Nervous signals are transmitted either electrically or chemically. While electrical signals operate inside the neuron, chemical mechanisms operate between different neurons, i.e., at the synapses. The electrical discharge starts at the cell body and then travels down the axon to the various synaptic connections. In the inactive state, the interior of the neuron (protoplasm) is negatively charged compared to its surroundings. This resting potential (-70 mV) is created by the property of the cell membrane, which is impenetrable for Na+ ions, causing a deficiency of positive ions in the protoplasm.
The signals, which arrive from the synaptic connections, resulting in a transient weakening or depolarization of the resting potential. When this is reduced below –60 mV, the membrane suddenly loses its impermeability against the Na+ ions. The Na+ ions now enter into the protoplasm and make it more positive when compared to its surroundings. The membrane then gradually recovers its original properties and regenerates the resting potential over a period of several milliseconds. During this recovery period, the neuron cannot get further excited. When the recovery is completed, the neuron is in its resting state and can fire again.
The discharge signal, which initially occurs in the cell body, propagates along the axon to the synapses. The speed of the discharge signal along the nerve fiber varies. In the cells of the human brain, the signal travels with a velocity of 0.5-2 m/s. To increase the speed of propagation, an electrically insulating myelin sheath covers the individual segments of the axons of peripheral neurons. This myelin sheath is interrupted at the Ranvier nodes. The myelin sheath causes the signal to propagate along the axon as in a wave, from one Ranvier node to the next, triggering almost instantaneous discharge within the whole myelinated segment.
This mode of propagation is called saltatory conduction and allows for transmission velocities of up to 100m/s. The discharge signal traveling along the axon comes to a halt at the synapses. Transmission of the signal across the synaptic gap is by means of chemical mechanisms. Substances called neurotransmitters are released in small amounts from vesicles present in the endplate. The transmitter release is triggered by the influx of Ca++ ions into the presynaptic axon during the depolarization caused by the flow of Na+ ions.
The neurotransmitter molecules travel across the synaptic cleft, and upon their arrival at special receptors, modifies the conductance of the postsynaptic membrane for certain ions (Na+, K+, Cl-, etc.), which then flow in or out of the neurons, causing a polarization or depolarization of the local postsynaptic potential. After their action, the neurotransmitter molecules are quickly broken up by enzymes, which are less potent in changing the membrane conduction.