Photo courtesy of "Focus on Life Science"
Every part of your body is controlled by nerve cells. Nerve cells run throughout your body. They carry messages back and forth between the brain and the rest of the body. Messages sent through nerves cause the contraction of smooth muscles in the digestive system and blood vessels.
The major centers in the nervous system are the brain and the spinal cord. They make up the central nervous system (CNS). Vision, hearing, taste, thinking and voluntary acts are controlled by the brain. Nerves outside the brain and spinal cord make up the peripheral, nervous system. These nerves carry message between the central nervous system and the rest of the body.
The brain contains three major parts-cerebrum, cerebellum and medulla. The cerebrum is the largest part of the brain. It is located in the upper part of the head. Intelligence is due to a highly developed cerebrum. The cerebrum is the origin of thinking and awareness. The cerebrum controls memory, learning and some voluntary movements. Seeing, hearing, touching, tasting and smelling are controlled by the cerebrum. If any one of these centers is destroyed, the activity or sense it controls is lost.
A neuron is a nerve cell that carries impulses. Each impulse is a "message" or signal that travels the length of the neuron. A typical neuron consist of a cell body to which dendrites are attached. Dendrites are structures that receive stimuli. A long fiber attached to a thickened area of the cell body is the axon. This structure ends in nerve branches. The axon carries a nerve impulse from the cell body to another neuron or cell. Some neurons have a coating of fatty material that forms a sheath that speeds the movement of nerve impulses.
There are three kinds of neurons in the human body, sensory, motor, and inter neurons. Sensory neurons carry impulses from receptors to the central nervous system. Receptors are those parts of the central nervous system that detect stimuli. Receptors are in sense organs such as taste buds, eyes, ears, nose and skin.
Image was taken from "Neuroscience Resources for Kids"
There are two cranial nerves that innervate the tongue and are used for taste: the facial nerve (cranial nerve VII) and the glossopharyngeal nerve (cranial nerve IX). The facial nerve innervates the anterior (front) two-thirds of the tongue and the glossopharyngeal nerve innervates that posterior (back) one-third part of the tongue. Another cranial nerve (the vagus nerve, X) carries taste information from the back part of the mouth. The cranial nerves carry taste information into the brain to a part used for the brain stem called the nucleus of the solitary tract. Taste information goes to the thalamus and then to the cerebral cortex. Like information for smell, taste information also goes to the limbic system(hypothalamus and amygdala). Another cranial nerve (the trigeminal nerve, V) also innervates the tongue, but is not used for taste. Rather, the trigeminal nerve carries information related to touch, pressure and pain.
The complete inability to taste is called ageusia and the reduced ability to taste is call gypogeusia. Ageusia is a rare disorder. It may be rare because there are 3 different nerves that carry taste information to the brain. Older people have reduced sense of taste. This probably occurs because the taste buds are not replaced as fast in older people.
Taste buds pay an important role in helping you enjoy food. Taste buds can recognize four basic kinds of tastes: sweet, salty, sour, and bitter. The salty/sweet taste buds are located near the front of your tongue; the sour taste buds line the sides of your tongue; and the bitter taste buds are found at the back of your tongue.
When light rays pass through your pupil, the muscle called the iris (colored ring) makes the size of the pupil change depending on the amount of light that's available. If there is too much light, your pupil will shrink to limit the number of light rays that enter. Likewise, if there is very little light available, the pupil will enlarge to let in as many light rays as it can. Just behind the pupil is the lens and it focuses the image through a jelly-like substance called the vitreous humor onto the back surface of the eyeball, call the retina. The retina is filled with approximately 150 million light-sensitive cells called rods and cones. Rods identify shapes and work best in dim light. Cones identify color and work best in bright light. Both of these types of cells then send the information to the brain by way of the optic nerve. The amazing thing is, when they send the image to the brain, the image is upside down. It is the brain's job to burn the image right side up and them tell you what you are looking at. The brain does this is a place called the visual cortex.
When an object makes a noise, vibrations are sent speeding through the air. These vibrations are then funneled into your ear canal by your outer ear. As the vibrations move into your middle ear, they it your eardrum and cause it to vibrate as well. This sets off a chain reaction of vibrations. Your eardrum vibrates the three smallest bones in your body: the hammer, then the anvil, and finally, the stirrup. The stirrup passes the vibrations into a coiled tube in the inner ear called cochlea. The cochlea contains thousands of hair-like nerve endings call cilia. When the stirrup causes the fluid in the cochlea to vibrate, the cilia move. The cilia change the vibrations into messages that are sent to the brain via the auditory nerve. The auditory nerve carries messages from 25,000 receptors in your ear to our brain. Your brain makes sense of the messages and tells you sound you are hearing.
Near the top of the cochlea are three loops called the semi-circular canals. The canals are full of liquid also. When you move your head, the liquid moves. It pushes against hairlike nerve endings, which send messages to your brain. From these messages, your brain can tell whether or how how your body is moving. When you feel dizzy it is because the hairs of the sensory cells bend in all different directions, so the cells' signals mix up the messages to your brain
The olfactory tract (cranial nerve) transmits the signals on to the brain to such areas as the olfactory cortex, hippocampus, amygdala, and hypothalamus. As you breathe, air flows pass the cilia (tiny little hairs that filter), through the nasal cavity, which is a place the air passes through on it's way to the lungs. The air passes through a thick layer of mucous to the olfactory bulb. The cells send signals along your olfactory nerve to the brain. Some of the areas of the brain are part of the limbic system. The limbic system is involved with emotional behavior and memory. Often times when you smell something it brings back memories.
Humans have seven primary odors that help them determine objects:
Camphoric(ex. moth balls)/Musky(ex. perfume,aftershave)/Roses(ex.floral)/Pepperminty(ex.mint gum)/Etheral(ex.dry cleaning fluid)/pungent(ex. vinegar)/putrid(ex. rotten eggs)
The skin is the largest sensory organ of the body. The skin is sensitive to many different kinds of "stimuli", like touch, pressure, and temperature. Within the skin, there are different types of "receptors" that are activated by different stimuli. When a receptor is activated, it triggers a series of nerve impulses. For a person to "feel" the stimulus,the nerve impulses must make their way up to brain.
Messages from skin allows us to identify several distinct types of sensation such as tapping, vibration, pressure, pain heat, and cold. Human skin contains different kinds of sensory receptors (cells) that respond preferentially to various mechanical, thermal, or chemical stimuli. (The receptors convey this information to the brain and spinal cord., also known as the central nervous system (CNS), to areas we receive as stimuli. Nerve endings of the sensory receptors convert, mechanical, thermal, or chemical energy into electrical signals. These electrical signals then travel along neuronal extensions called axons, to the CNS. We interpret or understand sensation is shaped not only by the properties of receptors and neurons, but also by previous experiences that are stored in our brains.
Photo taken from "Neuroscience for Kids"