Lec 12 - Sensory Overview

Lec 12 - Sensory Overview - An Overview of Sensory Systems...

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1 An Overview of Sensory Systems For the nervous system to react to an environmental stimulus, the energy form of that stimulus must be translated into an electrical signal, which can then be transmitted and processed in the nervous system. This translation, called sensory transduction , is carried out by the sensory receptor neurons (also called primary sensory receptors ), which sit at the interface between the nervous system and the outside world. Sensory Receptor Neurons Respond to Particular Types of Stimulus Energy The properties of our sensory receptors govern which aspects of the environment we are aware of and which aspects pass completely unnoticed. A bat seems to us to fly silently, but in fact it is shrieking frequently and loudly as it flies. Because the bat’s ultrasonic cries are beyond the frequency range of human hearing, they are not part of our sensory world (but they certainly are part of the bat’s sensory world).
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2 Sensory Receptor Neurons Respond to Particular Types of Stimulus Energy The bat’s ultrasonic cries are also part of the sensory world of moths, a favorite prey of bats. Sensory Receptor Neurons Respond to Particular Types of Stimulus Energy Moths have ultrasonic “bat detectors” that respond to echolocation pulses emitted by bats and trigger an evasive response.
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3 Sensory Modalities If an organism is able to respond to a particular type of environmental stimulus, then it must possess primary sensory receptors sensitive to the form of physical energy that constitutes the stimulus. The nervous system encodes the qualitative properties of a stimulus based on the types of sensory neurons that are activated by the stimulus. This is the perceived sensory modality of the stimulus (that is, whether it is perceived as light, touch, pain, temperature, etc.). Activity in neurons connected to photoreceptors will be interpreted as providing information about light, activity in neurons connected to muscle spindle receptors will be interpreted as giving information about muscle length, and so on. That is, the nervous system uses labeled lines to determine what kind of sensory information is arriving from the environment. Some General Features of Sensory Systems Coding of stimulus intensity – H ow information is provided about the strength of a stimulus Amplitude coding : the size of the electrical response produced by the transduction process in the primary sensory receptor cells increases with the strength of the stimulus. This graded electrical response in the primary receptor neuron is called the receptor potential . Frequency coding : the receptor potential ultimately gives rise to action potentials, either in the primary sensory neuron itself or in secondary neurons that receive synaptic input from the primary receptor cell. The stimulus intensity is then encoded by the frequency of action potentials during the stimulus.
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