The axial skeleton is comprised of all the bones that are oriented vertically (called the longitudinal axis). These include:
Bones of the skull (head)
Vertebra (bones of the spine) which includes the sacrum and coccyx (i.e., your tailbone)
The axial skeleton is distinguished from the appendicular skeleton which is made of the bones of the upper and lower extremities. In the upper extremity, the bones of the appendicular skeleton consist of the upper arm bone or humerus, the bones of the forearm, which are called radius and ulna, the wrist bones which are numerous and are also known as the carpal bones, plus the metacarpal bones, i.e., long bones that connect between the wrist bones and the finger bones. The finger bones, which are technically called the phalanges, are also a part of the bones of the appendicular skeleton in the upper extremity.
In the lower extremity, the story is similar. The bones of the appendicular skeleton are the femur or upper thigh bone, the kneecap, the lower leg bones (tibia and fibula) the ankle and heel bones, the metatarsals (similar to the metacarpal bones in the hand) and again the phalanges, which in this case are the toes.
Axial Back Pain
Pain due to problems in the spine can be confined to the axial skeleton or it can extend to an extremity as in the case of radiculopathy (irritation of a spinal nerve root).
Historically, axial back pain has referred to the pain that stays in the back, i.e., pain that does not affect nerves. Pain that does not affect nerves, and instead is related to tissue damage is called nociceptive pain.
When pain originating in the axial skeleton extends to an extremity, it's typically due to radiculopathy, (again, if the cause is an irritated spinal nerve root) or sciatica. Keep in mind, though, the sciatica is a less specific term that medical and lay people alike use to describe the symptoms associated with pain and electrical sensations that go down one leg or arm. When understood this way, sciatica can have a number of causes, which include, but are not limited to, a pressured or irritated spinal nerve root.
But in 2013, German researchers found that in 10% of study participants (1083 in total) axial low back pain had both nociceptive and nerve components. The researchers described a number of axial low back pain profiles based on the type of nerve symptoms present in these "mixed" cases. They say categorizing patients according to such profiles is likely a better way to accurately diagnose the pain and render the treatment that best fits the patient.
The researchers also found that depression, anxiety and sleep problems (called co-morbidities) tend to accompany axial low back pain, as well as low back pain in general. And finally, they found that those who had disc surgery were more likely to have a neuropathic component to their axial low back pain.