Biology
Chapter 29: 1-6
Lecture
The Five Senses (I)
WebLecture Topics
Sensory Perception
Philosophers and scientists have argued about where "sensation" ends and "perception" begins. Recognizing a symphony, the taste of your favorite chocolate dessert, or a painting by Miro requires specific biological processes: the activation of a sensory cell of the right type by the right stimulus, the transmission of that signal to the correct part of the brain, the recognition of the signal by the limbic system, and the interpretation of the memory and signal by the cerebral corte. We've already discussed how the brain receives and interprets input from the peripheral nervous system, so for this lecture, we focus on how the stimuli are detected by a variety of receptor cells.
Sensory Receptors
Most cells in the body respond to chemical signals or electrical signals from other parts of the body: tissue cells receive hormones that tell them to grow, macrophages and leukocytes receive signals that notify them to start fighting infection, nerve cells transmit electrical signals and information along the peripheral nervous system to the central nervous system. Sensory receptors are cells that respond to stimuli from outside the body. These cells can be classified by their location in the body, by the type of stimulus they receive, or by their shape and structure. It is important to recognize that these classifications overlap, and that types of receptors grouped together by location classification criteria may be in different groups when considered by stimulus type. Which criterion is used as the primary classification depends on purpose of a particular discussion.
| Classification of Receptor Cells: Different Criteria | ||
|---|---|---|
| Location | Exteroceptors | On or near the surface of the body, responding to touch, pain, temperature changes, light, sound, smell, and taste |
| Interoceptors | Receiving information from organs and blood vessels (autonomic nervous system monitors) | |
| Proprioceptors | Receiving information from muscles, tendons, ligaments, joints about physical conditions | |
| Stimulus Type | Mechanoreceptors | Pressure: touch or stretch sensations |
| Baroreceptors | Pressure sensitivity within blood vessels | |
| [Electromagnetic Radiation] Photoreceptors | Visible sensitivity | |
| [Electromagnetic Radiation] Infrared | Long wavelength sensitivity | |
| [Electromagnetic Radiation] UV receptors | Short wavelength sensitivity | |
| Electroreceptors | Electric field change sensitivity | |
| Magnetoreceptors | Magnetic field change sensitivity | |
| Hydroreceptors | Sensitivity to changes in humidity | |
| Osmoreceptors | Sensitivity to changes in concentrations and osmolarity of fluids | |
| Thermoreceptors | Temperature change | |
| Chemoreceptors | Dissolved chemicals: taste and smell, but also CO2 levels in the blood | |
| Nociceptors | Tissue damage, interpreted as pain | |
| Structure (classification by morphology or shape) | (Somatic) Free nerve ending | Nociceptors and thermoreceptors spread through the dermis and epidermis skin layers |
| (Somatic) Encapsulated nerve ending | Specialized receptors in the cutaneous layers | |
| Sensory cells | Photoreceptors such as rods and cones | |
| Peripheral processes on cell surface | Nerve cells connecting directly to the CNS | |
Regardless of location or type, sensory perception cells all operate with the same basic process. A stimulus or change in the environment triggers an internal change that results in a neural signal transmitted to neurons and through the PNS to the CNS for analysis.
Hearing
Hearing is the perception of vibrations, transmitted through the air or through solid materials, to the outer ear. The auricle folds direct and focus the sound into the ear canal (acoustic meatus), causing the tympanic membrane or eardrum to vibrate. This transmits the air vibrations to the middle ear chamber, called the tympanic cavity. The three bones of the middle ear transfer the vibrations to the fluids of the vestibule and cochlea spirals, where hairs lining the spirals bend with the vibrations, triggering nerve signals along the auditory nerve to the brain. Extremes of pressure are regulated by pressure in the auditory tube, which in turn is controlled by opening the mouth.
Wikipedia Commons, © 2013 by Bruce Blaus. Used per Creative Commons restrictions for educational purposes only.
A healthy human can hear frequencies between 20Hz and 20000Hz; bats can hear sounds with higher frequencies (ultrasound), while snakes, whales, and dolphins use infrasound (low pitches below human hearing range) to communicate.
You can test your own hearing with this five minute YouTube video, which displays the frequency as it changes.
The test ranges from 20Hz to 20kHz (20000Hz0. You should use headphones to maximize sound at the lower frequencies, but be sure to follow the instructions to decrease volume as the pitch goes up. Do not increase the volume at higher ranges if you can no longer here the test: while you may not detect the vibrations, they are still present and can still cause damage at high volume.
Can you hear sounds below 40Hz? Above 16kHz?
Equilibrioception (the sixth sense)
The same fluid that fills the inner ear for sound sensation also fills canals at the top of the inner ear to provide information about balance or the head's orientation with respect to the Earth's gravitational field. The loops of these semicircular canals are at right angles to each other. As with the cochlea, the loops are lined with hair cells that respond to pressure cause by the flow of the liquids in the canals. In other sections of the vestibule (the utricle and saccule), the fluid contains carbonate crystals that put additional pressure on the hair cells as the head changes position.
Anything that affects the ear and pressure on the ear can create balance detection problems. These can include diseases like Ménière's disease, an ear infection, a bad cold, movements resulting in rotational vertigo (spinning on a merry-go-round), or sudden acceleration vertigo (riding an elevator or a roller coaster), or ongoing conditions like high blood pressure or medication that changes blood chemistry.
An interesting way to test your sense of balance is to stand in a safe area (on a rug with no furniture nearby). Perform the next steps while standing on your left leg the whole time:
- Raise your right leg and extend it straight forward for 15 seconds.
- Without lowering your leg to the floor, move it so that it extends out to the right side and hold it in position for at least ten seconds.
- Without lowering the leg to the floor, move it so that it extends backwards and hold it in position for at least ten seconds (you can lean forward if this helps!)
- Without lowering your leg to the floor, bring your leg forward and bend your knee so that your foot is directly behind you.
- Put your foot down.
- Now try this again with your eyes closed!
Balance is a motor skill: it is a relationship between your prioreceptors and your balance sensitivity that can be trained and improved with practice.
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