Action Potential

In the mechanism of action potential, when
input is present, neurons have a membrane potential which is around -70
millivolts and without input, the resting potential will remain the same. However,
excitatory or inhibitory inputs, which can come into the soma or the axon, but
are more likely to enter through the dendrites, will cause changes to the
resting potential, also known as the graded potential. Graded potentials can be
both depolarizing, also known as excitatory potential, and hyperpolarizing,
also known as inhibitory potential. Action potentials on the other hand, always
cause depolarization and reversal of the membrane potential. Whether an excitatory
or inhibitory potential, movement to the membrane potential is involved.
Meaning, they can be closer to or farther away from the threshold potential, which
lands at around -50 millivolts. As polarization from both excitatory and inhibitory
input spreads along the membrane, the size of the graded potential gets smaller.
Spreading action potentials down the axons of neurons is what makes neurons capable
of transmitting information over a wide range of distances. Amplitude is
all-or-none when it comes to action potential. Another big difference between
graded and action potentials is that action potentials do not decay with
distance. Action potentials down an axon are unchanged no matter how long the
axon is and are usually very fast. Large-diameter axons lead to action potentials
being very fast. Axons that have a myelin sheath, known as myelinated axons, also
lead to action potentials being faster than axons that don’t have a myelin
sheath. This makes sense because myelin sheaths are usually on larger-diameter
axons. Action potentials run slower through the gaps between the myelinated
segments, known as the nodes of Ranvier than through the myelinated segment. This
occurrence is called saltatory conduction. The word “saltatory” comes from
a Latin word for jumping. When defining jumping in action potential, you are explaining
that action potential jumps from node to node rather than having a steady,
regular conduction along the axon.