Neurons: basic unit of nervous system, operate through electrical impulses (ap), communicate through chemical signals (nt) E. o wilson- 1 neuron is no neuron, 100 billion neurons that interact allows us to be self-directed, able to regulate our behaviour, have ideas, etc. 3 types: sensory (afferent), motor (efferent), interneurons (outnumber motor and sensory- facilitate transmission of info within cns) Resting membrane potential: when neuron is at rest, inside is slightly more (-) than outside. Ap travels along axon in a wave= propagation. Myelin- fatty insulation that surrounds cell axon polarization creates electrical e needed to fire neuron. Increases speed of neural impulses by preventing interference from adjacent cells. Deterioration of myelin sheath leads to multiple scerlosis, slows down neural impulses movement, sensation, coordination become impaired. Myelin , axons exposed and eventually break down. All or none principle: if the sum of excitatory and inhibitory signals leads to positive change in voltage that exceeds neuron"s firing threshold ap fires.