Effectors: Making Animals Move

Microtubules, Microfilaments, and Cell Movement Microfilaments are proteins that generate contractile forces by changing conformation. Microfilaments reach their highest level of organization in muscle cells. Myosin and actin are the proteins responsible for the contraction and relaxation of muscle.

Figure 47.1 Types of Vertebrate Muscle Tissue The three types of vertebrate muscle are smooth, cardiac, and skeletal.

Smooth muscle in human uterus. Muscle Contraction Smooth muscle provides contraction for internal organs, which are under the control of the autonomic nervous system. Smooth muscle moves food through the digestive tract, controls the flow of blood, and empties the urinary bladder. Smooth muscle cells are the simplest muscle cells structurally; they have a single nucleus and are usually long and spindle-shaped. Smooth muscle in human uterus.

Muscle Contraction Cardiac muscles are branched and appear striated because of the regular arrangement of their actin and myosin filaments. Branching creates a meshwork that resists tearing and allows the heart to withstand the high pressures of blood pumping without leaking. Intercalated discs provide strong mechanical adhesions between adjacent cells. Cardiac muscle cells are also in electrical contact with one another, and depolarization begun at one point in the heart rapidly spreads through the muscle mass. Human cardiac muscle

Myofibrils; bands of actin and myosin together appear darkest Muscle Contraction All voluntary movements are controlled by skeletal muscle. Skeletal muscle is also called striated muscle because of its striped appearance. Skeletal muscle cells are called muscle fibers. They are large and have many nuclei because they are a fusion of many individual cells. Each muscle fiber is packed with bundles of myofibrils, each made up of thin actin units surrounding thick myosin units. Human skeletal muscle Myofibrils; bands of actin and myosin together appear darkest

Figure 47.3 The Structure of Skeletal Muscle (Part 1)

Muscle Contraction Myofibrils consist of repeating units called sarcomeres. Each sarcomere is bounded by Z lines, which anchor the thin actin filaments. At the center is the A band, housing all the myosin filaments. The H zone and I band are areas where actin and myosin do not overlap and appear light. The M band contains proteins that support the myosin filaments.

Figure 47.3 The Structure of Skeletal Muscle (Part 2)

Muscle Contraction When a muscle contracts, the sarcomere shortens, the H zone and the I band become much narrower, and the Z lines move toward the A band as if the actin filaments were sliding into the region occupied by the myosin filaments. Actin and myosin slide past each other as the muscle contracts.

Muscle Contraction Each myosin molecule consists of two long polypeptide chains coiled together, each ending in a large globular head. A myosin filament is made of many such molecules arranged in parallel. An actin filament consists of two chains of actin molecules twisted together.

Muscle Contraction A myosin head binds to actin and its orientation changes. This exerts a force that causes the actin to slide. The myosin head then binds ATP and releases the actin. The myosin head returns to its original formation and can bind to actin again. Contraction of the sarcomere involves many cycles of interaction between many myosin heads and actin. Backsliding of actin does not occur because the many surrounding filaments create a system of interacting cycles.

Figure 47.6 The Release of Ca2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction

Muscle Contraction The ATP is needed to break the actin–myosin bonds, not to form them. The energy is actually used to stop muscles from contracting. This accounts for the stiffening of muscles (rigor mortis) after death. With no ATP being made, the actin–myosin bonds can’t be broken.

Muscle Contraction When the muscle is at rest, two proteins, tropomyosin and troponin, block the myosin binding sites on the actin filament. When Ca2+ is released to the sarcoplasm, it binds to troponin. Troponin and tropomyosin change shape, exposing the actin–myosin binding sites. With the binding sites exposed, the actin–myosin bonds are made, and the filaments are pulled past each other, resulting in muscle fiber contraction. If Ca2+ remains available, the cycle repeats and muscle contraction continues.

Figure 47.6 The Release of Ca2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction

Muscle Strength and Performance Slow-twitch fibers (red muscle) have many mitochondria and a lot of the oxygen-binding molecule myoglobin to provide steady, prolonged ATP production. Red muscle is also well supplied with blood vessels and fuel reserves (glycogen and fat). Long-term aerobic work such as running and swimming depend on this type of fiber. Fast-twitch fibers (white muscle) have fewer mitochondria and very little myoglobin. They develop maximum tension more rapidly and with greater tension, but fatigue rapidly.

Skeletal Systems The vertebrate skeleton provides support and protection for the body, and is capable of movement with the help of joints. Bones are connected by joints that allow a range of movements. The human body has 206 bones which work together with more than 600 muscles to provide movement for the body.

Skeletal Systems The appendicular skeleton is one of the two main anatomical categories of bones, consisting of the bones of the shoulder and pelvic girdles, the bones of the upper extremities, and the bones of the lower extremities. The axial skeleton is the other main categories of bones, consisting of the bones that form the body’s upright axis – the skull, the vertebral column, the ribs, and the sternum.

Figure 47.12 The Human Endoskeleton

Skeletal Systems Cartilage is connective tissue with an extracellular matrix of a rubbery mix of collagen and polysaccharide, which gives strength and resiliency. It is found in joints and in stiff but flexible structures such as the nose and ear. The embryonic skeleton of vertebrates is primarily cartilage, which is gradually replaced by bone during development. Bone is mostly extracellular matrix material of collagen fibers and crystals of calcium phosphate, which makes bone hard and rigid.

Figure 47.14 The Growth of Long Bones

Skeletal Systems A joint is where two bones meet. The human skeleton has several types of joints. Movement around joints is accomplished by antagonistic muscle pairs—one contracting, the other relaxing. One muscle is the flexor (bends the joint) and the other is the extensor (straightens the joint). Bones at a joint are held together by ligaments, flexible bands of tough, fibrous tissue that connects and supports bones. Straps of connective tissue called tendons attach the muscles to bones.

Figure 47.16 Joints, Ligaments, and Tendons

Skeletal Systems The vertebra is any of the 33 bones of the spinal column Includes the 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal vertebrae. The sacral and coccygeal vertebrae normally fuse to become 2 bones, the coccyx and the sacrum. Thus, in an adult, the spine usually consists of 26 vertebrae.