synaptic
side of the synaptic cleft
Receptors for neurotransmitter
are only located on the
post-synaptic membrane
synaptic vesicle
(contains neurotransmitter)
This arrangement allows
for the transmission of
impulses between neurons
in one direction only
post-synaptic membrane
(with receptors for neurotransmitter)
The nature of the
neurotransmitter determines
the response of the
post-synaptic membrane
During hyperpolarisation, the
post-synaptic membrane
potential becomes more negative
than its resting potential and
results from either the efflux of
positive charge or the influx
of negative charge
Inhibition occurs at
synapses where transmitter
release results in the
hyperpolarisation of the
post-synaptic membrane
Occlusion

On account of divergence one neuron may pass excitive
signals on the other neurons. Another neuron may excite
several neurons. But if from both neurons which is
divergented excitement will be simultaneously the total
quantity of excited neurons will be decrease.
Opposite inhibition
Lateral inhibition

If in a neurons' chain, which secure opposite inhibition
collaterals of axons of inhibition neurons form synaptic
connection with neighboring excitive cells in these cells
develop lateral inhibition
Spatial summation

The adding together of EPSPs
generated simultaneously at
many different synapses on a
dendrite.

Two or more presynaptic inputs
are active at the same time
A space (spatial) dependent
process.
Occurs in a Convergent Synapse


Temporal summation

The adding together of EPSPs
generated at the same synapse if
they occur in rapid succession,
within 1-15 msec of one another.

The same presynaptic fiber fires
AP in quick succession

A Time (Temporal) dependent
process

Occurs in a Divergent Synapse