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Sensory systems and Sensory receptors
The cutaneous receptors
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Touch and pressure is sensed by 4 types of mechanoreceptors
o Meissner corpuscles are dendrites encapsulated in connective tissue;
 they sense changes in texture and slow vibration
o Merkel cells are expanded dendritic endings
 they sense sustained pressure and touch
o Ruffini corpuscles are enlarged dendritic endings with elongated capsules
 they respond to sustained pressure
o Pacinian corpuscles are unmyelinated dendritic endings which are encapsulated in concentric layers of
connective tissue, in an “onion ring” sort of structure
 they respond to deep pressure and fast vibration
o these receptors encode the basic elements of sensory reception: modality, location, intensity and duration
o Merkel and Ruffini endings supply pressure sensation
o Pacinian and Meissner endings supply vibration and tingling
o Merkel cells and Meissner cells have the SMALLEST receptive fields
o Stimulus intensity is indicated by the firing rate of the activated receptor
Some nerve endings are not associated with any sort of specialized corpuscle.
Pain and temperature sensation is supplied from unmyelinated dendrites of sensory neurons, just sittling there.
Generation of impulses in cutaneous receptors
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PACINIAN CORPUSCLES:
o Onion-like bulb of connective tissues; nerve ending inside; one of the nodes of Ranvier is also inside
o When a small amount of pressure is applied, a small amount of potential is created- but it is not large enough
to be propagated.
o This is called the “generator potential” or “receptor potential”
o When the magnitude of the generator potential reaches 10mV, an action potential is generated the sensorty
nerve fires
o When the magnitude of the pressure increases, the nerve fires more rapidly
o The generator potential seems to be generated in the unmyelinated nerve terminal
Sensory coding
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The 4 basic attributes of a sensory stimulus:
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Modality is the type of energy transmitted by the stimulus- pressure, temperature etc
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Humans have 4 basic classes of receptor modalities: mechanical, thermal, electromagnetic (light) or
chemical.
A receptor is usually geared toward responding to only one kind of stimulus. However it can be made
to detect other types- but the stimulus needs to be much greater. For example, pressure on the eye
will eventually trigger rods and cones.
Location
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A single sensory axon and all of its peripheral branches is called a sensory unit
The receptive field of a sensory unit is the area it innervates
Representation of the senses in the skin is PUNCTATE: a sensation of touch is evoked from the skin directly
overlying the receptor, but no sensation is evoked from areas away from the receptor.
 In the cornea and acllera, one sensory unit supplies 50-20mm2 of surface
 Sensory unit areas overlap and interdigitate
LATERAL INHIBITION
 Information from sensory neurons whose receptors are at the edges of a stimulus is INHIBITED
 This enhances the contrast between the center of the stimulus, and its periphery
 This feature underlies two-point discrimination
TWO-POINT DISCRIMINATION
 The minimum distance between two stimuli required for them to be perceived as two discrete stimuli
 This distance is smalles where touch receptors are most abundant
 If your two stimuli are so close together that they are in the receptive field of one neuron, you will only perceive
a single stimulus.
 This can be used to test the dorsal column tract for integrity
This document was created by Alex Yartsev (dr.alex.yartsev@gmail.com); if I have used your data or images and forgot to reference you, please email me.
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Intensity is determined by the amplitude of the stimulus
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This amplitude is interpreted into frequency
The frequency of receptor firing increases as the amplitude of the stimulus increases
R = KSA
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Duration
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R is the sensation felt
K and A are constants
S is the intensity of the stimulus
When a sustained stimulus of the same intensity is applied to a receptor, the receptor firing rate
declines over time
This is called adaptation or desensitization
Some receptors are rapidly adapting (phasic ) receptors
 Meissner corpuscles
 Pacinian corpuscles
others are slowly adapting (tonic) receptors
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Merkel receptors
Ruffini nerve endings
Other examples are muscle spindles and nociceptors
There is a good reason for this: light touch would be distracting if it were persistent, so those
receptors are slowly adapting; likewise slow adaptation of a muscle spindle is required to
maintain posture.
Input from pain receptors would be useless if it adapted over time.
Law of Specific Nerve Energies
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The sensation produced by a stimulus depends on the part of the brain which the sensory pathway activates.
No matter whether it is the Pacinian corpuscle being squished, or the axon being irritated by a brain tumour- the
sensation felt in the cortex is that of light touch
This is the law of specific nerve energies
Law of projection
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No matter where along its path the sensory pathway is stimulated- the cortex will always map the
sensation produced to the body area supplied by that nerve.
Recruitment of sensory units
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As the strength of a stimulus is increased it “spreads” to neighbouring areas
i.e. the neurons surrounding the area of stimulus get “recruited” to feeling it as well even though their receptors aren’t
being affected
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References: Ganong Review of Medical physiology, 23 ed, chapter 8