Neural Pathways Of The Brain Essay, Research Paper
Everyday, we rely on our five senses: taste, touch, seeing, hearing, and smelling.
These are simple processes that we have accepted as part of our lives. However, the
process which lets our body actually use these senses is not so simple. In this paper I will
look at the ways which the nervous system and the brain work together to produce our
senses.
Seeing The Light
Humans are intensely visual animals. The eyes send millions of nerve signals every
second for analysis, interpretation, and recording. The nerve signals represent energy
transformed, or transducer, from the energy in light rays.
Inside the eye, light rays pass through the cornea and then the pupil in the center of
the iris. The fixed cornea and adjustable lens bend light rays to focus a clear image onto
the retina. As with an equivalent man-made lens, the image is inverted. But since this is the
case from birth, we never know any different, and so it is not a problem.
The retina has two types of light sensitive cells- rods and cones. The 125 million
rods detect shades of black and white. The 5-7 million cones detect color and fall into 3
types, each of which is most sensitive to one of the primary colors of light: red, blue, and
green. Most cones are in the center of the retina, especially in the fovea- a rod-free area
where vision is the sharpest. Rods, and some cones, are found in the rest of the retina
(Greenfield, 197).
Each rod cell is about 150-200 micrometers long. at one end, next to a layer of
pigment cells and facing the outside of the eyeball, it has a stack of disks studded with the
chemicals that play a part in transducing light energy. Toward its other end, it has a
nucleus and other standard cell parts, such as a mitochondria. At its bas, it connects with
dendrites of intermediary cells which link it to the retinal ganglion cells (Gregory, 96)
These send nerve signals to the brain, and their axons for the optic nerve. Each retina has a
blind spot, a receptor-free area where all the axons leave the eye.
Before light can reach rods and cones at the rear layer of the retina, it travels
through several other layers. These are made up of blood vessels and the so-called neural
cells of the retina- bipolar, horizontal, and ganglion cells (Gregory,98). Behind the rods
and cones is a layer of pigment cells which absorb any stray light and prevent it from
reflecting back to the retina.(See Figure 1)
Nerve signals from the 1 million ganglion cells in the retina of each eye pass along
the optic nerve to a half-crossover junction- the optic chiasma. The signals continue along
the optic tracts to paired parts of the thalamus known as lateral geniculate nuclei, or LCNs
(Kellog,57). They then continue along fan-shaped optic radiations to their main
destination, the visual cortex of each occipital lobe. These are sited at the lower central
back of the cerebrum.
The visual cortices are sight centers concerned with decoding and analyzing the
nerve signals from the retinal ganglion cells. Each region of visual cortex has a number, so
the primary visual cortex, the main reception area for visual signals is V1. The activities of
V1 s patchwork of neurons represents a pattern of signals sent in by the retinal ganglion
cells. Around it in the secondary visual cortex are regions V2, V3, etc (Kellog,59) They
sort the various aspects of vision, such as shape and form, color, contrast, distance and
depth, and movement or motion. The results are recombined as these cortical areas
communicate with other parts of the cerebral cortex-mainly the temporal lobe- plus the
language centers and other areas. By such interactions we become aware of the color,
shape, motion, distance, identity and meaning of what we see. (See Figure 2)
Sensing Sound
A sound that hits the ear takes a split second to register in the mind, but the
journey is long and complex.
Sound waves funneled into the ear canal bounce off the ear drum, making it
vibrate. The vibrations pass via the three tiny bones, the auditory ossicles- the malleus,
incus, and stapes- and from them to the oval window, another thin membrane which is part
of the wall of the fluid-filled cochlea. Different parts of the cochlea detect different
pitches-generally low-pitched sounds near its thin tip and shrill sounds at its wider base.
Inside the cochlea on the scala media, is the organ of Corti. Its hairs are arranged in two
rows on the basilar membrane. The hair tips contact the jellylike tectorial membrane. As
pressure waves shake the whole structure, the hairs move, making their cells fire nerve
signals. Nerve impulse pass from the hair cells along some 30,00 axons to the neurons in
the cochlear nerve. This runs beside the vestibular nerve from the ear s balance system as
the vetisbulocochlear nerve (Greenfield, 120). This nerve branched back into vestibular
and cochlear parts as it joins the medulla. The cochlear nerve fibers end in two nuclei on
their side of the medulla. Signals go from the cochlear nuclei to the superior olivery nuclei
on both sides in the medulla, so each side of the auditory cortex receives information from
both ears. The medial superior olive processes information about the localization of sound
to reach each ear. The intensity of the sound is processed by the lateral superior olive
(Restak, 257). This information enables us to tell where the sound is coming from. From
the superior olivery nuclei, more neurons carry the signals in a nerve tract up through the
pons. Some of them relay the signals to the inferior colliculus, which collects data from the
cochlear nuclei and the superior olivery nucleui, helping us recognize the location and
nature of sound. Others bypass and go straight to the thalamus, which focuses attention,
editing out extraneous sounds (Restak, 258). From the medial geniculate nucleus of the
thalamus, nerve signals are sent to the cerebral cortex along fan-shaped sets of fibers
known as auditory radiations. The primary auditory cortex- on the side of the brain- is the
main incoming and processing area for the nerve signals that represent sounds. The
secondary auditory cortex has links with the primary cortex and other parts of the brain, to
coordinate hearing with memories, awareness and the other senses. In both auditory areas,
the arrangement of neurons is tonotopic. This means sounds of different frequencies
stimulate neurons in different rows or columns. The general arrangement is that neurons at
the front, toward the face, respond most to high-pitched sounds and those at the rear to
low-pitched sounds.(See Figures 3 and 4)
Sense of Touch
The detection of physical contact with the body might seem straightforward. But
the fact that we can not only perceive but also tell the difference between a gentle stroke
of the skin and a rough pinch suggests there is more to it. Touch, together with sensations
from within the body about the position and posture of various muscles, tendons, and
joints, makes up the somatosensory system.
Skin sensors ( or cutaneous exeroceptors, are microscopic structures embedded
mainly in the dermis. There are about 6 main kinds and their distribution varies over the
body, from the lips and fingertips to the small of the back (Montagu, 66). Then stimulated
by mechanical distortion or thermal change, these sensors produce nerve signals that are
transmitted along axons. The axons gather into the peripheral nerves. These join the spinal
cord at dorsal nerve roots, and the signals are conveyed along the cord up to the brain.
Signals from touch sensations on the head and face are carried directly to the brain by the
sensory branches of the trigeminal nerves (cranial nerves).
In the brain, information about touch arrives at the somatosensory cortex, a strip
across the top of each hemisphere, just behind the motor cortex. Here it is analyzed and,
after further processing in the brain s association areas, details about the type of touch
enter our conscious awareness(Montagu, 76). (See Figure 5)
A Matter of Taste
Food without flavor would be like the beach without sunshine, so how do we taste
what we eat??
Taste, like smell, is a chemosense- it detects the presence of certain chemicals. The
individual tasters are chemosensory receptor cells, or chemosensors. They are shaped
like segments of an orange and are grouped with supporting cells known as a taste bud
(Kellog, 75).
The specialized chemosensors are short-lived, lasting only 10 days, but they are
replaced within 12 hours. Each taste bud is embedded in the covering layer, or epithelium,
with a small hole that opens to the surface. Dissolved chemicals from food and drink seep
through the hole to the chemosensors, whose tips have tufts of tiny hairs that detect
chemicals (Kellog,76).
Taste signals from the chemoreceptor cells on each side of the tongue and back of
the mouth travel along three pairs of nerves to the brain. From the front two-thirds of the
tongue, they go along a branch of the facial nerve; from the rear third, their rout is via the
lingual branch of the glossopharyngeal nerve; and from the palate and upper throat, it is
along the superior laryngeal branch of the vagus nerve (Greenfield, 54).
All the signals arrive in a region known as the nucleus solitarius in the medulla.
They then pass along more fibers tot the thalamus- the brain s relay station. This sends
signals tot he primary and secondary gustatory areas near the somatosensory area of the
cerebral cortex. Nerve fibers also connect the taste system to the hypothalamus and the
limbic system. This is why taste can affect feelings of hunger and mood (Greenfield, 55).
On the Scent
Like I mentioned before, like taste, smell is a chemosense: it detects the presence
of chemicals, in this case odors or odor molecules.
From the first sniff to recognition, an odor passes along many pathways: through
the olfactory system- from nose to olfactory cortex- to limbic system, thalamus, and
frontal cortex.
In, the olfactory bulb, nerve impulses from olfactory cells enter one of hundreds of
olfactory glomeruli- small ball-like tangles of axons, synapses, dendrites, and cell bodies.
Next the signal goes along the olfactory tract to the secondary olfactory cortex. The
anterior olfactory nucleus links the bulbs from the 2 nostrils via the anterior commisure.
The olfactory tubercle and the pyriform cortex project to other olfactory cortical regions
and to the medial dorsal nucleus of the thalamus. They are involved in conscious
perception of smell. The last two, with the amygdaloid complex and the entorhinal area,
which in turn projects to the hippocampus, are pathways to the limbic system, which is
why smells evoke memories and emotions.
Acting on signals from a region of about 25,000 olfactory receptor cells in the
nose, each glomerulus in the olfactory bulb react to certain odors (Greenfield, 24-26).
Messages are relayed from one glomerulus to the next, probably by periglomerular
cells, and a pattern of activity. For this information, which is in the form of a burst of
nerve signals, to get through to the rest of the brain, it has to be strong enough to survive
the inhibitory effects of cells, which form the olfactory tract, into the olfactory cortex.
Here the signals have to pass through an intermediate layer of cells, the superficial
pyramidal cells. They synapse with and excite stellate cells, as well as deep pyramidal cells.
But when excited, the stellate cells inhibit the deep pyramidal cells, creating a loop of
excitation and inhibition that has the effect of generating bursts of nerve signals which are
then transmitted to other brain regions (Greenfield, 27). (See Figure 6)
Bibliography
Greenfield, Susan A. The Human Mind Explained.
New York: Henry Holt and Co., 1996
Gregory, Richard L. Eye and Brain: The Psychology of Seeing.
Princeton, New Jersey: Princeton University Press, 1997.
Kellog, Ronald T. Cognitive Psychology.
London: Sage Publishers, 1995.
Montagu, Ashley. Touching: The Human Significance of the Skin. New York: Colombia
University Press, 1971.
Restak, Richard M.,MD. The Mind.
New York: Bantam Books, 1988.