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

Figure 15.23c

Taste fibersof cranialnerve

Connectivetissue

Gustatory(taste) cells

Tastepore

Gustatoryhair

Stratifiedsquamousepitheliumof tongue

(c) Enlarged view of a taste bud.

Basalcells

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

Figure 15.21a

Olfactory tract

Olfactory bulb

(a)

Nasalconchae

Route ofinhaled air

Olfactoryepithelium

Figure 15.23a

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

Fungiform papillae

Epiglottis

Palatine tonsil

Foliate papillae

Lingual tonsil

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.

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 posteromedialnucleus)

PonsSolitary nucleus inmedulla oblongata

Facial nerve (VII)

Glossopharyngealnerve (IX)

Vagus nerve (X)

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).

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.

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?

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.

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?)

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: ?

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

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

Somatosensory Cortex

Figure 5.1

Epidermis

Hair shaft

Dermis Reticularlayer

Papillarylayer

Hypodermis(superficial fascia)

Dermal papillae

Pore

Subpapillaryvascular plexus

Appendagesof skin • Eccrine sweat gland• Arrector pili muscle• Sebaceous (oil) gland• Hair follicle• Hair rootNervous structures

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

Cutaneous vascularplexus

Adipose tissue

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

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'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

Meissner's corpuscles

Meissner's corpuscles

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.

Pacinian corpuscles

Pacinian corpuscles

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

Merkel's discs

Ruffini endings

Ruffini endings

Non-hairy vs. hairy Receptors

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

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

Astrocytes

Astrocytes

3. Microglia

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

and pathogens• Not of neural origin; related to

macrophages (like osteoclasts)

Microglia

Microglia

4. Oligodendrocytes

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

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

Ependymal Cells

Ependymal Cells

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

Schwann Cells

Schwann Cells

Schwann Cells(“Unmyelinated”)

1. Satellite Cells

• Surround ganglia• Regulate environment around neuron

Satellite Cells

Satellite Cells

Neuron

Neural Cells

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

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?• association areas of the brain are important because they

integrate incoming (sensory) information coming into various parts of the sensory cortex, they also compare it with existing memory of the same sensations; the motor association areas refine movements by coordinating the signals from various parts of the motor cortex before initiating the movement.

Cerebellum

Association Areas

Interneuron