Module 6

• Receptors for the special senses


  Smell, taste, vision, hearing, and equilibrium are housed in complex sensory organs
• Ophthalmology


  The science that deals with the eye and its disorders
• Otorhinolaryngology


  The science that deals with disorders of the ear, nose, and throat
• Smell and taste are chemical senses
• Olfactory receptors


  • They are bipolar neurons in the nasal epithelium in the superior portion of the nasal cavity
  • They are first-order neurons of the olfactory pathway
  • Olfactory hairs are the parts that respond to the olfactory stimulus
  • Supporting cells are epithelial cells of the mucous membrane lining the nose
  • Basal stem cells produce new olfactory receptors
• Genetic evidence suggests there are hundreds of primary scents
• Olfactory reception


  1. Generator potential develops

  2. Transduction process triggers nerve impulses
• Odor thresholds and adaptation

  Adaptation to odors occurs quickly, and the threshold of smell is low: only a few molecules of certain substances need to be present in air to be smelled
• Olfactory pathway


  1. Olfactory receptors convey nerve impulses to olfactory nerves, olfactory bulbs, olfactory tracts, and the cerebral cortex and limbic system

  2. Primary olfactory area

  3. Orbitofrontal cortex
• Hyposmia, a reduced ability to smell, affects half of those over age 65 and 75% of those over 80. It can be caused by neurological changes, drugs, or the effects of smoking
• To be detected, taste molecules must be dissolved
• Taste stimuli classes

  • Sour
  • Sweet
  • Bitter
  • Umami
  • Salty
• Other "tastes" are a combination of the five primary taste sensations plus olfaction
• Taste buds


  • They consist of supporting cells, gustatory receptor cells, and basal cells
  • They are found in elevations on the tongue called papillae
  • Papillae include circumvallate, fungiform, and foliate papillae
  • Filiform papillae cover the tongue but act as tactile receptors instead of taste receptors
• Physiology of gustation


  1. When a tastant is dissolved in saliva it can make contact with the plasma membrane of gustatory receptor cells

  2. Receptor potentials developed in gustatory hairs cause the release of neurotransmitters that gives rise to nerve impulses

  3. Receptor potentials develop in response to chemicals in the food (i.e. sodium or hydrogen)
• Taste thresholds and adaptation

  Taste thresholds vary for each of the primary tastes. The lowest threshold is bitter, followed by sour, and then the other primary tastes. Adaptation to taste occurs quickly.
• Gustatory pathway


  Gustatory receptor cells convey nerve impulses to cranial nerves VII, IX, and X, the medulla, the thalamus, and the parietal lobe of the cerebral cortex
• Taste aversion causes individuals to avoid foods that upset their digestive system. Because cancer treatments cause nausea, cancer patients may lose their appetites because they develop taste aversion for most food
• More than half the sensory receptors in the human body are located in the eyes
• A large part of the cerebral cortex is devoted to processing visual information
• Visible light


  Wavelengths between 400 and 700 nm is the only part of the spectrum of electromagnetic radiation that can be detected by the eyes
• Eyelids


  • They shade the eyes during sleep, protect the eyes from excessive light and foreign objects, and spread lubricating secretions over the eyeballs
  • From superficial to deep, each eyelid consists of epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva
  • The tarsal plate gives form and support to the eyelids
  • The tarsal glands secrete a fluid to keep the eye lids from adhering to each other
  • The conjunctiva is a thin mucous membrane that lines the inner aspect of the eyelids and is reflected onto the anterior surface of the sclera, but not cornea
• Eyelashes and eyebrows


  • They help protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun
• Lacrimal apparatus


  • It consists of structures that produce and drain tears
• Extrinsic eye muscles


  • They move the eyeballs laterally, medially, superiorly, and inferiorly
• Layers of the eyeball

  • Fibrous tunic (sclera and cornea), vascular tunic (choroid, ciliary body, and iris), and retina (nervous tunic)
• Sclera


  • It is a white coat of dense fibrous tissue that covers the entire eyeball, except the most anterior portion—the iris. It gives shape to the eyeball and protects its inner parts. Its posterior surface is pierced by the optic nerve.
• Cornea


  • It is a nonvascular, transparent, fibrous coat through which the iris can be seen; the cornea acts in refraction of light.
• Choroid


  • It absorbs light rays so that they are not reflected and scattered within the eyeball; it also provides nutrients to the posterior surface of the retina.
• Ciliary body


  • It consists of the ciliary processes and ciliary muscle. The ciliary processes secrete aqueous humor. The ciliary muscle alters the shape of the lens for near or far vision.
• Iris


  • It is the colored portion seen through the cornea and consists of circular iris and radial iris smooth muscle fibers (cells) arranged to form a doughnut-shaped structure. The black hole in the center of the iris is the pupil, the area through which light enters the eyeball. A principal function of the iris is to regulate the amount of light entering the posterior cavity of the eyeball.
• Retina


  • It is the beginning of the visual pathway and lines the posterior three-quarters of the eyeball. It consists of a pigment epithelium (nonvisual portion) and a neural portion (visual portion). The photoreceptor neurons are called rods or cones because of the differing shapes of their outer segments.
• Rods and cones


  • Rods are specialized for black-and-white vision in dim light; they also allow us to discriminate between different shades of dark and light and permit us to see shapes and movement. Cones are specialized for color vision and sharpness of vision (high visual acuity) in bright light; cones are most densely concentrated in the central fovea.
• A detached retina may result in visual distortions or blindness
• Age related macular disease is a degenerative disorder of the retina and the pigmented layer in persons 50 years of age or older
• Lens, anterior cavity, and vitreous chamber

  • The lens fine tunes the focusing of light rays for clear vision. The anterior cavity is filled with aqueous humor that continually filters out of blood capillaries in the ciliary processes behind the iris. The vitreous chamber lies between the lens and the retina and contains a gel called the vitreous body.
• Excessive intraocular pressure, called glaucoma, results in degeneration of the retina and blindness
• Image formation on the retina

  Refraction of light rays by the cornea and lens, accommodation of the lens, and constriction of the pupil
• Images are focused upside-down (inverted) on the retina and also undergo mirror reversal; these inverted images are rearranged by the brain to produce perception of images in their actual orientation
• Accommodation


  Increase in the curvature of the lens, initiated by ciliary muscle contraction, which allows the lens to focus on near objects. To focus on far objects, the ciliary muscle relaxes, and the lens flattens.