Mechanoreception

Types of mechanical forces

• Outer mechanical forces

  • Touch

  • Pressure

  • Vibration

  • Air currents

  • Water currents

  • Sound

    Sensed by the ear and somatosensory system

• Inner mechanical stimuli

  • Pressure on blood vessels

  • Contraction of muscles

  • Limb flexions

    Proprioceptors deal with these forces

• Other

  • Gravity

  • Acceleration

    Sensed by equilibrium organs, found in inner ear in humans

Mechanoreceptors transduce mechanical forces and transmit the information to the brain.

Mechanical forces are directed to specific ion channels

High speed responses lack second messengers

Latency of mechanoreceptors = time between onset of a stimulus to the opening of the first ion channels. 10^-6 seconds (1000x faster than photoreceptors/vertebrate chemoreceptors)

Types of mechanoreceptor cells

• Receptor cells with cilia

  • Epithelial cells with at least one cilia

  • Found in

    • Cnidarian nematocytes

    • Inner ear of humans

    • Lateral line organ of fishes/amphibians

    • Wings, joints, antennae of arthropods and spiders

• Receptor cells without cilia

  • Nerve cells

    • Free nerve endings

    • Pacinian corpuscles in human skin

    • Ganglion cells

      • Crustacean stretch receptors

      • Annelid and mollusc touch receptors

      • Mechanoreceptors in human skin + tendons

      • Spindle organs in human muscle

Human skin

• Meissner’s corpuscles

  • Detect pressure on the skin

  • Detect slow vibration

• Merkel’s disks

  • Detect pressure and touch

• Pacinian corpuscles

  • Detect pressure on the skin

  • Detect rapid vibration (to feel texture of objects)

• Ruffini’s endings

  • Detect stretch and sustained pressure

• Free nerve endings

  • Detect pain, warmth, cold, tickle

• Hair follicle receptors

  • Detect tickle and light touch

  • Only found in hairy skin

Somatosensory cortex

• Large part of the brain

• Deal with the impressions on our body surface

• Are represented in somatotopic order

  • Neighbouring locations on the body are represented by neighbouring locations in the brain

    • Note that size may vary. Face, hands, and genitals are very sensitive to mechanical stimuli and thus correspond to larger parts of the somatosensory cortex

Lateral line organ

• Detects wave vibration and water currents

  • Guiding fishes during locomotion

  • Allows them to detect and locate prey, predators, and potential mates

• Neuromasts

  • Receptor cells

  • Usually located in canals under the body surface

    • Water passes through when currents flow around the fish’s body

    • Canals are connected to the external water by small openings

  • Each neuromast is a collection of hair cells with sensory hairs embedded in a jelly-like substance (cupula).

    • Projects upwards into the canal

    • Gets deformed when water flows through canal

      • Deformation cause hairs to bend, responding with action potentials

Proprioception

• Monitor tension of gut, blood pressure, position of joints, length and tension of muscles and tendons

In vertebrate muscles, there are muscle spindle organs, which are free nerve endings wrapped around specialised thin muscle fibres.

The afferent neuron senses stretch and sends action potentials to the spinal chord, where it synapses with the motor neuron controlling the muscle.

The efferent neuron sends its signal back to the muscle, maintaining the tone.

Balance

• Sensed by statolith organs

  • Sense body angle relative to gravity

  • Sense angular acceleration

    Both of these are in humans called the vestibular organ

    • Based on hair cells, like hearing

Mist animals use statocyst organs to sense direction of gravity

• Statoliths are small stonelike concretions in a fluid-filled chamber covered by ciliary mechanoreceptors

  • When tilting, the statoliths move and cause deformation to the hair cells, that respond with action potentials

  • This organ can be made in two ways

    • The statoliths move freely in the liquid-filled chamber

      • Entire chamber is lined with hair cells and filled with statoliths

    • The statolith is connected to the mechanoreceptor hairs

      • The direction and strength of bending of the cilia codes body angel

      • In vertebrates, the statoliths are called otoliths

        • The otolith organ consists of the utricle and saccule

In insects, gravity can be sensed also by

• Weight of body parts (flying insects) instead of statoliths.

• Air bubbles (water-living insects)

Vertebrate otolith organ

• Part of the vestibular system in the inner ear

• Two parts (saccule and utricle), both filled with endolymph

  • The hair cells are attached to a gelatinous mass (macula), where the otoliths are embedded

• In mammals:

  • The utricular macula is oriented horizontally

  • The saccular macula is oriented vertically

The hair cells have hair bundles embedded in a gelatinous matrix and connected to each other at the tips by tip-links. Bending of the hair bundle in one direction causes the opening of ion channels and a depolarisation of the cell (excitation). Bending in the opposite direction causes the closing of ion channels and a hyperpolarisation of the cell (inhibition).

Utricular macula

• Horizontal orientation

  • Monitor direction of gravity due to changes in head angle

Saccular macula

• Vertical orientation

  • Monitor linear acceleration

    • Either side-to-side movements or up-and-down movements

Animals that perform fast body movements often have separate organs to sense angular acceleration. In the highly evolved equilibrium organs of vertebrates, crustaceans and cephalopods, angular acceleration is sensed in a similar way to gravity. In humans, the three semicircular canals, filled with endolymph, each have an enlarged region called the ampulla. Inside the ampulla, a field of hair cells (similar to the hair cells in the otolith organs) project their cilia into a cupula. If the channel rotates, the endolymph – due to inertia – tends to remain stationary, thus deforming the cupula and its cilia. The hair cells to which the cilia are attached then indicate the direction of head rotation. If you turn your head in one of the three planes (X, Y or Z), the walls of the canal will move with the head and thus relative to the endolymph. This is analogous to turning a glass of water: the glass moves but the water remains stationary.