Chapter 50. You re on your own for: Sensory Reception Mechanoreceptors Gravity, Hearing and Equilibrium. Chemoreception taste and smell

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1 1 Sensory and Motor Mechanisms 2 Chapter 50 You re on your own for: Sensory Reception Mechanoreceptors Gravity, Hearing and Equilibrium Chemoreception taste and smell Photoreceptors vision It s interesting. Read it. 3 I. Skeletal muscle A. Skeletal muscle = attached to bones and responsible for skeletal movement Consists of smaller and smaller units Tissue is fibrous - each fiber is one cell with many nuclei Each fiber has many myofibrils Myofibrils have 2 kinds of microfilaments : Thin = actin Thick = myosin 4 Figure 49.31x1 Skeletal muscle 5 Myofibrils have 2 kinds of microfilaments: Actin Thin 2 strands of globular protein wrapped in a double helix Myosin Thick Long, fibrous tail Globular head (powers the contraction) Forms a cross bridge which will bind to actin Fig a Fig b 9 Myofibrils are striated = light and dark bands each repeating unit is called a sarcomere = functional unit of muscle

2 Z lines are the borders of the sarcomere; containing only thin filaments M lines are areas in the center; containing only thick (myosin) filaments 10 Fig B. Sliding Filament Model Neither filament changes length, but slide past each other to overlap more and sarcomere shortens 1.ATP binds to myosin head = low-energy conformation 2.Myosin head hydrolyzes ATP? = high-energy conformation ADP + P i Myosin heads bind to actin to form a cross- bridge 4. ADP + P i is released? myosin head changes back to low-energy configuration? actin slides to center of sarcomere = Contraction 5. New molecules of ATP returns myosin head to unattached position; cross bridge breaks Fig Fig Fig Fig Tropomyosin and the troponin complex are regulatory proteins Calcium (Ca 2+ ) binds to the troponin complex? causes tropomyosin to expose portions of actin filament for attachments of myosin heads 18 Fig Interesting facts: Most energy comes from creatine phosphate which supplies P to ADP? ATP - no new ATP = no relaxation

3 (rigor mortis) 20 B. Nervous Control of Contraction Motor neurons cause muscle contraction by triggering the release of Ca 2+ into the cytosol of muscle cells with which they form synapses Neuromuscular junctions = specialized synapses between neuron and muscle cell 21 Stimulation of contraction: 1.Acetylcholine (a neurotransmitter) is released Binding of acetylcholine to receptors on muscle fiber leads to depolarization 2.Action potential is triggered in muscle cell which spreads into interior of muscle through T (tranverse) tubules (infoldings of membrane) The action potential spreads farther entering modified ER called the sarcoplasmic reticulum is that stores calcium. 4.Action potential changes permeability of sarcoplasmic reticulum? releases calcium? calcium binds to the troponin complex = contraction of muscle fiber Fig a Fig b 25 When motor neuron input stops: Muscle cell relaxes Proteins reset the muscle for next contraction Ca 2+ pumped out of the cytosol and back in to SR Myosin-binding sites are blocked again 26 C. Strength of Contraction 2 mechanisms for graded contractions: 1. vary the number of muscle cells that contract Motor unit = 1 motor neuron may control many contract as a group muscle fibers; causes all fibers to

4 27 2. vary the rate at which muscle cells are stimulate 1 action potential = a single twitch (100 msec. or less) An overlapping series = tetanus = a single, smooth, sustained contraction (no jerky movements) Fig Figure 49.38x Motor units in a vertebrate muscle 31 Fig Types of skeletal muscle fibers: 1. Oxidative and glycolitic fibers Oxidative fibers have many mitochondria, a rich blood supply, and the oxygen storing protein myoglobin Glycolitic fibers Less myoglobin, fatigue more easily 2. Fast and slow twitch fibers: fast = rapid, powerful contractions slow = sustain long contractions Less SR and pumps Ca2+ more slowly helps us maintain posture 33 II. Cardiac Muscle Striated Cells are branched with intercalated discs where gap junctions allow for direct electrical contacts Cells can generate their own action potentials 34 III. Smooth Muscle Lack striations Have less myosin and it is not associated with specific actin strands Generate less tension, but contract over a greater range of lengths No T tubules or well developed sarcoplasmic ret.? calcium enters through the membrane Slow contractions 35 Muscle action is to always contract, extension is passive (the muscle relaxes). Muscles work in antagonistic pairs. 36 Figure The cooperation of muscles and skeletons in movement

5 37 A. Types of Skeletal System 1. Hydrostatic skeletons - cnidarians, flatworms, nematodes, annelids - consists of fluid under pressure in a closed body compartment 2. Exoskeletons - molluscs and arthropods - hard coverings deposited on surface of animal 3. Endoskeletons - sponges, echinoderms, chordates - rigid supporting elements embedded within the animal s body 38 Figure Peristaltic locomotion in an earthworm 39 Fig A. Types of Locomotion 1. Swimming challenge: overcoming friction 2. Moving on land challenge: support against gravity 3. Flight challenge: wings must generate enough lift to overcome