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Nerve Impulses
Neuron Physiology
Action Potentials- nerve impulses which are sent by
a change in electrical charge in the cell membrane.
Depends on ions:
• Sodium (Na+) highly concentrated outside of cells
• Potassium (K+) highly concentrated inside cells
Ion movement
• Ions move from high
concentration to a low
concentration passively
Na+/K+ pumps move ions
actively using ATP
Creating a Resting Potential
• Protein pumps
Active transport
– open and close
– let ions through
• Active pumps
– Ions move against a
gradient
– Use ATP to work
Passive pumps
– Ions move with the
gradient
– Ions move from high to
low concentration
Passive transport
Action Potential
Resting Membrane Potential (-70mV)
Stage 1: Resting Potential
Depolarization and Threshold Potential
Stage 2: Depolarizing neuron (-62mV)
Sodium rushes in when channels open which are
stimulated by neurotransmitters
Stage 3: Threshold is reached
once enough sodium rushes in
(-55 mV)
Stage 4: Neuron continues to depolarize
as sodium continues to rush in.
Na+
Na+
0mV to +20mV
Stage 5: Repolarization occurs as
K+ channels open and K+ moves
outward causing inside of
membrane to become negative
again.
K+
Potassium channels open
-70mV
A Nerve Impulse- a series of action
potentials
Action Potential
Refractory Period
For a short period
after the passage of
an impulse, the
threshold for
stimulation is raised,
so it limits the
frequency of impulses
and ensures unidirectional travel of
impulse.
Action Potential
What happens when the nerve impulse
reaches the end of the axon?
• Axon terminals
– Are found next to
another neuron (as
shown) or a muscle or
gland
• The gap is called a synapse
Synapse Action
• Neurotransmitters are
released at the synapse to
pass the message to the
next neuron.
Synapse
Neurotransmitters released at
synapse
Saltatory nerve impulse conduction
The action potential jumps large distances from node to node, a
process that is called saltatory propagation.
Myelinated vs. unmyelinated neurons
Speed of a Nerve impulse
• Temperature - higher the temperature= faster speed. Warm-blooded
animals have faster responses than cold-blooded.
• Axon diameter - larger the diameter= faster speed. Marine
invertebrates, (who live at temperatures close to 0°C), developed thick
axons to speed up their responses. This explains why squid have their
giant axons.
• Myelin sheath - Only vertebrates have a myelin sheath surrounding
their neurons. The voltage-gated ion channels are found only at the
nodes of Ranvier, and between nodes myelin sheath acts as a good
electrical insulator. Increases the speed of propagation dramatically.
 unmyelinated neurons –travel at about of 1 meters/second
 myelinated neurons-travel at about 100 meters/second
 Depending on the type of fiber, modern measurements are
from 6-122m/s
All or None Response
The strength of a response of a nerve cell or
muscle fiber is not dependent upon the strength
of the stimulus. If a stimulus is above a certain
threshold, a nerve or muscle fiber will fire. Full
response or no response at all.
"The all-or-none law guarantees that once an
action potential is generated it is always full
size, minimizing the possibility that
information will be lost along the way."
Multiple Sclerosis
• Autoimmune disease-body’s natural
defenses breaks down myelin in CNS
• Degeneration inhibits normal nerve
impulse transmission, may slow or even
stop in some cases.
Epilepsy
• Sudden disorderly discharge of brain neuronsimpulses fire continuously.
• Characterized by seizures
• Causes vary:
– by injuries , infections, tumors, with drawl from
drugs, etc.