2012 Group Project 2

From Embryology
Revision as of 17:03, 14 September 2012 by Z3332863 (talk | contribs) (Adult Central Somatosensory systems:)


Somatosensory Development

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

- Touch, Pain, Hot/Cold, Pressure Reception


Introduction

The somatosensory system is an important subdivision of the somatic nervous system comprising of a collection of receptors, tracts and nuclei. The system components convey the sensations of vibrations, light touch, pain and temperature to the consciousness (Creath, Kiemel, Horak, & Jeka, 2008) The system is important in conveying information about the body position and movements with significant influence on the body balance (Wong, Collins, & Kaas, 2010). The somatosensory system also plays an important role in motor control through conveying of feedback information about the muscular system dynamics including velocity of muscles, tension, length, joint position and movement and contact with the external environment. The system comprises of receptors in the muscles, skin, viscera and joints (Marani, 1994). The following picture shows the general organization of the somatosensory system.


(Lagercrantz, Hanson, Evrard & Rodeck, 2001) Understanding the development of this systems both structurally and functionally during the fetal life is crucial in understanding how a fetus develops the capacity to receive and experience sensations delivered by thermal, mechanical, tactile and noxious stimuli (Willis, 2007). The somatosensory systems development begins during the gestation period specifically the third week into the gestation period. By the end of the 9th week the fetus has a fully developed nervous system with sensory and receptors present at the skin level (Stiles, Reilly, Levine, Trauner, & Nass, 2012). Development of the system entails development of nerve fibers and receptors in the fetus body system. Development of the somatosensory system involves progressive changes in the structural alignment, neurochemical and functional changes with majority of the development changes taking place during the gestation period. Somatosensory receptors develop in the various parts of the body to enable detection and reception of stimuli which is then transmitted through the nerve fibers to the central nervous system (Nakamura & Morrison, 2008). Development of the somatosensory system also entails subsequent development of pathways including the dorsal column-medial lemniscal system.

History of Discoveries

Weber recognized for his role in the study of the nervous system including the establishment of the Weber’s law (Giclu, 2007). Some of the historical research conducted by Weber concerned the various aspects of nervous system including inhibition of impulse transmission, summation, adaptation and fusion. The shift from philosophy to physiology can be attributed to Weber’s research work through which he influenced the view on the human system. Other discoveries that followed Weber’s discoveries about the somatosensory system include the discovery that most receptor endings in the skin, the connection between the system and the spinal cord. The other important historical discovery about the somatosensory system include the discovery of different kinds of electrical potential in the nervous systems not covered by Weber as the pioneer in the understanding of the nervous system (Deco & Rolls, 2006).

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 and lateral spinothalamic tract:– receive signals from the rest of the body. [2] [3]

Dorsal column system and Lateral Spinothalamic tract:

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 or lateral spinothalamic tract up the spinal cord. [4] [3] From there, 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. [4] [3]

[File:Somatosensory.png]

Trigeminal System:

Sensory signals from the face are passed through the trigeminal nerve which passes signals to the trigeminal sensory nucleus. [4] 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.[4]

Development of the Primary Somatosensory Cortex:

Development of the primary somatosensory cortex is thought be controlled by both intrinsic factors and extrinsic factors. [4] 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’. [4] 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). [4] This patterning of the somatosensory cortex is the key step in its development. [4] 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.[4] 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. [4] The VP afferents receiving information from the face and jaw differentiate before birth. [4] 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. [4] This will be followed by the development of the medial regions of the somatosensory cortex that processes the information from the body. [4] 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.[5]

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 [5]


Stage 24:

  • the afferent axons have extended 1 segment rostrally and 1 segment caudally relative to the axons' point of entry [5]
  • the afferents start to grow within the white matter (periphery of Spinal Cord)[5]

Stage 28 –

  • unbranched afferent axonal fibres invade gray matter at the border of Dorsal horn [5]
  • 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)[5]

Stage 29:

  • afferent fibres have extended 100-200μm into gray matter of the Dorsal Horn [5]

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

With the current advancements in study and research on the nervous system, the mechanisms responsible for the sensation or the sensory component of pain are now well understood. Different nerve fibres involved in the transmission of the pain impulse have been identified including the A-delta fibres, C fibres and A-beta fibres (Nakamura & Morrison, 2008). The A-delta fibres have been identified with response to mechanical or thermal stimulation such as pin prick or scald while C fibres respond to thermal, mechanical and chemical stimulation (Silberstein, 2003). The C fibres are slower in response to simulation and particularly transmit the dull, thudding pain of injury, inflammation or disease. On the other hand, the A-beta fibres transmit touch and play a crucial role in the sensation of pain. Current research in the development of pain fibres has seen the classification of pain into fast and slow pain and the pain fibres responsible for transmission of the pain. Fast pain is transmitted by the A-delta fibers with the stimulus being more superficial stimulus. Slow pain starts one second or more after stimulation and increases slowly over seconds or minutes and has been found to be associated with tissue distraction as well as being felt in both superficial and deep tissues. The various nerve fibers carry somatosensory information from the body periphery to the spinal cord. According to Medina and Lebovic (2009), studies have revealed that some nerve fibers present in the endometriotic tissues are responsible for pain severity.

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)

- Pain and thermal sensations are conveyed by the spinothalamic tract of the anterolateral system


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

The development of pressure receptors takes place during the gestation period with the rapidly adapting pressure receptors developing first then followed by the slow adapting pressure receptors (Kinkelin, Stucky, L & Koltzenburg, 1999). Although these pressure receptors are present throughout the fetal life to adulthood, their depolarization responses to chemical irritants, mechanical injury and inflammatory mediators are been found to be similar in both the fetus and adults. Pressure-sensitive receptors of different types found in the skin where they detect changes in pressures. They respond to physical contact such as brushing against a wall, mosquito landing site or any form of touch on the skin. Bunched receptors known as Merkel’s discs have been found to specialize in conveying information about continuous pressure exerted on the surface of the skin (Ragert, Nierhaus, Cohen & Villringer, 2008). Pacinian corpuscles respond to quick changes in touch or pressure by firing off rapid bursts of signals before they decrease with time (Irie, Kato, Yakushji, Hirose & Mizuta, 2011). Baroreceptors are special pressure receptors found in the right atrium of the heart and play the role of detecting changes in blood pressure enabling the body to control the pressure and the amount of blood flowing into the heart (Ino-Oka, Sekino, Kajikawa, Inooka, Imai & Hashimoto, 2008). Different studies have established urinary bladder mechanoreceptors as responsible for detecting changes in bladder volume or intravesical pressure (Downie & Armour, 1992). Discovery of the pressure receptor locations in human bodies has been exploited in nursing interventions including the touch therapy or massage (Field, 1998). Massage therapy has been found to be effective in enhancing various clinical conditions including reduction of pain, diminishing of depression, enhancing attentiveness, promotion of immune function as well as promotion of growth and development of pre-term infants (Sanders, 2010). According to Field (1998), intentional stimulation of pressure receptors during massage therapy results in enhanced vagal activity associated with the diverse benefits accrued from massage therapy. Understanding of pressure receptors has been discovered to be important in the treatment of balance disorders. The pressure receptors in the skin detect bodily contact with the environment enabling control of the contact during treatment of balance disorders (George & Athanasios, 1999).

Current Research

Somatosensory Activation by Corneal Pain:

Investigation is currently done on to localize somatotopic representation of pain from the cornea. [6] This type of research gives insight into the mechanism of chronic pain development in various eye conditions. [6] This study shows processing of corneal pain information occur in localized regions of the primary somatosensory cortex. [6] When the cornea pain receptors are stimulated, these localized regions o the somatosensory cortex are activated. [6] The region of the somatosensory cortex that deals with corneal pain, also deals with blinking or photophobia. Such finding has been achieved using functional Magnetic Resonance Imaging (fMRI). [6] See figure


reference [6] refers to Eric A. Moulton1, Lino Becerra, 2012, PlosOne 'An Approach to Localizing Corneal Pain Representation in Human Primary Somatosensory Cortex'

Glossary

References

  1. <pubmed> 8440772</pubmed>
  2. <pubmed> 14485390</pubmed>
  3. 3.0 3.1 3.2 <pubmed>1127457</pubmed>
  4. 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 <pubmed>7962713</pubmed>
  5. 5.0 5.1 5.2 5.3 5.4 5.5 5.6 <pubmed>2918087</pubmed>
  6. 6.0 6.1 6.2 6.3 6.4 <pubmed>PMCIDPMC3433421</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