[Music] Neurons are the basic units of the brain. Their
main function is to send electrical signals over short and long distances in the body, and
they are electrically and chemically excitable. The function of the neuron is dependent
on the structure of the neuron. The typical neuron consists of the dendrites, cell body, axon (including the axon
hillock), and presynaptic terminal. Dendrites are processes that branch
out in a tree-like fashion from the cell body. They are the main target for
incoming signals received from other cells. The number of inputs a neuron receives depends
on the complexity of the dendritic branching. Dendrites may also have small protrusions
along the branches known as spines. Spines are the sites of some synaptic contacts. Spines
increase the surface area of the dendritic arbor, which is an important factor
for receiving communication. The cell body, also known as the soma,
contains the nucleus and cellular organelles, including endoplasmic reticulum, Golgi apparatus,
mitochondria, ribosomes, and secretory vesicles. The nucleus houses the DNA of the cell,
which is the template for all proteins synthesized in the cell. The organelles
are responsible for cellular mechanisms like protein synthesis, packaging of
molecules, and cellular respiration. The axon is usually a long, single process
that begins at the axon hillock and extends out from the cell body. The axon hillock
is located where the cell body transitions into the axon. Axons can branch in order to
communicate with more than one target cell. The axon transmits an electrical
signal, called an action potential, from the axon hillock to the presynaptic terminal
where the electrical signal will result in a release of chemical neurotransmitters
to communicate with the next cell. The action potential is a very brief change in
the electrical potential, which is the difference in charge between the inside and outside of the
cell. During the action potential, the electrical potential across the membrane moves from a
negative value to a positive value and back. Many axons are also covered by a myelin sheath, a fatty substance that wraps around portions of
the axon and increases action potential speed. There are breaks between the myelin segments
called Nodes of Ranvier, and this uncovered region of the membrane regenerates the action potential
as it propagates down the axon in a process called saltatory conduction. There is a high
concentration of voltage-gated ion channels, which are necessary for the action
potential to occur, in the Nodes of Ranvier. The length of an axon is variable depending on
the location of the neuron and its function. The axon of a sensory neuron in your big toe needs
to travel from your foot up to your spinal cord, whereas an interneuron in your spinal cord may
only be a few hundred micrometers in length. Axon diameter is also variable and can be used
to differentiate different types of neurons. The diameter affects the speed at which the action
potential will propagate. The larger the diameter, the faster the signal can travel. Larger
diameter axons tend to have thicker myelin. The axon terminates at the presynaptic terminal or
terminal bouton. The terminal of the presynaptic cell forms a synapse with another neuron
or cell, known as the postsynaptic cell. When the action potential reaches
the presynaptic terminal, the neuron releases neurotransmitters into the synapse. The
neurotransmitters act on the postsynaptic cell. Therefore, neuronal communication requires both
an electrical signal (the action potential) and a chemical signal (the neurotransmitter). Most
commonly, presynaptic terminals contact dendrites, but terminals can also communicate
with cell bodies or even axons. Neurons can also synapse on non-neuronal
cells such as muscle cells or glands. The terms presynaptic and postsynaptic
are in reference to which neuron is releasing neurotransmitters and which is
receiving them. Presynaptic cells release neurotransmitters into the synapse and those
neurotransmitters act on the postsynaptic cell. Although these typical structural components can
be seen in all neurons, the overall structure can vary drastically depending on the location
and function of the neuron. Some neurons, called unipolar, have only one branch from the
cell body, and the dendrites and axon terminals project from it. Others, called bipolar, have
one axonal branch and one dendritic branch. Multipolar neurons can have many processes
branching from the cell body. Additionally, each of the projections can take many forms,
with different branching characteristics. The common features of cell body, dendrites,
and axon, though, are common among all neurons. [Music]