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Synapse

Definition:

  1. Communication or Junction between two neurons i.e. ‘Interneuronal junction’
  2. Through release of chemical transmitters
  3. more than 50 compounds have been identified as transmitter substances

•Each neuron divides to form > 2000 synaptic endings
•  10,000 x Synaptic  knobs  on  a  single  spinal  motor neuron
(on cell body = 2000 & on dendrites = 8000)
“There  may be few hundred or as many as 20,000 synaptic
connections from input fibers”
•  In CNS = 2×1014  synapses
•  In Cerebral Cortex 98% synapse on Dendrites (mostly excitatory)
& 2% on Soma
•  Signals transmit in forward direction i.e.
‘one – way conduction’
from pre-synaptic neuron to postsynaptic neuron.

Types of Synapse

1. Functional:
a.  Electrical
b.  Chemical
2. Anatomical:
a.  Axodendritic e.g.  cerebral & cerebellar cortex
b.  Axosomatic e.g.  cerebellum, autonomic ganglia
c.  Axoaxonal e.g.  cerebral cortex
“i.e. Presynaptic terminals are located on: dendrites, soma
or axon of another neuron”
Synaptic Development:
Growth cones (at the tip of Growing Axons) migrate through tissues & guided by:
i.  attractants & repellants
ii. Glial cells in CNS
Thus, neurons find right target to make right synapse.

Structure of Synapse

Øi. Presynaptic terminal

Øii. Neurotransmitter

Øiii. Postsynaptic terminal

Structures important to the function of the synapse:
1. Presynaptic vesicles
qcontain neurotransmitter substances to excite or inhibit postsynaptic neuron
Three types:
ØSmall & clear synaptic vesicles    (Ach, GABA, Glycine)
ØSmall with dens core (Catecholamines)
ØLarge vesicles with dens core (Neuro peptides)
2. Mitochondria
provide energy to synthesize neurotransmitter

Synaptic Transmission

1. Arrival of AP at presynaptic terminal
2. Release of Neurotransmitter into the synaptic cleft

  • Opening of voltage-gated Ca++ channels in presynaptic terminal
  • Ca++ Influx
  • Binding of Ca++ to protein molecules inside presynaptic terminal to “Release sites”
  • Binding of transmitter vesicles to “Release sites”
  • Exocytosis of neuro transmitter into the “Synaptic cleft”
  • (2000-10,000 Ach molecules in each vesicle)
  • each presynaptic terminal contain enough vesicles to transmit > 10,000 action potentials
3. Binding of Neurotransmitter to receptor proteins

Ion channels:
1. Cation channels; Na+, K+
2. Anion channels;  Cl-
Excitatory Receptors:
↑ Na+ Influx, ↓ Cl- influx or ↓ K+  conductance
↑ No of Excitatory receptors, ↓ No of  Inhibitory receptors
Inhibitory Receptors:
↑ Cl- conductance , ↑ K+  conductance
↓ No. of excitatory receptors by the activation of receptor enzymes
4. Development of Excitatory post synaptic potential  or Inhibitory post synaptic potential

Intrasomal Membrane Potential
RMP of Neuronal soma =  – 65 mv
RMP of Large peripheral nerve fiber  =  – 90 mv
Significance of Less Negative Intrasomal RMP is that   smaller  change in  RMP  (-ve & +ve) makes Neuron more excitable or less excitable.

Outcome:

The nerve impulse may be;
a. Blocked
b. Changed from single to repetitive impulses
c. Integrated with impulses from other neurons

“Second Messenger”- Activators:

1. Prolong changes in neuron for sec to months
2. Process of Memory
3. Commonest type is a group of “G-Proteins”
4. Activated portion ‘α’ Performs four functions;
i.Opening of channels
ii.Activates membrane enzymes
iii.Activates gene transcription
iv.Activates one or more intracellular enzymes

I. Excitatory Postsynaptic Potential (EPSP)

How Develops?
ØRMP =  – 65 mv
ØRelease of excitatory transmitter
ØBinding of transmitter
ØConformational change in excitatory receptors
ØRapid influx of Na+ ions
ØRMP becomes less -ve i.e. RMP drifts to +ve side (from – 65 to – 45 mv).
lEPSP i.e. EPSP =  + 20 mv.
lGeneration of AP in postsynaptic Neuron
Discharge of single presynaptic terminal cannot increase neuronal potential to  – 45 mv
lSimultaneous discharge of 40-80 terminals in rapid succession & at the same time is necessary.

Threshold for Excitation / Development of AP

lEPSP = + 20 mV ;    →   Elicits A.P in Initial Segment (7 times greater no. of voltage gated Na+ channels than soma)
(350 – 500 Na+ Channels/µm2)
EPSP = + 30 to + 40 mV is required to develop AP in soma                          (50-75 Na+ Channels/µm2 )
Threshold for Excitation of Neuron =  – 45 mV or EPSP = + 20 mV
Properties of EPSP
EPSP is similar to EPP & Receptor potential
EPSP is confined to synapse;
Does not propagate.
It is mono phasic.
Does not obey all-or-non law.
Significance: EPSP of threshold level generates AP that spreads throughout axon.

II. Inhibitory Postsynaptic Potential (IPSP)

Definition
“An increase in negativity of membrane potential beyond RMP level is called IPSP”
How Develops?
ØNormal RMP = – 65 mv.
ØRelease of inhibitory transmitter by presynaptic terminal
ØOpening of Cl- channels or K+ channels in postsynaptic membrane
ØCl- influx or K+ out flux
ØIncreased negativity of ICF of soma i.e.  Hyper polarization
Membrane potential decreases from – 65 mv to –70 mv
IPSP =  – 5 mv inhibits transmission of nerve signal through the synapse

Significance

Synaptic inhibition in CNS
Offers restriction over neuron to react appropriately i.e. to select exact no. of impulses & to block the excess ones
ØPoisons (e.g. Strychnine) destroy inhibitory functions of synapse which leads to Convulsions
ØIn Parkinson’s disease: Impaired inhibitory system leading to Rigidity
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Synaptic transmission-Neurotransmitters and neuropeptides



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