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Peripheral Nervous System and Sensory Receptors

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  • Peripheral Nervous System (PNS) functional divisions
    The PNS is divided into sensory and motor divisions, which are further subdivided into somatic and visceral, and general and special categories.
  • Autonomic Nervous System (ANS) divisions
    The ANS, part of the general visceral motor PNS, has two divisions: parasympathetic and sympathetic.
  • Components of the Peripheral Nervous System
    The PNS includes sensory receptors, nerves (bundles of peripheral axons), ganglia (clusters of neuronal cell bodies), and motor endings that innervate muscles and glands.
  • Function of sensory receptors
    Sensory receptors pick up stimuli from inside or outside the body and initiate signals in sensory axons.
  • Two main categories of sensory receptors
    Free nerve endings of sensory neurons monitor general sensory information, while complete receptor cells are specialized epithelial cells or small neurons monitoring special sensory information.
  • Classification of sensory receptors by location
    Exteroceptors detect external stimuli; interoceptors detect internal stimuli; proprioceptors monitor body position and movement.
  • Exteroceptors
    Located at or near body surfaces, exteroceptors respond to stimuli like touch, pressure, pain, and temperature from outside the body.
  • Interoceptors
    Located in internal organs like the digestive tube and lungs, interoceptors monitor chemical changes, tissue stretch, temperature, and taste stimuli.
  • Proprioceptors
    Found in skeletal muscles, tendons, joints, and ligaments, proprioceptors monitor stretch and send information about body movement to the CNS.
  • Classification of sensory receptors by stimulus detected
    Includes mechanoreceptors (touch, pressure), thermoreceptors (temperature), chemoreceptors (chemicals), photoreceptors (light), and nociceptors (pain).
  • Mechanoreceptors
    Respond to mechanical forces such as touch, pressure, stretch, vibration, and itch; baroreceptors monitor blood pressure.
  • Thermoreceptors
    Sensory receptors that respond to changes in temperature.
  • Chemoreceptors
    Detect chemicals in solution, important for taste and internal chemical monitoring.
  • Photoreceptors
    Located in the eye, photoreceptors respond to light stimuli.
  • Nociceptors
    Respond to harmful stimuli that cause pain.
  • General sensory receptors structure
    Widely distributed nerve endings of sensory neurons that monitor touch, pressure, vibration, stretch, pain, temperature, and proprioception.
  • Two groups of general sensory receptors
    Free nerve endings and encapsulated nerve endings.
  • Free nerve endings
    Abundant in epithelia and connective tissue; respond mainly to pain and temperature.
  • Specialized free nerve endings
    Include epithelial tactile complexes (Merkel discs) for light touch and hair follicle receptors that rapidly adapt to touch.
  • Encapsulated nerve endings
    Sensory neuron end fibers enclosed in connective tissue; mainly mechanoreceptors including tactile, lamellar, bulbous corpuscles, and proprioceptors.
  • Tactile (Meissner’s) corpuscles
    Located in dermal papillae of hairless skin; rapidly adapting receptors for discriminative touch.
  • Lamellar (Pacinian) corpuscles
    Found in the hypodermis; sensitive to deep pressure and vibration; rapidly adapting.
  • Bulbous corpuscles (Ruffini endings)
    Located in the dermis; respond to continuous pressure and stretch; slowly adapting.
  • Proprioceptors types
    Include muscle spindles, tendon organs, and joint kinesthetic receptors.
  • Muscle spindles
    Measure changing muscle length; embedded in perimysium; contain intrafusal muscle fibers with anulospiral endings sensitive to stretch.
  • Tendon organs
    Located near muscle-tendon junctions; monitor tension within tendons.
  • Joint kinesthetic receptors
    Sensory nerve endings in joint capsules; include lamellar corpuscles, bulbous corpuscles, free nerve endings, and receptors like tendon organs.