Transcript File - Mr. Schmitt - Biology 12

Unit – M Neuron,
Impulse Generation,
and Reflex Arc
Structures and Functions
1. Dendrites
-Conduct a nerve impulse (message) towards a
cell body.
-Many dendrites enter a cell body.
2. Cell Body
-Contains the nucleus and cell organelles
needed to keep the cell alive.
-Relays impulse from Dendrite to Axon.
3. Axons
-Conduct a nerve impulse away from the cell
body.
4. Myelin Sheath
-Protective coating of Schwann Cells around
larger Axons
5. Nodes of Ranvier
-Interrupted areas on the Myelin Sheath
-Speeds up transmission of impulse.
6. Synaptic Terminal
-Junction through which neurons signal to
each other and to non-neuronal cells
such as those in muscles.
Poll
Types of Neurons
1. Sensory Neuron (see animation)
-Afferent Neuron: Moving away
from a central organ or point.
-Relays messages from receptors
to the brain or spinal cord
2. Motor Neuron
-Efferent Neuron: Moving toward a
central organ or point.
-Relays messages from the brain or
spinal cord to the muscles and organs.
3. Interneuron (associated neuron or Connector
Neuron)
-Relays message from sensory neuron to motor neuron.
-Make up the brain and spinal cord.
Simple Neuron Connection
Considering the direction of impulse, what is
structure X?
Bases of
Comparison
Length of
Fibers
Location
Function
Sensory Neuron
Interneuron
Motor Neuron
Long Dendrites
and short
Axon
Short Dendrites
and short or
long Axons
Short Dendrites
and long
Axons
Cell body and
Dendrite are
outside of the
spinal cord;
the cell body
is located in a
dorsal root
ganglion.
Entirely within
the spinal
cord or CNS.
Dendrites and
the cell body
are located
in the spinal
cord; the
Axon is
outside of
the spinal
cord.
Conduct impulse
to the spinal
cord.
Interconnect
the Sensory
neuron with
an
appropriate
Motor
Neuron.
Conduct
impulse to an
effector
(muscle or
***A nerve is composed of long fibers of a number of Neurons***
Reflex Arc
Reflexes are automatic, involuntary responses
to changes occurring inside or outside the
body. Some involve the brain (such as blinking
the eye), while others do not (such as moving
your hand away from a hot object).
Why does the brain not have to be involved?
If it were, by the time the impulse traveled to
the brain, the brain figured out what was
happening, and sent a response to the body,
serious damage might occur.
So the body evolved a method of by
passing the brain.
Stages of Reflex Arc
1. Receptor is stimulated and formulate
message. ie. nerve impulse
2. Sensory neuron takes the message to
the Central Nervous System. (spinal cord)
3. Interneuron passes the message to a
motor neuron.
4. Motor Neuron takes the message away
from the C.N.S. to the effector.
(muscle/organ)
5. The muscle receives the message
and contracts.
***The brain finds out later what had happened***
Impulse Generation (Action Potential)
Nerve impulses are electrical messages. If we
measure the voltage of a resting neuron using
a voltmeter, we will see a reading of –60
millivolts. Voltage is a comparison of electrical
charge between two points.
When the neuron is stimulated, the
charge changes briefly to +40 milivolts
(mv), then back to –60mv.
(-60mv means that the inside is 60mv
more negative than outside).
If we hook up our voltmeter to a machine
called an oscilloscope, we can see the
change in voltage over a period of time.
Stimulated Neuron Oscilloscope Reading
Why???
There is a difference in ion distribution on
either side of the membrane of a neuron.
At Rest
-Na+ outside the Neuron
-K+ and large negatively
charged organic molecules
inside the neuron.



The concentration of sodium ions Na+ is
greater outside the axon than inside.
The concentration of potassium ions K+ is
greater inside the axon than outside.
This unequal distribution is due to the
sodium-potassium pump which actively
transports Na+ out and K+ into the axon



The membrane is more permeable to K+ ions,
and some K+ diffuses back while Na+ does not.
This unequal charge distribution, along with of
the large negative molecules, causes the inside
to be more negative than the outside.
This situation is called Resting Potential. -60mv
When the axon or dendrite is stimulated, sodium gates
open which allows some Na+ to enter the axoplasm
(interior). Now, the inside becomes more positive
than the outside by 40 mv.
This is called the Upswing Phase of the action
potential. The charge changes from –60 mv to +40
mv. The change is called Depolarization.
After the sodium gates have opened, then potassium
gates open. K+ exit the axoplasm.
This is called the Downswing Phase of the action
potential. The charge returns to –60mv. The change
is called Repolarization.
**Note: Charge is back to normal, but ions are
reversed
Finally, there is a Recovery Phase in which the
sodium/potassium pump (ACTIVE TRANSPORT)
returns Na+ to the outside and K+ to the inside.
This is called the refractory period. During the
refractory period, another action potential cannot be
created.
After the recovery phase, the neuron is again at
resting potential and can fire again!
Action Potential Summary
Transmission of a signal – THINKING!!!

How is a signal transmitted down/along neuron?
Think Dominoes!
Transmission of a nerve signal

Neuron has similar system


units are set up
once 1st is tripped, the rest fall
in succession



all or nothing response
same force travels along neuron
have to re-set neuron to react again
So far we have only been looking at one
point on the axon or dendrite. The opening
of the sodium gates in one area causes the
sodium gates in the next area to open
because the sodium channels are voltage
gated (sensitive to changes in voltage).
We get a wave motion (chain reaction)
moving down the nerve fiber.
1. RESTING POTENTIAL
Charge is –60mv, -Na+ outside, K+ inside
2. STIMULUS REACHES THRESHOLD POTENTIAL
“all or none response”
3. UPSWING OF ACTION POTENTIAL
Depolarization, Na+ moves inside (sodium
gates opened), charge from –60mv back to
+40mv
4. DOWNSWING OF ACTION POTENTIAL
Repolarization, K+ moves outside (Potassium gates
opened), charge from +40mv back to –60mv
5. RECOVERY PHASE
Sodium/Potassium Pump, moves Na+ out and K+ inside,
charge is –60mv
****NOW NEURON CAN BE RE-STIMULATED ****
**REMEMBER THIS IS A WAVE MOTION DOWN THE NEURON**
Myelin Sheath
 Made from Schwann cells
signal

direction

insulate neuron
Increase speed of impulse
 signal hops from node to node
 “saltatory conduction”

150m/sec (530km/hr)
(vs. 5m/sec) unmyelinated
myelin sheath
Biology
2007
Biology
2007
Multiple Sclerosis
 T cells attack
myelin sheath
 loss of signal
2007
Synapse (see video)
Each axon branches off and ends with a
swelled tip or terminal knob that lies close to
but not touching the dendrite of another
neuron. (or an organ). The entire region is
called a synapse.
Transmission of nerve impulses across a Synaptic cleft
is carried out by chemicals called Neurotransmitters.
These substances are stored in vesicles at the end of
the axon. Noradrenalin (speeds up activity) and
acetylcholine (slows down activity) are examples of
neurotransmitters.
When an impulse reaches the end of the axon like it
usually would, not only does Na+ come into the axon,
but Ca+2 as well since voltage gated calcium
channels are opened.
This calcium binds with contractile proteins that pull the
neurotransmitter vesicles to the membrane surface. The
vesicles join with the cell membrane, forcing the
neurotransmitter into the cleft (exocytosis)
Neurotransmitter’s job is to increase the
permeability of the sodium ions on the
postsynaptic membrane.
The Neurotransmitter binds to specific
receptor sites on the dendrite of the next
neuron. If enough transmitter substance
is received, the neuron will “fire” and
continue the impulse.
Neurotransmitter diffusing across a synaptic
cleft
A neurotransmitter only has a short period to
work once it has been released into the
synaptic cleft.
Enzymes rapidly break down the transmitter
substance to clear the synapse so the next
impulse can be transmitted.
Monoamine oxidase breaks down noradrenaline
and Acetylcholinesterase breaks down
acetylcholine.(see video)
Pain killers such as Tylenol act as an enzyme to
break down the neurotransmitter to decrease
the pain impulse. A natural painkiller in the
body is prostaglandin.
An impulse can only travel across a synapse in
one direction. Only the axon contains
neurotransmitter vesicles, so the impulse can
only travel one way
AXON  DENDRITE across a synapse.
**** ALL OR NONE LAW (threshold): If
enough neurotransmitter is received by
the postsynaptic fiber, it will fire 100%
(all). If not enough substance is received,
it will not fire at all (none).
There are excitatory and inhibitory neurotransmitters in the
body. When two excitatory neurotransmitters work together to
cause an action potential, it is called summation.