Tension headaches are the most common form of headaches. They are caused by muscle tightening in the back of the neck and/or scalp, often triggered by emotional stress, fatigue, or depression.
The muscle contraction leads to a slight decrease in blood flow to the surrounding areas and an irritation of pain fibers in the skin, muscles, and blood vessel walls. If out-of-control muscle contraction (spasm) can be corrected then this progression of events can be reversed.
Muscles that control the motion of the head and neck contract in response to the activity of nerves which originate from the brainstem.
Greater stimulation means more contraction. Less stimulation means less contraction. There are different nerves on the right and left side of your body. This means that the right side muscles controlling the head and neck can be much tighter because of the right side nerves over functioning, or vice versa.
The brainstem rests, anatomically, within spinal canal created by the skull, atlas (C1) and often also the axis (C2). Poor alignment and function of these bones causes direct mechanical stress on the brainstem and all the nerves it contains. Just think how a garden hose responds when you start kinking and twisting it!