Within the cell, all neurons transmit electrical impulses in the same way. A neuron that senses an odor, for example, conveys information along its length in the same way as a neuron that regulates the movement of a bodily part does. The activated neuron's specific connections are what distinguishes the sort of information being conveyed.

Sorting neural pathways and connections is thus required when interpreting nerve signals. In increasingly sophisticated animals, this processing is mostly carried out by groupings of neurons arranged into a brain or smaller clusters known as ganglia.

• The term ganglia refer to more complex animals, this processing is carried out largely in groups of neurons organized into a brain or into simpler clusters called ganglia.

In all but the simplest animals, specialized populations of neurons handle each stage of information processing.

• Sensory neurons, like those in the snail’s siphon, transmit information about external stimuli such as light, touch, or smell or internal conditions such as blood pressure or muscle tension.

• Interneurons from the local circuits connecting neurons in the brain or ganglia. Interneurons are responsible for the integration (analysis and interpretation) of sensory input.

• Motor neurons transmit signals to muscle cells, causing them to contract. Additional neurons that extend out of the processing centers trigger gland activity.

The neurons that carry out the integration in many animals are arranged in a central nervous system (CNS). The peripheral nervous system is made up of neurons that transport information into and out of the CNS (PNS). Nerves are formed when the axons of neurons are tangled together.

A neuron's structure can range from simple to highly complicated depending on its role in information processing (as shown in the attached images). Some interneurons, for example, have extremely branching dendrites and can receive input through tens of thousands of synapses. Similarly, neurons that send information to a large number of target cells do so via heavily branching axons.

The snail explores its environment with its tubelike siphon to provide sensory input to the nervous system, collecting odors that may suggest a nearby fish (as shown in the attached image).

During the integration step, neural networks analyze this input to identify whether or not a fish is there and, if so, where the fish is positioned. The processing center's motor output subsequently starts the attack, activating neurons that cause the harpoon-like teeth to be released toward the target.

The resting potential of a neuron is established by ion pumps and ion channels.

Ionic gradients cause a voltage differential, or membrane potential, across a cell's plasma membrane. Outside, the concentration of Na+ is higher than inside; the opposite is true for K+. The plasma membrane of resting neurons includes numerous open potassium channels but few open sodium channels. The diffusion of ions, primarily K+, through channels produces a resting potential, with the interior being more negative than the outside.

Axons transmit messages through action potentials. Gated ion channels in neurons open and shut in response to inputs, causing changes in membrane potential. A hyperpolarization occurs when the amplitude of the membrane potential increases; a depolarization occurs when it decreases. Graded potentials are changes in membrane potential that vary continuously with the intensity of stimulation.

An action potential is a short, all-or-nothing depolarization of the plasma membrane of a neuron. When the membrane potential is brought to threshold by a graded depolarization, numerous voltage-gated ion channels open, causing an influx of Na+ to rapidly bring the membrane potential to a positive value.

A nerve impulse is propagated through the axon from the axon hillock to the synaptic terminals by a sequence of action potentials. Conduction speed rises with axon diameter and, in many vertebrate axons, with myelination.

Action potentials in myelin-insulated axons appear to hop from one Ranvier node to the next, a phenomenon is known as saltatory conduction.

Electrical current travels straight from one cell to another at an electrical synapse. Depolarization causes synaptic vesicles to fuse with the terminal membrane and release neurotransmitters into the synaptic cleft in a chemical synapse.

The neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane at numerous synapses, resulting in an excitatory or inhibitory postsynaptic potential (EPSP or IPSP). The neurotransmitter then diffuses out of the cleft, where it is either taken up by neighboring cells or destroyed by enzymes. On the dendrites and cell bodies of a single neuron, there are many synapses.

The temporal and spatial summation of EPSPs and IPSPs at the axon hillock determines whether or not a neuron gene is expressed.