BIO271 - Lecture 4 - Sensory physiology: Mechanoreception

why is mechanoreception important?

it is important for:

-cell volume control

-the sense of touch

-the sense of hearing

-the sense of balance

-regulating blood pressure in vertebrates

mechanoreceptors

-are specialized cells or organs

-detect and transform wide range of mechanical stimuli into electrical signals

-are distributed throughout the body, including in the skin, tendons, muscles, joint capsules and viscera

what subclass of mechanoreceptors are the most structurally complex?

-the proprioceptors

what can all organisms and cells do?

-they can all sense and respond to mechanical stimuli

what are the 2 types of mechanoreceptor protein in animals?

1. ENaC (epithelial sodium channels)

2. TRP (transient receptor potential channels) → type of polymodule

**both are ion channels

what were ENaC and TRP discovered in?

-they were found in invertebrates

what are mechanoreceptor proteins attaches to?

-they are attached to the cytoskeleton and to ECM proteins

what do mechanical stimulus cause?

-movement of the EC anchoring proteins relative to the cytoskeleton

-conformational change open/close channels

-changes in the cell membrane potential

what are the 3 classes of touch and pressure mechanoreceptors?

1. tactile receptors

2. proprioceptors

3. baroreceptors

tactile receptors

-exteroceptors

-present in both invertebrates and vertebrates

-their structure and function are different between these groups

exteroceptors

-that detect touch, pressure, and vibration on the body surface

proprioceptors

-monitor the position of the body

-found in both invertebrates and vertebrates

-their structure and function are different between these groups

baroreceptors

-interoceptors

-in the walls of blood vessels, part of the heart, digestive, reproductive and urinary tracts of vertebrates

-consist of free nerve endings that branch within elastic tissues

-respond immediately to a change in pressure, but adapt rapidly (adjust quickly)

interoceptors

-that detect pressure changes

insect mechanoreceptors

-can be found almost anywhere on the surface of an insect’s body (depends on the species you are looking at)

-may act as tactile receptors or proprioceptors

-are innervated by one or more sensory neurons that fire in response to stretching, bending, compression, vibration, or other mechanical disturbance of the exoskeleton

-may be phasic receptors or tonic receptors

phasic receptors

-active at the start or end and detect changes

-they fire once when activated and again when deactivated

tonic receptors

-firing repeatedly as long as a stimulus persists

what are the 2 types of tactile receptors (external surface mechanoreceptors)?

1. trichoid sensilla (singular sensillum)

2. campaniform sensilla

trichoid sensilla (singular sensillum)

-hair-like

-are tactile hairs (setae) that are innervated by a sensory neuron

-dendrites of the neuron attach near the base of the hair and generate a nerve impulse whenever they detect movement

-clusters of hairs often found behind the head, on the legs, or near joints (species specific though)

campaniform sensilla

-dome-shaped

-round or dome-shaped organs that act as proprioceptors (provide info about where the body is)

-found throughout the body — especially on the legs, near the base of the wings, and along sutures where two sclerites/plates of the exoskeleton meet

(ex. ~ 1200 campaniform sensilla on one insect)

what is the internal surface mechanoreceptors in insects?

1. scolopidia (scolopidium)

scolopidium

-are internal mechanoreceptors associated with internal surface of the cuticle

-function in proprioception

-found at nearly every joint, and between joints within individual limb and body segments

-the fundamental unit of a chordotonal organ is this

-is composed of 1–3 bipolar mechanosensory neurons and 2 accessory cell types (the scolopale cells) and cap cells, which envelop and anchor the sensory neurons, respectively

chordotonal organs

-scolopidia can be isolated or grouped to form these

what can scolopidia be modified into?

-it can be modified into tympanal organs for sound detection

-responds to sound waves and used to sense vibration in the air

schwann cells

-myelinate, so signal travels faster across neurons

what can vertebrate touch and pressure receptors be?

-can be free nerves endings or isolated sensory cells enclosed in accessory structures

root hair plexus

-free nerve ending wrapped around hair follicles;

-monitor distortions of the hair by mechanical stimuli

-adapt rapidly, ideal for detecting initial contact and subsequent movements

merkel disks

-where the epidermis meets the dermis

-fine touch and pressure receptors

-have very small receptive fields

-extremely sensitive tonic receptors

pacinian corpuscles (lamellated corpuscles)

-sensitive to deep pressure, pulsing or high-frequency vibrating stimuli

-fast-adapting receptors

ruffini corpuscles

-a polymodule type of receptor

-also sensitive to pressure, stretching of the skin and joint movement

-located in the reticular (deep) dermis

-tonic receptors that show little if any adaptation

-work with proprioceptors to provide information on the location of the body

free nerve endings

-thought to mediate nociception (the perception of noxious or painful stimuli)

-types: Aδ-fibers, C-fibers

-found in the skin, muscle, joint capsule, bone and major internal organs

-recently shown to identify with cutaneous Schwann cells

what are the 3 vertebrate tactile receptors?

1. root hair plexus

2. merkel disks

3. pacinian corpuscles (lamellated corpuscles)

4. ruffini corpuscles

5. free nerve endings

what are the 2 types of free nerve endings?

1. Aδ-fibers

2. C-fibers

Aδ-fibers

-myelinated free nerve ending

C fibers

-unmyelinated free nerve ending

what do specialized cutaneous schwann cells initiate?

-they initiate pain sensation in humans

what is the example of crocodile sense of touch?

-crocodile and alligator skin is extremely sensitive

-the skin is composed of a dermis and epidermis, with an outer keratinized layer

-uses a specialized organ called the Integumentary Sensory Organ (ISOs)

-thought to be involved in osmosensation or electrosensation

-using a mechanical stimulator, discovered these are complex mechanosensory organs

integumentary sensory organs (ISOs)

-specialized organ used by crocodile

-are located on the entire body of crocodiles, and on the head of alligators

vertebrate proprioceptors

-monitor the position of the body

what are 3 major groups of vertebrate proprioceptors?

1. muscle spindles

2. golgi tendon organs

3. joint capsule receptors

muscle spindles

-located on the surface of the muscle

-monitor muscle length

golgi tendon organs

-located at the junction between skeletal muscle and its tendon

-stimulated by tension in tendon

-monitor external tension developed during muscle contraction

joint capsule receptors

-free nerve located in the capsules that enclose joints

-detect pressure, tension, and movement at the joint

what do equilibrium and hearing utilize?

-they utilize mechanoreceptors

equilibrium/balance

-detecting position of the body relative to gravity

hearing

-detecting and interpreting sound waves

in vertebrates, what is responsible for both equilibrium and hearing?

-it is the ear

invertebrates equilibrium and hearing

-have 2 separate organisms for this

-Use statocysts for equilibrium and a variety of organs for hearing, (ex. trichoid sensilla, subgenual organ (on legs), tympanal organs (most sensitive in insects), Johnston’s organ)

tympanal organs

-very thin region of cuticle (tympanum) over an air space

-sound waves cause tympanum to vibrate; moves air to create sound

-frequency detected and vibration depends on thickness and tautness

-found all over the bodies

-modified version of scolopidia

statocysts

-organ of equilibrium in invertebrates

-hollow, fluid filled cavities lined with mechanosensory ciliated sensory neurons

-contain statoliths

statoliths

-dense particles calcium carbonate

-roll around and stimulate the hair cells

what is the process of invertebrate balance via statocysts?

1. mechanical stimulus

2. disturbs the statolith (roll around and stimulate the hair cells)

3. stimulates receptor proteins on cilia of neurons

4. depolarization

what is the example of invertebrate balance in octopus statocysts?

-have a more complex statocyst → similar to the organ of acceleration in vertebrates

-2 statocysts (one on each side) + statoliths

-1 macula → linear acceleration/ forward motion

-3 cristae (oriented in 3 different planes): angular acceleration/turning

what are the 4 components used for invertebrate hearing?

1. modified trichoid sensilla

2. subgenual organ

3. johnston’s organ

4. tympanal organs

modified trichoid sensilla

-sound waves bend these sensilla and send a signal to bipolar neuron

subgenual organ (SGO)

-detects vibrations carried through the ground

-is a modified chordotonal organ located in the legs

johnston’s organ

-detects sound waves in the air and bends the fine hairs on the antenna

-located at the base of the insect antennae

tympanal organs

-consists of a thin region of cuticle (tympanum)

-sound waves cause the tympanum to vibrate and stimulate a chordotonal organ inside the membrane

vertebrate hearing and balance (hair cells)

-use one organ, one system to sense hearing and balance

-uses mechanoreceptor cells for hearing and balance

-modified epithelial (hair cells)

-have extensive extracellular structures and cilia that extend from the apical end:

→ a long kinocilium and several short stereocilia connected by tip links

• sterocilia work together as a bundle

what are tip-links important for?

-important to ensure the whole group of stereocilia moves in the same way

what is the process of hearing + balance in hair cells of vertebrates?

1. mechanically-gated cation channels (TRP) are opened allowing K+ influx (extracellular conc. of K+ is high so that’s why flows in when this channels opens)

2. voltage-gated Ca2+ channels are open allowing Ca2+ inside the cell

3. this triggers exocytosis for the vesicles to release neurotransmitters to the primary afferent neuron (since the epithelial cell itself doesn’t produce an AP because it is not a neuron)

4. the neuron then produces an action potential

what can signal transduction in hair cells detect?

-they can detect movement and direction

what is the process of signal transduction in hair cells? (at rest)

hair cell → partially

AP frequency afferent neuron → intermediate

-basal level of APs when there’s no stimulus

what is the process of signal transduction in hair cells? (pressure signal #1= move toward the kinocilium (right))

hair cell → depolarized

AP frequency afferent neuron → high

what is the process of signal transduction in hair cells? (pressure signal #2 = move away from kinocilium (left))

hair cell → hyperpolarized

AP frequency afferent neuron → low

neuromasts

-cup-shaped sensory organs

-hair cell and accessory cupula

-detect water movement

-can be scattered over the entire body or organized into structures such as the lateral line

lateral line system

-array of neuromasts within pits or tubes running along the side of the body

what is the example of fish vertebrate neuromast?

-use hair cells in ears for hearing and for detecting body position and orientation

-neuromast can be scattered over the entire body or organized into structures such as the lateral line

cupula

-filled with viscous gel

-located on top of the hair cell (but only the stereocilia is in it not the epithelial part)

what are the 2 properties of sound and sound waves?

1. loudness

2. pitch (tone)

loudness

-related to amplitude of sound waves

-measured in decibels (ex. higher decibel for a higher amplitude)

pitch (tone)

-related to frequency (number of wave cycles/sec)

what does higher frequency mean?

-it means higher pitch

what does lower frequency mean?

-it means lower pitch

soft, low tone

amplitude (loudness) = low

frequency (pitch/tone)= low

loud, low tone

amplitude (loudness) = high

frequency (pitch/tone)= low

soft, high tone

amplitude (loudness) = low

frequency (pitch/tone)= high

loud, high tone

amplitude (loudness) = high

frequency (pitch/tone)= high

what are the 3 sections of the human ear?

1. outer ear = receiver

2. middle ear = amplifier

3. inner ear = transmitter

outer ear

-receiver

-receives and channels sound

structures:

• pinna

• auditory canal

middle ear

-amplifies sound

structures:

• eardrum

• malleus

• incus

• stapes

• oval window

• auditory

inner ear

1. sorts sounds by tone and converts them into impulses

structures: cochlea, vestibular canal, tympanic canal, cochlear duct, basilar membrane, hair cells, tectorial membrane, organ of Corti, round window

2. senses rotational movements and converts them into impulses

structures: semicircular canals, ampulla, cupula, hair cells

3. senses static (nonmoving) position and linear acceleration and deceleration; converts them into impulses

structures: vestibule, utricle, saccule, otoliths, hair cells

vertebrate ears

-function in both equilibrium and hearing

vestibular apparatus

-detects movements

-three semi-circular canals with enlarged region at one end (ampulla) and two sack-like swellings (utricle and saccule)

-all region contain hair cells to signal info to the CNS

-contain hair cells (mechanoreceptors) embedded in gel-like material

-hair cells bend in response to movement

lagena

-an extension off of the saccule

how is vestibular apparatus used in different animals?

• fish and amphibians → play a role in hearing

• birds → navigation

• mammals → hearing

how is the inner ear essential for balance?

1. vestibular apparatus

2. sensing rotational movement

3. head position and linear acceleration

what is sensing rotational movement?

-movement of fluid in semicircular canals bends hair cells (mechanoreceptors)

what is head position and linear acceleration?

-movement of otoliths (crystals) bends hair cells in vestibule (utricle and saccule

ampullae

-lack otoliths and contain cristae

cristae

-hair cells located in a cupula

utricle and saccule

-contain mineralized otoliths suspended in a macula covering >100,000 hair cells

-sensory receptors for static equilibrium

-one in each saccule wall and one in each utricle wall

-monitor the position of head in space, necessary for control of posture

-respond to linear acceleration forces, but not rotation

-contain supporting cells and hair cells

-stereocilia and kinocilia are embedded in the otolith membrane studded with otoliths (tiny CaCO3 stones)

what does the macula in the utricle respond to?

-it respond to horizontal movements and tilting head side to side

what does the macula in the saccule respond to?

-it responds to vertical movements

what do the hair cells in the macula of the ear synapse with?

-the hair cells synapse with vestibular nerve fibers

what does the maculae detect?

-it detects linear acceleration and tilting

what does the cristae detect?

-it detects angular acceleration

cristae

-sensory receptor for rotational acceleration

-one ampulla in each semicircular canal

-each crista has supporting cells and hair cells that extend into gel-like mass (ampullary cupula)

vertigo

-is the sensation of feeling like you are moving when you are not

-also called “Benign Positional Paroxysmal Vertigo” (like you are constantly stimulating your cristae

-can make people feel dizzy and nauseous

-due to the movement of otoliths into the semi-circular canals

what is the treatment for vertigo?

-the treatment is Epley Manoeuvre