Lecture 21: Somatosensory System Flashcards
Important thing to remember about dorsal root ganglia
There are no synapses within the dorsal root ganglia (DRG) because it is pseudounipolar cells
- the axon that comes off the cell body in the DRG towards the dorsal horn divides into a central and a peripheral branch - central branch enters spinal cord at dorsal root and makes 3 general types of connections i. local reflex: with interneurons and motorneurons at the same or different spinal levels ii. cerebral cortex: via several synapses, the sensory information finally reaches the thalamus and the cerebral cortex iii. cerebellum: via one or more synapses to provide feedback and sensory guidance of movements
What are Free nerve endings? Location?
Function: pain, temperature, crude touch
Rate of adaptation is slow while threshold of activation is high
Located in all skin (not bunched together)
What is the function of Golgi tendon organs and joint receptors?
Muscle tension and joint position respectively
What does it mean to be a slow adapting receptor?
When you have a prolonged stimulus, you are continually firing action potentials
What does it mean to be a rapidly (fast) adapting receptor?
Means that when you apply a continuous stimulus, you feel it right at the very beginning and then there are no more signals being sent (you get used to it) until the stimulus is removed
Example: Pacini’s corpuscle
What are the symbols used for sensory neurons to muscle?
Roman numerals (I as the biggest)
What are the symbols used for sensory neurons to skin?
Letters (A-beta, the biggest)
Which axons are most sensitive to damage from anoxia (lack of oxygen)?
The largest diameter axons (like Type I fibers)
Allodynia
pain due to a stimulus that does not normally provoke pain
Example: when you apply a tourniquet to your arm, a hairbrush at the very beginning won’t hurt…but once the anoxia takes out the biggest axon fibers, only the small pain axons remain…so 15 minutes later, patient would jump at hairbrush
Which axons are most sensitive to local anesthesia?
The smallest diameter axons (because that’s what is most sensitive to pain)
Anesthesia goal is to take out smallest axons first and leave large axons to leave pressure and temperature sensation…rarely occurs that way
Somatosensory Receptive Fields
the receptive field of a cell is that portion of the skin which when appropriately stimulated AFFECTS the FIRING of the cell
- receptive fields are characterized by location, size, shape and modality specificity and other properties - receptive fields of cutaneous dorsal root axons are roughly round or elliptical in shape and uniform in the sense that the appropriate stimulus applied in the receptive field excites the cell - dorsal root axons DIVERGE to synapse with many CNS neurons - each CNS neuron receives a CONVERGE of many dorsal root axons
How do you enhance a sensory system without diminishing the signal?
You do so through surround inhibition
-activated spot is activated but the areas surrounding it are INHIBITED so that your body knows absolutely that those spots are NOT activated…rather than just activating one spot and comparing that with potential activation from surrounding parts
Surround inhibition
serves to enhance or sharpen the detection of edges and to diminish the information about the absolute amount of stimulus energy
-sharpening borders by contrast enhancement is a principle used by other sensory sytems like visual/auditory
-bottom line is that the cells in the somatosensory system are wonderful at detecting stimulus edges
Example: Mach bands (different color bands in which we cannot tell the absolute amount of luminence)
Where is S1 and what thalamic nuclei send connections to it?
Answer: S1 is on postcentral gyrus and thalamic nuclei that project axons to it are VPL and VPM
What is the nature of the columnar organization of S1?
Vertical columnar organization of the somatosensory cortex (post central gyrus)
Rapidly adjusting and slowly adjusting strips are ALTERNATING in columns
Is S1 plastic? And if so, what’s the significance?
This cortical map is plastic and can be reorganized in an adult monkey
Example of experiment: amputated the third digit of the adult monkey and measured before and after of the somatosensory cortex…and shit was different!
Representation changes with trauma or injury or some new adjustment (amputation for instance)
The more you used to a sensory stimuli (i.e. touching a basketball repetitively), how is the S1 cortex representation affected? How is the receptive field area affected? Significance?
More cortical surface area
Smaller receptive fields
More able to discriminate
What is the Syndactyly example and what does it tell us about S1 plasticity?
Surgeons fused two digits of monkeys and found receptive fields spanning the two fused digits
- after digits were cut, the receptive fields that spanned two digits were still there! - due to temporal correlation (similarity between two signals), in that if there is similarity, receptive fields are formed, but if there is no similarity, receptive fields remain apart
Temporal Correlation
measures the similarity between two signals
-is only activated when two receptive fields are in close proximity and therefore receives similar signals
What is Sprouting?
Idea that new nerve connections being made
What is masking?
Idea that an already existing connection from thalamus to S1 that is normally “masked” by more dominant connections
What are Downstream effects of plasticity?
Plasticity of downstream structures responsible for different cortical representations, i.e. alteration of thalamic or dorsal column nuclei
-effects unlikely to account for all the changes seen in S1
How is pain processed?
In two ways…the sense of it and how much it bothers you
Unpleasantness vs. perceived intensity
Painful stimulus: intensity of stimulus but says nothing about the intensity of discomfort
Painful discomfort: you can separate the affective from the nocistimulus
What are of the brain is responsible for nociception?
Anterior cingulate gyrus
What are mechanisms for plasticity?
-Unmasking, sprouting, downstream effects, temporal correlation
What is the purpose of multiple maps and surround inhibition?
Multiple maps: to allow for plasticity
Surround inhibition: allows us to better enhance edges or to distinguish contrast
