PSL300H1 Lecture Notes - Lecture 9: Lamellar Corpuscle, Dorsal Root Ganglion, Dorsal Root Of Spinal Nerve
PSL300
Lecture 9: Somatosensory Pathways
Somatosensory Receptors
• The 4 somatic senses
o Touch
o Temperature
o Proprioception
▪ Awareness of the position of body parts relative to each other
o Nociception
▪ Detects tissue damage of the threat of it, and is perceived as pain or itch
▪ Sensations that we don’t like, but plays a role in protection from further damage
• Somatosensory receptor cells respond to touch, pressure etc.
o Receptors in the PNS carry info to the spinal cord through afferent fibres
o Enter through the dorsal root
• Receptors for somatic sensation below the chin have their cell bodies in the dorsal root ganglia
• Receptors for the head have their cell bodies in the brain
• The parts of these neurons that transduce touch, pressure etc., into electrical signals are in their nerve ending
o Tips of their fingers, in the skin, and viscera
Receptors in the Skin
• Free Nerve Endings
o Detect mechanical stimuli temperature, and chemicals
o Finger-like projections that are not that specialized
o Can be wrapped around a hair follicle and is sensitive to a
breeze over the skin or touch
o Embedded in the upper layers of skin and sense chemicals and
temperature
• Merkel Disks
o Mechanic receptor nerve endings
o In contact with epithelial cells called Merkel cells
o Merkel disks are in the upper more superficial layers of the skin
o At the end of each ending there is a specialized cell that responds to stimuli
• Encapsulated
o Meissner’s and Pacinian corpuscles are mechanoreceptors
o Protected inside the connective tissue
• At the bottom of the epidermis are saucer-shaped Merkel disks
o They are sensitive to deformation of the skin, and are more tonic than phasic
▪ They send sustained messages as long as the deformation persists
o They signal contact
Mechanoreceptors are usually phasic
• Given a sustained, constant stimulus, the nerve ending’s membrane depolarizes but then return to baseline in ~3
ms (registers changes, not steady levels)
• So you don’t perceive much unless the stimulation is changing: if you run your hand along a surface, you get a
vivid impression of its texture; after your hand stops, you sense far less
• At the top of the dermis are egg-shaped Meissner corpuscles
o They are mainly in the tongue and hairless skin –erogenous zones, palms, and fingertips
o Inside each corpuscle are many looping endings, like the coils of a spring → they detect sideways
shearing, as when you stroke a surface or life something with your fingertips
o They are phasic, so they sense changes in shear
• Deep in the dermis are onion- shaped Pacinian corpuscles
o The nerve endings are sheathed in many layers → can sense tiny displacements (10µm) it the motion is
quick
o They are phasic, and so they respond strongly to vibration and other fast-changing stimuli
