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MCB3208 Ch10 lec notes

MCB3208 Ch10 lec notes - MCB3208...

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MCB 3208 CH 10 NERVE SENSES: TOUCH AND VISION Types (receptors) Mechanoreceptors (pressure, hearing and balance, nociceptor for pain), thermoreceptors , chemoreceptors (odors, taste and blood chemistry), photoreceptors Another grouping: proprioceptors : Golgi tendon organs, stretch receptors joint receptors detect body posn cutaneous receptors: touch and pressure, cold and hot, pain special senses: sight, hearing and equilibrium Nociceptors detect variety of high intensity stimulation and are “felt” as pain. These receptors are turning out at the molecular level to be both more and less complicated than previously thought. For example, temperature receptors respond to both heating and to capsaicin, e.g. the chemicals in chilis that make them “hot.” Thus, leftover Mexican food that is eaten just after taking it from the refrigerator tastes spicier or hotter if it is heated up a bit. Coding: frequencies encode intensity and connections to cortex encode the perception Adaptation : Reduction in action potential frequency with constant stimulus to a receptor: phasic vs. tonic adaptation odor, touch and temperature adapt rapidly; pain adapts very little Law of Specific Nervous Energies: sensation characteristic of each sensory neuron is that produced by its normal stimulus. Example: "seeing stars" Receptor or generator potential : local change in the voltage triggered by the activity of the particular receptor. The receptor potential changes roughly linearly with the intensity. The change in the voltage is then transduced into a frequency of action potentials in the axon that leads from the receptor to the CNS. All receptors work similarly, turning some specific stimulus into a generator potential that is then transduced into action potentials, with increasing intensity coded in increasing frequency. Pacinian corpuscle and tactile sense Multiple types of cutaneous sensory receptors: free nerve endings and Ruffini endings, pacinian corpuscles (deep pressure), Meissner's corpuscles (texture), root hair plexus somatic sensory path to brain: receptor (Pacinian corpuscle), axon to the spinal cord then synapse in medulla oblongata of the brain stem and this second neuron then crosses over to the opposite side of the body at the thalamus (third relay), cortex and sensory homunculus (representation of the body at the periphery) in postcentral gyrus region of cortex. Experiments on intact animals and people showed the sensory homunculus , as described previously. 1
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Receptor potential of the Pacinian corpuscle is graded; frequency of action potentials (all or none) codes intensity Adaptation (reduction in the number of action potentials generated in a sensory nerve with constant stimulation. This can be fast or slow, yielding phasic or tonic adaptation Lateral inhibition sharpens discrimination. This occurs in the cortex, due to central nerves inhibiting the surrounding nerves. Two-point touch threshold is indication of tactile acuity.
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