\version "2.18.2"

{
  \override TextSpanner.bound-details.right.arrow = ##t
  \override TextSpanner.bound-details.left.text = "s.t."
  \override TextSpanner.bound-details.right.text = "s.p."
  \override TextSpanner.bound-details.left.stencil-align-dir-y = #CENTER
  \override TextSpanner.bound-details.right.stencil-align-dir-y = #CENTER

  a'1\startTextSpan 
  gis'1\stopTextSpan 
}