Somatosensory I Flashcards
What kinds of info do somatosensory receptors convey? (4)
Intensity
Sensory adaptation
Modality
Localization
What is meant by the intensity coding?
Increased stimulus results in increased amplitude of receptor potentials, which produces increased frequency of action potentials.
Thus, intensity of stimulus may be represented by the frequency of APs.
Describe sensory adaptation and what it communicates to the CNS.
Decrease in neural activity with sustained stimulus.
Tells CNS when the stimulus changes.
What impact do slow vs fast adapting receptors have on the message sent to the brain?
Fast adapting receptors have rapid onset and offset, they communicate that something occurred.
Slow adapting receptors gradually decrease activity and communicate the status of the stimulus, or what is occurring.
What are the different somatic modalities?
Tactile: touch, pain, temperature
Proprioception: joint position, muscle sense, and movement
How are sensory modalities distinguished by peripheral nerves?
Individual sensory neurons respond maximally to one modality.
Modalities can also be distinguished by grossly by conduction velocity.
What are cutaneous receptive fields and how do they contribute to tactile localization?
A cutaneous RF is the area of skin innervatd by the branches of a single sensory neuron. RFs of mechanoreceptors vary in size, so you have different degrees of acuity in different areas of the skin.
It’s easier to localize touch in areas of skin that have smaller RFs.
Describe the compound action potential and what it tells you. How is an unmyelinated axon designated in terms of the compound AP?
Recordings of the summed APs measured over time at a single point on a nerve. The APs are coming from different neurons in the same nerve. The farther down the nerve you measure, the more the peaks separate due to different conduction speeds of the axons. This allows you to judge how fast different axons are.
An unmyelinated axon would be more likely a C fiber, which are the slowest.
Which receptors for discriminative touch are slow-adapting and which are fast-adapting?
Slow: Merkel cell, Ruffini corpuscle
Fast: Meissner corpuscle, Pacinian corpuscle, Hair follicle
How are the tactile modalities (other than discriminative touch) sensed?
Free nerve endings, slow adapting, C and A-delta axons.
Temperature: within dermis
Pain: up to stratum granulosum
What are each of the discriminative touch receptors sensitive to? (5)
Merkel: form, texture (fingers scanning a surface)
Ruffini: skin stretch (hand shape and position)
Meissner: skin movement and slip (grip control)
Pacinian: Vibration
Hair follicle: motion/direction of stimulus
What modalities are transmitted along the dorsal column lemniscal system?
Discriminative touch
Proprioception
Describe the dorsal column lemniscal system.
First-order neurons are the A-beta sensory neurons from the periphery. They travel as the dorsal columns to the gracile and cuneate nuclei in the mudulla.
Second order neurons begin at the dorsal column nuclei and decussate at the level of the medulla, then travel up to synapse in the ventral posterolateral nucleus of the thalamus in the internal capsule.
Third order neurons convey the information from the thalamus to the primary somatosensory cortex.
What modalities are transmitted along the anterolateral system?
Pain
Temperature
Describe the path of the anterolateral system.
First-order (C and A-delta sensory) neurons synapse onto second-order dorsal horn neurons.
Second-order neurons are located in the dorsal horn. They decussate at the level of the spinal cord to the anterolateral system (in the white matter).
Then the path splits. The neospinothalamic tract projects to the lateral thalamus and then on to the cortex. The paleospinothalamic tract projects to the reticular formation and the medial thalamus, then the cortex and limbic system.
