Skin Flashcards
Two types of skin
Hairy and glabrous
Outer layer
Epidermis
Inner layer
Dermis
Functions of the skin
Protection, prevention of body fluid evaporation, direct contact with external environment
What are mechanoreceptors sensitive to
Physical distortion (bending, stretching)
Functions of mechanoreceptors
Monitor skin contact, pressure in heart and blood vessels, stretching of digestive organs and urinary bladder, and force against teeth
Structure of mechanoreceptors
Unmyelinated (grey matter) axon branches that are sensitive to stretching, pressure, vibration, or bending
Largest mechanoreceptor
Pacinian corpuscle
Where does the pacinian corpuscle lie
Deep in dermis, can be as long as 2mm and almost 1mm in diameter, each hand has around 2500 (highest density in fingers)
Ruffini’s endings
Found in hairy and glabrous skin
Meissner’s corpuscles
Located in fridges of glabrous skin (raised parts of fingertips)
Structure of Merkel’s disks
Located within epidermis, consists of nerve terminal and flattened, non-neural epithelial cell
Krause end bulbs
Lie in the border regions of dry skin and mucous membrane (around lips and genitals), nerve terminals look like knotted balls of string
Who developed methods to record from single sensory neurons in human arm
Ake Vallbo (Swedish neuroscientist)
Recording from single sensory axons in human arm
Can simultaneously measure sensitivity of mechanoreceptors in hand and evaluate perceptions produced by various mechanical stimuli; can map RF by moving probe around
RF of Meissner’s corpuscles
Small
RF of Merkel disks
Small
RF of Pacinian corpuscles
Large
RF of Ruffini’s endings
Large
Rapidly adapting receptors
Meissner’s and Pacinian corpuscles; respond quickly at first but stop firing when stimulus continues
Slowly adapting receptors
Merkel’s disks, Ruffini endings; generate a more sustained response during long stimulus
Meissner’s corpuscles and adaptation
Small, rapid
Pacinian corpuscles RF and adaptation
Large, rapid
Merkel’s disks RF and adaptation
Small, slow
Ruffini’s endings RF and adaptation
Large, slow
How can distortion of hair result in receptor activation
Hair follicles contain free nerve endings (terminals of axons) which wrap or run parallel. Bending of hair causes deformation of follicle and surrounding skin tissues. Stretches, bends, or flattens nerve endings. Increase or decrease AP firing frequency
Which mechanoreceptors are most sensitive to vibration at 200-300Hz
Pacinian’s corpuscles
What mechanoreceptors are most sensitive to vibration at 50Hz
Meissner’s corpuscles
Vibration and Pacinian corpuscle
20-70 concentric layers of connective tissue with a on terminal in middle. When capsule is compressed, energy is transferred to nerve terminal and the membrane is deformed, causing the mechanosensitive channels to open. Current flowing through creates a receptor potential (depolarising, graded). If depolarisation is large enough, AP is fired. If stimulus pressure is maintained, receptor potential dissipates. When pressure is released the events reverse.
Mechanosensitive ion channels
In membrane of unmyelinated axons. Convert mechanical force to change of ionic current. Forces applied alter gating and either enhance or decrease channel opening
Piezo2
Mechanosensitive channel in Merkel disks which opens in response to pressure and depolarises the cell. Depolarisation triggers synaptic release of unknown transmitter from cell, which excites nearby nerve ending. Nerve ending has second ion channel on membrane
Why are the fingertips better at 2-point discrimination than the back
1) higher density of mechanoreceptors
2) enriched in receptor types that have small receptor fields (merkel disks)
3) more brain tissue
4) special neural mechanisms devoted to high-resolution discriminations
Function of primary afferent axons
Bring information from the somatic sensory receptors to spinal cord or brain stem
Entry of primary afferent axons to the spinal cord
Via the dorsal roots; their cell bodies lie in dorsal root ganglia
Structure of primary afferent axons
Widely varying diameters, size correlated with type of sensory receptor (in order of decreasing size: Aa, Ab, Ad, C, or group I, II, III, IV)
Aa fibres (group I)
13-20um diameter (largest), myelinated, 80-120 m/sec, proprioceptors of skeletal muscle
Ab fibres (group II)
Myelinated, 6-12um, 35-75m/sec, mechanoreceptors of skin
Ad fibres
Myelinated, 1-5um, 5-30m/sec, pain/temperature
C fibres (group IV)
0.2-1.5um (smallest), unmyelinated, 0.5-2m/sec, temperature, pain, itch
How do peripheral nerves communicate with the CNS
Via the spinal cord, encased in bony vertebral column
Segmental organisation of spinal cord
Paired dorsal and ventral roots repeated 30 times down length of spinal cord
Spinal segments
Cervical (C): 1-8
Thoracic (T): 1-12
Lumbar (L): 1-5
Sacral (S): 1-5
Dermatome
Area of skin innervated by right and left dorsal roots of a single spinal segment
