Transcript MODEL OF WHOLE NEURON

MODEL OF
WHOLE
NEURON
MODEL OF WHOLE NEURON
This section
brings together
the entire
neuron,
combing the
dendrite, soma,
axon, and
presynaptic
terminal
MODEL OF WHOLE NEURON
• Neurons are
responsible for the
transmission and
analysis of all
electrochemical
communication within
the brain and other
parts of the nervous
system.
MODEL OF WHOLE NEURON
Dendrites and axons can be modeled as a
series of cylindrical compartments, each
connected together with an axial resistance
Note:
Both
dendrite and
axon are
connected to
the soma.
• Each neuron is composed of a cell
body called a soma, a major fiber
called an axon, and a system of
branches called dendrites.
• Axons, also called nerve fibers,
convey electrical signals away from
the soma and can be up to 1 m (3.3
ft) in length. Most axons are
covered with a protective sheath of
myelin, a substance made of fats
and protein, which insulates the
axon. Myelinated axons conduct
neuronal signals faster than do
unmyelinated axons. Dendrites
convey electrical signals toward the
soma, are shorter than axons, and
are usually multiple and branching.
MODEL OF WHOLE NEURON
• Note :
the dendrite and axon do not
have to have constantdiameter cylinders, but may
narrow toward the periphery
Figure 11.28 illustrates the axon compartment with active channels at
the axon hillock and the node of Ranvier
MODEL OF WHOLE NEURON
• To model the myelinated
portion of the axon, a set
of passive compartments,
like the dendrite
compartment, can be
used with capacitance,
passive ion channels, and
axial resistance
Figure 11.33 is a portion of the axon with myelin sheath, with three
passive channels, and an active component for the node of
Ranvier. The structure in Figure 11.33 can be modified for any
number of compartments as appropriate. The soma can be
modeled as an active or passive compartment depending on the
type of neuron.
To model the neuron in Figure 11.33,
Kirchhoff ’s current law is applied, giving
MODEL OF WHOLE NEURON
• Because neurons usually
have other channels in
addition to the three of the
squid giant axon, a model of
the neuron should have the
capability of including other
channels, such as a fast
sodium channel, delayed
potassium conductance, or
high-threshold calcium
conductance
Additional ion channels can be
added for each compartment in
Equation 11.49, by adding
for each compartment for
channels i ¼ 1, n. The values of
Cm, RTH, Ra,
and Gi are dependent on the
size of the compartment and the
type of neuron modeled.
MODEL OF WHOLE NEURON
• A complete model of the neuron can be constructed
by including as many dendritic branches as needed,
each described using Figure 11.17, each modeled by:
a soma with passive or active properties using
either:
MODEL OF WHOLE NEURON
an axon using Equation 11.49
• Except for the terminal
compartment, two inputs are
needed for the dendrite
compartment; the input defined by
the previous compartment’s
membrane potential and the next
compartment’s membrane
potential.
• Additional neurons can be added
using the same basic neuron,
interacting with each other using
the current from the adjacent
neuron (presynaptic terminal) to
stimulate the next neuron.
MODEL OF WHOLE NEURON
Many of the principles
•
discussed in this chapter also
apply to these other cells but
the action potential defining
equations are different. For
example, the cardiac action
potential can be defined with a
DiFranceso–Noble, Luo–Rudy,
or other models rather
than a Hodgkin–Huxley model
of the neuron.