Sense of taste Chemicals must dissolve in saliva, diffuse into the taste pore, then contact gustatory hairs. Bitter receptors are the most sensitive, followed by salty, and then sweet/sour receptors. Taste receptors adapt rapidly.
Transduction is the sensory mechanism bywhich stimulus energy is converted into anerve impulse.
Sense of smell To perceive a smell, the chemical must be volatile, and it must dissolve in the fluid coating of the olfactory epithelium. The resulting action potential is conducted to the olfactory bulbs. G-proteins may be involved with transduction in olfactory receptors.
Wavelength and color Visible light is that portion of the electro- magnetic spectrum with a wavelength range of approximately 400nm-700nm. Photons are small packets of light energy. The visible spectrum is a band of colors created when light passes through a prism.
Refraction and lenses Reflection of light occurs when light waves bounce off objects, and is responsible for the stimulation of photoreceptors. Refraction of light occurs when it contacts the surface of a different medium at an oblique angle.
When light waves are bent by a lens so thatthey converge at a single point, it is calledthe focal point.A real image is the image formed by aconvex lens, and is upside down andreversed.
Focusing of light on the retina The far point of vision is that distance beyond which no change in lens shape is needed for focusing. An emmetropic (normal) eye has a far point of 6 meters.
Accommodation is the process thatincreases the refractory power of the lendsso that diverging light rays are bent moresharply.The closest point on which we can focusclearly is called the near point of vision.
Accommodation pupillary reflex occurswhen the circular muscles of the irisenhance the effect of accommodation byreducing the size of the pupil.Convergence is the medial rotation of theeyeballs by the medial rectus muscles sothat each is directed toward the object beingviewed.
Homeostatic imbalances Myopia is a condition in which visual images are focused in front of the retina. Hyperopia is a condition in which visual images are focused behind the retina. Astigmatism is a condition in which unequal curvature in different parts of the lens leads to blurred vision.
Photoreception Photoreception is the process by which the eye detects light energy. Photopigments exist in the outer segment of photoreceptors, and are capable of changing shape as they absorb light.
Retinal is a light absorbing molecule thatcombines with proteins called opsins toform four types of photopigments.When bound to opsin, retinal has a bentshape called the 11-cis isomer.When the photopigment is struck by light,retinal twists into a new configurationcalled the all-trans isomer.
The visual pigment of rods is a deep purplepigment called rhodopsin.When rhodopsin absorbs light, retinalchanges to the its all-trans isomer, and theretinal-opsin combination then breaks downin a process known as the bleaching of thepigment.
Light is transduced into an electrical eventwhen cyclic GMP, which normally holdssodium channels open, is destroyed, thusclosing the sodium gates and causinghyperpolarization of the membrane.Transducin is a G protein subunit that bindswith free opsin, thus activating enzymesthat break down cGMP.
Light and dark adaptation Light adaptation occurs when large amounts of photopigment are broken down quickly as a result of switching from darkness to bright light. Dark adaptation occurs when rhodopsin accumulates after being bleached, as a result of switching from bright light to darkness.
Stereoscopic vision and depthperception Stereoscopic vision occurs due to an overlap of the visual field, causing each eye to see a different view. Depth perception occurs as a result of stereoscopic vision.
Visual processing Ganglion cells with on-center receptive fields are depolarized by light hitting the field center, and are inhibited by light hitting the periphery, while off-center fields are depolarized by the reverse. Unequal illumination of these fields caused the ganglion cells to change their rate of impulse conduction.
Hearing and equilibrium
Properties of sound Sound is a pressure disturbance originating from a vibrating object and propagated by the molecules of the medium. The distance between two consecutive crests is called the wavelength.
Frequency is the number of waves that passa given point in a given time.The term pitch refers to the perception ofdifferent sound frequencies.
The intensity of a sound is related to thepressure differences between its compressedand rarefied areas.The amplitude of a sound wave correspondsto its height.
Loudness refers to the subjectiveinterpretation of sound intensity.Sound intensity is measured in logarithmicunits called decibels (dB).
Excitation of hair cells in theorgan of Corti One row of inner hair cells and three rows of outer hair cells are sandwiched between the tectorial and basilar membranes. The hair cells protrude into the endolymph, where the longest are enmeshed in the tectorial membrane.
Homeostatic imbalances ofhearing Deafness is any hearing loss. Conduction deafness occurs when something interferes with the conduction of sound vibrations to the fluids of the inner ear.
Otosclerosis (hardening of the ear) occurswhen overgrowth of bony tissue fuses thestapes foot plate to the oval window orfuses to ossicles to one another.Sensorineural deafness results from damageto neural structures at any point from thecochlear hair cells to the auditory corticalcells.
Tinnitus is a ringing or clicking sound inthe ears in the absence of auditory stimuli.Meniere’s syndrome is a labyrinth disorderthat affects both the semicircular canals andthe cochlea, where the afflicted person hasrepeated attacks of vertigo and nausea.
Mechanisms of equilibrium andorientation Equilibrium receptors of the inner ear can be divided into two functional arms that monitor static and dynamic equilibrium.
The maculae and staticequilibrium The maculae are sensory receptors for static equilibrium which monitor the position of the head, and are located in the vestibule.
The crista ampullaris anddynamic equilibrium The crista ampullaris is an elevation in the ampulla of each semicircular canal that monitors dynamic equilibrium, which is excited by acceleration and deceleration of the head. The cupula is a network of gelatinous strands that contact each hair cell.
Vestibular nystagmus is a series of eyemovements that occurs during andimmediately after rotation of the body.
The equilibrium pathway to thebrain Motion sickness is an equilibrium disorder that is probably due to sensory input mismatch.