As we age, our sensory abilities change in predictable ways. Unfortunately, during our geriatric years, much of the change in sensation is associated with a decrement or loss in ability. Wi

Hearing.html

Hearing

The first part of hearing or auditory processing involves sound waves being funneled through the auditory canal. These waves must be transmitted all the way through the middle ear and into a structure called the cochlea. Once the sound waves make it to the end of the auditory canal, they cause a membrane called the tympanic membrane to vibrate. The vibrations then make the tiniest bones in your body move. These bones are collectively called the ossicles. The individual names of the ossicles are the hammer, the anvil, and the stirrup (in Latin, they are the malleus, the incus, and the stapes).

When the stirrup starts to vibrate, it makes a small membrane called the oval window move, and this energy is transmitted throughout the fluid-filled cochlea. After the waves make it to the cochlea, the fluid causes a chain reaction to produce electrical impulses. The basilar membrane is set in motion, and when it vibrates, it presses the hair cells against the membrane that is above it. This membrane is called the techtoral membrane. When the hair cells are pressed up against the techtoral membrane, transduction occurs. In other words, the mechanical stimulus that was initiated by the sound waves is transformed into an electrical impulse. The sound waves that are transformed into electrical impulses are characterized by pitch (the frequency of the sound) and amplitude (how loud the sound is). The pitch of a sound activates hair cells at different places along the basilar membrane, with higher frequency sounds occurring at the start (the base) of the membrane and lower frequency sounds occurring at the far end of the membrane.

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Vision.html

Vision

All of the sensory systems in the body involve a process in which a stimulus interacts with a receptor to create an electrical impulse. This process of changing an external stimulus into electricity is called transduction. Transduction is a necessary step for neural communication. After the stimulus is transduced, it is sent to the primary sensory cortex—a place where that specific sensory modality is first processed in the brain. The first sensory modality we will consider is vision. Visual sensation depends on the detection of light waves, which are a small part of the electromagnetic spectrum.

Light waves from images that we see enter the eye through the pupil and are projected onto the retina. The retina is like a small curved screen, but a visual stimulus is not the same; it is reduced and changed in orientation by the cornea. There are other parts of the eye such as the cornea and the lens, which help focus images on the retina. Once an image reaches the retina, it must be processed by photoreceptors.

An interesting thing about vision is that it is greatly influenced by nurture, which means it can be changed by developmental experience or the type of environmental stimulation. For example, animal studies have demonstrated that early lack of stimulation in one eye leads to the related synapses in the visual cortex becoming gradually unresponsive to input from that eye. It seems that synapses from the open eye inhibit those of the closed eye. Case studies show that when a person is born with strabismus (a lazy eye), the person’s vision is blocked and eventually decreased in the lazy eye as a result of the eye not responding properly to muscle movements.

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Refer to the following table for additional information regarding the structural components of the eye.

Olfaction.html

Olfaction

The neurons responsible for smell are olfactory cells that line the olfactory epithelium in the rear of the nasal passage. Receptors are on cilia, which extend from the cell body to the mucous surface of the nasal passage. Vertebrates have hundreds of olfactory receptors. When a smell binds to a receptor, proteins in olfactory receptors respond and trigger changes in G proteins inside the cell. The G proteins then trigger second messengers, and that leads to action potentials. The axons from olfactory receptors carry information to the olfactory bulb. Coding in the brain is determined by which part of the olfactory bulb is excited. It is interesting that the olfactory bulb sends axons to the somatosensory cortex, where messages are coded by location.

There is a large variation in olfactory ability between genders and also between individuals within each gender group. Women are able to detect odor more readily and have stronger brain responses. Although genetic differences might explain a portion of sensory differences between individuals, some of the variation that exists between people has to do with damage that occurs to the olfactory bulbs because of their location at the bottom of the brain near the skull

One highly controversial sense that is somewhat related to smell is pheromones. Pheromones are emitted by animals within a species to signal certain states and conditions such as fear or readiness for mating. Pheromones are perceived differently from normal olfaction because they are not processed with conscious awareness. The structure that is most sensitive to pheromones, which is found in nonhumans near the olfactory bulb, is called the vomeronasal organ.

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