The nonlinear properties of the neuronal output are due to located at the axonal
hillock voltage-gated Na+ channels that exhibit positive feedback. When a critical
threshold of -55mV of the transmembrane voltage is reached in the axonal hillock
more and more sodium ion channels do open. Considering the decrement along
the dendrites a rough estimate of 300-400 spatially and temporally summated
EPSPs are needed in order to elevate the voltage with 15-20mV in the soma and
to evoke axonal spike. The magnetic and the electric field strength are expected
to be the same as in the dendrites with maximal magnetic strength of 10-10 T and
maximal electric strength of 10 V/m.
Axonal morphophysiology
Neurons output information via long projections called axons. The diameter of
axons varies from 1μm to 25 μm in humans. Axons with small diameter could be
non-myelinated. However the larger axons in CNS are ensheathed by multiple
membrane layers known as myelin. Myelin is produced by supportive glial cells
called oligodendrocytes. Oligodendrocytic membrane rotates around the axon
and forms multiple-layered phospholipid structure that insulates the axon from
the surrounding environment. One axon is insulated by numerous
oligodendrocytes however there are tiny places where the axonal membrane is
non-myelinated. They are located between two oligodendrocytic membranes and
are called nodes of Ranvier. In the peripheral nervous system the myelin is
produced not by oligodendrocytes but by Schwann cells.
When voltage change in the axonal hillock reaches threshold potential of - 55mV,
action potential begins. Membrane becomes depolarized due to Na+ gates
opening, allowing Na+ to rush into cell through voltage-gated Na+ channels. This
is an all-or-none event, in that once threshold is reached, it will happen. When
inside of membrane is depolarized to +40mV, Na+ gates shut and K+ gates open.
K+ rushes out trough open K+ gates (Na+ gates are closed and inactive).
46
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