Somatosensory
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What does sensation entail?
• ability to transduce, encode and perceive info generated by external and internal stimuli
What do specialized neurons (receptors) do?
• convert energy into neural signals or afferent sensory signals
Afferent sensory signals activate central neurons capable of representing what aspects of the stimulus?
• qualitative (what it is) and quantitative (how strong it is) aspects • sometimes location of stimulus in space
What is an adequate stimulus?
• preferred or most sensitive stimulus modality (somatic, visual, auditory, electrosensory)
What are polymodal receptors?
• naturally sensitive to more than one stimulus modality
What are nociceptors?
• sensitive to extremely strong stimuli of various kinds (temp, pressure, chemicals) • many are polymodal
How do sensory neurons code stimulus intensity?
• through changes in AP frequency or graded membrane potential changes • strong stimuli = high frequency or strong depolarization = lots NT release
What is the dynamic range?
• range of intensities for which receptors can encode stimuli
What is threshold detection?
• weakest stimulus that produces a response in a receptor 50% of the time
What is saturation?
• top of the dynamic range; all available sensory transducing proteins have been stimulated or neuron can’t fire any faster
What is the graphical relationship between stimulus intensity and AP frequency?
• relationship is linear and can be across a large range of intensities or small range of intensities
How does a large change in stimulus effect AP frequency for a large range of intensities and how does this affect perception?
• large change in stimulus causes a small change in AP frequency • results in large dynamic range but poor sensory discrimination
How does a small change in stimulus affect AP frequency for a small range of intensities and how does this affect perception?
• small change in stimulus causes a large change in AP frequency • small dynamic range and high sensory discrimination
What is range fractionation?
• groups of receptors can work together to increase dynamic range without decreasing sensory discrimination
What are the three broad classes of receptors that encode stimulus duration and how do they do it?
• Phasic • Tonic non-adapting • Tonic slow adapting - encode stimulus duration in AP firing rates
How do phasic receptors record stimulus duration?
• produce APs only at the beginning or end of the stimulus - encode changes in stimulus
How do tonic non-adapting receptors record stimulus duration?
• produce APs as long as the stimulus continues
How do tonic slowly adapting receptors record stimulus duration?
• AP frequency decreases if stimulus intensity is maintained at the same level
All sensory receptors have ______. Only receptors that generate APs have _______.
• receptor potential - generator potential
Somatosensory afferents convey information from the ____ to ____.
• skin surface to central circuits
How does the ascending path start for “touch”?
• receptor endings • mechanosensory afferent fiber • dorsal root ganglion cells • ipsilateral at start of ascending path
How does the ascending path start for pain/temperature?
• receptor endings • pain and temperature afferent fiber • dorsal root ganglion cells • contralateral projecting neuron carries info to contralateral spinal cord then ascends
How does transduction in a mechanosensory afferent (Pacinian corpuscle) occur?
• deformation of the capsule leads to a stretching of the membrane afferent fiber, increasing the probability of opening mechanotransduction channels in the membrane • opening these channels leads to depolarization which if sufficient generates an action potential
For receptors that fire spikes what is detected for weak stimulus?
• nothing is “detected” if stimulus too weak to generate a spike