Basic question about neurons

dag.stenberg at nospam.helsinki.fi dag.stenberg at nospam.helsinki.fi
Fri Nov 23 03:34:51 EST 2001


chris ackerman <cma1114 at home.com> wrote:
> I am a neophyte so forgive the simpleness of my question, but why is it that
> I always read descriptions and see pictures of neurons as having many
> dendrites and few axon terminals? If this is representative of most neurons
> in the brain, how is it possible to have so many inputs and so few outputs?

Kandel says: "The average neuron forms about 1000 synaptic connections
and receives even more, perhaps as many as 10,000 connections. The
Purkinje cell of the cerebellum receives up to 100,000 inputs".
Also, a dendrite has many synaptic inputs, while an axon terminal is
considered to be one output. According to the textbook by Zigmond &al.,
"as many as 30,000 to 40,000 spines are present on the largest pyramidal
neurons .... more than 80,000 spines per Purkinje cell". 
  That leaves us with the question whether this can be used for a simple
arithmetic computation of the number of synapses in the brain. I must
pass on this one, but now it started to intrigue me. Kandel's textbook
statement seems to mean that there are 10 times as many inputs as there
are outputs. I thought a single spine (input) did not receive more than
one type of (transmitter) terminal. Could it be that many inputs are not
coupled directly to a terminal, but receives paracrine stimulation?
What is the simple standard answer? I have never worried about it before.

Dag Stenberg




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