A system for alleviating spasticity of a skeletal muscle having an associated motor nerve. The system may include a needle probe having at least one needle. The at least one needle has a proximal end, a distal end, and a needle lumen therebetween. The needle is configured for insertion proximate to the nerve. A cooling fluid supply lumen can extend distally within the needle lumen to a distal portion of the needle lumen. A cooling fluid source is couplable to the cooling fluid supply lumen to direct cooling fluid flow into the needle lumen. A controller having at least one processor configured to implement a spasticity treatment algorithm for controlling the cooling fluid source so that liquid from the cooling flow vaporizes within the needle lumen to provide a treatment phase to the motor nerve such spasticity of the skeletal muscle is mitigated.