How the Nervous System Works
The nervous system consists of the central nervous system (the brain, retina, and spinal cord), the peripheral nervous system (the sensory and motor nerve axons that connect the central nervous system to the limbs and organs). The peripheral nervous system also includes the autonomic nervous system (which regulates body processes such as digestion and heart rate), and the enteric nervous system, which controls the gastrointestinal system.
The important role of neurons
All the divisions of the nervous system are based universally on the functions of neurons. Neurons are specialized cells that process information. Like all cells, they are unbelievably complicated in their own right. All nervous systems in all animal species have four basic types of functional cells:
- Sensory neurons: These neurons tell the rest of the brain about the external and internal environment.
- Motor (and other output) neurons: Motor neurons contract muscles and mediate behavior, and other output neurons stimulate glands and organs.
- Projection neurons: Communication neurons transmit signals from one brain area to another.
- Interneurons: The vast majority of neurons in vertebrates are interneurons involved in local computations. Computational interneurons extract and process information coming in from the senses, compare that information to what’s in memory, and use the information to plan and execute behavior. Each of the several hundred distinguishable brain regions contains several dozen distinct types or classes of computational interneurons that mediate the function of that brain area.
What really distinguishes the nervous system from any other functioning group of cells is the complexity of the neuronal interconnections. The human brain has on the order of 100 billion neurons, each with a unique set of about 10,000 synaptic inputs from other neurons, yielding about a quadrillion synapses — a number even larger than the U.S. national debt in pennies! The number of possible distinct states of this system is virtually uncountable.
Computing in circuits, segments, and modules
The largest part of the brain, which is what you actually see when you look at a brain from above or the side, is the neocortex. The neocortex is really a 1.5 square foot sheet of cells wadded up a bit to fit inside the head. The neurons in the neocortex form a complex neural circuit that is repeated millions of times across the cortical surface. This repeated neural circuit is called a minicolumn.
The brain contains many specialized areas associated with particular senses (vision versus audition, for example) and other areas mediating particular motor outputs (like moving the leg versus the tongue). The function of different brain areas depends not on any particular structure of the minicolumns within it, but its inputs and outputs.
So even though the cell types and circuits in the auditory cortex are similar to those in the visual and motor cortices, the auditory cortex is the auditory cortex because it receives inputs from the cochlea (a part of the ear) and because it sends output to areas associated with processing auditory information and using it to guide behavior.
Many other parts of the nervous system also are made up of repeated circuits or circuit modules, although these are different in different parts of the brain:
- The spinal cord consists of very similar segments (cervical, thoracic, lumbar, and so on), whose structure is repeated from the border of the medulla at the top of the spinal cord to the coccygeal segments at the bottom.
- The cerebellum, a prominent brain structure at the back of the brain below the neocortex, is involved in fine-tuning motor sequences and motor learning. Within the cerebellum are repeated neural circuits forming modules that deal with motor planning, motor execution, and balance.
All the modules that make up the central nervous system are extensively interconnected. If you were to take a section through about any part of the brain, you’d see that the brain has more white matter, or pale-appearing axon tracts (the neural “wires” that connect neurons to each other) than darker gray matter (neural cell bodies and dendrites, which receive inputs from other neurons and do the neural computations).
Here’s why: The brain uses local interconnections between neurons to do computations in neural circuits. However, any single neuron contacts only a fraction of the other neurons in the brain. To get to other brain modules for other computations, the results of these computations must be sent over long distance projections via axon tracts of communication neurons.
What a charge: The role of electricity
Most neurons are cells specialized for computation and communication. They have two kinds of branches: dendrites (which normally receive inputs from other neurons) and axons (which are the neuron’s output to other neurons or other targets, like the muscles) emanating from their cell bodies.
Neuronal dendrites may be hundreds of micrometers in length, and neural axons may extend a meter (for example, axons run from single cells in the primary motor cortex in your brain down to the base of your spinal cord). Because the neuron is lengthened by the dendrites and axons, if the neuron is going to process signals rapidly, it needs mechanisms to help that intracellular communication along. That mechanism? Electricity, whose conduction down the axon is aided by myelin wrapping from glial cells.
Neurons use electricity to communicate what is happening in different parts of the neuron. The basic idea is that inputs spread out all over the dendrites and cause current flow from the dendrites into the cell body. The cell body converts this changing electrical current into a set of pulses sent down its axon to other neurons.
Understanding the nervous system’s modular organization
The nervous system has an overall modular organization. Neurons participate in local circuits consisting of several hundred neurons composed of a dozen or two (or three, or sometimes four!) different types of neurons. These local circuits perform neural computations on inputs to the circuit and send the results to other circuits as outputs via projection neurons.
Local circuits form modules that perform certain functions, like seeing vertical lines, hearing 10,000 Hz tones, causing a particular finger muscle to contract, or causing the heart to beat faster. Groups of similar modules form major brain regions, of which there are several hundred, give or take. Modules in the brain, spinal cord, peripheral nervous system, and autonomic nervous system all work together to maintain your survival by regulating your internal environment and managing your interaction with the external environment.
Of course, humans do more than just survive. They have feelings and memories and curiosity and spiritual yearnings. They are capable of language, self-reflection, technology, and curiosity about their place in the universe.