Nerve Impulses
Neuron Physiology
Action Potentials- nerve impulses which are sent by a change in
electrical charge in the cell membrane. Depends on ions inside
and outside of the neuron cell
A resting neuron:
• Sodium (Na+) highly concentrated outside of cells
• Potassium (K+) highly concentrated inside cells
Ion Movement
Passive transport-Ions move from high concentration to a low
concentration passively without using Energy (ATP) by 2
methods:
• Diffusion-molecule cross membranes from high
concentration to low concentrations
• Facilitated diffusion-Ions move with the gradient from
a high to low concentration when protein channels in
the membrane are open
Ion Movement
Active Transport- ions move against gradients
using pumps and ATP
•Na+/K+ pumps- move ions actively using ATP against their gradients
Na+ is moved from low concentrations to high concentration and K+ is
moved from low concentrations to high concentrations.
Creating a Resting Potential in a Neuron
(-70mV)
An Action Potential
The change in electrical potential associated with the
passage of an impulse along the membrane of a nerve cell.
Stages of Nerve Impulses
Stage 1:Resting Potential
•Neuron’s resting potential=-70mV
•Inside=high concentrations of negatively charged ions and high
concentrations of K+
•Outside=high concentrations of Na+
Stages of Nerve Impulses
• Stage 2: Depolarization (-62 mV)
•Gates in the axon called ion channels open when a neuron is
stimulated
•Na+ rushes into the axon giving it a brief positive electrical charge
and the axon becomes depolarized
Stages of Nerve Impulses
• Stage 3: Threshold (-55mV)
•Threshold- is the minimum for a neuron to fire
•It is reached once enough sodium rushes in to make the inside of the
neuron(-55 mV).
Stages of Nerve Impulses
• Stage 4: Repolarization
•This occurs as K+ channels open and K+ moves outward causing
inside of membrane to become negative again.
•At the same time Na+/K+ pumps move Na+ back out of the cell.
Stages of Nerve Impulses
Stage 5: Hyperpolarization.
•Hyperpolarization occurs when the voltage is below the resting potential
•A neuron cannot produce a new action potential
•This is the refractory period
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.
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."
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. This speeds up the
transmission of impulses.
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
Central Nervous System - Spinal
Cord
Figs 9.22, 9.23 & 9.24
What is a Reflex Arc?
• We need to detect a change in the
environment (a stimulus) and react to the
change (a response) in a way that maintains
homeostasis.
• When you do this without thinking, it is called
a reflex.
• Reflexes are not immediately processed by the
brain
Reflex Arc
Name the Neuron
Reflex Arcs
Stimulus
Response
Aroma of a burger
Salivation
Nasty odor
Nausea
Bright light in the eyes
Pupil dilates
Insect flying to the eye
Blinking
Muscles in the leg relax
Knee jerk