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bio midterm questions

Key Concepts:

Terms in this set (51)

Structural features:
• Thick filaments: run the entire length of an A band
• Thin filaments: run the length of the I band and partway into the A band
• Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
• H zone: lighter midregion where filaments do not overlap
• M line: line of protein myomesin that holds adjacent thick filaments together

Sliding filament model
• In a relaxed muscle fiber, the thin and thick filaments overlap only at the ends of the A band (Figure 9.6 1).
• The sliding filament model of contraction states that during contraction, the thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree.
• Neither the thick nor the thin filaments change length during contraction.

Here's how it works:
• When the nervous system stimulates muscle fibers, the myosin heads on the thick filaments latch onto myosin-binding sites on actin in the thin filaments, and the sliding begins.
• These cross bridge attachments form and break several times during a contraction, acting like tiny ratchets to generate tension and propel the thin filaments toward the center of the sarcomere.
• As this event occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens.
• At the microscopic level, the following things occur as a muscle cell shortens:
• The I bands shorten.
• The distance between successive Z discs shortens. As the thin filaments slide centrally, the Z discs to which they attach are pulled toward the M line
Articular cartilage ->
• Glassy-smooth hyaline cartilage covers the opposing bone surfaces as articular cartilage.
• These thin (1 mm or less) but spongy cushions absorb compression placed on the joint and thereby keep the bone ends from being crushed.
Joint (articular) cavity ->
• The joint cavity is a feature that is unique to synovial joints.
• It contains a small amount of synovial fluid.
• The joint cavity is a potential space because it is normally almost nonexistent, but can expand if fluid accumulates (as happens during inflammation).
Articular capsule ->
• The joint cavity is enclosed by a two-layered articular capsule, or joint capsule.
• The tough external fibrous layer is composed of dense irregular connective tissue that is continuous with the periostea of the articulating bones.
• It strengthens the joint so that the bones are not pulled apart. The inner layer of the joint capsule is a synovial membrane composed of loose connective tissue.
• Besides lining the fibrous layer internally, it covers all internal joint surfaces that are not hyaline cartilage.
• The synovial membrane's function is to make synovial fluid.
Synovial fluid->
• Occupies all free space within a joint cavity and is also found within the articular cartilages, provides a slippery, weight-bearing film that reduces friction between the cartilages.
• Without this lubricant, rubbing would wear away joint surfaces and excessive friction could overheat and destroy the joint tissues.
Reinforcing ligaments ->
• Synovial joints are reinforced and strengthened by a number of bandlike ligaments.
• Most often, these are capsular ligaments, which are thickened parts of the fibrous layer.
• In other cases, they remain distinct and are found outside the capsule (as extracapsular ligaments) or deep to it (as intracapsular ligaments).
• Since intracapsular ligaments are covered with synovial membrane, they do not actually lie within the joint cavity.
Nerves and blood vessels->
• Synovial joints are richly supplied with sensory nerve fibers that innervate the capsule.
• Some of these fibers detect pain, as anyone who has suffered joint injury is aware, but most monitor joint position and stretch.
• Monitoring joint stretch is one of several ways the nervous system senses our posture and body movements (see p. 493).
• Synovial joints are also richly supplied with blood vessels, most of which supply the synovial membrane.
• There, extensive capillary beds produce the blood filtrate that is the basis of synovial fluid.
Articular discs->
• improve the fit between articulating bone ends, making the joint more stable and minimizing wear and tear on the joint surfaces.