Special Senses

Chapter 10: The Senses

10.1: Introduction to the Senses

• Senses arise from sensory receptors detecting environmental changes and stimulating neurons.

• Nerve impulses are sent to the Central Nervous System (CNS) for processing.

• Responses manifest as feelings or sensations.

• Categories of Senses:

  • General Senses:

    • Widely distributed, structurally simple (e.g., touch, pressure, temperature, pain).

  • Special Senses:

    • Complex, specialized organs located in the head (e.g., vision, hearing, smell, taste, balance).

10.2: Receptors, Sensations, and Perception

• All action potentials initiated are identical, yet different sensory events are perceived due to various receptors.

• Types of Sensory Receptors:

  • Chemoreceptors: Sensitive to chemical concentration changes.

  • Pain Receptors (Nociceptors): Detect tissue damage.

  • Thermoreceptors: Detect temperature changes.

  • Mechanoreceptors: Respond to pressure or movement changes.

  • Photoreceptors: Respond to light and located in the eye as rods and cones.

10.3: Sensation and Perception

• Sensation: Occurs when receptors are stimulated and send impulses to the brain.

• Perception: Conscious awareness of stimuli.

• Projection: Enables sensation awareness back to its origin for localization.

• Sensations depend on the brain region that receives the impulses.

10.4: Sensory Adaptation

• The brain prioritizes incoming sensory impulses to avoid information overload.

• Sensory Adaptation: Brain becomes less responsive to maintained stimuli (e.g., clothing, persistent odors).

• Can result from receptor unresponsiveness or inhibition along CNS pathways.

10.5: General Senses

• General senses are widespread with receptors in skin, muscles, joints, and viscera.

• Types of Receptors:

  • Touch and Pressure:

    • Free nerve endings: Associated with itching; based in epithelial tissues.

    • Tactile (Meissner's) Corpuscles: Detect objects touching skin; abundant in hairless areas.

    • Lamellated (Pacinian) Corpuscles: Detect deep pressure, found in deeper dermis and subcutaneous layers.

10.6: Touch and Pressure Receptors

• Structure and function of various touch and pressure receptors are detailed in diagrams.

10.7: Temperature Senses

• Temperature Receptors:

  • Warm Receptors: Triggered by temperatures 25°C (77°F) to 45°C (113°F); above it, pain receptors activated.

  • Cold Receptors: Respond between 10°C (50°F) to 20°C (68°F); below evoke pain sensations.

• Rapid adaptation occurs with both receptor types after 1 minute of continuous stimulation.

10.8: Body Position, Movement, & Stretch Receptors

• Proprioception: Awareness of body position and spatial orientation.

• Proprioceptors: Involved in preventing muscle and tendon injuries.

  • Muscle Spindles: Monitor muscle contraction states.

  • Golgi Tendon Organs: Detect stretching of tendons during muscle contraction.

10.9: Sense of Pain

• Pain Receptors (Nociceptors): Free nerve endings activated by tissue damage and overstimulation.

• Substance P and Glutamate: Neurotransmitters involved in transmitting pain signals.

• Prostaglandin release inflates nociceptor sensitivity.

• Pain management can include drugs like aspirin and ibuprofen.

10.10: Visceral Pain

• Visceral pain receptors respond differently than surface tissue receptors.

• Common nerve pathways lead to referred pain; for instance, heart pain often felt in the shoulder or arm.

10.11: Fast and Slow Pain Fibers

• Fast Pain Fibers: Myelinated, relay sharp pain swiftly.

• Slow Pain Fibers: Unmyelinated, conduct dull pain slowly, continue after the stimulus ends.

• Pain experiences often relate to simultaneous stimulation of both fiber types.

10.12: Pain Pathways

• Pain impulses to the brain differ based on location:

  • Head: Via sensory fibers of cranial nerves.

  • Other body parts: Via spinal nerves; processed in the spinal cord's gray matter.

• Impulses then routed through the thalamus and limbic system for emotional interpretation.

10.13: Special Senses

• Special Senses: Include smell (olfactory), taste (taste buds), hearing (ears), and sight (eyes).

10.14: Sense of Smell

• Olfactory Organs: Located in the nasal cavity roof; includes olfactory receptors (bipolar neurons).

• Olfactory receptors sensitive to odorants that must be dissolved to stimulate.

10.15: Olfactory Pathways

• When stimulated, olfactory receptors connect to the olfactory bulbs and travel to cerebral interpretation areas.

• Impulses also project to the limbic system for emotional responses.

10.16: Sense of Taste

• Taste Buds: Located on the tongue and pharynx, housing 50-100 taste cells.

• Chemicals must dissolve in saliva to be tasted; involved in detecting five primary tastes: sweet, sour, salty, bitter, umami.

10.17: Taste Pathways

• Taste impulses travel through facial, glossopharyngeal, and vagus nerves to be interpreted in the gustatory cortex.

10.18: Sense of Hearing

• The ear has structural divisions: outer, middle, and inner.

• Outer Ear: Includes the auricle, external acoustic meatus (canal), and tympanic membrane (eardrum).

10.19: Middle Ear

• Middle Ear (Tympanic Cavity): Houses auditory ossicles: malleus, incus, stapes.

• Ossicles amplify sound waves from the tympanic membrane to the inner ear.

10.20: Auditory Tube

• Connects middle ear to nasopharynx to equalize air pressure.

10.21: Inner Ear

• Structure: A labyrinth comprising a bony and membranous labyrinth, housing cochlea (hearing) and semicircular canals (equilibrium).

10.22: Cochlea

• Composed of 3 chambers (scala vestibuli, cochlear duct, scala tympani) responsible for different frequencies.

10.23: Hearing Receptor

• Spiral organ (organ of Corti) responds to sound vibrations, generating action potentials.

10.24: Auditory Pathways

• Nerve fibers lead to auditory cortices for sound interpretation.

• Hearing loss can be conductive or sensorineural.

10.25: Steps in Generation of Sensory Impulses

• Outlines the transmission of sound waves through various structures to be processed.

10.26: Sense of Equilibrium

• Comprises static and dynamic equilibrium, aiding balance in various movements.

10.27: Static Equilibrium

• Located in the vestibule of the inner ear, contains maculae that respond to head positions.

10.28: Dynamic Equilibrium

• Senses rapid head movements via semicircular canals and cristae ampullaris.

10.29: Sense of Sight

• Eye structure encompasses protective and functional accessories and three layers: outer, middle, inner.

10.30: Visual Accessory Organs

• Include lacrimal apparatus for lubrication and cleansing, and extrinsic muscles for movement.

10.31: Structure of the Eye

• Fluid-filled, with distinct layers; focuses light via refraction.

10.32: Outer Layer of the Eye

• Composed of cornea (transparent, focuses light) and sclera (protective, opaque).

10.33: Middle Layer of the Eye

• Comprises vascular choroid coat, ciliary body (lens adjustment), and iris (light adjustment).

10.34: Inner Layer of the Eye

• Houses retina with photoreceptors; retina structure and function.

• Macula Lutea: Center for sharpest vision.

10.35: Light Refraction & Corrective Lenses

• Describes normal, nearsightedness, and farsightedness; corrective expectations.

10.36: Photoreceptors

• Rods (dim light) and cones (color vision) function and structure explained.

10.37: Photopigments

• Rhodopsin (in rods) and pigments in cones facilitate light absorption and nerve impulse generation.

10.38: Visual Pathways

• Pathway of nerve impulses from retina through the optic nerve to visual cortex for interpretation.