  We investigate spectral properties of the Laplacian in $L^2(Q)$, where $Q$ is
a tubular region in $\mathbb{R}^3$ of a fixed cross section, and the boundary
conditions combined a Dirichlet and a Neumann part. We analyze two
complementary situations, when the tube is bent but not twisted, and secondly,
it is twisted but not bent. In the first case we derive sufficient conditions
for the presence and absence of the discrete spectrum showing, roughly
speaking, that they depend on the direction in which the tube is bent. In the
second case we show that a constant twist raises the threshold of the essential
spectrum and a local slowndown of it gives rise to isolated eigenvalues.
Furthermore, we prove that the spectral threshold moves up also under a
sufficiently gentle periodic twist.
