The structural unit of nervous tissue is “Neuron”. The function of the neuron is to transmit impulses. Accordingly nerve cell can be divide into three types as Sensory neuron, Inter neuron and Motor neuron. Sensory neuron transmit impulses from sensory organs to central nervous system and Motor neuron transmit impulses from central nervous system to effector.  As well as Inter neuron connects the sensory neuron with motor neuron.
Accordingly we can conclude that neurons are the reason for your reactions when a thorn pricks, etc.