Published on February 19, 2014
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 endorgans, hair follicle receptors and some free nerve endings. – Rapidly adapting type I mechanoreceptors have multiple Meissner corpuscle endorgans. • 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
•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 19 Fig. 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 cellderived 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.
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