Have you ever experienced that annoying itch on your chin whenever you have a coughing fit? It’s a strange sensation that can make you wonder why these two seemingly unrelated things are connected. Well, let’s dive into the fascinating world of sensory neurons and explore the reasons behind this peculiar phenomenon.
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The Connection Between Cough and Itch
To understand why your chin itches when you cough, we need to delve into the respiratory tract and the sensory neurons that innervate it. In this complex network, there are specific fibers known as C-fibers that play a significant role in transmitting sensory information.
The Distinction of C-fibers
C-fibers can be classified as pulmonary C-fibers or bronchial C-fibers based on their blood supply. Pulmonary C-fibers are located in the lung interstitium near the pulmonary capillaries, while bronchial C-fibers are found in the large airways. The distinction between these two types of fibers is essential in understanding their different responses to various stimuli (Coleridge and Coleridge, 1984).
The Phenotypic Variations of C-fibers
Further categorization of C-fibers is based on their ganglionic origin. Vagal C-fibers, which innervate the respiratory tract, can be divided into jugular and nodose C-fibers. Jugular C-fibers, derived from the neural crest, contain more sensory neuropeptides and respond to a narrower range of chemical stimuli compared to nodose C-fibers (Undem et al., 2004).
Distribution of C-fibers
Interestingly, while the majority of C-fibers in guinea pigs terminate in the large extrapulmonary airways, there is an equal distribution of nodose and jugular C-fibers in the intrapulmonary tissues (Riccio et al., 1996). This consistency in the C-fiber phenotype throughout the respiratory tract suggests that their embryonic history plays a crucial role in their function (Undem et al., 2004).
Sensitivity of C-fibers
C-fibers are sensitive to mechanical forces, inflammatory mediators, and tissue acidification. They possess unique properties that distinguish them from other afferent nerves in the airways. These unmyelinated fibers respond directly to chemical substances, making them highly responsive to chemical stimuli (Widdicombe, 2001). Additionally, C-fibers are polymodal, meaning they can respond to both chemical and mechanical stimulation (Mazzone and Undem, 2016).
The Role of Neuropeptides
Certain subsets of C-fibers synthesize neuropeptides that are transported to their nerve terminals. These terminals form a vast network of fibers throughout the respiratory tract, including the airway epithelium, smooth muscles, glands, vasculature, and autonomic ganglia (Mazzone et al., 2009). Neuropeptide expression varies among species and depends on the ganglionic origin of the C-fiber (Mazzone and Undem, 2016).
The Intriguing Similarities of Itch and Cough
Now that we have a better understanding of C-fibers and their role in the respiratory tract, we can explore why your chin itches when you cough. Itch and cough are both mediated by sensory neurons, and they share common receptors that are activated in various instances (LaVinka and Dong, 2013).
The activation of these sensory neurons involves two types of receptors – ionotropic and metabotropic. Surprisingly, itch and cough utilize the same receptors in many cases, further blurring the lines between these two sensations.
Conclusion
The connection between a cough and an itchy chin lies within the intricate world of sensory neurons. C-fibers, with their distinct characteristics and distribution, play a crucial role in transmitting sensory information from the respiratory tract. While the exact mechanisms that link a cough and an itchy chin are still being explored, the similarities in receptor activation shed light on the fascinating overlap between these sensations.
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