Special Senses Part 1 (taste & Smell)

The Special Senses Part A

Objectives

 Describe the location, structure, and afferent pathways of taste and smell receptors, and explain how these receptors are activated.  Cranial Nerves and Functions

Chemical Senses

 5 Senses  Touch, Taste, Smell, Vision and Hearing  Touch Excluded as special sense  Various aspects of touch (Pain, heat, pressure) Somatic sense  Chemical senses – gustation (taste) and olfaction (smell)  The receptors for taste (gustation) and smell (olfaction) are Chemoreceptors.  Chemoreceptors respond to chemicals in aqueous solution  Taste – to substances dissolved in saliva  Smell – to substances dissolved in fluids

Taste Buds and the Sense of Taste

 The word taste comes from the Latin taxare, meaning “to touch, estimate, or judge.”  It is considered by many to be the most pleasurable of the special senses.  Taste Bud: Sensory receptor organs for taste  10,000 or so taste buds  Most of the them are found on the tongue(cheeks, pharynx)  Taste buds are found in papillae of the tongue mucosa  Papillae  Peglike projections of the tongue mucosa give slightly abrasive feel  Filiform papillae  Thin, long papillae "V"-shaped cones D  Don't contain taste buds but are the most

Types of Papillae

 Fungiform papillae  mostly at the apex (tip) of the tongue, as well as at the sides. Innervated by facial nerve. Contains most taste buds  Foliate papillae  Ridges and grooves towards the posterior part of the tongue found on lateral margins.  Facial nerve (anterior papillae)  glossopharyngeal nerve (posterior papillae).  Circumvallate papillae  Only about 3-14 of these papillae on most people,  Present at the back of the oral part of the

Anatomy of a Taste Bud

 Each Funnel-shaped taste bud consists of 50 to 100 three major cell types § Supporting cells – insulate the receptor § Basal cells – dynamic stem cells (dividing and differentiating into new gustatory cells) § Gustatory cells – taste cells  Long microvilli called gustatory hairs project from the tips of all gustatory cells and extend through a taste pore to the surface of the epithelium, where they are bathed by saliva.  The gustatory hairs are the sensitive

Taste Buds

Taste Sensations

 There are five basic taste sensations  Sweet – sugars, saccharin, alcohol, and some amino acids  Salt – metal ions  Sour – hydrogen ions  Bitter – alkaloids such as quinine and nicotine  Umami (Savoury) – elicited by the amino acid glutamate  Sweet, Bitter, and Umami work with a signal through a G protein-coupled receptor.  Salty and Sour, which work with ion

Physiology of Taste  In order to be tasted, a chemical:  Must be dissolved in saliva  Must contact gustatory hairs  Activation of Taste Receptors:  Binding of the food chemical to receptors  Depolarization the taste cell membrane,  Releasing neurotransmitter  Initiating an action potential  Threshold of activation  Bitter receptors detect substances present in minute amounts  Other receptors are less sensitive  Adaptation is rapid, with partial adaptation in 3–5 seconds and complete adaptation in

Taste Transduction

 Partly known  Each taste quality has its own special mechanism.  All of these mechanisms ultimately cause an increase in intracellular Ca2+, which increases neurotransmitter release  The stimulus energy is converted into a nerve impulse by:  Na+ influx in salty tastes  H+ influx in sour tastes by one of three ways  Directly entering the cell,  Opening cation channels,

Gustatory Pathway

 Cranial Nerves  VII(Facial), (Anterior two-thirds of the tongue)  IX(glossopharyngeal) (posterior third and the 

pharynx just behind) X (vagus) (epiglottis and the lower pharynx )

 carry impulses from taste buds to the solitary nucleus of the medulla  These impulses then travel to the thalamus, and from there fibers branch to the:  Gustatory cortex (taste)  Hypothalamus and limbic system (appreciation of taste)

Gustatory Pathway

Influence of Other Sensations on Taste

 Taste is 80% smell  When olfactory receptors in the nasal cavity are blocked by nasal congestion (or pinching your nostrils), food is bland  Thermoreceptors, mechanoreceptors, nociceptors also influence tastes  Temperature and texture enhance or detract from taste  “Hot” foods such as chili peppers actually bring about their pleasurable effects by exciting pain receptors in the mouth

Sense of Smell

 Olfact (To Smell)  The organ of smell is called the olfactory epithelium  Yellow-tinged patch (about 5cm2)  Pseudostratified epithelium  Located in the roof of the nasal cavity  Olfactory receptor cells are bipolar neurons with radiating olfactory cilia  Olfactory receptors are surrounded and cushioned by supporting cells  Basal cells lie at the base of the epithelium

Sense of Smell

Olfactory Pathway

 Olfactory receptor cells synapse with mitral cells  Glomerular mitral cells process odor signals  Mitral cells send impulses to:  The olfactory cortex  The hypothalamus, amygdala, and limbic system

Physiology of Smell

 Humans can distinguish10,000 or so odours  Olfactory receptors respond to several different odour causing chemicals  When bound to ligand these receptors initiate a G protein mechanism, which uses cAMP as a second messenger  cAMP opens Na+ and Ca2+ channels, causing depolarization of the receptor membrane that then triggers an action potential

Olfactory Transduction Process