3. Senses 1

Sense Organs

Free Nerve Endings

  • Sensory receptors for pain, temperature, tickle, and itch.

Tactile Discs

  • Detect fine touch and texture.

Hair Receptors

  • Respond to hair movement on the skin.

Sensation vs. Perception

  • Sensation: Awareness of stimuli through sensory receptors; includes unconscious stimuli (e.g., X-rays).

  • Perception: Conscious interpretation of sensations involving localization, identification, and memory storage.

Sensory Modalities

  • Types of Sensations: Touch, Pain, Temperature, Vibration, Hearing, Vision.

  • Classes of Sensory Modalities:

    • General Senses:

      • Somatic: Body wall sensations.

      • Visceral: Internal organ sensations.

    • Special Senses: Smell, taste, hearing, vision, balance.

Process of Sensation

  1. Stimulation: Activated by a stimulus.

  2. Transduction: Converts stimulus to graded potential.

  3. Action Potential Generation: Graded potential meets threshold.

  4. CNS Integration: Sensory input processed by CNS.

Characteristics of Sensory Receptors

  • Respond selectively to specific stimuli.

    • General Receptors: Simple structures (e.g., free nerve endings for pain).

    • Special Receptors: Complex structures (e.g., vision, hearing).

Properties of Receptors

  • Modality: Stimulus type.

  • Location: Identifies stimulus position.

  • Intensity: Action potential frequency and recruitment.

  • Duration: Changes in firing rates (phasic vs tonic).

Receptive Fields

  • Area of receptor's sensitivity linked to neurons.

Classification of Sensory Receptors

  • Structural: Free nerve endings, encapsulated endings, sensory cells.

  • Response Type: Generator vs receptor potentials.

  • Location: Exteroceptors, interoceptors, proprioceptors.

  • Stimuli Type: Mechanoreceptors, thermoreceptors, nociceptors, photoreceptors, chemoreceptors.

Adaptation of Sensory Receptors

  • Adaptation: Decreased response to prolonged stimuli.

  • Rapidly Adapting Receptors: Adjust quickly (e.g., smell).

  • Slowly Adapting Receptors: Continuous response (e.g., pain).

Somatic Tactile Sensations

  • Touch: Crude vs discriminative.

  • Pressure: Sustained sensation.

  • Vibration: Rapidly repetitive signals.

  • Itching: Chemical stimulation.

  • Tickle: Stimulation only from external sources.

Specialized Tactile Receptors

  • Meissner’s Corpuscle: Light touch and vibration.

  • Merkel’s Disc: Discriminative touch.

  • Ruffini Corpuscle: Continuous touch and pressure.

  • Pacinian Corpuscle: Pressure and high-frequency vibration.

Pain Sensations

  • Nociceptors: Receptors for intense stimuli (e.g., muscle spasms).

  • Types of Pain:

    • Fast Pain: Sharp and immediate.

    • Slow Pain: Dull and throbbing.

Pain Localization & Referred Pain

  • Superficial Somatic Pain: Skin origin.

  • Deep Somatic Pain: Muscles or joints.

  • Visceral Pain: Diffuse & poorly localized.

  • Referred Pain: Misinterpreted pain due to overlapping circuits.

Pain Relief and Management

  • Over-the-counter Analgesics: Aspirin, ibuprofen.

  • Local Anesthetics: Block nerve conduction.

  • Opioid Analgesics: Modulate brain pain perception.

Projection Pathway for Pain

  1. First-Order Neurons: In dorsal root ganglia.

  2. Second-Order Neurons: Cross to opposite side in spinal cord.

  3. Thalamic Projection: Signals to the somatosensory cortex.

CNS Modulation of Pain

  • Influenced by mental state; endogenous opioids involved.

  • Spinal Gating: Blocks pain transmission based on sensory input.

Proprioceptors and Kinesthetic Sense

  • Proprioception: Awareness of body position; minimal adaptation.

  • Muscle Spindles: Monitor muscle length.

  • Golgi Tendon Organs: Respond to muscle tension.

Balance and Equilibrium

  • Static Equilibrium: Position against gravity via macula receptors.

  • Dynamic Equilibrium: Balance during movement via crista receptors in semicircular ducts.

Auditory Structures and Pathways

  • External Ear: Auricle and external auditory canal.

  • Middle Ear: Contains ossicles for sound transmission.

  • Inner Ear: Houses cochlea and vestibular apparatus; contains endolymph and perilymph.

Physiology of Hearing

  • Sound transmission involves ossicles, basilar membrane vibration, and hair cell stimulation.

  • Cochlear Tuning: Sensitivity enhancement to frequencies.

Chemosensation: Taste and Smell

  • Taste: Five primary senses detected by taste buds (salty, sweet, sour, bitter, umami).

  • Smell: Sensitive olfactory receptors; pathways linked to emotional responses.