All this movement is directed 
by the brain. Each eye muscle is 
made to contract by the firing of 
motor neurons in a part of the 
brain called the brain-stem. To 
each of the twelve muscles 
there corresponds a small 
cluster of a few hundred motor 
neurons in the brainstem. 
These clusters are called 
oculomotor nuclei. Each motor 
neuron in an oculomotor 
nucleus supplies a few muscle 
fibers in an eye muscle. These 
motor neurons in turn receive 
inputs from other excitatory 
fibers. To obtain a movement 
such as convergence of the 
eyes, we would like to have 
these antecedent nerves send 
their axon branches to the 
appropriate motor neurons, 
those supplying the two 
internal recti. A single such 
antecedent cell could have its 
axon split, with one branch 
going to one oculomotor 
nucleus and the other to its 
counterpart on the other side. 
At the same time we need to 
have another antecedent nerve 
cell or cells, whose axons have 
inhibitory endings, supply the 
motor neurons to the external 
recti to produce just the right 
amount of relaxation. We would 
like both antecedent sets of 
cells to fire together, to produce 
the contraction and relaxation 
simultaneously, and for that we 
could have one master cell or 
group of cells, at still another 
stage back in the nervous 
system, excite both groups. This 
is one way in which we can get 
coordinated movements 
involving many muscles.

    Practically every movement 
we make is the result of many 
muscles contracting together 
and many others relaxing.