Sodium ions are positively charged (Na+) and enter the neuron starting at the axon hillock, which is where the axon and cell body come together. An influx of positively charged ions will cause the charge of the cell to increase.

When a neuron reaches threshold, it 'fires' an action potential down the axon using charge gradient to drive the propagation towards the axon terminal, where neurotransmitters will be released to relay the signal to the next cell.

Potassium ions are also positively charged (K+), but follow a gradient that promotes efflux over influx due to the equilibrium established by the sodium-potassium pump. As such, potassium flows out of a cells following depolarization, and actually results in the hyperpolarization observed immediately following an action potential.

The myelin sheath on neurons is a fatty sheath provided by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system that increases the speed of signal transmission through saltatory conduction. Myelin makes matter in the brain appear white. Areas of the axon that aren’t myelinated are called nodes of Ranvier.

Neuron function is highly dependent on ion concentrations. Sodium is required for depolarization, potassium for hyperpolarization and repolarization, and calcium is responsible for initiating neurotransmitter release. Vesicles of neurotransmitter are stored in the terminal end of the axon, opposite from the end with the axon hillock and cell body. When voltage-gated calcium channels open, the binding of calcium to these vesicles causes them to exocytose the neurotransmitter.

Myelin is the fatty substance that wraps around axons to create nodes of Ranvier. These nodes allow the depolarization signal to jump along the axon, rather than traveling fluidly. This method, known as saltatory conduction, allows the action potential to travel faster. Degeneration of myelin is associated with numerous neurological disorders.

It is true that the axon of one neuron transmits information to the dendrite(s) of another, but this contact is not direct. Neurons are separated by synapses, which are small gaps that neurotransmitters must cross in order to move from their original location to a new dendrite. The two neurons do not physically touch.

Rods and cones are contained in the human retina, but they differ in shape and function.

The functional difference between the two is that rods are more sensitive to light, but do not distinguish colors. Comparatively, cones are able to sense color and are less sensitive to distinctions fo light and dark. There are three different types of cone photoreceptors that have different sensitivity across the visible spectrum, with optimal responses to red, blue and green light, respectively.

Both types of receptors use rhodopsin as the visual pigment that is activated when exposed to light. This initiates a signaling cascade that causes cellular depolarization through sodium ion channels and the release of the neurotransmitter glutamate by bipolar neuron cells to further transmit the visual signal.

After neurotransmitters bind to the post-synaptic neuron, ion gated channels open. This allows for depolarization and the generation of another action potential.

The colloquial term “nerve” refers to a neuron—a nerve cell—that can transmit electrical signals throughout the human body. Nerves can be grouped into two categories: motor neurons and sensory neurons.

The somatic nervous system is a part of the peripheral nervous system and controls voluntary body movements. The central nervous system involves only the brain and spinal cord. The autonomic nervous system is not under voluntary control.

Glial cells are important in maintaining nervous system homeostasis. The major functions of glial cells include insulating neurons, holding them in place, and supplying them with nutrients and oxygen. Glial cells also degrade pathogens and dead neurons.

Ganglia are clusters of nerve cells that are a part of the autonomic nervous system, which controls the function of organs.

Signal transmission from presynaptic terminal to postsynaptic terminal occurs when neurotransmitters bind to receptors on the postsynaptic terminal. Once the neurotransmitter binds to the receptors on the postsynaptic neuron's cell membrane, the postsynaptic cell can be induced (excited) to fire another action potential, or inhibited based on the type of neurotransmitter and/or receptor that is bound.