C

Senses 1

Senses: General & Special

  • Sensation: Our conscious awareness of incoming sensory information.

    • We filter out a lot of information and only a small percentage is passed to the brain.

Sensory Receptors: Properties

  1. Transducers: Change energy forms (e.g., mechanical to electrical).

  2. Receptive Field: Location of sensitive ends of receptor cells varies by size and specificity.

  3. Response Types:

    • Tonic receptors: Respond at a constant rate (e.g., balance receptors in ear).

    • Phasic receptors: Detect changes in stimulus (e.g., skin), can undergo adaptation.

How Are Receptors Described?

  • Three Criteria:

    • Receptor Distribution: Where in the body?

    • Stimulus Origin: What tissues are they found in?

    • Modality of Stimulus: What is being detected?

1. Receptor Distribution

General Senses (Two Categories):

  • Somatic receptors: Located in body wall; respond to chemicals, temperature, pain, touch, proprioception, and pressure.

  • Visceral receptors: Located in walls of viscera; respond to chemicals, temperature, and pressure.

Special Senses:

  • Located only in the head in sense organs (e.g., gustation, olfaction, vision, equilibrium, and hearing).

2. Stimulus Origin

  • Exteroceptors: Detect external stimuli.

    • Cutaneous: Touch (skin/mucous membranes); includes special senses (vision, hearing).

  • Interoceptors: Detect internal body stimuli.

    • Chemoreceptors (taste & smell) detect stretching, temperature, and chemical changes in the viscera.

    • Pain signals indicate overstretching or lack of oxygen (heart) or trauma.

  • Proprioceptors: Associated with body movement; found in muscles, tendons, and joints.

3. Modality of Stimulus

  • Chemoreceptors: Detect chemicals (e.g., taste buds, blood vessels).

  • Thermoreceptors: Detect temperature changes.

  • Photoreceptors: Detect light intensity, color, and movement (eyes).

  • Mechanoreceptors: Detect physical deformations (touch, pressure, vibration, stretch).

  • Baroreceptors: Detect pressure changes in blood vessels/organs.

  • Nociceptors: Detect pain and tissue damage.

Tactile Receptors

Temperature and Pain Receptors:

  • Free nerve endings: In epidermis (unencapsulated).

Pressure Receptors:

  • Lamellated (Pacinian) corpuscles: Encapsulated.

Fine Touch Receptors:

  • Tactile (Meissner) corpuscles: 2-point discrimination (encapsulated).

  • Tactile (Merkel) disks: Light touch (unencapsulated).

  • Root hair plexus: Detects hair movement (unencapsulated).

Types of Pain

Phantom Pain:

  • Sensation associated with a missing body part; sensory cell bodies remain active, providing sensations to the CNS.

Referred Pain:

  • When sensations from viscera are perceived as originating from elsewhere, e.g., heart attack.

Gustation

  • Gustation: Ability to taste; gustatory receptors are chemoreceptors on gustatory cells in the tongue and soft palate.

  • Tongue also houses mechanoreceptors and thermoreceptors for texture and temperature.

Papillae and Taste Buds:

  • Four types of papillae on tongue.

Types of Papillae:

  • Filiform: Front 2/3 of tongue; no gustatory role.

  • Fungiform: Block-like projections on tip/sides with a few taste buds.

  • Foliate: Not well developed in adults; a few taste buds.

  • Vallate (circumvallate): Large taste buds on the posterior tongue.

Tongue Structure

  • Taste buds: Onion-shaped sensory organs with three cell types: gustatory (detect tastants), supporting (sustain gustatory cells), and basal cells (replace gustatory cells).

Gustatory Tracing

  1. Gustatory cells in tongue.

  2. Nerves VII (facial) for anterior 2/3rd and IX (glossopharyngeal) for posterior 1/3rd.

  3. Synapse in the medulla oblongata.

  4. Synapse in thalamus.

  5. Impulse travels to the insula of the cerebral cortex for interpretation.

Taste Categories

  • Recognized types: salty, sour, sweet, bitter, umami.

    • All areas of the tongue can respond, with specific and general taste cells.

Taste Sensitivity

  • Different receptor densities result in varying sensitivities to flavors, influenced by smell, temperature, and texture of food.

  • Salty/sour use Na+/H+ ions; sweet, umami, and bitter use ligands activating G-protein signaling.

Olfaction

  • Olfactory epithelium: Lines superior nasal cavity; contains olfactory receptor cells, which regenerate every 40-60 days.

  • Supporting (sustentacular) cells sustain olfactory receptors; basal cells continually replace them.

Olfactory Sense

  • Bipolar neurons detect odors, with axons forming bundled olfactory cranial nerve (I).

  • Located through foramens in the cribriform plate of ethmoid bone into olfactory bulb.

Olfactory Tracing

  1. Odorants bind to receptors leading to signal activation.

  2. Signals travel through olfactory tract for processing in the brain.

  3. Signals also reach regions associated with memory and emotion (e.g., amygdala).

Inner Ear Anatomy

Divisions:

  • Bony labyrinth: Contains perilymph.

  • Membranous labyrinth: Contains endolymph.

  • Cochlea: Converts sound waves to nerve impulses.

Cochlea:

  • Contains the spiral organ (Organ of Corti) for hearing; hair cells in tectorial membrane generate nerve impulses when bent.

Sound Properties

  • Loudness: Related to amplitude measured in decibels.

  • Pitch: Related to frequency; higher frequency = higher pitch.

Disorders of the Ear

Types of Deafness:

  • Nerve deafness: Damage to hair cells or vestibulocochlear nerve.

  • Conduction deafness: Damage to tympanic membrane or middle ear bones.

  • Otitis media: Inflammation of the middle ear.

Hearing Pathway

  1. Cochlear nerve axons.

  2. Cochlear nucleus in medulla oblongata.

  3. Inferior colliculus.

  4. Medial geniculate nucleus of thalamus.

  5. Primary auditory cortex.