Astronauts must often live and work in zero gravity. Weightlessness is not a restful state and it can put great stress on body systems, particularly the skeletal system. Many changes occur in zero gravity. With less work to do, body muscles begin to break down and the bones start to lose calcium at an increased rate. The number of red blood cells in the body falls and there are dramatic shifts in fluid distribution: the face becomes puffy and the legs become thinner (astronauts call this condition ‘bird’s legs). The lack of gravity also affects the spine. Without the constant downward force, the spine lengthens by as much as eight millimeters. This can lead to blocked nerves and back pain. Often, astronauts also lose their touch sensitivity.
NASA scientists are busy looking at forms of treatment and exercise that might prevent bone breakdown. Such information might also have practical benefits closer to home. By studying and finding ways to slow the accelerated changes produced in space, it may be possible to develop new treatments for bone diseases such as osteoporosis, which occurs when bones become brittle due to a loss of calcium.
The bodies of most multicellular organisms need some form of support: body tissue is soft and collapsible and so needs to be held in a rigid frame. Land-living animals need more support than those that live in water. Support in animals is usually provided by a skeleton. The skeletons of animals are of two basic types. Fluid or hydrostatic skeletons are used by a wide range of soft-bodied invertebrates such as earthworms, flatworms and sea anemones. Arthropods, echinoderms and vertebrates have rigid skeletons.
A rigid skeleton supports important body organs, enabling them to work efficiently (they cannot operate properly when squashed), it protects the internal structures from damage and it allows the animal to locomote, or move from place to place.
Locomotion is different from movement. It refers not to a simple shifting of the body parts, but to the movement of the whole organism from place to place
Remember that movement is a feature of all living things and occurs at all levels. Atoms move within molecules and cell contents move inside cytoplasm. Plants move when they tilt their leaves towards the Sun and when their flowers open and close. The human ribcage moves up and down as we breathe.
Bacteria, for example, move from place to place using flagellae. Many protoctists (single-celled organisms) move around by altering their cell shape. But the most dramatic and most efficient forms of movement result from muscular contraction and occur in higher animals.
Getting enough food and finding a mate are major priorities for most animals. Animals need to locomote to do both. Animals may also move to escape from predators or harmful stimuli, to disperse themselves through a particular environment, to avoid competition from other individuals, or to move to conditions that are more favourable (into the shade on a hot day, for example).
In this chapter, I will first describe the features of different types of animal skeletons and then look at how each type of skeleton is used in locomotion.
A hydrostatic skeleton consists of an enclosed, fluid-filled cavity, which provides support. Water is difficult to compress and so, when it fills body cavities such as the coelom, it provides a fluid mass against which muscles can contract.
The supporting properties of water are also seen in animals that have other types of skeleton. In mammals, amniotic fluid, the fluid inside the uterus, surrounds and supports the developing fetus, and the vitreous humour, the jelly-like material inside the eyeball, supports the structures inside the eye.
Animals such as earthworms, leeches, caterpillars, snails and slugs move using a hydrostatic skeleton. Earthworms are burrowing animals that need sustained and powerful locomotion. They can move because their muscles, attached to the body wall, pull against the fixed volume of fluid in the coelom.
The muscles are arranged in blocks and each enclosed segment of the earthworm’s body is controlled separately. Contraction of the longitudinal muscles, which run along the length of each segment, shortens that segment. Contraction of the circular muscles, which run around the circumference of each segment, extends it.
To move forward, the earthworm extends the segments at the front of its body. This part of the body moves forwards, the front segments anchor themselves to the ground using bristles, or chaetae, and then these segments contract. The following segments ‘catch up’ with the front section by relaxing, anchoring and then contracting in the same way.
Other animals that have a hydrostatic skeleton move in a very similar way. In leeches, the whole body acts as a single hydrostatic system.
Exoskeletons, literally ‘skeletons on the outside’, occur mainly in arthropods such as crustaceans. Arthropods have a hardened outer ‘skin’ that protects and supports the body. This outer covering, the cuticle, is made up of a rigid polysaccharide compound called chitin. In land-living arthropod groups such as insects, spiders, centipedes and millipedes, this cuticle is covered by a layer of wax which provides a waterproof outer coat. The cuticle of crustaceans such as crabs and lobsters does not have a waxy layer (understandable in these mainly aquatic animals) and contains calcium salts for greater strength. The cuticle is not completely smooth. Internal folds add to its strength and ridges provide sites to which muscles attach.
The arthropod leg is basically a jointed hollow cylinder. Tendons Attach muscles to projections on the inside of the exoskeleton and transmit the pulling force of the muscles to the hardened, chitinous cuticle. Areas of soft cuticle form flexible joints that allow the limbs to bend. Insects walk like us. They bend and straighten each of their six legs in turn, while moving their body weight forwards. We can stand on one leg because we’ve got big feet, but an insect, which has small feet, needs to keep at least three of them on the ground. Muscles act across a joint working in pairs, one to flex, or bend the limb, the other to extend, or straighten it. Two sets of muscles that oppose each other in this way are called antagonistic muscles. Vertebrate muscles also work antagonistically. Most of the propulsive force needed for forward movement is generated by large muscles at the top of the leg.
An endoskeleton is a rigid structure that is inside the body, enclosed by soft tissues. Although we are more familiar with vertebrate endoskeletons, some invertebrates also have internal skeletons. The soft bodies of sponges are supported by sharp, mineral rods called spicules, and starfish and sea urchins are supported by small internal plates called ossicles.
Vertebrate skeletons are made up of bone and cartilage, which are specialized connective tissues. Some vertebrate skeletons, such as those of the cartilaginous fish, are made almost entirely of cartilage. In a developing vertebrate with a bony skeleton, cartilage appears first and is then replaced by bone. We call this process ossification.
Mending damaged bones is a major part of a surgeon’s work yet acceptable substitutes for bone are hard to find. It is possible to use bone from another part of the patient’s body but only small amounts can be used. Artificial substitutes run the risk of being rejected by the body’s immune system.
A few species of coral have a porous structure similar to that of bone. When grafted into the body, the honeycomb texture of coral provides the conditions necessary for new blood vessels to grow into it, and this promotes new bone growth. In addition, coral is tough, caries no risk of infection and is unlikely to be rejected by the body.
‘Liquid bone’ can also be made from coral. This is based on a calcium-rich solution and a phosphate-rich solution. The surgeon mixes the compounds in a little acid before applying it, rather like toothpaste, to the site of a fracture. Within 12 minutes it has solidified, and after one hour the ‘new’ bone is as hard as real bone.
The human skeleton contains a total of 206 bones. It is divided into two parts; an axial skeleton that comprises the skull and vertebral column and an appendicular skeleton that is made up of the limbs and limb girdles. All vertebrates show skeletal modifications related to their lifestyle.
Since humans are bipedal (they walk on two legs), the hips and lower spine take most of the weight of the body and so the pelvic girdle (the hip) is larger and less flexible than the pectoral girdle (the shoulder).
The human skeleton has several functions:
In the structure and features of the human skeleton; there is a pectoral (shoulder) girdle, made up of scapula (shoulder blade) and clavicle (collar bone); a loose arrangement of bones providing flexibility. The cranium (protects the brain). The mandible (lower jaw the only movable bone in the skull, allows chewing movements). The ribs protect heart and lungs: intercostal (between the ribs) muscles and breathing movements. Vertebral column (spine); 26 individual bones held together by ligaments and separated by cartilage discs; provides support for the axis of the body, protects the nerve cord. Carpals are the small bones of the wrist. Pelvic (hip) girdle: made up of the pelvic bone (itself composed of three fused bones) together with the sacrum at the back (fused bones at the base of the spine); a solid arrangement of bones providing stability.
Bone is one of the hardest tissues in the human body and is second only to cartilage in its ability to absorb stress. It is made up of bone cells called osteocytes, which sit in a bone matrix. The matrix consists of inorganic matter (mainly compounds of calcium and phosphorus) interwoven with collagen fibres.
Bone needs to be tough and resilient. These properties are provided by two types of bone tissue: compact bone and spongy bone. Compact bone is deposited in sheets called lamellae arranged as cylinders inside cylinders. Nerves and blood vessels run in a central canal. The compacted structure of the lamellae gives immense strength. The lamellae of spongy bone are arranged in a criss-cross pattern, forming a spongy honeycomb. This structure has excellent shock-absorbing properties.
As in other vertebrates, the human skeleton is jointed. A joint is simply a place in the body where two bones meet. Most joints are movable, but some, such as between the bones making up the skull, the sacrum (base of the spine) and the pelvis, are immovable, or fused. Elastic ligaments bind bones together, while tough inelastic tendons attach muscles to bone. The human knee joint is lubricated by a viscous fluid, synovial fluid, secreted by the synovial membrane, the membrane that lines the joint. The ligaments and the synovial membrane together form a joint capsule which surrounds the end of the bones. Different types of joint allow for different kinds of movement.
In my next post, I will explain more on how the human body moves.
