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. (The dendrites and somata
typically lack voltage-gated channels, which are found in
abundance on the axon hillock and axolemma.)
• 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
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).
• If the initial amplitude of the GP is sufficient, it will spread all
the way to the axon hillock where Voltage-gated Na channels
are present. If threshold is reached here then AP will be
generated. If not, no AP will be generated.
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. 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.
5. 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.
6. 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.