Figure 10.3. Neuron drawings by Ramon y Cajal from his autobiography, first
published in 1917 as Recuerdos de mi vida. Growth cones are depicted on the
right, at the ends of the two axons.The growth cone (Fig. 10.3) progresses via the extension of
fingerlike structures called filopodia. This extension—as well as the
migration of entire cells as they find their appropriate places in the de-
veloping organism—is propelled by actions of the internal cytoskele-
tal structure of the cell. The dynamic cytoskeleton is composed of
elaborate ordered arrays of protein polymers—microfilaments made
of actin proteins and microtubules made of tubulin proteins (Fig.
10.4).
Microfilaments and microtubules form long strands that perform
multiple functions within cells (Fig. 10.5). These include growth and
movement of cell processes (such as axons, dendrites, and dendritic
spines), as well as moving materials around within the cell. The
insertion and removal of membrane proteins such as ion channels,
transporters, and neurotransmitter receptors are orchestrated via cy-
toskeletal microtubules and microfilaments.
It is highly probable that microtubules are involved in aspects of
cell function that are at present only dimly appreciated. For example,
conventional psychopharmacology assumes that the actions of drugs
can be understood by figuring out how drug molecules stick to and
impact various neural membrane proteins, such as neurotransmitter
receptors, reuptake transporters, and voltage-gated channels (many
examples were given in Chapters 8 and 9). However, drug molecules
also enter cells and interact with various intracellular structures,
such as microtubules and microfilaments. Perhaps these intracellular
interactions will eventually be understood to play significant roles in
the overall effects of a drug. For example, it has been suggested that