Although we are generally unaware of the fact, our nervous systems constantly monitor a variety of mechanical stimuli. Neurons in the spinal cord and brain stem extend sensory terminals to the body's surface and there provide us with sensitivity to touch. Other such neurons measure the tension in and extension of skeletal muscles. Sensory cells of the autonomic nervous system detect pressures within the body's hollow organs, including blood vessels, the bladder, and the gut. The most sensitive of our mechanical receptors are hair cells, the sensory receptors of the internal ear. Such cells underlie our sensitivities to sound, to linear accelerations (including that due to gravity), and to angular accelerations.