• The hydrostatic skeleton firms the animal body through fluid pressure against a resistant wall.  It also provides a base for muscle action.
• The coelenterates move and maintain their body shape and position by contractions of muscle fibers in the water-filled gastrovascular cavity.
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• Earthworms have complex muscle tissue arranged in two directions around their fluid-filled coelom. The body is extended by the squeezing action of circular muscles and shortened by contraction of longitudinal muscles.
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• Roundworms are fluid-filled with muscles directed longitudinally, permitting lashing movement only.

Exoskeletons

• Exoskeletons are outside, produced by ectodermally derived epidermis, while endoskeletons are inside and mesodermally derived.
• The arthropod chitinous exoskeleton or cuticle provides a muscle base, protects the soft parts, and prevents desiccation in terrestrial species.  In crustaceans, the cuticle is hardened by calcium salts, but is leathery and flexible at joints.
• Muscle attachments and action in arthropods and vertebrates are reversed and opposite with respect to each other.  Arthropod muscles are often attached to projecting apodemes.
• The great strength and agility of arthropods is only possible because of their minute size.
• Wing movement in flying insect species is either direct or indirect.  In the first, opposing direct flight muscles simply move the wings up and down, while the second, indirect flight muscles are vertically and longitudinally arranged and move the thorax up and down and the wings respond through a complex hinge.  A tilted angle provides lift.  Wingbeat rates can exceed neural impulses because of a resonating mechanism in the muscles.

• Mollusk  shells protect the body, but parts such as the fleshy foot and eyestalks are often moved through hydrostatic mechanisms.  Air is withdrawn from spaces in the spongy cuttlebone, a shell remnant in cuttlefish, making it an effective floating device.

• In mollusks, external shells are continuously secreted by the mantle and take one of three forms: a logarithmic spiral , simple cone, or spiraled cone.

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Endoskeletons

• The dermis of echinoderms secretes calcerous plates and spines that originate below the epidermis.

• The vertebrate body consists of four tissue types: epithelial , connective, nerve  and muscle
• Epithelium or epithelial tissue covers the body as epidermis and lines the internal organs as well.  Blood vessels have an endothelium, while  glands have a glandular epithelium.
• Connective tissue contains cells in a noncellular connective tissue matrix as seen in bone, cartilage, ligaments, tendons, and blood. Collagen is a common binding or tough matrix material.  Cells called fibroblasts secrete collagen and are active in bone mending.

Vertebrates: Endoskeletons of Bone

• The bony endoskeleton provides support, a muscle base, protection, a source of blood cells, and acts as a calcium reservoir.
• Bones are surrounded by the sheathlike periosteum.  Within regions of weblike spongy bone, containing red marrow where red blood cells form, and regions of compact bone, containing fatty yellow marrow.  Bone consists of hard calcium phosphate in a collagen matrix containing numerous bone cells, blood vessels, and nerves.

• Compact bone is organized into Haversian system or osteons, each with a central canal and hard, concentric, laminated lamellae.  Osteocytes (bone cells) reside in hollow lacunae, interconnected by weblike canaliculi.

The Skeleton

• Vertebrate skeletons consist of two divisions.  The axial contains the cranium (skull), vertebral column (backbones), and thorax (rib cage), and the appendicular contains the two girdles - pelvic (hip bones) and pectoral (shoulder bones and limbs).

        Sutures 

• Immovable joints include the fused sutures in the skull.  Movable joints have articulating surfaces of cartilage, and the bones are held in place by ligaments that often form the capsules of synovial joints.  Lubrication is provided by synovial fluid secreted by a synovial membrane lining the capsule.

• Movable joints include:
• gliding (wrist bones; light back and forth)
• pivotal (head and neck; rotation in one plane)
• ball-and-socket (shoulder; free rotation)
• hinge (knee; hingelike extension and flexion in one plane).

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The Axial Skeleton

• The skull or cranium in modern fishes includes many separate bones with little fusion.  Fusion increases in the amphibians, reptiles, and birds, and reaches its peak in mammals. Jaw mobility is also greatest in the mammals, although the maxilla (upper jaw) is fused to the skull.  Sensory receptors located on the cranium communicate with the brain through the foramina.  The foramen magnum at the base of the skull admits the spinal cord.

foramen magnum

• The number of vertebrae in the vertebral column varies among vertebrate classes. Reptiles have the most, while in birds many have fused.  Mammals have seven neck vertebrae.

• The 32 to 34 human vertebrae include three to five coccyx. The free vertebrae are separated by compressible, soft centered invertebral disks of fibrocartilage. The vertebral column houses and protects the spinal cord, which passes through the neural canal.
• The human vertebral column is divided into five regions: cervical (neck), thoracic (chest), lumbar (trunk), sacrum (hip) and coccyx (tail).  The first two vertebrae, the atlas and axis, support the skull and permit pivotal movements.  Thoracic vertebrae attach to the ribs, while lumbar vertebrae are massive and support the upper body.  Sacral vertebrae join the vertebral column to the pelvic girdle, while the coccyx is a vestigial tail.

• The thoracic bones include the rib cage and the three-part sternum (manubrium, body and xiphoid process). Amphibians lack rib development, while in the turtle the ribs are fused to the carapace.  In birds a large keel is fused to the sternum

Sternum. Turtle Carapace  Bird Keel 

The Appendicular Skeleton

• Each half of the human pelvic girdle contains an ilium, ischium,  and pubis.  On each side, the three bones form the acetabulum, or hip socket.

• The two pubi form the pubis symphysis in front, and the two ilia and the sacrum form the sacroiliac joint in the back.
• The pectoral girdle includes the two scapulae and clavicles, a loose assemblage that forms the shoulder joint.

• Human forelimbs are very generalized, except for subtle thumb modifications.  The humerus (upper arm) joins the ball-and-socket shoulder joint, while the radius and ulna (lower arm) form a complex pivoting hinge joint at both ends.

• The hands include the phalanges (fingers), long metacarpals (hand bones), and cubic carpals (wrist bones).

• The human leg contains the large femur (upper leg) and the tibia and fibula (lower leg). The hinged knee joint, formed by the femur, tibia and patella (knee cap), depends on a capsule of many tough ligaments for its strength.

• The ankle forms a rotating hinge joint where the talus meets the leg bones. The largest of the seven tasals in the ankle, the calcaneous, forms the heel and receives the Achilles tendon from the calf muscle.  The slender metatarsals and the phalanges (toes) complete the foot.  The arch is produced by ligaments binding the bones.

MUSCLE: ITS ORGANIZATION AND MOVEMENT -- College Lecture slides
Types of Muscle Tissue

• Smooth muscle (also involuntary or  visceral), is slow moving and rhythmic, occurring in the gut, blood vessels, hair roots, iris, uterus, and other places under the unconscious control of the autonomic nervous system.  Individual cells occur in sheets, are long and tapered, and have a single nucleus.
• Cardiac muscle (heart) is involuntary.  Its fibers are cylindrical and branching, with interlocking borders called intercalated disks, important to contraction.  It has rhythmic and tireless contractile characteristic in which isolated cells contract spontaneously.
• Skeletal muscle ( also voluntary muscle or striated), is controlled at the conscious level through the somatic nervous system.  Cells, called fibers, are striated and multinucleated.

Gross Anatomy of Skeletal Muscle

• Muscle fibers form bundles form bundles called fascicles, and a number of fascicles bond by connective tissue form muscle bundles.  Muscle contain a rich supply of blood vessels and nerves.  A sheathlike fascia surrounds the muscle, forming the tendons at its ends.  Tendons connect muscle to bone, forming insertions on the part to be moved and origins on the stationary part.
• Muscles also move the face and tongue, and others form ringlike sphincters in the gut.  Sheetlike muscles in the abdomen originate and insert via flattened tendons called aponeuroses.
• Muscle antagonists (opposing sets) account for opposite movements of the skeleton.

Ultrastructure of Skeletal Muscle

• Muscle fibers are surrounded by the sarcolemma, which receives motor neurons at neuromuscular junctions.  Each fiber contains several nuclei, many mitochondria, and an extensive sarcoplasmic reticulum.  Hollow transverse tubules, or  T-tubules, form boundaries for each contractile unit.  The pattern of contractile myofibrils, or fibrils, produces the striations.

• Myofibrils contain myofilaments, or filaments, made up of two kinds of protein: myosin and actin.  Each myosin filament is surrounded by six actins.  The striations are seen in the longitudinal view.  Prominent Z lines mark the sacromere, or contractile unit.  Within the Z lines lies a light I band (actin alone), and further in lines a darker A band (actin and myosin), ending in a lighter central H zone (myosin alone).

Contraction of the Ultrastructure

• Upon contraction, the Z lines move closer together, the I bands are reduced, and the H zone darkens.  However, the A band remains constant, indicating that it is the actin that moves, while the myosin is stationary.  The patterns change because minute myosin bridges contact each actin filament, pulling it inward.
• The myosin filaments are composed of spirally wound protein rods with regularly clustered, club-shaped heads.  Actin contains the assembled spheres of the protein actin and slender fibers of the protein tropomysin, ending with the globular protein troponin.

• In resting muscle, the troponin-tropomysin-actin complex inhibits bridge formation and the myosin knobs do not touch actin.
• In contraction, a neural impulse reaches the muscle fibers and travels along the T-tubules to the sarcoplasmic reticulum, which responds by suddenly releasing calcium ions into the contractile unit.  The ions alter the inhibition state, and the bridges connect and straighten sequentially along the actin filaments, pulling them inward as contraction continues.

• Restoration of the resting state begins when neural stimulation stops and the calcium ions are actively transported back into the sarcoplasmic reticulum.  The action occurs simultaneously and completely throughout the motor unit - the fibers served by the a motor neuron.