Sensory Systems: Chemical Senses

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16 Terms

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Basic tastes: salt, sour, sweet, bitter

Salt: NaCl

Sour: HCl

Sweet: Sucrose

Bitter: Quinine

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Sensation of Taste

Interaction with olfactory (smell) and tactile (texture) stimuli

perception of taste is a combination of stimuli not just taste (smell, texture, temp)

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Taste thresholds

Sweet: sucrose 10-2 M

Sour: HCl 9x10-4 M

Salty: NaCl 10-2 M

Bitter: quinine 8x10-6 M

High threshold: sweet and salty

Low threshold: sour and bitter (need to be easily detected for spoiled/poisonous food)

<p>Sweet: sucrose <strong>10-2 M</strong></p><p>Sour: HCl <strong>9x10-4 M</strong></p><p>Salty: NaCl <strong>10-2 M</strong></p><p>Bitter: quinine <strong>8x10-6 M</strong></p><p>High threshold: sweet and salty</p><p>Low threshold: sour and bitter (need to be easily detected for spoiled/poisonous food)</p>

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Taste Buds

Structure: receptor (specialized cell/not a neuron) and supporting cells

Receptor has synaptic vesicles at base that release neurotransmitters (communicate with afferent (sensory) nerve cells)

Cells are replaced about every 10 days (basal cells- maintenance and support)

<p>Structure: <strong>receptor </strong>(specialized cell/<strong>not a neuron)</strong> and supporting cells</p><p>Receptor has <strong>synaptic vesicles </strong>at base that release neurotransmitters (communicate with afferent (sensory) nerve cells)</p><p>Cells are replaced about <strong>every 10 days </strong>(basal cells- maintenance and support)</p>

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Signal Transduction in Taste Receptors

Membrane receptors on apical microvilli

Variety of primary/second messenger mechanisms

-Depends on chemical type

Salty (Na+) and Sour (H+/K+): ion channels → depolarize

Bitter and Sweet: 2nd messenger system (cyclic AMP → depolarize)

*Receptor cells do not generate action potentials. Instead, it receives a stimulus that depolarizes it, pushing the vesicle out. It is not all-or-nothing but instead is proportional to the number of receptors stimulated (generates membrane potential)

<p>Membrane receptors on apical microvilli</p><p>Variety of primary/second messenger mechanisms</p><p>-Depends on chemical type</p><p>Salty (<strong>Na+</strong>) and Sour (<strong>H+/K+</strong>): ion channels → depolarize</p><p>Bitter and Sweet: 2nd messenger system (cyclic AMP → depolarize)</p><p>*Receptor cells do <strong>not generate action potentials</strong>. Instead, it receives a stimulus that depolarizes it, pushing the vesicle out. It is not all-or-nothing but instead is proportional to the number of receptors stimulated <strong>(generates membrane potential)</strong></p>

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Synaptic contact generates action potential in cranial nerve ending

primary afferent is group III small myelinated nerve, with cell body in cranial nerve ganglion

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Classification of Peripheral Nerve Fibers

Largest/ Fastest/ Least resistance → Smallest/ Slowest/ More resistance

Group 1, 2 ,3 4 (unmyelinated)

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Taste Pathways (nerves → medulla)

Chorda tympani nerve to facial (VII) for front 2/3 of tongue (anterior)
- geniculate ganglion (outside brainstem: where cell bodies of facial nerves located)
Glossopharyngeal (IX) for back of tongue (1/3)
- petrosal or inferior ganglion
vagus (X) for throat
- inferior vagal (nodose) ganglion

Facial, glossopharyngeal, vagus nerves to solitary nucleus and tract of medulla

<ol><li><p><strong>Chorda tympani nerve</strong> to <strong>facial (VII)</strong> for front 2/3 of tongue (anterior)</p><ul><li><p><strong>geniculate ganglion </strong>(outside brainstem: where cell bodies of facial nerves located)</p></li></ul></li><li><p><strong>Glossopharyngeal </strong>(<strong>IX</strong>) for back of tongue (1/3)</p><ul><li><p><strong>petrosal </strong>or <strong>inferior ganglion</strong></p></li></ul></li><li><p><strong>vagus </strong>(<strong>X</strong>) for throat</p><ul><li><p><strong>inferior vagal (nodose) ganglion</strong></p></li></ul></li></ol><p>Facial, glossopharyngeal, vagus nerves to <strong>solitary nucleus and tract of medulla</strong></p>

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Taste Pathways (medulla→ parietal lobe)

From solitary nucleus (in brainstem/medulla):

Relayed to ventral posterior medial nucleus (VPM) of thalamus (relay station)
- Perceptual path ends at lateral surface of parietal cortex (Brodmann areas 3,1,2) (final perception)
Taste reflexes to salivatory nuclei (VII, IX) (vagus (X) not connected to salivary glands)
Additional path to hypothalamus & amygdala (limbic system: emotional center)
- Via parabrachial nucleus of pons
- Provides input to pleasure/pain, hunger/thirst/satiety centers

<p>From <strong>solitary nucleus </strong>(in brainstem/medulla):</p><ol><li><p>Relayed to <strong>ventral posterior medial nucleus </strong>(VPM) of <strong>thalamus </strong>(relay station)</p><ul><li><p>Perceptual path ends at <strong>lateral surface of parietal cortex</strong> (Brodmann areas 3,1,2) (final perception)</p></li></ul></li><li><p>Taste reflexes to <strong>salivatory nuclei (VII, IX) </strong>(vagus (X) not connected to salivary glands)</p></li><li><p>Additional path to<strong> hypothalamus & amygdala</strong> (limbic system: emotional center)</p><ul><li><p>Via <strong>parabrachial nucleus of pons</strong></p></li><li><p>Provides input to pleasure/pain, hunger/thirst/satiety centers</p></li></ul></li></ol><p></p>

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Sensation of Smell

Thought to be analogous to immune system

6 general groups: floral (rose), ethereal (pears), musky (musk), camphor (eucalyptus), putrid (rotten eggs), pungent (vinegar)

Odors may directly trigger instinctual behavior, strong emotional responses/memories in humans

Direct links to “emotional” brain centers (limbic system)

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Olfactory Epithelium

Structure: receptor neurons and supporting cells

*same cell is receptor and nerve fiber cell body (no separate ganglion)

Human olfactory epithelium is about 5 cm², dog may be 100 cm²

The olfactory receptor cell is a bipolar neuron - apical dendrites project cilia into the mucus layer, axon on the basal side conducts the information to the brain
Surrounded by a basal cell for regeneration, and supporting cell for maintenance and support
Does not have separate ganglion, the receptor cell is the receptor and the nerve fiber cell body
Axons bundle up and make up CN I (olfactory nerve)

<p>Structure: <strong>receptor neurons </strong>and supporting cells</p><p>*same cell is receptor and nerve fiber cell body <strong>(no separate ganglion)</strong></p><p>Human olfactory epithelium is about <strong>5 cm²</strong>, dog may be 100 cm²</p><ul><li><p><span>The <strong>olfactory receptor cell</strong> is a<strong> <u>bipolar neuron</u></strong> - apical dendrites project <strong><u>cilia</u></strong> into the <strong>mucus </strong>layer, <strong><u>axon</u> </strong>on the <strong>basal</strong> side conducts the information to the brain</span></p></li><li><p><span>Surrounded by a <strong><u>basal cell</u></strong> for regeneration, and supporting cell for maintenance and support</span></p></li><li><p><span>Does not have separate ganglion, the receptor cell is the receptor and the nerve fiber cell body</span></p></li><li><p><span>Axons bundle up and make up <strong><u>CN I (olfactory nerve)</u></strong></span></p></li></ul><p></p>

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Signal Transduction in Smell Receptors

Membrane receptors on cilia

Second messenger system produces depolarization (cAMP or cGMP)

Threshold for some substances very low

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In Sensation of Smell, Action potential travels via…

Action potential travels via unmyelinated fiber (CN I) (olfactory nerve)

primary afferent fiber is group IV axon

passes in bundles through cribriform plate of the ethmoid bone and synapses on olfactory bulb

<p>Action potential travels via <strong>unmyelinated fiber</strong> (<strong>CN I</strong>) (<strong>olfactory </strong>nerve)</p><p>primary afferent fiber is <strong>group IV</strong> axon</p><p>passes in<strong> bundles</strong> through <strong>cribriform plate </strong><span>of the ethmoid bone and <strong>synapses on olfactory bulb</strong></span></p>

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Olfactory Pathways

Fascicles of olfactory nerves (CN I) penetrate cribriform plate to enter olfactory bulb

Synapse on neurons of bulb (mitral cells)

Olfactory bulb (no longer CNI) processes and relays information (2nd order fiber) into the lateral and medial olfactory tracts

Seen on ventral surface of frontal lobe

Olfactory cell (a neuron) passes through the openings of the cribriform plate as bundles (forms CN I), enters the olfactory bulb → glomeruli of the mitral cells → olfactory tract to the olfactory cortex on parahippocampal gyrus (Brodmann area 28)
2 total neurons responsible for conducting message

<p>Fascicles of olfactory nerves (CN I) penetrate cribriform plate to enter olfactory bulb</p><p>Synapse on neurons of bulb <strong>(mitral cells)</strong></p><p>Olfactory bulb (no longer CNI) processes and relays information (<strong>2nd order fiber)</strong> into the<strong> lateral and medial olfactory tracts</strong></p><p>Seen on <strong>ventral surface of frontal lobe</strong></p><ul><li><p><span>Olfactory <strong>cell </strong>(a neuron) passes through the openings of the <strong>cribriform plate</strong> as bundles (forms <strong>CN I</strong>), enters the olfactory <strong>bulb </strong>→ <strong>glomeruli </strong>of the <strong>mitral cell</strong>s → olfactory <strong>tract </strong>to the olfactory <strong>cortex </strong>on <strong>parahippocampal gyrus </strong>(Brodmann area 28)</span></p></li><li><p><span><strong>2 total neurons </strong>responsible for conducting message</span></p></li></ul><p></p>

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Labeled line theory

specific receptor (i.e. floral) are capable of picking up a specific smell and sending it down a specific line of the pathway → activate a specific part of brain

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Olfactory tract to olfactory cortex

Olfactory tract to olfactory cortex on parahippocampal gyrus (Brodmann area 28), septal area, hypothalamus, limbic system

perception on parahippocampal

others for hunger/thirst/satiety, instinct memories , emotion centers

note no direct connection to sensory thalamus (no relay system)

<p><strong>Olfactory tract to olfactory cortex</strong> on <strong>parahippocampal gyrus</strong> (Brodmann area 28), s<strong>eptal area, hypothalamus, limbic system</strong></p><p><strong>perception </strong>on <strong>parahippocampal </strong></p><p>others for hunger/thirst/satiety, instinct memories , emotion centers </p><p>note<strong> no direct connection to sensory thalamus </strong>(no relay system)</p>