Organs Essay

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• General properties of sense organs • sense organs code several aspects of a stimulus • 1. Modality • all sense organs detect a specific form of energy of the stimulus • A. light (photoreceptors) • B. chemical (chemoreceptors) • taste (gustation) • smell (olfaction) • C. mechanical energy (mechanoreceptors) • touch and pressure • muscle stretch • auditory and balance receptors • baroreceptors • D. thermal energy (thermoreceptors) • hot and cold • also electro-receptors & magneto-receptors present in certain animals but not humans 2. Intensity amplitude of stimulus is coded by the frequency of action potentials (APs) (Weber –Fechner law) 500 AP frequency (Hz) 0 stimulus intensity (log scale) 3. Location -location of stimulus is detected by the receptive field of the sensory neuron -the area of the sensory surface of the body, eg, skin, that when stimulated increases the AP frequency of that neuron receptive field eg, in skin SN1 SN2 sensory neuron 1 sensory neuron 2 (SN1) (SN2) 4. Sense organs have the property of transduction • Means that the energy of stimulus is altered to electrical energy • energy of stimulus is usually increased (amplified) or reduced during transduction stimulus Sensory receptor potential -40 mV sensory potential (mv) -70 mV time Generation of sensory potential • the sensory stimulus generates a sensory receptor potential in the sensory cell • usually a depolarization • amplitude is proportional to stimulus • generated by Na influx, comonly via transient receptor potential channels (TRP) • sensory potential may decline if stimulus is prolonged (called sensory adaptation) • sensory receptors vary in degree of adaptation • tonic receptors are non or slowly adapting • phasic receptors are rapidly adapting Sensory adaptation • sensory receptors

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