D_G_S_I_N
1. U S O I
2. U O M A
3. E O I T
4. O G U U
Neural Control
2011 Spring
Lecture 38: Introduction to the Nervous System
4. How do neurons pass information along the length of the neuron? That is, how does an action potential propagate?
5. How do neurons communicate with other neurons or effectors?
1. Who has nerves?
2. What do nerves do?
3. Neuron design?
Multi-cellular Organism
Information highways
1. Hormones – slow
2. Neurons - fast
Why care about the nervous system?
• Below: Components of the animal mechanism for maintaining homeostasis in response to a change in internal environmental conditions.
RECEPTOR(e.g., nerve)
INTEGRATOR(e.g., brain)
EFFECTOR(e.g., gland)
Stimulus
Response (change)feedback
• Nervous system plays key role as receptors, pathways for transmitting information, integration and relaying commands for response by effector.
What is the nervous system?
• Organ system that
•detects stimuli
•integrates information and
•relays commands
• Fundamental units of forming tissues and organs: ∙ neurons
∙ glial cells
• In complex organisms, responsible for memory.
What organisms actually use a nervous system to detect and respond to stimuli?
• All living organisms detect & respond to stimuli
• Nervous system can only exist in multicellular organisms (select groups within Eukarya) but which ones?
MolluscaAnnelidaBrachiopodaGymnolaemataNematodaArthropodaRotiferaTrematodaEchinodermataUrochordataCephalochordataCraniataCnidariaCtenophora
Porifera
After Valentine (2005)
Animals
Sea
Anemone
‘lowest’
organisms to
have a nervous
system
What are the three general classes of neurons?
• 1. Sensory neurons
∙ Detect stimulus & relay stimulus to interneurons
• 2. Interneurons
∙ Integrators of information from sensory neurons
• 3. Motor neurons
∙ communicates signal to effectors
∙ signal can be;
∙ inhibitory ∙ excitatory
Organization in Higher Animals –Humans too
• Nervous System
– Central Nervous System
• Brain
• Spinal Cord
– Peripheral Nervous System
• Somatic – Control over muscles
• Autonomic
– Sympathetic
– Parasympathetic
What is the structure of a neuron cell? How does it reflect its function?
• 4 major zones:∙ 1. Input zone - composed of
dendrites
cell bodyINPUT ZONE
Fig 34.6
Motor neuron
∙ dendrites – cytoplasmic extensions receive signals (input)
∙ cell body – dendrites extend from cell body
What is the structure of a neuron? How does it reflect its function?
∙ 2. Trigger zone – patch of membrane adjoining input zone where input becomes encoded into ‘action potential’ if stimulus sufficiently large
dendrites
cell body
TRIGGER ZONE
INPUT ZONE
CONDUCTING ZONE
axon
Fig 34.6
Motor neuron
∙ 3. Conducting zone – ‘wire’ delivers signal from input location (where cell body /dendrites are) to destination location where axon terminates
∙ axon – cytoplasmic extension which propagates signal received at input (if strong enough) in form of ‘action potential’
∙ 4. Output zone – composed of
dendrites
cell body
TRIGGER ZONE
INPUT ZONE
CONDUCTING ZONE
OUPUT ZONEaxon
axon
endingsFig 34.6
Motor neuron
∙ Branched ending of many axons where action potential is converted to a chemical signal to be passed to neighboring cells
What is the structure of a neuron? How does it reflect its function?
Where is the longest human neuron?
• Answer: Base of spine to toe!
• Over 1m long
• Giant Squid neurons
How does a neuron (any neuron) pass along a signal?
• Involves 2 steps:
∙ no action potential, no signal propagation
1) input must generate an action potential (AP)
2) Action potential induces release of neurotransmitters
at output zone
∙ AP originates at trigger zone as a result of stimulus
received at input zone
∙ travels along axon to output zone
∙ neurotransmitters are chemicals released from axon endings
∙ generates response in next neuron or cells of effector
What is an action potential (AP)? How are they caused? How do they work?
• self-propagating electrical signal caused by an abrupt reversal in the voltage difference across a plasma membrane of a neuron
∙ At rest neurons (ie. unstimulated) maintain an electrical gradient across plasma membrane = resting potential = -70 mV
What is the electrochemical structure of the neuron? How does this enable an action potential?
Fig 34.8 Cytoplasm
Interstitial fluid
Plasma membrane
What causes this charge difference (i.e. resting potential)?
• Due to creation & maintenance of differences in concentration of potassium (K+) and sodium (Na+) ions on inside and outside of membrane
∙ Sodium higher outside ∙ Potassium higher inside
What creates the Na+ and K+ gradients in the first place?
• Sodium-potassium pump – a transmembrane protein that pumps Na+ ions to the outside and K+ ions into the inside (cytoplasm)
∙ Na+/K+ pumps require ATP = active transport∙ 3 Na pumped for every 2 K allowed in!
Na+/K+ pump
Interstitial fluid
(outside)
CytoplasmK+ pumped in
Na+ pumped out
Na+K+
Na+
K++
-
Plasma membrane -70mV
Why don’t Na+ and K+ flow back (into/out of the cell) along their concentration gradients? i.e. What maintains these gradients?
• K+ and Na+ can only move through channels (transport proteins), not plasma membrane
Passive transporters with voltage-sensitive gated channels
Lipid bilayer of neuron membrane
Interstitial fluid
Cytoplasm
a) Na+ movement controlled by passive transporters with voltage sensitive gated channels that are closed when neuron at rest.
∙ There is always the tiniest bit of Na+ leaking into cell via incompletely sealed channels
Na+
Na+leaking
+
-
Passive transporters with open channels
Interstitial fluid
Cytoplasm
b) K+ can only move via passive transporters with open channels
Na+
Na+K+
K+
∙ K+ can diffuse out of cell along concentration gradient when cell at rest
∙ BUT movement out makes cytoplasm slightly more negative attracting some K+ back into the cell
∙ K+ concentration gradient maintained by balance btw diffusion out and diffusion back in due to electrical gradient
+
-
Why don’t Na+ and K+ flow back (into and out of the cell) along their concentration gradients? I.e. What maintains these gradients?
So what maintains these gradients in neuron at rest despite the leaking?
Na+
Na+K+
K+ +
-
Interstitial fluid
CytoplasmK+ pumped in
Na+ pumped out
• Na+/K+ pumps which return leaked Na+ ions to the outside (and some K+ ions to the inside) to maintain the gradients and thus the voltage difference across the plasma membrane.
• Stimulus elicits an electrical disturbance in input zone.
∙ Causes some Na+ ions to flow into the neuron.
∙ If stimulus is intense enough or lasts long enough, disturbance reaches the trigger zone.
∙ Trigger zone is rich with gated sodium channels, creating potential for mass diffusion of Na+ down concentration & electrical gradients.
What determines whether or not an action potential occurs?
dendrites
cell body
TRIGGER ZONE
INPUT ZONE
CONDUCTING ZONE
OUPUT ZONEaxon
axon
endings
Motor neuron
• 1. Disturbance strong enough to elicit action potential.
∙ Happens when enough sodium channels open in trigger zone to trigger positive feedback loop leading more channels to open.
What can happen once the electrical disturbance reaches the trigger zone?
Na+ in
Cytoplasm becomes more positive
Causes more voltage gatedNa+ channels to opencytoplasm
Interstitialfluid
∙ Called depolarization - charge difference across membrane decreasing, i.e. not so polarized – value increasing from -70 to -10 to + 30
• Positive feedback loop initiated once reach threshold levelmembrane potential (i.e. depolarized to threshold potential).
What determines if sufficient depolarization to set positive feedback loop in motion?
∙ At threshold, Na+ gates opening because of positive feedback, no longer depends upon strength or duration of stimulus.
∙ Once reach threshold, positive feedback ensures action potential will happen.
• 2. What happens if stimulation of input zone is not sufficient to cause enough depolarization to reach threshold in trigger zone?
What can happen once the electrical disturbance reaches the trigger zone?
∙ action potential will not happen
∙ too few Na+ crossed the membrane to initiated positive feedback loop
∙ What restores resting potential?
Fig 34.9
∙ neuron will be returned to resting potential.
• Discovered gene that results in an inability to sense pain in family in Pakistan (Nature. 2006. 444: 831)
What is an example of a phenotype that results from inability to trigger an action potential?
∙ Individuals with condition (homozygous recessive) injure themselves all the time because lack signal that tissue damaged.
Why feel no pain?
• mutation in gene coding for Na+ channel in nerves responsible for pain detection.
∙ mutation results in non-functional Na+ channel
∙ unable to trigger action potential in pain neurons
• mutation in same gene causes over sensitivity to stimuli causing erythromelalgia – severe pain in response to mild stimuli, ex. mild warmth
• Enough sodium gates open to reverse membrane potential: - outside, + inside.
• With voltage reversal,
What happens after threshold reached to complete action potential?
∙ K+ diffuses out along concentration and electrical gradient
∙ Na+ gates close
∙ K+ gates open = gated K+ channel, differs from open channel responsible for resting potential
Fig 34.8
∙ Restores voltage difference to + outside & - inside (actually overshoots a bit).
K+
Na+
-
+
Fig 34.8
Another action potential cannot immediately occur in this location. Why?
• Voltage gated Na+ channels have refractory period.
∙ Brief time when Na+ gates cannot open again after they have opened and closed in course of previous action potential.
• Preceding about processes taking place in one location of membrane.
What causes the action potential to move along the axon away from the trigger zone?
• Disturbance caused by action potential in one part of membrane, initiates action potential at adjacent site.
• So signal is passed down axon from trigger zone to output zone as a series of action potentials, one initiating the next.
So how is the signal propagated down the axon?
Trigger zone Output zone
Team Questions: 5 points:A poison that specifically disables the Na+/K+ pumps is added to a
culture of neurons. What effect does this eventually have on the
neurons?
a) The resting membrane potential goes to
zero.
b) The inside of the neuron would become
more negative relative to the outside.
c) The inside of the neuron would become
positively charged relative to the outside.
d) Sodium would diffuse out of the cell and
potassium would diffuse into the cell.
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Team Questions: 5 points:
A(n) ___ in Na+ permeability and/or a(n) ___ in K+ permeability
across a neuron’s plasma membrane would cause a shift in the
membrane potential from -70 mV to -80mV.
a) increase; increase
b) increase; decrease
c) decrease; increase
d) decrease; decrease
e) none of the above
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Presynaptic cell
Postsynaptic cell
How is signal communicated from cell to cell?
• Signal must be passed from neuron to another neuron (ex. from sensory neuron to interneuron) or to an effector. How?
∙ Joint between output zone of one neuron and input zone of another neuron or between output zone & plasma membrane an effector cell.
What is a chemical synapse? Fig 34.10
Postsynaptic cell’s plasma membrane
Presynaptic cell
Synaptic cleft = gap
What happens when the action potential reaches the output zone?
• Action potential typically causes neuron to release molecules of a class of biochemicals called a neurotransmitters from synaptic vessel.
∙ Ex. serotonin, dopamine, melatonin
• Synaptic vessels fuse with plasma membrane of presynaptic cell & release neurotransmitter into synaptic cleft.
synaptic
vesicle
membrane
receptor
synaptic
cleft
Fig 34.10
What does the neurotransmitter do?
• Diffuses across synapse & binds to receptors on plasma membrane of postsynaptic cell, which opens ion channels.
∙ Ions (ex. Ca++, Na+, K+) diffuse into postsynaptic cell
neurotransmitter
receptor for neurotransmitter
gated channel protein
(postsynaptic cell)
∙ Biochemical signal converted back into electrical signal = flowing ions.
Ions (in synaptic cleft)
How do these ions affect the postsynaptic cell?
• May have excitatory effect on postsynaptic cell
∙ Act to depolarize membrane and thus, increase chances that trigger zone will receive sufficient stimulation to drive it to the threshold and initiate action potential.
∙ Move membrane farther away from threshold – increase polarization = hyperpolarization – reduce chances of action potential.
• May have inhibitory effect on postsynaptic cell
What determines whether postsynaptic cell is excited or inhibited?
∙ Ex. GABA and valium
• Type & concentration of neurotransmitter
• Number & type of receptors
• Number & type of voltage gated channels
• Type of cell it is
Which of the following choices best describes
what is happening at step four in the graph
below?
1. Some Na channels close.
2. Most Na channels open.
3. Some K channels close.
4. Most K channels open.
5. Na/K pumps are inactivated.
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