Transcript October 6

How Neurons Generate
Signals
The Neuron at Rest
Stepping on a Thumbtack
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Reflexive withdrawal of the foot – a simple
behavior controlled by a circuit with direct
connections between:
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Sensory neurons (responding to environment)
Motor neurons (controlling muscles)
Interneurons (to inhibit opposing muscles)
Projection neurons (carrying sensation to the
brain)
Messages are carried via action potentials.
Biochemical Reactions
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Ion – an electrically charged atom.
Polar covalent bonds – a molecule held
together by sharing electrons (H2O).
Ionic bond – a molecule held together by the
attraction of atoms with opposite charges
(NaCl – table salt).
Cation – ion with a net positive charge.
Anion – ion with a net negative charge.
The Neuron at Rest
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Neurons have potassium (K+) inside and
sodium (Na+) outside in the extracellular
fluid.
Ion channels in the cell wall (membrane) are
selectively permeable to potassium, sodium or
calcium.
Ion pumps maintain the cell’s inner
environment.
How Ions Cross the Membrane
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Diffusion – an ionic concentration gradient
exists
Differences in electrical membrane potential
and equilibrium potential
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Ionic driving force
Ion pumps
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Sodium/potassium, calcium
Electricity
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Opposite charges attract, like charges repel.
Current (I) – movement of electrical charge,
measured in amps.
Potential (voltage, V) – force exerted on a
particle, difference between terminals,
measured in volts.
Conductance (g) – relative ability of a charge
to migrate, measured in Siemens.
Resting Potential
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Membrane potential is voltage across the
neuronal membrane.
Resting potential is the point at which all of
the forces acting upon ions are in balance
(equilibrium).
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Diffusional and electrical forces are equal.
Vm = -65 mV
Two Important Equations
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Nernst equation – calculates the equilibrium
potential for a single ion, given knowledge of
its concentration inside & outside the neuron.
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Varies with body temperature.
Goldman equation – calculates the resting
potential of a neuron, given knowledge of its
permeability to various ions.
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Varies with ion concentrations.
Regulation of Potassium
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The neuron at rest is mostly permeable to
potassium (K+) so the resting potential is
close to EK.
Increasing potassium outside the cell leads to
depolarization (an increase in the resting
potential making it less negative).
To prevent this, potassium is tightly regulated
by the body via glia & blood-brain barrier.