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Study Guide
One of the jobs of the nervous system is to take in information from the environment and interpret and respond to it. The senses of taste, touch, smell, hearing and vision rely on your body's ability to receive information from the outside world and translate it into electrical impulses, the "language" of the nervous system. In general, senses rely on sensory neurons known as receptors. Your body has many different types of receptors that detect information from the environment, such as the pressure of touch, light, or chemicals in food, and translate it to a neurological (electrical) impulse.
Sensory receptors are connected to interneurons, which pass the electrical impulses generated by the receptor to the central nervous system (the brain and the spinal cord). The brain receives and makes sense of the information and directs our response to the stimulus. Responses can occur without input from the brain, an example is the reflex that causes you to withdraw from a painful stimulus, such as a hot object. This response involves input from pain receptors, a relay through interneurons in the spinal cord and output to the muscles. However, even simple reflexes can be modified by the brain. For example, the brain can shut down the withdrawal response in order to prevent you from dropping a hot pot.
For a more detailed description of the basics of the nervous system, visit the following website:
Nervous System Basics
The most common types of sensory receptors fall into three categories: chemoreceptors, photoreceptors and mechanoreceptors. Ion channels play an important role in the functioning of each of these receptor types. Ion channels are proteins that are responsible for allowing ions--charged particles--to flow through the cell membrane from one side to the other. Normally, there is a difference in the charge inside the membrane and outside the membrane. This charge difference means that there is an actual voltage--difference in electrical energy--across the membrane.
The opening of ion channels causes a decrease in voltage across a membrane. This decrease in voltage is known as a depolarization. The nerve impulse or action potential is the movement of this depolarization along the axon of nerve cells.
For an animation of this process, visit the following website:
Movement of ions during nerve impulse
Chemoreceptors
Receptors in your nose (olfactory receptors) and in the tastebuds in your mouth are examples of chemoreceptors, receptors designed to detect chemicals found in air, food and water. Chemoreceptors are believed to recognize chemicals according to their shape. Chemoreceptors in your tastebuds specifically recognize one of five tastes: sweet, salty, bitter, sour and umami (a taste associated with monosodium glutamate).
For more information about how taste receptors work, visit the following website:
Taste animation
Photoreceptors
Your ability to see is dependent on photoreceptors in the retina of your eyes. Most animals have two different types of photoreceptors in their eyes: rods and cones. Rods, which contain the light-sensitive pigment rhodopsin, are sensitive to low levels of light. Rods are responsible primarily for night vision. They are unable to distinguish color, and only allow black and white sight. Cones contain pigments similar to rhodopsin. However, the pigments found in the cones require more light, and respond to one of three colors: red, green or blue. The cones are responsible for color vision in bright conditions.
For an animation of this process, and for more information, visit the following websites:
Animation and description of how vision works
Animation of the conformation change of rhodopsin when hit by light
Mechanoreceptors
Mechanoreceptors respond to mechanical stimuli, such as motion, pressure, vibrations and sound waves. Our mechanoreceptors enable us to detect touch, sounds and the motion of the body, and they monitor the position of the muscles and joints (the sense of proprioception).
Most mechanoreceptor cells are hair cells, which have long clusters of cilia. The hair cells are directionally sensitive. Like a light switch, motion in one direction may open the ion channels, initiating an action potential, while motion in another direction will turn the ion flow off.
Dr. Zuker studies mechanoreceptors in fruit flies (Drosophila Melanogaster). Fruit flies are rich in mechanoreceptors; for example, during flight, mechanoreceptors at the base of bristles detect wind, permitting the fly to adjust its course accordingly. Although fruit fly mechanoreceptors are not structurally identical to mechanoreceptors in vertebrates like us, they are physiologically (functionally) similar. Since fruit flies are also relatively easy to manipulate genetically, flies are very useful as an experimental system to work out the details of mechanoreceptor function.
Green fluorescent protein reveals nerves that respond to mechanical stimuli, and are located at the base of bristles on a fruit fly. Credit: UCSD/Cell Press
The following video shows flies that have mutations in single genes that play essential roles in mechanoreceptor function. You will see that mutations in individual genes can cause profound behavioral problems, including lack of balance, poor coordination and inability to respond to certain features of the environment.
