Neurons

The nervous system is composed of neurons of different sizes and functions.  Before we begin describing the structure and function of neurons, it is helpful to learn about the neural doctrine.  Neural doctrine states that (1) the brain is composed of distinct neurons that (2) transmit information to other neurons across gaps.  These gaps are called synapses.  The discovery of the electron microscope, in the 1950’s confirmed the neural doctrine.


Structurally, the neuron can be divided into four functional zones: the input, integration, conduction and output zones.  The input zone is comprised of dendrites, which are activated by chemicals that they encounter.  The integration zone is the body of the neuron, where information is processed.  The conduction zone consists of the axon, which sends outgoing information away from the cell body.  There is typically one axon, but that axon can divide into multiple axon collaterals, which exert the effect of the axon on many other neurons.  Finally, the output zone, composed of axon terminals, send these messages out of the neuron through the cell membrane.


The hundreds of different neurons can be classified by shape, size and function.  Multipolar neurons have single axons, but multiple dendrites.  Bipolar neurons have only one axon and one dendrite.  Monopolar neurons have one extension that branch from a receptive end to output end.  Larger neurons typically convey information more rapidly.  Neurons can be classified as sensory-, motor- and interneurons.


In addition to neurons, there are different types of glial cells, which are typically smaller, and hold neurons in place, provide conductive insulation and remove dead tissue.  There are four types of glial cells: astrocytes, microglia, oligodendrocytes and Schwann cells.  Astrocytes serve as structural scaffolding for the neurons while microglia extract dead tissue from the nervous system.  Oligodendricytes insulate axons inside the brain, while Schwann cells insulate axons outside of the brain.  Both work primarily by enhancing the conduction of neural impulses.


When a neural impulse travels from one neuron to another, it has to cross a gap called the synapse.  A package of neurotransmitter travels to the outgoing membrane of the axon, referred to as the presynaptic membrane.  This package, called a vesicle, bursts open upon contact with the postsynaptic membrane and releases it’s contents into the synaptic cleft.  Once these chemicals are released into the cleft across the synaptic cleft they can make it across the cleft, where they activate a chemical reaction on the postsynaptic membrane.  The postsynaptic cleft is covered with dendritic spines, which are outgrowths that increase the receptive area of postsynaptic membrane.  The plasticity of dendritic growth plays a significant role in the neuroplasticity of the brain.  Together, neurons and glial cells form complex neural networks, which allow us to process information.