Sensing and acting Bats use sonar to detect their prey. Both bats and moths have complex sensory systems that facilitate their survival

Transcription

1 Sensing and acting Bats use sonar to detect their prey Moths, a common prey for bats can detect the bat s sonar and attempt to flee Both bats and moths have complex sensory systems that facilitate their survival The structures that make up these systems have been transformed by evolution into diverse mechanisms that sense various stimuli and generate the appropriate physical movement Concept: Sensory receptors transduce stimulus energy and transmit signals to the central nervous system Sensations are action potentials that reach the brain via sensory neurons Once the brain is aware of sensations it interprets them, giving the perception of stimuli. Sensations and perceptions begin with sensory reception, the detection of stimuli by sensory receptors Exteroreceptors Detect stimuli coming from the outside of the body Interoreceptors Detect internal stimuli 1

2 Functions Performed by Sensory Receptors All stimuli represent forms of energy Sensation involves converting this energy into a change in the membrane potential of sensory receptors Sensory receptors perform four functions in this process Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor. This change in the membrane potential is known as a receptor potential Amplification is the strengthening of stimulus energy by cells in sensory pathways Transmission: after energy in a stimulus has been transduced into a receptor potential some sensory cells generate action potentials, which are transmitted to the CNS Integration is the evaluation and coordination of stimuli to produce a response. It occurs at all levels of the nervous system. Hair cell found in vertebrates Hairs of hair cell Neurotransmitter at synapse Axon (b) Vertebrate hair cells have specialized cilia or microvilli ( hairs ) that bend when sur-rounding fluid moves. Each hair cell releases an excitatory neurotransmitter at a synapse Figure 49.2b Membrane potential (mv) 0 70 No fluid movement Action potentials Time (sec) More neurotransmitter Membrane potential (mv) with a sensory neuron, which conducts action potentials to the CNS. Bending in one direction depolarizes the hair cell, causing it to release more neurotransmitter and increasing frequency 0 70 Fluid moving in one direction Receptor potential Time (sec) Less neurotransmitter Membrane potential (mv) of action potentials in the sensory neuron. Bending in the other direction has the opposite effects. Thus, hair cells respond to the direction of motion as well as to its strength and speed.s 0 70 Fluid moving in other direction Time (sec) Types of Sensory Receptors Based on the energy they transduce, sensory receptors fall into five categories Mechanoreceptors: Chemoreceptors Electromagnetic receptors Thermoreceptors Pain receptors 2

3 Mechanoreceptors Mechanoreceptors sense physical deformation Caused by stimuli such as pressure, stretch, motion, and sound Mechanoreceptors The mammalian sense of touch relies on mechanoreceptors that are the dendrites of sensory neurons Cold Heat Light touch Pain Hair Epidermis Dermis Figure 49.3 Nerve Connective tissue Hair movement Strong pressure Chemoreceptors Chemoreceptors include General receptors that transmit information about the total solute concentration of a solution Specific receptors that respond to individual kinds of molecules Two of the most sensitive and specific chemoreceptors known are present in the antennae of the male silkworm moth. The males use their antennae to detect two components in pheromones released by females. Figure mm 3

4 Electromagnetic Receptors Electromagnetic receptors detect various forms of electromagnetic energy such as visible light, heat, electricity, and magnetism Some snakes have very sensitive infrared receptors that detect body heat of prey against a colder background (a) This rattlesnake and other pit vipers have a pair of infrared receptors, one between each eye and nostril. The organs are sensitive enough to detect the infrared radiation emitted by a warm mouse a meter away. The snake moves its head from side to side until the radiation is detected equally by the two receptors, indicating that the mouse is straight ahead. Many mammals and birds appear to use the Earth s magnetic field lines to orient themselves as they migrate Thermoreceptors Thermoreceptors, which respond to heat or cold help regulate body temperature by signaling both surface and body core temperature Figure 49.5b (b) Some migrating animals, such as these beluga whales, apparently sense Earth s magnetic field and use the information, along with other cues, for orientation. 4

5 Pain Receptors In humans, pain receptors, also called nociceptors are a class of naked dendrites in the epidermis They respond to excess heat, pressure, or specific classes of chemicals released from damaged or inflamed tissues Hearing, balance and the ear. The mechanoreceptors involved with hearing and equilibrium detect settling particles or moving fluid Hearing and the perception of body equilibrium are related in most animals Hearing and Equilibrium in Mammals In most terrestrial vertebrates the sensory organs for hearing and equilibrium are closely associated in the ear. The ear is divided into three areas: The outer ear which contains the auditory canal. The middle ear which includes the tympanic membrane and the three ear bones that transmit vibrations to the inner ear The inner ear which contains the cochlea where vibrations are converted to nerve impulses and sent to the brain as well as the organs of balance: utricle, saccule and semicircular canals. Structure of the human ear 1 Overview of ear structure 2 The middle ear and inner ear Incus Skull Semicircular bones canals for balance Middle Stapes Outer ear ear Inner ear Malleus Auditory nerve, to brain Pinna Tympanic membrane Eustachian Cochlea Auditory tube canal Oval Eustachian Tympanic Hair cells Tectorial window tube membrane membrane Round window Cochlear duct Bone Vestibular canal Auditory nerve Basilar Axons of To auditory membrane sensory neurons nerve Tympanic canal Organ of Corti Figure The organ of Corti 3 The cochlea 5

6 Hearing Vibrating objects create percussion waves in the air that cause the tympanic membrane to vibrate The three bones of the middle ear transmit the vibrations to the oval window on the cochlea Hearing These vibrations create pressure waves in the fluid in the cochlea that travel through the vestibular canal and ultimately strike the round window Stapes Cochlea Oval window Vestibular canal Perilymph Axons of sensory neurons Apex Base Round window Figure 49.9 Tympanic canal Basilar membrane Hearing The pressure waves in the vestibular canal cause the basilar membrane in the organ of Corti within the cochlea to vibrate up and down causing its hair cells to bend The bending of the hair cells depolarizes their membranes sending action potentials that travel via the auditory nerve to the brain where they are interpreted as sounds. Hearing The cochlea can distinguish pitch because the basilar membrane is not uniform along its length. Each region of the basilar membrane vibrates most vigorously at a particular frequency and leads to excitation of a specific auditory area of the cerebral cortex. Cochlea (uncoiled) Basilar membrane Apex (wide and flexible) 1 khz 500 Hz (low pitch) 2 khz 4 khz 8 khz Figure Base (narrow and stiff) 16 khz (high pitch) Frequency producing maximum vibration 6

7 Equilibrium Several of the organs of the inner ear detect body position and balance The utricle and saccule tell the brain which way is up. Semicircular canals in the inner ear detect angular movements of the head and function in balance and equilibrium Each canal has at its base The semicircular canals, arranged a swelling called an in three spatial planes, detect ampulla, containing a angular movements of the head. cluster of hair cells. Flow of endolymph Vestibular nerve Vestibule Utricle Saccule When the head changes its rate of rotation, inertia prevents endolymph in the semicircular canals from moving with the head, so the endolymph presses against the cupula, bending the hairs. Flow of endolymph Cupula Hairs Hair cell Nerve fibers Body movement The utricle and saccule tell the The hairs of the hair cells brain which way is up and inform it project into a gelatinous of the body s position or linear cap called the cupula. acceleration. Bending of the hairs increases the frequency of action potentials in sensory neurons in direct proportion to the amount of rotational acceleration. Odor detection in humans Olfactory receptor cells Are neurons that line the upper portion of the nasal cavity Odor detection in humans When odorant molecules bind to specific receptors a signal transduction pathway is triggered, sending action potentials to the brain Brain Action potentials Odorant Olfactory bulb Nasal cavity Bone Epithelial cell Odorant receptors Chemoreceptor Plasma membrane Cilia Figure Odorant Mucus 7

8 Vision Many types of light detectors have evolved in the animal kingdom ranging from simple eye cups of planarians (flatworms) to the compound eyes of insects and crustaceans to the camera-like eyes of vertebrates and cephalopods (octopuses and relatives). Structure of the Vertebrate Eye The main parts of the vertebrate eye are The sclera, a layer of tough connective tissue, which includes the transparent cornea which allows light into the eye and acts as a fixed lens. The choroid, a pigmented layer inside the sclera. The conjunctiva, that covers the outer surface of the sclera (except the cornea) and keeps it moist. Structure of the Vertebrate Eye The iris, which regulates the size of the pupil the hole in the middle of the iris that lets light in. The retina, which contains photoreceptors that detect light and convert it to nervous impulses. The lens, which focuses light on the retina The structure of the vertebrate eye Ciliary body Suspensory ligament Cornea Iris Pupil Sclera Choroid Retina Fovea (center of visual field) Optic nerve Aqueous humor Lens Figure Vitreous humor Optic disk (blind spot) Central artery and vein of the retina 8

9 Humans and other mammals focus light by changing the shape of the lens Ciliary muscles contract, pulling border of choroid toward lens Suspensory ligaments relax Lens becomes thicker and rounder, focusing on near objects (a) Near vision (accommodation) Ciliary muscles relax, and border of choroid moves away from lens Choroid Retina Front view of lens and ciliary muscle Lens (rounder) Ciliary muscle Suspensory ligaments Lens (flatter) The human retina contains two types of photoreceptors Rods are sensitive to light but do not distinguish colors Cones distinguish colors but are not as sensitive to light. Suspensory ligaments pull against lens Lens becomes flatter, focusing on distant objects Figure 49.19a b (b) Distance vision Sensory Transduction in the Eye Each rod or cone in the vertebrate retina contains visual pigments that consist of a lightabsorbing pigment called retinal bonded to a protein called opsin. Opsins vary in structure from one type of photoreceptor to another Rods contain the visual pigment rhodopsin, which changes shape when it absorbs light. Cones contain one or other of three different photopsins and are either red, green or blue cones. Rods are used under low light conditions and cones in bright light for color vision. Absorption spectra of the different types of cones overlap and the differential stimulation of the three types of cones allows the brain to produce the perception of color. For example, if both red and green cones are stimulated we may see orange or yellow depending on which cones are more strongly stimulated. 9

10 Processing Visual Information The processing of visual information begins in the retina itself When a cone or rod is struck it absorbs light and its retinal changes shape. This change in shape triggers a signal transduction pathway that generates an action potential and nerve impulses that travel to the brain. Light INSIDE OF DISK Active rhodopsin PDE CYTOSOL cgmp Inactive rhodopsin Transducin Disk membrane GMP 1 Light 2 Active 3 Transducin rhodopsin activates 4 Activated PDE isomerizes in turn the detaches cyclic retinal, guanosine which activates a enzyme G protein phosphodiestera monophosphate activates (cgmp) from rhodopsin. called transducin. e(pde). Na + channels in the plasma membrane by hydrolyzing cgmp to GMP. Figure EXTRACELLULAR FLUID Membrane potential (mv) Plasma membrane 0 Dark Light 40 Na + Hyper- 70 polarization Time 5 The Na+ channels close when cgmp detaches. The membrane s Na + permeability to Na+ decreases, and the rod hyperpolarizes. Signals from rods and cones travel from bipolar cells to ganglion cells The axons of ganglion cells are part of the optic nerve that transmit information to the brain Left visual field Right visual field Several integrating centers in the cerebral cortex are active in creating visual perceptions. You should be aware that the reality you perceive is a a simulation generated by your brain that builds a mental model of the world. Optic nerve Optic chiasm Lateral geniculate nucleus Primary visual cortex Left eye Right eye 10

11 Animal Skeletons Animal skeletons function in support, protection, and movement The various types of animal movements all result from muscles working against some type of skeleton Functions of the skeleton The three main functions of a skeleton are Support, Protection, and Movement Human skeleton The mammalian skeleton is an endoskeleton of bones buried within the soft tissue. It is built from more than 200 bones Some bones are fused together and others are connected at joints by ligaments that allow freedom of movement Shoulder girdle Sternum Rib Humerus Vertebra Radius Ulna Pelvic girdle Carpals key Axial skeleton Appendicular skeleton Clavicle Scapula Skull Examples of joints Head of humerus Scapula 1 Ball-and-socket joints, where the humerus contacts the shoulder girdle and where the femur contacts the pelvic girdle, enable us to rotate our arms and legs and move them in several planes. Humerus Phalanges Ulna Metacarpals Femur Patella 2 Hinge joints, such as between the humerus and the head of the ulna, restrict movement to a single plane. Tibia Fibula Tarsals Metatarsals Phalanges Ulna Radius 3 Pivot joints allow us to rotate our forearm at the elbow and to move our head from side to side. 11

12 Muscles move skeletal parts by contracting The action of a muscle is always to contract. Human Biceps contracts Extensor muscle relaxes Grasshopper Tibia flexes Skeletal muscles are attached to the skeleton in antagonistic pairs with each member of the pair working against each other e.g. triceps and biceps in the upper arm. Triceps relaxes Forearm flexes Flexor muscle contracts Biceps relaxes Extensor muscle contracts Tibia extends Triceps contracts Figure Forearm extends Flexor muscle relaxes Muscle Vertebrate skeletal muscle is characterized by a hierarchy of smaller and smaller units. A skeletal muscle consists of a bundle of long fibers running parallel to the length of the muscle A muscle fiber is itself a bundle of smaller myofibrils arranged longitudinally. Bundle of muscle fibers Single muscle fiber (cell) Plasma membrane Myofibril Light band Dark band Sarcomere Z line Nuclei Myofibrils are composed of myofilaments. Thick filaments (myosin) Thin filaments (actin) Figure TEM 0.5 µm I band A band I band M line Z line H zone Sarcomere Z line 12

13 The myofibrils are composed of two kinds of myofilaments Thin filaments, consisting of two strands of actin and one strand of regulatory protein Thick filaments, staggered arrays of myosin molecules Skeletal muscle is also called striated muscle because the regular arrangement of the myofilaments creates a pattern of light and dark bands Each repeating unit is a sarcomere bordered by Z lines. The Z lines are aligned vertically and are attached to the thin filaments (made of actin). The thin filaments project into the sarcomere. The thick filaments are aligned in the center of the sarcomere and align with and partially overlap with the thin filaments. Muscle At rest the thick and thin fibers only partially overlap and this produces a pattern of bands in the sarcomere that are identified by letters. The A-band is the area in the middle of the sarcomere where the thick filaments are located and appears dark. The I-band is the area at either end of the sarcomere where only thin filaments are found. This appears light. The H-zone in the middle is where only thick filaments occur. It appears less dark than the rest of the A-band. Thick filaments (myosin) Thin filaments (actin) Figure Bundle of muscle fibers Single muscle fiber (cell) Plasma membrane Myofibril Light band Dark band Sarcomere Z line TEM 0.5 µm I band A band I band M line Z line H zone Sarcomere Nuclei Z line 13

14 The Sliding-Filament Model of Muscle Contraction I band 0.5 µm According to the sliding-filament model of muscle contraction the filaments slide past each other longitudinally, producing more overlap between the thin and thick filaments. (a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bands and H zone are relatively wide. Z H A Sarcomere As a result of this sliding the I band and the H zone shrink. (b) Contracting muscle fiber. During contraction, the thick and thin filaments slide past each other, reducing the width of the I bands and H zone and shortening the sarcomere. (c) Fully contracted muscle fiber. In a fully contracted muscle fiber, the sarcomere is shorter still. The thin filaments overlap, eliminating the H zone. The I bands disappear as the ends of the thick filaments contact the Z lines. Figure 49.29a c Myosin-actin interactions underlying muscle fiber contraction The sliding of filaments is based on the interaction between the actin and myosin molecules of the thick and thin filaments Thin filaments Thick filament 1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration. The head of a myosin molecule binds to an actin filament forming a cross-bridge and pulling the thin filament toward the center of the sarcomere 5 Binding of a new molecule of ATP releases the myosin head from actin, ATP and a new cycle begins. Thin filament moves toward center of sarcomere. Myosin head (lowenergy configuration) ATP Myosin head (lowenergy configuration) Thick filament Thin filament 2 The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( P I ) and is in its high-energy configuration. Cross-bridge Actin binding site ADP Myosin head (high- P i energy configuration) ADP + P i 4 Releasing ADP and ( P i), myosin relaxes to its low-energy configuration, sliding the thin filament. ADP P i Cross-bridge 13 The myosin head binds to actin, forming a crossbridge. 14

15 The Role of Calcium and Regulatory Proteins A skeletal muscle fiber contracts Only when stimulated by a motor neuron When a muscle is at rest the myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin Tropomyosin Actin Ca 2+ -binding sites Troponin complex Figure 49.31a (a) Myosin-binding sites blocked For a muscle fiber to contract the myosinbinding sites must be uncovered This occurs when calcium ions (Ca 2+ ) bind to another set of regulatory proteins, the troponin complex The stimulus leading to the contraction of a skeletal muscle fiber is an action potential in a motor neuron that makes a synapse with the muscle fiber Motor neuron axon Mitochondrion Ca 2+ Synaptic terminal Myosinbinding site T tubule Figure 49.31b (b) Myosin-binding sites exposed Sarcoplasmic reticulum Myofibril Plasma membrane Figure of muscle fiber Ca 2+ released from sarcoplasmic reticulum Sarcomere 15

16 The synaptic terminal of the motor neuron releases the neurotransmitter acetylcholine, depolarizing the muscle and causing it to produce an action potential Neural Control of Muscle Tension Contraction of a whole muscle is graded Which means that we can voluntarily alter the extent and strength of its contraction. There are two basic mechanisms by which the nervous system produces graded contractions of whole muscles By varying the number of fibers that contract By varying the rate at which muscle fibers are stimulated In a vertebrate skeletal muscle each branched muscle fiber is innervated by only one motor neuron Each motor neuron may synapse with multiple muscle fibers A motor unit consists of a single motor neuron and all the muscle fibers it controls Recruitment of multiple motor neurons results in stronger contractions Spinal cord Motor Motor unit 1 unit 2 Synaptic terminals Nerve Motor neuron cell body Motor neuron axon Muscle Figure Tendon Muscle fibers 16

17 A twitch results from a single action potential in a motor neuron More rapidly delivered action potentials produce a graded contraction by summation Tetanus is a state of smooth and sustained contraction produced when motor neurons deliver a volley of action potentials. Tetanus Tension Single twitch Summation of two twitches Action Time Pair of potential action potentials Figure Series of action potentials at high frequency Cardiac muscle Cardiac muscle, found only in the heart Consists of striated cells that are electrically connected together. As a result an action potential in one part of the heart spreads to all cardiac cells and the heat contracts. Cardiac muscle can generate action potentials without neural input because the heart has its own pacemaker cells that cause rhythmic depolarizarions. Smooth muscle In smooth muscle, found mainly in the walls of hollow organs (e.g. the digestive tract) the contractions are relatively slow and may be initiated by the muscles themselves In addition, contractions may be caused by stimulation from neurons in the autonomic nervous system 17