Special Senses PDF

THE NERVOUS SYSTEM: PART 4 OLFACTION The sense of smell. ANATOMY OF OLFACTORY RECEPTORS  PHYSIOLOGY OF OLFACTION, THRESHOLDS AND ADAPTATION Odorants – stimuli Olfactory transduction – Depolarization occurs and triggers one or more nerve impulses Odorant binds to the receptor...

THE NERVOUS SYSTEM: PART 4 OLFACTION The sense of smell. ANATOMY OF OLFACTORY RECEPTORS  PHYSIOLOGY OF OLFACTION, THRESHOLDS AND ADAPTATION Odorants – stimuli Olfactory transduction – Depolarization occurs and triggers one or more nerve impulses Odorant binds to the receptor linked to G protein in olfactory cilium and activates the enzyme adenylate cyclase to produce a substance called cyclic adenosine monophosphate (cAMP) cAMP opens voltage-gated sodium channels to allow depolarization. Olfaction requires low threshold as few molecules are only needed to be present in the air to be perceived as an odor. Adaptation (decreasing sensitivity) to odors occurs rapidly. Olfactory receptors adapt by about 50% in the first second or so after stimulation but adapt very slowly thereafter. Still, complete insensitivity to certain strong odors occurs about a minute after exposure. OLFACTORY PATHWAY GUSTATION The sense of taste. ANATOMY OF TASTE BUDS AND PAPILLAE  PHYSIOLOGY OF GUSTATION, THRESHOLDS AND ADAPTATION Tastants – stimuli Once a tastant is dissolved in saliva, it can make contact with the plasma membranes of the gustatory microvilli, which are the sites of taste transduction. A receptor potential stimulates exocytosis of synaptic vesicles from the release of neurotransmitter. The hydrogen ions (H+) in sour tastants may flow into gustatory receptor cells via H+ channels. Some tastants bind to receptors on the plasma membrane that are linked to G proteins which in turn activate second messengers to cause depolarization. The pattern of nerve impulses determine the taste of the food. The threshold for bitter substances is lowest due to the fact that poisonous substances are bitter. Sour tastes have higher thresholds. Threshold for salty and sweet substances are similar and are higher than bitter or sour substances. Complete adaptation to a specific taste can occur in 1–5 minutes of continuous stimulation. GUSTATORY PATHWAY  VISION The sense of sight.  ACCESSORY STRUCTURES OF THE EYE Eyelids/Palpebrae Eyelashes Eyebrows Lacrimal apparatus Extrinsic eye muscles Superior rectus Inferior rectus Lateral rectus Medial rectus Superior oblique Inferior oblique  ANATOMY OF THE EYEBALL Fibrous tunic Cornea - anterior Sclera - posterior Scleral venous sinus (canal of Schlemm) – opening between the cornea and sclera; contains the aqueous humor Vascular tunic Choroid Ciliary body – contains the zonular fibers or suspensory ligaments that attach to the lens, and ciliary muscle Iris Pupil Retina Posterior three-quarters of the eyeball; contains the optic disc, a pigmented layer and a neural layer.  LAYERS OF THE RETINA Photoreceptor layer – contains specialized cells which convert light rays to nerve impulses Rods – 120 million, for night vision, do not provide color vision Cones – 6 million, produces color vision (blue, red, and green) Outer synaptic layer Bipolar cell layer Inner synaptic layer Ganglion cell layer Blind spot – area where the optic disc is located Macula lutea – exact center of the posterior portion of the retina, contains the fovea centralis  PHYSIOLOGY OF VISION Absorption of light by photopigment – a colored protein that undergoes structural changes when it absorbs light, in the outer segment of a photoreceptor. Rhodopsin – rods, regenerates in 30-40 minutes Cone photopigments – cones, regenerates in 90 seconds All photopigments associated with vision contain two parts: a glycoprotein known as opsin and a derivative of vitamin A called retinal. Isomerization – In darkness, retinal has a bent shape, called cis-retinal, which fits snugly into the opsin portion of the photopigment. When cis-retinal absorbs a photon of light, it straightens out to a shape called trans-retinal. Several chemical intermediates form and disappear leading to production of a receptor potential Bleaching – In about a minute, trans-retinal completely separates from opsin. Enzymatic conversion – Retinal isomerase converts trans-retinal back to cis-retinal Regeneration – Cis-retinal binding to opsin, resynthesis of a photopigment  LIGHT AND DARK ADAPTATION Light adaptation – The visual system adjusts in seconds to the brighter environment by decreasing its sensitivity Dark adaptation – The sensitivity of the visual system increases slowly over many minutes As the light level increases, more and more photopigment is bleached. While light is bleaching some photopigment molecules, however, others are being regenerated. In daylight, regeneration of rhodopsin cannot keep up with the bleaching process, so rods contribute little to daylight vision. In contrast, cone photopigments regenerate rapidly enough that some of the cis form is always present, even in very bright light. If the light level decreases abruptly, sensitivity increases rapidly at first and then more slowly. In complete darkness, full regeneration of cone photopigments occurs during the first 8 minutes of dark adaptation. During this time, a threshold (barely perceptible) light flash is seen as having color. Rhodopsin regenerates more slowly, and our visual sensitivity increases until even a single photon (the smallest unit of light) can be detected. RELEASE OF NEUROTRANSMITTER VISUAL PATHWAY HEARING AND EQUILIBRIUM The sense of hearing and balance. ANATOMY OF THE EAR ANATOMY OF THE EAR PHYSIOLOGY OF HEARING AUDITORY PATHWAY  PHYSIOLOGY OF BALANCE The inner ear senses balance. With head motion or pressure impulses of sound, the endolymph vibrates and creates a stimulus for the receptors of the vestibular system - the utricle and saccule. Inside the utricle and saccule are maculae containing hair cells with a membranous covering of microscopic otoconia that detect motion of the endolymph. Those in the saccule help sense vertical accelerations whereas those in the utricle sense horizontal accelerations. With changes in position, and thus changes in fluid motion, the shifting of these hair cells causes opening of receptor channels leading to action potentials propagating from the hair cells to the auditory nerve. The rate of fluid motion, plus the quality of the fluid, gives us more information about the motion. While the utricle and saccule detect linear motion, the semicircular ducts detect rotations in a similar fashion. EQUILIBRIUM PATHWAY