The Special SensesPart 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

coating nasal membranes

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

numerous. Mechanical and not involved in gustation.

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 tongue. Form an inverted V at the back of the tongue. Innervated by the glossopharyngeal nerve.

Anatomy of a Taste Bud

Each Funnel-shaped taste bud consists of 50 to 100 three major cell types1. Supporting cells – insulate the receptor 2. Basal cells – dynamic stem cells (dividing

and differentiating into new gustatory cells)3. 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 portions (receptor membranes) of the gustatory cells.

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 channels.

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 1–5 minutes.

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,Blockade of K+ channels

G-protein mediated (Gustducin) Bitter causes Ca2+ release from intracellular stores Sweet causes closure of K+ channels,( depolarizing

the cell )

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