Musculoskeletal System Overview

Changes in movement occur because muscles pull against a support structure.
Three types recognized by zoologists:
- Hydrostatic skeletons
- Exoskeletons
- Endoskeletons

Found primarily in soft-bodied invertebrates (both terrestrial and aquatic).
- Example: Earthworms
- Locomotion involves a fluid-filled central cavity (hydrostatic skeleton) and surrounding circular and longitudinal muscles.
- Movement mechanism: A wave of circular muscle contractions followed by longitudinal muscle contractions moves fluid down the body.

Surrounds the body as a rigid hard case.
- Made of chitin in arthropods.
Provides protection for internal organs and serves as a site for muscle attachment.
It must be periodically shed for growth (process called ecdysis).
Less strong compared to bony skeletons.
The respiratory system limits body size due to the necessity of gas exchange.

Rigid internal skeletons that provide structure and muscle attachment.
Types of Endoskeletons:
- Echinoderms: Have a skeleton made of calcium carbonate.
- Vertebrates: Have bones made of calcium phosphate.

Comprise both bone and cartilage.
Key characteristics:
- Bone is much stronger than cartilage and less flexible.
- Both tissues are living and can remodel in response to stress or injuries.

Bone Characteristics: Hard yet resilient connective tissue unique to vertebrates.
Development modes:
1. Intramembranous Development
- Initiated by osteoblasts.
- Some cells become trapped in the bone matrix and change into osteocytes (found in lacunae).
- Communicate through canals called canaliculi.
- Osteoclasts break down the bone matrix.
1. Endochondral Development
- Bones begin as cartilaginous models.
- Progression involves adding bone to the outside and replacing interior cartilage.
- Calcification starts with a fibrous sheath (periosteum) where osteoblasts turn into osteocytes.

Components of Bone:
- Epiphysis: Ends of bone containing red marrow.
- Growth plate indicates lengthwise growth.
- Medullary cavity: Contains marrow.
- Osteon: Structural unit of compact bone, containing Haversian systems and canaliculi.

Types of joints and movement capabilities include:
- Ball-and-Socket Joints
- Hinge Joints
- Gliding Joints

Skeletal muscles include numerous muscle fibers that enclose bundles called myofibrils (4-20 structures each).
Myofibrils contain thick and thin myofilaments leading to a striated appearance under a microscope.

Sarcomere: The basic contractile unit defined as the distance between two Z lines.
Relaxed State: Sarcomeres have wider I bands due to minimal overlap of filaments.
Contracted State: Increased overlap narrows the H and I bands. Involves a sliding filament mechanism where thick and thin filaments slide relative to each other without shortening.

Locomotion Types:
- Appendicular locomotion: Movement generated by oscillating appendages.
- Axial locomotion: Movement through body undulation or peristaltic waves.
Constraints: Gravity and frictional drag affect all environments differently.

In water, buoyancy minimizes gravitational effects, while frictional drag retards movement.
- Hydraulic propulsion: Used by some marine invertebrates.
- Swimming Examples: Eels use their entire body while trout use primarily their posterior half.

Involves primarily overcoming gravitational forces and is facilitated through jointed appendages (legs) pushing against the ground.
- Terrestrial tetrapods use limb movement for swimming.
Walking Patterns: Quadrupeds typically have diagonal footfalls for balance and efficient running.

Flight has evolved independently among four groups: Insects, Pterosaurs, Birds, and Bats.
Each has uniquely modified forelimbs into wing structures optimized for flight, despite different anatomical arrangements.

Understanding the structure and function of the musculoskeletal system is crucial in biology, especially in locomotion and adaptability across environments.