TrkA is a member of the neurotrophic tyrosine kinase receptor family. It is a membrane-bound receptor that upon neurotrophin binding, phosphorylates itself and members of the MAPK pathway. TrkA is required for high-affinity binding to nerve growth factor (NGF), neurotrophin-3 and neurotrophin-4/5 but not brain-derived neurotrophic factor (BDNF). TrkA leads to cell differentiations and may play a role in specifying sensory neuron subtypes. It has a crucial role in the development and function of the nociceptive reception system as well as establishment of thermal regulation via sweating. SUBUNIT: Exists in a dynamic equilibrium between monomeric (low affinity) and dimeric (high affinity) structures. SUBCELLULAR LOCATION: Cell membrane; single-pass type I membrane protein. Endocytosed to the endosomes upon treatment of cells with NGF. ALTERNATIVE PRODUCTS: 2 named isoforms produced by alternative splicing. Both isoforms have similar biological properties. TISSUE SPECIFICITY: Isoform TrkA-II is primarily expressed in neuronal cells. Isoform TrkA-I is found in non-neuronal tissues. Mutations in TrkA have been associated with congenital insensitivity to pain, anhidrosis, self-mutalating behaviour, mental retardation and cancer.