communication CNS brain spinal cord reflex PNS nerve somatic NS afferent sensory efferent motor autonomic NS sympathetic parasympathetic neuron dendrite cell body axon glial cell Schwann cell myelin sheath sensory neuron motor neuron interneuron impulse electrochemical differentially permeable active transport Na+/K+ pump electrical polarity at rest action potential depolarization repolarization recovery refractory synapse neurotransmitter reflex arc