Biology, Sensory system

Raven Biology 13th edition, CH43

Senses, Sensory receptors give an organism the senses of

vision, hearing, taste, smell, and touch; senses provide information about the environment

Overview of Sensory Receptors, Sensory receptors provide information from internal and external environments

exteroceptors sense external stimuli; interceptors sense internal stimuli

Response to Stimuli, Sensory cells respond to stimuli via stimulus-gated ion channels

depolarization of receptor cell occurs (receptor potential) analogous to EPSP

Receptor Potential, Receptor potential is like a graded potential

larger stimulus causes greater depolarization and higher frequency of action potentials

Overview of Mechanoreceptors, Mechanoreceptors are stimulated by physical or mechanical forces

give rise to senses of touch and hearing; provide input for balance and body position

Mechanoreceptors in Skin, Several types of mechanoreceptors in skin detect touch

phasic receptors (intermittently activated) include hair follicle receptors, Meissner corpuscles, Pacinian corpuscles; tonic receptors (continuously activated) include Ruffini corpuscles, Merkel's disks

Proprioceptors, Monitor muscle length and tension

muscle spindles monitor stretch (knee jerk reflex); Golgi tendon organs monitor tension on tendons (prevents damage)

Baroreceptors, Monitor blood pressure at carotid sinus and aortic arch

detect tension/stretch in vessel walls; decreased blood pressure decreases impulse frequency, increasing heart rate and vasoconstriction

Mechanoreceptor in Hearing, Vibration, and Balance, These receptors detect vibrations through water or air (hearing/lateral line system)

and gravity/acceleration (balance and body position)

Hearing, Sound is the result of vibration traveling through a medium

detection of sound waves is possible through specialized mechanoreceptors that first evolved in aquatic organisms

Lateral Line System in Fish, Sense objects that reflect pressure waves and low-frequency vibrations

consists of hair cells within a longitudinal canal; hair cell processes project into cupula; hair cells innervated by sensory neurons to brain

Lateral Line System, Bending of stereocilia in direction of kinocilium has stimulatory effect

bending in opposite direction is inhibitory

Hearing Structures in Fish, Called otoliths composed of calcium carbonate crystals

contained in otolith organs of membranous labyrinth; otoliths vibrate against stereocilia producing action potentials

Ear Structure of Terrestrial Vertebrates, Air vibrations channeled through ear canal to tympanic membrane

three ossicles (malleus, incus, stapes) vibrate; stapes vibrates against oval window leading to inner ear

Inner Ear Structure of Land Vertebrates, Inner ear consists of cochlea with vestibular canal above and tympanic canal below

all chambers filled with fluid; pressure waves travel to round window

Pressure Waves in the Ear, Pressure waves cause cochlear duct to vibrate

organ of Corti contains basilar membrane with hair cells; stereocilia project into tectorial membrane; bending depolarizes hair cells sending action potentials to brain

Ear Structure of Land Vertebrates, Basilar membrane consists of elastic fibers that respond to different sound frequencies

hair cell depolarization greatest in region responding to particular frequency; brain interprets as pitch

Navigation by Sound, Bats, shrews, whales, dolphins use echolocation

emit sounds and determine time for sounds to return; sonar and radar based on same principles

Detection of Body Position, Most invertebrates orient with gravity using statocyst (ciliated hair cells with statoliths)

vertebrates have gravity receptors (utricle and saccule) in membranous labyrinth

Utricle and Saccule, Utricle more sensitive to horizontal acceleration

saccule more sensitive to vertical acceleration; both cause cilia to bend producing action potentials

Semicircular Canals, Three semicircular canals detect angular acceleration in any direction

ampullae contain groups of cilia with tips embedded in gelatinous cupula

Vestibular Apparatus, When head rotates, semicircular canal fluid pushes against cupula causing cilia to bend

bending toward kinocilium causes receptor potential; utricle, saccule, and semicircular canals are vestibular apparatus

Chemoreceptors, Bind to particular chemicals in extracellular fluid

membrane of sensory neuron depolarizes producing action potentials; used in taste and smell and monitoring blood chemistry

Taste (gustation), Broken down into five categories

sweet, sour, salty, bitter, and umami (hearty); taste buds are collections of chemosensitive cells with afferent neurons

Taste, In fish taste buds scattered over body surface; in land vertebrates located on tongue/oral cavity within papillae

salty and sour act through ion channels; other tastes act through G protein-coupled receptors

Smell (olfaction), In land vertebrates involves neurons in upper nasal passages

receptors project into nasal mucosa; axons project directly to cerebral cortex; particles must dissolve before activating receptors

pH, Peripheral chemoreceptors in aortic and carotid bodies sensitive to plasma pH

central chemoreceptors in medulla oblongata sensitive to cerebrospinal fluid pH; increased CO2 lowers pH stimulating respiratory control center

Thermoreceptors, Nonspecialized sensory receptors sensitive to temperature changes

warm and cold receptors; vertebrates have most thermoreceptors in dermis of skin

Sensing infrared radiation, Pit vipers have heat-detecting pit organs between eye and nostril

locate heat sources including prey in darkness; paired pits appear stereoscopic forming thermal image

Nociceptors, Transmit impulses perceived as pain

sensitive to tissue damage; free nerve endings located throughout body; use neurotransmitters substance P and glutamate; endorphins/enkephalins block substance P release

Electroeceptors and Magnetoreceptors, Elasmobranchs have ampullae of Lorenzini sensing electrical currents from prey

eels, sharks, bees, birds navigate along Earth's magnetic field lines

Animals with Well-Developed Eyes, Members of annelids, mollusks, arthropods, and chordates evolved image-forming eyes

similar in structure but evolved independently (convergent evolution)

Structure of the Vertebrate Eye, Sclera is white tough connective tissue

cornea is transparent where light enters; iris is colored portion controlling pupil size; lens completes focusing onto retina

Near versus Distance Vision, Lens attached to ciliary muscles by suspensory ligament

near vision: ciliary muscles contract, lens more rounded; distance vision: ciliary muscles relax, lens flattened

The Retina, Contains two types of photoreceptors

rods (black-and-white vision in dim light) and cones (color vision and high acuity); fovea has sharpest image

Rods and Cones, Both have inner segment rich in mitochondria and vesicles

connected by stalk to outer segment packed with flattened disks containing photopigments

Photopigments, Photopigment in rods is rhodopsin

photopigments in cones are photopsins; humans have three kinds of cones each with different absorption maximum

Cell Layers in the Retina, Three layers: external layer (rods/cones), middle layer (bipolar cells), layer closest to eye cavity (ganglion cells)

photoreceptors stimulate bipolar cells which stimulate ganglion cells; ganglion cells transmit to brain via optic nerve

Sensory Transduction, In dark: photoreceptors release inhibitory neurotransmitter hyperpolarizing bipolar neurons

In light: photoreceptors stop releasing inhibitory neurotransmitter, stimulating bipolar cells which stimulate ganglion cells

Visual Processing, Action potentials relayed from retina to lateral geniculate nuclei of thalamus

then projected to occipital lobes of cerebral cortex; each hemisphere receives input from both eyes

Visual Acuity, Fovea has one-to-one connections = high acuity

outside fovea many rods converge on bipolar cells; more sensitive to dim light at expense of acuity and color vision

Color Vision, Color blindness due to inherited lack of one or more cone types

normal vision trichromats have all three cones; color blind dichromats; sex-linked recessive trait more common in men

Binocular vision, Primates and predators have eyes on each side of face with overlapping fields

parallax permits 3-D perception and depth; prey animals have eyes on sides enlarging receptive field

Evolution of Eyes, Incremental improvements in function built complex structures through natural selection

morphologists concluded eyes are example of convergent evolution
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