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lect fin fall 2020

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somatosensory.md
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## Somatic sensory system
#### Roles
* Monitoring the external and internal forces acting on the body at any moment
* Discrimination of shapes and textures of objects
* Detection of potentially harmful circumstances
#### Includes
1. tactile input (touch– vibration, pressure, stretch)
2. proprioceptive input (positioning of self– muscle fiber feedback )
1. proprioceptive input (positioning of self– muscle fiber feedback )
* Monitoring the external and internal forces acting on the body at any moment
2. tactile input (touch– vibration, pressure, stretch)
* Discrimination of shapes and textures of objects
3. nociceptive input (pain– injury, extreme temperatures, pH)
* Detection of potentially harmful circumstances
<div style="font-size:0.5em;">
<!-- date: -->
</div>
Note:
Today we will focus on the somatic sensory system also called the somatosensory system.
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## Overview of somatic sensory system
* Specific receptor neurons located in skin or joints receive stimuli
* Specific receptor neurons located in skin, muscle, or joints receive stimuli
* Information is carried to brain via the spinal cord, brainstem, thalamus, to the post central gyrus of the parietal lobe, which in turn project to higher order cortical areas
* Projections are topographic with respect to body region, and the amount of cortical space allocated to various body parts is proportional to the density of sensory receptors in that area
Note:
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## Dermatomal segments
...from the embryonic somites
<div><figcaption class="big">Human embryo at three to four weeks</figcaption><img src="figs/IMG_0186_human_embryo_day24_6e71b8a.jpg" width="400px"><figcaption>24d, 2.6mm, 19somites; 26d, 3.6mm, 25somites; *Dev Anatomy* Arey 1954</figcaption></div>
<div><figcaption class="big">Human embryo at four to six weeks</figcaption><img src="figs/IMG_0187_human_embryo_day35_fd26b22.jpg" width="400px"><figcaption>28d, 5mm; 35d, 8mm; 42d, 12mm; *Developmental Anatomy* Arey 1954</figcaption></div>
Note:
somites, metameres
: mesoderm separates into localized structures called somites
: origin of the dermatomal segments
: happens around neural tube closure, third week of gestation in human
: develop in rostral to caudal direction
: positioning regulated by the highly conserved hox genes
: formed every 90 minutes in chick embryo
: chicken 50 somites, mice 65 somites, snakes 500, human 39-44
: somites become the dermatomes and myotomes that make up the vertebrae, tissue regions of skin and muscle
Developmental Anatomy 1924, 1954 Arey
p. 98
> Fig. 74 Human embryos of twenty-fout and twenty-six days, viewed from left side. A, Atwell embryo of 26 mm, with ninetween somites (Streeter; x23); B, 3.6mm, embryo with twenty-five somites (x16)
Developmental Anatomy 1924, 1954 Arey
p. 99
> Fig. 75 Human embryos of four and five weeks, viewed from the left side. A At 5 mm. B, at 8 mm
> Fig. 76. Human embryo of six weeks (12 mm). viewed from the right side.
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## Dermatomal segments of the body
<div><figcaption class="big">the human dermatome</figcaption><img src="figs/dermatomes-Grant_1962_663.png" height="450px"><figcaption>Grant, *An atlas of anatomy* 1962 [Public Domain](https://en.wikipedia.org/wiki/Public_domain)</figcaption></div>
Note:
image is public domain. Grant, John Charles Boileau - An atlas of anatomy, / by regions 1962 <https://en.wikipedia.org/wiki/Dermatome_(anatomy)#/media/File:Grant_1962_663.png>
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## Various types of somatic sensory receptors
## Somatic sensory receptors
Can vary characteristically by
* Functions– pain, temperature, touch, and proprioception
* Morphologies– free nerve endings or encapsulated
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Tab. 1 after Rosenzweig 2005
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## Slowly adapting and rapidly adapting mechanoreceptors respond differently to stimulation
<figure><img src="figs/Neuroscience5e-Fig-09.04-0_b85b14e.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 9.4</figcaption></figure>
Note:
Another type of somatosensory afferent variability I mentioned was rate of adaptation– this figure highlights this difference where if we were performing extracellular electrode recordings close to somatic sensory we find that some types adapt slowly, with sustained spiking as a stimulus stays on, whereas others adapt rapidly with their spiking activity strong at the beginning of the stimulus but quiet as the stimulus is maintained.
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: no displacment, no work
: no work in direction orthongonal to displacement
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## Slowly adapting and rapidly adapting mechanoreceptors respond differently to stimulation
<figure><img src="figs/Neuroscience5e-Fig-09.04-0_b85b14e.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 9.4</figcaption></figure>
Note:
Another type of somatosensory afferent variability I mentioned was rate of adaptation– this figure highlights this difference where if we were performing extracellular electrode recordings close to somatic sensory we find that some types adapt slowly, with sustained spiking as a stimulus stays on, whereas others adapt rapidly with their spiking activity strong at the beginning of the stimulus but quiet as the stimulus is maintained.
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## Cutaneous mechanoreceptors
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* **Receptive field** (RF)– the area in the periphery within which sensory stimulus can modulate the firing of the sensory neuron
* Spatial resolution of the RF:
* Size– smaller RF, higher resolution
* Density– higher density, higher resolution
* "Two-point discrimination test"
* Density– higher cell density, higher resolution
* Two-point discrimination test
</div>
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Note:
<!-- ## Discrimination can also be at the level of the primary or secondary sensory neuron
<!--
## Discrimination can also be at the level of the primary or secondary sensory neuron
<div><img src="figs/image3_04b7a83.png" height="100px"><figcaption></figcaption></div>
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<div><img src="figs/image6_dbace8a.jpg" height="100px"><figcaption></figcaption></div>
-->
-->
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Note:
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## Dorsal column-medial lemniscus system
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## Dorsal column-medial lemniscus system
<figure><img src="figs/neuroscience3e-dorsoleminiscal-sys_a8e9735.png" height="200px"><figcaption></figcaption></figure>
<figure><img src="figs/neuroscience3e-dorsoleminiscal-sys_a8e9735.png" height="400px"><figcaption>Neuroscience 3e</figcaption></figure>
Note:
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[http://www.pnas.org/content/109/Supplement_1/10647/F3.expansion.html](http://www.pnas.org/content/109/Supplement_1/10647/F3.expansion.html)
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## Whisker 'barrels' in rodent cortex
<figure><img src="figs/image14-rodent-barrels_391df8c.png" height="300px"><figcaption></figcaption></figure>
Note:
---