Pain

Pain is an illusion, what you're really feeling is what your brain abstracts from nociceptive nerve receptors. Nociceptive nerve receptors respond to noxious stimuli (damaging stimuli). There's 3 different types: Polymodal = C fibres, affected by chemical, thermal and mechanical stimuli. They're unmyelinated fibres that give you that throbbing pain after the initial fast hurt. The other two kinds of receptors are thermal and mechanical, information about these are carried by thinly myelinated A-delta fibres.

These are excitatory (glutaminergic - AMPAr) neurons with a fast synaptic potential that can be slowed by neuropeptides. This neuropeptide is called substance P that causes the slowing of the information to the brain. Substance P may also travel vast distances, allowing other areas to be modulated. This gives the unlocalized character of pain.

These fibres project into the ventral horn of the spinal cord which is arranged in layers or laminae. Laminae 5 is responsible for most of the information sent upwards to the brainstem and thalamus. It also gets input from visceral structures and this is why some people may feel pain in wide spread areas for deeper or chronic pain as compared to localized.

There are 5 different tracts:

Spinothalamic: laminae I & V ->contra-anterolateral white matter ->thalamus
Gives information about location of injury

Spinoreticular: laminae VII and VIII-> anterolateral quadrant->reticular formation and thalamus

Spinomesencephalic: I & V-> anterolateral quadrant -> mesencephalic reticular formation ->periaqueductal grey

Cervicothalamus: III, IV-> contralateral ascending-> medial leminscus, ->ventroposterior thalamus -> posteromedial thalamus

Spinohypothalamus: I, V, VII -> usual pathways

Within the cerebral cortex, the cingulate gyrus is responsible for emotional response to pain, insular cortex is responsible for the autonomic component of the pain response from the medial, ventral and posterior thalamus, possibly integrating sensory affective and cognitive components.