Lecture 16: Locomotion

1. Three organ systems play key roles in movement. The nervous system issues commands to the muscular system, which exerts force against a firm skeletal system. Some animals remain in place and let the world come to them. Sponges use flagellated collar cells to move water through their bodies. Some cnidarians (such as Hydra) remain attached and move slowly during feeding activities. Locomotion in all its forms requires an animal to overcome two forces: friction and gravity. Water supports against gravity but offers considerable frictional resistance. Air offers little resistance but provides little support. Swimming involves legs as oars (many aquatic insects and mammals), jet propulsion (squids), whole body side to side (fishes), and up and down (whales).

Land animals not only must overcome gravity but must maintain balance while they move forward. Terrestrial locomotion includes hopping on springlike back legs, and quadrupedal or bipedal walking and running (Figures 30.lB, 30.lC). Some land animals crawl (by undulating movements or by peristalsis. During peristalsis, longitudinal muscles shorten and thicken some regions, while circular muscles constrict and elongate other regions. In an earthworm, bristles anchor the short, thick regions, and regions anterior to them lengthen (Figure 30.lD).

2. True flight has evolved only in insects, birds, extinct reptiles, and mammals (bats). To fly, animals move wings in patterns that provide lift. Bird wings have cross-sectional shapes of airfoils. Air flowing past an airfoil has lower pressure above relative to below, providing lift (Figure 30.lE). NOTE: Insects, bats (most of the time), and some birds (hummingbirds) produce lift in a different way ("fluttering," which is more like the lift a helicopter produces) by pushing their wings down against the air during a power stroke and slipping them up through the air during a retum, nonpower stroke. This type of flight enables these animals to hover, a feat the rest of the birds cannot do without fluttering, and then inefficiently. Some other animal groups (fishes, amphibians, and other mammals) have evolved gliding, which moves the animal through the air or water without producing lift.

3. All types of movement are based on either the contraction of microtubules (see cilia and flagella in Module 4.18) or the contraction of microfilaments (amoeboid movement and muscle contraction).

4. Skeletons function in support, movement, and protection (Module 30.2). There are two types of skeletons: exoskeletons, and endoskeletons. A hydrostatic skeleton consists of a volume of fluid held under pressure in a body compartment. Such skeletons work well for aq ' uatic animals and animals that burrow by peristalsis. Earthworms have a body composed of fluid-filled sections. Hydra plays muscle cell contractions against a hydrostatic skeleton of its closed gastrovascular cavity (Figure 30.2A). An exoskeleton consists of a rigid, external, armor-like covering. Muscles are attached to the inner surface of the exoskeleton. At joints, the exoskeleton is thin and flexible. Clams and snails have exoskeletons (shells) that are enlarged by secretions from the body margin (mantle). The hollow, tubular exoskeletons of arthropods (Module 18.12) are extremely light for their strength, but they do not grow with the animal. Periodically, during molting, the old skeleton is lost, and, following body growth, a new skeleton is hardened (Figures 30.2B, 30.2C). At this time, these animals are particularly vulnerable to predators, and remain so until the new exoskeleton hardens. Although most shell-bearing mollusks move by manipulating a muscular foot, the scallop moves by rapid opening and closing of its shells, producing a jet-propulsive movement that is somewhat random.

5. An endoskeleton consists of rigid, internal supports, usually consisting of noncellular material secreted by surrounding cells. Sponges support their cells on spicules. Spicules are made of materials such as calcium salts or silica. Echinoderms have an endoskeleton of calcium plates under their skin (Figure 30.2D). Vertebrates have endoskeletons of bone, cartilage, or a combination of the two (Figure 30.2E).

The human skeleton is a unique variation on an ancient theme (Module 30.3). All vertebrate skeletons have consistent features, both the overall pattem described below and the number, shape, and articulation of the individual bones. The basic patterns are modified according to the needs of each animal.

In contrast to the frog skeleton, which supports a quadruped that moves by hopping, the human skeleton supports a biped that walks or runs. The axial skeleton consists of a skull protecting the brain, the backbone (vertebral column) protecting the spinal cord and supporting the remaining skeletal elements, and the rib cage surrounding the lungs and heart. The appendicular skeleton consists of the bones of the appendages (arms, legs, fins) and the bones that link the appendages to the axial skeleton (the shoulder [pectoral] and pelvic girdles). The shoulder girdle consists of the clavicle and scapula. Coming off the shoulder girdle are the humerus, radius and ulna, carpals, metacarpals, and phalanges. The pelvic girdle is formed by the coxal bone (os coxa), which consists of three fused bones: the ilium, the ischium, and the pubis. Coming off the pelvic girdle are the femur, patella (kneecap), tibia and fibula, tarsals, metatarsals, and phalanges.

A human skeleton can be determined to be that of a female or male by examining the pelvic girdle. There are several differences, but one of the easiest to use is the angle of the pubic arch. If the angle is greater than 90, then it is the skeleton of a female; if the angle is less than 90, then it is the skeleton of a male.

The versatility of the vertebrate skeleton comes in part from its movable joints. Ball-and-socket joints allow movement in all directions. Hinge joints are strong and restrict movement to one plane. Pivot joints allow bones to rotate, providing ease of manipulation (Figure 30.3C).

6. Skeletal disorders afflict millions (Module 30.4). Lower back problems stem from the uneven distribution oi we iiht veificaay on the backbone. The S-shape in this area cushions vertical loads but cannot bear the lateral forces during lifting. One form of arthritis, inflammation of the joints, seems to be a normal part of aging, as joints become stiff and cartilage between bones wears down. Crippling, rheumatoid arthritis is an autoimmune disease (Module 24.16) in which the immune system attacks and degrades the joints following stress or an infection. Osteoporosis is due to hormonal changes (greatly reduced estrogen levels) during aging, particularly in women following menopause. It is characterized by the bones becoming thinner, more porous, and easily broken. For women, bone density begins to decline at about age 30 to 35. Ca2, intake and exercise to offset this decline should be of concem to all woman, both pre- and postmenopausal.

7. Bones are complex living organs (Module 30.5; Figure 30.5). Bones are composed of other tissues besides bone and cartilage. These tissues intermix with tissues of the circulatory system (vessels and blood) and nervous system (nerves). Most of the outside surface is covered with fibrous connective tissue. When bones break or crack, this tissue is able to form new bone. At either end of most bones, cartilage replaces connective tissue, forming a surface that cushions the joint (Figure 20.5E). Bone itself is mostly a noncellular matrix of calcium salts (which resist compression) and protein fibers (which resist cracking) surrounding the cells that secrete these materials (Figure 20.5F). The shafts of long bones are made of compact bone, with a dense matrix surrounding a hollow cavity containing stored fat (yellow bone marrow). The ends of long bones are made of an outer layer of compact bone and an inner area of spongy bone. Within cavities in the matrix of the spongy bone, specialized tissues produce blood cells (red bone marrow). The cavity of long bones reduces the weight of the body and makes movement easier.

8. Bone growth is a major feature of human development (Module 30.6). Bones begin to form about one month after conception (Figure 30.6A). The bones of the skull form from sheets of connective tissue. The bones of the remaining skeleton form and grow by replacing cartilage. In long bones, this starts with the laying down of a ring of bone (bone collar) around the shaft. Bone also replaces cartilage at the center of the shaft. The bone grows in length and thickness, and blood vessels penetrate the shaft. Within the shaft, the yellow marrow cavity begins to form. Blood vessels also penetrate the cartilage at the ends of the bone, and bone formation takes place there. The bone continues to grow as long as new cartilage is added to the region of cartilage between the shaft and the ends of the long bone (the dark blue area in Figure 30.6B). Skeletal growth stops at about age 18 in women and age 21 in men.

9. Muscle Contraction and Movement. The skeleton and muscles interact in movement (Module 30.7). Muscles are connected to bones by tendons (Module 20.5; Figure 20.5D). At joints, bones are held together by ligaments. A muscle can only contract. To extend, it must be pulled by the contraction of an opposing muscle. Thus, movement of most parts of the body requires antagonistic pairs of muscles (Figure 30.7). Nerves that innervate antagonistic muscle pairs have a built-in circuitry that prevents both muscles of a pair from contracting at the same time (this is referred to as reciprocal innervation). A strong electrical shock can bypass this circuitry and cause both nerves to induce their muscles to contract at the same time. This can break bones.

Each muscle cell has its own contractile apparatus (Module 30.8).

Striated skeletal muscle tissue was introduced in Module 20.6 (Figure 20.6A). Each muscle fiber is a single ceR with many nuclei. Within each fiber are numerous, long myofibrils (Figure 30.8).

A myofibril is composed of contracting units called sarcomeres, joined end to-end at Z lines.

Each sarcomere is composed of thin filaments (coiled strands of two actin proteins and one regulatory protein) and thick filaments (parallel strands of myosin protein). This structure produces a pattem of light and dark bands in the muscle tissue. The dark bands (Figure 30.8) consist of thick filaments and thin filaments (which do not extend to the center of the dark band). The light bands have only thin filaments and straddle the Z lines that connect adjacent thin filaments. The regulatory protein wrapped around actin is actually two proteins, a complex of troponin and tropomyosin. Tropomyosin physically blocks binding sites for myosin on actin. These sites are unblocked when Ca2+ binds to troponin, forcing a conformational change that in turn moves tropomyosin from its position blocking the binding sites (Module 30.9). This mechanism works essentially the same way in cardiac muscle fibers. In smooth muscle fibers Ca2+ binds to calmodulin, which is on the thick filament.

A muscle contracts when thin filaments slide across thick filaments (Module 30.9). In the 1950s, Huxley proposed the sliding filament model of muscle contraction. The model has been supported by considerable subsequent research, and many molecular details have been added to it.

The model originally attempted to explain one set of observations seen in living muscle: When the muscle contracts, the dark bands stay the same length, while the light bands decrease in length. When the muscle is fully contracted, an even darker band appears in the middle of the dark bands.

Contraction shortens the sarcomere but does not shorten the thick and thin filaments, which slide between each other (Figures 30.8 and 30.9A). Energy-consun-dng interactions between the myosin molecules of the thick filaments and the actin molecules of the thin filaments cause them to slide along one another. The myosin molecules of the thick filament expose about 350 swollen "heads" per filament. These "walk" along the actin filaments with the expenditure of ATP. Each head can repeatedly move at about five movements per second. The process continues until th@ muscle fiber stops contracting or is fully contracted (Figure 30.9B).

Data suggest that ATP attaches to each head and, upon hydrolysis to ADP and phosphate, adds potential energy to the head ("cocks" it). Ca 2+ opens a binding site on the adjacent actin molecule. When ADP and phosphate are released from the bound head, its energy is released, pulling the actin in a power stroke.

10. Motor neurons stimulate muscle contraction (Module 30.10). Each muscle fiber is stimulated by just one neuron, but a single neuron can stimulate many fibers, up to several hundred in a large muscle moving the appendicular skeleton. Each such group of muscle fibers is known as a motor unit because each is stimulated to contract together (Figure 30.10A). The fewer the number of muscle fibers per motor unit, the greater the degree of fine control over the muscle. A weak contraction is produced by the stimulation of one motor unit. A strong contraction involves the simultaneous contractions of several motor units. The synapses between neuron and muscle fiber are called neuromuscular junctions. The action potential is transmitted to the fiber through the release of the neurotransmitter acetylcholine. At the cellular level, muscle fiber stimulation proceeds as follows. The released acetylcholine changes the permeability of the muscle fiber's plasma membrane. This induces an action potential along the muscle cell membrane and into tubular infoldings of the plasma membrane into the cell. Within the cell, the action potentials cause the endoplasmic reticulum (ER) to release Ca 2, into the cytoplasm, and this triggers the binding of myosin to actin. When action potentials stop, Ca2, moves back into the ER (Figure 30.10B).