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The central nervous system contains around 1011 nerve cells and 10 times as many glia cells such as oligodendrocytes, astrocytes, ependy-mal cells, and microglia A). Oligodendrocytes (ODC) form the myelin sheath that surrounds axons of the CNS (^ A).

Astrocytes (AC) are responsible for extracellular K+ and H+ homeostasis in the CNS. Neurons release K+ in response to high-frequency stimulation (^ B). Astrocytes prevent an increase in the interstitial K+ concentration and thus an undesirable depolarization of neurons (see Nernst equation, Eq. 1.18, p. 32) by taking up K+, and intervene in a similar manner with H+ ions. Since AC are connected by gap junctions (^ p. 16ff.), they can transfer their K+ or H+ load to nearby AC (^ B). In addition to forming a barrier that prevents transmitters from one synapse from being absorbed by another, AC also take transmitters up, e.g. glutamate (Glu). Intracellular Glu is converted to glu-tamine (GluNH2), then transported out of the cell and taken up by the nerve cells, which convert it back to Glu (transmitter recycling; ^ B).

Some AC have receptors for transmitters such as Glu, which triggers a Ca2+ wave from one AC to another. Astrocytes are also able to modify the Ca2+ concentration in the neuronal cytosol so that the two cell types can "communicate" with each other. AC also mediate the transport of materials between capillaries and neurons and playan important part in energy homeostasis of the neurons by mediating glycogen synthesis and breakdown.

During embryonal development, the long processes of AC serve as guiding structures that help undifferentiated nerve cells migrate to their target areas. Glia cells also play an important role in CNS development by helping to control gene expression in nerve cell clusters with or without the aid of growth factors such as NGF (nerve growth factor), BDGF (brain-derived growth factor), and GDNF (glial cell line-derived neurotropic factor). GDNF also serves as a trophic factor for all mature neurons. Cell division of glia cells can lead to in scarring (epileptic foci) and tumor formation (glioma).

Immunocompetent microglia (^A) assume many functions of macrophages outside the CNS when CNS injuries or infections occur (^ p. 94ff.). Ependymal cells line internal hollow cavities of the CNS (^ A).

Gustatory pathways. The taste buds (^ D) consist of clusters of 50-100 secondary sensory cells on the tongue (renewed in 2-week cycles); humans have around 5000 taste buds. Sensory stimuli from the taste buds are conducted to endings of the Vllth, IXth and Xth cranial nerves, relayed by the nucleus tractus solitarii, and converge at a high frequency on (a) the postcentral gyrus via the thalamus (^ p. 323 B, "tongue") and (b) the hypothalamus and limbic system via the pons (^ C).

The qualities of taste distinguishable in humans are conventionally defined as sweet, sour, salty, and bitter. The specific taste sensor cells for these qualities are distributed over the whole tongue but differ with respect to their densities. Umami, the sensation caused by monosodium-L-glutamate (MSG), is now classified as a fifth quality of taste. MSG is chiefly found in protein-rich foods.

Taste sensor cells distinguish the types of taste as follows: Salty: Cations (Na+, K+, etc.) taste salty, but the presence of anions also plays a role. E.g., Na+ enters the taste sensor cell via Na+ channels and depolarizes the cell. Sour: H+ ions lead to a more frequent closure of K+ channels, which also has a depolarizing effect. Bitter: A family of > 50 genes codes for an battery of bitter sensors. A number of sensory proteins specific for a particular substance are expressed in a single taste sensor cell, making it sensitive to different bitter tastes. The sensory input is relayed by the G-protein a-gustducin. No nuances but only the overall warning signal "bitter" is perceived. Umami: Certain taste sensor contain a metabotropic glutamate receptor, mGluR4, the stimulation of which leads to a drop in cAMP conc.

Taste thresholds. The threshold (mol/L) for recognition of taste stimuli applied to the tongue is roughly 10-5 for quinine sulfate and saccharin, 10-3 for HCl, and 10-2 for sucrose and NaCl. The relative intensity differential threshold Al/l (^ p. 352) is about 0.20. The concentration of the gustatory stimulus determines whether its taste will be perceived as pleasant or unpleasant (^ E). For the adaptation of the sense of taste, see p. 341 C.

Function of taste. The sense of taste has a protective function as spoiled or bitter-tasting food (low taste threshold) is often poisonous. Tasting substances also stimulate the secretion of saliva and gastric juices (^ pp. 236, 242).

J Astrocytes

J Astrocytes

Ependymal Cells

Ependymal cells

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Ependymal cells

Postcentral gyrus

Postcentral gyrus

Insulo Thalamus: Nucleus ventralis posteromedials Hypothalamus

N. petrosus major Chorda tympani

Insulo Thalamus: Nucleus ventralis posteromedials Hypothalamus

N. petrosus major Chorda tympani

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How To Reduce Acne Scarring

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