Have your physician check this out. It could just be an enlarged lymph node or something more serious. Not a sign of Arthritis or brain tumor, but may signal cancer depending on locatin (your description is vague).
Brain stem
The brain stem is what connects the spinal cord to the rest of the brain. More specifically, the medulla oblongota is what directly connects it.
The spinal cord connects tissues and organs to the brain.
The Brain stem connects these two portions of the brain.
There is no joint between brain and spinal cord. The spinal cord is a mass of neurones that is situated in the backbone which feeds in to the brain via neurones.
Midbrain-Apex
The cerebellum connects the cerebrum and the spinal cord.
They are both brains.
medulla oblangata
It connects your cornea to brain. And tells the brain what to see.
The spinal cord is attached to the brain stem located at the base of the brain. This stem is connected to the cerebellum.
Optic nerve. Well, it connects the retina to the brain. The muscles of the eye also connect to the brain, but not through the optic nerve.