Sensory Physiology

Concepts To Understand Receptor Potential Amplitude Coding Frequency Coding Activation/Inactivation Neural Adaptation Synaptic Depression Receptive Field Primary: Always On Secondary:Center:On/Surround:Off Lateral Inhibition 2-point Discrimination Sharpening Receptive Field

Classification of Sensory Receptors Nomenclature based on sensory stimulus source Exteroreceptor Interoreceptor Propioceptive receptor Nomenclature based on type of sensory stimulus Mechanoreceptor Chemoreceptor Thermoreceptor Nocireceptor Photoreceptor Electroreceptor

How to Code the Stimulus Strength Amplitude Coding Frequency Coding Population Coding –How many neurons responded to stimulus? Sensitivity of Neurons –Did you activate neurons with low as well as high threshold for activation?

Receptor Potential: the neuron’s electrical response to the Sensory stimulus Leads to an action potential in the primary or 2 nd sensory neuron

Amplitude Coding The amplitude of the receptor potential increases as the sensory input increases It modifies the firing rate of action potential The size of the receptor potential gives information about the strength of the sensory stimulus.

Action Potential Firing Rate The steady state rate of action potential firing can increase or decrease in frequency known as “On” pathway and “Off” pathway. Secondary neurons can receive inputs from both “on” and “off” neurons and that leads to more sensitivity.

Frequency Coding Firing rate of action potential encodes the strength of the stimulus.

Concepts To Understand Receptor Potential Amplitude Coding Frequency Coding Activation/Inactivation Neural Adaptation Synaptic Depression Receptive Field Primary: Always On Secondary:Center:On/Surround:Off Lateral Inhibition 2-point Discrimination Sharpening Receptive Field

Receptive Field: Portion of skin that can elicit a response in a single neuron with sensitivity to that particular sensory modality Two Point Discrimination

Lateral Inhibition Interneurons inhibit adjacent 2 nd neurons Refine receptive fields Of 2 nd neurons by Decreasing receptive Field of 2 nd neuron Lead to increased discrimination of sensory stimulus at next level Occurs at each Ascending level of the pathway

Increased Information Input 2 nd Sensory Neurons Can be influenced by Input outside the receptive Field for their primary Receptor neurons by synaptic Connection from interneurons Activated by adjacent primary Receptor neurons.

2 nd sensory neurons have Smaller more refined Receptive fields than the Primary sensory receptor neuron

Somatosensory neuron in somatosensory cortex has a more refined receptive field area than the primary sensory receptor neuron in the periphery that responds to the stimuls. Cortical somatosensory neurons also have more complex and selective stimulus requirements for responses, such as a specific of movement

Direction Sensitive Coritical Neurons in S1 somatosensory cortex

Inhibitory Interneuron Has only 1 synapse On adjacent neuron