Transcript Chapter 39
Chapter 39 Learning Objectives • • • • Describe sensory circuit List five basic type of sensory receptors Define sensation in neurological terms Identify the role of each of the various mechanoreceptors for pressure, vibration, etc. • Explain the function of proprioceptors, electroreceptors and magnetoceptors Learning Objectives • Describe how the ear transmits sound • Describe the morphology and function of the human eye • Define chemoreceptors • Map the human tongue and the process of taste • Diagram the human nasal cavity and the process of olfaction • Describe the function of nociceptors Sensory Receptors (1) • Formed by endings of afferent neurons or specialized cells adjacent to neurons • Detect stimuli (various forms of energy) – Mechanical pressure – Sound waves – Light – Specific molecules or chemical conditions Sensory Receptors (2) • Sensory transduction – Axons of afferent neurons carry action potentials generated by receptors to pathways leading to specific parts of the brain • Brain processes signals into sensory sensations a. Sensory receptor formed by dendrites of an afferent neuron Stimulus Stimulus opens gated Action ion channels potential Afferent neuron (to CNS) Dendrites forming sensory receptor In sensory receptors formed by the dendrites of afferent neurons, a stimulus causes a change in membrane potential that generates action potentials in the axon of the neuron. Temperature and pain receptors are among the receptors of this type. Fig. 39.1a, p. 886 b. Sensory receptor formed by a cell that synapses with an afferent neuron Stimulus Sensory receptor cell or structure Diffusion of neurotransmitter or first messenger Action potential Neurotransmitter or first messenger opens gated ion channels In sensory receptors consisting of separate cells, a stimulus causes a change in membrane potential that releases a neurotransmitter from the cell. The neurotransmitter triggers an action potential in the axon of a nearby afferent neuron. Mechanoreceptors, photoreceptors, and chemoreceptors are examples of receptors of this type. Fig. 39.1b, p. 886 Types of Receptors • 5 basic types – Mechanoreceptors – Photoreceptors – Chemoreceptors – Thermoreceptors – Nociceptors • Some animals have receptors that detect electrical or magnetic fields Sensory Perception • Routing of information from sensory receptors to particular brain regions identifies a specific stimulus as a sensation • Intensity of a stimulus is determined by – Frequency of action potentials along neural pathways – Number of afferent neurons carrying action potentials Sensory Adaptation • In many systems, frequency of action potentials decreases while a stimulus remains constant • Some sensory receptors (those related to tissue damage) show little or no sensory adaptation 39.2 Mechanoreceptors and the Tactile and Spatial Senses • Receptors for touch and pressure occur throughout the body • Proprioceptors provide information about movements and position of the body Mechanoreceptors • Detect mechanical energy – Touch, pressure, acceleration, vibration • Touch and pressure receptors – Free nerve endings – Encapsulated nerve endings of sensory neurons 39.2 Mechanoreceptors and the Tactile and Spatial Senses • Receptors for touch and pressure occur throughout the body • Proprioceptors provide information about movements and position of the body Mechanoreceptors • Detect mechanical energy – Touch, pressure, acceleration, vibration • Touch and pressure receptors – Free nerve endings – Encapsulated nerve endings of sensory neurons 39.2 Mechanoreceptors and the Tactile and Spatial Senses • Receptors for touch and pressure occur throughout the body • Proprioceptors provide information about movements and position of the body Mechanoreceptors: constant feedback on tactile senses • Detect mechanical energy – Touch, pressure, acceleration, vibration • Touch and pressure receptors – Free nerve endings – Encapsulated nerve endings of sensory neurons Shaft of hair inside Skin surface follicle Epidermis Dermis Myelinated neuron Free nerve endings around hair root plexus Free nerve endings: light touch Pacinian corpuscle: deep pressure and vibrations Ruffini endings: deep pressure Meissner’s corpuscle: light touch, surface vibrations Fig. 39.2, p. 888 Proprioceptors: constant feedback on spatial position • Proprioceptors provide information about movements and position of the body • The vestibular apparatus of the human ear • Stretch receptors in muscles (muscles spindles) • Proprioceptors of tendons (Golgi tendon organs) for balance Vestibular apparatus Anterior semicircular canal Posterior semicircular canal Utricle Saccule Lateral semicircular canal Fig. 39.5a, p. 890 Ampulla of a semicircular canal Direction of body rotation Endolymph pulls cupula in this direction Cupula Sensory hair cells Afferent neurons Fig. 39.5b, p. 890 Muscle Spindles and Golgi Tendon Organs Hearing • Hearing relies on sensory hair cells in organs that respond to the vibrations of sound waves Terrestrial Vertebrate Ear (1) • Outer ear (pinna) – Directs sound to the eardrum • Eardrum (tympanic membrane) – Transmits vibrations through one or more bones in the middle ear to the fluid-filled inner ear Terrestrial Vertebrate Ear (2) • Middle ear – Malleus, incus, stapes – Oval window • Inner ear – Transmits vibrations through structures that bend stereocilia of hair cells – Cochelea, organ of Corti, round window – Bursts of action potentials determine frequency of sound waves Semicircular canals Pinna Bone of skull Eustachian tube leading to throat Location of the human ear in the head Oval window (behind stapes) Auditory nerve Stapes Incus Malleus Auditory canal Round Cochlea window Eardrum Outer ear Middle ear Inner ear Internal structures of the outer, middle, and inner ear Fig. 39.8a, p. 893 39.4 Photoreceptors and Vision • Vision involves detection and perception of radiant energy • Mammalian retinas contains rods and cones and a complex neuronal network • Three kinds of opsin pigments underlie color vision • The visual cortex processes visual information Retina Cornea Lens Pupil Iris Fig. 39.11, p. 895 Photoreceptors of the Eye • Contain pigment molecules – Absorb energy of light – Generate changes in membrane potential • Retinal – Light-absorbing pigment in all animals The Vertebrate Eye (1) • Cornea – Transparent, admits light • Iris – Behind the cornea – Controls diameter of pupil – Regulates amount of light that strikes the lens The Vertebrate Eye (2) • Lens – Focuses image on the retina • Retina – Lines the back of the eye – Photoreceptors and neurons integrate information detected by photoreceptors Sclera Choroid Ciliary body Iris Lens Retina Fovea Blind spot Pupil Cornea Aqueous humor Part of optic nerve Ciliary muscle Vitreous humor Fig. 39.12, p. 896 Focus • In birds, mammals, and most reptiles, light is focused on the retina by the combined effect of the cornea and adjusting lens shape Fig. 39.13a, p. 896 Fig. 39.13b, p. 896 Region of overlap of the two visual fields Visual field of left eye Visual field of right eye Region of overlap of two visual fields Right eye Left eye Optic nerve Optic chiasm Lateral geniculate nucleus of the thalamus Visual cortex Fig. 39.17, p. 900 Photoreceptors of the Retina • Rods – Specialized for detection of low-intensity light • Cones – Specialized for detecting light of different wavelengths (colors) Photopigment Molecules • Absorb light in photoreceptor cells – Consist of retinal combined with an opsin protein – 3 photopigments (photopsins) Pigment absorbs light, retinal changes form – Reactions alter amount of neurotransmitter released by photoreceptor cells a. Structure of cones and rods Cone Back of retina Outer segment (houses discs that contain light-absorbing photopigment) Outer segment Rod Discs Lightabsorbing photopigment Discs Inner segment (houses cell’s metabolic machinery) Inner segment Synaptic terminal (stores and releases neurotransmitters) Synaptic terminal Front of retina Fig. 39.14a, p. 897 b. How rhodopsin functions Rhodopsin in the dark (inactivated) Rhodopsin in the light (activated) Light absorption Retinal changes shape Enzymes cis-Retinal trans-Retinal Fig. 39.14b, p. 897 The Retina and Visual Processing • Rods (low light) and cones( bright and color) – Linked to neurons in the retina – Perform initial integration and processing of visual information • Processed signal is sent via the optic nerve through the lateral geniculate nuclei to the visual cortex Color-blindness • • • • Genetic disorder 8% males, 0.5% females Red-green is most common Type of color deficiency depends on wavelength of light that is not being detected (long, medium, or short wavelength detecting cone). Retina: Initial Integration Chemoreceptors • Chemoreceptors respond to the presence of specific molecules in the environment • In vertebrates, they form parts of receptor organs for taste (gustation) and smell (olfaction) Taste Receptors • Detect molecules from food or other objects that come into direct contact with the receptor • Are used primarily to identify foods Papilla (cutaway) Taste bud Sensory hair of taste receptor Papillae Tongue Afferent nerve Taste buds Papilla Fig. 39.20, p. 902 Olfactory tract from receptors to the brain Olfactory bulb Nasal cavity Bone Olfactory receptors Supporting cells Sensory hairs of olfactory receptors Mucus Fig. 39.21, p. 902 39.6 Thermoreceptors and Nociceptors • Thermoreceptors occur in warm and cold forms • Nociceptors protect animals from potentially damaging stimuli Nociceptors • Detect stimuli that can damage body tissues • Located on body surface and interior • Information from receptors is integrated in the brain into the sensation of pain Other Sensory Receptors Found in Some Vertebrates • Electroreceptors – Detect electrical currents and fields • Magnetoreceptors – Detect magnetic fields