Simply put: from the International Association for the Study of Pain http://www.iasp-pain.org/Taxonomy
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.
So that back pain that you perceive can be a warning system to alert you to damage that may occur if you keep doing that activity or that that group of activities was more than you could handle.
There are multiple systems that carry a signal from a Nociceptor https://en.wikipedia.org/wiki/Nociceptor. A Nociceptor changes a Nociceptive stimulus* [An actually or potentially tissue damaging event transduced and encoded by nociceptors] these stimuli being; back pain, pins and needles radiating down the arm, a hot flame – heat, icy cold, a cut to a splinter, inflammation from a bruise or intervertebral disc etc into an electrical signal that the brain can perceive and interpret as being a noxious or nonnoxious pain.
Yes, PAIN is only perceived in the head, because this is where we have our sensory map to tell us where the stimulus is coming from, our limbic system to emotionally respond (is this bad pain or good pain) our hypothalamus to release adrenaline, cortisol, and insulin to ramp up the nerve system to respond intensely.
Nociception is detected by receptors that specialise in thermal, mechanical and chemical Nociception* [The neural process of encoding noxious stimuli.]
THE BRAIN: PERCEIVES ————– INTERPRETS—————RESPONDS
When you perceive pain within the brain—-you interpret the situation——you respond.
The flame is warm — this is nice because I’m cold—- stay near fire
The flame is hot —- take hands away —- because will burn my hand
I lifted ten bags of cement onto a trailer and felt great after, I lifted ten bags of cement onto a trailer and felt really sore with intense low back pain. It’s telling you that you will do damage because your back can’t do this job properly. This can also be sitting for to 8 hours, carry a baby on one side. If you load the body excessively it may not be able to cope and will alert you in this way. G
Note: The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment. Pain is always subjective. Each individual learns the application of the word through experiences related to injury in early life. Biologists recognize that those stimuli which cause pain are liable to damage tissue. Accordingly, pain is that experience we associate with actual or potential tissue damage. It is unquestionably a sensation in a part or parts of the body, but it is also always unpleasant and therefore also an emotional experience. Experiences which resemble pain but are not unpleasant, e.g., pricking, should not be called pain. Unpleasant abnormal experiences (dysesthesias) may also be pain but are not necessarily so because, subjectively, they may not have the usual sensory qualities of pain. Many people report pain in the absence of tissue damage or any likely pathophysiological cause; usually, this happens for psychological reasons. There is usually no way to distinguish their experience from that due to tissue damage if we take the subjective report. If they regard their experience as pain, and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain. This definition avoids tying pain to the stimulus. Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause.