The Sensory Processes

VISION
In order to see, we transduce light energy (in the form of electromagnetic waves) into neural messages that our brain processes.

First, light passes through the cornea, which bends the light to focus. Then, the iris adjusts the amount of light that enters the pupil. Once through the pupil, the light travels through the lens, which focuses the light in an image on the retina. The retina has two types of receptor cells, rods (which enable peripheral vision, have a high sensitivity in dim light, and are located in the periphery of the retina) and cones (which have high color sensitivity/detail sensitivity and are located in the center of the retina). Chemical changes in the rods and cones activate bipolar cells, which activate ganglion cells. The axons of the ganglion cells form the optic nerve, which carries visual information to the thalamus. From there, the thalamus distributes the visual information to the visual cortex in the occipital lobe, where it will be processed. Feature detector cells in the visual cortex respond to different features of the image, and send this information to supercell clusters which respond to these complex patters and features. More generally, the brain divides the visual scene into several different aspects, such as color, motion, form, and depth. This is known as parallel processing.

 

HEARING

The stimulus energy of noise is sound waves. In order to hear, our ears transform sound waves into nerve impulses which our brain processes as sounds. Sound waves first travel through the outer ear's auditory canal, which leads the energy into the middle ear. In the middle ear, the sound waves meet the eardrum, which begins to vibrate. The vibrations pass through the piston, composed of the hammer, anvil, and stirrup. From there, the vibrations enter the cochlea, located in the inner ear. The cochlea's membrane and the fluid within begin to vibrate, creating ripples in the basilar membrane, which bends hair cells on its surface. The movement of hair cells creates impulses in nearby nerve cells, whose axons form the auditory nerve. Through the thalamus, neural messages are sent to the auditory cortex in the temporal lobes.

 

TOUCH

All skin sensations are various combinations of four basic sensations: pressure, warmth, cold, and pain. Some parts of skin are especially sensitive to one of the four basic sensations, making some sensations more strong. However, there are no specialized nerve endings for the specific sensations; there are only identifiable receptors for pressure. Touch is a bottom-up property of senses and a top-down product of expectations, because expectations effect how you respond to a sensation.

 

VESTIBULAR SENSE

The vestibular sense monitors position and movement of your head, basically your sense of equilibrium, using "biological gyroscopes" in your inner ear. Vestibular sacs, which connect the semicircular canals with the cochlea, are filled with fluid that moves when your head or body moves. When you move, hair-like receptors are activated and send neural messages to the cerebellum about your position. The cerebellum allows you to maintain balance.

 

KINESTHESIS

Kinesthesis is your sense of position and movement of your body parts. There are sensors in your joints, tendons, bones, ears, and even your skin which enable kinesthesis. It also interacts with vision; if you are in the dark or close your eyes, your sense of balance and position will become slightly compromised. Without kinesthesis, you wouldn't be able to sense the location or position of your limbs. Kinesthesis works with the vestibular sense.

 

PAIN

Pain varies depending on physiology, experiences, attention, and culture, much like other sensations. Unlike other sensations, however, there isn't just one stimulus that creates pain. Instead, there are many nocireceptors, which detect harmful sensations (high temperatures, burning chemicals, intense pressure). One theory of pain is the gate-control theory, which involves the fibers in the spinal cord that conduct signals to the brain. The gate-control theory says that during a painful experience (tissue damage), small fibers in the spinal cord become activated and open the "gate," allowing pain signals to be transferred to the brain to be processed. When large fibers in the spinal cord become activated, they close the "gate" and prevent pain signals from reaching the brain.

 

 

TASTE

Similar to touch, which has four basic sensations, taste has five basic sensations: sweet, salty, sour, bitter, and umami. All other tastes are a mixture of these five, basic taste sensations. Essentially, taste is a chemical sense. All over tongues are bumps, which each contain 200+ taste buds. Taste buds collect chemicals from the food eaten. Inside the taste buds are 50-100 receptor cells with hair-fibers that sense food molecules. Some receptors are more sensitive to a specific taste sensation, such as sour. Much like other sensations, taste varies based on expectations and experiences.

 

SMELL

Like taste, smell is a chemical sensation. Because the smell chemicals are so different, there are many receptor proteins which work together to detect these scents. When an odor molecule binds to a receptor, which are located on nasal cavity neurons, the olfactory receptor cells send electrical signals to high regions of the brain via the combined axons of the olfactory receptor cells.