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Lecture

Physiology 3120 Lecture Notes - Lamellar Corpuscle, Dorsal Root Ganglion, Axon Terminal


Department
Physiology
Course Code
PHYSIO 3120
Professor
Tom Stavraky

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Human Physiology
Monday, October 19, 2009
Transduction of Environmental Information
Humans have an array of sensory receptors on the skin (i.e. free nerve ending; Pacinian corpuscle)
Afferent axon = INTO the CNS
Becomes the presynaptic terminal on other axons that travel up into higher brain regions
Cell bodies located in dorsal root ganglia near spinal cord
Axon terminal of Pacinian corpuscle is surrounded by connective tissue
Sensory receptor: free/specialized nerve endings that is particularly sensitive to detect one form
of environmental energy (i.e. modalities)
Receptors for pain & temperature are different
Adequate stimulus: particular form of energy to which a receptor is most sensitive
Receptors can still respond to different forms of energy, but we perceive any stimulation
of these receptors as that of its adequate stimulus
Mechanisms of transduction
Mechanical stimulation: ion channels open up due to stretching
Chemical stimulation (i.e. pain nociceptors): binding of ligand with receptor opens ion
channels
Sometimes the same stimulus can be painful & non-painful (i.e. if your attention is
elsewhere)
Thus, they are called nociceptors, not pain receptors
Photoreceptor: photon absorption leads to closing of ion channel
Generator (receptor potentials)
Stimulus causing opening of ion channel in axon terminal
Both sodium & potassium are free to move; sodium rushes in & causes a depolarization in the
axon terminal
We only sense the stimulus if threshold is reached & an action potential is fired
4 characteristics:
Depolarization due to an increase in permeability to sodium & potassium
Local, not propagated
Graded/proportional to the magnitude of the stimulus
Can summate
Patterns of action potentials
Rapidly adapting (dynamic) receptor
Detects onset & offset
Offset of stimulus causes a mechanical shift of the membrane, and these receptors
are extremely sensitive
Doesn’t fire during the applied touch
Gives information about the occurrence/rate of skin indentation
Slowly adapting (tonic) receptor (Merkel or Ruffini ending)
Low threshold or high threshold
Sends a frequency code to the brain; the higher the frequency of action potentials, the
bigger the mechanical stimulus (thus signal amplitude of indentation)
Brain gets information from the entire population (thus produces a population code
Changes discharge over time
Why is a Pacinian corpuscle rapidly adapting?
Inner lamellae slip back in spite of maintained stimulus
Fluid between layers of connective tissue creates great elasticity in the lamellae
Occurs within 1ms
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