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.

The signaling cascade in response to light  in the human eye is caused by two types of photoreceptors present in the retina—rods and cones. Rods provide dark and light vision (black and white) and cones are capable of discerning color along the visible spectrum.

Rhodopsin is the visual pigment in photoreceptors and is made up of a light-absorbing vitamin-like molecule (retinal) that is bound to a cell membrane protein called opsin. The absorbtion of light by rhodopsin results in chemical bond shifting and a change in the molecule's shape. This causes rhodopsin to activate. The active rhodopsin initiates a G-protein cascade that causes sodium channels on the receptor's cell membrane to close. This prevents sodium ions from entering the cell, resulting in a build-up of ions in the extracellular space in comparison to the cell interior. The membrane potential is directly related to this difference in concentration; a higher concentration of positive ions outside the cell will result in hyperpolarization.

In response to being in a hyperpolarized state, the photoreceptor cell will stop its release of the neurotransmitter glutamate across the cell synapse with retinal neurons. Glutamate is an inhibitory neurotransmitter; halting the release of glutamate will leave the neurons capable of stimulation by visual signals.

Nervous tissue is responsible for the transmission of electrical impulses throughout the human body. This transmission occurs in nerve cells called neurons. It regulates sensory input, muscle control, homeostasis, and mental activity.

The PNS is the part of the nervous system that is comprised of all the nerves located outside of the central nervous system. The PNS nerves are not protected by bone; therefore, they are susceptible to toxins and injury. The PNS can be further divided into the somatic and autonomic nervous systems.

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.

Neuron cells are functional units of nervous tissue that transmit electrical signals. Neurons typically are composed of a soma, dendrites, and an axon. The soma is the body of the cell, the dendrites are branched projections that receive signals, and the axon conducts signals away from the cell body.

Axons are the parts of neurons that transmit electrical signals away from the cell body and to other neurons. The axon branches off of the soma and is long with a constant radius. Parts of the axon are myelinated, meaning that it is insulated to allow for salutatory conduction. Gaps in insulation are referred nodes of Ranvier.

In neurons and many other cells, electrical signal transmission requires action potential. Action potential can be defined as the rapid rise and fall of membrane potential that allows for signal propagation. In the case of neurons, reaching action potential allows the electrical signal to travel down to axon, which permits signal transduction.

The CNS is composed of the brain and spinal cord. It receives and regulates information from within the body.

Ganglia are described as clusters of nerve cells that are found in the autonomic nervous system.