AfferentsAfferents

The chemical senses

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 by which stimulus energy is converted into a nerve 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.

Vision

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 that they converge at a single point, it is called the focal point.

A real image is the image formed by a convex lens, and is upside down and reversed.

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 that increases the refractory power of the lends so that diverging light rays are bent more sharply.

The closest point on which we can focus clearly is called the near point of vision.

Accommodation pupillary reflex occurs when the circular muscles of the iris enhance the effect of accommodation by reducing the size of the pupil.

Convergence is the medial rotation of the eyeballs by the medial rectus muscles so that each is directed toward the object being viewed.

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 that combines with proteins called opsins to form four types of photopigments.

When bound to opsin, retinal has a bent shape called the 11-cis isomer.

When the photopigment is struck by light, retinal twists into a new configuration called the all-trans isomer.

The visual pigment of rods is a deep purple pigment called rhodopsin.

When rhodopsin absorbs light, retinal changes to the its all-trans isomer, and the retinal-opsin combination then breaks down in a process known as the bleaching of the pigment.

Light is transduced into an electrical event when cyclic GMP, which normally holds sodium channels open, is destroyed, thus closing the sodium gates and causing hyperpolarization of the membrane.

Transducin is a G protein subunit that binds with free opsin, thus activating enzymes that 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 depth perception

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 pass a given point in a given time.

The term pitch refers to the perception of different sound frequencies.

The intensity of a sound is related to the pressure differences between its compressed and rarefied areas.

The amplitude of a sound wave corresponds to its height.

Loudness refers to the subjective interpretation of sound intensity.

Sound intensity is measured in logarithmic units called decibels (dB).

Excitation of hair cells in the organ 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 of hearing

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) occurs when overgrowth of bony tissue fuses the stapes foot plate to the oval window or fuses to ossicles to one another.

Sensorineural deafness results from damage to neural structures at any point from the cochlear hair cells to the auditory cortical cells.

Tinnitus is a ringing or clicking sound in the ears in the absence of auditory stimuli.

Meniere’s syndrome is a labyrinth disorder that affects both the semicircular canals and the cochlea, where the afflicted person has repeated attacks of vertigo and nausea.

Mechanisms of equilibrium and orientation

Equilibrium receptors of the inner ear can be divided into two functional arms that monitor static and dynamic equilibrium.

The maculae and static equilibrium

The maculae are sensory receptors for static equilibrium which monitor the position of the head, and are located in the vestibule.

The crista ampullaris and dynamic 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 eye movements that occurs during and immediately after rotation of the body.

The equilibrium pathway to the brain

Motion sickness is an equilibrium disorder that is probably due to sensory input mismatch.