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C o n c e p t 501 sensory receptors transduce

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C O N C E P T50.1Sensory receptors transducestimulus energy and transmitsignals to the centralnervous systemAll sensory processes begin with stimuli, and all stimuli rep-resent forms of energy. A sensory receptor converts stimulusenergy to a change in membrane potential and thereby regu-lates the output of action potentials to the central nervoussystem (CNS). Activating a sensory receptor does not neces-sarily require a large amount of stimulus energy. Indeed,some sensory receptors can detect the smallest possible unitof stimulus; most light receptors, for example, can detect asingle quantum (photon) of light.When a stimulus is received and processed by the nervoussystem, a motor response may be generated. One of the sim-plest stimulus-response circuits is a reflex, such as the knee-jerk reflex shown in Figure 49.3. Many other behaviors relyon more elaborate processing that involves integration of sen-sory input. As an example, consider how the star-nosed mole
in the plasma membrane. The resultingflow of ions across the membranechanges the membrane potential.The conversion of a physical or chemi-cal stimulus to a change in the mem-brane potential of a sensory receptor iscalledsensory transduction, and thechange in membrane potential itself isknown as areceptor potential. Recep-tor potentials are graded potentials; theirmagnitude varies with the strength of thestimulus.TransmissionSensory information travels throughthe nervous system as nerve impulses,or action potentials. For many sensoryreceptors, transducing the energy in astimulus into a receptor potential initi-atestransmissionof action potentialsto the CNS.Some sensory receptors are them-selves specialized neurons, whereas others are specialized cellsthat regulate neurons(Figure 50.3). Neurons that act directlyas sensory receptors produce action potentials and have anaxon that extends into the CNS. Non-neuronal sensory recep-tor cells form chemical synapses with sensory (afferent) neu-rons and typically respond to stimuli by increasing the rate at1086UNIT SEVENAnimal Form and FunctionFood presentMole bites.Mole foragesalong tunnel.Molemoves on.Food absentMotor outputIntegrationSensory inputContact with an objectactivates touch receptors onthe mole’s nose, whichtransmit information to thebrain along sensory nerves.Circuits of neuronsin the brainintegrate the inputand generateaction potentials inmotor neurons.Musclescontract, causingthe mole to bitedown on food ormove fartheralong the tunnel.Figure 50.2A simple response pathway: foraging by a star-nosed mole.To CNSTo CNSAfferentneuronAfferentneuronSensoryreceptorcellSensoryreceptorReceptorproteinNeurotransmitterStimulusleads toneuro-transmitterrelease.StimulusStimulus(a) Receptorisafferentneuron.

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Term
Fall
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phloem, Xylem, Plant anatomy, Vascular cambium

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