Oo tubulin proteinsFigure 10.5. Microfilaments (top) are long polymers of actin proteins. Each
actin protein is composed of about 375 amino acids. Microtubules (bottom)
are long polymers of alpha-tubulin and beta-tubulin proteins. Each tubulin
protein is composed of around 450 amino acids. Microtubules are cylindrical,
with a hollow core. Microfilaments and microtubules can grow to be hun-
dreds of times longer than their diameters and are composed of millions of
atoms.
Microtubule structure and function have even been invoked in an
important hypothesis about the nature of mind and consciousness
(see Chapter 22). Whatever the details turn out to be, it is likely that
the intricate structure of the cellular interior is involved in numerous
aspects of the life process—up to and including mind and conscious-
ness—that at this point we have little understanding of.
Decades after Ramon y Cajal’s observations, how axons know where