Download Sending Messages:

Sending Messages:
Where an electrical signal is received by a
neuron and then transferred down the
processes to the next neuron.
This carries the signal from the stimulus
detected by the PNS to the brain in the CNS
to interpret and then back to the motor
neurons in the PNS to respond.
A: The resting neuron membrane
At rest, the neuron plasma membrane is
chemically and electrically polarized (more
positive ions are outside than inside the cell)
So, there is a charge/voltage across the plasma
membrane.
Outside: Positively charged with Sodium ions
Inside: Negatively charged with potassium ions
B: Arrival of a Stimulus (Depolarization)
A stimulus is detected by:
the environment (detected by the skin)
Neurotransmitter (from skin or another axon terminal)
Light (from neurons in the eye)
This starts an action potential: Sodium moves in
making the inside of the cell slightly positive.
The process of depolarization takes only 1/1000 of a
second!
http://www.wisc-online.com/objects/index_tj.asp?objid=AP1201
C: Repolarization
After the depolarization wave passes, the sodium
gates close and potassium gates open, allowing
potassium to diffuse to the outside of the cell.
During this step, there is a short time period when a
neuron is not able to fire because it must restore
those charges. This is called the refractory period.
This restores the electrical differences to the way
they were at rest. The ions are switched…
Outside is positive
Inside is negative.
http://www.blackwellpublishing.com/matthews/actionp.html
D: Restoration of the resting state
An active transport pump (Sodium-Potassium
Pump) begins working to pump sodium ions back
to the outside and potassium ions back to the
inside of the fiber.
ATP is used to accomplish this.
This is the ONLY part of the process that requires
energy.
http://www.brookscole.com/chemistry_d/templates/student_resources/shared_re
sources/animations/ion_pump/ionpump.html
E. THE SYNAPSE
• Bridging the gap between the axon terminal of a neuron and
another neuron, a muscle or a gland:
•
Synapse- the point between the axon of one neuron and the
dendrite of the next where they are almost in contact, but not
quite touching. Also can be between the axon of a neuron and
the organ it is stimulating (such as a muscle fiber at a
neuromuscular junction).
Synaptic cleft (or gap)- the actual space between the axon
terminal and the dendrite of the next neuron or the organ to be
stimulated.
When an impulse reaches the end of an axon: a chemical called a
Neurotransmitter is released and travels across the synaptic
cleft to the second neuron or muscle or gland.
•
If the organ stimulated is a muscle or a second neuron, it will
begin to depolarize. If the organ to be stimulated is a gland, the
chemical stimulates the release of whatever substance the
gland produces.
•
It only takes a few hundredths of a second for the impulse to
cross a synapse.
Why is this synapse arrangement important?
This arrangement insures that the action potential goes in only
one direction.
Also, it is easier for the body to control. A cell can stop the
signal by breaking down the neurotransmitter using enzymes at
the cell membrane.
The Intensity of the Stimulus:
The “firing” of a neuron (generation of an
action potential) is an “all-or-none
phenomenon”.
Each neuron has a minimum level of stimulus
it needs to get started.
Either it fires completely or not at all.
A strong stimulus depolarizes the membrane
quickly
The Intensity of the Stimulus:
Weaker stimuli must be applied for a longer
period of time to achieve the crucial amount
of current flow.
Very weak stimuli are unable to trigger an
action potential, because the local current
flows they produce are so weak they just die
out.
The Intensity of the Stimulus:
The amount of stimulus needed to cause
action potential to be self-propagating is
called the threshold stimulus.
This is about 15-20 mV.
When an action potential is large enough to
keep going on its own, it is self-propagating.
How the Brain Perceives the Stimulus
The stimulus can be perceived as just sensation
or strong enough to be actual pain.
The intensity of the sensory input depends on 2
factors:
1. Number of sensory neurons fired
(stimulated)
2. Frequency of the stimulation
Medical Applications:
In order for this system to work, a neuron
must be able to change permeability.
Calcium is needed to keep the Na and K
gates closed between stimulations.
The neuron also needs sufficient amounts of
sodium and potassium.
If a person is deficient in these 3 ions, their
nervous system will not work.
Medical Applications:
Anesthetics work by decreasing the
membrane permeability to sodium. The
sodium can’t get into the cell so the neuron
won’t “fire” and the person feels nothing.
Medical Applications:
Alcohol and sedatives also block nerve
impulses by reducing membrane
permeability to sodium ions.
▪
▪
Since cold temperatures can interrupt blood
circulation and cut off oxygen to neurons,
drinking alcohol “to feel warm” while out in the
cold is dangerous
You feel better because you can’t feel the cold,
but you also don’t know to get in a warmer area.
This could result in frost bite.
E.
The Synapse: This step transfers the signal from:
How it works:
when an impulse reaches the end of the axon, it trigger the cell to release:
this chemical moves across the:
to:
If the chemical reaches a muscle or another neuron, what will happen?
If the chemical reaches a gland what will happen?
A synapse functions to:
1.
2.
The intensity of the nervous signal can vary as well. This is based on the intensity of the
_______________________.
Strong stimuli start an action potential instantly. Weak stimuli need repeated stimulus to get an action
potential going.
The minimum level of stimulus that is needed to start a reaction is the:
If a stimulus is strong enough to keep an action potential going it is called:
A weak stimulus can be perceived as a sensation; strong stimuli are perceived as pain. How the brain
perceives the stimulus depends on two factors:
1.
2.
Medical Applications:
1.
Nutrient defiencies in the following ions can hault nerve impulses:
2.
Anesthetics limit the cell’s ability to fire because:
3.
Alcohol and sedatives work the same way as anesthetics. Dangers of using these in cold temperatures: