Transcript electrochemical impulse

ELECTROCHEMICAL
IMPULSE
1. How does a neuron transfer a
signal?
2. What causes neuron excitation?
3. How does the brain interpret
different strength signals?
1. How does a neuron transfer a
signal?
• When the neuron is not relaying information, it is said to
be at rest. In this situation, there is a charge difference
across the membrane due to the unequal concentration
of positive ions on either side of the membrane. This
charge difference or resting membrane potential is -70
mV (millivolts).
• Potassium ions are concentrated on the inside of
the cell and sodium ions are concentrated
outside the cell. Both ions move across the
membrane but the membrane is 50 times more
permeable to potassium ions, so more
potassium ions diffuse out of the cell than
sodium ions diffuse in. Because of this unequal
movement, the exterior of the cell becomes
positive relative to the interior. So, at rest, the
membrane is said to be polarized.
• The nerve impulse is an electrochemical message
generated by the movement of ions through the membrane.
• When the nerve is excited, the potential charge difference
across the membrane changes to +40 mV. This is called
the action potential.
• When excitation of the nerve occurs, the membrane
becomes more permeable to sodium ions. It is believed that
the membrane has sodium ion channels embedded within
it. As the sodium gates open, the potassium gates close.
This allows for the rapid flow of sodium ions into the cell,
causing a charge reversal which is referred to as
depolarization.
• Once the interior of the nerve cell becomes positive, the
sodium and potassium gates reverse themselves and a
sodium-potassium pump restores the ion concentrations to
their resting levels.
• The nerve cannot send another impulse again until the
membrane has been restored or repolarized. The refractory
period can last from 1 to 10 milliseconds.
• The wave of depolarization sweeps down the length of the
axon in sections. A similar analogy would be a crowd doing
the wave in a stadium.
2. What causes neuron excitation?
• When a sensory neuron detects a change in the environment
known as a stimulus, it has to be strong enough to trigger the
depolarization of the membrane.
• The intensity of the stimulus must reach a set level called the
threshold level before the signal will be sent. This threshold is
important for it prevents small changes that don’t have an
effect from sending a signal to the brain. Without the
threshold, the sensory neurons would send signals
continuously which would overwhelm the brain.
• Once the threshold level is reached the neuron will fire at the
same intensity and the same speed for any stimulus.
• The size of the change cannot affect these factors of the
nerve impulse. A greater stimulus does not cause a greater
depolarization of the membrane. This is known as the all-ornone response. The neuron either sends the signal or it does
not.
3. How does the brain interpret a
signal of different strengths?
• The brain is able to interpret intensity by the frequency of
the impulses being sent. Therefore, if the stimulus is
small a few signals will be sent, one after the other.
• When the stimulus is intense, the signal will be sent
repeatedly as soon as the membrane has repolarized.
http://www.youtube.com/watch?v=jcZLtH-Uv8M&feature=related
SYNAPTIC TRANSMISSION
1. How are signals transferred
between neurons?
2. What is a reflex arc?
1. How are signals transferred
between neurons?
• Information from sensory neurons must be passed to
interneurons (by synaptic transmission) which can then relay
the information to the brain.
• The small space between the ends of the neurons is called the
synapse.
• When the signal reaches the end of the neuron, chemical
neurotransmitters (small molecules that are held in tiny vesicles
at the end of the first neuron) move across the space or
synaptic cleft.
• The vesicles move to the end of the neuron, fuse with the
membrane and dump their contents into the cleft. The next
neuron in line, called the post-synaptic neuron, has receptors
on its end where the neurotransmitter can bind.
• Once they bind, the threshold level is reached and the signal
moves down the length of the neuron. The diffusion of the
transmitter across the cleft slows down the transmission of the
signal so the greater the number of synapses, the slower the
signal travels.
• Acetylcholine is a common neurotransmitter that is capable of
opening the sodium ion channels in the post-synaptic neuron.
Once the message has been transmitted, the enzyme
cholinesterase is released into the synaptic cleft and breaks
the acetylcholine down.
http://www.youtube.com/watch?v=9vZI_XKca88
2. What is a reflex arc?
• In most situations, the nervous system works
quickly enough to respond to changes in the
environment. But, there are some situations
where an almost instantaneous response is
required to keep the organism from harm. This is
the function of the reflex arc.
• Each reflex arc contains five necessary
components: the receptor, the sensory neuron,
the interneuron in the spinal cord, the motor
neuron and the effector.
• The sensor is the portion of the nervous system that is able to
detect the environmental change. It activates the sensory
neuron which sends the signal towards the brain.
• Once the signal reaches the spinal cord, specialized
interneurons that are pre-programmed with a specific
response, send a message back through the motor neuron to
the effector which immediately makes the change.
• At the same time, the signal travels along other interneurons
to the brain for processing.
• This system cuts down on the lag time required for the signal
to reach the brain, by having the brain process the information
and then send the appropriate response.
http://www.youtube.com/watch?v=eZundDVPIYw