Which fibers are affected earliest in spinal anaesthesia?

Local anaesthetic solution injected into the subarachnoid space blocks conduction of impulses along all nerves with which it comes in contact, although some nerves are more easily blocked than others. Stimulation of the motor nerves causes muscles to contract and when they are blocked, muscle paralysis results. Sensory nerves transmit sensations such as touch and pain to the spinal cord and from there to the brain, whilst autonomic nerves control the caliber of blood vessels, hea rate, gut contraction and other functions not under conscious control. Generally, autonomic and sensory fibres are blocked before motor fibres.