Lecture 11 Senses

Overview

Lecture 11 covers the sensory system, emphasizing special senses in animals: taste, smell, hearing, and vision. Notes include structures, receptor types, functions, and clinical relevance.

General Concepts of Sensation

• Senses monitor internal and external environments via specialized receptors.
• Receptors convert mechanical, thermal, electromagnetic, and chemical stimuli into nerve impulses.
• Impulses reach the central nervous system (brain and spinal cord) and are perceived as sensations.
• Special senses have receptors concentrated in the head, aided by accessory structures.
• Normal receptor function supports awareness, movement, protection, and homeostasis.

Clinical Relevance

• Damage to receptors or nerves impairs sensing and responses to the environment.
• Consequences include corneal ulcers, spinal injuries, chronic ear infections, and homeostatic disruption.
• Clinical signs can include loss of reflexes, blindness, and altered pain perception.

Special Senses: Summary Table

Gustatory Sense (Taste)

• Chemical sense relying on chemoreceptors detecting dissolved molecules.
• Taste buds contain taste pores; receptor cell microvilli project into pores.
• Dissolved substances interact with microvilli, generating impulses interpreted as tastes.

Olfactory Sense (Smell)

• Chemical sense detecting inhaled airborne molecules.
• Receptor cells in nasal epithelium; microvilli project into the mucosal layer.
• Odor molecules dissolve in mucus; signals travel to brain via olfactory nerve.

Auditory Sense (Hearing) and Balance

• Mechanical sense using receptors activated by touch, pressure, vibration, and sound waves.
• Ear enables hearing and contributes to balance; structures mostly in temporal bones.
• Sound pathway: external ear collects waves, middle ear amplifies/transmits, inner ear converts to impulses.

Ear Anatomy and Function

• External ear:
  • Pinna (ear flap) captures and directs sound waves.
  • External auditory canal transmits sound to tympanic membrane.
  • Tympanic membrane (eardrum) is thin connective tissue; vibrates with sound.
• Middle ear:
  • Air-filled cavity medial to tympanic membrane.
  • Ossicles: malleus (attached to tympanic membrane), incus, stapes (contacts cochlea).
  • Eustachian tube links middle ear to pharynx; equalizes pressure during swallowing.
• Inner ear:
  • Cochlea: fluid-filled, snail-shaped; Organ of Corti houses receptor cells.
  • Fluid motion distorts microvilli on receptor cells; impulses carried via cochlear nerve.
  • Vestibule and semicircular canals monitor balance and head position.

Visual Sense (Sight)

• Electromagnetic sense detecting visible light via specialized eye receptors.
• Eye components function to gather and focus light onto photoreceptors.
• Light stimulation generates impulses to brain via optic nerve.

Eye Structures and Functions

• Cornea: clear window at rostral eye; admits and focuses light rays.
• Sclera: dense, white outer layer; supports and shapes the globe.
• Iris: colored muscular diaphragm behind cornea; controls pupil size.
• Pupil: central opening in iris; constricts in bright light, enlarges in dim light.
• Lens: located caudal to iris; accommodation for near and distant vision.
• Retina: inner, multilayered caudal eye lining; forms visual images.
  • Photoreceptors:
    • Rods: long, narrow; very light-sensitive; dim light vision; poor detail/color.
    • Cones: flask-shaped; detect detail and color.
  • Optic disc: convergence point of nerve fibers forming the optic nerve.

Accessory Structures of the Eye

• Conjunctiva:
  • Thin membrane lining eyelids and covering outer eyeball.
  • Assessed for anemia, inflammation, or jaundice by color and appearance.
• Eyelids and eyelashes:
  • Upper (dorsal) and lower (ventral) eyelids meet at medial and lateral canthi.
  • Protect eyes; blinking spreads tears; lashes shield from debris and bright light.
• Third eyelid (nictitating membrane):
  • Cartilage plate covered by conjunctiva; located medially between eyelids.
  • Present in cats and various vertebrates; rare in primates; humans have vestigial equivalent.
• Lacrimal apparatus:
  • Produces and drains tears; located dorsal to lateral canthus.
  • Tears flow over eye; blinking distributes; drained via lacrimal puncta at medial canthus.

Key Terms & Definitions

• Sensory receptor: Specialized nerve ending converting stimuli into nerve impulses.
• Chemoreceptor: Receptor responding to chemical stimuli (taste, smell).
• Mechanical receptor: Receptor responding to touch, pressure, vibration, sound.
• Tympanic membrane: Eardrum; vibrates with sound waves.
• Ossicles: Malleus, incus, stapes; transmit vibrations to inner ear.
• Eustachian tube: Connects middle ear to pharynx; equalizes pressure.
• Cochlea: Inner ear structure for hearing; contains Organ of Corti.
• Organ of Corti: Houses hair cell receptors; generates auditory impulses.
• Vestibule/Semicircular canals: Inner ear structures for balance.
• Cornea/Sclera: Outer eye layers; clear window and white supportive layer.
• Iris/Pupil: Colored diaphragm and central opening controlling light entry.
• Lens: Adjusts focus for near and far vision (accommodation).
• Retina: Inner layer with rods and cones; forms visual images.
• Optic disc: Site where optic nerve fibers exit the eye.
• Conjunctiva: Membrane covering eyelids and outer eyeball.
• Lacrimal puncta: Small openings draining tears at medial canthus.

Action Items / Next Steps

• Review textbook Table 7.4 comparing general and special senses.
• Study Figure 7.23 (ear anatomy) and Figure 7.24 (eye anatomy).
• Practice labeling cornea, sclera, retina, iris, pupil, and lens on an eye diagram.
• Correlate clinical signs (reflex loss, blindness, pain changes) with receptor and pathway functions.