Skeleton + Muscles

Skeleton + Muscles

1. Animals move by muscles working against a skeleton.

   • Hydrostatic
     • Fluid held under pressure in a closed body compartment. Suitable in aquatic environments. Hydra move by closing mouth and elongation. Flatworms exert pressure on the interstitial fluid by localized forces. Roundworms hold fluid under high pressure and use longitudinal muscles to thrash and move.
       • Earthworms have circular and longitudinal muscles to perform peristalsis.
   • Exoskeleton
     • A hard encasement found on most mollusks and anthropods. Shed exoskeleton and have growth spurts.
       • Mollusks. Calcium Carbonate shell, not flexible, enlarged by adding layers to the outer edge, secreted by the mantle. Shells attached by muscles on the inside
       • Anthropods. Cuticle made of 30-50% chitin the rest is protein. Secreted by the epidermis. Muscles attached to knobs and plates on the inside. Leg joints are thin and flexible
   • Endoskeleton
     • Hard supporting material (bones) buried in soft tissue
       • Sponges are reinforced by hard spicules or softer fibers of protein
       • Chordates have cartilage or bone. Humans have an axial skeleton (down the axis of the body) and appendicular body (limbs). Joints include ball and socket, hinge and pivot
       • Echinoderms have hard plates called ossicles. Can be tightly bound like in sea urchins or loosely bound like in sea stars

2. Muscles move skeletal parts by contracting

   • Arm flex. Antagonistic muscle pair work against each other (triceps and biceps). the hinge joint allows the elbow to act as a lever. Work in both endo and exoskeleton
     • Wings have independent rhythmic contractions so can beat faster.
   • Vertebrate Skeletal Muscle
     • Muscles made up of a bundle of fibres, a single muscle fiber is made up of many myofibrils. Each microfibril consists of many subunits called sarcomeres
     • Can vary the rate of fibers contracting or number of fibers contracting= graded contractions
       • Recruitment of multple motor units strengthens contractions. Motor units take turns e.g. muscles in maintaining posture.
       • Single AP produces a twitch, two twitches sum= greater tension. tetanus is a smooth sustained contraction by a volley of AP's delivered by motor neurons.
     • Fast fibers for brief rapid contractions. Slow fibers for maintaining posture and long contractions, twitches last longer, less SR.
       • Glycolic fibers use glycolysis to produce ATP, all are fast fibers. Oxidative fibers are fast or slow and use aerobic respiration to produce ATP
       • Cardiac muscle= longer AP, AP in one part spreads throughout the whole heart. Smooth muscle= walls of hollow organs and lack striations, different actin and myosin layout.
   • Myofibril consist of two types of myofilaments. 1. Thin Filaments composed of 2 actin strands and 1 protein strand. 2. Thick filaments are made of staggered arrays of myosin molecules. Skeletal muscle is striated because of this.
     • At rest= thin and thick don't completely overlap. I band= only thin. H band= only thick.
     • A single muscle fiber can show how a who muscle contracts by the sliding filament model. Longitudinally slide past each other so overlap increases and I and H bands decrease. No change in the filament length. But the sarcomere shortens with contraction
       • Sliding filaments based on interaction with myosin and actin. Head of a myosin molecule binding to n actin filament creating a cross bridge. Pulls actin to the center of the sarcomere
         • Myosin head is bound to ATP (low energy configuration). It hydrolyses ATP to ADP + P (high energy configuration). Mysoin the binds to actin forming a cross bridge. releasing ADP + P it returns to its low energy state
         • The energy for contractions are stored as glycogen (glycolysis 1 min and aerobic respiration 1 hour) and creatine phosphate (15 s)
     • They only contract when activated by a motor neuron. At rest myosin binding sites are blocked by tropomyosin. must be uncovered for binding to occur. When calcium bind to troponin receptors binding sites are available.
       • For muscles to contract Calcium must be present in the cytosol. When calcium concentration drops sites are covered and contraction stops
       • The stimulus that causes contraction is an action potential in a motor neuron that makes a synapse with a muscle fiber
         • Synaptic terminal releases the neurotransmitter acetylcholine. Depolarises the muscle causing it to create an action potential. Travels along T (transverse) tubules. Causes sarcoplasmic reticulum to release calcium which will bind to troponin recpetors. At rest, SR pumps calcium from cytosol back RS

3. Locomotion requires energy to overcome friction and gravity

   • Swimming: Must overcome friction, gravity not an issue. Streamlined use legs as oars. Take water in and squirt it out. Move tail sided to side or up and down
   • Waking and running: On land gravity is main obstacle. Strong skeleton and muscles more important than streamlined shape. Balance is vital.
   • Flying: Must have wings large enough to carry body, use wing shape to alter air currents. Reduced mass, streamlined shape, hollow strong bones.
   • Costs: depends on mode of locomotion and environment. Animals specialized for swimming use the least energy per meter. flying animals use the most energy per minute. Large animals move more efficiently. Total energy expended per minute is greater for larger animals. Humming bird uses a lot of energy per minute.

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