During contraction, myosin heads repeatedly bind to and detach from actin. ATP binding allows detachment; the heads then continue the cross-bridge cycle as long as the necessary conditions are present.

Smooth muscle is a type of muscle tissue. Blood, bone, and cartilage are connective tissues.

Resting skeletal muscle can maintain low-level tension called muscle tone. A motor unit consists of a motor neuron and the muscle fibers it innervates. Closely spaced stimuli can produce summation and tetanus. In an ideal isometric contraction, muscle length stays constant; in an ideal isotonic contraction, tension stays approximately constant while length changes.

A and B are true. Evaporating sweat cools the body, and dilation of skin blood vessels promotes heat loss. C and D are false. Pituitary and thyroid glands are endocrine glands that release hormones without ducts. Mammary and salivary glands are exocrine glands that release products through ducts.

I and III are correct. Bones protect organs and support the body. Red bone marrow also produces blood cells. Aged circulating red blood cells are removed mainly by macrophages in the spleen and liver; bone is not the primary clearance site. This does not mean that marrow macrophages never break down cells.

Epithelial tissue covers body surfaces and lines internal spaces. Its cells may be squamous, cuboidal, or columnar. Connective tissue supports and connects structures; muscle tissue produces force and movement. Nervous tissue contains neurons and supporting glial cells and is a major component of organs such as the brain and spinal cord.

Rods are highly sensitive to dim light and do not distinguish colors. Cones support color vision and function best in brighter light.

The A band spans the thick filaments, whose length remains constant. As thin filaments slide farther between thick filaments, the I bands and H zone shorten. The filaments themselves do not shorten.

The iris changes pupil size to regulate incoming light. The retina contains rods and cones that detect light. The optic disc lacks photoreceptors and creates the blind spot. The sclera is the tough outer coat, while the cornea is the transparent front surface that admits and refracts light.

The picture shows cuboidal epithelium. Cuboidal epithelial cells are approximately as tall as they are wide and commonly perform secretion and absorption.

Tendons typically connect muscle to bone, while ligaments connect bone to bone at joints. Both are composed largely of collagen-rich connective tissue.

The axial skeleton includes the skull, vertebral column, ribs, and sternum. The pelvic girdle belongs to the appendicular skeleton.

B and D are correct. In darkness, cGMP keeps cation channels open, allowing inward current that maintains a relatively depolarized membrane. Photoreceptors continuously release glutamate. Light activates visual pigments, lowers cGMP, closes these channels, and hyperpolarizes the cells.

The branched cells, central nuclei, and intercalated discs identify cardiac muscle.

Epiphyseal growth plates contain cartilage that supports lengthwise growth until the plates close. Central (Haversian) canals contain blood vessels and nerves. Osteoclasts resorb bone. The shaft and medullary cavity are other structures of a long bone.

All four statements are correct. The pinna collects sound, and the eardrum transmits vibrations to the ossicles. Cochlear hair cells transduce sound-driven motion into electrical signals. The semicircular canals detect rotational acceleration and contribute to balance.

Articular cartilage reduces friction and helps distribute forces at joints. Chondrocytes maintain a matrix rich in collagen and proteoglycans. Cartilage also supports structures such as the nose, external ear, and trachea. Cuboidal and squamous tissues are epithelial types, while skeletal muscle produces movement.

Skeletal muscles are the only voluntary muscles, and cardiac and smooth muscles have involuntary functions. The skeletal muscle is striated and has multiple nuclei with an array of actin and myosin filaments. The cardiac muscle is striated with intercalated discs. The smooth muscle is not striated and can be found in arteries, the urinary bladder, and the digestive tract.

Thin filaments contain mainly actin, along with regulatory proteins such as tropomyosin and troponin. Thick filaments contain myosin. Repeating sarcomeres form myofibrils within a muscle fiber.

The middle ear contains three ossicles: the malleus, incus, and stapes. They transmit eardrum vibrations to the oval window and increase pressure through mechanical advantage, improving energy transfer into inner-ear fluid. The semicircular canals belong to the inner ear.