Special senses Smell Vision Hearing Taste Equilibrium
Receptors Specialized cells for detecting particular changes in the environment Exteroceptors Proprioceptors Receptors are not absolutely specific for a given sensation; strong stimuli can cause various sensations, even pain, even though the inciting stimuli are not necessarily painful
Adaptation denotes the diminution in rate of discharge of some receptors on repeated or continuous stimulation of constant intensity the sensation of sitting in a chair or walking on even ground is suppressed.
Exteroceptors Affected mainly by the external environment: Meissners corpuscles, Merkels corpuscles, and hair cells for touch Krauses end-bulbs for cold Ruffinis corpuscles for warmth Free nerve endings for pain
Ending Type Receptor Location Responds to TypeEndings with Merkel Glabrous and Touchaccessory endings hairy skinstructures Endings Hairy skin Touch around hairsFree nerve Ubiquitous Pain,endings temperature, light touch
The receptors that provide the information we use for the discriminative aspects of touch-assessing the shape and texture of objects, and the direction of movement across the skin-all have large-diameter axons and encapsulated endings or endings with accessory structures
Examples -Meissner corpuscles, pacinian corpuscles, and Merkel endings The packing density of receptors like this, especially the Meissner corpuscles and Merkel endings, determines the tactile acuity of a given area of skin.
This varies – the fingertips and lips;we can distinguish between two small objects separated by only a few millimeters the skin of the legs and trunk;objects can be separated by a few cm and still feel like a single object.
Meissner corpuscles, found in the dermal papillae of glabrous (hairless) skin, are encapsulated structures important for detecting the details of things moving across the skin. come into play when we move our fingertips across something, or when something in our grasp begins to slip and distorts the skin.
Pacinian corpuscles rapidly adapting receptor that responds briefly at the beginning and end of a mechanical stimulus. good at detecting rapidly changing stimuli, such as vibrations
Free nerve endings unmyelinated (C) fibers respond best to slow, gentle brushing of the skin. probably more important for the pleasurable feelings associated with this kind of touch than for its explicit detection
Merkel endings basal layer of the epidermis sensitive, slowly adapting receptors important for detecting the shape and texture of stationary objects touching the skin.
Nociceptors detect events that damage or threaten to damage tissue two groups correspond to everyones experience with pain as a two-part sensation.. Fast pain is initiated by firing of Aδ nociceptors, and the delayed onset of slow pain is directly related to the slower conduction velocity of the C fibers that mediate it.
A physically painful event (e.g., touching a very hot pot, missing a nail and hitting a finger with the hammer) elicits first a sensation of sharp, well- localized fast pain followed by a dull, poorly localized, aching sensation of slow pain Fast pain- firing of Aδ nociceptors Delayed onset -slower conduction velocity of the C fibers that mediate it
Tissue damage is more complicated than something like skin indentation or muscle stretch. Multiple things can cause it Once it occurs a series of chemical changes in the damaged tissue ensue.
Nociceptors transduce multiple aspects of painful stimuli. Aδ nociceptors respond specifically to intense mechanical stimulation (e.g., pinprick), to painful heat or cold, or to both. C-fiber nociceptors respond to all of these, as well as to a variety of substances released in damaged tissue=polymodal nociceptors
CONNECTIONS A chain of three long neurons and a number of interneurons conducts stimuli from the receptor or free ending to the somatosensory cortex
First-Order Neuron The cell body lies in a dorsal root ganglion or a somatic afferent ganglion (eg, trigeminal ganglion) of cranial nerves.
Second-Order Neuron The cell body lies within the neuraxis -spinal cord or brain stem; Axons of these cells usually decussate and terminate in the thalamus.
Third-Order Neuron The cell body projects rostrally to the sensory cortex. The networks of neurons within the cortex, in turn, process information relayed by this type of neuron; they interpret its location, quality, and intensity and make appropriate responses.
Variable Dorsal Column- Anterolateral Medial Lemniscus Pathway PathwayCourse in spinal cord Dorsal and Anterior and dorsolateral funiculi anterolateral funiculiSpecificity of signal Each sensation Multimodal (severalconveyed carried separately; sensations carried in precise localization of one fiber system) sensationDiameter of nerve Large-diameter Small-diameterfiber primary afferents primary afferentsSensation transmitted Fine touch, joint Pain, temperature, sensation, vibration crude touch, visceral painSynaptic chain Two or three synapses Multisynaptic to cortexSpeed of transmission Fast Slow