Mechanoreception & Electroreception

Define mechanoreceptor and know the 2 types of mechanoreceptor proteins and their characteristics

Mechanoreceptors: sensory receptors that transduce mechanical stimuli into electrical signals (required for touch, hearing, balance, and vertebrate blood pressure)

• 2 known types of mechanoreceptor proteins:

  • ENaCs (epithelial sodium channels)

  • TRP (transient receptor potential) channels

• Mechanoreceptor signal transduction

  • Mechanical forces cause displacement of extracellular anchoring proteins and produces a conformational change in ion channels

  • Absence of intracellular cascade

Understand that there are many types of mechanoreceptors (vertebrates and invertebrates)

Types of mechanoreceptors

• Touch and pressure

  • Baroreceptors (vertebrates)

  • Tactile receptors (vertebrates and invertebrates)

  • Proprioceptors (vertebrates and invertebrates)

• Equilibrium and hearing

  • Statocysts (invertebrates)

  • Various hearing organs (invertebrates)

  • Hair cells (vertebrates)

  • Neuromasts (bony fish)

Describe the mechanoreceptors involved in touch and pressure in vertebrates and invertebrates (baroreceptors, tactile receptors, proprioceptors)

Baroreceptors

• Vertebrates

Tactile receptors

• Vertebrates

  • Free nerve endings

  • Merkel’s Disks

  • Ruffini Corpuscle

  • Root hair plexus

  • Pacinian Corpuscle

• Invertebrates

  • Trichoid sensilla

  • Campaniform sensilla

Proprioceptors

• Vertebrates

  • Muscle spindles

  • Golgi tendon organs

  • Joint capsule receptors

• Invertebrates

  • Chordonal organ

Describe baroreceptors

Baroreceptors (vertebrates)

• Detect pressure changes (stretch-sensitive) in walls of blood vessels, parts of the heart, digestive, reproductive, and urinary tracts

• E.g. Carotid sinus baroreceptors: monitor blood pressure to the brain

Describe tactile receptors

Tactile Receptors

(vertebrates)

• Detect touch, pressure, vibration on the body

• Free nerve endings

  • Sensory neurons with dendrites interspersed among epidermal cells

  • Receptor proteins on the dendrites

• Merkel’s Disks

  • Enlarged epidermal cell (Merkel cells) associated with free nerve endings

  • Small receptive field (fine discrimination)

• Tonic receptor cells

  • detect light touch and pressure on skin surface

  • Slowly adapting

• Ruffini Corpuscle

  • dendrite endings with elongated capsule

  • located in connective tissue of skin and with connective tissue of joints and limbs

  • sensitive to stretching of skin and mvt of joints

  • help detect position of body in space (proprioceptor)

• Root hair plexus

  • Nerve endings wrap around base of hair follicles (responds when hair is displaced)

  • Phasic receptor

  • Detect changes in movement across body surface

• Pacinian Corpuscle

  • Sensory dendrite surrounded by lamellae

  • Located in skin (deep), muscles, joints, and internal organs

  • Phasic receptors, detect changes in pressure

  • Large receptive field, poor discrimination, especially sensitive to vibration

• Phasic receptor cells

  • Detect movement or pressure on skin surface

• Trichoid sensilla

  • Hair-like projection of cuticle, bends in response to touch or vibration

  • Accessory structure transfer mvt of sensilla to dendrite of bipolar sensory neuron

  • Open stretch-sensitive TRP ion channels

  • Very sensitive: detect small changes in air movements

• Campaniform sensilla

  • Similar to trichoid sensilla except lack hair shaft

  • Dome-shape projection of cuticle

  • Found in clusters, especially near joints of limbs

  • Detect cuticle deformation as insect moves

  • Allows coordinated movements

Describe proprioceptors

Proprioceptors (vertebrates)

• encode information about body position; essential for motor coordination

• Tonic receptors: NO adaptation constant message to CNS

Muscle spindles

• surface of skeletal muscles

• consist of intrafusal fibers: modified muscle fibers enclosed in connective tissue capsule

• monitor muscle length (AP firing rate related to spindle stretch)

Golgi tendon organs

• at junction between skeletal muscle and tendon

• sense tendon tension

Joint capsule receptors

• located in capsules that enclose the joints

• many types similar to Pacinian corpuscles, golgi tendon organs, free nerve endings, etc.

• Detect pressure, tension and mvt in joint

Chordotonal organ

• detects bending of the cuticle

• grouping of functional units called scolopidia, consisting of a bipolar sensory neuron and scolopale cells (complex accessory cells) that surround sensory neuron dendrites

Describe the structure involved in equilibrium and hearing in vertebrates

• vertebrate hair cells

• Equilibrium

  • Mammalian inner ear

    • Vestibular apparatus

      • 3 semicircular canals

      • 2 vestibular sacs (utricle and saccule)

• Hearing

  • Mammalian outer ear

  • Mammalian middle ear

  • Mammalian inner ear

Vertebrate hair cells

• distributed in different sensory organs serving diverse functions

• Epithelial sensory receptor cells

  • Synapse with afferent neuron

  • Transduce mechanical stimulus into electrical signal

• 1 long kinocilium (absent in mammals)

• Many stereocilia in tight bundle:

  • contain few actin filament at base (flexible)

  • Linked by small fibers: tip links

    • connected to ion channels via elastic proteins (springs)

    • pull open channel or push it closed

Equilibrium:

• mammalian inner ear

  • vestibular apparatus

    • 3 semicircular canals

      • each arranged in a separate plane perpendicular to each other

      • acceleration of fluid in a particular canal is dependent on the plane of movement

      • Ampulla:

        • at rest: hair cells partially depolarized

        • with movement of head: fluid in canals moves and causes stereocilia to pivot

          • displacement in one direction depolarizes

          • displacement in other direction hyperpolarizes

    • 2 vestibular sacks (utricle and saccule)

      • contain otoliths (mineralized particles) which are suspended in gelatinous matrix above membrane called macula

      • physical movements induce displacement of otoliths, pulls on macula, causing deformation of hair cells

      • Macula contains large number of hair cells (>100 000)

        • utricle macula: oriented horizontally

          • detects motion in the horizontal plane

        • saccule macula: oriented vertically

          • detects motion in the vertical plane

Hearing

• Mammalian outer ear

  • outer ear:

    • pinna and auditory canal

      • funnel and concentrate sound wave

• Mammalian middle ear

  • Middle ear amplifies sound, allowing sensitive detection of sound

    • Tympanic membrane

      • Larger surface area than oval window

      • Vibrations from sound

    • Malleus, incus, and stapes

      • Hinge-like connections between malleus, incus, stapes act as levers, amplifying vibrations

      • Transfer vibrations initiated at tympanic membrane across the middle ear space to oval window

• Mammalian inner ear

  • Mammalian inner ear receives and transfers vibrations to hair cells in the cochlea, where they are transduced

    • Cochlea

      • 2 fluid filled compartments

        • Outer: vestibular and tympanic ducts (perilymph)

        • Inner: cochlear duct (endolymph, high [K+])

          • cochlear duct is separated by basilar membrane and houses tectorial membrane

    • Organ of Corti

      • Space between basilar and tectorial membranes, contains hair cells

      • Hair cells are embedded in basilar membrane, and stereocilia contact the tectorial membrane’

    • Vibrations transmitted from stapes to oval window

    • Causes waves of pressure differences in fluid of vestibular duct (perilymph)

    • Waves travel to round window

    • Cause vibrations of basilar membrane

    • Hair cells embedded in basilar membrane move relative to stationary tectorial membrane

Electroreception in fish

Electroreception

• Electroreceptors - Modified hair cells similar to neuromasts

• Passive electrolocation

  • Detect fields (flow, animal muscle, or nerve activity)

  • “Electroreceptive” - detect fields only

• Active electrolocation

  • Detect and produce fields (electric organ produces a field, electroreceptors detect perturbations in the field)

  • “Electrogenic” detect and produce fields