Synaptic Transmission of Impulse

SYNAPTIC TRANSMISSION OF IMPULSE

Structural properties common to all synaptic functions 

Presence of synaptic cleft 

  • There is no continuity of the cytoplasm across  the synapse.  There is a synaptic cleft (200-300 micron width) between the pre and post synaptic membranes.
  • Thickened and modified portions both on the synaptic knob membrane and post synaptic membrane.
  • Rich presence of mitochondria in the cytoplasm of the synaptic knob.
  • Presence of synaptic vessicle (300-600 micron dia.) storing the chemical transmitter in the synaptic knob. The vessicles are absent in the post synaptic cell.

Mechanism of synaptic transmission  

mechanism_of_synapse

Transmitter release 

  • The membrane of the synaptic knob contains large number of voltage gated calcium channels  (the other area of the nerve fiber has very few channels). The action potential  travelling along the fiber depolarizes the terminal membrane.  Large number of Ca++ along with Na+ from the extracellular fluid enter the synaptic knob.  It is postulated that when the Ca++ enter the knob bind themselves with the protein receptors present on the inside of the knob membrane (synaptic membrane).  The receptor sites are known as release sites. This Ca++ receptor combination causes the nearby vesicles to adhere and fuse with the synaptic membrane, and finally the vesicles to open to the exterior (into the cleft) by a process called exocytosis. A single action potential often opens several hundred vesicles and release their transmitter into the cleft.

Fate of the released transmitter  

  • It is accompanied by three ways
    •  Reincorporation of the transmitter by or into the vesicles. The opened vesicle invaginates towards the inside of the knob and pinch off to reconstitute the vesicle.  The appropriate transport proteins still present in the vesicle membrane shift into  and concentrate the transmitter in the vesicle.
    • Resynthesis of the transmitter in the cytoplasm of the knob and their immediate absorption into the vesicles.  This is a continuous process.  By this mechanism, the exhaustion of the transmitter  is minimised. As the vesicles by these methods of replenishing are used again this method is referred to as recycling of the vesicles.
    • But, when both the vesicle and the mitochondria are aged they disintegrate.  Both, new vesicles and the mitochondria are continually transported from the soma down the axon to the presynaptic knob.  They move along the axon at a velocity from 1cm to 40 cm / day.
  • Action of the transmitter on the post-synaptic membrane
    • The transmitter acts on the receptor proteins present on the post synaptic membrane. The receptor have two functional components
      • A binding component, which protrudes from the membrane to the exterior into the cleft and binds with the neurotransmitter released into the cleft. 
      • An ionophore component which through the membrane  protrudes to the interior of the post-synaptic membrane or neuron.
Last modified: Thursday, 9 June 2011, 5:05 AM