Our Sneaky Senses

Objectives

• Describe how sound is perceived by the human body.

• Describe how the brain helps us create images from what we observe.

• Describe how the nervous system responds to a stimulus.

• Describe how the body detects and responds to different smells.

• Outline how the body’s taste buds detect different tastes.

• Identify and explore the five basic senses.

• Explore the connection between different senses.

• Investigate the body’s sense of touch.

Materials

• See individual activities for materials.

Background

Our senses are the physiological methods of perception. A broadly acceptable definition of a sense would be “a system that consists of a sensory cell type that responds to a specific kind of physical energy, and that corresponds to a defined region within the brain where the signals are received and interpreted”. Several senses have been identified. The five most familiar to us – sight, hearing, smell, taste, and touch were first defined by Aristotle (384 – 322 BC).

Our Current Understanding of the Senses

• Seeing (vision) describes the ability to detect light and interpret it as “sight”. [Neuroanatomists generally regard it as two senses, given that different receptors are responsible for the perception of colour (the frequency of light) and brightness (the energy of light)].
• Hearing (audition) is the sense of sound perception and results from tiny hair fibres in the inner ear detecting the motion of atmospheric particles within (at best) a range of 20 to 20000 Hz. Sound can also be detected as vibration by tactition. Lower and higher frequencies than can be heard are detected only this way.
• Taste (gustation) is one of the two “chemical” senses. The five well-known taste bud receptors detect sweet, salt, sour, bitter, and umami (savoury).
• Smell (olfaction) is the other “chemical” sense. Sensory chemoreceptors that respond to airborne chemicals are located in the olfactory epithelium — a patch of tissue about the size of a postage stamp located high in the nasal cavity. Olfactory neurons or nerve cells differ from most other neurons in that they die and regenerate on a regular basis.
• Touch (tactition) is the sense of pressure perception. The sense of touch is found all over because it originates in the bottom layer of the skin called the dermis. The dermis is filled with many tiny nerve endings that give you information about the things your body touches.

Other senses include:

• Thermoception is the sense of heat and the absence of heat (cold). How hot or cold something feels does not only depend on temperature, but also on specific heat capacity and heat conductance. For example, warm metal feels warmer than warm wood, and cold metal feels colder than cold wood, because metal has a higher thermal conductivity than wood.
• Nociception is the perception of pain. The three types of pain receptors are cutaneous (skin), somatic (joints and bones) and visceral (body organs).
• Equilibrioception is the perception of balance and is related to cavities containing fluid in the inner ear.
  
• Proprioception is the perception of body awareness and is a sense that people rely on enormously, yet are frequently not aware of. More easily demonstrated than explained, this sense is the “unconscious” awareness of where the various regions of the body are located at any one time.

Why are our senses important?

What our senses do for us is allow us to react to our surroundings. They tell us what is out there, where it is, how much there is and whether it changed in some respect. So most of what we think day-to-day is based on perceptions that have come through our sensory systems.

Sight

From the moment you wake up in the morning to the time you go to sleep at night, your eyes are acting like a video camera. Everything you look at is then sent to your brain for processing and storage much like a data file. This is a very simplified explanation, but the sense of sight is actually considered the most complex of the five senses.

When you look at an object what you are actually seeing are beams of light bouncing off of the object and into your eyes. Light enters our eyeballs through the pupil. The colorful circle around the pupil is called the iris. It controls how much light is let in by making your pupil bigger or smaller. Since too much light can actually hurt our eyes, our irises will make our pupils shrink up to tiny dots on bright, sunny days. After the pupil, light goes through the lens, which functions like the lens of a camera or telescope. The lens focuses the light onto the back of the eyeball. This part is called the retina. The retina has a lot of special nerve cells that sense the light and carry signals to the brain to let us know what we’re seeing.

When images are focused on your retina, your lens turns them upside down. So, for you to see properly, your brain has to turn them the right way up again. Your brain also needs to merge the two slightly different images captured by each of your eyes into one. By doing so, your brain creates a 3D picture. A newborn baby sees the world upside down because it takes some time for the baby’s brain to learn to turn the picture right-side up.

Smell and taste

Odour and food molecules activate membrane receptors in our nose and tongue. The complicated processes of smelling and tasting begin when molecules detach from substances and float into noses or are put into mouths. In both cases, the molecules must dissolve in watery mucous in order to bind to and stimulate special receptor cells. These cells transmit messages to brain areas where we perceive odours and tastes, and where we remember people, places, or events associated with these smell and taste sensations. The neural systems for these two chemical senses can distinguish thousands of different odours and flavors!

Although the neural systems for taste and smell are distinct from one another, the sensations of flavors and aromas often work together, especially during eating. Much of what we normally describe as flavor comes from food molecules wafting up our noses.

Another similarity between these systems is the constant turnover of receptor cells. After about 10 days, taste receptor cells die and are replaced by cells that differentiate from a sort of stem cell in the taste bud. More surprising is the story of smell sensory cells. These are not epithelial cells like taste cells, but neurons, which until recently were not known to be generated in adults. These neurons are not only replaced every 60 days or so, but each must also grow an axon to the correct place in the brain. Researchers are investigating how taste perception and odour recognition are maintained in the face of this turnover and new axon growth.

Hearing

When something makes a noise, it sends vibrations, or sound waves, through the air. The human eardrum is a stretched membrane, like the skin of a drum. When the sound waves hit your eardrum, it vibrates and a chain reaction is set off. Your eardrum sends the vibrations to the three smallest bones in your body. First the hammer, then the anvil, and finally, the stirrup. The stirrup passes those vibrations along a coiled tube in the inner ear called the cochlea. Inside the cochlea there are thousands of hair-like nerve endings called cilia. When the cochlea vibrates, the cilia move. Your brain is sent these messages (translated from vibrations by the cilia) through the auditory nerve. Your brain then translates all that information and tells you what you are hearing.

Touch

The sense of touch develops in embryos before all other senses and is the main way in which infants learn about their environment and bond with other people. This sense never turns off or takes a break, and it continues to work long after the other senses fail in old age. Compared to the other senses, touch is very hard to isolate because tactile sensory information enters the nervous system from every single part of the body.

Our skin acts as the protective barrier between our internal body systems and the outside world, but also has the ability to perceive touch sensations, giving our brains a wealth of information about the environment around us, such as temperature, pain, and pressure. Our sense of touch is controlled by a huge network of nerve endings and touch receptors in the skin known as the somatosensory system. This system is responsible for all the sensations we feel – cold, hot, smooth, rough, pressure, tickle, itch, pain, vibrations and more. Within the somatosensory system, there are four main types of receptors: mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors.

The sensations felt by the somatosensory system then need to reach the brain and the nervous system of the body takes up this important task. Neurons, which are specialized nerve cells that are the smallest unit of the nervous system, receive and transmit messages with other neurons so that messages can be sent to and from the brain. This allows the brain to communicate with the body.

For instance, when your hand touches an object, the mechanoreceptors in the skin are activated, and they start a chain of events by signaling to the nearest neuron that they touched something. This neuron then transmits this message to the next neuron which gets passed on to the next neuron and on it goes until the message is sent to the brain. Now the brain can process what your hand touched and send messages back to your hand via this same pathway to let the hand know if the brain wants more information about the object it is touching or if the hand should stop touching it.

Vocabulary

Audition: sense of hearing

Binocular vision: where visual fields overlap

Blind spot: an area in the eye containing no cone or rod cells where the optic nerve connects to the retina

Depth perception: the ability to judge objects that are nearer or farther than others

Equilibrioception: perception of balance

Gustation: sense of taste

Limbic area: primitive part of the brain thought to be connected with emotion

Neural pathway: connect sone part of the nervous system with another, usually via neurons

Neuron: a nerve cell that processes and transmits information by electrical and chemical signaling

Nociception: perception of pain

Obscertainer: a container that obscure its contents

Olfaction: sense of smell

Perception: the process of attaining awareness or understanding of the environment by organizing and interpreting sensory information

Proprioception: sense one’s own position and movement

Receptor: a molecule most often found on the surface of a cell, which receives chemical signals originating externally from the cell.

Somatosensory: a vast network of nerve endings and touch receptors in the skin

Tacitation: sense of touch

Tactile: relating to the sense of touch

Taste bud: located in the mouth, they contain the receptors for taste

Thermoception: sense of heat and cold

Vestibular system: the sensory system that provides the leading contribution about movement and sense of balance

Vision: sense of sight

Other Resources

Southwest Educational Development Laboratory | Five Senses Lesson Plan

Social Issues Research Centre | The Smell Report

The Children’s University of Manchester | The Brain and Senses

University of Washington Education | Neuroscience for Kids | Blind Spot Testers

University of Washington Education | Neuroscience for Kids | Interactive Reaction Time Tester