Page 351 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Glia Sense of Taste
The central nervous system contains around Gustatory pathways. The taste buds (! D) con-
11
10 nerve cells and 10 times as many glia cells sist of clusters of 50–100 secondary sensory
such as oligodendrocytes, astrocytes, ependy- cells on the tongue (renewed in 2-week cy-
mal cells, and microglia (! A). Oligodendro- cles); humans have around 5000 taste buds.
cytes (ODC) form the myelin sheath that sur- Sensory stimuli from the taste buds are con-
rounds axons of the CNS (! A).
ducted to endings of the VIIth, IXth and Xth
Central Nervous System and Senses rons release K in response to high-frequency (a) the postcentral gyrus via the thalamus
Astrocytes (AC) are responsible for extra-
cranial nerves, relayed by the nucleus tractus
cellular K and H homeostasis in the CNS. Neu-
solitarii, and converge at a high frequency on
+
+
+
stimulation (! B). Astrocytes prevent an in-
(! p. 323 B, “tongue”) and (b) the hypotha-
+
crease in the interstitial K concentration and
lamus and limbic system via the pons (! C).
The qualities of taste distinguishable in
thus an undesirable depolarization of neurons
(see Nernst equation, Eq. 1.18, p. 32) by taking
humans are conventionally defined as sweet,
up K , and intervene in a similar manner with
sour, salty, and bitter. The specific taste sensor
+
+
cells for these qualities are distributed over the
H ions. Since AC are connected by gap junc-
tions (! p. 16ff.), they can transfer their K or
whole tongue but differ with respect to their
+
+
is
now
ing a barrier that prevents transmitters from
(MSG),
monosodium-L-glutamate
one synapse from being absorbed by another,
classified as a fifth quality of taste. MSG is
chiefly found in protein-rich foods.
AC also take transmitters up, e.g. glutamate
12 H load to nearby AC (! B). In addition to form- densities. Umami, the sensation caused by
(Glu). Intracellular Glu is converted to glu-
tamine (GluNH 2), then transported out of the Taste sensor cells distinguish the types of taste as
+
+
follows: Salty: Cations (Na , K , etc.) taste salty, but
cell and taken up by the nerve cells, which con- the presence of anions also plays a role. E.g., Na en-
+
vert it back to Glu (transmitter recycling; ! B). ters the taste sensor cell via Na channels and
+
+
depolarizes the cell. Sour: H ions lead to a more
Some AC have receptors for transmitters such as frequent closure of K channels, which also has a
+
Glu, which triggers a Ca 2+ wave from one AC to depolarizing effect. Bitter: A family of ! 50 genes
another. Astrocytes are also able to modify the Ca 2+ codes for an battery of bitter sensors. A number of
concentration in the neuronal cytosol so that the two sensory proteins specific for a particular substance
cell types can “communicate” with each other. AC are expressed in a single taste sensor cell, making it
also mediate the transport of materials between sensitive to different bitter tastes. The sensory input
capillaries and neurons and play an important part in is relayed by the G-protein α-gustducin. No nuances
energy homeostasis of the neurons by mediating gly- but only the overall warning signal “bitter” is per-
cogen synthesis and breakdown. ceived. Umami: Certain taste sensor contain a
During embryonal development, the long metabotropic glutamate receptor, mGluR4, the
processes of AC serve as guiding structures that help stimulation of which leads to a drop in cAMP conc.
undifferentiated nerve cells migrate to their target
areas. Glia cells also play an important role in CNS Taste thresholds. The threshold (mol/L) for
development by helping to control gene expression recognition of taste stimuli applied to the
in nerve cell clusters with or without the aid of tongue is roughly 10 – 5 for quinine sulfate and
growth factors such as NGF (nerve growth factor), saccharin, 10 – 3 for HCl, and 10 – 2 for sucrose
BDGF (brain-derived growth factor), and GDNF (glial and NaCl. The relative intensity differential
cell line-derived neurotropic factor). GDNF also threshold ∆I/I (! p. 352) is about 0.20. The
serves as a trophic factor for all mature neurons. Cell concentration of the gustatory stimulus deter-
division of glia cells can lead to in scarring (epileptic
foci) and tumor formation (glioma). mines whether its taste will be perceived as
Immunocompetent microglia (! A) assume pleasant or unpleasant (! E). For the adapta-
many functions of macrophages outside the CNS tion of the sense of taste, see p. 341 C.
when CNS injuries or infections occur (! p. 94ff.). Function of taste. The sense of taste has a
Ependymal cells line internal hollow cavities of the protective function as spoiled or bitter-tasting
CNS (! A). food (low taste threshold) is often poisonous.
338 Tasting substances also stimulate the secretion
of saliva and gastric juices (! pp. 236, 242).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.