MadSci Network: Neuroscience

Re: How do motor neuron circuits work

Date: Wed Feb 19 05:11:47 2003
Posted By: Jocelyn Wishart, Lecturer, Education,
Area of science: Neuroscience
ID: 1021510160.Ns

Much of the following  information is from a lecture by Dr Kilburn of Old 
Dominion University

Roeder’s theory is that animal nervous systems are organized into command 
centers.  A command center is a unit of the nervous system that could 
include a variety of components, such as innate releasing mechanisms, 
pattern generators, song control.

He found ganglia (clusters of nerves) in each body segment suggesting that 
they controlled muscles of each segment and presumed these to be the 
command centers. He confirmed this by experimental evidence from cutting 
connections and stimulating isolated ganglia. There are many examples of 
these innate, continuously firing circuits of neurons in the body – 
animals  continuously respire, digest, pump blood and so on.

These command centers are interconnected via a hierarchy of inhibitory 
relationships: activating one inhibits the other. Roeder found that some 
brain cells were responsible for inhibiting activity in abdominal ganglia –
 preventing muscles from acting until “ordered” to do so by other command 
centers in brain. He showed that the protocerebral ganglion (brain) 
typically inhibits abdominal ganglia, suppressing muscle activity. 

Thus cutting off the insect’s head stops the inhibition and the abdominal 
ganglia continues its innate pattern of firing moving the limbs attached 
to its segment. These circuits are able to fire continuously as one of the 
activating (output) neurons is connected to an excitatory (input) neuron 
forming a feedback loop. At the synapses and neuromuscular junctions 
neurotransmitters are synthesized, used and broken down by enzymes in the 
usual way.

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