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continued from the Brain Pageof
Neurons and Nerves
The Brain & Spinal Cord
Peripheral Nervous System
Autonomic Nervous System
Taste & Tongue Sensation,
Memory , Memory
types, Creation of Memory,
Continued from Brain
Continue to next page the cranial nerves
Papillae - The papillae are those small elevations visible to
the naked eyes. There are three types of papillae located from the
back of the tongue toward the tip. Filiform papillae are generally
conical or pointed; fungiform papillae are flat-toped; vallate
papillae are larger with an outer groove (see Figure 20). Many taste
buds lie along the walls of the papillae. Isolated ones also are
present on the palate, the pharynx, and the epiglottis.
- Continue to taste
Taste (see location of the various
components in Figure 09):
- Tongue - Embedded within the surface of the tongue are four
types of taste receptors localized in specific regions on the
||tongue (see Figure 19). Each detects a
different class of chemical: sweet (sugars), sour
(acids), bitter (complex organics), and salty (salts).
The "hot" sensation of foods such as chili peppers is
detected by pain receptors, not chemical receptors. But
a report in 2006 reveals that contrary to popular
belief, there is no tongue map. Responsiveness to the
five basic modalities - bitter, sour, sweet salty and
umami (a Japanese word
|meaning the savory or meaty taste of
amino acids) is present in all areas of the tongue.
Taste buds - The tasting, or gustatory, cells in the
buds have hairy tips which detect chemicals in solution
(secreted by the gland at the bottom of papilla). When
stimulated by flavor molecules, these cells generate nerve
signals, which they send to the taste center on the brain's
cortex, and also to the hypothalamus, which is concerned
with appetite and the salivating reflex.
Taste nerve pathway - The nerve signals are carried by
three nerves in each side of the tongue (cranial nerves) to
a small part of the medulla (brain stem). The signals then
travel to parts of the brain, such as the hypothalamus, the
thalamus, and the gustatory part of the sensory cortex - the
"taste center", where the signals are interpreted (Figure
21). The thalamus acts like a relay station, shunting the
data onto appropriate cortical areas for processing. The
sense of taste tells us what is good to eat. It evolved to
pick out sweet, ripe fruits and energy-packed sugars
||and starches. Likewise, taste is is
extremely sensitive to bitter flavors, because many
poisonous berries, fruits and fungi are bitter-tasting.
Sensations (see location of the
various components in Figure 09):
- Skin - Skin has a thin epidermis, which is mainly for protection, and
a thicker dermis below. In addition to small blood vessels and
sweat glands, it has tiny nerve endings in the various type of
touch receptors (see Figure 09).
- Receptors -
- Bulb of Krause - These are multi-layered capsules with
many branched nerve endings. They are quick-change
mechanoreceptors, triggered by rapid alterations in shap
caused by pressure or vibrations, and may also help us to
feel extreme cold.
- Free nerve endings - They have a treelike branching
system of naked nerve fibers. They are the most common
sensory endings in the skin and detect just about anything -
light touch, heavy pressure, heat, cold, and importantly,
pain. Slight stimulation of these nerve endings may elicit
the sensation that is known as itching.
- Meissner's endings - They are found in the uppermost
part of the dermis, especially on the hands, feet, lips, and
inner surfaces of the eyelids. They are shaped like eggs and
are both quick- and slow-change mechanoreceptors, detecting
light touch and vibrations.
- Merkel's endings - They are like tiny disks stuck in the
underside of the epidermis, where they feel slight changes
in its shape, thereby detecting light touch. They are both
quick- and slow-change mechanoreceptors.
- Pacinian endings - They have layers like an onion and
are sited deep in the dermis. They pick up heavy pressure
and also fast vibrations, such as those from a tuning fork.
- Ruffini endings - They respond to sustained stress or
gradually altering shape. This means that they are
slow-change mechanoreceptors. They are found mainly in hairy
skin and are sausage- or spindle-shaped. It is thought that
they may also detect extreme heat.
- Proprioceptors - The sense of position and movement of limbs
is dependent upon receptors termed proprioceptors (Figure 22a).
They are located in the joints and associated ligaments and
tendons that respond to stretching, pressure, and pain. Nerve
endings from these receptors are integrated with those received
from other types of receptors so that we know the position of
- Sensory nerves - Nerve impulses may reach the somatosensory
cortex for analysis before a response is decided. These result
in voluntary actions - a deliberate response. Sometimes the
stimulus require immediate action (such as from the burning
sensation), a reflex action is taken without the conscious
control of the brain. These are the involuntary actions directed
by the spinal cord. We only become aware of them when other
impulses are sent to the brain to "inform" what has happened.
The path which impulses travel along during a reflex action is
reflex arc. Not all the body parts
||receive the same attention of the brain.
The relative importance is often represented by mapping
over the length of the sensory or motor cortex. These
cortical maps (Figure 22b) are not drawn to scale;
instead they are variously distorted to reflect the
amount the neural processing power devoted to different
regions. This accounts for the grotesque appearance of
the human body in the homun-culus, which is a
translation of the body's sensory map into the human
Balance is an ongoing process that keeps our two-legged posture
stable. Four main sets of sensory input are involved:
- Information from the skin is important, especially from the touch and
pressure sensors on different parts of the feet, which tell the
brain if you are leaning. This sense is not available in a free
falling environment such as in a spacecraft.
- Eyesight is used to judge verticals and horizontals to which
your body should be parallel and at right angle respectively.
- The body's proprioceptive sense of stretch in muscles,
tendons, and joints tell the brain about the positions and
angles of the arms, legs, torso, and neck.
- The sensory parts dedicated to balance is located deep
inside each inner ear, next to the cochlea (see
Figure 09). These parts are known
collectively as the vestibular apparatus and are part of the
same network of fluid-filled chambers as the cochlea. They
consist of the utricle, the saccule, and the semicircular canals
(Figure 23a). In certain parts of their linings are tinny hairs,
whose roots are embedded in lumpy crystals or gels. The crystals
or gels are attracted downward by gravity, and they are also
pushed to and fro by the fluid in the chambers, which swirls as
the head changes its position.
The functions of the these organs are shown in Figure
- (a) The ampullae of the semicircular canals contain hair cells with
cilia embedded in a gelationous material.
- (b) When the head rotates, the material is displaced
and the bending of the cilia initiates nerve impulses in
sensory nerve fibers for maintaining dynamic
- (c) The utricle and saccule are sacs that contain
hair cell with cilia embedded in the gelationous
- (d) When the head bends, otoliths are displaced,
causing the gelationous material to sag and the cilia to
bend. This initiates nerve impulses in sensory nerve
fibers for maintaining static equilibrium.
|The vestibular nerve feeds its information
chiefly to the cerebellum and to four structures in the
medulla known as vestibular bodies. Using these data, as
well as input from the other three sensory sources, the
brain works out what to do, usually subconsciously.
||It turns out that such structure of hair
within gel to detect disturbance has been around hundred of
million years in the shark and fish (Figure 23b). This is
the neuromasts embedded in the skin of fish. They give the
fish information about the flow of water. Amphibians and
reptiles have a simple uncoiled inner ear. Jawless fish has
only one semicircular canal instead of three in mammals (for
detecting three dimensional movement). Ultimately, it is the
Pax 2 gene that give rise to these structures. It is
also known that the Pax 6 gene is responsible for the
development of eye. The connection to ancient creatures goes
even deeper when it is
|found that the box jellyfish carries a gene
which is the combination of Pax 2 and Pax 6.
box jellyfish is an amazing animal with more than 20 eye
pits and many eyes very similar to ours. They seem to double
for ears as well.
||As shown in Figure 03a,
the ability to modify our behaviour in response to life's
experiences is shared by all animals including the bacteria
E. coli. Such feat requires the brain's willingness to
learn. Learning results in the formation of memories and in
humans this process reaches its most sophisticated form,
allowing us creatively to associate different reflections on
the past, to generate new ideas, and most importantly to
acquire language as a medium of expression and
communication. Memory requires the brain to be physically
altered by experience and it is this remarkable property
that makes thought, consciousness, and language possible.
The basic mechanism of memory formation is highly
||billion years of biological evolution. The
difference in humans is that we have a lot more of the
stuffs. There are about 100 trillion synaptic connections in
||There are many ways to classify the memory.
The concept of explicit and implicit memory refers to
whether or not the recollection is produced consciously and
intentionally. While the scheme of declarative and
nondeclarative memory depend on the retrieval that can be
declared verbally or not. Associative memory is triggered by
clues; nonassociative memory can be habitual or sensitive.
There are also short term and long term memory. One of the
classification schemes is shown in Figure 24a. Table 06 is
an attempt to put them all together. In the table, the
declarative, and the procedural memory are explicit with the
rest of nondeclarative memories being implicit. Only the
working memory belongs to the category of short term memory
fading away in hours, while the others are long term, and
available for retrieval in years. Figure 24b shows the
||locations, and pathways for many types of
continue to TMemory types
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