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Nervous System

Senses :

Sight, Hearing, Smell, Taste, Sensations, Balance

Senses Cerebrum Mapping Senses organs receive external and internal stimuli; therefore, they are called receptors. Each type of receptor is sensitive to only one type of stimulus as listed Table 06, while Figure 09 shows many types of receptors. When a receptor is stimulated, it generated nerve impulses that are transmitted to the spinal cord and/or the brain, but we are conscious of a sensation only if the impulses reach the cerebrum.

Figure 09 Senses
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Figure 10 Cerebrum Mapping [view large image]

Receptor Type Sense Stimulus
Ruffini's endings, Krause end bulbs Radioreceptor Hot-cold Heat flow
Merkel's and Meissner's endings Mechanoreceptor Touch Mechanical displacement of tissue
Pacinian corpuscles Mechanoreceptor Pressure Mechanical displacement of tissue
Free nerve endings Chemoreceptor Pain Tissue damage
Proprioceptors Mechanoreceptor Limb placement Mechanical displacement
Eye Radioreceptor Sight Light
Ear Mechanoreceptor Hearing Sound wave
Olfactory cells Chemoreceptor Smell Chemicals
Taste buds Chemoreceptor Taste Chemicals

Table 06 Receptors

The general receptors distribute all over the skin. They are usually grouped together as sensation. The special receptors locate only at certain part of the body in the head. Altogether, they are referred to as the five senses. The followings present a further break down into components, and functions.

  • 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.
  • Sense of Taste
  • 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
  • Figure 21 Sense of Taste
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    and starches. Likewise, taste is is extremely sensitive to bitter flavors, because many poisonous berries, fruits and fungi are bitter-tasting.


      Balance is an ongoing process that keeps our two-legged posture stable. Four main sets of sensory input are involved:

    1. 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.
    2. Eyesight is used to judge verticals and horizontals to which your body should be parallel and at right angle respectively.
    3. 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.
    4. 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 23a:
    • (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 equilibrium.
    • (c) The utricle and saccule are sacs that contain hair cell with cilia embedded in the gelationous material.
    • (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.

    Figure 23a Balance
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    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.

    Neuromast 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

    Figure 23b Neuromast
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    found that the box jellyfish carries a gene which is the combination of Pax 2 and Pax 6.
    The 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.

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