2.5 Motors of eukaryotic organisms
Figure 2.6: Schematic diagram of a vertebrate striated muscle. (A) Whole muscle that
consists of muscle bres (B), which each contain many myobrils (thin
lines). (C) The basic motor unit is the sarcomere, of which two are shown.
One sarcomere that is limited by so-called Z-disks (horizontal lines) consists
of thick myosin laments with cross bridges and thin actin laments that
are xed to the Z-disks. (D) The thick laments are made of myosin
molecules, the structure of which allows them to arrange into laments
and to bind with actin in order to contract the muscle. Sketch adapted
from Schmidt-Nielsen 1997, p. 403 and Alexander 2003, p. 17.
principles. In eukaryotic agella the main proteins are dynein and tubulin, while
in muscles, myosin and actin make up the essential force-producing part of the motor
Schmidt-Nielsen, 1997; Vogel, 2013. A agellum has the constriction of a set diameter,
which limits its use at larger body size to pump uid eciently. By combination of
agellar motors one can mainly scale the motor up in two dimensions by arranging
agella next to each other as linings of surfaces. Muscles, however, are more exible
to use as motors across many scales in multicellular organisms from millimetre-sized
copepods to metre-sized whales Schmidt-Nielsen, 1997.
The base unit of a muscle is the sarcomere (gure 2.6 C). By sliding the movable
actin laments across the xed myosin laments through actin-myosin cross links the
two ends of the sarcomere (Z-disks) are brought closer together, which leads to muscle
contraction Alexander, 2003; Vogel, 2013. This mechanism is similar to the dyneintubulin
interaction in agella, which results in only local contraction and extension
and thus an overall bending. A typical sarcomere unit is 2.5 µm long in the relaxed
state and long chains of sarcomeres (myobrils) are bundled together into muscle bres
(gure 2.6). A bundle of a certain number of bres are controlled by a single nerve
cell and thus build an eective motor unit. The subunits (sarcomeres, brils, bres)
can vary in number and size, depending on the type of muscle Schmidt-Nielsen, 1997;
Alexander, 2003; Vogel, 2013.
In a very simplied dynamical model a thicker muscle with a large cross-section
A L2 can produce a high force F = max L2 with maximum tension max due to many
units working in parallel, while a longer muscle can produce higher speeds v = fmax L