Difference between revisions of "2012 Group Project 2"

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Ruffini Endings
 
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These mechanoreceptors are found within the dermal and subcutaneous layers of the skin and contribute to touch sensations in response to changes in joint movement, stretching of the skin and pressure applied to skin surfaces. This allows human beings to effectively hold and grip objects via these dendritic endings that are located within the fingers of individuals.  Alterations in pressure and mechanics of the skin, joints and fingers, such as the sensation of an object slipping from one's hand are recognized by these receptors.
  
 
'''Embryonic Development'''
 
'''Embryonic Development'''

Revision as of 23:24, 10 September 2012


Somatosensory Development

--Mark Hill 12:23, 15 August 2012 (EST) This is a better project title.

- Touch, Pain, Hot/Cold, Pressure Reception


Introduction

History of Discoveries

Central Somatosensory Differentiation

Adult Central Somatosensory systems:

Ascending components of the Central Somatosensory system include;

  • the primary somatosensory cortex of the brain,
  • the trigeminal system: – receives sensory signals from the face; [1]
  • the dorsal column system:– receives signals from the rest of the body. [2]

Dorsal column system:

Peripheral sensory neurons enter the spinal cord via the dorsal root ganglion. The sensory signal then get passed onto collateral fibres in the spinal cord which ascend via the dorsal column up the spinal cord, ending at the dorsal column nuclei. [3] From these dorsal column nuclei, fibres go the lateral regions of the ventroposterior nucleus (VP) of the thalamus. From the thalamus, 3rd order neurons project out and into the primary somatosensory cortex so information can be processed. [3]

Trigeminal System:

Sensory signals from the face are passed through the trigeminal nerve which passes signals to the trigeminal sensory nucleus. [3] Axons from this trigeminal sensory nucleus go to the medial regions of the VP of the thalamus. From there fibres conduct the signals to the primary somatosensory cortex.[3]


Development of the Primary Somatosensory Cortex:

Development of the primary somatosensory cortex is thought be controlled by both intrinsic factors and extrinsic factors. [3] Development of this region begins in late embryonic period and continues post-natally. The primary somatosensory cortex has separate functional groups of layer IV neurons called ‘barrels’. [3] In the adult, the barrels are arranged in a pattern, isomorphic to the pattern of somatosensory receptors on the face and body surface (see figure). [3] This patterning of the somatosensory cortex is the key step in its development. [3] These layer IV neuron barrels receive inputs from the afferents coming from the ventroposterior nucleus (VP) thalamus. These thalamocortical afferents of the VP provide information that patterns the developing primary somatosensory cortex.[3] This extrinsic signalling by the VP afferents from the thalamus may cause graded gene expression in the cortical neurons to pattern the somatosensory cortex.

VP afferents develops just prior to the development of the area of the somatosensory cortex that will process the information from these VP afferents. [3] The VP afferents receiving information from the face and jaw differentiate before birth. [3] Then the lateral regions of the somatosensory cortex develop. Within 24hrs after birth, the VP afferents receiving sensory information from the rest of the body develops. [3] This will be followed by the development of the medial regions of the somatosensory cortex that processes the information from the body. [3] Consequently, there’s a lateral to medial gradient of somatosensory cortex development which controlled by the VP afferents from the thalamus.


Making Connections between Afferent Sensory Fibres and the Central Nervous System (CNS)

This is the process where sensory afferents synapse the neurons in the spinal cord so peripheral somatosensory information can be transmitted through the spinal reflex arc or up to the primary somatosensory cortex where the information can be processed. Sensory afferents from the periphery, with their cell bodies (soma) in the dorsal root ganglion, grow towards the spinal cord in stages to make these connections with the CNS.[4]

Stage 23;

  • Axons of primary afferent neurons extend to the spinal cord. When these afferent neurons reach the CNS, axons of these afferent neurons bifurcate and begin to extend into the Primordium of the dorsal funiculus [4]


Stage 24:

  • the afferent axons have extended 1 segment rostrally and 1 segment caudally relative to the axons' point of entry [4]
  • the afferents start to grow within the white matter (periphery of Spinal Cord)[4]

Stage 28 –

  • unbranched afferent axonal fibres invade gray matter at the border of Dorsal horn [4]
  • axonal fibres extend rostrally and caudally and start sending fine collateral fibres into the gray matter of spinal cord (the cellular, central region of spinal cord)[4]

Stage 29:

  • afferent fibres have extended 100-200μm into gray matter of the Dorsal Horn [4]

Touch

The sense of touch allows individuals to perform a myriad of functions through the receptors deep within dermal and epidermal layers of the skin. This sensory modality, though it’s development is not greatly understood among the five acknowledged sense subsets, it is essential for survival and development throughout life. Receptors that are established throughout embryonic development linked to touch are mechanoreceptors/transducers such as Pacinian Corpuscle, Meissner’s Corpuscle, Merkel-cell-neurite complexes and Ruffini endings. Function and development of these receptors will be discussed in this section.

Touch Receptors

Pacinian Corpuscle

These receptors and nerve endings are found in the subcutaneous tissue of the skin and are also referred to as lamellar corpuscles. When stimulated these nerve endings result in action potentials which respond to the detection of changes in pressure against the skin in relation to vibrations sensations. This can allow for the ability of individuals to establish distinctions between rough and smooth surfaces.

Link to Pacinian Corpuscle image

1. http://thediagram.com/3_1/pacinian.html

2. http://www.biologymad.com/nervoussystem/nerveimpulses.htm

Meissner’s Corpuscle

With similar sensory function as the Pacinian Corpuscle, these receptors are responsible for the detection of vibrations. However, Meissner's (tactile) corpuscles are more sensitive, and able to detect light touch sensations. Found in the dermal papillae under the epidermis of the skin, these receptors are distributed in many areas of the body, specifically the fingertips and lips.

Links to Meissner’s Corpuscle Images

1. http://www.siumed.edu/~dking2/intro/images/IN038b.jpg

2. http://www.virtualworldlets.net/Worlds/Listings/BodySenses/Texture-MeissnerCorpuscle.jpg

Merkel-cell-neurite complexes

Similarly to the Meissner's Corpuscles these skin receptors are able to detect 'light touch' sensations via somatosensory afferents. However, specifically, these receptors are involved in spatial differentiation; establishment of shapes, sizes, textures of objects, in relation to touch. These receptors are also located in the epidermis of the skin in the stratum basale, in close proximity to the fingertips of mammals. This particular cells has been associated with abnormalities of growth and therefore in rare cases leads to Merkel-cell carcinoma.

Ruffini Endings

These mechanoreceptors are found within the dermal and subcutaneous layers of the skin and contribute to touch sensations in response to changes in joint movement, stretching of the skin and pressure applied to skin surfaces. This allows human beings to effectively hold and grip objects via these dendritic endings that are located within the fingers of individuals. Alterations in pressure and mechanics of the skin, joints and fingers, such as the sensation of an object slipping from one's hand are recognized by these receptors.

Embryonic Development

Neural Components

References

1. [1]

2. [2]

3. [3]

4. [4]

5. [5]

Pain

Hot/Cold

Thermoreceptors(1)

- Convey information regarding the temperature of the nerve ending in the tissue

o Free nerve endings

o Ion channels responsible for action potentials are called Transient Receptor Potential channels. Their activity is modulated by temperature

- Two type, warm and cold

- Warm- 30-45

- Cold – 20 – 30

- Frequency of action potentials codes for the relevant temperature

o As temperature increases, the frequency of warm thermoreceptors increases

o As temperature decreases, the frequency of cold thermoreceptors increases.

- Receptors

o Warm – TYPV1 to V4

o Cold – TRPM8 and TRPA1

Some current research (2)

From:

1. Stanfield, C. L., and W. J. Germann. 2011. Principles of human physiology. Pearson/Benjamin Cummings, San Francisco, CA.

2. Hjerling-Leffler, J., F. Marmigere, M. Heglind, A. Cederberg, M. Koltzenburg, S. Enerback, and P. Ernfors. 2005. The boundary cap: a source of neural crest stem cells that generate multiple sensory neuron subtypes. Development 132:2623-2632.


- The thermosensitive neurons travel in the dorsal root ganglion, as with other sensory neurons such as proprioceptive, mechanosensitive and nociceptive neurons.

- DRG neurons pseudounipolar

o One process detects the stimuli in the tissues

o The other relays it into the dorsal horn

- Mixture of small-diameter, slow, unmyelinated C fibers and larger, faster Aδ fibers

- This article is great for understanding how temperature sensation works, but not to great on the embryology behind it(1)

- Another good article for a more molecular understanding(2)

- Understanding – more recent(3)

- TRPV 1 and 2 respond to painful levels of heat, whilst 3 and 4 respond to non-painful levels

- TRPM8 responds to non-painful cold, TRPA1 responds to painful cold(4)


1. Patapoutian A, Peier AM, Story GM, Viswanath V. ThermoTRP channels and beyond: mechanisms of temperature sensation. Nature reviews Neuroscience. [Review]. 2003 Jul;4(7):529-39.

2. Bandell M, Macpherson LJ, Patapoutian A. From chills to chilis: mechanisms for thermosensation and chemesthesis via thermoTRPs. Current opinion in neurobiology. [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov't Review]. 2007 Aug;17(4):490-7.

3. Schepers RJ, Ringkamp M. Thermoreceptors and thermosensitive afferents. Neuroscience and biobehavioral reviews. [Review]. 2010 Feb;34(2):177-84.

4. Vay L, Gu C, McNaughton PA. The thermo-TRP ion channel family: properties and therapeutic implications. British journal of pharmacology. [Research Support, Non-U.S. Gov't Review]. 2012 Feb;165(4):787-801.

Pressure

Current Research

Glossary

References

  1. <pubmed> 8440772</pubmed>
  2. <pubmed> 14485390</pubmed>
  3. 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 <pubmed>7962713</pubmed>
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 <pubmed>2918087</pubmed>


External Links

External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name. Links to any external commercial sites are provided for information purposes only and should never be considered an endorsement. UNSW Embryology is provided as an educational resource with no clinical information or commercial affiliation.

--Mark Hill 12:22, 15 August 2012 (EST) Please leave the content listed below the line at the bottom of your project page.


2012 Projects: Vision | Somatosensory | Taste | Olfaction | Abnormal Vision | Hearing