lose up, cross section view of a mammalian tongue, with taste buds Image: © 2011 iStockphoto

There are four basic sensations of taste: sweet, bitter, sour and salty. Recently a fifth taste was identified. Umami is the perception of “meaty” or “brothy” taste based on the glutamate molecule.

Humans have between 50 and 10,000 taste buds located on the surface of the tongue and surrounding areas inside the mouth. Each taste bud has around 20 receptors designed to bond with chemical particles of food dissolved in saliva and stimulate neurons leading through the thalamus to the cortex.

The ability to taste is an innate ability; however, taste preferences are determined by the foods available within a culture. In Western European culture it is perfectly acceptable to eat cows and chickens. Would we eat goat blood or whale blubber? In spite of their foreign nature, these are considered delicacies in other parts of the world.

While taste buds function to detect chemicals, the perception of flavor is also influenced by our sense of smell.


iagram: cross section of human head with olfactory bulb and nerves h Image © 2006: Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist, via Wikimedia Commons.

Similar to our taste receptors, our sense of smell (olfaction) is based on chemical detection. However, olfaction is the detection of particles in the air rather than in our foods.

Olfaction  is special in that it is the only sensation system not routed through the thalamus. Chemicals carried in the air pass into the nasal cavity and are dissolved in the mucous lining. Hair cells called olfactory cilia bind with the chemicals and stimulate cells in the  olfactory bulb , triggering neurons in the cortex for processing.

Skin Sensations

iagram: cross section of human skin with labels: skin layers, somest Image: © 2011 iStockphoto

The skin is the largest organ in the human body and the first line of defense against bacterial infection. Likewise, the skin is infused with  somesthetic  receptors designed to detect pressure, warmth, cold, and pain, amongst other less understood stimuli.

Pain is our warning system when our body is in immediate danger. Pain signals appear to travel along two pathways to the brain for perception. The first is a quick message resulting in immediate sharp sensation, like the focused sting when you first cut yourself. The second signal is slower and results in an enduring burning sensation surrounding the afflicted area.

The perception of pain can be influenced by the individual’s expectations, experiences, mood, and other cognitive processes. Even something as simple as distracting one’s attention can reduce the sensation of pain. Pain perception’s subjective nature suggests a complex system of signals coming from the damaged area as well as ways to block these signals. Gate-control theory proposes that pain impulses pass through a hypothetical passageway in the spinal cord in route to the brain. This gate can inhibit pain signals by confusing them with impulses from the brain or those from surrounding sensory receptors. (That is why a wound hurts less when you rub the area around it. Those messages disrupt the pain signals at the spinal cord gate.)


oman in yoga pose. © 2010 Jupiterimages Corporation

Close your eyes and position your arms into an outstretched pose.

How do you know where your arms are if you cannot see them?

The  kinesthetic system  monitors the body’s positions in space. Receptors in the joints and muscles relay information to the brain regarding the body’s posture and positioning. These messages follow the same pathways as tactile sensory signals, but they are two distinct systems.

Vestibular Sense

iagram of the vestibular system, with the semicircular canals, cochl © 2010 NASA

Perception of balance and equilibrium are overseen by the  vestibular sense . Located in the inner ear, displaced fluid in the semicircular canals stimulates hair cells that send messages to the brain as the head is repositioned. Imagine these tubes to be like a water bottle. Anytime the bottle is shifted, the water makes contact with different areas of the bottle.

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