- 1 Somatosensory Development
- 1.1 Introduction
- 1.2 History of Discoveries
- 1.3 Central Somatosensory Differentiation
- 1.4 Touch
- 1.5 Glossary
- 1.6 References
- 1.7 External Links
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; 
- the dorsal column system and lateral spinothalamic tract:– receive signals from the rest of the body.  
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.   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.  
Sensory signals from the face are passed through the trigeminal nerve which passes signals to the trigeminal sensory nucleus.  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.
Development of the Primary Somatosensory Cortex:
Development of the primary somatosensory cortex is thought be controlled by both intrinsic factors and extrinsic factors.  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’.  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).  This patterning of the somatosensory cortex is the key step in its development.  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. 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.  The VP afferents receiving information from the face and jaw differentiate before birth.  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.  This will be followed by the development of the medial regions of the somatosensory cortex that processes the information from the body.  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.
- 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 
- the afferent axons have extended 1 segment rostrally and 1 segment caudally relative to the axons' point of entry 
- the afferents start to grow within the white matter (periphery of Spinal Cord)
Stage 28 –
- unbranched afferent axonal fibres invade gray matter at the border of Dorsal horn 
- 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)
- afferent fibres have extended 100-200μm into gray matter of the Dorsal Horn 
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. 
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.  Cite error: Closing
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Links to Meissner’s Corpuscle Images
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.  
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. 
Mechanoreceptors are involved with the primary afferent pathways and terminals, which allow for the detection of tactile sensation. With particular reference to the receptors of touch; found in the skin, action potentials are triggered through alterations in skin, such as stretch, vibrations or larger or small stimuli.
Ruffini Endings, Pacian and Meissner’s Corpuscles are activated via the surrounding components of their terminals. These mechanoreceptors are surrounded by a single capsules and specific cells and tissues (such as laminar ells in the Meissner’s corpuscles). Cite error: Closing
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<ref> tag This type of research gives insight into the mechanism of chronic pain development in various eye conditions.  This study shows processing of corneal pain information occur in localized regions of the primary somatosensory cortex.  When the cornea pain receptors are stimulated, these localized regions o the somatosensory cortex are activated.  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). See figure
reference  refers to Eric A. Moulton1, Lino Becerra, 2012, PlosOne 'An Approach to Localizing Corneal Pain Representation in Human Primary Somatosensory Cortex'
- <pubmed> 8440772</pubmed>
- <pubmed> 14485390</pubmed>
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