Organismal Biology #32
SKELETON and MUSCLES

Click here for the
ONLINE CLASSROOM VERSION


SKELETON IN GENERAL
Various types of skeletal system can help support the body's shape. Fluid-filled cavities can form a hydrostatic skeleton. Skeletons can also be built of rigid structures on the outside (exoskeletons) or on the inside (endoskeletons). Skeletal proportions (and therefore body proportions) are often dictated by area/volume relationships.

Hydrostatic skeleton (as in many worms): a series of fluid-filled body cavities (coelomic cavities) can form a skeleton. Such a skeleton can be very useful in burrowing through loose sand of soil: those segments contricted by circular fibers protrude forward and lengthen the body, while segments compressed along the body axis by longitudinal fibers bulge sideways and anchor those parts of the body against slipping backward.

Exoskeleton (as in insects and other arthropods): body parts are hollow, with hardened tissue on the outside. In small organisms (most insects, spiders), exoskeletons can be made of chitin, but in larger organisms (lobsters), chitin layer is usually strengthened by calcium salts.
  • Muscles are arranged on the inside, spanning joints. Most muscles span one joint at a time.
  • The weight of the exoskeleton limits its size, especially on land.
  • Growth is a problem once the skeleton hardens, so animals with exoskeletons molt at certain intervals: the animal cracks through and sheds its skeleton, exposes a new skeletal layer underneath, inflates its body, and allows its new, larger skeleton to harden.

Endoskeleton: a skeleton on the inside, as in vertebrates. Muscles are arranged surrounding the bones of the skeleton. Animals with endoskeletons are generally larger than those with exoskeletons. Growth is less of a problem because skin can remain flexible.

Area-volume relationships: Skeletal proportions are often dictated by area-volume relationships, which limit body shapes and sizes.
  • Mass and muscle strength are both related to volume.
  • Efficiency of digestion, respiration, or heat exchange are related to surface area, so large animals need more heavily folded surfaces.
  • Strength of supporting legs is related to cross-sectional area, while the weight needing support is related to volume. Thus, large animals (elephants, brontosaurs) need disproportionately thicker, cylindrical, and more vertical legs.

VERTEBRATE SKELETON
The skeleton contains both bone and cartilage tissues.
Most cartilage later turns to bone.

 Cartilage tissue: not as strong as bone, but resists shock better. Smoother surface is better at moving joints. No blood supply; cells nourished by diffusion only. Grows much more rapidly than bone, until diffusion cannot meet nutritional needs; then cells die or are replaced by bone. Types of cartilage are:
  • Hyaline cartilage: simple matrix
  • Fibrocartilage: collagen fibers added to matrix
  • Elastic cartilage: elastic fibers added
  • Calcified cartilage: calcium salts added

Bone tissue: stronger than cartilage, but less resistant to shock and grows more slowly. Internal blood supply nourishes interior and keeps cells alive, allowing constant restructuring, repair, and healing of injuries.

Compact (lamellar) bone tissue: made of layers, usually organized as Haversian systems (concentric cylinders surrounding a central Haversian artery). As bones restructure, new Harversian systems align parallel to greatest stress.

Spongy (cancellous) bone tissue: built of struts (trabeculae) separated by spaces containing blood or marrow.


Illustrations: Skeletons and Muscles



MUSCLES
Muscle tissues are specialized for contraction. Contractuion results from the sliding of thin filaments of actin lengthwise between thick myosin filaments.

Muscle tissues are specialized for contraction
  • Smooth muscle:   involuntary cells with tapering ends but no cross-banding; smooth, rhythmic contractions; nuclei located centrally; occurs in digestive organs, reproductive organs, etc.
  • Cardiac muscle:   involuntary fibers with cross-striations; cylindrical in shape but branching and coming together repeatedly; nuclei located centrally; cell boundaries marked by intercalated disks; rhythmic contractions; occurs in heart only
  • Skeletal muscle:   voluntary cells with cylindrical shape; cross-striations caused by alignment of actin and myosin fibers; many nuclei per fiber; no cell boundaries (each fiber is thus called a syncytium); rapid, forceful contractions, but fatigues easily; occurs in muscles; always attaches to connective tissues

Individual muscles are organs made of many tissues.
  • The belly or fleshy part contains skeletal muscle tissue surrounded by layers of connective tissue (and also blood vessels and nerve endings).
  • Connective tissue usually extends beyond the belly to form tendons.
  • Most muscles span at least one joint.
  • A muscle that bends a joint is called a flexor.
  • A muscle that straightens a joint is called an extensor.


  • REVIEW:         Study guide and vocabulary

    • Index             Syllabus
    Prev rev. March 2020 Next