The nerve impulse is the sum of mechanical, chemical and electrical disturbances created by a stimulus in a neuron. The conduction of the nerve impulse can be divided into two main phases – resting membrane potential and action membrane potential. Neurons are excitable cells. They may be stimulated by physical, mechanical, chemical or electrical stimuli. The axoplasm inside the axon contains a high concentration of potassium ions, negatively-charged proteins and a low concentration of sodium ions. On the contrary, the extracellular fluid outside the axon contains a low concentration of potassium and a high concentration of sodium. This differential permeability is maintained by a sodium-potassium pump present inside the membrane. The electrical potential difference across the neural membrane in an unexcited nerve fibre is called resting potential, and the neuron is called a polarised nerve fibre. The reversal of polarity across the two sides of the membrane is called depolarization. The action potential travels as a wave of depolarisation along the length of a nerve fibre in a particular direction and is called a nerve impulse. A junction helps transmit the nerve impulse from one neuron to another. These junctions are called synapses. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft. There are two types of synapses on the basis of nature of transfer of information. These are chemical synapses and electrical synapses. Each synaptic vesicle contains neurotransmitter chemical molecules such as Acetyl choline. The neurotransmitters thus released bind to their specific chemoreceptors present on the post-synaptic membrane of the dendron. This binding opens sodium ion channels that allow the entry of ions to generate a new potential in the post-synaptic neuron.