Neural Pathways Of The Brain Essay Research

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 vestibula

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