Substance P belongs to the tachykinin family of peptides. Which of the following peripheral tissues contain substance P?
**Core Concept**
Substance P is a neuropeptide belonging to the tachykinin family, which plays a crucial role in transmitting pain signals. It is a key neurotransmitter involved in various physiological processes, including pain transmission, smooth muscle contraction, and vasodilation.
**Why the Correct Answer is Right**
Substance P is primarily found in the peripheral terminals of sensory neurons, particularly in the trigeminal nerve and dorsal root ganglia. It is stored in small vesicles and released upon stimulation, binding to its receptor, neurokinin 1 (NK1) receptor, which mediates its effects. Substance P is involved in transmitting pain signals from the periphery to the central nervous system, making it a key player in pain perception.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because substance P is not primarily found in the adrenal medulla. The adrenal medulla contains epinephrine and norepinephrine, which are catecholamines involved in the fight-or-flight response.
**Option B:** This option is incorrect because substance P is not primarily found in the parasympathetic ganglia. The parasympathetic ganglia contain acetylcholine, which is involved in the parasympathetic nervous system's functions, such as promoting digestion and reducing heart rate.
**Option C:** This option is incorrect because substance P is not primarily found in the posterior pituitary gland. The posterior pituitary gland releases oxytocin and vasopressin, which are hormones involved in lactation and water reabsorption, respectively.
**Clinical Pearl / High-Yield Fact**
Substance P is often referred to as a "pain peptide" due to its role in transmitting pain signals. Its release and binding to the NK1 receptor are key steps in the pain transmission pathway.
**Correct Answer:** C. Dorsal root ganglia.