Page 83 - 84 human physiolofy part-1

Senses

The supporting cells are mostly arranged like the
staves of a cask, and form an outer envelope for the
bud. Some, however, are found in the interior of the
bud between the gustatory cells. The gustatory cells
occupy the central portion of the bud; they are spindle-
shaped, and each possesses a large spherical nucleus
near the middle of the cell. The peripheral end of the
cell terminates at the gustatory pore in a fine hair-like
filament, the gustatory hair.

The central process passes toward the deep
extremity of the bud, and there ends in single or
bifurcated varicosities.

The nerve fibrils after losing their medullary
sheaths enter the taste bud, and end in fine extremities
between the gustatory cells; other nerve fibrils ramify
between the supporting cells and terminate in fine Semidiagrammatic view of a portion of the mucous
extremities; these, however, are believed to be nerves membrane of the tongue. Two fungiform papillæ are
of ordinary sensation and not gustatory. shown. On some of the filiform papillæ the epithelial
prolongations stand erect, in one they are spread out,
and in three they are folded in.
Types of Taste

Salt
Arguably the simplest receptor found in the mouth is the salt (NaCl) receptor. An ion channel in
the taste cell wall allows Na+ ions to enter the cell. This on its own depolarizes the cell, and
opens voltage-regulated Ca2+ gates, flooding the cell with ions and leading to neurotransmitter
release. This sodium channel is known as EnAC and is composed of three subunits. EnAC can be
blocked by the drug amiloride in many mammals, especially rats. The sensitivity of the salt taste
to amiloride in humans, however, is much less pronounced, leading to conjecture that there may
be additional receptor proteins besides EnAC that may not have been discovered yet.

Sour
Sour taste signals the presence of acidic compounds (H+ ions in solution). There are three
different receptor proteins at work in sour taste. The first is a simple ion channel which allows
hydrogen ions to flow directly into the cell. The protein for this is EnAC, the same protein
involved in the distinction of salt taste (this implies a relationship between salt and sour receptors
and could explain why salty taste is reduced when a sour taste is present). There are also H+
gated channels present. The first is a K+ channel, which ordinarily allows K+ ions to escape from
the cell. H+ ions block these, trapping the potassium ions inside the cell (this receptor is
classified as MDEG1 of the EnAC/Deg Family). A third protein opens to Na+ ions when a
hydrogen ion attaches to it, allowing the sodium ions to flow down the concentration gradient
into the cell. The influx of ions leads to the opening of a voltage regulated Ca2+ gate. These
receptors work together and lead to depolarization of the cell and neurotransmitter release.

Bitter
There are many classes of bitter compounds which can be chemically very different. It is
interesting that the human body has evolved a very sophisticated sense for bitter substances: we

Wikibooks | 83