INTRODUCTION
1. Sensory receptors monitor important characteristics of the internal and external environment.
2. The receptor may be a free nerve ending, an encapsulated nerve ending, an accessory cell, or a modified neuron.
PRINCIPLES OF RECEPTOR FUNCTION
1. Every cell membrane functions as a receptor, changing its properties in response to environmental stimuli. Differences in membrane structure account for the differences in cellular sensitivity.
2. Sensory receptors form the interface between the environment and the nervous system. Each receptor responds best to certain stimuli and not to others.
The Classification of Receptors
1. Receptors for the general senses are scattered throughout the body. The special senses are located in relatively small and specialized areas.
2. Receptors may be categorized as exteroceptors, proprioceptors, or interoceptors. A more detailed breakdown recognizes nociceptors, chemoreceptors, thermoreceptors, mechanoreceptors, and photoreceptors.
Receptor Physiology and Sensory Coding
1. The arrival of an appropriate stimulus triggers a series of events leading to the production of an action potential in an afferent fiber.
2. The arriving information is processed to determine what neurons are active, and what the pattern of their activity indicates.
3. The sensory modality is indicated by the identity of the stimulated neuron. The location of the stimulus can be determined because the sensation is projected to a specific portion of the cerebral cortex.
4. Localization is best when each sensory neuron monitors a receptor with a very small receptor field.
5. Information about the stimulus may be conveyed in the frequency of action potential generation, by the rate of change, and by various combinations of the two.
6. Tonic receptors are always sending signals to the CNS; phasic receptors become active only when the local situation is changing.
Central Processing and Adaptation
1. A sensation is provided by an afferent fiber. Perception refers to the conscious awareness of the stimulus.
2. Adaptation to sensory stimuli may involve changes in receptor sensitivity (peripheral/sensory adaptation) or inhibition along the sensory pathways (central adaptation).
THE GENERAL SENSES
Nociception
1. Nociceptors are free nerve endings sensitive to temperature, mechanical trauma, and chemicals in the extracellular fluids.
2. There are two nociceptor populations, producing sensations of fast (prickling) and slow (burning) pain. Neither permits accurate localization. Slow pain sensations from deep or visceral structures are particularly difficult to assess.
Chemoreception
1. Chemoreceptors respond to water-soluble and lipid-soluble compounds that contact their cell membranes. Chemoreceptive neurons in the CNS respond to changes in the pH and carbon dioxide
concentrations of the cerebrospinal fluid; those along arterial trunks monitor the oxygen content of the blood.
Thermoreception
1. There are several populations of thermoreceptors. Their histological identities are not known, but warm receptors outnumber cold receptors. Thermoreceptors show a phasic response to stimulation; this enables us to adapt to a new temperature relatively quickly.
Mechanoreception
1. Mechanoreceptors respond to physical distortion of the cell membrane. Tactile receptors provide touch, pressure, and vibration sensations. Baroreceptors monitor pressures within the walls of blood vessels.Proprioceptorsreport on the positions of joints and muscles.
Tactile Receptors
1. Tactile receptors provide fine touch and pressure sensations as well as crude touch and pressure sensations. Tactile receptors can be very simple or very complicated in structure.
2. Important tactile receptors include: free nerve endings, the root hair plexuses, Merkel's discs, Meissner's corpuscles, Pacinian corpuscles, and Ruffini corpuscles.
3. Tactile perception may be altered by infection, disease, or damage to afferents or central pathways.
Baroreception
1. Baroreceptors in the wall of major arteries and veins respond to changes in blood pressure. The information reaches the CNS over cranial nerves IX and X. Receptors along the digestive tract are important in coordinating reflex activities important to digestion.
Proprioception
1. Proprioceptors include the tendon organs, receptors in the walls of joints, and the muscle spindles.
2. The afferent fiber monitoring a muscle spindle responds to stretch or compression. The afferent synapses on motor neurons controlling the surrounding extrafusal fibers.
3. Passive stretch leads to increased afferent activity and a rise in muscle tone; passive compression causes a decrease in afferent activity, and a reduction in muscle tone.
4. During voluntary contraction of a muscle, gamma motor neurons order the contraction of myofibrils within the muscle spindle fibers. This tenses the region monitored by the afferent fiber, and prevents a reduction in muscle tone as the muscle contracts.
THE SPECIAL SENSES
Olfaction
1. The olfactory organs contain the olfactory epithelium.This epithelium has supporting cells, basal (stem) cells, and olfactory receptors. The surface is coated with the secretions of the olfactory glands.
2. The receptors are specialized neurons that function as chemoreceptors. There are 10-20 million olfactory receptors.
3. There are at least 50 different primary smells.Olfactory sensitivity diminishes with increasing age.
Gustation
1. The taste buds are scattered over the surface of the tongue, the pharynx, and the larynx.
2. Each taste bud contains gustatory cells with sensory microvilli, or taste hairs.
3. The largest number of taste buds are associated with the circumvallate papillae.
4. The primary taste sensations are sweet, sour, bitter, and salt.
5. The taste buds are monitored by CN VII, IX, and X, which relay information to the nucleus solitarius of the brain stem. From here the information ascends within the medial lemniscus before being projected to the primary sensory cortex by thalamic neurons.
6. Much of our perception of taste represents the central integration of gustatory and olfactory information. Gustatory sensitivity also decreases with age; there are also genetic differences in taste sensitivity.
Equilibrium and Hearing
General Anatomy and Organization
1. The external ear includes the pinna, the external auditory meatus, and the external auditory canal that ends at the tympanic membrane.
2. The middle ear contains the auditory ossicles .The pharyngotympanic (Eustachian) tube connects the middle ear to the pharynx.
3. The mechanoreceptors of the inner ear are surrounded by the endolymph of the membranous labyrinth and protected by the bony labyrinth of the temporal bone.
4. The vestibule contains the organs of equilibrium; the cochlea contains the organ of hearing.
5. The vestibule includes two chambers, thesaccule and utricle, and three semicircular canals.
6. The coiled cochlea contains a slender extension of the membranous labyrinth, the cochlear duct.
Receptor Function
1. A hair cell responds to distortion of its cilium and stereocilia by external forces.
The Vestibular Sense Organs
1. The anterior, posterior, and lateral semicircular canals are continuous with the utricle.
2. In a semicircular canal the hair cells are found within an ampulla where the cilia contact a gelatinous cupula. Movement of fluid within the canal distorts the cupula and affects hair cell activity.
3. Each canal responds to movement in a single plane.
4. The saccule and utricle are connected by a passageway continuous with the endolymphatic duct. Endolymph is reabsorbed into the circulation at the endolymphatic sac.
5.In the saccule and utricle the hair cells are found within maculae where their cilia contact a gelatinous mass that contains densely-packed mineral crystals, or otoconia. When gravity or acceleration pushes against the otoconia, the sensory processes are distorted.
6. The vestibular receptors activate sensory neurons of the vestibular ganglion. Their axons form the vestibular branch of the acoustic nerve, CN VIII. The information then travels to the vestibular nucleus. Reflexive motor commands are forwarded to cranial nerve nuclei and the spinal cord; the information also reaches the cerebellum and the cerebral cortex.
The Cochlea
1. Sound travels through the air in a series of pressure waves. The energy content of a sound determines its intensity, measured in decibels.
2. Sounds that vibrate the tympanic membrane move the auditory ossicles.
3. The auditory ossicles amplify the degree of motion and convey the movement to the oval window. The tensor tympani and the stapedius muscles contract to reduce the amount of motion when very loud sounds arrive.
4. Pressure waves in the perilymph of the scala vestibuli and scala tympani distort the basilar membrane of the scala media cochlear duct).
5. The higher the frequency of the pressure waves, the closer to the oval window the area of maximal distortion will be located.
6. The hair cells of the cochlea are arranged along the axis of the basilar membrane. Distortion of the membrane pushes the hair cells against the tectorial membrane and provides the necessary stimulation.
7. Young children have the greatest hearing range; many factors contribute to decreasing auditory sensitivity in later years.
8. Conductive deafness results from conditions in the middle ear that block sound transmission to the oval window. Nerve deafness reflects problems in the cochlea or along central pathways.
9. The sensory neurons are located in the spiral ganglion, and their axons form the cochlear branch of the acoustic nerve
10. From the cochlear nucleus the information passes to the inferior colliculus. From there instructions and/or information is relayed to cranial nerve nuclei, to the spinal cord, or to the cerebral cortex via the medial geniculate of the thalamus.
Vision
Accessory Structures of the Eye
1. The accessory structures of the eye include the palpebrae, the ocular epithelium, and the associated glands.
2. The eyelids are separated by the palpebral fissure. The free margins contain enlarged hairs, the eyelashes.
3. The eyelashes are associated with large sebaceous glands and the Meibomian glands that lie along the inner margin of the eyelid; these glands secrete lipid-rich secretions. The lacrimal caruncle contains glands that produce a thick, granular material.
4. The ocular conjunctiva and the corneal epithelium cover the exposed surface of the eye and the palpebral conjunctiva lines the inner surface of the eyelids.
5. The secretions of the lacrimal gland bathe the anterior surface of the eye. The tears are slightly alkaline and contain electroltyes, lysozymes , and lipids.
6. Tears collect at the medial canthus in the region known as the lacus lacrimalis. The tears reach the inferior meatus of the nose after passing through the lacrimal puncta, the lacrimal canals, the lacrimal sac, and the nasolacrimal duct.
Anatomy of the Eye
1. The eye has three layers: a fibrous tunic, a vascular tunic, and a neural tunic.
2. The sclera forms the outer covering of the eye and is continuous with the cornea.
3. The uvea includes the iris, the ciliary body, and the choroid.
4. The iris forms the boundary between the anterior and posterior chambers of the eye.
5. The ciliary body contains the ciliary muscle and the ciliary processes attached to the suspensory ligaments.
6. The ciliary processes secrete aqueous humor, a fluid similar in composition to cerebrospinal fluid.Aqueous humor circulates within the eye and reenters the circulation after diffusing through the walls of the anterior chamber and into thecanal of Schlemm.
7. The neural tunic includes a pigment layer and an inner retinathat lines the vitreous chamber.
8. The vitreous body fills the chamber and provides mechanical support to the retina. The vitreous body has a gelatinous consistency due to the presence of collagen fibers and proteoglycans. Aqueous humor can circulate through the vitreous chamber, diffusing through the vitreous body.
9. The surface of the retina contains a network of blood vessels that radiate from the optic disc. The fovea contains a high concentration of photoreceptors.
10. The lens consists of concentric layers of cells encased in an elastic capsule. When no external force is applied, it assumes a spherical shape; tension in the suspensory ligaments normally flattens it to some degree.
11.A cataract is a lens that has lost its transparency.
12. The photoreceptors can only provide useful information if a focused image arrives at the retina.
13. Refraction occurs as light passes through the cornea and lens.
14. The focal length of a lens changes depending on its shape.A round lens shortens the focal length, a flat lens increases it. The focal length also increases as objects move toward the lens.
15. In the eye the focal length must be kept constant, regardless of the position of the object in view. Normal accommodation involves changing the shape of the lens.
16. When the ciliary muscles contract, the suspensory ligaments relax and the lens becomes more spherical. This brings nearby objects into focus.
17. Visual acuity is tested by comparing an individual's visual performance with standard values. Normal vision is indicated as "20/20."
18. The visual image arrives at the retina upside down; the brain adjusts to this, and we are not consciously aware of the adjustments.
Light and Photoreception
1. Aphotonrepresents a single energy packet of visible light.
2. Rods are very sensitive photoreceptors that respond to almost any photon. They are important for vision under dim light conditions.
3. Cones have specific sensitivities and provide color vision. They must receive relatively strong stimulation, and only function well under daylight or other brightly lit conditions.
4. The retina contains several layers of cells, with the photoreceptors farthest away from the vitreous chamber.
5. Each visual receptor contains an outer segment with membranous discs. The visual pigments are bound to the membranes of the discs.
6. The visual pigments are derivatives of the compound rhodopsin, composed of a pigment (retinene) and a protein (opsin). The absorbed energy changes the shape of the visual pigment, and this somehow affects the transmembrane potential of the receptor.
7. Shortly after this, the molecule begins to break down. Restoration of the rhodopsin takes time and requires enzymes and ATP. If excess retinene accumulates it reverts to vitamin A. Vitamin A appearing in the retina gets absorbed and stored by the pigment layer of the choroid.
8.If retinal detachment occurs the retina and choroid become separated, and the photoreceptors can be damaged or destroyed.
9. Dark-adapted vision is characterized by dilation of the pupil, facilitation of central pathways, and restoration of full rhodopsin concentrations in the visual receptors of the eye.
10.Light-adapted vision is characterized by constriction of the pupil and bleaching of the visual pigments.
11. Rods form a broad band around the outer portions of the
retina. The area around the fovea, where the visual image normally arrives, contains abundant cones. Objects in dim light are best seen by looking slightly to one side, so that the image is focused on rods rather than cones.
12. Night blindness results from a vitamin A deficiency; the carotene pigments in many vegetables can be converted to vitamin A inside the body.
13. The retina usually contains 16 percent blue cones, 10 percent green cones, and 74 percent red cones.Their differential responses provide a perception of color.
14.Color blindness is the inability to detect certain colors. It usually indicates that the cones are unable to manufacture one or more of the necessary visual pigments.Total color blindness is extremely rare.
Retinal Processing
1. The direct line to the CNS proceeds from the photoreceptors to bipolar cells, then to ganglion cells, and to the brain via the optic nerve.
2. Horizontal cells and amacrine cells modify the signals passed between other retinal components.
3. Considerable convergence exists in the visual system, especially among rods.The ratio of rods to ganglion cells may be 1000:1, although cones at the fovea are often communicating 1:1 with ganglion cells.
4. At the optic disc the axons from the ganglion cells converge before penetrating the tunics of the eye. This area does not contain photoreceptors, and it is known as the blind spot.
Central Processing
1. At the optic chiasm a partial decussation occurs. Nerves monitoring the left half of the visual field pass to the lateral geniculate of the left side, and those monitoring the right half are directed to the right side.
2. The information gets projected to the visual cortex, which contains a sensory map of the composite visual field.
3. Collaterals from the optic tracts synapse at the superior colliculus and trigger extrapyramidal motor commands that descend in the tectobulbar and tectospinal tracts.
4. Collaterals entering the suprachiasmatic nucleus of the hypothalamus are important in establishing a daily circadian rhythm to visceral functions.