Sensory Physiology

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SENSORY PHYSIOLOGY

A STIMULUS represents a change in the environment. RECEPTORS are structures which respond to specific stimuli by producing generator potentials.

Receptors act as TRANSDUCERS (i.e. they convert energy in various forms into the energy of nerve impulses.

[FIG. 7-1] A RECEPTOR may be simply the ending of an AFFERENT neuron [as in (a)], or it may be a specialized cell or organ [as in (b)].

A SENSORY MODALITY is a quality of sensation. The various modalities are encoded by the receptors and interpreted by the brain.

Receptors can be classified as either special or general.

Special senses: e.g. vision, hearing, equilibrium

General senses: e.g. touch, heat, pain, pressure

Receptors can be classified functionally:

    1. Chemoreceptors:
    2. Taste, smell, pain, blood carbon dioxide receptors, etc.

    3. Photoreceptors
    4. Thermoreceptors
    5. Mechanoreceptors:

Touch, pressure, vibration, proprioception, equilibrium, etc.

Tonic receptors adapt only slowly to prolonged stimulation. Phasic receptors, in contrast, rapidly adapt to prolonged stimulation. This response is called SENSORY ADAPTATION.

[see Figure]

[Fig. 7-2] Note that the RECEPTOR POTENTIALS are graded and localized. These, to some degree, reflect the properties of the STIMULUS. The resulting ACTION POTENTIALS, in contrast, are ALL OR NONE (non-graded and propagated).

[FIG. 7-5] Information about the INTENSITY of a stimulus (in this case, PRESSURE) is coded by the FREQUENCY of the action potentials in the afferent neuron. This is FM coding. There is no AMPLITUDE coding, because the amplitude of an actionl potential is invariant (i.e. ALL or NONE).

Information about the LOCATION of a stimulus is also very important. Several factors combine to provide this information, including:

a. The location of the stimulated receptor and the size of its RECEPTIVE FIELD.

b. The point within the receptive field where the stimulus is applied.

c. The comparative responses of overlapping receptive fields.

d. The phenomenon of LATERAL INHIBITION.

[Fig. 7-6] Because of its smaller receptive field, neuron (a) can provide more precise information about the location of a point stimulus.

[Fig. 7-7] The frequency of action potentials reaching the central nervous system can depend on the position stimulated within a receptive field. Here, stimulus A affects more nerve endings and will likely result in a higher frequencey of action potentials.

[Fig. 7-8] Location information is also provided through a comparison of the outputs of adjacent and overlapping receptive fields. Here, neuron B is generating a higher frequency of action potentials, revealing that the point stimulus is more within the receptive field of B, than those of A or C.

[Fig. 7-10] Lateral inhibition helps to localize a stimulus site by markedly sharpening the contrast at the edges of the affected area. Lateral inhibition can occur at any of several levels within a sensory pathway.

HEARING

Sound travels as pressure waves in air or water, and hearing involves the detection of such waves. Basically, pressure waves within the COCHLEA (inner ear) cause displacement of the "hairs" of hair cells and their subsequent production of generator potentials.

[Fig. 7-33]

[see Figure] A. low intensity, pure tone

  1. twice the intensity of A, same pure tone
  2. same intensity as A, twice the pitch, pure tone
  3. same intensity as A, more complex tone
  4. white noise

[see Figure] The intensity of sound can be expressed in DECIBELS (dB).

Each 10 dB increase represents a 10-fold increase in loudness. Thus, as sound at 70 dB is 10 x that at 60 dB, and a rock concert at 120 dB is 1,000,000 times as loud as average conversation.

A hair cell is a common type of mechanoreceptor. Deflection of the "hair"

Results in a distortion of the plasma membrane, thereby changing the membrane potential.

[see Figure]

[Fig. 7-34]

[Fig. 7-35] A diagram of the middle and inner ear. The receptors (hair cells) are located within the coiled COCHLEA.

[Fig. 7-36] The bones of the middle ear transmit and amplify the pressure waves and convert the vibrations into fluid waves within the cochlea.

[Fig. 7-37] Fluid waves moving through the cochlea and crossing into the cochlear duct cause the basilar membrane (within the Organ of Corti) to vibrate. Low frequency sould travels greater distances into the cochlea.

[Fig. 7-37] Movement of the basilar membrane stimulates (i.e. causes bending of the hair cells attached to the tectorial membrane, thereby activating receptors and generating action potentials in the afferent neurons.

The intensity (loudness) of the sound is encoded in the frequency of the action potentials in the auditory nerve. The frequency (pitch) of the sound depends primarily on the region of the cochlea maximally stimulated by the fluid wave.

SENSE OF EQUILIBRIUM

The fluid-filled VESTIBULAR APPARATUS and SEMICIRCULAR CANALS of the inner ear contain hair cell receptors which respond to changes in rotational, vertical, and linear acceleration.

Gravity and acceleration (not sound waves) activate these hair cells.

[Fig. 7-39] The receptors of the semicircular canals are housed within chambers known as ampullae.

The receptors of the utricle and saccule (the maculae) include a gelatinous mass with embedded calcium carbonate crystals (otoliths).

[see Figure] The semicircular canals are at right angles to one another, and thus can respond to rotational acceleration in any direction.

[see Figure] Due to inertia acting on the fluid (endolymph) within the canals, the fluid lags behind during angular acceleration, causing the hair cells to bend in the opposite direction. This is comparable to the bristles of a paint brush. During deceleration, the fluid moves ahead because of its momentum, causing deflection in the direction of deceleration.

[Fig. 7-41] The cupula is a gelatinous mass at the top of a tuft of hair cells.

The deflection of the hair cells in the opposite direction during acceleration is depicted on the bottom.

[see Figure] The maculae (within the utricle and saccule) respond to linear acceleration and provide information about head position.

The otoliths magnify the responsiveness of the maculae.

[Fig. 7-42] The hair cells of the vestibular apparatus are sensitive to the direction of bending, which may either increase or decrease the output. The latter is possible only because there is tonic output at rest.

VISION

Light rays travel (as waves) through a vacuum at 300,000 km/s, but progressively slower through a gas, a liquid, and a transparent solid.

Light is a form of radiant energy, and can be described in terms of wavelengths and frequencies.

[see Figure] There is an inverse relationship between wavelength and energy.

Visible light (for humans and most vertebrates) has wavelengths from a bit less than 400 to a bit more than 700 nm.

[see Figure] Light rays are bent (REFRACTED) when they enter (or leave), at an angle, the surface of a transparent object with a different density.

[see Figure] Because of REFRACTION, concave lenses will diverge parallel light rays and convex lenses will converge (i.e. focus) parallel rays.

For a convex lens, the FOCAL DISTANCE is the distance from the center of the lens to the focal point.

[Fig. 7-22 (a)] The vertebrate eye has a lens system consisting of the transparent CORNEA (where most of the refraction occurs), the AQUEOUS HUMOR, the LENS, and the VITREOUS HUMOR.

The focal point is on the RETINA (ordinarily on the FOVEA CENTRALIS).

There actually are four points of refraction:

The interfaces between:

    1. the air (or water) and the cornea
    2. the cornea and the aqueous humor
    3. the aqueous humor and the lens
    4. the lens and the vitreous humor

[Fig. 7-23] Every point on a viewed object is focused on the retina as a point of light. The image projected on the retina is inverted (and reversed right for left).

[see Figure] Light rays from distant (i.e. > 20 feet) objects are essentially parallel when they reach the eye. However, light rays from close objects are diverging when they strike the eye. To maintain the proper focus, the lens must become more rounded in order to maintain the proper focal distance.

This is ACCOMMODATION.

[see Figure] At rest, the tension of the zonular fibers pulls the lens into a flattened oval. The is appropriate for distant vision.

To accommodate for near vision, the ciliary muscles contract and thereby lessen the tension on the zonular fibers and allow the lens (via elastic recoil) to assume a more spherical shape.

[Fig. 7-26] Many individuals suffer from MYOPIA (nearsightedness), where the focal distance for distant objects is in front of the retina. A concave lens can correct this condition. Other suffer from HYPEROPIA (farsightedness) where the focal distance (especially that for near objects) is behind the retina. A convex lens can correct this condition.

[see Figure] The RETINA consists of three layers of cells: The photoreceptors (rods and cones) at the back, the bipolar cells, and the ganglion cells. Axons from the ganglion cells combine to form the optic nerve, which exits the eyeball.

There are also important cross-connections with horizontal and amacrine cells.

[Fig. 7-27]

[see Figure] The primary pathway involves just the three main layers of cells: a point of light strikes a group of rods and cones which converge on a few bipolar cells, which, in turn, converge on a ganglion cell.

Here, the center of a receptive field in the retina is responding positively to light (ON CENTER).

[see Figure] Some cells in the retina are directionally selective. They respond to motion in a particular direction anywhere within the receptive field.

This slide depicts a cell which responds to movement to the right.

PHOTORECEPTORS

Rods are responsible for nighttime (SCOTOPIC) vision, which is monochromatic (i.e. black and white) and "coarse-grained".

Cones are responsible for daytime (PHOTOPIC) vision, which is in color and high acuity.

Except in the fovea centralis (an area of concentrated cones), rods outnumber cones by about 20 : 1.

PHOTOTRANSDUCTION

Rods contain a visual pigment called RHODOPSIN, which consists of a membrane-bound protein (opsin) that surrounds an associated light-sensitive component (retinal). When struck by light, retinal is structurally altered (i.e. "bleached"), and this, through a cascading series of events, leads to a decrease in the "second messenger" cyclic GMP.

The decrease in cGMP, causes cation channels to close, resulting in a hyperpolarization of the cell. There is a decreased release of an inhibitory neurotransmitter (glutamate) and this excites some and inhibits other bipolar cells.

In bright light, rhodopsin is essentially all bleached. Full recovery from bleaching depends on the enzymatic regeneration of rhodopsin, and this can take 20 minutes or more. Thus, moving from bright light into a dark room results in a temporary "blindness" until the necessary adjustment (DARK ADAPTATION) occurs.

[See Figure] The three primary colors that you may have learned about in school are RED, YELLOW, and BLUE. This is based on paint-mixing and uses a system called SUBTRACTIVE COLOR. In this system, all colors mixed together make a muddy brown/black.

The primary colors of light, however, are RED, GREEN, and BLUE. These are also the colors used by TVs and computer monitors (The RGB color mode). These are ADDITIVE COLORS, and, based on the physics of light, black is the absence of color and all colors mixed together produce white.

[Fig. 7-30]

The three types of cones contain different photopigments consisting of different types of opsin. Different cones are maximally sensitive to blue, green, or red light. Phototransduction is similar to that for rods, with light causing a hyperpolarization and a decreased release of glutamate.

There is a great deal of CONVERGENCE within the retina. Information from more than 100 million photoreceptors is condensed down to only one million axons leaving the eye. There is generally much more convergence for the rods than for the cones, which is why the rods are much more responsive in dim light, but provide a "grainy" image.

[see Figure] This illustrates convergence of many rods to one bipolar cell and of multiple bipolar cells to one ganglion cell.

Each eye has both a NASAL (inner) and a TEMPORAL (outer) portion of the retina. The nasal retina primarily contributes to peripheral vision, whereas the overlap of the visual fields from the temporal retinas provides BINOCULAR vision.

Fiber tracts from the nasal retinas cross over (at the OPTIC CHIASM) before reaching the visual cortex at the back of the brain. However, the temporal fiber tracts DO NOT cross over.

[see Figure] Consider what would happen if (a) the optic nerve was severed on one side, (b) if the optic tract was severed on one side, (c) if there was a front-to-back lesion through the optic chiasm.

[for a similar figure, see Fig. 7-29]

COMPARATIVE SENSORY PHYSIOLOGY

Hearing:

Bat 1,000 - 210,000 Hz

Porpoise 150 - 200,000 Hz

Dog 15 - 50,000 Hz

Human 20 - 20,000 Hz

Elephant 5 - 18,000 Hz

Frog 50 - 10,000 Hz

Pigeon 0.1 - 15,000 Hz

A dog has 17 muscles to raise, lower, or swivel each ear. Humans have only 9 muscles, none of which work very well in most people.

The sperm whale appears to be the champion of producing loud sound, being capable of generating up to 265 dB (enough to stun large fish and kill small ones).

Taste and Smell:

Catfish 100,000 taste buds

Cow 35,000 taste buds

Rabbit 17,000 taste buds

Pig 15,000 taste buds

Human 10,000 taste buds

Bat 800 taste buds

Birds 200 taste buds

Sheepdog 220,000,000 olfactory cells

Dachshund 125,000,000 olfactory cells

Human 5,000,000 olfactory cells

Vision:

The eye of a giant squid contains about one billion photoreceptors (10 x the number in a human eye). However, squid vision is monochromatic.

Dogs have only two types of cones and cannot easily distinguish red from green.

Bees can detect UV light and polarized light.

Peregrine falcons can detect a pigeon from five miles away.

Temperature Sensors:

Rattlesnakes are very sensitive to infrared radiation (i.e. heat) and can detect heat changes as small as 0.002 C.

Electric Sensors:

Many fishes can detect electric fields and can use them to locate (e.g. buried in sand) prey invertebrates and fishes.

The platypus uses sensors in its bill to detect the electric fields (as small as 0.5 mV) generated by freshwater shrimp.

Magnetic Sensors:

Birds, honeybees, and snails are sensitive to magnetic fields and use these for orientation.

Pressure Sensors:

Pigeons can determine changes in altitude by monitoring barometric pressure.

Penguins and sharks use pressure sensitive organs to monitor water depth.

 

 

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