Download Nerve Cell Physiology

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 ‫الفياض‬
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 repolarization 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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