Unit 3-Nervous System

Chapter 8


1. Receptor Organs:

     A receptor organ is one that detects changes in the external or internal environment. A listing of these organs and of the types of sensation which originate in them far exceeds the traditional sensory organs and sensory modalities. Thus, the eye, particularly suited to the detection of electromagnetic radiation in the wavelength range from 4000 to 7000 Angstrom units is recognized as a sense organ which serves vision. The ear, which detects mechanical vibrations in the frequency range from 18 to 18,000 cycles per second, is recognized as the organ of hearing. The tongue and the nasal mucosa are very sensitive to chemical changes, the tongue when the chemicals touch it in solid or liquid form, the nasal mucosa when the chemicals are brought to it by way of the air, though they must be dissolved before they are detected. The skin is sensitive to touch.

     But the skin is also sensitive to heat and cold, two separate senses, as well as pain, another separate sensation. The ear is sensitive to angular acceleration and to changes in position of the head. Muscles, tendons, and joints contain sense organs which sense position of parts of the body. The internal organs detect stretch, a form of particular interest is the stretching of special parts of the arterial system. The internal organs are also chemically sensitive, unusual chemical agents on their outside being easily detected. They detect internal chemical changes as well, usually when they are diseased, as in a peptic ulcer, but sometimes when they are not, as in heart burn. Vibrations which are audible are detected, possibly by the same sense organs which detect pressure. The eye, which detects light, may serve to distinguish colors or intensities, and in fact there are at least two types of receptors for these functions of the eye. Any part of the body can give rise to pain, for which there are some specialized receptor organs, but any overstimulated receptor can cause painful sensations. The special sensory receptors of the genital organs have not been clearly identified, but no one can doubt that specific genital sensations exist. This list is by no means complete--many more sensory modalities have been described as well as many more sensory receptors, but it may suffice to show that the usual "special senses" do not limit our ways of being aware of the environment.

     It may be added parenthetically that our society favors the usually named sensory modalities. Others are not the proper subject of conversation, even thought. It may be that these sensations are prevented from reaching cortical levels, but such suppression must be learned. The "mind-expanding" drugs may work by diminishing cortical suppression. New experiences, actually old ones which have been suppressed, now come to the cortex and are reckoned with. The process seems to be one of forgetting to forget things once forgotten. This gives the subjective impression of enlarging awareness, but there is no evidence that this is so. Evidence indicates that some of these drugs act in specific chemical ways on the central nervous system, but whether they result in better or worse adjustment to the environment is not, by any means, clear, and probably depends very largely on the environment rather than the central nervous system effects.

2. General Properties of Perception:

     Receptors of any kind transform one kind of energy into another. Regardless of the type of energy sensed, the receptor transforms it into a nerve impulse, which, as has been noted before, is probably electrical in nature or can be detected electrically.

     Each receptor organ appears to be adapted to transform a particular type of energy. Those of the ear respond best to sound; those of the eye respond to light; those of the skin to touch. Insensitive to light or sound, touch receptors of the skin discharge when they are subjected to slight mechanical disturbance. The separation is not, however, perfect, for very strong stimuli may activate the wrong receptor. For example, there are specific temperature receptors in the skin; but when the skin temperature is very high or very low, they may give rise to pain. In fact, pain may be elicited by over stimulation of any receptor, though there are also specific pain receptors, some of which are shown in Figure 165.

     The law of specific nerve energies states that direct stimulation of the receptor nerve produces a sensory response characteristic of the receptor organ. If the optic nerve is directly stimulated by pressure, the sensation perceived is vision, not pressure. Stimulation of the cochlear nerve by heat is perceived as sound. Electrical stimulation of the nerves serving the sense of taste results in taste sensation. A very cold object applied to an area of the skin where there are heat receptors is perceived as hot.

     The last two paragraphs are not inconsistent with each other. Any sense organ can be made to respond to any stimulus, but it is most easily stimulated by the kind of stimulus for which it is fashioned.

     All sensory organs have a threshhold. Stimuli below this threshhold are said to be inadequate. As the strength of the stimulus increases to adequate levels, the sense organ discharges, as its membrane, or membranes, become depolarized. In some receptors, the membrane repolarizes quickly despite continued application of the stimulus. Others remain depolarized during the application of the stimulus.

     Depolarization of the receptor does not necessarily lead to neural discharge. In fact, such depolarization may not be communicated to the sensory nerve fiber. In the sense organs which detect pressure, the Pacinian corpuscles, the continued application of pressure may lead to a non-propagated potential called the generator potential. More intense pressure may lead to greater generator potentials, and these may excite the nerve fiber. The larger the generator potential, the more frequent will neural discharges be, though, characteristically, the neural discharges are always of the same size.

     Some sense organs which repolarize after they are stimulated, and the generator potential decays to a level adequate to stimulate the neuron are said to show adaptation. A steady stimulus affecting such an organ is not perceived after a few moments. It is possible that the physical composition of the receptor is altered by the stimulus; for example when a touch receptor is touched, it may assume a new shape in which its membranes are no longer deformed and are, therefore, not depolarized. Other mechanisms for adaptation have been proposed. One of the most interesting is the possibility that autonomic fibers may hyperpolarize the receptors of the membrane.

     These receptors in which adaptation is most rapid are called phasic. These are particularly useful in detecting rapid changes in the environment. Tonic receptors, which adapt slowly, show large generator potentials which initiate many neural discharges as long as they are stimulated. Touch receptors belong to the first group; position receptors in muscles and joints arc considered tonic.

3. General Somatic Sensation:

     Touch and pressure receptors are found all over the body. Free nerve endings in the skin, Meissner's corpuscles, Pacinian corpuscles, and sensory endings attached to the hairs all are capable of detecting light touch and pressure. The skin also contains receptors sensitive to heat, Ruffini's end organ, and cold, Krause's corpuscle. Any sensory ending in the skin can give rise to pain, and the free nerve endings are most sensitive to painful stimuli.

     Muscles, tendons, and joints contain tonic receptors which respond to stretch of the affected parts. The muscle receptor is called the muscle spindle. The tendons contain the Golgi tendon apparatus in large number, while the joints contain many nerve endings. See Figure 166.

     The information conveyed by these receptors travels with the mixed spinal nerve. On reaching the spinal cord the sensory and motor fibers are separated; the former go to the dorsal root ganglion, where their neuron is, and thence to the cord by way of the dorsal root. Their subsequent course has already been described (Chapter 5). It should be reemphasized that much of the information crosses over to the opposite side of the brain, but not all of it does. The level at which crossing occurs is also variable. For example, the spinothalamic tracts, which carry information from sense organs concerned with pain and temperature, cross over immediately. The tracts concerned with touch and pressure information remain on the same side of the cord, crossing over in the medulla. The spinocerebellar tracts, carrying information relative to position, rise in the cord from both sides. Their representation is in the cerebullum, which is a midline structure, so position sense is rarely lost on one side.

     The organs of special sense will be discussed in Parts 5 through 10 of this chapter. Pain will be considered in more detail in Part 12.

4. General Visceral Sensation:

     There is no doubt whatever that the internal organs can send sensory information to the central nervous system (see Chapter 6). Some of this sensory information originates in the smooth muscle. In correspondence with the variable potentials of smooth muscles and the slowness of conduction through these muscles, this type of sensation is vague and shifting. It usually travels to the central nervous system by way of very slowly conducting tracts. Its course and central representation are not well known.

     Sensory information originating from the outermost layer of the viscera is usually painful. It is more rapidly propagated and more accurately defined in location. The reasons for this are given in Part 12.

     Some very unfamiliar information is normally carried by visceral afferents. For example, the blood pressure is monitored by stretch receptors in the arch of the aorta and the carotid sinuses. This information, which almost never reaches conscious awareness, is nevertheless used in the correction of altered blood pressure (See Chapters 11 and 12). Disease of these receptors or their pathways may lead to high blood pressure and occasionally to low blood pressure.

     Low oxygen and high carbon dioxide stimulate the aortic and carotid bodies, increasing respiratory activity. A high concentration of blood solutes, reaching the hypothalamus, may act as a stimulus for the retention of water and perhaps thirst (Chapter 23).

     Visceral afferents are difficult to study, and the view which once prevailed that none existed did not help to encourage research in the field. There are probably many more than those indicated in this brief list.

5. Hearing:

     The organ of hearing has been studied extensively. The process of hearing is basically one in which sound waves are converted into nerve impulses which reach the central nervous system by way of the cochlear branch of the vestibulocochlear nerve.

     The transformation of sound waves begins in the external ear, which being somewhat funnel shaped, amplifies them. The auditory canal ends at the tympanic membrane, which is set into vibration by the vibrating air. The vibrations of the tympanic membrane are converted into movement of the bones of the middle car. These little bones, or ossicles, lie in a small cavity which communicates with the throat by the narrow Eustacian tube. This tube, though usually open, may become shut.

     The ossicles beginning at the tympanic membrane are called the malleus, incus, and the stapes, the Latin terms for hammer, anvil, and stirrup, which the bones resemble. The ossicles are closely coupled to each other, so vibrations of the tympanic membrane are faithfully transmitted to the stapes.

     The stapes rests on the oval window, the entrance to the inner ear. As it vibrates, the oval window also vibrates, and the fluid of the inner ear, the endolvmph, also vibrates. Since fluid is incompressible, vibration would not be possible if there were not a yielding structure at its end which communicated with air. This structure, the round window, connects to the middle ear.

     Before the vibrations begun at the oval window reach the round window, they travel through a spiral structure with two and one half turns called the cochlea, which is the actual organ of hearing.

     The vibrations of the endolymph are picked up by the part of the cochlea which resonates with them. This is the basilar membrane, which is long at the beginning of the cochea and correspondingly vibrates with low pitched sounds, and is quite short at the top of the cochlea, where it resonates in response to sounds of high pitch. The basilar mernbrane is covered with hair cells; when a portion of the membrane vibrates, its hair cells vibrate, and in vibrating, they rub against a structure called the tectorial membrane. The rubbing stimulates the cells, which communicate this information to branches of the cochlear nerve. The structure which includes the basilar membrane, the hair cells, the tectorial membrane, and the cochlear nerve fibers is called the organ of corti.

     The idea that the cochlear nerve carries the same frequencies that are heard is called the frequency theory. Unfortunately for this theory, the cochlear nerve is never able to carry frequencies greater than 2000 cycles per second, yet a person with normal heating can detect up to 20,000 cycles per second. It must be concluded that it is the place stimulated in the cochlea, not the frequency of cochlear nerve, discharge which determines the pitch heard. The fact that loudness and pitch can both be discriminated in a pure tone is further evidence for the place theory. The nerve impulse is all-or-none, so it cannot vary its size. It can, however, vary its frequency, which supplies information concerning its loudness, and it can be derived from different parts of the cochlea, which gives information concerning its pitch. Presumably, if the cochlear nerve fibrils supplying the apex were stimulated slowly, a faint, high-pitched sound would be heard, while if the base were stimulated with high frequencies, a loud, low-pitched sound would be sensed.

     The central connections for hearing show medullary synapses, the cochlear nuclei. Secondary synapses occur in the inferior coiliculi both ipsilaterally and contralaterally. They are presumably concerned with auditory reflexes. Other cells proceed to the thalamus and thence to the auditory cortex in the superior portion of the temporal lobe.

     The cochlea is part of the labyrinth. Like the rest of the labyrinth, the cochlea consists of two parts. The bony labyrinth is a set of channels in the temporal bone. The membranous labyrinth, the true sense organ, fits closely within the bony labyrinth, is surrounded by perilymph, and filled with endolymph. See Figure 169.

6. Semicircular Canals, Utricle, and Saccule:

     The semicircular canals, utricle, and saccule are connected to the cochlea, but have entirely different functions. A schematic view of their relationships to each other and to the cochlea is shown in Figure 170. The three semicircular canals, at right angles to each other, all end in the utricle, dilating into ampullae before so doing, the utricle communicates by way of a small duct, the utriculo-saccular duct, with the saccule, which in turn has an endolymphatic connection with the cochlea. The vestibule is that part of the bony labyrinth that contains both utricle and saccule.

     Within the ampulla of each semicircular canal is a little valve-like structure which can separate the semicircular canal from the utricle or the ampulla from the canal. This structure is named the cupula. See Figure 171. Through movements of the cupula, pressure changes can be induced in the ampulla when the body is rotated in the plane of any semicircular canal. Such pressure changes affect the receptor organs of the ampulla, the cristaampullaris and through these vestibular nerve fibers. The fact that the semicircular canals are at right angles to each other and involve three planes implies that cristas on both sides will be involved in changing the speed of rotation in any plane. At constant speed of rotation, the cupula comes into midposition, and there are no pressure changes in the cristas.

     The utricles and saccules contain sensory areas called macules. Small dense particles called otoliths are attached to the hair cells of these macules. Linear acceleration in any direction changes the relationship of the otoliths to the macules, and fibers of the vestibular branch of the vestibulo-cochlear nerve are stimulated.

     The vestibular nerve fibers pass through the vestibular ganglion, where their cell bodies are, and synapse in the vestibular nuclei or pass into the cerebellum. These fibers which synapse in the vestibular nuclei pass downward to motor neuron in the spinal cord and upward to the motor nuclei of the cranial nerves concerned in eye movement. Some fibers may pass upward into the thalamus, and synapsing there, they may convey information to the cortex.

     The functions of the semicircular canals, utricle, and saccule may be summarized by the statement that they are concerned with acceleration and movement, whether linear or rotational. Through their activities, the eyes are fixed as well as possible, and the position of the head in relation to the body is optimized.

7. The Eye as an Optical Instrument:

     The sense of vision as observed in man requires:

     These subjects will be covered in Parts 7, 8, and 9 of this chapter. This section is concerned solely with image formation.

     Figure 172 shows those structures of the eye which are involved in the formation of the image. Of particular importance are the transparent cornea, the iris, lens, ciliary bodies, and aqueous humor, the vitreous humor, the retina, the choroid, and the fovea.

     When light rays pass at an angle from a medium of low refraction, such as air, to a medium of high refraction, such as water, they are so bent that the rays become more nearly vertical. See Figure 173. If the medium entered is properly shaped, such as a biconvex lens, parallel light rays will be brought to focus at the focal point. Light rays originating at a point source in front of the lens, will also be brought to a focus, but behind the focal point of the lens. See Figure 174.

     The focal point depends on the curvature of the lens and the refractive index of the material of which the lens is made. For example, a highly curved lens made of a highly refractive material will have a focal point closer to the lens than will a less curved lens of less refractive material.

     Real images are produced by objects in front of a biconvex lens in a plane behind the focal point of the lens. In order to understand this, we must define the optical center of the eye. This is that point on the optical axis of the eye through which the light ray is not bent at all. It is usually near the center of the eye. See Figure 175. If the focal point is known, the distance of the real image and the object itself are given by the relationship:

1   1   1

F   Dobject   Dimage

     The use of this equation will be illustrated by numbers which characterize a real eye. In such an eye, F = 15 mm. If the distance of the object looked at is very great, say, 50,000 mm, the value of 1 / Dobject becomes negligible, and it can be predicted that a real image will form 15 mm behind the optical center of the eye. It happens that in the real normal eye the retina is located exactly at this distance behind the optical center of the eye, so a real image forms on the retina.

     The way in which F changes is related to changes in the power of the lens. As noted above, the focal point of a lens depends on its curvature as well as its materials. In the diagrammatic eye of Figure 175, the lens is shown to be suspended by zonal fibers attached to the ciliary muscle. The attachment is such that as the ciliary muscle contracts, the tension on these fibers is reduced. The lens in a slack container bulges. Its curvature is increased and its focal distance reduced. The eye is now better accommodated for near vision. Relaxation of the ciliary muscle increases the tension on the zonal fibers and has the opposite effect.

     A convenient unit for describing the overall optical properties of the eye is the diopter. A lens with a power of one diopter has a focal distance of 1000 mm. The dioptric power of a lens system is given as the ratio:

1000 mm   D

Focal Distance (mm)  

For the normal eye with a focal distance of 15 mm, D = 66.7. This value represents a very powerful lens, but most of the dioptric power is derived from the cornea rather than the lens. The lens contributes a little; in a young person of 20 years, it can add 10 Diopters to the power of the eye by bulging when the ciliary muscles are relaxed. In the older person of 40 years, the lens bulges less and adds only 5 Diopters to the power of the eye. At 60 years, changes in the shape of the lens alter power by only 1 Diopter.

     It is well known that lens systems produce the clearest images in their principal axis. The more peripheral portions of the lens system produce rather blurred images. This phenomenon is seen in physical lens systems as well as in the eye and is spoken of as spherical aberration. Even when spherical aberration is corrected by reshaping the lens, chromatic aberration occurs; that is to say that lights of different colors come to a focus at different points behind the lens.

     In physical lens systems, spherical and chromatic aberrations arc dealt with by using a shutter which excludes most of the borders of the lens. Doing this, of course, reduces the amount of light which passes through the system. Increased accuracy of image formation is, so to speak bought, by faintness of the image.

     The eye has a device similar to the shutter and suffers from the same disadvantages. The shutter of the eye is called the iris, whose muscular fibers contract to make the optical path quite narrow, and less light reaches the retina. This contraction occurs autonomically, when the eye is used for near vision, and it also occurs when the object examined is bright. In the first case, the eye is said to have accommodated for near vision, while in the second case the eye has accommodated for light. The afferent pathways for the two types of accommodation are different; they will be discussed in Part 12. Accommodation for near vision can be advantageous provided the object is brightly illuminated as well, but if the illumination is poor, it can obscure the object.

     The iris and the ciliary muscle have adequate blood supplies of their own. The cornea, lens, lens capsule, and zonal fibers of the lens have no independent blood supply. Their nutritional requirements are low, but they do exist. These requirements are met largely by the aqueous humor, a fluid derived from the ciliary body which fills the entire space in front of the zonal fibers. It is divided into an anterior and posterior chamber by the iris. It moves from the ciliary body to the posterior chamber through the hole in the iris, the pupil, and into the anterior chamber. It leaves the anterior chamber by way of the canal of Schlemm. See Figure 176.

     The optical properties of the eye make possible semimicroscopic examinations of the retina. The instrument used is called the ophthalmoscope. In the usual ophthalmoscopic examination, light directed into the patient's eye acts as a high-powered lens which magnifies the structures at its back end. In fact, the lens is so high powered that retinal structures may be magnified as much as a hundred times. NOTE WELL: The lenses of the opthalmoscope contribute little or nothing to the magnification. Almost all of the magnification is achieved by the subject's eye itself. The ophthalmoscope lenses adjust the eye of the physician to that of the patient.

     The use of the ophthalmoscope makes possible the detailed examination of the retina. Very small blood vessels can be seen and the point of entry of the optic nerve is easily identified. Figure 177 shows the arrangement in ophthalomoscopy. Figure 178 shows the view of the eye obtained by the observer

8. Properties of the Retina:

     It was shown in Chapter 6 that the eye could be understood as an optical instrument to a camera. The "film" of this camera is, of course, the retina. It was actually used so in1878 by the German physiologist Kühne. He was able to obtain rough pictures of the last objects seen by a rabbit by killing it, removing its retina, and putting it into alum. This kind of picture is called an optogram. Kühne made one optogram in man--the subject was a beheaded criminal. There was, in fact, an image on the retina, but no one could ever interpret it.

     The pigment responsible for the optogram was rhodopsin, or visual purple. The retina contains rhodopsin and other pigments, but its most striking characteristic is that it contains so may cells. The anterior surface of the retina contains optic nerve fibers; just behind these are the cell bodies which give rise to these fibers. These cells, called ganglion cells, have innumerable dendrites which are supplied by a still deeper layer of bipolar neurons. These in turn connect to the cells which contain the rhodopsin and other visual pigments. The cellular arrangement of the retina is shown in diagrammatic form in Figure 179. The cells superficial to those of the pigment-containing cells are multiply connected to each other, so that the retinal connections are not only from the pigment cells to the bipolar and ganglion cells, but also from one area of the retina to another. Thus, visual information which is inadequate at each of two retinal areas may summate, and a visual stimulus may be perceived.

     Pursuing this line of investigation leads to some strange findings: Some retinal areas do not summate with but rather inhibit the response of the retina to other stimuli. Usually the areas which produce summation in the adjacent areas are themselves inhibited by light. Those which are stimulated by light tend to produce inhibition in the adjacent areas. Thus, by using a certain area of the retina to look at something, one may be blinded to things around it. In contrast, one may "not look" at something by using retinal cells which are inhibited by light, things around these unseeing areas being better perceived than before.

     Two types of retinal receptors have been described. They are called rods and cones. There are 250 million rods and 12 million cones between the two eyes. Altogether, there are less than a million optic nerve fibers. The huge difference between the number of receptors and the number of nerve fibers suggests that the optic nerve impulses depend on multiple stimulation of receptors and the right pattern of summation and inhibition.

     There is one area, however, which is exceptional. This is called the fovea and is the part of the retina used when one looks at something. Here there are no rods at all. Instead, each cone in the fovea has a single bipolar cell which connects to a single ganglion cell.

     There are probably three types of pigment in the cones, though only two have been identified. One is called erythrolabe, the red catching pigment; a green sensitive pigment has been called chlorolabe; and the existence of a blue sensitive pigment, cyanolabe, is suspected, but has not been proved. Whether each cone contains all three pigments, two of them, or just one type of pigment per cone is quite uncertain.

     The probable sequence of events in the retina has been established in the rods which are sensitive to light and dark but not to color. The rods contain rhodopsin, which is easily broken down by light to retinene and a protein called scotopsin. Retinene is derived from Vitamin A. Scotopsin is not broken down. Some of the retinene is broken down and stimulates the rods, which in turn stimulate the bipolar cells; these stimulate the ganglion cells to discharge, and the end result is optic nerve fiber stimulation. The original condition is restored by retinene resynthesis, which may require Vitamin A. The breakdown of rhodopsin is very easy, and it has been suggested that one photon is enough to achieve the breakdown of one molecule of rhodopsin.

     However, it is more probable that at least two hundred and forty retinene molecules must be broken down before a nerve fiber is stimulated. This figure is probably too small, for foveal fixation probably inhibits ganglion discharge. Nevertheless, it represents a minute amount of light energy. This may explain why the visual fields are so sensitive to light and dark. Rhodopsin can be almost completely destroyed by bright light. Light-dark sensitivity is, however, restored in 20 minutes of dark exposure, as the rhodopsin is reconstituted. This phenomenon which increases rod sensitivity to light no less than 3000 times is called dark adaptation. Unfortunately, this much increased sensitivity to light is not seen in the fovea, where rods are absent, so that precise vision is not nearly so much increased as one would like. It is routine for radiologists doing fluoroscopy and air plane pilots flying at night to dark-adapt first.

     The pigments of the cones are not nearly so well understood. It may be guessed that the same kind of process is involved as that seen in the rods, but since one is dealing with unknown pigments in an anatomically different area, conclusions must be guarded. However, it is established that at least one billion times more light energy is required to stimulate cones than rods.

9. Binocular Vision and Central Pathways:

     The position of the eyes at the front of the head results in the duplication of information in the two eyes. Thus both the right and left eyes detect visual signals on the right side of the body. The right eye does so with the medial portion of its retina, while the left eye uses the lateral part of its retina. When an object is fixated by the two foveas, both foveas convey corresponding information. An object not so fixated is seen in non-corresponding parts of the two retinas and is usually seen doubly. This rarely gives rise to trouble because foveal vision is almost always favored, while information from the non-foveal parts of the retina is usually assigned very little importance. See Figure 180.

     Since each eye has a slightly different foveal view of the thing observed, depth perception is much improved in binocular vision. Nevertheless, depth perception persists in monocular vision, meaning near and distant objects are distinguished readily by one-eyed men. This may be accomplished by perceiving how much effort is required to bring the objects into focus. On the other hand, previous learning may enter into this awareness. For example, a distant quarter may project the same size retinal image as a close dime. If, however, the coins are recognized, the one-eyed man has no problem in estimating that the distance of the quarter is greater than that of the dime, and he may be able to estimate how much greater as well as the man with binocular vision.

     When the ganglion cells of the optic nerve are stimulated, the appropriate nerve fibers discharge. In the eye itself, the fibers course over the front of the retina, entering the optic nerve at its head. This site, which contains no visual receptors, is blind. It lies 150 nm to the nasal side of the fovea, and there is a corresponding blind spot 150 nm lateral to the point fixed by the fovea.

     The optic nerve courses backward and medially. The two optic nerves join to form the optic chiasma (Figure 138) where their fibers are reassorted so that fibers arising from the right visual field go to the left side of the brain while fibers from the left visual field go to the right side. This means that information concerning the opposite visual field is carried in each optic tract, so the optic nerves carry information from the eye on the same side.

     Each optic tract goes to both the midbrain (superior colliculi) and the occipital cortex by way of lateral geniculate body of the thalamus. Synapses just rostral to the superior colliculi are concerned with visual reflexes. The fibers which go to the occipital cortex are concerned with seeing. The nature of seeing-in the cerebral cortex is by no means clear. It should be recalled that one million fibers make up the optic tracts. There are many more neurons in the visual (occipital) cortex. These are probably brought to bear on the visual function through the facts that the fibers of optic tracts are multiply connected and that seeing also involves movements of the eyes. Thus the neurons of the occipital cortex receive patterns of information which are partially dependent on retinal connections and partially on the movements of the eyes.

     The fovea is exceptionally well represented in the cerebral cortex, which is as much as to say that one is rather more likely to receive cortical information from the things one looks at, than the things around them. This heavy representation of the fovea is illustrated in Figure 181.

     Like any other cortical area, the visual cortex has extensive connections with other parts of the brain. There is much truth in the saying that one sees what he wishes to see. The remarkable thing about the visual cortex is not that it "sees" so much, but rather that it "sees" so little. In general, caricatures convey more information than photographs. Line diagrams are more instructive than the thing diagrammed. Some very instructive experiments suggest that information relevant to boundaries between light and dark is cortically represented first by simple cortical neurons which "like" such boundaries to be at certain angles and relay the fact that boundaries of the correct angle are present to complex cortical neurons, which receive information from many simple neurons with similar special preferences. In the same way, some cortical neurons "like" boundaries which move in certain ways and relay this information to complex cortical neurons which have many connections with similar simple ones, whose preference is for the right boundary to be moving in the right way.

     In the end, the visual cortex is remarkably impoverished with respect to the things seen. The skeptical reader may ask himself how much of what he has just seen remains with him when he shuts his eyes, or even how many things are seen at once with open eyes, in fixed gaze. We have seen that the retina can be used as a camera film as an optogram, though not a very good one. The visual cortex conveys even less information than the retina, though many more cells are involved. It may be that the visual cortex underrepresents the visual field because that field contains too much information to be useful to the organism. A similar function for thalamo-cortical synapses was suggested before (Chapter 5). It should, however, always be recalled that cortical facilitation may make almost any variety of sensory information available. In the case of vision, we may consider ourselves nearly blind until we become interested, when we become as good at seeing the thing of interest as it is necessary to be.

10. Taste and Smell:

     These sensory modalities are both concerned with detecting chemical changes. The principal organ of taste is the taste bud of the tongue. Figure 182. There are about 10, 000 such taste buds, each consisting of a group of cells surrounding a central pore. The taste producing substance, in solution, enters the taste bud through the pore and, by making a weak bond to the surface of the receptor cells which make up the bud, depolarizes them. Nerve impulses from the stimulated taste buds of the front of the tongue travel to the brain by way of the chorda tympani, a branch of the seventh cranial nerve. Taste buds at the back of the tongue are carried by the ninth cranial nerve.

     The idea that there were specific taste buds for salt, sweet, sour, and bitter tastes, and that these were the specific parts of the tongue, has not been well supported. In a very general way, it can be said that the front of the tongue is responsive to sweets and the back to bitter substances. Salty tastes are detected in the area behind the sweet area and sour in front of the bitter area. However, there is much variation from individual to individual and even from time to time that it seems unwise to make even the roughest of taste maps. This is particularly so if it is recalled how profoundly the sense of taste is influenced by the sense of smell.

     The sense of smell is very poorly understood. The sense organ itself lies in the upper part of each nostril. It appears to consist of true neurons whose dendrites reach the olfactory area. Each such neuron produces an axon. The axons pass into the skull through the cribri form plate, where they end in the olfactory bulb. The axons are remarkable in two respects: they are extremely small, at 0.2 micra in diameter, and clusters of such axons, closely packed, are surrounded by a single Schwann sheath. The structure of the olfactory epithelium is shown in Figure 184.

     The neurons, when stimulated, give rise to generator potentials whose magnitude increases with the increase in stimulus. This is an unexpected finding for true neurons, though it is quite characteristic of sensory receptors (See Part 3). The reason for the peculiar behavior of the olfactory receptors has not yet been explained.

     In any case, these generator potentials, which show extremely rapid adaptation, are elicited by unimaginably small amounts of chemical substances. For example, 4 x 10-10 grams of methyl mercaptan per liter of air can be detected. When it is considered that very little of the air is actually brought into contact with the olfactory epithelium, less is dissolved, and still less is brought into contact with the specific receptors for this substance, it begins to appear possible that only a few molecules of some substances are required to excite the sense of smell. Even more remarkable is the fact that man, an animal who depends very little on his sense of smell, as compared to, say, the dog, which can distinguish several thousand odors. This is much greater than the number of colors which can be distinguished, and about the same as the number of pitches which can be discriminated by the trained ear.

     The structure of the olfactory bulb suggests that its neurons communicate extensively with each other (Figure 185). They are gathered together and proceed to the cerebral cortex of the same side, where they enter the medial surface of the cortex by way of the limbic system. Unlike other sensory fibers, they have no thalamic connection.

     The limbic system is partially concerned with emotional behavior. It has been speculated that the strong affective role of the sense of smell is related to the closeness of its central relationships to the rest of the limbic system, and perhaps certain perfumes used by women are, for this reason, attractive to men. This may indeed be true, but the argument might be more effective if it could be shown that a stuffy nose, which, by covering the olfactory epithelium, produces a loss of the sense of smell, altered emotional behavior in any way, or if it could be shown that animals in which the olfactory epithelium had been destroyed were less attracted to animals of the opposite sex. If there is any such evidence, this writer is not aware of it, though a few years of research on the subject could be financed by the price of a few hours of television time dedicated to the proposition that man is a bad-smelling animal, almost instinctively detested by others of his species when his natural odors remain unsuppressed. There is, of course, the appalling possibility that unwashed, undeodorized men and women are more appealing to each other than those who scrub with the latest soaps and use the latest anti-perspirantS. It is possible that the emotional content of this proposition is at least as great as the emotional content of odors, at least to the manufacturers of deodorant compounds and the producers of television commercials.

11. Pain Receptors:

     Although pain can be elicited by overstimulation of any sensory organ, there are pain fibers that probably serve no other function than the detection of pain. These fibers, which may originate in the skin or in the internal organs, usually begin as undifferentiated, unsheathed nerve fibers which terminate at surfaces.

     In general, the pain fibers of a nerve are of two types. Some are fairly large (2 - 5 micra) and myelinated; others are small (. 5 - 1. 0 micra) and unmyelinated. Fibers in the first group conduct impulses at about 20 meters per second, while those of the second group conduct at about 1 meter per second.

     The fact that fibers of both types exist leads to the interesting phenomenon of double pain. If a painful stimulus is applied to the finger, the "fast" fibers will carry information to the cord in 0.05 seconds, assuming a 1 meter long arm, while "slow" pain will arrive at the cord one second after stimulation. Thus two pain impulses will arrive at the central nervous system after only one painful stimulus.

     Pain fibers from the internal organs are very like those of the skin, but they are much fewer in number. They travel via autonomic nerves, both sympathetic and parasympathetic, but their cortical representation is very small. As has been noted before, such pain, perhaps because it is unfamiliar, is usually not well localized, and when it is localized, may be localized incorrectly.

12. Abnormalities in Sensation:

     These sensory modalities may become disordered as the result of disease of their receptors, their afferent pathways to the cord, or their pathways in the central nervous system. The modalities with which we will be concerned here are those of the skin (touch, heat, cold, pain), of voluntary muscle, tendons, and joints (position sense and deep pressure), and those of the lining of the body cavities (pain).

     Skin: There are no known diseases of the skin receptors as such. In general, the loss of skin sensation should be interpreted as disease of the sensory nerve fibers from the area involved or disease of the central nervous system. There may be exceptions, but they are not clearly delineated. For example, some persons with intact temperature sense in an area may not be able to detect light touch in the same area, and the converse may also occur. Loss of the sense of pain is reported in some cases, but it is almost always associated with loss of other sensory modalities. Local anesthetics tend to abolish pain first and the other sensory modalities later. This is probably due to the fact that these agents reach the pain sensing receptors, the naked nerve fibers, first.

     Outside of touch, skin sensation crosses over in the spinal cord at the level of the corresponding dematome. This crossing occurs in the region of the central portion of the cord. When this is diseased, as in a very rare disease called syringomyelia, the skin may become quite anesthetic to temperature and pain, but not to touch. Remarkable anesthesia can be produced in the skin and elsewhere by hypnosis. Such anesthesia is central and is not dependent on the sense organs.

     General anesthetics, in sufficient amounts, abolish all sensation centrally. There is a certain folklore about this: it has been suggested that these agents do not abolish sensation, but only the memory of sensation. Verification of this statement is obviously impossible, though it is asserted with great confidence.

     Muscles, tendons, and joints: These structures cross sarcomeres, and it is virtually impossible to abolish the sensations from them except through the central nervous system. Deep pressure and position sense are, however, lost in tabes dorsalis and in cerebellar diseases as mentioned in Chapter 5. These sensations are lost in general anesthesia.

     Body cavities: The inside of the chest is lined by the parietal pleura. The inside of the abdomen is lined by the parietal peritoneum. Both are richly innervated with pain endings, which convey information to the mixed spinal nerve concerned with that sarcomere.

     Normally, these pain endings are not stimulated. When, however, disease of an internal organ produces an inflammation in the adjacent area of the pleura or peritoneum, the pain is localized with great accuracy. For example, the lung may be diseased without producing pain, but if the disease extends to its outer layer, the visceral pleura, and then to the parietal peritoneum, a very sharply localized pain is experienced, pleurisy. In the same way, an inflamed appendix (Sec Chapter 6) produces pain just below the sternum, until its outermost layer, the visceral peritoneum, is affected. This usually causes inflammation of the adjacent parietal peritoneum, so the pain can now be located with pin-point accuracy.

     There are conditions in which the pleura or peritoneum produce painful stimuli in the absence of disease of the internal organs. The parietal pleura pain endings may be stimulated in epidermic pleurodynia, and the parietal peritoneum may become painful in familial mediterranean fever. These diseases often mimic diseases of the internal organs and may result in unnecessary surgery. Fortunately, they are quite rare.

     The belated recognition that there were sensory endings in the viscera has left this field virtually unexplored except for pain, which will be considered in Part 12. A few visceral receptors have been intensively studied. Recent work suggest very strongly that the stretch receptors of the arch of the aorta and the carotic sinuses do not function correctly in high blood pressure. See also Part 4 of this Chapter and Chapters 12 and 16.

     Losses of hearing can occur at any point in the auditory pathway. The external canal can be plugged with wax or foreign objects. The tympanic membrane may be thickened due to repeated infections. The middle ear may become filled with fluid when the Eustachian tube is closed off, so that the ossicles cannot vibrate freely.

     A condition which is both common and distressing is inflammation of the middle ear, the ofitis media. Since the middle ear is connected to the throat by the Eustachian tube, inflammations of the throat tend to spread to the middle ear. Some of them close off the tube, the sealed cavity invites infection, the ossicles may be destroyed, and the tympanic membrane may rupture. The infection may spread into the mastoid process, which is very porous and communicates with the middle ear. Mastoiditis was once a dreaded complication of middle ear infection.

     In flying and diving, there are great pressure changes in the respiratory passages and the middle ear. When the Eustachian tube is open, there are no problems. If the Eustachian tube is closed and middle ear pressure becomes higher than the pressure in the outside air, the tympanic membrane may rupture; low pressures in the middle ear may cause it to fill with fluid. Yawning, which tends to open the Eustachian tube, is often helpful when the pressure is high, and when the pressure is low, it is sometimes useful to open the tube by blowing one's nose. Losses of hearing due to any of the above conditions are called conduction deafness.

     Nerve deafness may occur when a portion of the organ of Corti is damaged by prolonged exposure to a particular pitch. The cochlear nerve may be damaged by dihydrostreptomycin. Tumors and strokes, which injure the auditory portions of the brain, may also produce nerve deafness.

     The two types of deafness are usually distinguished by comparing detection of sound produced over bone to that produced in air. In conduction deafness, there is greater hearing loss through air than through bone. Nerve deafness shows the opposite picture.

     Disorders of these organs arc not readily distinguished from disorders of their nerve or of the brain. The best known disease of the receptors is Meniere's disease. This usually occurs along with deafness and is associated with distention of the membranous labyrinth and degeneration of the inner ear. The most distressing aspect of the disease is the paroxysmal vertigo. The patient profits from bed rest, usually finding a position in which vertigo is not present. The course of the disease is variable, but when persistent attacks of vertigo occur, surgical destruction of the labyrinth may be required.

     A specific disorder of labyrinthine function which does not involve the cochlea is the "positional vertigo of Baranyi," which is believed to be due to disease of the otolithic apparatus. It is a temporary condition.

     Infections of the mernbranous labyrinth usually affect both hearing and position senses together. The course depends on the cause and the treatment.

     One would presume that astronauts in the weightless state would have considera trouble in adjusting to the loss of information from the utricle and saccule, which are very dependent on gravity for the relationship of the otoliths to the maccules. Before there were any astronauts, some scientists speculated that their vestibular apparatus would have to be destroyed to prevent intolerable vertigo and nausea, which are common symptoms of labyrinthine disease. Actually, few if any astronauts experience any such symptoms whatever. It has been reported that one Russian astronaut developed disease of the labyrinth after re-entry, but the reliability of the report is questionable.

     The fact that symptoms originating in the labyrinth are not troublesome in orbit is probably attributable to the multiple sensory cues which are available to the central nervous system. Visual and bodily proprioreceptive cues are probably coordinated with labyrinthine information, and all are used together to assess the position and movement of the body.

     Streptomycin tends to damage the vestibular branch of the eighth nerve, just as dihydrostreptomycin damages the cochlear division. The cause is unknown. When untoward reactions toward either drug develop, they should be discontinued immediately.

     Diseases of the brain, such as strokes, tumors, etc. in the area of central representation of the cochlear and vestibular nerves may produce deafness, vertigo, nausea, loss of position sense, and losses in the perception of angular rotation. Often such disease leads to faulty evaluation of information from the receptor organs. In this case, the information is not lost, only inappropriately acted upon.

     In the normal eye, the curvature of the cornea, aqueous humor, lens, vitreous humor, and the index of refraction of these structures produce a "reduced eye" in which the focal point is 15 mm behind the optical center of the eye, which in turn, is 7 mm behind the cornea. The retina is usually 22 mm behind the cornea. These values all refer to a normal eye accommodated for far vision.

     In many persons, the curvature of the "reduced eye" is greater than normal while the retina is in normal position. Conversely, the retina may be more than 22 mm in back of the cornea, or both circumstances may exist together. In persons with such eyes distant objects are focussed in front of the retina, and the retina itself receives a blurred image. Recalling the equation of Part 7:

1   1   1

F   Dobject   Dimage

and assuming that F is 15 mm, which is a normal value, but that the retina is 16 mm behind the cornea, it is obvious that a distant object, say, 10,000 mm away, will come to a focus in front of the retina. Thus,

1   1   1

15   10,000   Dimage

so Dimage = 15.001, and Dretina = 16 mm.

     Exactly the same is true if the "reduced eye" has too great a curvature, so that, for example, the focal length is 14 mm, the shape of the eye being normal otherwise. In this case,

1   1   1

14   10,000   Dimage

so Dimage = 14.001, and Dretina = 15 mm.

     In either case, distant images will not be perceived accurately, but close images are brought to a focus on the retina. For example, an object 240 mm away will focus on the retina of the first eye.

1   1   1

15   240   16

     In the second eye, an object at 210 mm will focus exactly on the retina:

1   1   1

15   210   14

     Note that in both cases, clearest vision is for objects less than a foot away. Such people are called near sighted, or myopic. See also Figure 126.

     If the lens system is too strong, or if the retina is too close to the cornea, the opposite situation prevails. Near objects are poorly perceived, while distant objects are in focus. This condition, popularly called far-sightedness, is also referred to as hypermetropia.

     Abnormalities in focussing the image on the retina are usually a consequence of abnormal axial length of the eyeball, meaning the retina is too far from or too near to the cornea. Abnormalities of dioptric power are only half as common. These abnormalities can be corrected by spectacles, which alter the dioptric power of the eye, or contact lenses, which do the same by changing the curvature of the eye.

     Some eyes may show irregular curvature, having greater dioptric power in some planes than others, or conversely, the retina is located at different distances from the optical center of the eye in different planes. This defect, called astigmatism, can be corrected by the use of spectacles which correct the faulty curvature, being themselves curved differently in different planes.

     In the above, it has been assumed that the lens adds nothing to the dioptric power of the eye. Actually, it does add a little. In a young person, the lens can add 10 diopters, but in old age, changes in shape of the lens contribute very little to the dioptric power. The near point recedes as the lens loses its ability to change its shape. Ordinary glasses do not help since the eye may be perfectly all right for distant vision and lenses which increase dioptric power may blur distant objects. The lost ability of the lens to change its power may require three sets of glasses, one for near vision, one for intermediate vision, and one for far vision. These glasses can be combined in trifocals. The voluntary use of one part or another of these glasses takes the place of the automatic accomodation of the lens displayed by younger people. The condition, characteristically one of old age, is called presbyopis.

     Apart from the changes of presbyopia, the lens, which is normally quite transparent, may develop opacities. These may involve the entire lens or be quite small. When the opacities become large or numerous enough to interfere with vision, one is said to have a cataract. Most cataracts require surgical removal. This procedure is relatively easy, and the lens-less eye can function usually with the aid of spectacles or contact lenses, which add the lost dioptric power. Of course, bifocal or trifocal lenses must be used as in presbyopia to replace the lost abilities of the lens to change its power. See Figure 187.

     Cataracts may develop early in life as a result of faulty nutrition or inflammation, but more commonly cataracts develop only in older persons. They may be due to diminished metabolism of the lens, perhaps due to diminished nutrition, heat, or exposure of the lens to ionizing radiation, for example X-rays. At one time dinitrophenol was used to reduce weight, but it was found to be a very dangerous drug, producing many deaths. In addition, about one percent of persons taking dinitrophenol developed cataracts. The drug is no longer used in medicine, and the possibility that dinitrophenol exaggerates whatever biochemical process is involved in the formation of cataracts in normal persons has not been intensively studied.

     The most dramatic of retinal diseases occurs after obstruction of the main artery to the retina. Blindness occurs immediately without pain. Ophthalmoscopic examination characteristically shows the blood vessels of the retina to be pale and thin. Ordinarily, this type of obstruction results in retinal degeneration and permanent blindness.

     Retinal detachment from the choroid which nourishes it is partial at first. Later it becomes complete if untreated. It sometimes occurs spontaneously, especially in myopic persons, and it can be produced in any eye by injury. Early recognition is essential for proper treatment. The sudden appearance of a blind spot and the sensation of a curtain moving across the eye are characteristic. The detached area usually remains blind, but spreading of the detachment can be prevented by fusing the retina to the choroid around the detachment, which can now be done by laser beams.

     Retinopathies are retinal disorders secondary to other diseases. For example, in hypertensive retinopathy, the arterioles are seen at ophthalmoscopic examination to be narrowed. Hemorrhages and exudates are common in advanced, untreated hypertension. There are corresponding blind spots.

     In diabetic retinopathy, changes are seen similar to those described above. In both cases, the treatment should be directed to the underlying disease.

     The optic disc, where the optic nerve enters the eye, is usually cupped. It may be swollen, usually due to increased pressure in the brain. The condition, called papilledema, calls for treatment of the underlying disease. This must be considered serious unless proved otherwise. The appearance of papilledema (choched line) is shown in Figure 189.

     Glaucoma: The aqueous humor is produced in the ciliary body. Ordinarily it moves forward through the aperture in the iris and is reabsorbed in the canal of Schlemn. Overproduction and underabsorption results in increased intraocular pressure. The pressure may be so great that blood can no longer be delivered to the retina, which may suffer irreversible damage. This condition, called glaucoma, should always be considered a medical emergency, since it may result in blindness within a few hours.

     One should always suspect glaucoma in patients over 40 who have had frequent changes of spectacles, vague visual disturbances, and especially a severe, throbbing pain in the eye. The diagnosis is usually made by the observation of elevated pressure within the eye, which requires the use of a device called a tonometer, which is placed in contact with the cornea, although the procedure is not unpleasant in skilled hands. If intraocular pressure is found to be elevated, there are two methods of treatment: decreasing the formation of the aqueous humor, and increasing its drainage through the canal of Schlemn. The details are described in textbooks of medicine and ophthalmology.

     These usually represent brain damage and little can be done to relieve them, though surgery to remove tumors may be helpful.

     These are not usually serious. Many persons are congenitally unable to detect certain odors or tastes. Hereditary factors may be involved. Schizophrenics tend to have hallucinations involving these senses, but the diseased nerve is of central, not peripheral origin.

     Position sense begins in propriocetors in muscles, tendons and joints. It travels in the dorsal columns of the spinal cord. In one form of neurosyphilis, these columns or their central nuclei in the medulla may be affected, tabes dorsalis. This is usually associated with lightning-like pains and a loss of sense of position of the legs, and, to a lesser degree, the arms. Persons so afflicted guide their legs by the sense of sight. They have a typical broad-based gait. There is no treatment for the damage done, but further damage may be prevented by treating the neurosyphilis.

     The stimulus for visceral pain may be chemical: pain is elicited in the beating heart suddenly deprived of blood, presumably because of something it produces in its activity, helpfully called the P substance, for pain, stimulates the pain endings.

     It has been noted that stretch tends to depolarize smooth muscle cells. These cells, when depolarized, may stimulate pain endings within them. This may lead to a sequence of events which terminates in each unberable pain as follows.

     Assume that a hollow organ's outflow is obstructed. The organ stretches and depolarizes. By depolarizing its muscle, it stimulates its pain endings, but it also contracts more vigorously because its muscle has been depolarized. Obstruction leads to more pain and contraction. This process may continue until the obstruction is relieved.

     A familiar example of such a sequence is biliary colic. The opening of the gall bladder is obstructed, perhaps by a stone. The stretched gall bladder contracts more and more vigorously and gives rise to more and more pain. If untreated, the pain may terminate when the gallstone has been passed or no longer produces obstruction. Rupture of the gall bladder is always a possibility.

     A very effective means of treating visceral pain is the administration of morphine or meperidine (demerol). These drugs appear to act on the central nervous system rather than on the peripheral receptors. Somehow they appear to reduce the fear and anxiety associated with the pain. This may, in turn, result in messages from the central nervous system carried via the autonomies which produce hyperpolarization of the affected structures. The attack is aborted by this means. There is an element of risk in such treatment, because both drugs may produce dependency in some people. However, persons receiving the drugs for pain alone usually do not develop such dependency, though many persons who are drug dependent claim that their dependency began with the use of drugs in the treatment of pain. Actually, drug dependency rarely occurs in persons who receive drugs for the treatment of pain, and most people dislike the effects of either drug when they are not actually in pain. There is considerable evidence that drug dependency depends on the persona of the user rather than on prior usage, and it seems irrational to withhold the drugs when pain is to be treated simply because of the possibility of drug dependence.

     Pain from internal organs is often poorly localized. For example, the pain of an inflamed appendix tends to localize just below the sternum, at first, though later it may be exactly localized. The reasons for this poor localization may have to do with the distribution of the visceral afferents, or it may be correlated with the unfamiliarity of the pain. It is safest to say that the poor localization is caused by unknown factors. Figure 190 shows the localization of visceral pain from a number of organs.

     It should be reemphasized that the internal organs may be extensively diseased without causing pain. For example, tuberculosis may not cause any pain at all. Cancers, growing slowly, may invade organ after organ without producing pain. It is unfortunate that this is so; if disease of the internal organs were signalled by pain, its course might be interrupted before irreparable damage was done. Somatic pain is very well localized, its source being easily located, so the indicated treatment may be begun immediately.

Continue to Chapter 9.