Mechanoreception

Föreläsning 13/11/2023

Latency

Time between the onset of a stimulus and the opening of the first ion channels

How are mechanical forces transmitted?

Transmitted to ion channels directly, without intervening second messangers

The transduction channel detects the deflection of an external structure relative to an internal (eg cytoskeleton). This deflection changes the tension in the system, changing the likelihood of the transduction channel being open, thus altering the flow of ions into/out of the cell, which changes the receptor potential.

Types of mechanoreceptor cells

Receptor cells with cilia

Epithelial cells, found in nematocytes, human inner ears, lateral line organ in fishes/amphibians, and wings/joints/antannae of arthropods/spiders.

Used for detecting touch, pressure, sounds, balance, vibrations, water flow/waves.

Receptor cells without cilia

Nerve cells with cell body in one of the body’s ganglia. May be free nerve endings or Pacinian corpuscles (both in human skin).

Ganglion cells include crustacean stretch receptors, annelid and mollusc touch receptors, human skin/tendons mechanoreceptors, and spindle organs in human muscle.

Somatosensory system

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Sensory receptors in human skin

Belong to the mechanoreceptors without cilia

1. Meissner’s corpuscles

   Detect pressure and slow vibration

2. Merkel’s disks

   Detect pressure and touch

3. Pacinian corpuscles

   Detect pressure and rapid vibration

4. Ruffini’s endings

   Detect stretch and sustained pressure

5. Free nerve endings

   Detect pain, warmth/cold, and tickle

6. Hair follicle receptors

   Detect tickle and light touch. Only found in hairy skin

Somatosensory cortex

Part of the brain dedicated to deal with impressions on our body surface.

Body parts are respresented in somatotopic order, meaning neighboring locations on the body are repersented by neighboring locations in the brain.

Mechanoreceptors in invertebrates

Long thin hairs

Common receptor in arthropods

Neuromasts

In fish (lateral line organ), usually located in canals under the body service, through which water can pass when currents flow around the body.

Each neuromast is a collection of hair cells, with sensory hairs embedded in a jelly-like substance (cupula), which gets deformed when water flows through the canal. This deformation causes the hairs to bend, causing an action potential.

Proprioception

Monitor blood pressure, joint position, length/tension of muscles, tendons, and guts.

1. Vertebrate muscle spindle organs

   Free nerve endings wrapped around specialised thin musclel fibrers within a muscle.

   Measure muscle length

2. Golgi tendon organs

   Sensory receptors in tendons (attach muscles to bones).

   Supply CNS with information about the forces generated by muscles

3. The abdomen stretch receptor in crayfish

   Each consists of 2+receptor strands (fine strands of muscle).

Balance

Usually sensed by statolith organs

Sense angle of body relative to gravity and angular acceleration

Statocyst organs

Hold statoliths: small stonelike concretions in a fluid-filled chamber covered by ciliary mechanoreceptors

When tilting the animal, the statoliths move inside the chamber, causing physical deformation of the mechanoreceptive hair cells below the statoliths, triggering action potentials in the neurons.

The statoliths can either move freely in the liquid-filled chamber (mollusc etc), or be connected to the mechanoreceptor hairs (hydrozoans, cephalopods, vertebrates)

In vertebrates, the statoliths are called otoliths, with the otolith organ consisting of the utricle and saccule

Gravity organs in insects

Flying insects use the weight of body parts instead of statoliths

Water-living insects often have air bubbles attached to the mechanoreceptors to sense gravity

Otolith organ

Part of the vestibular system (balansorganet) in the inner ear, along with the semicircular channels (respond to angular acceleration).

The utricule and saccule are both hollow structures filled with endolymph, and a hollow region with hair cells, connected to the macula (gelatinous mass), where the otoliths are embedded.

In mammals, the utricular macula is oriented horizontally (measures direction of gravity), while the saccular macula is vertical (measures linear accelaration).

The hair cells are connected to each other at the tips via tip-links, where the bending of the hair bundle in one direction causes opening of the ion channels (depolarisation of cell= excitation), while bending in other direction, against the shortest hair cell, causes closing of the ion channels and thus hyperpolarisation (inhibition).

Perception of angular acceleration in humans

Three semicircular canals filled with endolymph, each with the ampulla, containing a field of hair cells that project their cilia into a cupula.

When the channel rotates, inertia keeps the endolymph stationary, causing the cupula and the cilia to deform, indicating head rotation.

Note that this is a phasic response, action potentials are only produced at the start of the turn as the head accelerates

Pitch

Frequency of sound = number of times the object vibrates per second. Measured in Hertz. Frequencies below 16 Hz are infrasounds, while sounds above 20 000 Hz are ultrasounds

Sound intensity

The energy the sound contains. Depends on the vibration’s amplitude. Measured in decibel

Invertebrate ears

Cockroaches have mechanoreceptors on the cerci to detect air-flow caused by predators

Mosquitoes have mechanoreceptors in the Johnston’s organ in the second antennal segment.

Crickets have ears in the forelegs, while grasshoppers and cicadas have them in their abdomen.

Moths have ears in the abdomen, used to avoid bats

Human ears

The pinna collects auditory stimuli and funnels them into the auditory canal toward the eardrum.

The middle ear works as an amplifier, as the sound causes the eardrum to vibrate, and the movement is transferred via the three bones (ossicles) (hammaren, städet, stigbygeln) to the oval window of the fluid-filled cochlea. The amplification is needed to overcome the barrier from air to water.

Movement of the oval window causes movement of the fluid in the cochlea, and thus stimulates the hair cells.

Vertebrate ears

Birds, reptiles, and amphibians lack pinna, and the eardrum is exposed. Birds and reptiles only have 1-2 middle ear bones (columella), while amphibians also have a third bone (operculum).