Chapter 5 - Sense Organs PDF

1 ANATOMY AND PHYSIOLOGY CHAPTER 5: Sense Organs BIO 343  OUTLINE 1) CLASSES OF SENSES and TYPES OF SENSORY RECEPTORS Anatomy and Physiology of sense organs responsible for: 2) GUSTATION 3) OLFACTION 4) AUDITION & EQUILIBRIUM 5) VISION...

1 ANATOMY AND PHYSIOLOGY CHAPTER 5: Sense Organs BIO 343  OUTLINE 1) CLASSES OF SENSES and TYPES OF SENSORY RECEPTORS Anatomy and Physiology of sense organs responsible for: 2) GUSTATION 3) OLFACTION 4) AUDITION & EQUILIBRIUM 5) VISION 2  SENSE ORGAN AND CLASSES OF SENSES  A sense organ is a structure that combines nervous tissue with other tissues that enhance its response to a certain type of stimulus (e.g. eyes, ears, tongue, skin, nose)  There are two broad classes of senses: General senses: employ widely distributed receptors in the skin, muscles, tendons, joints and viscera. They include touch, pressure, stretch, heat, cold and pain Special senses: distribution of receptors is limited to the head, are innervated by the cranial nerves, and employ relatively complex sense organs. The special senses are taste, smell, hearing, equilibrium, and vision 3  SENSORY RECEPTORS  The body contains several sensory receptors that detect physical or chemical events outside of the cell membrane  Some of these receptors combine with nervous tissue and other types of tissues to form sense organs (e.g. ear, eye)  Example of sensory receptors include: Chemoreceptors: react to various chemicals (e.g. odors, tastes, glucose, carbon dioxide, etc.) Mechanoreceptors: react to mechanical stimuli (e.g. pressure, stretch, vibration, etc.) Thermoreceptors: react to a change in temperature Nociceptors: pain receptors that react to tissue damage Photoreceptors: receptors found in the eye that react to light 4  TASTE  Gustation (sense of taste) Sensation that results from the © McGraw-Hill action of chemicals on taste buds 4,000 taste buds mainly on the tongue, but also inside cheeks, on the soft palate, the pharynx, and the epiglottis Taste buds are located in the papillae of the tongue (Fungiform, vallate and foliate) A vallate papilla of the tongue, where most taste buds are located (SEM) Tongue 5 TASTE (NO TASTE BUDS)  All taste buds look alike  Lemon-shaped groups of 40 to 60 taste cells, supporting cells, and basal cells: © McGraw-Hill 6 TASTE BUDS:  Taste cells Have taste hairs (hair-like projection) that serve as receptor surface for taste molecules. The taste hairs project through pits called the taste pores. Taste cells are epithelial cells, not neurons, but they synapse with sensory nerve fibers at their base, where they have synaptic vesicles and release neurotransmitters © McGraw-Hill 7  TASTE BUDDS  Supporting cells Resemble taste cells but have no synaptic vesicles or sensory role  Basal cells Stem cells that replace taste cells that have died (every 7 to 10 days). © McGraw-Hill 8  PHYSIOLOGY OF TASTE Two mechanisms of action: Some tastants activate G protein-coupled receptors on taste cell membrane Sugars, alkaloids, and glutamate Activate second-messenger systems Other tastants can depolarize cells directly Sodium (Na+) and acids (H+) are cations Enter taste cells through specific channels Either mechanism results in release of neurotransmitters that stimulate dendrites at base of taste cells 9  PROJECTION PATHWAY OF TASTE Three cranial nerves carry taste information: Facial nerve (CN VII) Anterior two-thirds of tongue Glossopharyngeal nerve (CN IX) Posterior one-third of tongue Vagus nerve (CN X) Palate, pharynx, and epiglottis Fibers from all three cranial nerves collect sensory info from taste buds and synapse in the solitary nucleus of the medulla oblongata NOTE: Hot pepper stimulates nociceptors, not taste buds; fibers travel in trigeminal nerve (CN V) 10  PROJECTION PATHWAY OF TASTE From the medulla oblongata, signals are sent to two destinations: Hypothalamus and amygdala Control autonomic reflexes, Examples: salivation, gagging, vomiting Thalamus Conscious sense of taste Relays signals to postcentral gyrus of cerebrum, then to the orbitofrontal cortex (OFC) which integrates gustatory, olfactory and visual information to form impression of flavor and palatability of food 11  OLFACTION: THE SENSE OF SMELL  Olfaction, the sense of smell, is a response to airborne chemicals called odorants These are detected by receptor cells in a patch of epithelium within the roof of the nasal cavity called the olfactory mucosa  Olfactory mucosa Contains 10 to 20 million olfactory cells © McGraw-Hill Unlike taste cells, olfactory cells are neurons (BUT they are replaceable) Head bears 10 to 20 cilia called olfactory hairs Cilia are immobile but have binding sites for odorant molecules They lie in a tangled mass in a thin layer of mucus The axons of the neurons synapse with Olfactory mucosa in the nasal cavity neurons in olfactory bulb, then send the info through the olfactory tract into the brain Olfactory cell (cranial nerve I). 12  PHYSIOLOGY OF OLFACTION Olfactory transduction Odorant binds to G protein-coupled receptor on olfactory cell Activates cAMP second-messenger system Opens ion channels for Na+ or Ca2+ causing membrane depolarization Triggers action potential, send message to interneuron in bulb, then message travels to the brain (through CN I) Reaches primary olfactory cortex in the inferior surface of the temporal lobe (TL) TL relays signal to the OFC without passing through the thalamus (sensation of odor) Signal is also sent from TL to secondary destinations: hippocampus, amygdala, hypothalamus and OFC: Identify odors, integrate with taste, evoke memories, emotions, and visceral reactions NOTE: Olfactory receptors adapt quickly, due to synaptic inhibition in olfactory bulbs NOTE: Some odorants act on nociceptors of trigeminal nerve eg. Ammonia, menthol, chlorine, and capsaicin of hot peppers 13  HEARING AND EQUILIBRIUM  Hearing is a response to vibrating air molecules  Equilibrium is the sense of motion, body orientation, and balance  Both senses reside in the inner ear, a maze of fluid-filled passages and sensory cells  Fluid is set in motion and the sensory cells convert this motion into an informative pattern of action potentials 14  ANATOMY OF THE EAR  The ear has three sections: outer, middle, and inner ear Outer and middle ear are concerned only with the transmission of sound to the inner ear Inner ear converts vibrations to nerves signals  Outer ear: Auricle: the visible part of the ear, shaped by cartilage Auditory canal: leads from the outside of the ear to the tympanic membrane (or eardrum) Internal anatomy of the ear 15  ANATOMY OF THE EAR  Middle ear: Contains auditory ossicles (malleus, incus, stapes): 3 smallest bones in the body, connecting the tympanic membrane to the inner ear.  Inner ear: Contains spiral organ or organ of Corti  Located in the cochlea (snail-like structure within the inner ear)  Receptor organ for hearing that converts vibrations into nerve impulses Contains the semicircular ducts and the vestibule (marks the entrance of the Internal anatomy of the ear labyrinths), which are important for the sense of balance and equilibrium 16  PHYSIOLOGY OF HEARING - Ossicles in middle ear concentrate the energy of the vibrating tympanic membrane - Vibration of ossicles causes pressure waves in the inner ear fluid - Mechanical Stimulation of hair cells in cochlea - Single transmembrane protein at tip functions as a mechanically gated ion channel - K+ flows in—depolarization causes release of neurotransmitter (note: the fluid surrounding the cells in the labyrinth is very concentrated in K+) - Stimulates sensory dendrites of spiral ganglion neuron (bipolar) and generates action potential in the cochlear nerve (CN VIII) 17  PROJECTION PATHWAY OF HEARING Unlike most other sensory pathways, it involves four neurons: First order Bipolar sensory neurons with somas in spiral ganglia in cochlea Their axons run from cochlea, through CN VIII to medulla oblongata Second order Axons travel to pons and some other to midbrain Third order Axons travel to thalamus Fourth order Axons travel to primary auditory cortex in the temporal lobe (conscious perception of sound) 18  VISION  Vision (sight) is the perception of objects in the environment by means of the light that they emit or reflect.  Three layers (tunics) of tissue form the wall of the eyeball, while the inside is divided into fluid-filled chambers  From outer to inner: Fibrous tunic (sclera, cornea) Vascular tunic (choroid, ciliary body, iris) Nervous tunic (retina, optic nerve)  Optical components admit and focus light while neural components transmit signal to the CNS © BrainKart 19  SENSORY TRANSDUCTION IN THE RETINA  Structure of retina Neural components of the retina from the back to the front of the eye Photoreceptor cells—absorb light and generate a chemical or electrical signal Rods, cones, and certain ganglion cells Bipolar cells—synapse with rods and cones and are known as the first-order neurons of the visual pathway Ganglion cells—largest neurons in the retina and are known as the second-order neurons of the visual pathway. Histology of retina 20  PROJECTION PATHWAY OF VISION One example: Light detection by rods (pigment: rhodopsin) - In dark, rods steadily release the neurotransmitter glutamate. - When rhodopsin absorbs light, glutamate secretion decreases, - Bipolar cells detect fluctuations in light intensity and pass this information along to the ganglion cells - Ganglion cells produce action potentials. - Their axons form the optic nerve → optic chiasm → optic tract - Visual signals reach the thalamus (3rd order neuron)→ primary visual cortex in occipital lobe (conscious visual sensation). NOTE: Ganglion cells are the ONLY cells that generate action potential. Remaining cells use various signaling transduction pathways involving second messengers (such as cGMP, among others) to communicate with each other 21 The End 22