WordNet sense 1 (any sharply pointed projection):

spur, acantha

WordNet sense 2 (a small sharp-pointed tip resembling a spike on a stem or leaf):

pricker, prickle, sticker, thorn, spikelet

WordNet sense 3 (the series of vertebrae forming the axis of the skeleton and protecting the spinal cord):

back, backbone, rachis, spinal column, vertebral column