2 Movement

2.1 Human Movement and Muscles on the Molecular Scale

In this chapter, voluntary movement is discussed. As with the human senses, human vol-

untary movement happens in response to a signal. When the signal occurs, the muscle

contracts. Depending on the muscle force needed, a different number of muscle fibers

contract. In most cases, this contraction takes place over a short time period and the

muscle will quickly return to its extended, relaxed state.

Molecularly-speaking, how does a muscle contract? A muscle contains bundles of

parallel muscle fibers called fascicles (Figure 2.1a) [1]. Each muscle fiber is actually a cell

with its own cell membrane and nucleus, as well as storage granules containing glycogen

(see 1.2, Structure and function of molecules – sugars and polysaccharides). The muscle

cell has several important special features: it contains the sarcolemma, which is a large,

membrane-covered storage space for calcium ions (in fact, there are basically no cal-

cium ions present in the cell with the exception of the calcium ions in the sarcolemma).

Additionally, the cell membrane of muscle cells is special in that it is charged, and it can

change its charge as a nerve cell does by pumping sodium and potassium ions in and

out of the cell.

The signal for contraction comes from nerve cells originating in the spinal cord. If

these nerve cells release their neurotransmitter acetylcholine, which activates ion chan-

nels in the cell membrane, ions will be released and thus the charge on the membrane

is changed (depolarization) (Figure 2.1b) [1]. As soon as the charge of the membrane

changes, calcium channels in the membrane are activated to pump a small amount of

calcium ions into the muscle cell. Those few calcium ions are sufficient to activate cal-

cium channels in the sarcolemma, which then pump many calcium ions into the muscle

cell. As seen with many signals in cells, the stepwise activation leads to the amplification

of the signal and thus a fast change, as needed for voluntary muscle contraction [2]. Now

that we have the signal, how does the signal lead to an actual force?

To understand that, another special set of features of the muscle cells must be ex-

plained: the myofilaments in the myofibrils, actin and myosin (Figure 2.1). Actin is a fiber

that is stiff and fixed in the cell. Myosin is also a stiff fiber but has a lot of heads that can

move [3]. Myosin can walk with those heads along the actin fibers and pull the whole

fibers and thus cells with it, 10 nanometers at a time (Figure 2.2). This occurs via lever

action, part of the myosin head protein being built like a lever [4–6]. So each muscle con-

traction is a combination of a lot of concurrent 10 nm lever actions all parallel to each

other and in the same direction [7].

When the muscle is at rest, the actin fiber is covered and does not allow the myosin

to bind [1]. Calcium ions in the muscle cell essentially pull the covers of the actin fibers

away, exposing binding sites for the myosin heads. The myosin heads are always acti-

vated, i. e. ready for the next pull, when the muscle is at rest. Therefore, as soon as it

is possible, the myosin heads will bind the actin fibers and move the head so that the

https://doi.org/10.1515/9783110779196-002