Week 5, Friday

Sensory Receptors pdf (continued)

Classification of Somatic Senses:

• somatic senses collect different types of information from all over the body

• Divided into:

  • Mechanoreceptors

    • Stimulated by mechanical displacement/ deformation

      • Tactile: touch, pressure, vibration, tickle, itch

    • Proprioceptive

      • Detect movement, joint positions

        • Muscles spindles in muscles, Golgi tendon organs in tendons, joint receptors in joints

  • Thermoreceptors

    • Detect changes in body temperature

      • Heat or cold

      • Also in skin

  • Nocioceptors

    • Nerve endings

      • Detect tissue damage or the release of pain

        • Mediating molecules

          • I.e. in skin

Classification of sensation based on microscopic structures:

• 3 types:

  1. Free nerve endings

     • For detection of pain, temp., tickle, itch and some light touch

     • First-order neurons

       • Carry information from receptors to CNS

       • The cell bodies of these neurons are in the dorsal root ganglia

     • Second-order neurons

       • Located within the CNS

  2. Encapsulated nerve endings

     • used to detect deep pressure, vibration and some intense touch

     • Contains a lamellated corpuscle on Soma

       • Ex. Pacinian Corpuscle

     • First-order sensory neurons

       • With encapsulated nerve endings

  3. Separate cells

     • Sensory receptors for some special senses include:

       • Gustatory receptor cells in taste buds

       • Photoreceptors in the retina of the eye

       • Hair cells in the inner ear for hearing

Receptor classification base on Location and Activating Stimuli

• Extreoceptors

  • Located on or near body surface

  • Provide info about external environment

  • Convey visual, smell, taste, touch, pressure vibration, thermal, and pain sensations

• Visceral receptors

  • Located in internal organs

    • I.e. blood vessels, visceral organs, etc.

  • Provide info about internal environment

  • Impulses usually are not consciously perceived but occasionally may be felt as pain or pressure

• Proprioceptors

  • Located in muscles, tendons, joints and inner ear

  • Provide info about body position, muscle length and tension, position and motion of joints and equilibrium (balance)

Classification based on type of Stimulus Detected:

• Photoreceptors

  • Detect photons of light that strikes the retina of the eye (also called electromagnetic receptors)

• Chemoreceptors

  • Detect chemicals in mouth (taste), nose (smell), O2, CO2 levels and body fluids

• Osmoreceptors

  • Detect osmotic pressure/osmolarity of body fluids

    • Usually amount of electrolytes like Na+ ions

  • Detected by hypothalamus

Adaptation in Sensory Receptors over time:

• Adaptation

  • The receptor potential can decrease in amplitude during a maintained, constant stimulus over time

  • The perception of a sensation may fade or disappear even though the stimulus persists either because of specific properties of the receptor, or because of accommodation

    • Sodium channels inactivate over time and no longer generate receptor potentials

  • 2 types of adaptation

    • Rapidly adapting receptors (phasic/rate)

      • Adapt Very quickly

      • Specialized for signaling changes in a stimulus intensity

      • Receptors associated with pressure, some touch, and small and in semicircular canals in ears

      • Examples of Phasic/rate receptors:

        • Hair receptor in ear

        • Pacinian Corpscle

    • Slowly adapting receptors (tonic)

      • By contrast, adapt slowly and continue to trigger nerve impulses as long as the stimulus persists

      • Monitor stimuli associated with pain, body position, and chemical composition of the blood

      • Examples of Tonic receptors:

        • Macula in the vestibular apparatus (ear)

        • Pain receptors

        • Baroreceptors of arteries

        • Chemoreceptors in the body, Golgi tendon organs, muscle spindles in muscles

Tactile Receptors:

• Touch, pressure, vibration

• Glabrous= hairless skin like palm of hand, finger tips

• Merkel cells and Discs= very soft touch

  • In the epidermis

  • Slow

  • Saucer-shaped free nerve endings

  • In hairy and non-hairy skin

  • Continuous touch

• Meissner’s Corpuscle= low frequency vibrations ad very sensitive to light touch

  • In the dermal papillae

  • Rapid

  • Capsule surrounds mass of dendrites

  • Non-hairy skin

  • Epidermis of finger tips, lips

• Hair Root Plexus= movements on skin surface that disturbs hairs

  • Rapid

  • Free nerve endings wrapped around hair follicles in skin

• Pacinian Corpuscles= high-frequency vibrations

  • Most rapid

  • Oval, layered capsule surrounds dendrites

  • Present in Deep dermis and subcutaneous layer, submucosa tissues, joints, periosteum ad some viscera

• Ruffini Corpuscles= continuous skin stretching and pressure

  • Slow

  • Elongated capsule surrounds dendrites deep in dermis

Action Potentials are transmitted via sensory neurons:

• proprioceptors

  • Muscle spindle in skeletal muscles and Golgi tendon organs in tendons

  • Transmit info about body position through the Aa type sensory neurons

• Somatic motors neurons

  • Innervate skeletal muscles and cause muscle contraction

  • Transmit info through the Aa type motor neuron

• Touch and pressure

  • Transmitted through A beta neurons

• Fast pain, cold

  • Transmitted through A delta neurons

• Slow pain, heat

  • Transmitted through the C neurons (unmyelinated)

Classification of nerve fibers

• A - myelinated

  • Alpha

    • Somatic motor

    • Proprioception

  • Beta

    • Touch, pressure

  • Gamma

    • Motor to muscle spindles

  • Delta

    • Pain (fast)

    • Cold

    • Touch

    • Thin myeliation

• B - myelinated

  • Preganglionic sympathetic

• C - unmyelinated

  • Dorsal root

    • Pain receptors, reflex, Hot, other mechanoreceptors

    • Free nerve endings

    • Temp, slow pain, itch, tickle

  • Sympathetic

    • Postganglionic