Sensory Systems

Sensory systems are systems in the body that allow humans to perceive and interpret different types of sensory information from our environment. The five main sensory systems are: Visual, Auditory, Olfactory, Gustatory, and Somatosensory.

The visual system is responsible for processing information from our eyes. First, light passes through the cornea. The cornea is shaped like a dome and bends light to help the eye focus. Afterwards, light enters the eye through the pupil. The iris controls how much light the pupil lets in by contracting or relaxing. Next, light passes through the lens, and works together with the cornea to focus light correctly on the retina. The retina focuses the light on photoreceptors that turn the light into electrical signals. The photoreceptor layers consist of the rods and cones, which generate action potentials. The ganglion cell layer and nerve fiber layer serve as the foundation of the optic nerve; the former contains the cell bodies, and the latter contains the axons as they stream across the retina. The nerve is surrounded by the dura, which is in continuation of that of the brain, allowing free movement of the eye in its socket. After, it enters the optic canal, a bone-encased tunnel intended to protect the nerve, and is transferred to the brain for processing into the images we see.

The auditory system is responsible for processing sound information from the ears. Sound waves enter the outer ear and travel through the ear canal, which leads to the eardrum. The eardrum vibrates from the incoming sound waves and sends these vibrations to three tiny bones in the middle ear. These bones are called the malleus, incus, and stapes. These bones amplify, or increase, the sound vibrations and send them to the cochlea in the inner ear. An elastic partition runs from the beginning to the end of the cochlea, splitting it into an upper and lower part. This partition is called the basilar membrane because it serves as the base, or ground floor, on which key hearing structures sit. Once the vibrations cause the fluid inside the cochlea to ripple, a traveling wave forms along the basilar membrane. Hair cells near the basilar membrane ride the wave. The cells near the wide end of the snail-shaped cochlea detect higher-pitched sounds whereas those closer to the center detect lower-pitched sounds, such as a large dog barking. As the hair cells move up and down, microscopic hair-like projections (known as stereocilia) that perch on top of the hair cells bump against an overlying structure and bend. Bending causes pore-like channels, which are at the tips of the stereocilia, to open up. When that happens, chemicals rush into the cells, creating an electrical signal. The auditory nerve carries this electrical signal to the brain, which turns it into a sound that we recognize and understand.

The olfactory system is responsible for processing information from the nose, and more generally, with scent. Your ability to smell comes from specialized sensory cells, called olfactory sensory neurons, which are found inside the nose. These cells connect directly to the brain. Each olfactory neuron has one odor receptor. Thus, you have thousands of olfactory neurons to process the thousands of smells you perceive. Once the neurons detect scent molecules, they send messages to your brain, which identifies the smell. The scent can come through two ways: your nostrils or the connection between the roof of your throat to the nose. Chewing food releases aromas that access the olfactory sensory neurons through the second channel. If the channel is blocked, such as when your nose is stuffed up by a cold or flu, odors can’t reach the sensory cells that are stimulated by smells. As a result, you lose much of your ability to enjoy a food’s flavor. It should be noted that there are more smells in the environment than there are receptors, and any given molecule may stimulate a combination of receptors, creating a unique representation in the brain. These representations are registered by the brain as a particular smell. Without the olfactory sensory neurons, familiar flavors such as chocolate or oranges would be hard to distinguish. Without smell, foods tend to taste bland and have little or no flavor.

The gustatory system is responsible for processing from the tongue, or taste. A chemical substance comes into contact with a nerve cell which then transmits messenger substances, which in turn activate further nerve cells. These nerve cells then pass information for a particular perception of flavor on to the brain. Taste papillae on the tongue, contain many sensory cells with a special structure: together with other cells they make up a bud that looks a bit like an orange with its sections arranged around a center. Adults have between 2,000 and 4,000 taste buds in total. The sensory cells in the taste buds are renewed once a week. The final step in perceiving taste is transfer to the nervous system. This is done by several cranial nerves. All information is carried along the cranial nerves to part of the lower section of the brainstem (the medulla oblongata). About half of the sensory cells react to several of the five basic tastes. They only differ by having varying levels of sensitivity to the different basic tastes. Each cell has a specific palette of tastes with fixed rankings: this means that a particular cell might be most sensitive to sweet, followed by sour, salty and bitter, while another has its own ranking. The full experience of a flavor is produced only after all of the sensory cell profiles from the different parts of the tongue are combined. It is the job of these cells to transmit information on the intensity of the stimulus – how salty or sour something tastes. Assuming 5 basic tastes and 10 levels of intensity, 100,000 (105) different flavors are possible.

The somatosensory system is responsible for processing information from the skin and other organs. Somatosensation begins when mechano- and thermosensitive structures in the skin or internal organs sense physical stimuli such as pressure on the skin. Activation of these structures, or receptors, leads to activation of peripheral sensory neurons that convey signals to the spinal cord as patterns of action potentials. Sensory information is then processed locally in the spinal cord to drive reflexes, and is also conveyed to the brain for conscious perception of touch and proprioception. The neural pathways that go to the brain are structured such that information about the location of the physical stimulus is preserved. In this way, neighboring neurons in the somatosensory cerebral cortex in the brain represent nearby locations on the skin or in the body, creating a map.