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Nerves, Taste, Touch

Nerves, Taste, Touch. BIOL241 Last lecture. Taste. Tastants taste receptor cells taste buds five primary taste sensations Properties of the taste system Na + , H + , Ca ++ 80% Smell. Taste Bud & Receptors. Connective tissue. Gustatory hair. Taste fibers of cranial nerve.

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Nerves, Taste, Touch

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  1. Nerves, Taste, Touch BIOL241 Last lecture

  2. Taste • Tastants • taste receptor cells • taste buds • five primary taste sensations • Properties of the taste system • Na+, H+, Ca++ • 80% Smell

  3. Taste Bud & Receptors

  4. Connective tissue Gustatory hair Taste fibers of cranial nerve Stratified squamous epithelium of tongue Basal cells Gustatory (taste) cells Taste pore (c) Enlarged view of a taste bud. Figure 15.23c

  5. Primary Taste Sensations • salty • sour • sweet • bitter • Umami • Dissolve in saliva, diffuse into the taste pore, and contact the gustatory hairs

  6. Taste Components • Thermoreceptors • Mechanoreceptors • Nociceptors • Hot • Ansomias • Uncinate Fits • Papillae

  7. Olfactory epithelium Olfactory tract Olfactory bulb Nasal conchae Route of inhaled air (a) Figure 15.21a

  8. Epiglottis Palatine tonsil Lingual tonsil Foliate papillae Fungiform papillae (a) Taste buds are associated with fungiform, foliate, and circumvallate (vallate) papillae. Figure 15.23a

  9. Circumvallate papilla Taste bud (b) Enlarged section of a circumvallate papilla. Figure 15.23b

  10. Properties of the taste system • A single taste bud contains 50–100 taste cells representing all 5 taste sensations (so the classic textbook pictures showing separate taste areas on the tongue are wrong). • Each taste cell has receptors on its apical surface. These are transmembrane proteins which • admit the ions that give rise to the sensation of salty; • bind to the molecules that give rise to the sensations of sweet, bitter, and umami.

  11. Properties of the taste system, cont. • A single taste cell seems to be restricted to expressing only a single type of receptor (except for bitter receptors). • Taste receptor cells are connected, through an ATP-releasing synapse, to a sensory neuron leading back to the brain. • However, a single sensory neuron can be connected to several taste cells in each of several different taste buds. • The sensation of taste — like all sensations — resides in the brain

  12. Gustatory cortex (in insula) Thalamic nucleus (ventral posteromedial nucleus) Pons Solitary nucleus in medulla oblongata Facial nerve (VII) Vagus nerve (X) Glossopharyngeal nerve (IX) Figure 15.24

  13. Gustatory Pathway • Cranial nerves VII and IX carry impulses from taste buds to the solitary nucleus of the medulla • Impulses then travel to the thalamus and from there fibers branch to the: • Gustatory cortex in the insula • Hypothalamus and limbic system (appreciation of taste)

  14. Salty • In mice, perhaps humans, the receptors for table salt (NaCl) is an ion channel that allows sodium ions (Na+) to enter directly into the cell. This depolarizes it allowing calcium ions (Ca2+) to enter [Link] triggering the release of ATP at the synapse to the attached sensory neuron and generating an action potential in it. • In lab animals, and perhaps in humans, the hormone aldosterone increases the number of these salt receptors. This makes good biological sense: The main function of aldosterone is to maintain normal sodium levels in the body. • An increased sensitivity to sodium in its food would help an animal suffering from sodium deficiency (often a problem for ungulates, like cattle and deer).

  15. Sour • Sour receptors are transmembrane ion channels that detect the protons (H+) liberated by sour substances • (Why?)

  16. Sweet • Sweet substances (like table sugar — sucrose) bind to G-protein-coupled receptors (GPCRs) at the cell surface. • Each receptor contains 2 subunits designated T1R2 and T1R3 and is • coupled to G proteins. • The complex of G proteins has been named gustducin because of its similarity in structure and action to the transducin that plays such an essential role in rod vision.

  17. Gustducin • Activation of gustducin triggers a cascade of intracellular reactions: • activation of adenylyl cyclase • formation of cyclic AMP (cAMP) • the closing of K+ channels that leads to depolarization of the cell. • The mechanism is similar to that used by our odor receptors [View].

  18. Leptin • The hormone leptin inhibits sweet cells by opening their K+ channels. This hyperpolarizes the cell making the generation of action potentials more difficult. • Could leptin, which is secreted by fat cells, be a signal to cut down on sweets?

  19. Bitter • The binding of substances with a bitter taste, e.g., quinine, phenylthiocarbamide [PTC], also takes place on G-protein-coupled receptors that are coupled to gustducin. • In this case, however, cyclic AMP acts to release calcium ions from the endoplasmic reticulum [Link], which triggers the release of neurotransmitter at the synapse to the sensory neuron.

  20. Bitter, cont. • Humans have genes encoding 25 different bitter receptors ("T2Rs"), and each taste cell responsive to bitter expresses a number (4–11) of these genes. (This is in sharp contrast to the system in olfaction where a single odor-detecting cell expresses only a single type of odor receptor.) • Despite this — and still unexplained — a single taste cell seems to respond to certain bitter-tasting molecules in preference to others.

  21. Bitter, cont. • The sensation of taste — like all sensations — resides in the brain. • Transgenic mice that express T2Rs in cells that normally • express T1Rs (sweet) respond to bitter substances as though they were sweet; • express a receptor for a tasteless substance in cells that normally express T2Rs (bitter) are repelled by the tasteless compound. • So it is the activation of hard-wired neurons that determines the sensation of taste, not the molecules nor the receptors themselves.

  22. Umami • Umami is the response to salts of glutamic acid — like monosodium glutamate (MSG) a flavor enhancer used in many processed foods and in many Asian dishes. Processed meats and cheeses (proteins) also contain glutamate. • (What is Glutamic Acid?)

  23. Umami The binding of amino acids, including glutamic acid, takes place on G-protein-coupled receptorsthat are coupled to heterodimers of the protein subunits T1R1 and T1R3. • Another umami receptor (at least in the rat's tongue) is a modified version of the glutamate receptors found at excitatory synapses in the brain.

  24. Taste Receptors in Other Locations • Taste receptors have been found in several other places in the body. • Examples: ?

  25. Taste Receptors in Other Locations • Bitter receptors (T2Rs) are found on the cilia and smooth muscle cells of the trachea and bronchi [View] where they probably serve to expel inhaled irritants; • Sweet receptors (T1Rs) are found in cells of the duodenum. When sugars reach the duodenum, the cells respond by releasing incretins. These cause the beta cells of the pancreas to increase the release of insulin. • So the function of "taste" receptors appears to be the detection of chemicals in the environment — a broader function than simply taste.

  26. Touch • What? • How? • Where? • Cells • Nerves • 4 distinct somatic modalities: • touch • proprioceptive sensations • pain • thermal sensations

  27. Central somatic pathways • 2 major pathways to the somatosensory cortex • dorsal column-medial lemniscal system -- tactile sensation and arm proprioception • anterolateral system -- pain and temperature a bit of tactile information • the body surface is represented in the brain in an orderly fashion

  28. Central somatic pathways

  29. Somatosensory Cortex

  30. Hair shaft Dermal papillae Subpapillary vascular plexus Epidermis Papillary layer Pore Appendages of skin Dermis • Eccrine sweat gland Reticular layer • Arrector pili muscle • Sebaceous (oil) gland Hypodermis (superficial fascia) • Hair follicle Nervous structures • Hair root • Sensory nerve fiber Cutaneous vascular plexus • Pacinian corpuscle Adipose tissue • Hair follicle receptor (root hair plexus) Figure 5.1

  31. Layers of the Dermis: Papillary Layer • Papillary layer • Areolar connective tissue with collagen and elastic fibers and blood vessels • Dermal papillae contain: • Capillary loops • Meissner’s corpuscles • Free nerve endings

  32. Functions of the Integumentary System • Body temperature regulation • ~500 ml/day of routine insensible perspiration (at normal body temperature) • At elevated temperature, dilation of dermal vessels and increased sweat gland activity (sensible perspirations) cool the body • Cutaneous sensations • Temperature, touch, and pain

  33. MechanoreceptorSensory Neurons • Meissner's corpuscles detect changes in texture (vibrations around 50 Hz) and adapt rapidly. small receptive field: 2-4mm • Pacinian corpuscles detect rapid vibrations (about 200–300 Hz). • Merkel's discs detect sustained touch and pressure. small receptive field: 2-4mm • Ruffini'scorpuscle

  34. Meissner's corpuscles • Meissner's corpuscles are located at the tips of the dermal papillae. Each corpuscle consists of a number of flattened layers of cells, each with an elongated nucleus. The neuron within is coiled among these cells, but is not easily seen. When the corpuscle is deformed by pressure, the nerve endings are stimulated, registering the sensation of touch

  35. Meissner's corpuscles

  36. Meissner's corpuscles

  37. Meissner's corpuscles

  38. Pacinian corpuscles • Each corpuscle is an egg-shaped structure consisting of many concentric layers of tissue. Embedded within this structure is a free nerve ending. When the corpuscle is deformed by pressure, an action potential is initiated in the nerve ending. Pacinian corpuscles are found in many areas of the body, including the skin, the mesenteries surrounding the gut, and joint capsules. The Pacinian corpuscles in joints provide the CNS with information on the position of the joints. As such they play an important role as proprioceptors.

  39. Pacinian corpuscles

  40. Pacinian corpuscles

  41. Merkel's discs • found in the deepest layer of the epidermis (stratum basale).  • Always found associated with a sensory receptor nerve ending for touch.

  42. Merkel's discs

  43. Merkel's discs

  44. Ruffini endings

  45. Ruffini endings

  46. Non-hairy vs. hairy Receptors

  47. Nervous System Cells • Sensory neurons: • afferent neurons of PNS • Motor neurons: • efferent neurons of PNS • Interneurons: • association neurons • Ependymal cells • Astrocytes • Microglia • Oligodendrocytes • Satellite cells (amphicytes) • Schwann cells (neurilemmacytes)

  48. Neurogliaare supporting cells

  49. 2. Astrocytes • Maintain blood–brain barrier (isolates CNS) • Create 3-dimensional framework for CNS • Repair damaged neural tissue • Guide neuron development • Control interstitial environment

  50. Astrocytes

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