Section Summary

The basic cellular building blocks of the nervous system are neurons and glia. While sharing all features of eukaryotic cells, including membrane bound organelles and the central dogma, neurons and glia differ in some important ways that allow them to serve the functions of the nervous system. Neurons are specialized for communication via chemical and electrical signals with other cells. Glia serve in a number of support roles that are vital for neurons, including signaling. Our ability to visualize these cells and their architecture has revolutionized modern neuroscience.

Nervous systems vary across the animal kingdom in design and complexity. From neural nets to ganglia to centralized brains, there is a great deal of diversity. At the same time, due to evolution, all nervous systems share fundamental similarities. The human nervous system is a typical vertebrate system divided into a peripheral and central nervous system. Both are composed of white (myelinated axons) and gray matter (neuron cell bodies, dendrites and unmyelinated axons). Neurons do not operate in isolation but rather interact with each other in neural circuits that can vary in complexity. Neuroscientists have been working to understand the neural circuitry found in nervous systems by developing wiring maps or connectomes. There are several large circuit mapping projects underway seeking to map the human brain.

The CNS is composed of the brain and spinal cord and is responsible for receiving and processing information from the PNS and ultimately controlling the entire body. It develops from a section of the ectoderm called the neural plate which eventually becomes the neural tube and ultimately separates into different brain regions and the spinal cord. The spinal cord is divided into 5 anatomical regions and houses ascending and descending nerve tracts, horns and roots.

The human brain is derived from 3 major embryonic divisions (the forebrain, midbrain and hindbrain), which is further subdivided into 5 regions during development. The adult brain derivatives can be organized according to these 5 regions. While we can assign some general functions to major brain regions, we should view the brain as having built in ‘degeneracy’—a behavior or brain function can be supported by multiple regions of the brain rather than a stringent one-to-one function to region ratio.

The peripheral nervous system is composed of the somatic and autonomic divisions and includes all the nerves and ganglia outside of the brain and spinal cord. The somatic division is under voluntary control and sends commands from the central nervous system to muscles, for example. The autonomic division is generally involuntary and controls functions such as heart rate and digestion. The autonomic system can be further divided into the sympathetic, parasympathetic and enteric nervous systems.