Tutorial presentation

Cutaneous mechano-receptors and their mechanism of action.

Presented by: Dr. Mahesh

Introduction

• Mechanoreceptors– sensory receptor that responds to mechanical

pressure or distortion.• Normally there are four main types in glabrous

skin:– Pacinian corpuscles, – Meissner's corpuscles,– Merkel's discs, and – Ruffini endings.

• The nerve networks of skin contains somatosensory and sympathetic autonomic nerve fibres.

• The sensory fibres alone (free nerve ending) or in conjunction with specialized structures (corpuscular receptors) functions as receptor of touch, pain, temperature, itch and mechanical stimuli.

• Receptors are particularly dense in hairless areas such as the areola, labia, and glans penis.

• Sympathetic motor fibers are codistributed with the sensory nerves in the dermis until they branch to innervate

• the sweat glands,• vascular smooth muscle, • the arrector pili muscle of hair follicles, and• Sebaceous glands.

Types• In glabrous (hairless) skin, there are four

principal types of mechanoreceptors, each shaped according to its function.

• The tactile corpuscles (meissner) respond to light touch, and adapt rapidly to changes in texture (vibrations around 50 Hz).

• The bulbous corpuscles (ruffini) detect tension deep in the skin and fascia.

• The Merkel nerve endings detect sustained pressure.

• The lamellar corpuscles(pacinian) in the skin and fascia detect rapid vibrations (of about 200–300 Hz).

• Receptors in hair follicles sense when a hair changes position. Indeed, the most sensitive mechanoreceptors in humans are the follicular receptors for the hair cells in the cochlea of the inner ear; these receptors transduce sound for the brain.

• Mechanoreceiving free nerve endings detect touch, pressure, and stretching.

INNERVATION• Cutaneous mechanoreceptors provide the senses

of touch, pressure, vibration, proprioception and others. They are located in the skin, like other cutaneous receptors.

• They are all innervated byAβ fibers, except the mechanorecepting free nerve endings, which are innervated by Aδ fibers.

• Cutaneous mechanoreceptors can be categorized by morphology, by what kind of sensation they perceive, and by the rate of adaptation. Furthermore, each has a different receptive field.

• Slowly adapting: Slowly adapting mechanoreceptors include Merkel and Ruffini corpuscle end-organs, and some free nerve endings.– Slowly adapting type I mechanoreceptors have

multiple Merkel corpuscle end-organs.– Slowly adapting type II mechanoreceptors have

single Ruffini corpuscle end-organs.• Intermediate adapting: Some

free nerve endings are intermediate adapting.

• Rapidly adapting: Rapidly adapting mechanoreceptors include Meissner corpuscle end-organs, Pacinian corpuscle end-organs, hair follicle receptors and some free nerve endings.–Rapidly adapting type I mechanoreceptors

have multiple Meissner corpuscle end-organs.

• Rapidly adapting type II mechanoreceptors (usually called Pacinian) have single Pacinian corpuscle end-organs

Mechanism of action• Ability of mechanoceptors to detect

mechanical cues relies on the presence of mechano-transducer channels on sensory nerve endings.

• They rapidly transform mechanical forces into electric signals and depolarise the receptive field .

• This local depolarization, called the receptor potential, can generate action potentials that propogate towards CNS.

• The afferent neurons transmit messages through synapses in the dorsal column nuclei, where second-order neurons send the signal to the thalamus and synapse with third-order neurons in the ventrobasal complex.

• The third-order neurons then send the signal to the somatosensory cortex

• 2 pathways• Medial

lemniscal pathway–Propriocept

ion & touch perception

• Spinothalamic pathway– Temperatur

e and pain

•Hair follice receptors.

•Light Touch – Meissner’s Corpuscle

•Strong Pressure – Merkel’s Disk

•Pain – Nociceptors

•Heat/Cold – Thermoreceptors, some Nociceptors

• Merkel receptor

• Meissner corpuscle

• Ruffin cylinder • Pacinian

corpuscle

ch 14 19Fig. 14-1, p. 331

Pacinian corpuscle• Lies in the deep dermis and subcutaneous

tissue.• Characteristic lamellar and capsular

wrappings.• Perineural capsule is organised into 30 or

more concentric layer of cells and fibrous connective tissue.

• The middle subcapsular zone is composed of collagen and fibroblasts.

• The inner core consists of Schwann cell-derived hemilamellae.

• Pacinian corpuscles serve as rapidly adapting mechanoreceptors:responds briefly to beginning and end of stimuli.

• Detects pressure changes esp.to vibrational stimuli.

Characteristic perineural capsule (onion skin appearance)

Meissner’s corpuscles

•Highly concentrated in areas sensitive to light touch.• Fingertips, lips,

tongue, soles, genitals

Meissner's corpuscle:capsule

and inner core located in the

dermal papilla.

• Meissner's corpuscles are elongated or ovoid mechanoreceptors.

• located in the dermal papillae of digital skin and oriented vertically toward the epidermal surface.

• One to six axons enter the corpuscle, ramify extensively, and terminate in bulboid endings that are surrounded by lamellae.

Merkel’s disk•Slow adaption because of rigid structure.• Sustainable response – 30 min. in humans• Irregular firing in sustained.

•Large receptive field.• Small, sharp pressure: fast firing rate.• Large, flat pressure: slow rate

•Located in hairless skin and in hair follices.• Not in skin surrounding follicle.

• Free nerve endings are also associated with individual Merkel cells.

• Merkel cell-nerve complexes also known as:• touch domes,• hederiform endings, • Iggo's capsule, • Pinbus corpuscles,• Haarscheibe• In palmoplantar skin, these complexes are

found at the site where the eccrine sweat duct penetrates a glandular epidermal papilla

Ruffini ending•Only in glabrous skin•Sensitive to skin streching.•Contributes to fine motor control.• Contributes

significantly to finger positioning.

Free nerve endings

• Free nerve endings include the penicillate and papillary nerve fibres.

• Most widespread and important sensory receptors of the body.

• Particularly found In papillary dermis.

Penicillate fibres• rapidly adapting receptors that function in the

perception of touch, temperature, pain, and itch.

• In haired skin: Because of overlapping innervations, discrimination tends to be generalized.

• In non-haired skin: project individually without overlapping distribution.

Papillary fibres

• Found at the orifice of the follicle.• thought to be particularly receptive to cold

sensation.

Thanks