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Receptors and transduction
-The ability to receive and process information about surroundings, and to respond to this, is essential for survival and reproduction.
-Sensory receptors are responsible for detecting various stimuli, with this information then potentially leading to either subconscious or conscious adjustments.
-The receptors convey four basic types of information about a stimulus: modality (type of energy), location, intensity and timing.
-A single stimulus usually activates many receptors.
-Information that is processed by the cerebral cortex leads to conscious perception of the stimuli and a conscious 'decision' about the appropriate response.
-Only a very small proportion of the total receptor stimulation leads to conscious perception of the stimulatory factor.
-Receptors can be defined as rapidly or slowly adapting. Rapidly adapting receptors respond to changes in stimulus intensity but there is very little firing if that stimulus is maintained.
-Slowly adapting receptors do not respond as rapidly when the stimulus strength changes but they continue to fire, indicating that the stimulus is still present.
-The main classes of receptor are: mechanoreceptors, thermoreceptors, chemoreceptors and nociceptors. a) Mechanoreceptors: Touch, proprioception
-Receptors for touch and proprioception terminate in a nonneural capsule- sense mechanical stimuli that indent/physical deform the receptive surface
-These may be responsive to one or several of a very broad range of stimuli including vibrations or pressure on the skin, sound (vibrations), gravity, muscle stretch, vascular pressure, and speed of movement of a joint.
-Sense physical deformation of tissue- Mechanical distension such as pressure on skin/ stretch of muscles-transduced into electrical signal by deforming and opening stretch sensitive ion channels- increase Na+ , Ca2+ ? depolarise the membrane
-Mechanical stimuli: forces conveyed through lipid tension in cell membrane (osmotic swelling), forces conveyed through structural proteins linked to ion channels, indirectly activated by forces conveyed to a force sensor: use a second messenger pathway, slow to activate and inactivate-outlasts the stimulus, but sensory signal amplified
-Located superficially in dermal ridges of glabrous (hairless) skin such as fingertips- about 1/16th the size of Pacinian corpuscles
-Detect stroking/fluttering- rapidly adaptive with small receptive fields- nerve ending is surrounded by Schwann cells which can modify the sensory input
-Can detect light touch Merkel cells- innervated by slowly adapting type 1 fibres (fire in response to steady pressure on the skin). Detect initial contact of hand with objects, slippage of objects, motion of hand over textured surface, low frequency vibration
-Located superficially with attachments to basemement membrane underlying epidermis and are dense in fignertips
-Nerve terminal surrounded by an epitheial cell- derived from neural crest cells and contains many neuroactive substances. Slow receptive fields and are slowly adapting touch receptors Pancinian corpuscles
-Largest and best studied of the mechanoreceptors in the skin- measure up to 2mm long an 1mm in diameter
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