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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

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Taste Bud & Receptors

Figure 15.23c

Taste fibers of cranial nerve

Connective tissue

Gustatory (taste) cells

Taste pore

Gustatory hair

Stratified squamous epithelium of tongue

(c) Enlarged view of a taste bud.

Basal cells

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Primary Taste Sensations

•  salty •  sour •  sweet •  bitter •  Umami •  Dissolve in saliva, diffuse into the taste

pore, and contact the gustatory hairs  

Taste Components

•  Thermoreceptors •  Mechanoreceptors •  Nociceptors •  Hot •  Ansomias •  Uncinate Fits •  Papillae

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Figure 15.21a

Olfactory tract Olfactory bulb

(a)

Nasal conchae

Route of inhaled air

Olfactory epithelium

Figure 15.23a

(a) Taste buds are associated with fungiform, foliate, and circumvallate (vallate) papillae.

Fungiform papillae

Epiglottis

Palatine tonsil

Foliate papillae

Lingual tonsil

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Figure 15.23b

(b) Enlarged section of a circumvallate papilla.

Taste bud

Circumvallate papilla

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.

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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

Figure 15.24

Gustatory cortex (in insula)

Thalamic nucleus (ventral posteromedial nucleus)

Pons Solitary nucleus in medulla oblongata

Facial nerve (VII)

Glossopharyngeal nerve (IX)

Vagus nerve (X)

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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)

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).

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Sour

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

•  (Why?)

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.

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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].

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?

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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.

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.    

 

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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.

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?)

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Umami  

�The  binding  of  amino  acids,  including  glutamic  acid,  takes  place  on  G-­‐protein-­‐coupled  receptors  that  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.  

Taste Receptors in Other Locations

•  Taste receptors have been found in several other places in the body.

•  Examples: ?

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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.    

Touch •  What? •  How? •  Where? •  Cells •  Nerves •  4 distinct somatic modalities:

–  touch – proprioceptive sensations – pain –  thermal sensations

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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

Central somatic pathways

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Somatosensory Cortex

Figure 5.1

Epidermis

Hair shaft

Dermis Reticular layer

Papillary layer

Hypodermis (superficial fascia)

Dermal papillae

Pore

Subpapillary vascular plexus

Appendages of skin • Eccrine sweat gland • Arrector pili muscle • Sebaceous (oil) gland • Hair follicle • Hair root Nervous structures

• Sensory nerve fiber • Pacinian corpuscle • Hair follicle receptor (root hair plexus)

Cutaneous vascular plexus

Adipose tissue

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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

Functions of the Integumentary System

2.  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

3. Cutaneous sensations –  Temperature, touch, and pain

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Mechanoreceptor Sensory 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's corpuscle

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

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Meissner's corpuscles  

Meissner's corpuscles  

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Meissner's corpuscles  

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.

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Pacinian corpuscles  

Pacinian corpuscles  

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Merkel's discs

•  found in the deepest layer of the epidermis (stratum basale).

•  Always found associated with a sensory receptor nerve ending for touch.

Merkel's discs  

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Merkel's discs  

Ruffini endings

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Ruffini endings  

Non-hairy vs. hairy Receptors

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Nervous System Cells •  Sensory neurons:

–  afferent neurons of PNS •  Motor neurons:

–  efferent neurons of PNS •  Interneurons:

–  association neurons 1.  Ependymal cells 2.  Astrocytes 3.  Microglia 4.  Oligodendrocytes 5.  Satellite cells (amphicytes) 6.  Schwann cells (neurilemmacytes)

Neuroglia are supporting cells

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2. Astrocytes

•  Maintain blood–brain barrier (isolates CNS)

•  Create 3-dimensional framework for CNS •  Repair damaged neural tissue •  Guide neuron development •  Control interstitial environment

Astrocytes  

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Astrocytes

Astrocytes  

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3. Microglia

•  Migrate through neural tissue •  Clean up cellular debris, waste products,

and pathogens •  Not of neural origin; related to

macrophages (like osteoclasts)

Microglia

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Microglia  

4. Oligodendrocytes

•  Processes contact other neuron cell bodies •  Wrap around axons to form myelin sheaths

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Oligodendrocytes

1. Ependymal Cells

•  Form epithelium called ependyma •  Line central canal of spinal cord and

ventricles of brain: – secrete cerebrospinal fluid (CSF) – have cilia or microvilli that circulate CSF – monitor CSF – contain stem cells for repair

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Ependymal Cells

Ependymal Cells  

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1. Schwann Cells

•  Form myelin sheath around peripheral axons (nerves)

•  1 Schwann cell sheaths 1 segment of axon: – many Schwann cells sheath entire axon

Schwann Cells

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Schwann Cells  

Schwann Cells  

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Schwann Cells (“Unmyelinated”)  

1. Satellite Cells    

•  Surround ganglia •  Regulate environment around neuron

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Satellite Cells

Satellite Cells  

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Neuron

Neural Cells  

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Purkinje Cells

Interneurons •  Most are located in brain, spinal cord, and

autonomic ganglia: – between sensory and motor neurons

•  Are responsible for: – distribution of sensory information – coordination of motor activity

•  Are involved in higher functions: – memory, planning, learning

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Some questions •  Is the Cerebellum part of the Brain Stem? •  Not really (similar location) •  Into what muscles are Injections given? •  Gluteus minimus, Deltoid, Vastus lateralis,

Ventrogluteal (Dorsalgluteal – “avoid”) •  Association Areas vs. Interneurons? •  associaKon  areas  of  the  brain  are  important  because  they  

integrate  incoming  (sensory)  informaKon  coming  into  various  parts  of  the  sensory  cortex,  they  also  compare  it  with  exisKng  memory  of  the  same  sensaKons;  the  motor  associaKon  areas  refine  movements  by  coordinaKng  the  signals  from  various  parts  of  the  motor  cortex  before  iniKaKng  the  movement.

Cerebellum

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Association Areas

Interneuron