Nerve Cell Physiology
by
Dr.Rafea Al-Fayyadh
Al-fallujah Medicine College
2016
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The Cells of the Nervous System
• The human nervous system is comprised of two kinds of cells:
•
Neurons
•
Glia
• The human brain contains approximately
100 billion
individual
neurons.
• Behavior depends upon the communication between neurons.
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The Cells of the Nervous System
• Like other cells in the body, neurons contain the following
structures:
1. Membrane
2. Nucleus
3. Mitochondria
4. Ribosomes
5. Endoplasmic reticulum
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The Cells of the Nervous System
• The
membrane
refers to the structure that separates the inside of
the cell from the outside environment.
• The
nucleus
refers to the structure that contains the chromosomes.
• The
mitochondria
are the structures that perform metabolic
activities and provides energy that the cells requires.
• Ribosomes
are the sites at which the cell synthesizes new protein
molecules
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Embedded in the membrane are protein channels that permit
certain ions to cross through the membrane at a controlled rate.
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The Cells of the Nervous System
• Neuron cells are similar to other cells of the body but have a
distinctive shape.
• A
motor neuron
has its soma in the spinal cord and receives excitation
from other neurons and conducts impulses along it axon to a muscle.
• A
sensory neuron
is specialized at one end to be highly sensitive to a
particular type of stimulation (touch, temperature, odor etc.)
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The components of motor neuron.
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The components of sensory neuron
Note that the soma is located on a stalk off the main trunk of the axon
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The Cells of the Nervous System
• All neurons have the following major components:
1. Dendrites.
2. Soma/ cell body.
3. Axon.
4. Presynaptic terminals.
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The Cells of the Nervous System
• Dendrites
: branching fibers with a surface lined
with synaptic receptors responsible for
bringing
in information from other neurons
.
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The Cells of the Nervous System
• Soma:
contains the nucleus, mitochondria, ribosomes, and
other structures found in other cells.
responsible for the metabolic work of the neuron
.
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The Cells of the Nervous System
• Axon:
thin fiber of a neuron responsible for
transmitting nerve
impulses
away to other neurons, glands, or muscles.
• Some neurons are covered with an insulating material called the
myelin sheath
with interruptions in the sheath known as
nodes of
Ranvier.
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The Cells of the Nervous System
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Nodes of Ranvier
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Nodes of Ranvier
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The Cells of the Nervous System
• Presynaptic terminals
refer to the end points of an
axon
responsible
for
releasing
chemicals
to
communicate with other neurons.
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The Cells of the Nervous System
• Terms used to describe the neuron include the
following:
• Afferent axon
- refers to bringing information into a
structure.
• Efferent axon
- refers to carrying information away from a
structure.
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The Cells of the Nervous System
• Glia
are the other major component of the nervous
system and include the following:
• Astrocytes
helps synchronize the activity of the axon by
wrapping around the presynaptic terminal and taking up
chemicals released by the axon.
• Microglia
- remove waste material and other microorganisms
that could prove harmful to the neuron.
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Shapes of some glia cells
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The Cells of the Nervous System
• (Types of glia:
• Oligodendrocytes
&
Schwann cells
- build the myelin sheath
that surrounds the axon of some neurons.
• Radial glia
- guide the migration of neurons and the growth of
their axons and dendrites during embryonic development.
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blood-brain barrier
• The
blood-brain barrier
is a mechanism that surrounds the brain
and blocks most chemicals from entering.
• Because neurons in the brain generally do not regenerate, it is
vitally important for the blood brain barrier to block incoming
viruses, bacteria or other harmful material from entering.
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The blood-brain barrier
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The Cells of the Nervous System
• Active transport
is the protein mediated process by which useful
chemicals are brought into the brain.
• Glucose, hormones, amino acids, and vitamins are brought into the
brain via
active transport
.
• Glucose is a simple sugar that is the primary source of nutrition for
neurons.
• Thiamine
is a chemical that is necessary for the use of glucose.
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Basic schematic of excitable cell
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Types of ion channels
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Types of ion channels
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Important ions equilibrium potential
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Resting membrane potential
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Resting membrane potential
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Sodium-Potassium Exchange Pump
What is the purpose of pumping sodium
and potassium across a membrane?
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Who win the battle?
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The Na+/K+ ATPase pump
• The
sodium-potassium pump
is a
protein complex
that continually
pumps
three sodium ions
out of the cells while drawing
two
potassium ions into
the cell.
• Function
: it helps to maintain the electrical gradient and restore
the cell from action potential to resting potential state(
maintaining a
low intracellular concentration of Na+, keeping the inside of the cell negative
)
.
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The sodium and potassium gradients for a resting membrane
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Resting membrane potential
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Resting membrane potential
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Resting membrane potential
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The action potential
The action potential is a rapid depolarization followed by a
repolarization (return of membrane potential to rest).
The function is:
• Nerves: conduct neuronal signals
• Muscle: initiate a contraction
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The action potential
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The action potential phases
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The Nerve Impulse
• A
nerve impulse
is the electrical message that is transmitted
down the axon of a neuron.
• The impulse does not travel directly down the axon but is
regenerated at points along the axon.
• The speed of nerve impulses ranges from approximately
1 m/s
to 100 m/s
.
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The Nerve Impulse
• The
resting potential
of a neuron refers to the state of the neuron
prior to the sending of a nerve impulse.
• The membrane of a neuron maintains an
electrical gradient
which is a
difference in the electrical charge inside and outside of the cell.
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The Nerve Impulse
• The membrane is
selectively permeable,
allowing some
chemicals to pass more freely than others.
• Sodium, potassium, calcium, and chloride pass through
channels in the membrane.
• When the membrane is at rest:
• Sodium channels are closed
.
• Potassium channels are partially closed.
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Ion channels in the membrane of a neuron
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Voltage-gated Ion Channels
Voltage-gated (fast) Na+
Channels
The opening of these channels is
responsible for the
rapid
depolarization phase
(upstroke)
of the action potential.
It has 2 gates and 3
conformational states
.
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Voltage-gated Ion Channels
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Voltage-activated channels
• Voltage-activated channels
are membrane channels whose
permeability depends upon the voltage difference across the
membrane.
• Sodium channels are voltage activated channels.
• When sodium channels are opened, positively charged sodium
ions rush in and a subsequent nerve impulse occurs.
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Voltage-activated channels
• After an action potential occurs,
sodium channels are quickly
closed
.
• The neuron is returned to its resting state by the
opening of
potassium channels.
• potassium ions flow out due to the concentration gradient and take
with them their positive charge.
• The sodium-potassium pump later restores the original
distribution of ions.
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Voltage-gated Ion Channels
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Subthreshold Stimulus
Important points regarding these stimuli are:
The degree of depolarization is related to the magnitude of the stimulus.
The membrane repolarizes (returns to rest).
It can summate, which means if another stimulus is applied before repo-
larization is complete, the depolarization of the second stimulus adds onto
the depolarization of the first (the 2 depolarizations sum together).
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Subthreshold Stimulus
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Threshold Stimulus
If the initial stimulus is
great enough to depolarize
the neuron to threshold,
then an action potential
results.
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Steps of the action potential
1. At threshold, a critical mass of fast Na+ channels open, resulting in
further depolarization and the opening of more fast Na+ channels.
2. Because Na+ conductance is high, the membrane potential rapidly
approaches the equilibrium potential for Na+ (- +70 m V).
3. As membrane potential becomes positive, fast Na+ channels begin
to inactivate, resulting in a rapid reduction in Na+ conductance.
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Steps of the action potential
4. Voltage-gated K+ channels open in response to the depolarization,
but since their kinetics are much slower, the inward Na+ current
(upstroke of the action potential) dominates initially.
5. K+ conductance begins to rise as more channels open. As the rise
in membrane potential approaches its peak, fast Na+ channels
are inactivating, and now the neuron has a high K+ conductance
and a low Na+ conductance.
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Steps of the action potential
6. The high K+ conductance drives membrane potential toward K+
equilibrium ( -95 mV) resulting in a rapid repolarization.
7. As membrane potential becomes negative, K+ channels begin to
close, and K+ conductance slowly returns to its original level.
However,
because
of
the
slow
kinetics,
a
period
of
hyperpolarization occurs.
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Steps of the action potential
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Steps of the action potential
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the action potential phases
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the action potential phases
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Key points in action potential
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Properties of action potentials
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Refractory period
• After an action potential, a neuron has a
refractory
period
during which time the neuron resists another
action potential.
• The
absolute refractory period
is the first part of the
period in which the membrane
can not
produce an
action potential.
• The
relative refractory period
is the second part in which
it take a stronger than usual stimulus to trigger an action
potential.
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Properties of action potentials
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Neuromuscular Junction (NMJ)
The synapse between the axons of an
alpha-motor neuron
and a
skeletal muscle fiber
is called the
neuromuscular
junction
(NMJ). The terminals of alpha-motor neurons
contain acetylcholine (Ach).
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Neuromuscular Junction (NMJ)
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Sequence of events in neuromuscular junction
1) The
action potential
travelling down the motor neuron depolarizes
the presynaptic membrane.
2) This depolarization
opens voltage-gated Ca2+ channels
in the
presynaptic membrane, resulting in Ca+ influx into the presynaptic
terminal.
3) The rise in Ca+ causes synaptic vesicles to
release their
contents(Ach)
.
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Sequence of events in neuromuscular junction
4) Ach binds to a nicotinic receptor
located on the muscle membrane lead to
depolarization
. This depolarization is called an end-plate potential (EPP).
5) The resulting depolarization
opens fast Na+ channels
on the muscle
membrane (sarcolemma) causing an
action potential in the sarcolemma
.
6) The actions of Ach are terminated by
acetylcholinesterase (AchE),
an enzyme
located on the postsynaptic membrane that breaks down Ach into
choline and
acetate.
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Synapses Between Neurons
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Postsynaptic potential types
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Postsynaptic potential types
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Chapter summery
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Chapter summery
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