Skeletal & Muscle Systems

Hydrostatic Skeletons:
- Rely on fluid pressure within a cavity for support and movement.
- Muscles interact with fluid-filled cavities to maintain body shape.
- Common in soft-bodied invertebrates (e.g., earthworms).
Exoskeletons:
- Rigid external coverings made of chitin (e.g., insects, crustaceans).
- Provide protection and structural support.
- Require molting (ecdysis) for growth.
Endoskeletons:
- Internal frameworks made of bone or cartilage (e.g., vertebrates).
- Provide structural support, protect internal organs, and allow muscle attachment.
- Allow for continuous growth without shedding.

Provides structural support, protection, and enables movement.
Bones house bone marrow, which produces blood cells and acts as a reservoir for calcium and phosphorus.
Axial Skeleton: Skull, vertebral column, and rib cage.
Appendicular Skeleton: Limbs and girdles.
Bone formation and maintenance depend on osteoblasts (form bone), osteocytes (mature bone cells), and osteoclasts (break down bone).
Bones adapt to physical demands, becoming stronger when stressed and weaker when not.

Types of Muscles:
- Skeletal: Responsible for voluntary movements of bones and body. Striated appearance.
- Cardiac: Powers heartbeat. Striated, involuntary, and rhythmically contractile.
- Smooth: Controls involuntary actions in organs and blood vessels. Non-striated, slow, and sustained contractions.
Muscle Anatomy:
- Muscle fibers are wrapped in connective tissue sheaths.
- Muscle fibers contain myofibrils, which are composed of sarcomeres.
- Sarcomeres consist of thick filaments (myosin) and thin filaments (actin).

Contractile units within muscle cells are called sarcomeres.
Thin filaments: Primarily composed of actin, along with regulatory proteins like troponin and tropomyosin.
Thick filaments: Made of myosin; myosin heads attach to actin filaments to generate force.
Sarcomeres are bordered by Z lines and arranged in repeating patterns along the length of the myofibril.
Skeletal muscle has its striated appearance, and enable coordinated contraction.

Muscles attach to bones and create movement by contracting across joints.
Joints act as pivot points, and bones function as levers.
Ligaments hold bones together, and tendons connect muscles to bones.
Hinge Joints: (e.g., elbow) Allow movement in one plane.
Ball and Socket Joints: (e.g., shoulder) Allow movement in multiple planes and rotation.

Movement is achieved through alternating contraction of muscle pairs (antagonistic muscles).
Examples: Biceps and triceps, quadriceps and hamstrings.

Myosin heads attach to actin filaments, bend to pull actin, then detach and repeat.
This action slides the actin filaments past the myosin, drawing $Z$ discs together and shortening the muscle fiber.
$ATP$ is required for each cycle.