Von Frey's hairs--a review of their technology and use--a novel automated von Frey device for improved testing for hyperalgesia.

We describe a device which allows the mechanical sensitivity of trigeminovascular sensory neurons to be monitored over extended time periods. The device can be used to stimulate either the skin or dura mater and consists of a solenoid-driven plunger to which are fixed interchangeable von Frey hairs. The solenoid can be attached to a stereotaxic carrier and mounted on a stereotaxic frame to allow precise positioning over the receptive field. The device is driven from the synchronization signals of a standard stimulator via TTL circuitry and a relay driver, to allow stimulation by a single or multiple stimuli. The advantages of the device over manual stimulation include the reproducibility of the site of stimulation; the ability to apply a known force for a known time; the ability to measure response latencies to millisecond precision and to compare them to latencies to other stimuli and; easy interface with computer-control. We discuss some of the drawbacks of the von Frey system as usually used and illustrate the use of the new device with results from experiments on peripherally induced sensitization.