Color Atlas of Physiology 2003 thieme

Sense of Smell

olfactory bulb. The glomeruli therefore function as convergence centers that integrate and relay signals from the same sensor type. Their respective sensor protein also determines which glomerulus newly formed sensor axons will connect to. Periglomerular cells and granular cells connect and inhibit mitral and bristle cells (!A2). Mitral cells act on the same reciprocal synapses (!A, “+/–”) in reverse direction to activate the periglomerular cells and granular cells which, on the other hand, are inhibited by efferents from the primary olfactory cortex and contralateral anterior olfactory nucleus (!A2, violet tracts). These connections enable the cells to inhibit themselves or nearby cells (contrast), or they can be disinhibited by higher centers. The signals of the axons of mitral cells (1) reach the anterior olfactory nucleus. Its neurons cross over (in the anterior commissure) to the mitral cells of the contralateral bulb and (2) form the olfactory tract projecting to the primary olfactory cortex (prepiriform cortex, tuberculum olfactorium, nucleus corticalis amygdalae). The olfactory input processed there is relayed to the hypothalamus, limbic system (see also p. 330), and reticular formation; it is also relayed to the neocortex (insula, orbitofrontal area) either directly or by way of the thalamus.

Thresholds. It takes only 4 !10-15 g of methylmercaptan (in garlic) per liter of air to trigger the vague sensation of smell (perception or absolute threshold). The odor can be properly identified when 2 !10– 13 g/L is present (identification threshold). Such thresholds are affected by air temperature and humidity; those for other substances can be 1010 times higher. The relative intensity differential threshold I/I (0.25) is relatively high (!p. 352). Adaptation to smell is sensor-de- pendent (desensitization) and neuronal (!C).

The sense of smell has various functions. Pleasant smells trigger the secretion of saliva and gastric juices, whereas unpleasant smells warn of potentially spoiled food. Body odor permits hygiene control (sweat, excrement), conveys social information (e.g., family, enemy; !p. 330), and influences sexual behavior. Other aromas influence the emotional state.

B. Transduction of olfactory stimuli

C. Adaptation of smell and taste

(After Engel and Ekman et al.)

Sense of Balance

stimulation of the semicircular canals usually reflects the rotational velocity.

The pressure difference across the cupula disappears when the body rotates for longer periods of time. Deceleration of the rotation causes a pressure gradient in the opposite direction. When bending of the cilia increased the AP frequency at the start of rotation, it decreases during deceleration and vice versa. Abrupt cessation of the rotation leads to vertigo and nystagmus (see below).

342 The saccule and utricle contain maculae (!A1, A4) with cilia that project into a gelatinous

membrane (!A4) with high density (!3.0) calcite crystals called statoconia, statoliths or otoliths. They displace the membrane and thereby bend the embedded cilia (!A4) due to changes of the direction of gravity, e.g. when the head position deviates from the perpendicular axis. The maculae respond also to other linear (translational) accelerations or decelerations, e.g. of a car or an elevator.

Central connections. The bipolar neurons of the vestibular ganglion synapse with the vestibular nuclei (!A, B). Important tracts extend from there to the contralateral side and to ocular muscle nuclei, cerebellum (!p. 326), motoneurons of the skeletal muscles, and to the postcentral gyrus (conscious spatial orientation). Vestibular reflexes (a) maintain the balance of the body (postural motor function,

!p. 328) and (b) keep the visual field in focus despite changes in head and body position (oculomotor control, !B and p. 360).

Example (!C): If a support holding a test subject is tilted, the activated vestibular organ prompts the subject to extend the arm and thigh on the declining side and to bend the arm on the inclining side to maintain balance (!C2). The patient with an impaired equilibrium organ fails to respond appropriately and topples over (!C3).

Since the vestibular organ cannot determine whether the head alone or the entire body moves (sense of movement) or changed position (postural sense), the vestibular nuclei must also receive and process visual information and that from propriosensors in the neck muscles. Efferent fibers project bilaterally to the eye muscle nuclei, and any change in head position is immediately corrected by opposing eye movement (!B). This vestibulo-ocular reflex maintains spatial orientation.

Vestibular organ function can be assessed by testing oculomotor control. Secondary or postrotatory nystagmus occurs after abrupt cessation of prolonged rotation of the head around the vertical axis (e.g., in an office chair) due to activation of the horizontal semicircular canals. It is characterized by slow horizontal movement of the eyes in the direction of rotation and rapid return movement. Rightward rotation leads to left nystagmus and vice versa (!p. 360). Caloric stimulation of the horizontal semicircular canal by instilling cold (30 !C) or warm water (44 !C) in the auditory canal leads to caloric nystagmus. This method can be used for unilateral testing.

12 Central Nervous System and Senses

344

Eye Structure, Tear Fluid,

Aqueous Humor

Light entering the eye must pass through the cornea, aqueous humor, lens and vitreous body, which are collectively called the optical apparatus, before reaching the retina and its light-sensitive photosensors (!A). This produces a reduced and inverse image of the visual field on the retina. All parts of the apparatus must be transparent and have a stable shape and smooth surface to produce an undistorted image, which is the main purpose of tear fluid in case of the cornea. Tears are secreted by lacrimal glands located in the top outer portion of orbit and their mode of production is similar to that of saliva (!p. 236). Tears are distributed by reflex blinking and then pass through the lacrimal puncta and lacrimal canaliculi (or ducts) of the upper and lower eyelid into the lacrimal sac and finally drain into the nasal sinuses by way of the nasolacrimal duct. Tear fluid improves the optical characteristics of the cornea by smoothing uneven surfaces, washing away dust, protecting it from caustic vapors and chemicals, and protects it from drying out. Tears lubricate the eyelid movement and contain lysozyme and immunoglobulin A (!pp. 96ff. and 232), which help ward off infections. In addition, tears are a well known mode of expressing emotions.

The entry of light into the eye is regulated by the iris (!A; p. 353 C1), which contains annular and radial smooth muscle fibers. Cholinergic activation of the sphincter muscle of pupil leads to pupil contraction (miosis), and adrenergic activation of the dilator muscle of pupil results in pupil dilatation (mydriasis).

The bulbus (eyeball) maintains its shape due to its tough outer coat or sclera (!C) and intraocular pressure which is normally 10–21 mmHg above the atmospheric pressure. The drainage of aqueous humor must balance its production to maintain a constant ocular pressure (!C). Aqueous humor is produced in the ciliary process of the posterior ocular chamber with the aid of carbonic anhydrase and active ion transport. It flows through the pupil into the anterior ocular chamber and drains into the venous system by way of the trabecu-

lar meshwork and Schlemm’s canal. Aqueous humor is renewed once every hour or so.

Glaucoma. Obstruction of humor drainage can occur due to chronic obliteration of the trabecular meshwork (open-angle glaucoma) or due to acute block of the anterior angle (angle-closure glaucoma) leading to elevated intraocular pressure, pain, retinal damage, and blindness. Drugs that decrease humor production (e.g. carbonic anhydrase inhibitors) and induce meiosis are used to treat glaucoma.

The lens is held in place by the ciliary zonules (!C). When the eye adjusts for far vision, the zonules are stretched and the lens becomes flatter, especially its anterior surface (!D, top). When looking at nearby objects (near vision), the zonules are relaxed due to contraction of the ciliary muscle, and the lens reassumes its original shape due to its elasticity (!D , bottom, and p. 346).

The retina lines the interior surface of the bulbus except the anterior surface and the site where the optical nerve (!A) exits the bulbus via the optic papilla (!A). The fovea centralis

(!A) forms a slight depression across from the pupillary opening. The retina consists of several layers, named from inside out as follows (!E): pigmented epithelium, photosensors (rods and cones), Cajal’s horizontal cells, bipolar cells, amacrine cells, and ganglion cells. The central processes of the ganglion cells (n !106) exit the bulbus as the optical nerve (retinal circuitry; !p. 355ff.).

Photosensors. Retinal rods and cones have a light-sensitive outer segment, which is connected to a inner segment by a thin connecting part (!p. 349 C1). The inner segment contains the normal cell organelles and establishes synaptic contact with the neighboring cells. The outer segment of the rod cells contains ca. 800 membranous disks, and the plasma membrane of the outer segment of the cones is folded. Visual pigments are stored in these disks and folds (!p. 348). The outer segment is continuously regenerated; the old membranous disks at the tip of the cell are shed and replaced by new disks from the inner segment. The phagocytic cells of the pigmented epithelium engulf the disks shed by the rods in the morning, and those shed by the cones in the evening. Some ganglion cells contain a light-sensitive pigment (!p. 334).

Optical Apparatus of the Eye

Accommodation. When the eyes are adjusted for far vision, parallel light rays from a distant point meet at Fp (!B1, red dot). Since the retina is also located at Fp, the distant point is clearly imaged there. The eye adjusted for far vision will not form a clear image of a nearby point (the light rays meet behind the retina, !B1, green dot) until the eye has adjusted for near vision. In other words, the curvature of the lens (and its refractive power) increases and the image of the nearby point moves to the retinal plane (!B2, green dot). Now, the distant point cannot not be sharply imaged since Fp does not lie in the retinal plane any more (!B2).

The refractive power around the edge of the optical apparatus is higher than near the optical axis. This spherical aberration can be min-

346imized by narrowing the pupils. The refractive power of the eye is the reciprocal of the ante-

rior focal length in meters, and is measured in diopters (dpt). In accommodation for far vision, focal length = anterior focal point (Fa)—princi- pal point (P) = 0.017 m (!B1). Thus, the corresponding refractive power is 1/0.017 = 58.8 dpt, which is mainly attributable to refraction at the air–cornea interface (43 dpt). In maximum accommodation for near vision in a young person with normal vision (emmetropia), the refractive power increases by around 10–14 dpt. This increase is called range of accommodation and is calculated as 1/near point – 1/far point [m– 1 = dpt). The near point is the closest distance to which the eye can accommodate; that of a young person with normal vision is 0.07–0.1 m. The far point is infinity (!) in subjects with normal vision. The range of accommodation to a near point of 0.1 m is therefore 10 dpt since 1/! = 0. It decreases as we grow older (to 1–3.5 dpt in 50-year-olds) due to the loss of elasticity of the lens. This visual impairment of aging, called presbyopia (!C1–3), normally does not affect far vision, but convex lenses are generally required for near vision, e.g., reading.

Cataract causes opacity of the lens of one or both eyes. When surgically treated, convex lenses (glasses or artificial intraocular lenses) of at least + 15 dpt must be used to correct the vision.

In myopia (near-sightedness), rays of light entering the eye parallel to the optical axis are brought to focus in front of the retina because the eyeball is too long (!C4). Distant objects are therefore seen as blurred because the far point is displaced towards the eyes (!C5). Myopia is corrected by concave lenses (negative dpt) that disperse the parallel light rays to the corresponding extent (!C6 ). Example: When the far point = 0.5 m, a lens of [– 1/0.5] = [– 2 dpt] will be required for correction (!C7). In hyperopia (far-sightedness), on the other hand, the eyeball is too short. Since the accommodation mechanisms for near vision must then be already used to focus distant objects (!C8), the range of accommodation no longer suffices to clearly focus nearby objects (!C9). Hyperopia is corrected by convex lenses (+ dpt) (!C10–11).

Astigmatism. In regular astigmatism, the corneal surface is more curved in one plane (usually the vertical: astigmatism with the rule) than the other, creating a difference in refraction between the two planes. A point source of light is therefore seen as a line or oval. Regular astigmatism is corrected by cylindrical lenses. Irregular astigmatism (caused by scars, etc.) can be corrected by contact lenses.

B. Eye: Accommodation for (1) far vision and (2) near vision

C. Presbyopia, myopia and hyperopia

Plate 12.19 Optical Apparatus of the Eye

347

Visual Acuity, Photosensors

red curve). In a dark-adapted eye, on the other hand, the sensitivity of the retina (!B2, blue curve) is completely dependent on the rod distribution (!B1, purple curve). The color-sen- sitive cones are therefore used for visual perception in daylight or good lighting (day vision, photopic vision), while the black and whitesensitive cones are used to visualize objects in darkness (dim-light vision, night vision, scotoptic vision). The high light sensitivity in night vision is associated with a high loss of visual acuity (!p. 354).

Photosensor Function

Light-absorbing visual pigments and a variety of enzymes and transmitters in retinal rods and cones (!C1) mediate the conversion of light stimuli into electrical stimuli; this is called photoelectric transduction. The membranous disks of the retinal rods contain rhodopsin (!C2), a photosensitive purple-red chromoprotein (visual purple). Rhodopsin consists of the integral membrane protein opsin and the aldehyde 11-cis-retinal. The latter is bound to a lysine residue of opsin which is embedded in this protein; it is stably kept in place by weak interactions with two other amino acid residues. Photic stimuli trigger a primary photochemical reaction in rhodopsin (duration, 2 · 10– 14 s) in which 11-cis-retinal is converted to all-trans-retinal (!C3). Even without continued photic stimulation, the reaction yields bathorhodopsin, the intermediates lumirhodopsin and metarhodopsin I, and finally metarhodopsin II within roughly 10– 3 s (!D1).

Metarhodopsin II (MR-II) reacts with a Gs- protein (!p. 274) called transducin (Gt-pro- tein), which breaks down into αs and "γ subunits once GDP has been replaced by GTP (!D1). Activated αs-GTP now binds the inhibitory subunit of cGMP phosphodiesterase (IPDE) (!D2). The consequently disinhibited phosphodiesterase (PDE) then lowers the cytosolic concentration of cyclic guanosine monophosphate (cGMP). The activation of a single retinal rhodopsin molecule by a quantum of light can induce the hydrolysis of up to 106 cGMP molecules per second. The reaction cascade therefore has tremendous amplifying power.