Ramesh Rajan; Week 2 MED1022; Physiology
Discriminative somatic sensations are mechanoreception (touch pressure and flutter vibration), proprioception. Non-discriminative somatic sensations are mechanoreception (crude touch), thermoreception and nociception. Fine touch has at least 3 dimensions- form perception, texture perception and vibration perception (flutter frequencies <40Hz; vibration 40-400Hz).Meissner's discs and Merkel's discs are more superficial, Ruffini's corpuscles are deeper and Pacinian corpuscles are the deepest. Free nerve endings can also detect touch. Hair follicles can replace Meissner's discs and detect touch, are innervated by free nerve endings.
Density varies between the receptors. Merkel SA I is about 100/cm2. Meissner 150cm2, Pacinian 20cm2, Ruffini 20cm2. Meissner and Merkel are concentrated in the fingertips. In receptors providing fine touch, transduction involved stretch. All skin mechanoreceptors send signals with Abeta nerve fibres. Microneurography can be used to detect nerve fibres and their endings.
The smaller the receptors field size the shallower the receptors are inthe skin. Meissner's corpuscles and Merkel's discs are shallow. Meissner adapts rapidly and detects stroking. Merkel adapts slowly and detects pressure. Pacinian and Ruffini are deep in skin, Pacinian adapts quickly and detects vibration and Ruffini adapts slowly and detects skin stretch. Pacinian adapts faster than Meissner, allows them to respond to higher rates of vibration.
The receptor field is the size of the area of skin from which a sensory nerve fibre (not receptor) is activated. Neurons with small RFs have many receptors in a small area, multiple can be stimulated at once, large RFs have few receptors stimulated at once. Merkel RA is 2mm, Meissner is 5mm, Pacinian is 8-10mm, Ruffini is 8 (large, fuzzy borders). Merkel's discs provide the best information on fine detail (SAIs are Merkel afferents); RAIs (Meissner) and PC respond to the stimuli but RAIs provide a representation of the pattern and PC almost no representation. SAII (Ruffini afferents) do not respond well to these stimuli, instead respond to skin stretch.
RAs adapt quickly, only respond at beginning and end of stimulus but respond well to high frequency vibration. RAI is Meissner and PC is Pacinian. SA respond throughout sustained displacement, code duration and magnitude of mechanical stimuli. SAI is Merkel, SAII is Ruffini.
Active and passive touch (you touch something or something touches you) are very different, but in discriminative terms they are the same. Brain plasticity can improve discrimination (blindness). Apply pressure measures hardness, static contact gives temperature information, enclosure with global shape and volume (all active touch).
Proprioception is provided by cutaneous mechanoreceptors, join receptors, muscle spindles and Golgi tendon organs. SAII respond to skin stretch as they terminate along collagen fibrils, are important around hands, mouth and feet. In joints there are Ruffini type and PCs which are not essential. Muscle spindles have type Ia and II axons coiled around intrafusal fibres which respond to muscle stretch. They can be rapidly adapting or slow adapting. Golgi tendon organs respond to muscle stretch/force, are Ib.