Muscular Movement

The movement of large multicellular organisms, such as ourselves, is achieved through the contraction of muscles, which are themselves composed of numerous specialized cells. These are eucaryotes, of course, so the ultimate control is by ions, just as for the simpler eucaryotic cilia and flagella discussed earlier. However, in the case of the larger animals, simple diffusion would be too slow a process to fulfil the need of responding sufficiently rapidly to changes in the environment, so evolution has augmented that process by developing nervous systems. We thus see signals being swiftly dispatched to remote regions of the body, via the nerve cells (neurons) with only the final (and very small) step in the signalling process being entrusted to diffusion. Muscle action is triggered by the electrical signals received from a motor nerve. The nerve cell has one highly elongated extension, or process, which can have a length of many centimetres, and this terminates in intimate contact with the membranes of the muscle cells. It is known as an axon and it is surrounded, over most of its length, by myelin which acts as a sort of electrically insulating layer. The myelin is occasionally interrupted, at sites known as nodes of Ranvier, at which the electrical signal receives a boost. A nerve signal travels much faster along such a myelinated axon than it would if the sheath were absent. Nerve cells can also receive signals from other nerve cells by way of synaptic contacts to their dendrites, or minor extensions. It is worth emphasizing that muscles can only be made to contract, not expand, by stimulation from a nerve signal. Muscular expansion is thus achieved through the contraction of an opposing muscle. Skeletal muscles, which are those used to move the limbs, are thus arranged in pairs of flexors (used to bend a joint – as does the biceps muscle of a human arm) and extensors (used to straighten out the joint – as does the triceps muscle of a human arm).

The advent of transmission electron microscopy, and its use particularly by Hugh Huxley in the 1950s, has permitted muscular microstructure to be investigated at high resolution, and muscles have been studied both in their expanded and contracted states. The data thereby acquired have shed decisive light on the issue of muscle function.

Define the words

Eucariotic, cilia, to trigger, to entrust, a flexor, advent, an extensor, to expand.

Complete the sentences

1. The movement of large multicellular organisms, such as ourselves, is achieved through…

2. However, in the case of the larger animals,...

3. We thus see signals…

4. The nerve cell has one highly elongated extension, or process,...

5. It is worth emphasizing that muscles…

6. Muscular expansion is thus achieved…

7. The advent of transmission electron microscopy…

Put the following words and word combinations into the gaps. Some of them may be used more than once.

sheath / shed light / augment / limbs / motor / cilia / contraction / extensor / terminate / diffusion

Blood is usually pushed through the circulatory system by ______________ of one or more muscular hearts.

Ctenophores propel themselves through water with eight comblike plates of fused ______________ that beat in a coordinated fashion.

Among the unicellular protists, oxygen and nutrients are obtained directly from the aqueous external environment by simple ______________.

The proteins of the complement system can ______________ the effects of other body defenses.

At its origin, a spinal nerve separates into sensory and ______________ components.

At its far end, the axon may branch to ______________ on more than one cell.