SYNAPSE:
Dr. Ayisha Qureshi
MBBS, Mphil
Department of Physiology
Synapse:
Definition:
A synapse is a region of functional contact and anatomical
differentiation between two neurons.
OR
It is a point of contact between two adjacent neurons.
•
•
•
Action potentials cannot cross the synaptic cleft present between
2 neurons.
Nerve impulse is carried by neurotransmitters which transmit the
nerve impulse from one nerve cell to the next across the synapse.
The structure of synapse consists of:
–
–
–
presynaptic ending (from where neurotransmitters in vesicles are
synthesized & released)
post synaptic ending (has neuroreceptors in the membrane)
synaptic cleft
CLASSIFICATION OF SYNAPSES:
Classification
Physiological/
functional
Chemical
synapse
Electrical
synapse
Mixed synapse
Types of Synapses:
1. Chemical Synapse (transmission thru chemicals
i.e. NT)
2. Electrical Synapse
• Impulse conducted without release of NT
• Synaptic gap only 2-3 nm
• No synaptic delay
• Unidirectional & Bidirectional conduction
3. Mixed Synapse i.e. having both electrical &
chemical regions
CLASSIFICATION OF SYNAPSES:
Anatomical classification of Synapses:
1. Axo-dendritic
2. Axo-somatic
3. Axo-axonic
4. Somato-dendritic
5. Dendro-dendritic
6. Somato-somatic
7. Reciprocal
8. Serial
9. Triad
STRUCTURE OF A SYNAPSE:
Structure Of a Synapse
SYNAPSE= Presynaptic terminal + Synaptic cleft + Postsynaptic terminal
• Presynaptic terminal: is the first part of the synapse & is
usually (not always) the Axon terminal. The axon terminals are
also called the bouton terminaux or synaptic knob. The
synaptic knobs have synaptic vesicles that contain the NT
(neurotransmitters). The NT are produced in the body &
conducted along the axon (anterograde flow). The NT can be
inhibitory or excitatory.
• Synaptic cleft or gap: is app. 20nm. It is a non-anatomical
continuity between the post and pre-synaptic ends.
• Postsynaptic terminal: is the name given to the last part of
the synapse. It is usually comprised of the dendrite or the cell
body on which the axon synapses.
Mechanism Of Conduction of an
Impulse in a chemical synapse
•
action potential reaches the PRESYNAPTIC terminal
↓
• voltage-gated Ca2+ channels open
↓
• influx of Ca2+
↓
• synaptic vesicles fuse with the pre-synaptic membrane (exocytosis)
↓
• neurotransmitters are released into SYNAPTIC TERMINAL cross it and diffuse to
the POST-SYNAPTIC terminal
↓
• neurotransmitter binds to neuroreceptor on postsynaptic membrane
↓
• causes Na+ channels to open, and Na+ flows into postsynaptic membrane
↓
• if threshold is reached then action potential is initiated
↓
• neurotransmitter is broken down by specific enzymes in the synaptic cleft.
Fate of the Neurotransmitter:
Dissociates from the Receptor & can have either
of the 3 fates:
• Enzymatic Degradation: A portion of it is
inactivated by the enzymes present in high
concentration at the postsynaptic membrane.
• Re-uptake of remaining NT by Pre-synaptic
neuron and Re-used.
• Diffusion into the blood stream.
Fate of Neurotransmitters:
POSTSYNAPTIC POTENTIAL
CHANGES AT THE CELL
MEMBRANE :
GRADED POTENTIAL
• Let’s consider a stimulus at the dendrite of a neuron. The stimulus
reaches the dendrite (postsynaptic neuron) from the axon
(presynaptic neuron) with the help of a NT.
• The NT leads to opening of simple ligand-gated channels that are
present in the postsynaptic membrane, either Na+ or K+ channels
which leads to Na or K or Influx; this could lead to depolarization or
repolarization.
• However, dendrites and somata typically lack voltage-gated
channels, which are found in abundance on the axon hillock and
axolemma. They only contain Na channels that depend upon the
NT….
– So what cannot occur on dendrites and somata?
An Action Potential cannot occur in the soma & dendrites
• Thus, the question we must answer is,
“what does the depolarization that results due
to the opening of the ligand gated Na channels
do?”
This depolarization leads to the generation of a
a GRADED POTENTIAL….
Graded Potentials
• The positive charge carried by the Na+ spreads as a wave of
depolarization through the cytoplasm (much like the ripples
created by a stone tossed into a pond).
• As the Na+ drifts, some of it will be moved back into the ECF by
the NA-K pump.
– What this means is that the degree of depolarization caused by the
graded potential decreases with distance from the origin unlike the AP.
Graded Potentials (GP)
• Their initial amplitude may be of almost any size – it
simply depends on how much Na+ originally entered
the cell, which depends on how many NT molecules
were released.
• If the initial amplitude of the GP is sufficient, it will
spread all the way to the axon hillock where Voltagegated Na channels reside.
• If the arriving potential change is threshold or
suprathreshold, an ACTION POTENTIAL will be
initiated in the axon hillock and it will travel down
the axon. If the potential change is subthreshold,
then NO AP will initiated and NOTHING will happen.
The kind of ion that enters the Postsynaptic
terminal (whether Na or K) will determine the
kind of potential that will be generated:
Depolarization OR Repolarization.
EPSP
IPSP
(Excitatory postsynaptic potential)
(Inhibitory postsynaptic potential)
• Opening of Na channels
• K & cl channels are not
opened.
Both the above actions cause
increased positivity of the
neuron and so excitation
that can lead to
depolarization & AP….
• Opening of Chloride
channels & chloride ion
influx
• Increased efflux of K ions
Both cause increased
negativity inside the neuron
leading to repolarization
and inhibition of the
neuron. Thus, AP will NOT
be initiated…
GRADED POTENTIAL
EPSP
IPSP
PROPERTIES OF SYNAPSES
1. DALE’S LAW:
This law states:
At a given chemical synapse only one type of
neurotransmitter is released and thus only
one effect, either excitatory or inhibitory, is
possible.
2. IRREDUCIBLE SYNAPTIC DELAY
Definition:
It is the time taken for the neurotransmitter to
be released from the presynaptic membrane,
diffuse across the synaptic cleft to reach the
post synaptic membrane and bind to the
neuroreceptors there.
It is about 0.5 msec.
3. ONE-WAY TRAVEL
In a chemical synapse the impulse always travels
from the presynaptic to the postsynaptic cell
as the neurotransmitter is only released from
the presynaptic terminal.
4. CONVERGENCE
Usually the postsynaptic neuron receives
afferents from a large no. This means that a
number of neurons will synapse on a single
neuron. It is very rare to find that only a single
neuron synapses on another single neuron.
1:1 convergence is rare.
5. SPATIAL SUMMATION
Summation of stimuli from two different
presynaptic elements reaching a neuron
simultaneously, which by adding up results in
excitation or facilitation, of a postsynaptic
neuron is called SPATIAL SUMMATION.
6. TEMPORAL SUMMATION
Summation of stimuli from two different
presynaptic impulses reaching a neuron one
after the other, which by adding up results in
the excitation or facilitation of a postsynaptic
neuron is called TEMPORAL SUMMATION.
7. EFFECTS OF CHEMICAL CHANGES IN THE BLOOD:
• Acidosis depresses while alkalosis increases
the neuronal activity.
• Hypoxia exerts a depressing effect.
8. OCCLUSION:
The deficit in the effect of the discharge of the
two preganglionic neurons on being
simultaneously stimulated is called
OCCLUSION.
(Figure: 2-20, page: 136, Essentials of Medical Physiology by
Mushtaq)
9. INHIBITION:
• Feed- Forward Inhibition
• Presynaptic Inhibition
• Recurrent Inhibition
(Figure: 2-21, page: 137, Essentials of Medical Physiology by
Mushtaq, 5th edition)
10. FATIGUE
If there is continuous stimulation of the
presynaptic synapse, this leads to the
neurotransmitter supply being exhausted. This
causes the synaptic transmission to stop.