Our Senses

~ Kim Vander Vegte ~

Our senses are controlled by five main organs: the ears, eyes, tongue, nose and skin. On this page the emphasis is on the main ideas and overview of the sensory system. There will be more specific focus on vision and hearing on other pages but here there will be some focus on taste, smell, and touch.

Action potential

Action potentials are created by neurons when a stimuli is present. This is caused by sodium and potassium ion flow. The stronger the stimuli is, the more action potential will be created. This idea is known as rate coding. So a very strong smell will have a faster rate of action potential than a weak smell. Our senses sense changes in stimuli which means that if you are in the presence of a strong smell and you stay there for a long time you get use to it and no longer notice that it is present.

Sensory Receptors

There are five main types of sensory receptors:

1. Mechanoreceptors: detect mechanical energy (pressure)
2. Photoreceptors: detect light energy
3. Chemoreceptors: detect chemical levels and molecules
4. Thermoreceptors: detect thermal energy
5. Nociceptors: detect pain

1. MECHANORECEPTORS

Mechanoreceptors are used mainly in the ears and the skin. The ear uses them to detect sound, and maintain balance. They are located in the inner ear. In the skin there are two ways that they are used. First, for sensing movement on the skin and second for feeling pressure on the skin. The first is immediate and the second lasts as long as the object is present.

2. PHOTORECEPTORS

Photoreceptors are found in the retina of the eye. They are cells that capture light and send signals through the optic nerve to the brain. There are two different types: cone and rod.

Cone: enable us to see colour and fine details, work better in higher lighting
Rod: enable us to see changes in lighting and movement, work more in lower lighting

3. CHEMORECEPTORS

Chemoreceptors are located in our taste buds and an organ in our nose. On our tongue there are different receptors for different tastes: good or bad, sweet or sour... etc. The taste bud will release a different signal for each chemical taste that activates it. In our nose the chemoreceptors work in a similar way to recognize smells and send signals to the brain.

4. THERMORECEPTORS

Thermoreceptors monitor temperature in the skin. When there is a higher concentration of thermoreceptors, there is a higher sensitivity to temperature change. For example: our palms are more sensative to heat than our thighs which means there is a higher concentration of thermoreceptors in the palm than the thigh. There are two different types of thermoreceptors: ones that detect heat and ones that detect the cold. At 52 degrees Celsius and above and 8 degrees Celsius and below instead of the thermoreceptors picking up hot or cold signals the nociceptors detect it as pain.

5. NOCICEPTORS

Nociceptors are pain receptors which respond to damaged stimuli and translate it to the brain as pain. Unlike other receptors which are used for Homeostasis, these are instinctive to protect oneself from immediate dangers. Often times your body will respond to pain before you've even registered the pain. Interestingly, there are the more pain receptors than any other receptor in the body. "For each square centimetre of skin, there are around 200 pain receptors, 15 pressure receptors, 6 cold receptors and 1 heat receptor" (Fraser, 2012, p. 546).

Papillae: little pimples on the tongue

Thalamus: a part of the brain which processes messages

Afferent Neurons: send sensory information to the brain

Cerebral Cortex: part of the brain involved in memory and the funtions of the nervous system

Taste

Chemoreceptors in the tongue are in our taste buds. A taste bud is a pear-shaped pore connected to a nerve on one side. The other is open to the mouth and has little hairs to do the actual tasting. There are approximately 10,000 taste buds on a tongue. They are located on the papillae. That is the reason that our tongue has it’s rough texture. Taste buds have five main groups of tasting: sweet, sour, bitter, salty, and savoury. Signals from the taste buds are sent to the thalamus through afferent neurons. From there it is sent to the cerebral cortex to differentiate between tastes. Sometimes it is then sent to the brain stem. When there the brain can give an emotional response by saying it's a good taste which might make you salivate and give you feelings of enjoyment or it could taste bad to you and make you gag.

Smell

The olfactory cells are found in the nose. Each one has around fifteen hairs which protrude into a layer of mucus. In order to smell anything air molecules must dissolve in the mucus. Then like with taste buds the signal is sent to the brain to differentiate between different smells and maybe produce an emotional response. These cells are different from all others because they interact directly with the brain interneurons while other receptors interact with afferent neurons instead.

Interneurons: a neuron (messenger) that sends messages to other neurons

Touch

Back to mechanoreceptors, which are used for touch. Mechanoreceptors are found in the skin and other surface tissues. There are four different types of mechanoreceptors:

1. Merkel's disks: very sensitive mechanoreceptors found in the top levels of the dermis and epidermis in areas with little hair (for example palms, lips and the bottom of your feet). They are slow adapting and they are thought to discern textures and edges.
2. Meissner's corpuscles: sensitive and found in the top levels of the dermis and epidermis in areas with little hair. They are fast adapting and discern movement against the skin.
  1. - The ridges of fingertips contain many Merke's disks and Meissner's corpuscles which produces a lot of information about the texture of an object.
3. Ruffini's corpuscles and : found deeper in the dermis and along joints and tendons. Not a lot is known about these.
4. Pacinian corpuscles: found deeper in the dermis and along joints and tendons. They feel deep vibrations and movement of arms and legs and are mainly involved in pressure.

Free nerve endings are also involved in the sense of touch. They are wrapped around the hair follicles which then react when the hair is moved or bent.

Fast Adapting: feels the touch of an object immediately

Slow Adapting: feel the continuos pressure of an object but does not differentiate when the touch started or ended

Sensory Disorders

Some have problems with processing, transmitting or receiving stimuli. This could happen if something happens to the spine where many of the messages are sent to the brain. It is also possible that the sensory organ is damaged. For example a damaged eye will lose its ability to see.

A Sensory Processing Disorder (SPD) is another possibility. This occurs when information comes in normally but something along the way interrupts it. This in turn could cause hypersensitivity or hyposensitivity. Hypersensitivity is when a person's stimuli is overwhelming. A soft shirt could feel like it is chafing against the skin, or a normal light could seem blinding. Hyposensitivity is when a person has little or no response to stimuli. A person wouldn't feel the pain of a burn or a cut in their hand. Unfortunately SPD is incurable but with therapy one can lessen the symptoms.

Works Cited:

Bhandari, S. (2017, January 8). Sensory Processing Disorder. Retrieved April 11, 2017, from

http://www.webmd.com/children/sensory-processing-disorder#1

Chudler, E. H. (n.d.). Neuroscience For Kids. Retrieved April 11, 2017, from https://faculty.washington.edu/chudler/retina.html

Dijkgraaf, S. (2017, January 06). Mechanoreception. Retrieved April 11, 2017, from

https://www.britannica.com/science/mechanoreception

Fraser, D., DiGiuseppe, M., Vavitsas, A., & White-McMahon, M. (2012). Nelson Biology 12: university preparation. Toronto, Ont.: Nelson

Education.

Home Science Tools. (n.d.). Sense of Touch. Retrieved April 11, 2017, from https://www.homesciencetools.com/a/skin-touch

Mechanoreceptors. (2014, December 1). Retrieved April 11, 2017, from http://www.biology- pages.info/M/Mechanoreceptors.html

Steele, E. (n.d.). Chemoreceptors: Definition, Location & Function - Video & Lesson Transcript.Retrieved April 11, 2017, from

http://study.com/academy/lesson/chemoreceptors-definition-location-function.html

The Senses. (n.d.). Retrieved April 11, 2017, from https://canvas.brown.edu/courses/851434/pages/the-senses

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