Transcript 6.5 Nervous system part1

Nervous Systems
IB Assessment Statement
• State that the nervous system consists of the
central nervous system (CNS) and peripheral
nerves, and is composed of cells called neurons
that can carry rapid electrical impulses.
• Crash Course Video on the Nervous System
• https://www.youtube.com/watch?v=x4PPZCLnVkA
Organization of the Body
• Nervous System
• Structures: Brain,
spinal cord, peripheral
nerves
• Function: Recognizes
and coordinates the
body’s response to
changes in its internal
and external
environments
Overview: Command and Control Center
• The human brain contains about 100 billion
nerve cells, or neurons
•
Functional magnetic resonance imaging is a technology that can
reconstruct a three-dimensional map of brain activity
•
Brain imaging and other methods reveal that groups of neurons
function in specialized circuits dedicated to different tasks
Parts of the Nervous System
The nervous system consists of :
– the central nervous system (CNS) Brain and
spinal cord
– and peripheral nerves (PNS), Nerves they
run to all parts of the body.
– and is composed of cells called neurons that
can carry rapid electrical impulses.
Neuron Cell Structure
Neuron Cells have the following structures:
– Cell Body
– dendrites
– axon
– myelin sheath
– Node of Ranvier
– Motor end plates
IB Assessment Statement
• Draw and label a diagram of the structure of a
motor neuron. Include dendrites, cell body with
nucleus, axon, myelin sheath, nodes of Ranvier
and motor end plates.
Cell Body
• The largest part of a typical neuron is the cell
body.
• It contains the nucleus and much of the
cytoplasm.
Cell body
Dendrites
• Dendrites extend from the cell body
• and carry impulses from the environment toward
the cell body.
Dendrites
Axon
• The axon is the long fiber that carries impulses
away from the cell body.
Axon
Motor end plates
• The axon ends in motor end plates.
Motor end
plates
Axon
Myelin Sheath
• The axon is surrounded by an insulating lipid membrane
called the myelin sheath.
• Myelin sheaths have high electrical resistance
Myelin sheath
Nodes of
Ranvier
Myelin Sheath
• There are gaps in the myelin sheath, called nodes of
Ranvier, where the membrane is exposed.
• Electrical Impulses jump from one node of Ranvier to the
next.
Myelin sheath
Nodes of
Ranvier
Neurons
• Structures of a Neuron
Nucleus
Dendrites
Motor end
plates
Cell body
Myelin sheath
Node of
Ranvier
Axon
• OK YOUR TURN
• Draw and label a diagram of the structure of a
motor neuron. Include
–
–
–
–
–
–
dendrites,
cell body with nucleus,
axon,
myelin sheath,
nodes of Ranvier
and motor end plates
IB Assessment Statement
• State that nerve impulses are conducted from
receptors to the CNS by sensory neurons, within
the CNS by relay neurons, and from the CNS to
effectors by motor neurons.
The Path of Nerve Impulses
•
There are various receptor around the body such as skin and the eye.
•
Stimuli (think of them as energy forms) are detected by the receptors and
turned into an nerve impulse (chemical energy).
•
Nerve impulses from sensory nerves are conducted to the central nervous
system along sensory neurons.
•
The impulse is sent to the relay neurons that move it around inside the
central nervous system (brain and spine).
•
Motor neurons take the relayed nerve impulse to the effectors (often
muscles) which then produce the response.
Path of Nerve Impulses
•
This is a cross section through the vertebrate spinal column.
•
The receptor is deep in the biceps muscle.
•
Sensory neuron conducts nerve impulses from the receptor to the
central nervous system.
•
The relay nerve conduct the impulse through the spinal cord and in a
reflex back to the motor neuron.
•
The motor neuron connects to the effector which in this case is the
biceps muscle.
Reflex Arc Video
Reflex Arc Animation
• http://www.sumanasinc.com/webcontent/animation
s/content/reflexarcs.html
IB LEARNING Objective
• Define resting potential and action potential
(depolarization and repolarization).
Definitions:
• Resting potential is the negative charge registered when
the nerve is at rest and not conducting a nerve impulse.
• Action potential is the positive electrochemical charge
generated at the nerve impulse. Normally this is seen as
the 'marker' of the nerve impulse position.
• Depolarisation is a change from the negative resting
potential to the positive action potential.
• Re-polarisation is the change in the electrical potential
from the positive action potential back to the negative
resting potential.
IB Assessment Statement
• Explain how a nerve impulse passes along a nonmyelinated neuron. Include the movement of Na+
and K+ ions to create a resting potential and an
action potential.
How Neuron cells transmit an impulse
• Neurons transmit information in the form of
electrical impulses
• An electrical impulse is transmitted along nerve
fibers.
Dendrites
Cell body
Nucleus
Axon
Signal
direction
Myelin sheath
Synaptic
terminals
Synapse
Electrical Potential Differences
• An impulse is a change in electrical potential
difference in the membrane of a neuron cell.
• A potential difference is when there are more
positive ions (+) on one side of a membrane than
negative ions(-)
– Positive Ion examples: Sodium (Na+),
Potassium (K+), Calcium (Ca2+)
– Negative Ion examples: Chlorine (Cl-)
Resting Potential
• Normally cells has a rest potential of -70 millivolts
(mV)
– This means a neuron cell will have more
negative ions on the inside of it means than
the outside.
– This is called its resting potential.
Resting Potential
• where
–
Em is the membrane potential, measured in volts
–
EX is the equilibrium potential for ion X, also in volts
–
PX is the relative permeability of ion X in arbitrary units (e.g. siemens for
electrical conductance)
–
Ptot is the total permeability of all permeant ions, in this case PK+ +
PNa+ + PCl-
An electrical Impulse in neuron cells
An electrical impulse is a momentary reversal in
electrical potential difference in the neuron cell
membrane
Between conduction of one impulse the cell is said
to be ‘resting.’
The ‘resting’ neuron membrane maintains the
electrical potential difference between the inside
and outside of the cell using active transport.
Resting Potential is maintained by TWO processes
1. Active transport of potassium ions (K+) in
across the membrane and sodium (Na+) out
across the membrane.
2. Facilitated Diffusion of potassium ions (K+) out,
and sodium ions (Na+) back in.
LE 48-9
Microelectrode
–70 mV
Voltage
recorder
Reference
electrode
Active Transport across Neuron Cell Membranes
The sodium-potassium pump in the nerve cell use
ATP and active transport to pump two ions:
1. Pumps 3 sodium (Na+) ions out of the cell
2. 2 potassium (K+) ions into the cell by
means of active transport.
As a result, the inside of the cell contains more K+
ions and fewer Na+ ions than the outside.
Sodium-Potassium Pump
Sodium-Potassium ATPase -- Protein Pump
•
This uses the energy from ATP splitting to simultaneously pump 3 sodium ions out of the
cell and 2 potassium ions in.
•
If this was to continue unchecked there would be no sodium or potassium ions left to
pump, but there are also sodium and potassium ion channels in the membrane.
•
These channels are normally closed, but even when closed, they “leak”, allowing sodium
ions to leak in and potassium ions to leak out, down their respective concentration
gradients.
•
The combination of the Na +K +ATPase pump and the leak channels cause a stable
imbalance of Na + and K + ions across the membrane.
•
This imbalance causes a potential difference across all animal cell membranes, called
the membrane potential.
•
The membrane potential is always negative inside the cell, and varies in size from –20 to
–200 mV in different cells and species.
LE 48-10
Inside Cell
Outside Cell
[Na+]
15 M
[Na+]
150 M
[K+]
150 M
[K+]
5M
M = Concentration
Plasma
membrane
Facilitated diffusion across cell membranes
• Because K+ ions are in higher concentration
inside than outside the cell, they slowly diffuse
OUT across the membrane via facilitated
diffusion
• Because Na+ ions are in higher concentration
outside the cell they do not diffuse out.
• This produces a negative charge on the inside and
a positive charge on the outside.
• The electrical charge across the cell membrane of
a neuron at rest is known as the resting
potential.
The Moving Impulse
An impulse begins when a neuron is stimulated by
another neuron or by the environment.
The Nerve Impulse
1. At the leading edge of the impulse, gates in the
sodium channels open allowing positively
charged Na+ ions to flow inside the cell
membrane.
Sodium Channels
• Sodium Channels/ Gates –
– Globular proteins that are imbedded in the
neuron cell membrave
– They have a central pore that they can open
and close
– Only sodium can diffuse through them
– This is facilitated diffusion.
The Nerve Impulse
2. The inside of the membrane temporarily becomes
more positive than the outside, reversing the
resting potential. This is called depolarization.
Depolarization & Action Potential
• Action potential causes local & temporary
depolarization of the neuron.
– Depolarization - positive ions (sodium Na+)
flow back into the cell via sodium channels.
• Sodium will passively diffuse down it
electrochemical gradient (from high
concentration to low concentration)
• This depolarizes the neuron from -70mV
(resting potential) to +40mV (action
potential)
The Nerve Impulse
3. This reversal of charges f is called a nerve
impulse, or an action potential.
The Nerve Impulse
4. As the action potential passes, gates in the
potassium channels open, allowing K+ ions to
flow out restoring the negative potential inside the
axon.
Potassium Channels
• Potassium channels/ gates
– Similar to sodium channels/ gates
– Globular proteins that are imbedded in the
membrane of the neuron cell.
– All only potassium to diffuse in and out of cell
via facilitated diffusion.
The Nerve Impulse
5. Then the gates in the potassium channels
CLOSE, and the resting potential is re-established
by sodium/ potassium pumps and facilitated
diffusion. This is called repolarize.
Repolarize
• Repolarize means to return back to resting
potential.
– Resting potential -70mV
– More negative ions on the inside than out of
neuron
– More potassium (K+) inside of cell
– More sodium (Na+) outside of cell
The Nerve Impulse
6. Meanwhile, the impulse continues to move along
the axon.
An impulse at any point of the membrane causes an
impulse at the next point along the membrane.
Nerve Impulse More Summary
Axon
1. Action potential is generate Na+
flows in cell. Depolarizing the
cell.
Action
potential
Na+
An action potential is generated as Na+ flows
inward across the membrane at one location.
K+
Action
potential
Na+
K+
The depolarization of the action potential spreads to
the neighboring region of the membrane, reinitiating the action potential there. To the left of
this region, the membrane is repolarizing as K+
flows outward.
K+
2. Action potential spreads to
neighboring cell and depolarizes
this cell. Initial cell undergoes
repolarization as K+ flows
outward.
Action
potential
Na+
K+
The depolarization-repolarization process is repeated
in the next region of the membrane. In this way, local
currents of ions across the plasma membrane cause
the action potential to be propagated along the length
of the axon.
3. Depolarization & Repolarization
process is repeated in the next
region of the membranes.
Nerve Impulse Animations/ Tutorials
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter1
4/animation__the_nerve_impulse.html
• http://bcs.whfreeman.com/thelifewire/content/chp4
4/4402s.swf
• http://www.mrothery.co.uk/images/nerveimpulse.s
wf
• http://www.psych.ualberta.ca/~ITL/ap/ap.htm
• http://www.sumanasinc.com/webcontent/animation
s/content/actionpotential.html
IB LEARNING OBJECTIVE
• Explain the principles of synaptic transmission.
– Include the release, diffusion and binding of
the neurotransmitter, initiation of an action
potential in the post-synaptic membrane, and
subsequent removal of the neurotransmitter.
LE 48-15
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
The Synapse
• The Synapse
• At the end of the neuron cell , the impulse
reaches the motor end plates. At this site,
one neuron cell makes contact with another
neuron cell.
• The neuron passes the impulse along to
the second cell.
• The location at which a neuron can transfer
an impulse to another neuron cell is called a
synapse.
LE 48-5
Dendrites
Cell body
Nucleus
Axon hillock Axon
Presynaptic cell
Signal
direction
Motor end
Myelin sheath plates
Synapse
Postsynaptic cell
The Synapse
• A Synapse is a
link between two
neuron cells.
• A cell carrying and
transmitting an
impulse is called
the pre-synaptic
neuron
Pre-synaptic neuron
The Synapse
Pre-synaptic neuron
• The synaptic cleft is a
gap that separates the
motor end plates from
the dendrites of the
adjacent cell.
•
Pre-synaptic neuron
opens up calcium
channels and calcium
moves into the cell
Synaptic cleft
The Synapse
Pre-synaptic neuron
• Calcium causes
vesicles of
neurotransmitters
to fuse with presynaptic membrane.
Vesicle
Synaptic cleft
The Synapse
• Neurotransmitters
are chemicals used
by a neuron to
transmit an impulse
across a synapse to
another cell.
Vesicle
Neurotransmitter
The Synapse
As an impulse reaches the
motor end plates, vesicles
send neurotransmitters
into the synaptic cleft.
These diffuse across the
cleft
The cell membrane receiving
the impulse is called the
post-synaptic
membrane.
Receptor
The Synapse
• Sodium ions then rush across the membrane,
stimulating a nerve impulse (action potential/
depolarization) in the next cell, the postsynaptic
cell.
• Once an action potential has been generated in
the post-synaptic cell membrane an enzyme
inactivates the neurotransmitter.
• The inactivated neurotransmitter is packaged for
re-use later by the Golgi apparatus.
Inactivation of Neurotransmitter
• The neurotransmitter must be rapidly broken down
to prevent continuous synaptic transmission of
impulse.
• Calcium ions will started to be pumped out of the
pre-synaptic neuron into the synaptic cleft.
LE 48-17
Presynaptic
cell
Postsynaptic cell
Synaptic vesicles
containing
neurotransmitter
Na+
K+
Presynaptic
membrane
Neurotransmitter
Postsynaptic
membrane
Sodium
ion channel
Voltage-gated
Ca2+ channel
Postsynaptic
membrane
Ca2+
Na+
Synaptic cleft
Sodium
ion channels
Neurotransmitter
LE 48-5
Dendrites
Cell body
Nucleus
Axon hillock Axon
Presynaptic cell
Signal
direction
Synaptic
Myelin sheath terminals
Synapse
Postsynaptic cell
Synaptice Transmission Videos / Tutorials
•
http://outreach.mcb.harvard.edu/animations/synaptic.swf
•
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter14/animation__transmission_across_a
_synapse.html
•
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter14/animation__chemical_synapse__qui
z_1_.html
•
https://www.youtube.com/watch?v=Tbq-KZaXiL4
•
http://www.sumanasinc.com/webcontent/animations/content/synapse.html
LE 48-16
Synaptic
terminals
of presynaptic
neurons
5 µm
Postsynaptic
neuron
50 µm
LE 48-7