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Sensory and Motor Mechanisms

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1 Sensory and Motor Mechanisms
Chapter 50 Sensory and Motor Mechanisms

2 Overview: Sensing and Acting
Detection/processing of sensory information and generation of motor output provides physiological basis for all animal activity Bats use sonar to detect their prey Moths, a common prey for bats, can detect the bat’s sonar and attempt to flee Both organisms have complex sensory systems that facilitate survival that include diverse mechanisms that sense stimuli and generate appropriate movement

3 Concept 50.1: Sensory receptors transduce stimulus energy and transmit signals to the central nervous system All stimuli represent forms of energy Sensation involves converting energy into change in membrane potential of sensory receptors Sensations are action potentials that reach brain via sensory neurons Brain interprets sensations, giving perception of stimuli

4 Sensory Pathways 4 functions of sensory pathways
Stimulation of stretch receptor in crayfish is first step in sensory pathway Bending activates stretch receptor in muscle Stretch receptor converts muscle deflection to change in membrane potential in cell body (receptor potential) Receptor potential triggers action potentials that travel along axon of stretch receptor Processing of action potentials that reach brain via axon of stretch receptor produces perception of body bending 4 functions of sensory pathways

5 Sensory Reception and Transduction
Sensations/perceptions begin with sensory reception (detection of stimuli by sensory receptors) Sensory receptors (usually specialized neurons/epithelial cells that respond to specific stimuli) detect stimuli outside (heat/light/pressure/ chemicals) and inside body (blood pressure/body position) Most are specialized neurons/epithelial cells Sensory transduction is conversion of physical/chemical stimulus energy into change in membrane potential of sensory receptor (receptor potential) Receptor potential are graded potentials (magnitude varies w/strength of stimulus) Many sensory receptors are very sensitive (detect smallest physical unit of stimulus, light photon of light by light receptors)

6 Transmission After energy has been transduced into receptor potential, some sensory cells whose axons extend into CNS generate transmission of action potentials (nerve impulses) to CNS Sensory cells without axons release neurotransmitters at synapses with sensory neurons Larger receptor potentials generate more frequent action potentials if receptor is sensory neuron (more neurotransmitter released if not, increasing production of action potentials by postsynaptic neuron) Integration (processing of sensory information) of sensory information begins when information is received Some receptor potentials are integrated through summation

7 Perception Perceptions are brain’s construction of stimuli (color, smells, sounds, tastes) Stimuli from different sensory receptors travel as action potentials along different neural pathways Brain distinguishes stimuli from different receptors by area in brain where action potentials arrive

8 Amplification and Adaptation
Transduction of stimuli by sensory receptors subject to two types of modifications Amplification is strengthening of stimulus energy by cells in sensory pathways to produce action potentials or release neurotransmitter at synapse w/sensory neuron Often require signal transduction pathways involving second messengers Amplify signal strength through formation of many product molecules by single enzyme molecule May also take place in accessory structures of complex sense organ Sensory adaptation is decrease in responsiveness to continued stimulation (don’t sense stimuli after while)

9 Receptor protein for neuro- transmitter
Fig. 50-UN1 To CNS To CNS Afferent neuron Afferent neuron Receptor protein for neuro- transmitter Neuro- transmitter Sensory receptor Stimulus leads to neuro- transmitter release Sensory receptor cell Stimulus Stimulus (a) Receptor is afferent neuron. (b) Receptor regulates afferent neuron.

10 Types of Sensory Receptors
Based on energy transduced, sensory receptors fall into five categories Mechanoreceptors Chemoreceptors Electromagnetic receptors Thermoreceptors Pain receptors

11 Mechanoreceptors Mechanoreceptors sense physical deformation caused by stimuli such as touch, pressure, stretch, motion, sound Bending/stretching of external structure (cilia)  tension that alters permeability of ion channels  membrane potential altered  depolarization or hyperpolarization Sense of touch in mammals relies on mechanoreceptors that are dendrites of sensory neurons, often embedded in layers of connective tissue Structure of connective tissue/location of receptors affect type of mechanical energy (light touch, vibration, strong pressure) that best stimulates them

12 Heat Gentle touch Pain Cold Hair Epidermis Dermis Hypodermis Nerve
Fig. 50-3 Heat Gentle touch (Responds to light touch or vibration) Pain Cold Hair Epidermis Dermis Figure 50.3 Sensory receptors in human skin Hypodermis Nerve Connective tissue Hair movement (like whiskers providing detailed information about nearby objects) Strong pressure/ Vibration

13 Chemoreceptors General chemoreceptors transmit information about total solute concentration of solution Specific chemoreceptors respond to individual kinds of molecules (glucose/oxygen/carbon dioxide/amino acids) When stimulus molecule binds to chemoreceptor, initiates changes in ion permeability (becomes more or less permeable to ions) Antennae of male silkworm moth have very sensitive specific chemoreceptors

14 Electromagnetic Receptors
Electromagnetic receptors detect electromagnetic radiation such as Light (photoreceptors) Electricity (electroreceptors of some fish/platypus to detect presence of prey) Magnetism (Earth’s magnetic field for migration/ iron-containing magnetite) Heat (infrared receptors of some snakes to detect body heat of prey)

15 Thermoreceptors Thermoreceptors, which respond to heat or cold, help regulate body temperature by signaling both surface and body core temperature Located in skin/anterior hypothalamus Send information to body’s thermostat (posterior hypothalamus) Capsaicin (jalapeno/cayenne peppers) activates chemoreceptors AND thermoreceptors (why we think spicy food is hot) Mammals have at least 6 different thermoreceptors , each for particular temperature range

16 Pain Receptors In humans, pain receptors, or nociceptors, are class of naked dendrites in epidermis (some also associated w/other organs) Respond to excess heat (thermal), pressure (mechanical), or chemical stimuli (prostaglanins) released from damaged or inflamed tissues

17 Low frequency of action potentials More pressure
Fig. 50-UN2 Gentle pressure Sensory receptor Low frequency of action potentials More pressure (a) Single sensory receptor activated High frequency of action potentials Gentle pressure Fewer receptors activated Sensory receptors More pressure More receptors activated (b) Multiple receptors activated

18 Concept 50.2: The mechanoreceptors responsible for hearing and equilibrium detect moving fluid or settling particles Hearing and perception of body equilibrium (balance) are related in most animals Settling particles or moving fluid are detected by mechanoreceptors, produce receptor potentials

19 Sensing Gravity and Sound in Invertebrates
Most invertebrates maintain equilibrium using sensory organs called statocysts Contain mechanoreceptors that detect movement of granules called statoliths (grains of sand/other dense granules) found in chamber surrounded by layerok of ciliated receptor cells Gravity causes statoliths to settle to low point in chamber, stimulating mechanoreceptors

20 Many arthropods sense sounds with
Body hairs that vibrate in response to sound waves Hairs of different stiffnesses/lengths vibrate at different frequencies Localized “ears” consisting of tympanic membrane  receptor cells  nerve impulses to brain

21 Hearing and Equilibrium in Mammals Hearing
In most terrestrial vertebrates, sensory organs for hearing and equilibrium are closely associated in ear Vibrating objects create percussion waves in air that cause tympanic membrane to vibrate Hearing: perception of sound in brain from vibration of air waves Vibrations of moving air  three bones of middle ear  oval window on cochlea  pressure waves in fluid in cochlea  vestibular canal  vibrates basilar membrane, bending its hair cells  depolarizes membranes of mechanoreceptors  action potentials to brain  auditory nerve

22 Semicircular canals (equilibrium) Incus
Fig. 50-8a Middle ear Outer ear Inner ear Skull bone Stapes Semicircular canals (equilibrium) Incus Malleus Auditory nerve to brain Figure 50.8 The structure of the human ear Cochlea (hearing) Oval window Eustachian tube (equalizes pressure between middle ear/ atmosphere) Pinna Auditory canal Round window Tympanic membrane


24 With each vibration, hairs bend when surrounding fluid moves
Fig. 50-9 With each vibration, hairs bend when surrounding fluid moves Mechanoreceptors in hair cells respond by opening/closing ion channels in plasma membrane Each hair cell releases an excitatory neurotransmitter at synapse w/sensory neuron, which conducts action potential to CNS “Hairs” of hair cell Neuro- trans- mitter at synapse More neuro- trans- mitter Less neuro- trans- mitter Sensory neuron –50 –50 Receptor potential –50 –70 –70 –70 Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) Action potentials Signal Signal Signal –70 –70 –70 Figure 50.9 Sensory reception by hair cells Time (sec) Time (sec) Time (sec) (a) No bending of hairs (b) Bending of hairs in one direction depolarization  increases neurotransmitter released and frequency of action potentials to brain along auditory nerve (c) Bending of hairs in other direction hyperpolarization  reduces neurotransmitter released and frequency of auditory nerve sensations

25 http://highered. mcgraw-hill
Vibrations of stapes against oval window produce pressure waves in perilymph (fluid of cochlea) Waves travel to apex of cochlea through vestibular canal and back toward base through tympanic canal Energy in waves causes basilar membrane to vibrate, stimulating hair cells Fluid waves dissipate when they strike round window at end of tympanic canal Prevents reverberation within ear and prolonged sensation/ resets apparatus for next vibration

26 Ear conveys information about
Volume (amplitude/loudness of sound wave) Large-amplitude sound wave  more vigorous vibration of basilar membrane  greater bending of hairs on hair cells  more action potentials Pitch (frequency of sound wave) 

27 Cochlea distinguishes pitch because basilar membrane
Cochlea distinguishes pitch because basilar membrane is not uniform along its length Variations in stiffness of basilar membrane along its length “tunes” specific regions to specific frequencies Relatively narrow and stiff at base near oval Wider and more flexible at apex Different frequencies of pressure waves cause different portions to vibrate, stimulating particular hair cells and sensory neurons that leads to excitation of specific auditory area of cerebral cortex Selective stimulation of hair cells perceived in brain as sound of certain pitch

28 Equilibrium Several organs of inner ear detect body position and balance In vestibule behind oval window Utricle/saccule contain calcium carbonate granules (otoliths) that allow us to detect gravity/linear movement (acceleration) Otoliths press on hairs protruding into gel when you tilt your head  deflection of hairs transformed into change in output of sensory neurons  brain signaled of angle change

29 Hairs of hair cells project into gelatinous cap (cupula)
Fig Hairs of hair cells project into gelatinous cap (cupula) When head starts or stops rotating, fluid in semicircular canals presses against cupula, bending the hairs Flow of fluid Semicircular canals detect angular movement of head (each canal has swelling at base containing cluster of hair cells) Vestibular nerve Cupula Hairs Hair cells Vestibule Figure Organs of equilibrium in the inner ear Axons Utricle & Body movement Bending of hairs increases frequency of action potentials in sensory neurons in direct proportion to amount of rotational acceleration Saccule both tell brain which way is up/inform it of body’s position or linear acceleration

30 Three semicircular canals, connected to utricle
Contain fluid Allow us to detect angular acceleration (turning head) Each canal in different spatial plane, allowing us to detect motion in any direction Each canal has hair cells in single cluster Hairs project into cupula (gelatinous cap) Motion deflects hairs  impulse sent to brain

31 Hearing and Equilibrium in Other Vertebrates
Unlike mammals, fishes have only pair of inner ears near brain (no external opening/eardrum/cochlea) Water vibrations/vibrations in air-filled swim bladder conducted through head’s skeleton to pair of inner ears, setting otholiths in motion and stimulating hair cells Most fishes/aquatic amphibians also have lateral line system along both sides of their body  Contains mechanoreceptors with hair cells that detect and respond to low-frequency waves Terrestrial vertebrates’ ears main organ of hearing/ equilibrium

32 by moving objects, low-frequency sounds conducted through water
Fig Lateral line enables fish to monitor water currents (direction/velocity), pressure waves produced by moving objects, low-frequency sounds conducted through water Surrounding water Scale Lateral line canal Opening of lateral line canal Cupula Epidermis Sensory hairs Figure The lateral line system in a fish Hair cell Supporting cell Segmental muscles Lateral nerve Axon Fish body wall Water flowing through system bends hair cells  transduce energy into receptor potentials (depolarizes hair cells)  action potentials conveyed to brain

33 Concept 50.3: The senses of taste and smell rely on similar sets of sensory receptors
In terrestrial animals, chemoreceptors needed to detect specific chemicals in environment Gustation (taste) is dependent on detection of chemicals (tastants) Olfaction (smell) is dependent on detection of odorant molecules Chemicals must be dissolved to bind to proteins on plasma membrane No distinction between taste/smell in aquatic animals Insects’ taste receptors in sensory hairs (sensilla) located on feet/in mouth parts/are capable of smelling airborne odorants using olfactory hairs located in antennae

34 Taste in Mammals In humans, receptor cells for taste are modified epithelial cells organized into taste buds Five taste perceptions: sweet, sour, salty, bitter, and umami (elicited by glutamate) Each type of taste can be detected in any region of tongue that has taste buds Each taste cell has only one type of receptor and detects tastants representing only one of five tastes When taste receptor is stimulated, signal transduced to sensory neuron

35 Sugar molecule binds to receptor protein on sensory receptor cell
Fig Sugar molecule binds to receptor protein on sensory receptor cell G protein Binding initiates signal transduction path- Way involving G protein/phospholipase C Sweet receptor Tongue Phospholipase C SENSORY RECEPTOR CELL Sugar molecule Taste pore PIP2 Phospholipase C activity generates second messenger IP3 which binds to calcium channel in ER, opening it. Ca2+, another second messenger flows into cytosol Taste bud Sensory receptor cells IP3 (second messenger) Sensory neuron IP3-gated calcium channel Nucleus Figure Sensory transduction by a sweet receptor Sodium channel opened, allowing Na+ to diffuse into taste receptor cell ER Ca2+ (second messenger) Na+ Depolarization activates sensory neuron through process not fully understood

36 Smell in Humans Olfactory receptor cells are neurons (unlike gustation) that line upper portion of nasal cavity Binding of odorant molecules to receptors triggers signal transduction pathway, sending action potentials along axons extend directly to olfactory bulb of brain >1000 genes code for membrane proteins that bind to specific classes of odorants/each receptor cell appears to expreess only one of those genes Receptive ends contain cilia that extend into layer of mucus coating nasal cavity Odorant diffuses into region  binds to specific GPCR protein (odorant receptor/OR) on plasma membrane of olfactory cilia  transduction  cAMP production  Na+/Ca2+ channels open  depolarization of membrane  action potential

37 Brain Olfactory bulb Odorants Nasal cavity Bone Epithelial cell
Fig Brain Action potentials Olfactory bulb Odorants Nasal cavity Bone Epithelial cell Odorant receptors Chemo- receptor Figure Smell in humans Plasma membrane Cilia Odorants Mucus

38 Concept 50.4: Similar mechanisms underlie vision throughout the animal kingdom
Many types of light detectors have evolved in the animal kingdom Simple cluster of cells that detect direction and intensity of light Complex organs that form images

39 Vision in Invertebrates
Most invertebrates have light-detecting organ One of simplest is ocellus (eyecup or eyespot of planarians) Light-sensitive photoreceptors determine direction of light/planaria moves away from source

40 Two major types of image-forming eyes have evolved in invertebrates
Image-forming compound eyes found in insects/crustaceans/ some polychaete worms Consist of up to several thousand light detectors (ommatidia) Each has light-focusing lens Very effective at detecting movement Excellent color vision/some see UV range

41 Single-lens eyes found in some jellies/polychaetes, spiders, many mollusks
Work on camera-like principle: Iris changes diameter of pupil to control how much light enters  focuses on layer of photoreceptors

42 The Vertebrate Visual System Structure of the Eye
In vertebrates, eye detects color/light, but brain assembles information and perceives image Main parts of vertebrate eye Sclera: white outer layer, including cornea (transparent front/ lets light in/acts as fixed lens) Choroid: pigmented inner layer which includes Iris (colored portion of eye/regulates size of pupil) Retina: innermost layer/has layers of neurons/photoreceptors Lens: focuses light on retina Optic disk: blind spot (no photoreceptors) in retina where optic nerve attaches to eye

43 Fovea (center of visual field)
Fig Sclera Choroid Retina Ciliary body Suspensory ligament Fovea (center of visual field) Cornea Iris Optic nerve Pupil Aqueous humor Figure Structure of the vertebrate eye Lens Central artery and vein of the retina Vitreous humor Optic disk (blind spot)

44 Eye divided into two cavities separated by lens (transparent disk of protein used to focus by changing its shape) and ciliary body Anterior cavity filled with watery aqueous humor Ciliary body produces aqueous humor Glaucoma: blockage of ducts that drains aqueous humor, increasing pressure that can damage optic nerve/cause blindness Posterior cavity filled with jellylike vitreous humor Most of eye’s volume

45 Ciliary muscles contract.
Fig Ciliary muscles relax. Ciliary muscles contract. Choroid Suspensory ligaments pull against lens. Suspensory ligaments relax. Retina Lens becomes thicker and rounder. Lens becomes flatter. Figure Focusing in the mammalian eye (a) Near vision (accommodation) (b) Distance vision

46 Human retina contains two types of photoreceptors
Rods: light-sensitive but don’t distinguish colors Enable us to see at night Humans have ~125 million In humans, concentrated in fovea (center of visual field-~150,000 cones/mm2) Cones: distinguish colors but are not as sensitive to light 3 types, each w/different sensitivity across visual spectrum Humans have ~6 million In humans, more concentrated around periphery of retina

47 Sensory Transduction in the Eye
Each rod/cone contains visual pigments consisting of light-absorbing molecule (retinal, derivative of vitamin A) bonded to protein (opsin) Rods contain pigment rhodopsin (retinal combined w/specific opsin), which changes shape when absorbing light Absorption of destabilizes/activates rhodopsin (turns it purple to yellow, called “bleaching”)

48 Membrane potential (mV)
Fig 1. Light isomerizes retinal, activating rhodopsin 2. Active rhodopsin activates G protein (transducin) INSIDE OF DISK EXTRACELLULAR FLUID Light Disk membrane Active rhodopsin Phosphodiesterase Plasma membrane CYTOSOL Sodium channel cGMP Inactive rhodopsin Transducin GMP 3. Transducin activates phosphodiesterase (activates G protein) Na+ 4. Activated phosphodiesterase detaches cGMP from Na+ channels in plasma membrane by hydrolyzing cGMP to GMP Dark Figure Receptor potential production in a rod cell Light Membrane potential (mV) –40 Hyper- polarization Na+ –70 Time 5. Na+ channels close when cGMP detaches. Membrane’s permeability to Na+ decreases, rod hyperpolarizes (response in rods saturated). Remains bleached in very bright light and takes minutes to regain full responsiveness once light decreases (rhodopsin returns to inactive form when enzymes convert retinal back to cis form)

49 In humans, three pigments (photopsins) detect light of different wave lengths
Formed from binding of retinal to three distinct opsin proteins Absorption spectrums of visual pigments (red, green, blue) overlap, so brain’s perception of intermediate hues depends on different stimulation of two or more classes of cones Abnormal color vision typically results from alterations in genes for one or more photopsin protein Genes for red/green on X chromosome

50 Processing of Visual Information
Processing of visual information begins in retina, where absorption of light triggers signal transduction pathway In dark, rods/cones are depolarized and continually release neurotransmitter glutamate into synapses with neurons (bipolar cells) Some bipolar cells hyperpolarized/others depolarized in response to glutamate Response determined by glutamate receptor present on surface at synapse In the light, rods/cones hyperpolarize, shutting off release of glutamate (release less neurotransmitter) Depolarized bipolar cells hyperpolarize/hyperpolarized one depolarize

51 Bipolar cell either depolarized or hyperpolarized
Fig Dark Responses Light Responses Rhodopsin inactive Rhodopsin active Na+ channels open Na+ channels closed Rod depolarized Rod hyperpolarized Figure Synaptic activity of rod cells in light and dark Glutamate released No glutamate released Bipolar cell either depolarized or hyperpolarized Bipolar cell either hyperpolarized or depolarized

52 Interaction among different cells results in lateral inhibition
Three other types of neurons contribute to information processing in retina Ganglion cells transmit signals from bipolar cells to brain along optic nerves (made of ganglion cell axons) Horizontal cells/amacrine cells help integrate visual information before it is sent to brain Interaction among different cells results in lateral inhibition When horizontal cells illuminated, they inhibit more distant photoreceptors/bipolar cells not illuminated Light spot appears light/dark surroundings even darker Form of integration sharpens edges/enhances contrast of image


54 Many rods/cones  information sent to single ganglion cell
Reception field (part of visual filed to which ganglion can respond) Smaller reception field = sharper image (more precise information going to retina) Ganglion cells of fovea very small , high visual acuity

55 Optic nerves (each contain ~million axons) meet at optic chiasm near cerebral cortex
Here, axons from left visual field (from both left/right eye) converge and travel to right side of brain Axons from right visual field travel to left side of brain Most ganglion cell axons lead to lateral geniculate nuclei Relays information to primary visual cortex in cerebrum Several integrating centers in cerebral cortex are active in creating visual perceptions

56 Lateral geniculate nucleus
Fig Neural Pathways for Vision Right visual field Optic chiasm Right eye Left eye Figure Neural pathways for vision Left visual field Optic nerve Primary visual cortex Lateral geniculate nucleus

57 Evolution of Visual Perception
Photoreceptors in diverse animals likely originated in earliest bilateral animals All photoreceptors contain similar pigment molecules Many invertebrates/vertebrates share genes associated w/embryonic development of photoreceptors Melanopsin (pigment in ganglion cells) may play role in circadian rhythms in humans

58 Concept 50.5: The physical interaction of protein filaments is required for muscle function
Muscle activity is response to input from nervous system Microfilament movements bring about contraction Extension occurs only passively

59 Fig Vertebrate Skeletal Muscle characterized by hierarchy of smaller and smaller units Muscle consists of bundle of long fibers running parallel to length of muscle Bundle of muscle fibers Single muscle fiber is single cell w/multiple nuclei Nuclei Plasma membrane Muscle fiber contains bundle of smaller Myofibril arranged longitudinally. Each myofibril is composed of thin filaments and thick filaments. Functional (contractile) unit of muscle Borders lined up in adjacent myofibrils and contribute to striations Thin filaments attached at Z lines and project toward center of sarcomere, while thick filaments attached at M lines centered in sarcomere At rest, thick/thin filaments only partially overlap (near edge, only thin/center only thick) Sarcomere Z lines Skeletal muscle also called striated muscle because regular arrangement of myofilaments creates pattern of light/dark bands Figure The structure of skeletal muscle TEM 0.5 µm M line Thick filaments are composed of staggered arrays of myosin molecules Thin filaments consist of two strands of actin and two strands of regulatory protein coiled around one another Z line Z line Sarcomere

60 The Sliding-Filament Model of Muscle Contraction
According to sliding-filament model, filaments slide past each other longitudinally, producing more overlap between actin of thin/myosin of thick filaments, which never change length “Head” of myosin molecule energized by ATP binds to actin filament, forming cross-bridge and pulling thin filament toward center of sarcomere Each of ~350 heads of thick filament forms/reforms ~5 cross-bridges/sec Energy for repeated contractions comes from Creatine phosphate transfers phosphate group to ADP to synthesize additional ATP (sustained contractions for ~15 sec) Glycogen  glucose by glycolysis (~1 minute of sustained contraction)/aerobic respiration (up to hour of sustained contractions) generate ATP needed to sustain muscle contraction

61 Fully contracted muscle
Fig Sarcomere 0.5 µm Z M Z Relaxed muscle Contracting muscle Fully contracted muscle Figure The sliding-filament model of muscle contraction For the Cell Biology Video Conformational Changes in Calmodulin, go to Animation and Video Files. For the Cell Biology Video Cardiac Muscle Contraction, go to Animation and Video Files. Contracted Sarcomere

62 1. Starting here, myosin head bound to ATP/in low-energy conformation
Fig Thick filament Thin filaments 1. Starting here, myosin head bound to ATP/in low-energy conformation Thin filament 5. Binding of new molecule of ATP releases myosin head from actin, new cycle begins Thin filament moves toward center of sarcomere. Myosin head (low- energy configuration ATP 2. Myosin head hydrolyzes ATP  ATD/Pi/in high- energy configuration Myosin binding sites ATP Thick filament Actin Myosin head (low energy configuration 4. Releasing ADP/Pi, myosin returns to low-energy configuration, sliding the thin filament ADP Myosin head (high- energy configuration 3. Myosin head binds to actin, forming cross-bridge P i Figure Myosin-actin interactions underlying muscle fiber contraction ADP ADP + P i P i Cross-bridge

63 The Role of Calcium and Regulatory Proteins
Skeletal muscle fiber contracts only when stimulated by motor neuron When muscle is at rest, myosin-binding sites on thin filament are blocked by regulatory protein tropomyosin, preventing actin/myosin from interacting

64 For muscle fiber to contract, myosin-binding sites must be uncovered
For muscle fiber to contract, myosin-binding sites must be uncovered Occurs when calcium ions (Ca2+) bind to set of regulatory proteins (troponin complex) Actin strands shift position and expose myosin-binding sites on thin filament Muscle fiber contracts when concentration of Ca2+ is high; muscle fiber contraction stops when concentration of Ca2+ is low

65 Sarcoplasmic reticulum (SR)
Fig a Synaptic terminal Motor neuron axon T tubule Mitochondrion Sarcoplasmic reticulum (SR) Myofibril Plasma membrane of muscle fiber Figure 50.29a The regulation of skeletal muscle contraction Ca2+ released from SR Sarcomere

66 Synaptic terminal of motor neuron
Fig b Stimulus leading to contraction of muscle fiber is action potential in motor neuron that makes synapse with muscle fiber  1. Acetylcholine released at synaptic terminal of motor neuron diffuses across synaptic cleft/binds to receptor proteins on muscle fiber’s plasma membrane  depolarization  action potential Synaptic terminal of motor neuron T (transverse) Tubule Synaptic cleft Plasma membrane 2. Action potential propagated along plasma membrane/down T tubules ACh SR Ca2+ ATPase pump 3. Action potential triggers Ca2+ release from sarcoplasmic reticulum (SR) Ca2+ ATP 4. Calcium ions bind to troponin in thin filament; myosin-binding sites exposed CYTOSOL 7. Tropomyosin blockage of myosin-binding site is restored; contraction ends/muscle fiber relaxes Figure 50.29b The regulation of skeletal muscle contraction Ca2+ 6. Cytosolic Ca2+ removed by active transport into SR after action potential ends ADP P i 5. Myosin cross-bridges alternately attach to actin/detach, pulling thin filament toward center of sarcomere; ATP powers sliding of filaments

67 Diseases causing paralysis
Amyotrophic lateral sclerosis (ALS), formerly called Lou Gehrig’s disease Interferes with excitation of skeletal muscle fibers Usually fatal Myasthenia gravis Autoimmune disease that attacks acetylcholine receptors on muscle fibers As number of receptors decreases, synaptic transmission between motor neurons/muscle fibers declines Treatments exist for this disease

68 Nervous Control of Muscle Tension
Contraction of whole muscle is graded (extent/strength of its contraction can be voluntarily altered) Two basic mechanisms by which nervous system produces graded contractions Varying number of fibers that contract Varying rate at which fibers are stimulated

69 Spinal cord Motor unit 1 Motor Motor unit consists of single motor
Fig Spinal cord Motor unit 1 Motor Motor unit consists of single motor unit 2 neuron/all muscle fibers it controls Synaptic terminals Nerve Motor neuron cell body Motor neuron axon Each branched motor neuron may form synapses w/many muscle fibers Figure Motor units in a vertebrate skeletal muscle Muscle Muscle fibers Each muscle fiber controlled by only one motor neuron Tendon

70 Some muscles always partially contracted
When motor neuron produces action potential, all muscle fibers in its motor unit contract as group Strength of resulting contraction depends on how many muscle fibers motor neuron controls Nervous system regulates strength of contraction by determining how many motor units are activated Force developed by muscle progressively increases (contraction stronger) as more motor neurons controlling muscle are activated (recruitment) Some muscles always partially contracted To prevent fatigue, nervous system alternates activation among motor units, reducing length of time any one set of fibers is contracted

71 Gradation also achieved by varying rate of muscle fiber stimulation
Twitch results from single action potential in motor neuron (lasts ~100msec) If second action potential arrives before muscle fiber completely relaxed, two twitches add up, resulting in greater tension More rapidly delivered action potentials produce graded contraction by summation Tetanus is state of smooth and sustained contraction produced when motor neurons deliver volley of action potentials (not jerky like individual twitches)/muscle fiber cannot relax at all between stimuli

72 Summation of two twitches Tension
Fig Tetanus Summation of two twitches Tension Single twitch Figure Summation of twitches Time Single action potential Dashed lines show tension that would have developed if only first action potential occurred Pair of action potentials Series of action potentials at high frequency

73 Types of Skeletal Muscle Fibers
Skeletal muscle fibers can be classified As oxidative or glycolytic fibers, by source of ATP As fast-twitch or slow-twitch fibers, by speed of muscle contraction

74 Oxidative and Glycolytic Fibers
Oxidative fibers use aerobic respiration to generate ATP (dark meat in chicken) Have many mitochondria, rich blood supply, and much myoglobin (protein that binds oxygen more tightly than hemoglobin does) Glycolytic fibers use glycolysis as primary source of ATP (light meat in chicken) Have larger diameter, less myoglobin, fatigue much more readily

75 Fast-Twitch and Slow-Twitch Fibers
Muscle fibers vary in speed with which they contract Slow-twitch fibers contract more slowly, but sustain longer contractions (~5x as long) All are oxidative Often found in muscles that maintain posture Fast-twitch fibers contract more rapidly (2-3x faster), but sustain shorter contractions Can be either glycolytic or oxidative Used for brief, rapid, powerful contractions Most skeletal muscles contain both in varying ratios

76 Other Types of Muscle In addition to skeletal muscle, vertebrates have
Cardiac muscle (only in heart) consists of striated cells Have ion channels in plasma membrane that cause rhythmic depolarizations, triggering action potentials without neural input (unlike skeletal muscle fibers) Plasma membranes of adjacent cardiac muscle cells interlock at intercalated disks that provide direct electrical coupling between cells Action potential generated by specialized cells in one part of heart spread to all other cardiac muscle cells, causing whole heart to contract Long refractory period prevents summation and tetanus

77 Smooth muscle (mainly in walls of hollow organs)
Smooth muscle (mainly in walls of hollow organs) Actin/myosin filaments not regularly arrayed along length of cell, so no striations Contractions are relatively slow May be initiated by muscles themselves, without neural input (electrically coupled to one another) May also be caused by stimulation from neurons in autonomic nervous system

78 Concept 50.6: Skeletal systems transform muscle contraction into locomotion
Skeletal muscles are attached in antagonistic pairs, with each member of pair working against other Skeleton provides rigid structure to which muscles attach Skeletons function in support (maintains shape), protection, and movement

79 Extensor muscle relaxes Biceps contracts Tibia flexes
Fig Human Grasshopper Extensor muscle relaxes Biceps contracts Tibia flexes Flexor muscle contracts Forearm flexes Triceps relaxes Biceps relaxes Extensor muscle contracts Tibia extends Figure The interaction of muscles and skeletons in movement For the Discovery Video Muscles and Bones, go to Animation and Video Files. Forearm extends Flexor muscle relaxes Triceps contracts

80 Types of Skeletal Systems
Three main types of skeletons are Hydrostatic skeletons (lack hard parts) Exoskeletons (external hard parts) Endoskeletons (internal hard parts)

81 Hydrostatic Skeletons
Hydrostatic skeleton consists of fluid held under pressure in closed body compartment Main type of skeleton in most cnidarians, flatworms, nematodes, annelids Annelids use their hydrostatic skeleton for peristalsis (type of movement on land produced by rhythmic waves of muscle contractions) Well suited for aquatic life, cushion internal organs from shocks, provide support for crawling/burrowing in terrestrial animals Cannot support terrestrial activities when body held off ground (walking, running)

82 Longitudinal muscle relaxed (extended) Circular muscle contracted
Fig Longitudinal muscle relaxed (extended) Circular muscle contracted Circular muscle relaxed Longitudinal muscle contracted Bristles Longitudinal muscles contracted/circular muscles relaxed making body Segments at head/rear short/thick and anchored to ground. Other segments thin/elongated (circular muscles contracted/ longitudinal muscles relaxed) Head moves forward because circular muscles in head segments have contracted. Segments behind head/at read now thick/anchored, preventing worm from slipping backward. Head segments thick again/anchored in new positions. Rear segments Released hold on ground/pulled forward. Head end Head end Figure Crawling by peristalsis Head end

83 Exoskeletons Exoskeleton is hard encasement deposited on surface of animal and found in Most mollusks (calcium carbonate shell secreted by mantle) Arthropods (made of cuticle secreted by epidermis, can be strong/flexible, polysaccharide chitin is often found in cuticle)

84 Endoskeletons Endoskeleton consists of hard supporting elements, such as bones, embedded within animal’s body and are found in Sponges Echinoderms (hard plates composed of magnesium carbonate/calcium carbonate crystals called ossicles beneath skin) Chordates (consists of cartilage, bone) Bones reinforced by inorganic material/softer fibers made of protein Mammalian skeleton has >200 bones, some fused; others connected at joints by ligaments that allow freedom of movement

85 Figure 50.34 Bones and joints of the human skeleton
Head of humerus Examples of joints Skull Scapula 1 Clavicle Shoulder girdle Scapula Sternum 1 Ball-and-socket joint Rib 2 Humerus 3 Vertebra Radius Ulna Humerus Pelvic girdle Carpals Ulna Phalanges Metacarpals Figure Bones and joints of the human skeleton 2 Hinge joint Femur Patella Tibia Fibula Ulna Radius Tarsals Metatarsals Phalanges 3 Pivot joint

86 Size and Scale of Skeletons
Animal’s body structure must support its size Body posture (position of legs relative to main body) more important in support body weight than leg size relative to body size Size of animal’s body scales with volume (function of n3), while support for that body scales with cross-sectional area of legs (function of n2) As objects get larger, size (n3) increases faster than cross-sectional area (n2); this is principle of scaling

87 The skeletons of small and large animals have different proportions because of the principle of scaling In mammals and birds, the position of legs relative to the body is very important in determining how much weight the legs can bear

88 Types of Locomotion Most animals are capable of locomotion (active travel from place to place) to search for food, escaping from danger, finding mate In locomotion, energy is expended to overcome friction and gravity

89 Swimming In water, friction is bigger problem than gravity
Fast swimmers usually have streamlined shape to minimize friction Animals swim in diverse ways Paddling with their legs as oars Jet propulsion Undulating their body and tail from side to side, or up and down

90 Locomotion on Land Walking, running, hopping, or crawling on land requires animal to support itself and move against gravity Air poses little resistance (up to moderate speeds) Powerful muscles (must overcome inertia by accelerating from standing start, must propel/keep from falling down)/strong skeletal support important Diverse adaptations for locomotion on land have evolved in vertebrates Maintain balance Energy to overcome friction problem w/crawling

91 Flying Flight requires that wings develop enough lift to overcome downward force of gravity Wing shape is key All types of wings are airfoils (structures whose shape alters air currents to help object stay afloat) Many flying animals have adaptations that reduce body mass For example, birds lack teeth/urinary bladder and have relatively large bones w/air-filled spaces to lessen weight

92 Energy Costs of Locomotion
Energy cost of locomotion Depends on mode of locomotion/environment Running expends more energy to overcome gravity Swimming most energy-efficient (if animal specialized for it) Flying uses more energy than swimming/running Larger animals more efficient than smaller ones specialized for same mode of locomotion Can be estimated by rate of oxygen consumption or carbon dioxide production Energy in food used for locomotion not available for other activities (growth/reproduction) Structural/behavioral adaptations that maximize efficiency of locomotion increase evolutionary fitness of organism

93 You should now be able to:
Distinguish between the following pairs of terms: sensation/perception; sensory transduction/receptor potential; tastants/odorants; rod/cone cells; oxidative/glycolytic muscle fibers; slow-twitch/fast-twitch muscle fibers; endoskeleton/exoskeleton List the five categories of sensory receptors and explain the energy transduced by each type Explain the role of mechanoreceptors in hearing and balance Give the function of each structure using a diagram of the human ear Explain the basis of the sensory discrimination of human smell Identify and give the function of each structure using a diagram of the vertebrate eye Identify the components of a skeletal muscle cell using a diagram Explain the sliding-filament model of muscle contraction Explain how a skeleton combines with an antagonistic muscle arrangement to provide a mechanism for movement

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