Regarding the significance of phox2 expression in the visceral nervous system, any interpretation of the data in terms of a simple one-to-one cell type homology is difficult. Which of these would correspond to any of the mollusk neurons, which, among others, innervate mollusk-specific organs such as the mantle or the osphradia, special kinds of respiratory organ? In their favor previous studies showed that the over-expression of phox2 in non-visceral cells suffices to induce a visceral-motor axon phenotype [ 13 ]; and in phox2 knockout mice visceral-sensory neurons change their axonal projections to those of somatic-sensory neurons [ 14 ].
Also, the authors argue that both the viscero-motor autonomic ganglia in vertebrates and the stellate ganglion in cephalopods are considered new characters synapomorphies and thus should not find a counterpart outside the respective groups; yet, they both express phox2. Once they evolved, the partaking neurons of these ganglia might have started to express phox2 anew, thus becoming visceral and connecting to the pre-existing visceral circuits. However, this interpretation is problematic, too.
This scenario assumes that neurons that partake in a given circuit have arisen by segregation of cellular functions to different sister cell types, which diverge but remain interconnected by axons. This concept allows for neuron types to be gradually relocated to remote locations during evolution.
Then, the resulting post-ganglionic type would have been relocated to the periphery while the resulting pre-ganglionic neuron might have remained in the palliovisceral ganglion. This concept implies that there is not a simple one-to-one but rather a many-to-many homology-relationship between phox2 neuron types of the circuits compared.
The work of Nomaksteinsky et al. If both systems were in place in stem bilaterians, did they evolve independently or by duplication from a single, simple sensory-motor precursor circuit? And what were the most ancient sensory modalities and motor outputs of these two circuits, once they evolved? The most fascinating perspective comes from the fact that the urbilaterian ancestor was a marine animal as this has profound implications for what is regarded as external somatic and internal visceral.
For example, the somatic circuit processed all information relevant for locomotion that is, mechanical stimuli and olfaction ; part of the visceral circuit instead processed chemical information relevant for feeding and accordingly controlled the movement of the gut. Future work in simple marine invertebrates should elucidate these fascinating questions further. National Center for Biotechnology Information , U. BMC Biol. Published online Apr Select personalised content. Create a personalised content profile.
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Measure content performance. Develop and improve products. List of Partners vendors. Somatic pain and visceral pain are two distinct types of pain, and they feel different. Somatic pain comes from the skin. Both somatic pain and visceral pain are detected in the same way. Pain-detecting nerves called nociceptors send an impulse from the painful site up through the spinal cord and to the brain for interpretation and reaction.
This is called nociceptive pain , and it differs from neuropathic pain , which is caused by nerve damage. Though they are detected in similar ways, somatic pain and visceral pain do not feel the same. Somatic pain is generally described as musculoskeletal pain. Because many nerves supply the muscles, bones, and other soft tissues, somatic pain is usually easier to locate than visceral pain. It also tends to be more intense. The main difference between somatic and visceral reflex is that the somatic reflex occurs in skeletal muscles whereas, the visceral reflex occurs in soft tissue organs.
Furthermore, the somatic nervous system produces somatic reflexes while autonomic nervous system produces visceral reflexes. Somatic and visceral reflex are two types of reflex arcs or nerve circuits produced by the different systems of the peripheral nervous system. Both types of reflexes are important for the coordination between the central nervous system and the effector organ. Somatic reflex is the reflex of the somatic nervous system, which produces unconscious motor responses in skeletal muscles.
Here, it uses alpha motor neurons, which control the voluntary muscular movements. This can happen either directly, as in the case of monosynaptic reflexes, or via interneurons. Efferent neurons carry an impulse from the integration center out to the appropriate muscles.
We also talked about how the somatic motor division of the PNS involves mostly conscious muscle movements and how the visceral motor division ANS handles involuntary muscle movements.
So tune in next week for an in-depth discussion of the divisions of the ANS and the completion of the map—same VB time, same VB channel. Be sure to subscribe to the Visible Body Blog for more anatomy awesomeness! Are you a professor or know someone who is? We have awesome visuals and resources for your anatomy and physiology course! When you select "Subscribe" you will start receiving our email newsletter.
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