sense

17.1 Sensory Receptors

  • Definition of Key Terms
      - Receptor: A specialized structure that detects stimulus and generates nerve impulses in response to it.
      - Sense Organ: A nerve ending with connective, epithelial, or muscular tissue that gives response to a stimulus.

  • Classification of Sensory Receptors
      - Three main ways to classify sensory receptors:
        - By Modality: The type of stimulus they respond to (e.g., light, sound, pressure).
        - By Organ of Stimuli: Where the receptors are located (e.g., eyes for vision, ears for hearing).
        - By Distribution in the Body: General senses versus special senses.

  • General Sense: These receptors are distributed throughout the body, including skin, muscles, tendons, joint capsules, and organs. They detect various stimuli such as:
      - Touch
      - Pressure
      - Stretch
      - Heat or Cold

  • Types of Receptors in General Sense:
      - Unencapsulated Nerve Endings: Lack connective tissue wrapping and include:
        - Free Nerve Endings: Detect heat, cold, and pain.
        - Tactile Discs: Receptors for light touch and pressure on skin.
        - Hair Receptors: Dendrites wrap around hair follicles, detecting hair displacement.

  - Encapsulated Nerve Endings: Dendrites are wrapped in glial cells or connective tissue for specialized functions such as detecting touch, pressure, and stretch. Include:
    - Tactile Corpuscles: Sensitive to light touch.
    - End Bulbs: Detect warmth or cold.
    - Bulbous Corpuscles: Sense continuous pressure and distortion.

  • Sensory Neurons Receptive Fields:
      - Definition: The area that a single sensory neuron can detect stimuli.
      - Relevance: Receptive fields determine sensitivity; smaller fields (like fingertips) allow for more precise localization of stimuli than larger fields (like the back).

  • Pathways for General Senses:
      - Sensory information travels through:
        - First-order neurons (afferent fibers)
        - Second-order neurons (typically in the spinal cord or brainstem)
        - Third-order neurons (in the thalamus).
      - Major pathways include the spinothalamic tract and the reticulospinal tract, which are involved in the perception of pain.

  • Types of Pain:
      - Nociceptors detect pain and are specialized receptors.
      - Pain projection pathways:
        - Spinothalamic Tract: Carries signals of sharp pain and temperature to higher brain centers.
        - Reticulospinal Tract: Associated with dull pain and arousal responses.

  • Distinction between General and Special Senses:
      - General senses have widespread distribution and involve simple receptors, while special senses are localized in organs with complex structures and are innervated by cranial nerves.

  • Specific Receptor Classifications:
      - Full Bladder Receptors: Classified as general interoceptors that respond to stretch.
      - Taste Receptors: Classified as special senses found in taste buds.

  • Stimulus Modalities Detected by Free Nerve Endings:
      - Heat, cold, mechanical deformation (touch), and pain.

  • Encapsulated Nerve Endings:
      - Four types include:
        - Tactile Corpuscles: Light touch
        - End Bulbs: Warmth
        - Bulbous Corpuscles: Stretch
        - Lamellar Corpuscles: Vibration and pressure.

  • Connection of Neurons:
      - Most second-order somatosensory neurons synapse with third-order neurons in the thalamus.
      - Spinothalamic tract primarily handles sharp pain, while reticulospinal tract is more involved in dull, chronic pain.

17.2 The Chemical Senses: Taste and Smell

  • Anatomy of Taste and Smell Receptors:
      - Taste (Gustation):
        - Taste Buds: Sensory structures located in tongue, cheek, soft palate, pharynx, upper esophagus, and epiglottis.
        - Lingual Papillae: Physical structures on the tongue that house taste buds; visible to the naked eye.
        - Various papillae types:
          - Fusiform Papillae: Tiny spikes without taste buds.
          - Foliate Papillae: Ridges on the side of the tongue.
          - Fungiform Papillae: Taste and food texture receptors on tongue apex.
          - Vallate Papillae: V-shaped structures at the back of the tongue.
        - Taste Cells: Epithelial cells with microvilli (taste hairs) that contain binding sites for tastants.
        - Support Cells: Flat cells providing structural support, while basal cells serve as stem cells for replacing taste cells.

  • Pathway for Taste Signals:
      - Sequence: Tongue → Brainstem → Thalamus → Insula (taste processing).
      - cranial nerves involved:
        - Facial Nerve
        - Glossopharyngeal Nerve
        - Vagus Nerve

  • Smell (Olfaction):
      - Olfactory fibers project to the olfactory bulb, where they synapse with mitral and tufted cells, which then send signals to the brain.
        - Each glomerulus corresponds with a unique odor.
      - Conscious and Subconscious Processing:
        - The Primary Olfactory Cortex is responsible for conscious perception of odor.
        - Other regions (e.g., amygdala, insula, and hypothalamus) relate odor to memory and evoke emotional responses.
        - The Orbitofrontal Cortex integrates smell with other sensory information to form flavor perception.
        - There are feedback mechanisms to modify the perception of smell based on context.

  • Basic Functionalities:
      - Salivation, gagging, and vomiting responses are regulated by regions like the hypothalamus and amygdala in reaction to taste.

17.3 The Ear, Hearing, and Equilibrium

  • Anatomy of the Ear:
      - Three sections: Outer ear, Middle ear, and Inner ear.
        - Outer Ear: Functions as a funnel for sound vibrations.
        - Middle Ear: Contains the tympanic cavity within the temporal bone housing the tympanic membrane, which vibrates in response to sound.
          - Also includes auditory ossicles (malleus, incus, stapes) which amplify sound.
          - Eustachian tube connects middle ear to throat, important for pressure equalization.
        - Inner Ear: Comprised of bony labyrinth and membranous labyrinth, which contain fluids (endolymph and perilymph) and structures crucial for hearing and balance.

  • Function of the Ear:
      - Sound wave vibrations cause the tympanic membrane to vibrate → auditory ossicles transmit vibrations to the oval window of the cochlea → fluid within the cochlea moves causing hair cells to transduce mechanical signals into nerve impulses.
      - Vibrational motion causes the basilar membrane to oscillate, affecting hair cell movements which release neurotransmitters → generating auditory signals.

  • Auditory Pathway:
      - Cochlear nerve + vestibular nerve = vestibulocochlear nerve.
      - Signals from the cochlear nerve are relayed to the superior olivary nucleus, which has roles in:
        - Cochlear tuning (feedback to cochlea).
        - Protective tympanic reflex by signaling stapedius and tensor tympani muscles.
        - Binaural hearing for directional sound identification.

  • Vestibular Apparatus for Balance:
      - Contains sensory structures for equilibrium, including three semicircular ducts and the saccule and utricle.
        - Maculae in saccule and utricle contain support cells and otoliths which aid in detecting body position and movement.

17.4 The Eye and Vision

  • Anatomy of the Eye:
      - Tunics:
        - Fibrous Layer (Outer):
          - Sclera: White part of the eye, provides structure.
          - Cornea: Transparent front part that admits light into the eye.
        - Vascular Layer (Uvea): Contains blood vessels and tissues that nourish the eye.
          - Choroid: Rich in blood supply, supports retina.
          - Ciliary Body: Supports the lens and controls its shape via ciliary muscle.
          - Iris: Controls pupil diameter.

  • Optical Components of Eye:
      - Aqueous Humor: Fluid filling the anterior segment, maintaining pressure and nourishment.
      - Lens: Transparent structure whose curvature is adjusted for focusing on objects at varying distances (accommodation).
      - Vitreous Body: Jelly-like substance filling most of the eye, helping maintain its shape.

  • Neural Components:
      - Retina: Contains photoreceptor cells (rods and cones).
        - Macula Lutea: A central part of the retina involved in high acuity vision.
        - Fovea Centralis: Center of the macula, where vision is sharpest.
        - Optic Disc: Area where optic nerve fibers exit the eye, creates a blind spot as it lacks photoreceptors.

  • Vision Process:
      - Light from distant objects causes lens to flatten (less curvature); for close objects, ciliary muscles contract allowing lens to bulge (more curvature).
      - Rods provide low-light vision (scotopic), while cones are responsible for color and daylight vision (photopic).
      - Different types of cones:
        - Short-wavelength: Deep blue
        - Medium-wavelength: Green
        - Long-wavelength: Orange and yellow

  • Duplicity Theory of Vision:
      - Suggests that one type of retinal photoreceptor cannot simultaneously provide high sensitivity and high resolution; rods are more sensitive to light but have low resolution, whereas cones provide high resolution but less sensitivity.

  • Visual Projection Pathway:
      - Sequence: First-order neurons → synapse to second-order neurons → optic nerve → optic chiasm → optic tracts → undergo hemidecussation.