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 [view large image]

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 body parts.
• 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 called a 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 form.
  Figure 22a Propriocep-tors [view large image]	Figure 22b Homunculus [view large image]

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 [view large image]

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

Figure 23b Neuromast [view large image]

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.

Memory

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 conservative over

Figure 24a Memory Classification [view large image]

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 our brain.

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 components,

Figure 24b Types of Memory [view large image]

locations, and pathways for many types of memory.

continue to TMemory types