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Flashcards in Module 10: Special Senses Deck (313)
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151

Transmission to CNS (Circumvallate papillae, Posterior mouth and Throat)

• Circumvallate papillae, Posterior mouth and Throat -> Glossopharyngeal nerve (CN IX) -> Tractus Solitarus (NTS) -> VPM -> lower tip of the postcentral gyrus) -> sylvian fissure -> opercular insular area

152

Transmission to CNS (Base of the tongue, epiglottis)

• Base of the tongue, epiglottis -> Vagus nerve ->Tractus Solitarus (NTS) -> VPM -> lower tip of the postcentral gyrus -> sylvian fissure -> opercular insular area

153

Transmission to CNS (From NTS)

• From the NTS divides into -> Superior salivatory nucleus and Inferior salivatory nucleus
then activates the -->>Submandibular, Sublingual and Parotid glands

154

• Choosing certain types of food over the other
• Helps control the diet he eats
• Usually changes in accordance to body’s needs
• Previous experience with unpleasant or pleasant taste plays a key factor
• Taste AVERSION: negative taste preference

Taste Preference

155

Experiments (TASTE):

1. Adrenalecomized, salt-depleted animals automatically select drinking water with high concentration of NaCl in preference to pure water

2. Animal given injections of insulin choses sweetest food among many samples

3. Calcium depleted parathyroidectomized animals choose water with high calcium chloride

156

• Least understood of our senses
• Subjective phenomenon
• Cannot be studied with ease in lower animals
• Smell is poorly developed in humans vs lower animals
• Pregnant women and young adults have higher sense of smell
• Olfactory receptors are the only sense that is not routed through the CNS
• Suggests that smell is more important than other senses

OLFACTION

157

• Located at the -Superior part of each nostril
• Medially: downward along the surface of the superior septum
• Laterally: folds over superior turbinate, upper surface of middle turbinate
• 2.4sq m per nostril

Olfactory Membrane

158

Olfactory Cells

• Receptor cells for smell sensation
• Bipolar nerve cells (derived originally from CNS)
- Each olfactory receptor is a neuron
• 100M olfactory cells (OC) in the olfactory epithelium
• Interspersed among sustentacular cell
• Ends of OC forms a knob from which 4 to 25 Olfactory Hairs (olfactory Cilia) project into the olfactory membrane
- Olfactory mucosa is the place in the body where NS is closest to the external world
• Interspersed among Bowman’s gland in the olfactory membrane
• The Olfactory portion of the brain were among the first brain structures developed in primitive animals, and much of the remainder of the brain developed around these olfactory beginnings.
• Limbic system –part of the brain that originally subserved olfaction

159

• reacts to odors in the air and stimulate olfactory cells, upper portion of nasal passages –contain the receptors

Olfactory cilia

160

• secrete mucus onto olfactory membrane

Bowman’s gland

161

• lies over the cribriform plate
• one bulb has several thousand Glomeruli

Olfactory bulb

162

• separates the brain cavity form the upper nasal cavity
• has several perforations through which nerves pass upwards to reach the olfactory bulb

Cribriform plate

163

• glomerular structure in olfactory bulb where short axons from olfactory cells terminate
• one bulb usually terminal for 25,000 axons from olfactory cells
• also the terminus for dendrites from ~25 large mitral cells and ~60small tufted cells
• different glomeruli different odor

Glomeruli

164

Olfactory Signal Transduction

1. Odorant binds to its receptor
2. Receptor activates G protein
3. G protein activates adenyl cyclase
4. cAMP opens a cation channel, allowing Sodium and Calcium influx causing depolarization

165

Importance of Olfactory Stimulation

• Multiplies even the weakest odorant
• Only volatile substances can be sniffed into the nostrils can be smelled
• Stimulating substance must be at least slightly water soluble so that it can pass through the mucus into the cilia
• Helpful if substance is slightly lipid soluble

166

Membrane potential

• Membrane potential of Unstimulated Olfactory cells = -55 millivolts
- Slow action potentials at 1 every 20 seconds; 2-3 per second

• Membrane potential of Stimulated Olfactory cells = -30 millivolts or less (can be positive)
- Action potential of 20-30 per second

167

• Olfactory receptors adapt 50% in the first second after stimulation
• After that they adapt Very little and Very Slowly
• Adapt almost to EXTINCTION within minutes
- Psychological adaptation

Rapid adaptation of olfactory sensations

168

3 Reasons for Rapid Adaptation

1. The Granule cell
- specialized inhibitory cell in the olfactory bulb
- from which a large number of centrifugal nerve fibers terminate on

2. Feedback inhibition by the CNS
- After a strong stimulus the CNS suppress the smell signals through the olfactory bulb

3. Odor receptors
- 1000 different types or more

169

7 odor system

1. Camphoraceous
2. Musky
3. Floral
4. Pepperminty
5. Ethereal
6. Pungent
7. Putrid

170

• Lack of the appropriate receptor protein in olfactory cells for that particular substance

Odor Blindness

171

Affective Nature of Smell

• Pleasant
• Unpleasant
• Previous experience affects sense of smell

172

• Organ is not well developed in humans
• Very well developed in rodents
• This organ is concerned with perception of odors that act as pheromones

Vemeronasal organ

173

Threshold for smell

• Methylmercaptan – can be smelled when only one 25 trillionth of a gram is present in each mililiter of air. (low threshold)
• Mixed with natural gas to give an odor to be detected in case of gas leaks
• Humans can recognize more than 10,000 different odors but determination of intensity is poor
• Smell is concerned more with the presence or absence of odors rather than with quantitative detection of their intensities

174

Role of Pain Fibers in the Nose

• Many trigeminal pain fibers are found in olfactory membrane
• They are stimulated by irritating substances
• Are responsible for initiating sneezing, lacrimation and other reflex responses.

175

• enters the brain at the anterior junction between the mesencephalon and cerebrum
• Divides into:
- Medial olfactory area
- Lateral olfactory area

Olfactory tract

176

• The Very Old Olfactory System
• Subserves basic olfactory reflexes
• Consists of a group of nuclei located at the midbasal portion of the brain immediately anterior to the hypothalamus and other primitive portions of the brain’s limbic system
• Concerned with basic behavior.

Medial Olfactory area

177

• The Less old olfactory system
• Composed of prepyriform and pyriform cortex plus the cortical portion of the amygdaloid nuclei.
• Some signal pathways from LOA also feed into the Paleocortex (anteromedial portion of the temporal lobe)
• Only area in the entire cerebral cortex where sensory signals pass directly to the cortex without passing first through the thalamus.
• Has many connections to the limbic system!
• Its connection with the hippocampus is important in learning to like or dislike certain foods.
• Automatic but partially learned control of food intake and aversion to toxic and unhealthy foods
• Odor memory

Lateral Olfactory Area

178

Removal of lateral olfactory areas in animals:

• did not affect more primitive responses to olfaction such as licking of the lips, salivation and other feeding responses caused by the smell of food or primary emotional drives associated with smell
• Abolishes more complicated olfactory conditioned reflexes

179

The Newer Pathway

• Helps in the conscious analysis of odor (studies in monkeys)
• Passing through the Thalamus -> dorsomedial thalamic nucleus -> lateroposterior quadrant of the orbitofrontale cortex
• Comparable to other cortical systems
• Used for conscious perception and analysis of olfaction

180

Centrifugal Control of Activity in the Olfactory Bulb by the CNS

• Brain to the olfactory tract to the olfactory bulb. Hence OUTWARD direction.
• i.e. CENTRIFUGALLY from the brain to the periphery
• These tracts terminate on granule cells located among mitral and tufted cells in the olfactory bulb.
• Granule cells send inhibitory signals to Tufted and Mitral cells.