Transcript Nervous system

Biology 103 - Main points/Questions
1. What does a neuron look like?
2. Why do membranes have charges?
3. How can these charges change?
Functions of the Nervous System
• Process and coordinate:
– sensory input:
• from inside and outside body
– motor commands:
• control activities of peripheral organs (e.g.,
skeletal muscles)
– Integration – occurs in the central nervous
system
– higher functions of brain:
• intelligence, memory, learning, emotion
Coordinating all the
different body
systems and
interacting with the
external world are
the job of the
body’s control
systems – the
nervous system
and the endocrine
(hormone) system.
Aplysia (sea slug) neurons
• Neurons are nerve cells that transfer
information within the body
• Neurons use two types of signals to
communicate:
– electrical signals (long-distance) and
– chemical signals (one cell to the next - short)
• Nervous systems process information in
three stages: sensory input, integration,
and motor output
Sensory input
Integration
Sensor
Motor output
Effector
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Integration
Sensory
information
White
matter
Response
Spinal cord
Sensory neuron
Motor neuron
Interneuron
• These stages use
three basic types of
neurons –
– sensory
– association and
– motor
Three types of neurons
Neuron Structure and Function
• Most of a neuron’s organelles are in the
cell body
• Most neurons have dendrites, that receive
signals from other neurons
• The axon is typically a longer extension
that transmits signals to other cells
• Many axons are wrapped by other cells
(glial cells) to speed signaling
Glial Cells
A typical neuron & formation of the myelin sheath
Big idea: Neuron membranes have a
charge.
• Every cell has a voltage (difference in
electrical charge) across its plasma
membrane called a membrane potential
• Messages are transmitted as changes in
membrane potential
• The resting potential is the membrane
potential of a neuron not sending signals
The Resting Potential
• Why do neurons have a resting potential?
• Lets look at one ion - potassium (K+) – that
is found in your neurons
• Cells have large amounts of potassium
inside them and small amounts outside.
• Neurons have channels that let potassium
cross the membrane – what does this do?
Electrochemical
Gradients
Figure 12–9c, d
Electrochemical
Gradients
Figure 12–9a, b
Inner
chamber
–90 mV
Outer
chamber
140 mM
KCI
5 mM
KCI
K+
Cl–
Potassium
channel
(a) Membrane selectively permeable to K+
• Potassium stops
moving when
charge is -90mV
– Why?
The Resting Potential
• Of course there are more charged ions and
molecules inside a neuron
• Sodium (Na+) is a key player in neuron
signaling.
• There is lots of sodium outside the cell!
OUTSIDE [K+]
CELL
5 mM
INSIDE [K+]
CELL 140 mM
(a)
[Na+]
150 mM
[Na+]
15 mM
[A–]
100 mM
• Two key ions for
neurons
• Other molecules
and ions add
negative charge
to the inside of a
neuron.
The Resting Potential
• In your neuron the concentration of K+ is
greater inside the cell, while the
concentration of Na+ is greater outside
• How do your neurons maintain this
difference?
Active resting in neurons
• Neurons are constantly working to maintain
“resting” conditions
• This is because the membrane leaks ions
• A neuron at rest contains many open K+
channels and few open Na+ channels; so
lots of K+ diffuses out of the cell
• Active transport allows cells to maintain
concentration gradients that differ from their
surroundings
• The sodium-potassium pump is one type
of active transport system
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
CYTOPLASM
Na+
[Na+] low
[K+] high
1 Cytoplasmic Na+ binds to
the sodium-potassium pump.
Na+
Na+
Na+
P
ADP
ATP
2 Na+ binding stimulates
phosphorylation by ATP.
Na+
Na+
Na+
P
3 Phosphorylation causes
the protein to change its
shape. Na+ is expelled to
the outside.
P
P
4 K+ binds on the
extracellular side and
triggers release of the
phosphate group.
5 Loss of the phosphate
restores the protein’s original
shape.
K+ is released, and the
cycle repeats.
• K+ constantly leaks out of the neuron
• The flow of K+ ions out of the cell helps to
maintain the resting potential
• A neuron at rest has a potential about -70 mV
Big idea: Action potentials are the
signals conducted by axons
• Signals are passed down an axon as
spikes in membrane potential
• These spikes, that briefly reverse
membrane polarity, are called action
potentials
• These action potentials are the basic
form of communication for neurons
(a) Gentle
touch
1
fires slowly
2
silent
2
1
Changing membrane potential
• Neurons contain gated ion channels that
open or close in response to stimuli
• Membrane potential changes in response to
opening or closing of these channels
• What would happen if K+ permeability
increased?
3 Conditions of Gated
Channels
1. Closed, but capable of opening
2. Open (activated)
3. Closed, not capable of opening
(inactivated)
Stimuli
Membrane potential (mV)
+50
0
–50 Threshold
Resting
potential
–100
Hyperpolarizations
0
(a) hyperpolarizations
1 2 3 4 5
Time (msec)
• When gated K+
channels open, K+
diffuses out, making
the inside of the cell
more negative
• This is called
hyperpolarization
• What if Na+ gates
open?
Stimuli
Membrane potential (mV)
+50
0
–50 Threshold
Resting
potential
Depolarizations
–100
0 1 2 3 4 5
Time (msec)
(b) depolarizations
• If gated Na+
channels open and
Na+ diffuses into the
cell
• This causes a
depolarization, a
reduction in the
membrane potential
Stimuli
Membrane potential (mV)
+50
0
–50 Threshold
Resting
potential
Depolarizations
–100
0 1 2 3 4 5
Time (msec)
(b) depolarizations
• If enough open the
membrane in this
region reaches
threshold
• At this point a large
number of Na+
channels open and
sodium pours in
• What would this do
to membrane
potential?
• Membrane polarity
flips!
Action
potential
• Then these channels
shut & K+ open
• Potential drops back
as K+ ions flow out
• This spike in charge
is an action potential!
Strong depolarizing stimulus
Membrane potential (mV)
+50
0
–50 Threshold
Resting
potential
–100
0
(c) Action potential
1 2 3 4 5
Time (msec)
6
• This flipping and returning of the
membrane potential is passed along a
neuron down it’s axon
• The action potential flows down the axon
as depolarization is pushed ahead of the
action potential (propagation)
Big idea: Action potentials
•
starts with a slight depolarization
membrane (closer to 0mv)
of
– often no action potential is fired if
Threshold isn’t hit
•
gated
at ~ -50mv
channels open –
allowing Na+ to pour (in/out)
Big idea: Action potentials
•
at ~ -50mv gated channels open – allowing
Na+ to pour (out!)
– this causes membrane potential to flip
– They slam shut
after a very short time
(~1msec.)
•
K+ channels also respond to voltage – but
they are much slower
– K+ pours (in/out) – reversing the charge again
– They shut after driving charge below resting
Axon
Action
potential
Na+
Plasma
membrane
Cytosol
Axon
Plasma
membrane
Action
potential
Cytosol
Na+
K+
Action
potential
Na+
K+
Axon
Plasma
membrane
Action
potential
Cytosol
Na+
K+
Action
potential
Na+
K+
K+
Action
potential
Na+
K+
• Because the sodium gates lock shut an
action potential cannot move “backwards”
• During the refractory period after an
action potential, a second action potential
cannot be initiated
• The refractory period is a result of a
temporary inactivation of the Na+ channels
Figure 34.5 How an
action potential is
generated
Generation of Action Potentials
What happens at the end of the
axon?
• Axons end at a synapse
• This is a small gap between one neuron
and another (or sometimes another cell)
• Chemicals called neurotransmitters carry
information across the gap
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Presynaptic
cell
Axon
Synapse
Synaptic terminals
Postsynaptic cell
Neurotransmitter
A synapse between two neurons
5
Synaptic vesicles
containing
neurotransmitter
Voltage-gated
Ca2+ channel
Postsynaptic
membrane
1 Ca2+
4
2
Synaptic
cleft
Presynaptic
membrane
3
Ligand-gated
ion channels
6
K+
Na+
• The presynaptic neuron synthesizes and
packages neurotransmitter in synaptic
vesicles located in the synaptic terminal
• The action potential causes the release of
the neurotransmitter
• The neurotransmitter diffuses across the
synaptic cleft and is received by the
postsynaptic cell
Synaptic
terminals
of presynaptic
neurons
5 µm
Postsynaptic
neuron