Animal Movement and Support

Describe the structure of a striated muscle cell, and the arrangement of the filaments.

A striated muscle cell is a long, multinucleated fiber packed with myofibrils. Myofibrils contain repeating units of actin (thin) and myosin (thick) filaments arranged in a regular pattern, creating alternating dark (A bands) and light (I bands) stripes. Actin attaches to Z-lines; myosin lies in the center.

What is a sarcomere?

A sarcomere is the basic contractile unit of striated muscle, extending from one Z-line to the next. It contains organized thick (myosin) and thin (actin) filaments whose sliding past each other shortens the muscle during contraction.

With the aid of Figure 50.32 in Campbell, 4th Cdn. Ed., (Figure 50.29 in Campbell 3rd

Cdn. Ed.), describe the myosin and actin interactions during the contraction of a

muscle fibre.

Myosin heads bind to exposed sites on actin, forming cross-bridges. The release of ADP + Pi triggers the power stroke, pulling actin toward the sarcomere centre. A new ATP binds to detach the myosin head, and ATP hydrolysis re-cocks the head for another cycle. Repeated cycles slide actin filaments past myosin, shortening the muscle.

Where does the energy for muscle contraction come from?

ATP provides the energy for muscle contraction. ATP binds to myosin, detaching it from actin and powering the re-cocking of the myosin head after hydrolysis. Muscles regenerate ATP using creatine phosphate, glycolysis, and aerobic respiration.

Explain the role of tropomyosin, troponin and Ca2+ in muscle contraction.

Tropomyosin blocks the myosin-binding sites on actin when muscles are relaxed. Ca²⁺ binds to troponin, causing tropomyosin to shift and expose these binding sites, allowing myosin heads to attach to actin and initiate contraction. When Ca²⁺ is removed, tropomyosin returns to its blocking position and the muscle relaxes.

With the aid of Figures 48.13, 50.31 and 50.32 in Campbell, 4th Cdn. Ed., (Figures 48.14, 50.31 and 50.32 in Campbell 3rd Cdn. Ed.), describe the regulation of skeletal muscle contraction. Starting from the arrival of an action potential at the motor neuron axon terminal to the relaxation of the muscle cell.

An action potential reaches the motor neuron terminal, opening voltage-gated Ca²⁺ channels. Ca²⁺ entry triggers release of acetylcholine (ACh) into the synaptic cleft. ACh binds to receptors on the muscle fiber membrane (sarcolemma), generating an action potential that travels down T-tubules. This signal opens Ca²⁺ channels in the sarcoplasmic reticulum, releasing Ca²⁺ into the cytosol. Ca²⁺ binds to troponin, exposing actin sites for contraction. When stimulation stops, ACh is broken down, Ca²⁺ is pumped back into the SR, and the muscle relaxes.

How does the structure of the plasma membrane and sarcoplasmic reticulum facilitate muscle contraction?

The sarcolemma (plasma membrane) contains T-tubules that transmit action potentials deep into the muscle fiber. The sarcoplasmic reticulum stores Ca²⁺ and releases it in response to signals from T-tubules, coordinating rapid contraction and relaxation throughout the fiber.

Location within the animal body:Skeletal,smooth,cardiac muscle

Skeletal muscle is attached to bones; cardiac muscle is in the heart wall; smooth muscle lines internal organs and blood vessels.

Shape of the cells, number of nuclei, and specialized regions or features.: Skeletal,smooth,cardiac muscle

Skeletal fibers are long, cylindrical, and multinucleated; cardiac cells are branched with one nucleus and connected by intercalated discs; smooth muscle cells are spindle-shaped with a single nucleus.

Presence of striations (bands) in the cells: Skeletal,smooth,cardiac muscle

Skeletal and cardiac muscles are striated; smooth muscle is non-striated.

Function: Skeletal,smooth,cardiac muscle

Skeletal muscle produces voluntary movement; cardiac muscle pumps blood; smooth muscle controls internal organ movements and vessel diameter.

Whether it can be voluntarily controlled or not:Skeletal,smooth,cardiac muscle

Skeletal muscle is under voluntary control; cardiac and smooth muscles are involuntary.

Compare and contrast hydrostatic skeletons, exoskeletons and endoskeletons.

Hydrostatic skeletons use fluid-filled cavities and muscle contractions for support and movement (e.g., earthworms). Exoskeletons are rigid outer coverings made of chitin or calcium carbonate (e.g., arthropods, molluscs). Endoskeletons are internal frameworks made of bone or cartilage that support large body sizes (e.g., vertebrates).

Explain how the earthworm uses the hydrostatic skeleton, circular muscles and longitudinal muscles in locomotion (see Figure 50.37 in Campbell, 4th Cdn. Ed., and Figure 50.37 in Campbell 3rd Cdn. Ed.). Provide examples of other animals with this type of skeleton.

Earthworms move by peristalsis: circular muscles contract to elongate the body, while longitudinal muscles contract to shorten it. Setae anchor segments to the ground as waves of contraction move forward. Other animals with hydrostatic skeletons include cnidarians and nematodes.

Compare the shell of snails and the exoskeleton of arthropods based on their chemical composition and their function.

Snail shells are made of calcium carbonate and provide rigid protection. Arthropod exoskeletons consist of chitin and protein, offering strength and flexibility. Both provide protection and muscle attachment but differ in material and molting process.

Referring to Figures 50.38 and 50.39 in Campbell, 4th Cdn. Ed., (50.38 and 50.39 in Campbell, 3rd Cdn. Ed.), identify the major bones, and some of the movable joints of the human skeleton. Describe the kind of movements allowed by each of these joints.

Major bones include the skull, vertebral column, ribs, pelvis, and limbs. Joints include hinge joints (elbow, knee) allowing bending; ball-and-socket joints (shoulder, hip) allowing rotation; pivot joints (neck) for turning; and gliding joints (wrist) for sliding motion.

What bones form the shoulder and pelvic girdles, and what is the function of the girdles?

The shoulder girdle includes the clavicle and scapula, supporting arm movement and muscle attachment. The pelvic girdle consists of fused hip bones (ilium, ischium, and pubis) that support body weight, protect organs, and attach the legs.

Give examples of animals other than vertebrates, with endoskeletons.

Echinoderms such as sea stars and sea urchins have internal calcium-rich ossicles forming an endoskeleton. Sponges have internal spicules made of silica or calcium carbonate for structural support.

Explain how the principle of muscular antagonism applies to: a) The movement of the human forearm b) The movement of the tibia by the grasshopper.

In antagonistic muscle pairs, one muscle contracts while the other relaxes to produce opposite movements. (a) In the human forearm, the biceps (flexor) bends the arm while the triceps (extensor) straightens it. (b) In the grasshopper leg, the flexor tibiae bends the tibia, and the extensor tibiae straightens it for jumping.