Sensory systems summary

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Biological Sciences
Stephen Reid

Lecture 11 Notes: Sensory Systems Sensory Receptors There are sensory receptors for many different modalities. There are receptors that sense, for example, chemical stimuli (chemoreceptors), mechanical movement (mechanoreceptors), light (photoreceptors), temperature (thermal receptors), noxious substances (nocioceptors), etc In general, a receptor protein on a sensory (receptor) cell detects the stimulus. This usually causes the opening of ion channels either alone or in conjunction with activation of second messenger systems. Ultimately there is a change in membrane potential that leads a signal being sent to an integrating centre in the brain. The sensory receptor itself may send the signal to the brain (i.e., sensory neurons) or the sensory cell may release neurotransmitters onto another neuron (interneuron) that conveys the signal to the brain (i.e., epithelial sensory receptor cells). One important integrating centre is the nucleus of the solitary tract (nucleus tractus solitarius; NTS) located in the brainstem at the junction between the medulla oblongata and the spinal cord. Tonic and Phasic Receptors Tonic receptors are those that fire action potentials for the duration of the time that the receptor cell is stimulated. However, if the stimulus lasts a very long time, the receptors may become desensitized or down-regulated resulting in a reduction (or cessation) of action potential activity. Phasic receptors are those which fire APs either at the beginning of a stimulus, at the end of a stimulus or at both the beginning and end of a stimulus. Pulmonary Stretch Receptors Pulmonary (lung) stretch receptors (PSR) are located in the walls of the lungs. They respond to the stretching of the lungs during inspiration. There are three types of pulmonary stretch receptors. Rapidly-adapting pulmonary stretch receptors fire APs immediately when the lungs begin to inflate and then cease firing. Slowly-adapting PSR begin to fire action potentials at around the time when the lung inflation reaches its maximum. They fire until the lung volume returns to normal. Mixed rapidly-adapting and slowly-adapting PSR exhibit a rapid and a slow component. There is activity immediately upon lung inflation (rapid firing of APs) followed by a second phase of APs that are slower than the initial phase. These mixed PSR regulate the
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