Answers to review questions – chapter 29
1.      For rapid movement of a part of the skeleton, is it better to have a muscle inserting close to or far away from the joint over which it passes and has its effect? (p. 705 Box 29.2)
The closer the muscle is inserted to the joint over which it has an action, the faster the resulting action can be made. For example, small changes in the length of the biceps brachii muscle in the upper arm result in large displacements of the forearm and these movements can occur quickly. If the muscle were situated further away from the joint (more distal), larger muscle movements would have to occur (and hence a slower overall response) to achieve the same effect.

2.      What are the basic components of skeletal muscle and how do these interact during a muscle-shortening event? (p. 708 Box 29.2)
Electronmicrographs taken at high magnification show a regular arrangement of filaments contained within intracellular structures called myofibrils. The myofibrils are tubular and are divided into compartments by discs (Z-discs), which lie perpendicular to the long axis of the myofibril. The distance between two adjacent discs is known as a sarcomere. Within each myofibril there are two types of myofilament: thick and thin. Each thick myofilament is composed of the protein myosin and the thin myofilaments are predominantly the protein actin. A myosin filament consists of a bundle of hundreds of myosin molecules, each of which is composed of two identical proteins. Each protein is shaped like a golf club, with the ‘shafts’ lying parallel to the myosin filament and the double heads projecting laterally. The actin filament is composed of two strands of fibrous actin, tropomyosin and troponin molecules. Each sarcomere contains two sets of actin filaments extending from the Z-discs, and one set of myosin filaments in the centre of the sarcomere. These thread-like structures are arranged parallel to one another and, in regions where the two populations of filaments overlap, actin filaments surround each myosin filament in a hexagonal arrangement.

The sliding filament model explains the mechanism of contraction. In this model it is proposed that the actin filaments, which slide relative to the myosin filaments, move towards the middle of the sarcomere and pull the Z-discs, to which they are attached, towards one another. This results in a shortening of the sarcomere and hence the muscle contracts. Individual filaments do not change length or contract but simply slide relative to one another. The whole process requires an input of energy, which is derived from the breakdown of ATP. The basis for the sliding filaments is the interaction between the heads of myosin molecules and specific sites on adjacent actin filaments, for which myosin heads have a high affinity. When a muscle is in its resting state, these active sites on the actin filament are obscured by tropomyosin, a regulatory protein associated with the actin filament. In this condition, the myosin heads are unable to form attachments or cross-bridges with the actin.

Muscle contraction can occur only when these binding sites are exposed, and this requires the presence of calcium Ca2+ ions, which are normally stored within modified endoplasmic reticulum, the sarcoplasmic reticulum. Upon neural stimulation great enough to produce muscle contraction, an action potential spreads across the muscle cell membrane and along the T-tubule system formed by invagination of the plasma membrane. The T-tubules conduct the action potential deep into the fibre, causing a wave of depolarisation to cross the membrane of the sarcoplasmic reticulum. This increases membrane permeability to Ca2+, thus releasing these ions into the sarcoplasm. The Ca2+ binds to troponin and causes a conformational change in the tropomyosin-troponin complex, which uncovers the binding sites, allowing contraction to occur. Myosin heads now bind to actin filaments, forming cross-bridges. A subsequent change in shape of the myosin heads results in the actin filament being pulled towards the centre of the sarcomere. (It is at this stage of the cycle that mechanical work occurs—that is, the muscle shortens and/or generates force.) The binding of ATP to a myosin head causes it to dissociate from the actin filament and return it to its original ‘primed’ condition before reattachment to another binding site further along the actin filament.

This cycle, which lasts for tenths or hundredths of a second, can then be repeated, resulting in further muscle shortening. The energy needed to fuel one complete cycle is ultimately derived from the hydrolysis of one ATP molecule. Muscle relaxation also requires an input of ATP, but in this case it is used in the active transport of Ca2+ back into the sarcoplasmic reticulum. A reduction in the Ca2+ concentration within the sarcoplasm allows the tropomyosin-troponin complex to re-establish its position, blocking the actin binding sites once again and preventing the establishment of new cross-bridges.

3.      What is the energy source for muscle contraction? What happens when muscle operates anaerobically? (pp. 710–712)
The direct source of energy for muscular contraction is ATP and is produced by aerobic or anaerobic respiration. The amount of ATP stored in a resting muscle cell is low but other energy stores are present within the cell. Glucose is converted into glycogen and this is stored in the cell. When muscle activity occurs, glycogen can be broken down to yield glucose. Creatine phosphate is an important energy storage compound in muscle and is formed by the transfer of a high-energy phosphate from ATP to creatine. It can be utilised for the rapid conversion of ADP to ATP in conditions of sudden demand.

Using the example of human runners, short periods of intense activity (sprinting) exhaust the supply of ATP present in resting muscle almost immediately. Anaerobic respiration and creatine phosphate breakdown produce enough ATP for further contraction (for about 20 seconds). One outcome of sprinting is the build-up of high concentrations of lactic acid within muscle fibres. This and the exhaustion of creatine phosphate stores effectively limit the time over which sprinting can be maintained. Aerobic respiration is used to produce ATP in long-distance running, where the muscular effort is sustained at submaximal levels for long periods of time. Under these conditions, fatty acids form a more important energy source than does glucose, although glucose is also metabolised.

4.      Why is it that most large animals tend to stand and move using more upright limb postures compared to small animals? (p. 707)
A straight-legged (upright) limb posture is an adaptation that reduces both the muscular energy costs of standing and locomotion. It also reduces the forces that act on the limb bones well aligned with the long axis of the bones during locomotion and standing still. If the forces were less well aligned, the limb bones would have to be bigger if they were to be as strong. This posture does have the disadvantage of reduced manoeuvrability and ability to accelerate rapidly as the limbs are almost fully extended when at rest. Small animals can get away with increased muscular expenditure of flexed limb posture, gaining rapid acceleration in exchange.

5.      What is the general response of living bone to increased loading? Why is this a desirable response? (pp. 704–705)
Living bone responds to increased loading by increasing both in thickness and density. This remodelling allows the skeleton to adapt to new situations where the pattern of magnitude of the forces acting on it is altered. Good examples can be found in the increase in thickness of the bone in a professional tennis player’s racquet arm, and in the loss of density in a limb immobilised in a plaster cast.

6.      Why do many jumping animals have relatively long limbs? (pp. 701–702)
The limbs involved in jumping are often long as this allows the feet to remain in contact with the ground for a relatively long period of time, during which limb extension accelerates the body upwards.

7.      Why is a bipedal stance intrinsically less stable than a quadrupedal one? (pp. 699–701)
Any animal, when it stands still, has to maintain its body’s centre of gravity directly above the area bounded by the outlines of the feet, otherwise it will fall over. Animals with four feet are inherently more stable than those with only two feet, as it is much harder to accidentally cause the centre of gravity to move outside the support base.

8.      What is the ‘aspect ratio’ of a bird and what influences does it have on gliding performance? Why is the ability to hover limited to small animals? (pp. 695–696)
The ‘aspect ratio’ of a bird is the tip-to-tip wing length divided by the average width. Animals with large aspect ratios are able to glide at shallow angles relative to the ground and are thus relatively much more efficient at gliding.

Hovering is extremely energetically demanding and, as the power requirements are proportionally higher for larger animals, hovering is effectively limited to small animals.

9.      Which groups of animals use jet propulsion to locomote? (pp. 691–692)
Three groups of animals use what could be called jet propulsion to locomote. The most well known are the cephalopod molluscs—the octopuses, squid, cuttlefish and Nautilus—which forcefully contract the circular muscles in the walls of the mantle cavity, expelling water rapidly through the funnel and accelerating the animal in the opposite direction. Many jellyfish also use jet propulsion, contracting circular subumbrellar muscles and resulting in the ejection of a pulse of water from the bell. While not very efficient, this does serve to keep them in the surface waters. Finally, while most bivalves are sedentary, some, such as scallops and file shells, can hop and swim by clapping their shells together. Muscles attached to the internal surfaces of the shells provide the power by pulling the shells shut, ejecting water rapidly.

10.    Define ‘buoyancy’. Explain how it might be achieved and the possible benefits gained by neutrally buoyant animals. (pp. 690–691)
Buoyancy is the tendency for objects to float, usually in water. Negative buoyancy is inherent in all animals as tissues, bone and muscles are denser than water. In order to counteract this, many water-dwelling animals have in-built buoyancy mechanisms which counteract the tendency to sink and give them neutral buoyancy. Neutral buoyancy allows animals to hover in mid-water without having to swim constantly. In addition, a neutrally buoyant animal requires less power (work per unit time) to swim at a particular speed than if it were negatively buoyant. Positive buoyancy can be achieved by trapping gas within various cavities in the animal. The extreme example to this is Physalia (the so-called bluebottle jellyfish or Portuguese man-of-war), where one highly modified individual in the colony forms a very conspicuous gas-filled sac that acts as a float. The mollusc Nautilus has a spiral shell divided into numerous chambers that are gas-filled, conferring buoyancy. A similar approach is adopted by many teleost fish which have a gas-filled sac, called the swim bladder, to control buoyancy. A different approach is adopted by many sharks, which secrete lipid that is less dense than sea water into their large livers and this then acts as an in-built flotation device. Even so, many sharks are slightly denser than sea water and use their fins in conjunction with constant forward swimming to produce lift to keep them off the bottom.