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Nucleus
Dendrites Cell body
Axon
Collect
electrical
signals
Passes electrical signals
to dendrites of another
cell or to an effector cell
Integrates incoming signals
and generates outgoing
signal to axon
The membrane potential drives the responsiveness to stimulation.
How are signals conducted along the length of a neuron?
OUTSIDE
INSIDE -70mV
Na+
440 mM
K+
20 mM
Na+
50 mM
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400 mM
1. Depolarization
phase
2. Repolarization
phase
Threshold potential
Resting potential
3. Hyperpolarization phase
Figure
45.6
Action potentials propagate by positive feedback.
Speed is critical: (1) large diameter and (2) myelination
Action potentials jump down axon.
Nodes of Ranvier
Schwann cells (glia)
wrap around axon,
forming myelin sheath
WHY ACTION POTENTIALS JUMP DOWN MYELINATED AXONS
Axon
Schwann cell membrane
wrapped around axon
Schwann cell
1. As charge spreads down
an axon, myelination (via
Schwann cells) prevents
ions from leaking out across
the plasma membrane.
Node of
Ranvier
2. Charge spreads
unimpeded until it reaches
an unmyelinated section of
the axon, called the node
of Ranvier, which is packed
with Na+ channels.
3. In this way, electrical
signals continue to jump
down the axon much faster
than they can move down
an unmyelinated cell.
Sample problem.
The distance from your
toe to your spinal column
is about 1m. If your
sensory axon is 5 um in
diameter, how much time
elapses before your CNS
receives the signal?
How much time would
elapse if your nerve was
not myelinated?
What you should understand
How the generation of an action potential represents
an example of positive feedback.
How voltage gated channels generate and keep brief
the action potential.
The flows of major ions during resting,
depolarization, repolarization, and hyperpolarization.
How myelination leads to rapid propagation
velocities.
Synapses: Calcium mediates synaptic vesicle fusion
with SNARE, SNAP AND SYNAPTOTAGMIN
Neurotransmitters lead to either
Excitatory or Inhibitory Postsynaptic Potentials:
EPSPs and IPSPs
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presynaptic membrane
Neurotransmitter
Enzyme recycler
Receptor
Myasthenia gravis
Acetylcholine (Ach) binds to receptors
Positive ions flow in – depolarizing postsynaptic cell
Acetylcholinesterase breaks Ach into acetate +
choline
These are transported back into cell
Very fast (~25,000/sec)!
Summation: EPSPs and IPSPs from multiple inputs sum at postsynaptic cells
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Temporal summation
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A neuron in your spinal
column receives input
from a sensor in your leg.
Under resting conditions,
that sensor sends a
signal every 10 seconds.
Under extreme stretch of
your leg, it sends signals
every second. Why
would our spinal nerve
only respond to the more
frequent stimulus ?
Worksheet
Neurotransmitter
Enzyme recycler
Receptor
What you should understand
The roles of neurotransmitters, postsynaptic receptor
molecules and enzyme recycling components of
synapses.
Summation of IPSPs and EPSPs by postsynaptic
cells (temporal and spatial)
The consequences of up- and down-regulation of
postsynaptic receptor molecules.
Sensory systems
• Stimuli are transduced into changes in membrane
potential by ionotropic and metabotropic
mechanisms
• Four characteristics of the stimulus are encoded
–
–
–
–
Intensity: spike rate
Frequency: tuning curves
Location: receptive fields
Modality: labeled line
• Sensory systems are diverse and adapted for their
specific tasks…and amazing!
From stimulus to action potential: ionotropic
example
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From stimulus to action potential:
metabotropic example
Na+
G protein
Adenylate cyclase
ATP cAMP
GTP GDP
receptor
cAMP activates
many channels
Amplification:
1 active receptor ~10 GTP conversions each
GTP powers ~10 cAMP
about 1:100
Vision: also metabotropic
transducin
Phosphodiesterase
Disk
membrane
cGMP 5’cGMP
GTP GDP 5’ cGMP changes
many ion channels
Amplification:
1 active receptor ~500 transducin
activations -> each one converts 103 GMPs
Rhodopsin
How does a single sensory neuron
encode stimuli?
Characteristics of the stimulus: intensity, frequency, location, modality
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Ways the nervous system encodes these: spike rate, tuning curves,
receptive fields, labeled line
Spike rate
Stimulus intensity is encoded by
spike rate
Intensity
(brightness, concentration,
loudness, pressure, temperature)
Different neurons respond best to different frequencies
Threshold (dB SPL)
Louder
Quieter
Frequency
Shape of curve = selectivity for frequency
Receptive fields: area of space in which the presence of a
stimulus will alter the firing of a sensory neuron
These receptive fields form spatiotopic maps of the world on the sensory
organ… and these maps usually translate to areas of cortex as well
If all neurons communicate using action potentials, how can
we keep the modalities apart?
Specific sensory cells with specific receptors project to specific parts of the
thamalus…which project to specific parts of cortex. LABELED LINE.