Characteristics of Sensory Receptors 242

I. Sensory receptors may be categorized on the basis of their structure, the stimulus energy they transduce, or the nature of their response.

A. Receptors may be dendritic nerve endings, specialized neurons, or specialized epithelial cells associated with sensory nerve endings.

B. Receptors may be chemoreceptors, photoreceptors, thermoreceptors, mechanoreceptors, or nociceptors.

1. Proprioceptors include receptors in the muscles, tendons, and joints.

2. The senses of sight, hearing, taste, olfaction, and equilibrium are grouped as special senses.

C. Receptors vary in the duration of their firing in response to a constant stimulus.

1. Tonic receptors continue to fire as long as the stimulus is maintained; they monitor the presence and intensity of a stimulus.

2. Phasic receptors respond to stimulus changes; they do not respond to a sustained stimulus, and this partly accounts for sensory adaptation.

II. According to the law of specific nerve energies, each sensory receptor responds with lowest threshold to only one modality of sensation.

A. That stimulus modality is called the adequate stimulus.

B. Stimulation of the sensory nerve from a receptor by any means is interpreted in the brain as the adequate stimulus modality of that receptor.

III. Generator potentials are graded changes (usually depolarizations) in the membrane potential of the dendritic endings of sensory neurons. A. The magnitude of the potential change of the generator potential is directly proportional to the strength of the stimulus applied to the receptor.

B. After the generator potential reaches a threshold value, increases in the magnitude of the depolarization result in increased frequency of action potential production in the sensory neuron.

Cutaneous Sensations 244

I. Somatesthetic information—from cutaneous receptors and proprioceptors—is carried by third-order neurons to the postcentral gyrus of the cerebrum.

A. Proprioception and pressure sensations ascend on the ipsilateral side of the spinal cord, synapse in the medulla and cross to the contralateral side, and then ascend in the medial lemniscus to the thalamus; neurons in the thalamus in turn project to the postcentral gyrus.

B. Sensory neurons from other cutaneous receptors synapse and cross to the contralateral side in the spinal cord and ascend in the lateral and ventral spinothalamic tracts to the thalamus; neurons in the thalamus then project to the postcentral gyrus.

II. The receptive field of a cutaneous sensory neuron is the area of skin that, when stimulated, produces responses in the neuron.

A. The receptive fields are smaller where the skin has a greater density of cutaneous receptors.

B. The two-point touch threshold test reveals that the fingertips and tip of the tongue have a greater density of touch receptors, and thus a greater sensory acuity, than other areas of the body.

III. Lateral inhibition acts to sharpen a sensation by inhibiting the activity of sensory neurons coming from areas of the skin around the area that is most greatly stimulated.

Taste and Smell 248

I. The sense of taste is mediated by taste buds.

A. There are four well-established modalities of taste (salty, sour, sweet, and bitter); a fifth, called umami, which is stimulated by glutamate, is now also recognized.

B. Salty and sour taste are produced by the movement of sodium and hydrogen ions, respectively, through membrane channels; sweet and bitter tastes are produced by binding of molecules to protein receptors that are coupled to G-proteins.

II. The olfactory receptors are neurons that synapse within the olfactory bulb of the brain.

A. Odorant molecules bind to membrane protein receptors. There may be as many as 1,000 different receptor proteins responsible for the ability to detect as many as 10,000 different odors.

B. Binding of an odorant molecule to its receptor causes the dissociation of large numbers of G-protein subunits. The effect is thereby amplified, which may contribute to the extreme sensitivity of the sense of smell.

Vestibular Apparatus and Equilibrium 251

I. The structures for equilibrium and hearing are located in the inner ear, within the membranous labyrinth.

A. The structure involved in equilibrium, known as the vestibular apparatus, consists of the otolith organs (utricle and saccule) and the semicircular canals.

B. The utricle and saccule provide information about linear acceleration, whereas the semicircular canals provide information about angular acceleration.

C. The sensory receptors for equilibrium are hair cells that support numerous stereocilia and one kinocilium.

1. When the stereocilia are bent in the direction of the kinocilium, the cell membrane becomes depolarized.

2. When the stereocilia are bent in the opposite direction, the membrane becomes hyperpolarized.

II. The stereocilia of the hair cells in the utricle and saccule project into the endolymph of the membranous labyrinth and are embedded in a gelatinous otolithic membrane.

A. When a person is upright, the stereocilia of the utricle are oriented vertically; those of the saccule are oriented horizontally.

B. Linear acceleration produces a shearing force between the hairs of the otolithic membrane, thus bending the stereocilia and electrically stimulating the sensory endings.

III. The three semicircular canals are oriented at nearly right angles to each other, like the faces of a cube.

A. The hair cells are embedded within a gelatinous membrane called the cupula, which projects into the endolymph.

B. Movement along one of the planes of a semicircular canal causes the endolymph to bend the cupula and stimulate the hair cells.

C. Stimulation of the hair cells in the vestibular apparatus activates the sensory neurons of the vestibulocochlear nerve (VIII), which projects to the cerebellum and to the vestibular nuclei of the medulla oblongata.

1. The vestibular nuclei in turn send fibers to the oculomotor center, which controls eye movements.

2. Spinning and then stopping abruptly can thus cause oscillatory movements of the eyes (nystagmus).

The Ears and Hearing 255

I. The outer ear funnels sound waves of a given frequency (measured in hertz) and intensity (measured in decibels) to the tympanic membrane, causing it to vibrate.

II. Vibrations of the tympanic membrane cause movement of the middle-ear ossicles—malleus, incus, and stapes— which in turn produces vibrations of the oval window of the cochlea.

III. Vibrations of the oval window set up a traveling wave of perilymph in the scala vestibuli.

A. This wave can pass around the helicotrema to the scala tympani, or it can reach the scala tympani by passing through the scala media (cochlear duct). B. The scala media is filled with endolymph.

1. The membrane of the cochlear duct that faces the scala vestibuli is called the vestibular membrane.

2. The membrane that faces the scala tympani is called the basilar membrane.

IV. The sensory structure of the cochlea is called the spiral organ or organ of Corti.

A. The organ of Corti rests on the basilar membrane and contains sensory hair cells.

1. The stereocilia of the hair cells project upward into an overhanging tectorial membrane.

2. The hair cells are innervated by the vestibulocochlear nerve (VIII).

B. Sounds of high frequency cause maximum displacement of the basilar membrane closer to its base, near the stapes; sounds of lower frequency produce maximum displacement of the basilar membrane closer to its apex, near the helicotrema.

1. Displacement of the basilar membrane causes the hairs to bend against the tectorial membrane and stimulate the production of nerve impulses.

2. Pitch discrimination is thus dependent on the region of the basilar membrane that vibrates maximally to sounds of different frequencies.

3. Pitch discrimination is enhanced by lateral inhibition.

The Eyes and Vision 261

I. Light enters the cornea of the eye, passes through the pupil (the opening of the iris) and then through the lens, from which point it is projected to the retina in the back of the eye.

A. Light rays are bent, or refracted, by the cornea and lens.

B. Because of refraction, the image on the retina is upside down and right to left.

C. The right half of the visual field is projected to the left half of the retina in each eye, and vice versa.

II. Accommodation is the ability to maintain a focus on the retina as the distance between the object and the eyes is changed.

A. Accommodation is achieved by changes in the shape and refractive power of the lens.

B. When the muscles of the ciliary body are relaxed, the suspensory ligament is tight, and the lens is pulled to its least convex form.

1. This gives the lens a low refractive power for distance vision.

2. As an object is brought closer than 20 feet from the eyes, the ciliary body contracts, the suspensory ligament becomes less tight, and the lens becomes more convex and more powerful.

III. Visual acuity refers to the sharpness of the image. It depends in part on the ability of the lens to bring the image to a focus on the retina.

A. People with myopia have an eyeball that is too long, so that the image is brought to a focus in front of the retina; this is corrected by a concave lens.

B. People with hyperopia have an eyeball that is too short, so that the image is brought to a focus behind the retina; this is corrected by a convex lens.

C. Astigmatism is the condition in which asymmetry of the cornea and/or lens causes uneven refraction of light around 360 degrees of a circle, resulting in an image that is not sharply focused on the retina.

Retina 268

I. The retina contains rods and cones— photoreceptor neurons that synapse with bipolar cells.

A. When light strikes the rods, it causes the photodissociation of rhodopsin into retinene and opsin.

1. This bleaching reaction occurs maximally with a light wavelength of 500 nm.

2. Photodissociation is caused by the conversion of the 11-cis form of retinene to the all-trans form that cannot bind to opsin.

B. In the dark, more rhodopsin can be produced, and increased rhodopsin in the rods makes the eyes more sensitive to light. The increased concentration of rhodopsin in the rods is partly responsible for dark adaptation. C. The rods provide black-and-white vision under conditions of low light intensity. At higher light intensity, the rods are bleached out and the cones provide color vision.

II. In the dark, a constant movement of Na+ into the rods produces what is known as a "dark current."

A. When light causes the dissociation of rhodopsin, the Na+ channels become blocked and the rods become hyperpolarized in comparison to their membrane potential in the dark.

B. When the rods are hyperpolarized, they release less neurotransmitter at their synapses with bipolar cells.

C. Neurotransmitters from rods cause depolarization of bipolar cells in some cases and hyperpolarization of bipolar cells in other cases; thus, when the rods are in light and release less neurotransmitter, these effects are inverted.

III. According to the trichromatic theory of color vision, there are three systems of cones, each of which responds to one of three colors: red, blue, or green.

A. Each type of cone contains retinene attached to a different type of protein.

B. The names for the cones signify the region of the spectrum in which the cones absorb light maximally.

IV. The fovea centralis contains only cones; more peripheral parts of the retina contain both cones and rods. A. Each cone in the fovea synapses with one bipolar cell, which in turn synapses with one ganglion cell.

1. The ganglion cell that receives input from the fovea thus has a visual field limited to that part of the retina that activated its cone.

2. As a result of this 1:1 ratio of cones to bipolar cells, visual acuity is high in the fovea but sensitivity to low light levels is lower than in other regions of the retina.

B. In regions of the retina where rods predominate, large numbers of rods provide input to each ganglion cell (there is a great convergence). As a result, visual acuity is impaired, but sensitivity to low light levels is improved.

V. The right half of the visual field is projected to the left half of the retina of each eye.

A. The left half of the left retina sends fibers to the left lateral geniculate body of the thalamus.

B. The left half of the right retina also sends fibers to the left lateral geniculate body. This is because these fibers decussate in the optic chiasma.

C. The left lateral geniculate body thus receives input from the left half of the retina of both eyes, corresponding to the right half of the visual field; the right lateral geniculate receives information about the left half of the visual field.

1. Neurons in the lateral geniculate bodies send fibers to the striate cortex of the occipital lobes.

2. The geniculostriate system is involved in providing meaning to the images that form on the retina.

D. Instead of synapsing in the geniculate bodies, some fibers from the ganglion cells of the retina synapse in the superior colliculus of the midbrain, which controls eye movement.

1. Since this brain region is also called the optic tectum, this pathway is called the tectal system.

2. The tectal system enables the eyes to move and track an object; it is also responsible for the pupillary reflex and the changes in lens shape that are needed for accommodation.

Neural Processing of Visual Information 275

I. The area of the retina that provides input to a ganglion cell is called the receptive field of the ganglion cell.

A. The receptive field of a ganglion cell is roughly circular, with an "on" or "off" center and an antagonistic surround.

1. A spot of light in the center of an "on" receptive field stimulates the ganglion cell, whereas a spot of light in its surround inhibits the ganglion cell.

2. The opposite is true for ganglion cells with "off" receptive cells.

3. Wide illumination that stimulates both the center and the surround of a receptive field affects a ganglion cell to a lesser degree than a pinpoint of light that illuminates only the center or the surround.

B. The antagonistic center and surround of the receptive field of ganglion cells provide lateral inhibition, which enhances contours and provides better visual acuity.

II. Each lateral geniculate body receives input from both eyes relating to the same part of the visual field.

A. The neurons receiving input from each eye are arranged in layers within the lateral geniculate.

B. The receptive fields of neurons in the lateral geniculate are circular, with an antagonistic center and surround—much like the receptive field of ganglion cells.

III. Cortical neurons involved in vision may be simple, complex, or hypercomplex.

A. Simple neurons receive input from neurons in the lateral geniculate; complex neurons receive input from simple cells; and hypercomplex neurons receive input from complex cells.

B. Simple neurons are best stimulated by a slit or bar of light that is located in a precise part of the visual field and that has a precise orientation.

Sensory Physiology

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