Joints and bone regulation

What is a bursa?

A small, fluid-filled sac lined with synovial membrane.

What is the function of a bursa?

Reduces friction between structures that rub against each other.

Where are bursae located?

Where moving structures rub against one another.

What is a labrum?

A ring of fibrocartilage attached to the rim of certain articular surfaces.

What is the function of a labrum?

Deepens shallow joint sockets and improves stability.

What is a tendon sheath?

An elongated, tube-like sac lined with synovial membrane that surrounds a tendon.

What is the function of a tendon sheath?

Reduces friction and allows tendons to glide smoothly through confined spaces.

How is a tendon sheath related to a bursa?

A tendon sheath is essentially an elongated bursa that wraps around a tendon.

What is a tendon?

A band of dense regular connective tissue that usually connects muscle to bone.

What is the main function of a tendon?

Transmits muscle force to the skeleton to produce or control movement.

What is an aponeurosis?

A broad, flat sheet of dense regular connective tissue.

What is the function of an aponeurosis?

Distributes muscle force across a broad area.

What structures can aponeuroses connect?

Muscle to bone, fascia, or another muscle.

What is the rectus sheath?

A fibrous envelope surrounding the rectus abdominis muscle.

What forms the rectus sheath?

Aponeuroses of the three lateral abdominal muscles.

What is a retinaculum?

A thickened band of deep fascia that holds tendons close to bones and joints.

What is the function of retinacula?

Prevent tendons from pulling away from bones (bowstringing) and guide their movement.

Where are retinacula especially common?

Around the wrist and ankle.

What are the two major hormones regulating calcium homeostasis?

Parathyroid hormone (PTH) and calcitriol (active vitamin D).

Why is blood calcium tightly regulated?

It is essential for muscle contraction, nerve signaling, and blood clotting.

What is the body's largest calcium reservoir?

Bone.

What is the primary effect of PTH on blood calcium?

Raises blood calcium levels.

How does PTH affect bone resorption?

Indirectly stimulates osteoclast activity, releasing calcium into the blood.

What does PTH stimulate osteoblast-lineage cells and osteocytes to increase?

RANKL.

What does RANKL bind to?

RANK on osteoclast precursors.

What happens when RANKL binds to RANK?

Osteoclast precursors differentiate into active osteoclasts, increasing bone resorption.

How does PTH affect the kidneys?

Increases calcium reabsorption and stimulates calcitriol production.

What is the effect of continuous or high PTH exposure?

Favors bone resorption.

What is the effect of intermittent PTH exposure?

Can stimulate bone formation.

Where does vitamin D production begin?

In the skin through UVB exposure, or vitamin D can be obtained from the diet.

What does UVB radiation do in the skin?

Converts 7-dehydrocholesterol into pre-vitamin D3.

What is pre-vitamin D3 converted into?

Vitamin D3 (cholecalciferol), an inactive form.

Where does the first activation step of vitamin D occur?

Liver.

What is vitamin D converted into in the liver?

Calcidiol, also called 25-hydroxyvitamin D [25(OH)D].

Which form of vitamin D is measured clinically to assess vitamin D status?

Calcidiol [25(OH)D].

Where does the second activation step of vitamin D occur?

Kidney.

What stimulates the kidney to produce calcitriol?

PTH.

What is the main function of calcitriol?

Increases intestinal absorption of calcium and phosphate, supporting bone mineralization.