p. 378, slide 1 - are you trying to show that there is a threshold for every sensory
experience, and this threshold lies at the 50% detection level?
The point of the figure is to show that there is no exact all or none threshold to any
sensory experience but rather that the probability of detecting a stimulus goes up from 0
to 100% with increasing intensity in the region of the threshold and that the common
definition of threshold is the intensity where one can detect 50% of the stimuli. The
second graph illustrates that this curve can shift a bit to the right or left depending on
conditions, such as fatigue, distraction.
p. 378, slide 2 - where does the "n" in your formula come from? what determines n (type
of fibre? Type of sensory modality (e.g. is proprioception's different from vibration sense
different from discriminative touch?).
n is the value of the exponent in the formula (a power function) is the slope of the line
when the function is plotted on a log log plot. N varies with modality because it depends
on the properties of the receptor and central processing
p. 380, slide 3 - should we memorize the relative diameters of the axons, or does it suffice
just to know that different size exist?
This figure lists typical lengths not diameters and you do not need to know this. With
respect to diameters, you do not need to memorize the details but you should know that C
fibers conduct at approximately 1 m/sec, A-delta about 10 m/s and A-beta at 100 m/s
(actually a bit lower, but this is good enough)
p. 382, slide 1 - is this definition for receptive field correct: the area that is sensed by a
single afferent neuron.
This was not meant to be a definition. Receptive field is the region of skin where a
stimulus will evoke a response in the sensory afferent (or for a single neuron in the CNS).
Perceptive field is the region of skin from which a sensation is perceived as arising from
(as a result of for example electrical stimulation of a single sensory afferent)
p. 382, slide 1 - what are submodalities? what is a population code?
Submodalities is term used to refer to more specific attributes of a general sensation. So
for example a tactile stimulus can evoke separate sensations of pressure or vibration
depending on the stimulus. The code to differentiate these would be the information
arising from what population of afferents have been activated – i.e. pacinian receptors or
SA1 receptors
p. 385, slide 3 - could you please explain the 2 types of plots shown here? what are the
axes? where is the adaptation?
These show recordings from two different receptors (A and B) . The top line of each pair
shows the action potentials (so y axis is voltage). The line underneath it shows the timing
and intensity of the mechanical stimulus (so y axis would be displacement). The amount
of displacement (indentation) is given by the number on the left in micrometers. On the
right the numbers refer to the rate of rise of the displacement in mm per second. The y
axis is time.
p. 387, slide 3 - what are you trying to compare in A? in B? in C?
A shows examples of individual receptive fields on the hand. The graph is a cumulative
plot of the receptive field sizes and shows the curves for each of the 4 types of receptors.
B shows a ‘countour plot’ as in a topographic map where the lines represent isothresholds
– ie. The threshold to evoke a response is equal at any point on the line but is higher for
lines more distant from the center where the lowest threshold is. The graph to right
shows threshold for evoking the response as a function of distance along the skin across
the receptive field (the horizontal line shown on the contour map)
C shows the density of receptors at the tips of the digits, at the base of the digits and on
the palm
p. 389, slide 2 - you said in class that one can perceive a single AP in some parts of skin,
and in other parts you must have several APs to perceive. what determines whether it's
high or low sensitivity to an AP? This probably depends on the extent of magnification
of that area of skin in primary somatosensory cortex – ie. Tips of fingers have much
larger representation than equivalent area of skin on palm.
p. 389, slide 3 - by definition, does innervation density = receptive field density?
In this particular case, yes. However the term receptive field density is not usually used
and I don’t believe I used it.
p. 391, slide 1 - what is the adapting response seen here? what does each "little spike" on
the diagram represent?
Each spike represent the occurrence of one action potential. The receptor fires very
rapidly to the initial change in temperature (so rapidly that in this figure one can not make
out the individual spikes during the initial period). However after the first few seconds
the firing rates decreases (i.e. adaptation has taken place). At lower temperatures the
receptor tends to fire in bursts but this is not functionally significant.
p. 394, slide 3 - what do "SI" and "SII" stand for? Do we have to memorize the
numbering systen of these cortical pathways (e.g. 3a, 3b, 1, 2,... as shown in p. 404, slide
3)?
Yes, you should know about the 4 areas of primary somatosensory cortex – SI. The
secondary somatosensory cortex is abbreviated SII
p. 399, slide 1 - of the 3 neurons (primary, secondary, tertiary) in each sensory pathway,
what are relay neurons? what are projection neurons? what is the difference between the
two? The relay neurons are neurons that communicate information to other brain regions
– i.e. project out of the nucleus. So relay and projection neurons are equivalent terms.
These terms are generally used for neurons in the CNS rather than sensory afferents since
in the periphery all neurons are relay or projection neurons. The other type of neuron is
the local circuit neuron or interneuron. These neurons are involved only in local
processing and their axons remain within the nucleus. In the diagram, the black neurons
are interneurons.
p. 400, slide 3 - I don't understand this diagram at all - what are the axes? the
interpretation? what does it all mean?
The two long horizontal bars are raw recordings of neural activity in the thalamus. They
are photographs of an oscilloscope trace. The y axis is voltage and x axis time and the
time scale is given by the bar with ‘1 sec’ underneath. The inset in the top left shows
superimposed action potentials taken from the raw recording and expanded in time so
that the shape of the potential can be seen – you can ignore this. The figurine shows the
location of the receptive of the neuron in each recording. In each recording the vertical
bars are actually the extracellularly recorded action potentials arising from a single
neuron in VPL. The top trace shows on and off responses to a tactile stimulus delivered
to the receptive field. The bottom trace shows the slowly adapting responses of a neurons
to application of a tactile stimulus delivered to the tip of digit 5.
p. 403, slide 3 - are these collaterals inhibitory? are they excitatory? are there about
equal amounts of both types?
The collaterals of collaterals of thalamic and cortical projection neurons. These
projection neurons release glutamate and are excitatory. They would excite the neurons
in the reticular nucleus of the thalamus. However the reticular neurons release GABA
and so when they are excited they inhibit the cells they project to, in this case the
projection neurons in thalamus – so this is a negative feedback circuit. Knowledge of
relative numbers is still sketchy but is clear that both are substantial.
Lastly, what textbook(s) do you recommend for your section of this course?
Kandel Schwartz and Jessell. Principles of Neuroscience.