SYNAPTIC TRANSMISSION
A synapse appears as the thin, 
dark area near the bottom 
center in this electron 
microscope picture of a section 
through cerebellar cortex of a 
rat. To the left of the synapse, 
an axon cut in cross section is 
filled with tiny round synaptic 
vesicles, in which 
neurotransmitter is stored. To 
the right a dendritic process 
(called a spine) can be seen 
coming off of a large dendritic 
branch, which runs 
horizontally across the picture 
near the top. (The two sausage-
like dark structures in this 
dendrite are mitochondria.) The 
two membrane surfaces, of the 
axon and dendrite, come 
together at the synapse, where 
they are thicker and darker. A 
20-nanometer cleft separates 
them.

How are impulses started up in 
the first place, and what 
happens at the far end, when 
an impulse reaches the end of 
an axon?

    The part of the cell 
membrane at the terminal of an 
axon, which forms the first 
half of the synapse (the 
presynaptic membrane), is a 
specialized and remarkable 
machine. First, it contains 
special channels that respond 
to depolarization by opening 
and letting positively charged 
calcium ions through. Since 
the concentration of calcium 
(like that of sodium) is higher 
outside the cell than inside, 
opening the gates lets calcium 
flow in. In some way still not 
understood, this arrival of 
calcium inside the cell leads to 
the expulsion, across the 
membrane from inside to 
outside, of packages of special 
chemicals called 
neurotransmitters. About 
twenty transmitter chemicals 
have been identified, and to 
judge from the rate of new 
discoveries the total number 
may exceed fifty. Transmitter 
molecules are much smaller 
than protein molecules but are 
generally larger than sodium or 
calcium ions. Acetylcholine 
and noradrenaline are 
examples of neurotransmitters. 
When these molecules are 
released from the presynaptic 
terminal they quickly diffuse 
across the 0.02-micrometer 
synaptic gap to the postsynaptic 
membrane.