Chemical Senses: Gustation

I. Background

A. Chemical senses

1. Mechanism by which we can detect chemicals in both the internal and external environment

2. Taste and olfaction are the most familiar chemical senses

3. Many types of chemically sensitive cells

a. Chemoreceptors

i. Distributed throughout the body

ii. Report subconsciously and consciously about our internal state

B. Types

1. Chemoreceptors in skin and mucus membranes warn us about irritating chemicals.

2. Nerve endings in the digestive organs detect many types of ingested substances

a. Viral agents may release chemicals into the GI tract that cause discomfort, activate vomiting reflexes, etc.

3. Chemical receptors in the arteries in the neck measure CO2 and O2 levels in the blood.

4. Sensory endings in the muscles respond to acidity

a. Burning sensation experienced during anaerobic exercise results from lactic acid formation

C. Taste (Gustation) and Olfaction have similar tasks

1. Detection of environmental chemicals

2. Both are required to perceive flavor

3. Both have strong and direct connections to our most basic needs

a. Thirst, hunger, emotion, sex, and certain forms of memory

4. Systems are separate and different and only merge at higher levels of cortical function

            a. Have different chemoreceptors

            b. Use different transduction pathways

            c. Have separate connections to the brain

            d. Have different effects on behavior


II. Gustation

A. Basic categories

1. Salty

2. Sour

3. Sweet

4. Bitter

B. Complex flavors

1. Each food activates a different combination of basic tastes

2. Most foods have a distinctive flavor as a result of their taste and smell occurring simultaneously

3. Other sensory modalities may contribute to a unique food-tasting experience

a. Texture, temperature, pain sensitivity (some hot and spicy flavors are actually a pain response)

C. Organs of taste

1. Tongue

a. Primary organ

2. Pharynx, palate and epiglottis have some sensitivity

3. Nasal passages are located so that odors can enter through the nose or pharynx and contribute to the perception of flavor

D. Anatomy of the tongue

1. Basic tastes

            a. Bitter across the back

            b. Sour on side closest to the back

            c. Salty on side more rostral than sour

            d. Sweet across front

2. Taste distribution

a. Most of the tongue is receptive to all basic tastes

i. Regions are most sensitive to a given taste

3. Papillae

a. Small projections

b. Each papillae has one to several hundred taste buds

4. Taste buds

            a. Each taste bud has 50-150 taste cells

5. Taste cells

            a. Taste cells are only 1% of the tongue epithelium

E. Taste receptor cells

1. Not neurons

a. Form synapses with the endings of gustatory afferent axons near the bottom of the taste bud


III. Gustatory Transduction

A. Basic process

1. When taste receptor is activated by the appropriate chemical, its membrane potential changes

a. Receptor potential

2. Depolarizing receptor potential cause Ca++ to enter the cytoplasm

a. Triggers the release of NT

3. Taste stimuli may:

            a. Pass directly through an ion channel (salt and sour)

            b. Bind to and block ion channels (sour and bitter)

            c. Bind to and open ion channels (some sweet amino acids)

            d. Bind to membrane receptors that activate 2nd messenger systems that in turn open or close ion channels (sweet and bitter)

B. Salt

1. Na+ flows down a concentration gradient into the taste receptor cell (most salts are Na+ salts--NaCl)

2. Na+ increase within the cell depolarizes the membrane and opens a voltage dependent Ca++ channel

3. Ca++ increase causes the release of NT

C. Sour

1. Foods that are sour have high acidity (low pH)

a. Acids (HCl) when dissolved in water generate H+ ions

2. H+ ions pass through the same channel that Na+ does

(How do we discriminate between salt and sour then?)

3. H+ also blocks a K+ channel

4. Net movement of + into the cell depolarizes the taste cell

a. Opens a Ca++ channel

b. Causes NT release

D. Sweetness

1. Molecules that are sweet bind to specific receptor sites and activate a cascade of 2nd messengers in certain taste cells

2. Molecules bind receptor

3. G-protein activates an effector enzyme-adenylate cyclase (cAMP produced)

4. cAMP causes a K+ channel to be blocked

5. Cell depolarizes

6. Ca++ channel opens and Ca++ in

7. NT released

E. Bitter

1. Chemicals in the environment that are deleterious often have a bitter flavor

a. Senses have evolved primarily to protect and preserve

b. Ability to detect bitter has two separate mechanisms

i. May result from this evolutionary pressure

2. System I

a. Bitter tastants can directly block a K+ channel (same transduction mechanisms as acids)

b. Cell depolarizes

c. Ca++ channel is opened and Ca++ in

d. NT released

3. System II

            a. Bitter tastant binds bitter receptor

            b. G-protein activates an effector enzyme-phospholipase C

            c. Ca++ is released from intracellular storage

            d. Ca++ increase causes NT release

IV. Taste Neural Pathway

A. Circuit

1. NT release from taste cells causes an AP in the gustatory afferent axon

2. Three different cranial nerves (VII, IX and X) innervate the taste buds and carry taste information from the tongue, palate, epiglottis and esophagus

a. Efferent target of this information is gustatory nucleus in the medulla

3. Information is relayed to the thalamus (VPM--ventral posterior medial nucleus)

4. Information then goes to the primary gustatory cortex (parietal lobe)



Chemical Senses: Olfaction

I. Background

A. Olfaction--sense of smell

1. As many as 100,000 unique odors can be discriminated

a. 80% of which are noxious

b. Odors perceived to be noxious are often deleterious (rotting meat, etc.).

B. Organs of smell

1. Not the nose

2. Olfactory epithelium

a. Thin sheet of cells high up in our nasal cavity

b. Size of the olfactory epithelium is proportionate to olfactory acuity

i. Man has 10 cm2

ii. Dog has 170 cm2

iii. Dogs also have 100x a many receptors per cm2

C. Olfactory receptors

1. Only receptor discussed thus far that are neurons

a. Fire action potentials

b. Only neurons in the nervous system that are replaced regularly throughout life

i. Every 4-8 weeks

2. Olfactory receptors are neurons and continuous with the CNS

3. Ends of the olfactory receptors are a mucus (water soluble)

            a. This mucus contains cells of the immune system and is shed every ten minutes

                        i. Individual with an infection (cold, flu, etc.) one sympton is a runny noise

                        ii. Mucus is shed more frequently to protect the olfactory receptors from infection

4. 500-1000 different odor binding proteins

a. Each olfactory receptor cell expresses only one type of binding protein

5. Receptor is G-protein-coupled

            a. Receptor binding activates an effector enzyme (either adenylate cyclase or phospholipase C, depending on the nature of the odorant)

b. 2nd messenger (cAMP or IP3) opens a Ca++ channel

c. Ca++ influx (unlike taste) does not cause NT release

i. It opens a Cl- channel

            d. Cl- leaves the cell and the membrane is depolarized

            e. Sufficient depolarization causes an AP results


II. Olfactory Pathway

A. Projects directly to the cortex

1. Cortex then projects to the thalamus and other cortical structure

            a. Unique

B. Circuit

1. Olfactory receptor cell axons leave the olfactory epithelium, coalesce to form a large number of bundles (together this is the olfactory nerve, cranial nerve I)

a. Run directly into the olfactory bulb

2. In the olfactory bulb, primary synapses between the olfactory receptor axons and mitral cells (the projection neuron of the olfactory system)

a. Glomeruli

i. Spherical arrangement of mitral cells

            b. Within the bulb, there are a number of other cells that contribute to the formation of special circuits for processing olfactory information (e.g., granule and periglomerular cells)

3. Axons of the mitral cells form a bundle known as the lateral olfactory tract

a. Projects primarily to the pyriform cortex

b. Minority projections to the accessory olfactory nuclei, the olfactory tubercle, the enterorhinal cortex, and the amygdala

4. Pyramidal cells in the pyriform cortex in turn project to the thalamus, neocortical regions, the hippocampus and the amygdala



Somatic Sensory System

I. Background

A. Differences between somatic senses and other senses

1. Receptors are distributed throughout the body as opposed to being concentrated at small, specialized locations

2. Responds to many kinds of stimuli (usually mechanical)

3. At least four senses (not one)

a. Temperature

b. Body position

c. Touch

d. Pain

4. Place, pressure, sharpness, texture, and duration can be accurately gauges

B. Types of somatic sensation receptors

1. Mechanoreceptors--sensitive to physical distortion

2. Nociceptors--respond to damaging stimuli

3. Thermoreceptors--sensitive to changes in temperature

4. Proprioceptors--monitor body position

5. Chemoreceptors--respond to certain chemicals

C. Classification

1. Free nerve endings

a. Nociceptors

b. Thermoreceptors

2. Encapsulated

a. Most cutaneous receptors

D. Mechanism of function

1. Stimuli applied to skin deform or change receptor

a. Alters the ionic permeability of the receptor creating generator potentials

i. Trigger action potentials


II. Mechanical Senses

A. Mechanical energy

1. Easily differentiated

            a. Stimulus frequency

            b. Stimulus pressure

            c. Receptive field

B. Types of receptors

1. Mechanoreceptors

a. Pacinian

i. Sensitive to vibration (250-350 Hz)

ii. Involved in the fine discrimination of texture or other moving stimuli that cause vibrations

            b. Meissner's corpuscle

i. Most common receptor in glabrous skin (smooth, hairless)

ii. Sensitive to vibration (low frequency, 30-50 Hz)

            c. Ruffini's ending-not well understood

            d. Mercel's disks

i. Light pressure and tactile discrimination

e. Hair follicle receptor


2. Nociceptors

a. Free, unmyelinated nerve endings

b. Signal that body tissue is being damaged

c. In most tissues, not brain

d. Types of damage detected

                        i. Mechanical--strong pressure (sharp objects)

            ii. Thermal (different from temperature)--active when tissues begin to be destroyed

            iii. Chemical--environmental agents or those from tissues itself--pH, histamine, etc.

3. Thermoreceptors

a. Brain temperature is tightly regulated

i. Close to 37C

ii. Brain function changes above and below that temperature

            b. Specialized receptors in our skin that can perceive changes in temperature as small as 0.01C.

c. Two types:

                        i. Warm--begin firing at 30C up to 45C (above causes damage and pain)

                        ii. Cold--below 35C to 10C .


Note: Like other sensory receptors, temperature receptors adapt. They respond to sudden changes in temperature.


Experiment--three beakers of water: one cold, one hot, one lukewarm. One finger from one hand into hot; one finger from the other hand into cold. After some time period, immerse both simultaneously into the lukewarm. The finger from the hot senses the water to be cold and the finger from the cold senses the same water to be hot. Why? Adaptation--the hot and cold receptors adapted (stopped firing). When immersed in lukewarm, only the unadapted receptors were available. You need both to sense lukewarm, etc.


4. Proprioceptors

a. Body position

i. Where the body is

ii. Direction of movement

iii. Speed of movement

b. Receptors in the skeletal muscles (more in movement lecture)

c. Two different mechanosensitive proprioceptors:

            i. Muscle spindles-consist of specialized intrafusal muscle fibers distributed among ordinary (extrafusal) muscle fibers; detect changes in muscle length

ii. Golgi tendon organs-distributed among collagen fibers in tendons and detects changes in muscle tension


III. Organization of Somatic Sensory Information

A. Structure of spinal cord (see Neuroanatomy Lecture)

B. Spinal segments

1. 30 spinal segments consisting of paired dorsal and ventral roots

2.  Spinal segments are divided into 4 groups: cervical, thoracic, lumbar, sacral

3. Each segment is named after the vertebra from which the nerves

a. Cervical: C1 - C8

b. Thoracic: T1 - T12

c. Lumbar: L1 - L5

d. Sacral: S1 - S5

C. Dermatomes

1. Segmental organization of the spinal nerves and sensory innervation of skin are related

2. Area of skin innervated by the dorsal roots of a single spinal segment is a dermatome

3. Characteristics

            a. Overlap between the dermatomes

            b. Cervical dermatomes

i. Above the sternum

            c. Thoracic dermatomes

i. Top of sternum to waist

            d. Lumbar dermatomes

i. Front of legs and stomach

            e. Sacral dermatomes

i. Back of legs and genitals


IV. Somatic Sensory Pathways

A. Two basic systems

1. Pain and temperature

2. Touch and Proprioception

B. Pathways

1. Dorsal column-medial lemniscal pathway

a. Touch and proprioception

2. Spinothalamic pathway

a. Pain and temperature

C. DCML Pathway

1. In the DCML pathway information ascends through the dorsal column on the ipsilateral side of the spinal cord

2. Synapses in the medulla

3. Crosses over and ascends via the medial lemniscus to the thalamus (VP)

4. Synapses in VP thalamus

5. Projects to the cortex

D. ST Pathway

1. Information crosses to the contralateral side in the spinal cord

2. Ascends via the spinothalamic tract

3. Synapses in the thalamus (VP)

4. Projects to the cortex.

E. Information carried in each pathway remains separate

1. Segregated all the way to the cortex

2. Thalamus

a. Ventral posterior (VP) nucleus receives the information and projects to the somatosensory cortex


V. Somatosensory Cortex

A. Anatomy

1. Parietal lobe

            a. Post-central gyrus

                        i. Most complex processing occurs in the cortex

B. Somatotopy

1. Mapping of the body's surface sensations onto a brain structure

2. Features of the map:

a. Not continuous

b. Not scaled to the human body

c. Relative size of the cortex devoted to each body part is correlated with the density of sensory input (i.e., lips versus the skin on your calf).

d. Size is related to the importance of the sensory input (i.e., finger tip versus elbow)

C. Posterior parietal lobe

1. Primary somatosensory cortex receives simple segregated streams of sensory information

2. Integration takes place in the posterior parietal cortex


VI. Pain and Its Control

A. Nociception

1. Sensory process that provides signals that trigger pain

B. Characteristics

1. Pain is influenced cognitively

2. Hyperalgesia

a. Tissue already damaged is much more sensitive to pain

i. Nociceptors are sensitized by various substances released by damaged tissue (protaglandins, histamines, etc.)

C. Regulation of pain

1. Pain can be modified by non-painful sensory input (i.e., rub the skin around a bruise)

a. Gate Theory of Pain-circuit in spinal cord dorsal root

2. Several brain regions can act to suppress pain

            a. PAG (periacqueductal gray matter) project to the raphe (serotonin) that sends axons to the spinal cord (5-HT is inhibitory, block synaptic activity)

3. Brain chemicals

a. Endorphins

i. Share many opioid properties and bind to opioid receptors in the brain

ii. Opioid receptors are throughout the body, but especially in the brain and particularly in brain areas that process and modulate nociceptive information (PA, raphe, and spinal cord)