1. Chemoreceptors respond to particular chemical structures.

2. Photoreceptors respond to light.

3. Thermoreceptors respond to heat, including infrared radiation.

4. Mechanoreceptors respond to mechanical changes such as touch, pressure, or sound waves.

5. Electroreceptors, which only certain fishes possess, respond to changes in electric currents.

6. Each receptor translates, or transduces, the stimulus into a change in membrane potential.

7. The change in membrane potential is converted into a common language of nerve impulses, the only language the nervous system can interpret (Figures 43.1 and 43.2).

1. Exteroceptors such as those in the eyes and ears respond to stimuli outside the body.

2. Interoceptors monitor internal factors such as blood glucose concentration and blood pressure.

3. Proprioceptors are mechanoreceptors that detect the orientation of the body.

1. Generator potential is always a graded potential proportional to the energy of the stimulus impinging on the receptor.

2. Some receptors are neurons themselves, and in these, the changes in membrane potential affect the frequency of action potentials the neurons send out along their axons (Figure 43.3).

3. Other receptors are cells that synapse with neurons.

4. The frequency of these impulses reflects the stimulus intensity, as reflected by the Law of Intensity Coding: The frequency of nerve impulses from a receptor increases with the intensity of the stimulus.

1. Some touches are too light to feel, some sounds are too soft to hear.

2. Each sense organ has an absolute threshold, the minimum energy a stimulus must have, conventionally defined as the strength it must have to be detected half the time (Figure 43.4).

1. Receptors of the phasic type produce action potentials for only a short time after being stimulated and then stop responding; this behavior is sensory adaptation.

2. A tonic receptor continues to respond even to constant stimuli.

3. If an animal no longer responds to stimuli arising from a tonic receptor, it is not undergoing adaptation but rather a more complex process of habituation through a change in the nervous system downstream of the receptor.

1. The differential threshold (DS) is the change in stimulus strength (S) that will be detected as a new stimulus half the time.

2. In 1834, Ernst Weber discovered a basic law of sensory physiology: Within limits, DS/S (the Weber fraction) is a constant for each sense.

1. Olfaction means the reception of many distinct molecules, while taste means the reception of a few general qualities such as sweetness or saltiness.

2. What we call tasting is largely smelling, because some air normally rises from the back of the throat to the olfactory receptors while we chew food.

3. Aquatic animals like fish also have different types of cells that correspond to taste and smell receptors and are connected to different brain centers.

1. Steroids and other substances normally found in human sweat seem to be pheromones that have subtle effects on human behavior.

2. Sweetness indicates a source of energy and tends to attract animals and stimulate feeding.

3. Bitterness may warn of potentially harmful compounds to avoid.

1. The papillae tend to be specialized for each taste (Figure 43.5), and each one may bear 200 or more taste buds, each made of several taste receptors.

2. Only dissolved substances can be tasted.

3. Molecules that elicit one taste sensation, such as bitterness, presumably have common chemical features of shape and electrical charge that allow them all to bind to the same type of receptor protein.

1. Olfactory receptors are the only neurons known to regenerate; in humans, each one lasts for an average of 60 days and is then replaced by the differentiation of basal cells in the olfactory epithelium.

2. Because odor is so important to certain industries, notably the perfume trade, there have been several attempts to devise a good vocabulary for describing an odor, but no one system is generally accepted.

3. People who have a specific odor-blindness (anosmia), comparable to color-blindness, provide evidence that specific odor receptors exist.

4. Current olfaction theory postulates that molecules of every odorant (a substance we can smell) bind stereospecifically to several proteins, each on a different olfactory receptor cell (Figure 43.7).

1. Mechanoreceptors are concentrated in the skin, especially in our hands and in particularly erotic areas such as the lips, nipples, and genitals.

2. Movements of hairs convey a lot of sensory information about events on or near the skin, because even the slight bending of a hair will stimulate nerve endings wrapped around its base and several other receptors located nearby (Figure 43.8).

3. Pacinian corpuscles are buried deep in the skin where they apparently detect deep touch and vibration, while Meissner's corpuscles and Merkel's discs detect lighter touches.

1. We depend on proprioception for balance and for knowing, without having to look, just how each muscle or finger is tensed and oriented.

2. Information about muscle tension comes from stretch receptors called muscle spindles (Figure 43.9).

1. Suddenly striking the knee tendon stretches spindles in the thigh muscles, causing sensory neurons in the muscle spindles to send signals quickly along sensory fibers and back through motor neurons to make the muscles contract, so your leg kicks (Figure 43.10).

2. The kick allows the thigh muscles to contract enough to release the tension in their intrafusal fibers and stop them from signaling.

1. The "hair" is really a set of cilia that bend under slight pressure (Figure 43.11).

2. A hair cell is in contact with a sensory neuron, and by releasing neurotransmitters into the synapse between them at a certain rate when the cilia are in a neutral position, it causes the neuron to send out a steady stream of nerve impulses.

3. By changing its rate of releasing neurotransmitters, the hair cell causes the neuron to fire at a higher frequency when the cilia are bent one direction and at a lower frequency when they are bent the other direction.

4. The angular position of the hair cell's cilia is encoded in the firing frequency of the sensory neuron.

1. A statocyst consists of a cavity lined by hair cells with a grain of sand (statolith) in the middle (Figure 43.12).

2. As gravity pulls the grain downward, it presses against the hair cells beneath it, so by their arrangement and connection to the brain, they continually tell the animal which way is down.

1. These cells respond to small water displacements, especially to low-frequency vibrations in the water.

2. The lateral line system reports on events in the immediate environment, including vibrations from the movements of other animals.

1. Humans experience two forms of acceleration: from movement and from gravity.

2. By detecting all accelerations, the labyrinth provides the information needed for orientation in equilibrium.

3. The layers of hair cells are arranged in two different planes, so the utricle detects mainly horizontal acceleration and the saccule mainly vertical acceleration, telling an animal how its head is oriented.

1. As hair cells in the canals are pushed one way or another by the movements of the endolymph, they detect accelerations, not steady movement.

2. Humans feel little sensation of movement in a car moving steadily along a straight highway, but if the car changes speed quickly or swerves to one side, the endolymph in the semicircular ducts is shifted in one direction, giving you a sensation of movement.

1. The vertebrate ear is a device for delivering sound waves to hair cells, which are well-suited for detecting vibrations (Figure 43.15).

2. Sound waves initially set up vibrations of the tympanic membrane, or eardrum.

3. In mammals, these vibrations are then transmitted through the three small bones of the middle ear–hammer, anvil, and stirrup–that link the eardrum with the oval window of the cochlea, a snail-shaped cavity in the mastoid bone of the skull containing the innervated organ where vibrations are converted into nerve impulses.

4. These bones also increase the strength of the vibrations they carry.

5. The cochlea is divided lengthwise into three fluid-filled canals: vestibular and tympanic canals contain the same fluid and join at the distal end of the cochlea.

6. A smaller middle canal contains a different fluid and partly encloses the ear's sensory tissue, the organ of Corti.

1. The upper limit declines sharply with age in varying degrees.

2. Some of the loss is due to a slight decline in the mobility of the middle-ear chain.

3. Continuing exposure to noise distorts and destroys the arrangements of delicate cilia in the organ of Corti (Figure 43.16).

1. Some mammalian thermoreceptors are just free nerve endings in the skin; others have distinctive shapes, like the end-bulb of Krause that detects cold and the Ruffini ending that detects warmth.

2. Thermoreceptors adapt rapidly to changing environments.

3. Perhaps the most extensively analyzed heat receptors are the infrared detectors of snakes, particularly those of pit vipers, like rattlesnakes, that have heat-sensitive membranes in the pits along the sides of their heads (Figure 43.17).

1. Most animals have photoreceptors.

2. Invertebrates like earthworms and certain clams have photoreceptor cells scattered over their bodies that can only report the general intensity of light.

3. Other animals have complex eyes with lenses that collect and focus light on clusters or sheets of photoreceptors.

4. A very interesting area of study is that of parallel and convergent evolution concerning independent evolution of eyes with similar structures in several phyla of animals (Figure 43.18).

1. Visual photopigments consist of a protein, opsin, bound to a chromatophore, the small organic molecule that absorbs light.

2. The main mammalian chromatophore is the carotenoid retinal, a derivative of vitamin A.

3. Rhodopsin is made of opsin bound to the colored 11-cis-retinal (Figure 43.19).

1. Rhabdomeric receptors found in the eyes of molluscs and arthropods, have the outer segment of the cell membrane folded into a dense array of microvilli that contain the photopigment (Figure 43.20).

2. The ciliary receptors of a vertebrate eye are modified cilia, which show their ancestry by retaining a ninefold right of microtubules (Figure 43.19).

1. Vertebrate rods are remarkably sensitive to dim light.

2. Cones respond poorly to dim light, and are the receptors for color vision (Figure 43.21).

1. Just inside the sclera is the choroid layer containing blood vessels and a dark pigment that absorbs light, preventing internally reflected light from blurring the visual image.

2. Just behind the cornea, the choroid extends inward to make the iris, the colored ring that gives an eye its characteristic color.

3. The center of the iris is the pupil, a round hole that admits light to the body of the eye.

4. Changes in the ambient light intensity stimulate the pupillary reflex, a contraction or dilation of the iris that keeps the amount of light entering the eye relatively constant.

5. Just behind the iris is the lens, made of elongated cells filled with proteins called crystallins.

1. Rods and cones form the outside layer, farthest from the source of light, and synapse with an intermediate layer of cells that, in turn, synapse with cells of the ganglion layer (Figure 43.22).

2. Light passes through two layers of cells which are not stimulated before it reaches the photoreceptors.

3. At one spot called the fovea centralis these overlying cells are displaced so light does not have to pass through them before reaching the photoreceptors.

4. Rods and cones are particularly dense in the fovea, and these features combine to make it a spot of particular visual acuity.

5. The axons of the ganglion cells converge to form the optic nerve leading to the brain.

1. Because light passing from one medium to another is bent, or refracted, the curved cornea acts as a fixed lens with just the right shape to focus parallel light rays onto the retina.

2. Light rays from a source more than 5—6 meters away are essentially parallel, and an eye is best adapted to seeing objects at such distances, with the lens quite flat so as to produce minimal additional refraction of the incoming light.

3. To view a closer object, the lens has to accommodate by changing its shape through muscle movement.

4. Accommodation generally becomes more difficult with age, as the lens loses its flexibility.

5. Nearsightedness (myopia) and farsightedness (hyperopia) are the result of eyeballs being either too long or too short, respectively, to focus light properly (Figure 43.24).

1. Visual information is not just presented to the brain as a pattern of light and dark, but from the beginning, even in the retina itself, it is being analyzed into patterns and forms.

2. The patterns are being synthesized into increasingly complex forms.

3. Visual information is processed in the sequence: Light -> Rods and Cones -> Intermediate cells -> Ganglion cells -> via optic nerve -> lateral geniculate nucleus of the thalamus -> Visual cortex -> Visual association cortex.

1. The information from the receptors is processed through intermediate neurons so as to accentuate the pattern of light and dark falling across the retina.

2. The visual field of an eye is the entire space that the eye may be focused on at any time.

3. Within the visual field, investigators map out the receptive fields of single neurons: the area or form within the visual field to which the neuron responds.

1. The receptive fields of both types have a circular center with a surrounding ring.

2. A cell with an on-center field fires when the center is illuminated and the surrounding ring is dark.

3. A cell with an off-center field fires when the center is dark and the ring is illuminated (Figure 43.25).

4. The stimulus that makes a ganglion cell fire must reflect the way it is connected to a group of receptors (Figure 43.26).

5. The ganglion cells then pass visual information on to brain centers.

1. The eyes send information from the left halves of both retinas to the left side of the brain and from the right halves to the right side.

2. Each LGN receives information about the same image from both eyes and somehow correlates this information, allowing us to have stereoscopic vision, which depends on information from two independent views of space.

1. One cell might respond strongly to a line oriented at 40 degrees to the horizontal, another at 50 degrees, and so on.

2. Each cell discriminates so well that changing the angle of the line by only 4 degrees makes it ineffective as a stimulus for that cell.

3. Collectively, the cortex cells respond to lines in every part of the visual field and with every possible orientation (about 4 degrees apart).

4. The simple cells synapse with complex cells that also respond only to lines with specific orientations, but each complex cell responds to lines located anywhere in a large part of the visual field and to lines moving through the field with the right orientation (Figure 43.28).

1. A hypercomplex cell responds maximally to a line in a particular orientation that has a definite end somewhere, or perhaps two definite ends (Figure 43.28).

2. Hypercomplex cells respond to the length of a line as well as to its orientation.

3. They also respond to corners with certain orientations.

4. Other kinds of hypercomplex cells respond maximally to more complicated figures, such as a tonguelike bar of a certain length and width, but oriented at any angle and moving in any direction.

5. Figure 43.29 shows how a series of LGN cells whose receptive fields lie in a straight line could then be connected to stimulate one simple cell, if and only if they are all stimulated at once.