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Lecture

Lecture 1


Department
Biology
Course Code
BIO271H1
Professor
Ohana

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Lecture 1
Structure & Function of Neurons; Actions Potentials
Central Nervous System
Peripheral Nervous System
Organization of the Nervous System
oMade up of neurons and glia
oNeurons can be further divided
Sensory neurons provide input (external or internal
world)
Interneurons perform computational processing
Motor neurons efferent fibers, provide output from
brain
Neurons
oVary in structure and properties
oUse same basic mechanisms to send signals
o**Neurons vary in shape and size, but most neurons are divided
into four functional regions, each specialized for a particular
task; signal reception, signal integration, signal conduction, or
signal transmission to other cells
oSignal reception reception of input from outside world, receives
signals
oSignal integration integration of signal
oSignal conduction conducts integrated signals along the
neuron, potentials across long distances
oSignal transmission neurons transmits signal to effector or to
next neuron
oTherefore, signals are transmitted from one end of the neuron to
the other, but not in the opposite direction (specific polarity in
neuron)
Neural Zones
oFour functional zones motor neuron (sends signals from CNS
to skeletal muscles, control animal movement)
Signal reception
Dendrites and the cell body (soma)
Incoming signal received and converted to change
in membrane potential
Signal integration
Axon hillock located at junction of the cell body
and the axon; incoming signals from dendrites and
the cell body are conducted to the axon hillock
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Strong signal is converted to an action potential
(AP)
A change in membrane potential initiates APs
Signal conduction
AP conducted to axon terminals (some wrapped in
myelin sheath which aids conduction)
AP travels down axon
Signal transmission
Axon terminals
Release of neurotransmitter transmits a signal to
the target cell
oSummary notes
Membrane Potential
oLike all cells, neurons have a resting membrane potential, which
is negative at rest
oNeurons have resting potential, negative at rest
oPotential a difference between charges of the inside and
outside
oWhat determines membrane potential?
Differences in ion concentrations ([X]in/[X]out)
Selective permeability of the membrane not any ion can
go through any channel at any time
Membrane Separates Charges
oMembrane acts as a capacitor maintains separation of charges
oIons pass to the other side of the membrane through ion
channels, and are transported or pumped across by other
proteins
oPumps and transporters are active utilize energy (ATP in
pumps)
oNormal at rest is -70 volts
oWhen further away from membrane, dont have potential (no
differences seen)
Ion Channels
oAllow passive transport of ions across the membrane
oExhibit ion selectivity
oSome channels are open most of the time, while others are gated
by stimuli (ligand-gated channels, voltage-gated channels)
Electrochemical Equilibrium
Electrochemical Potential
oWhen ions diffuse across membrane they carry a net charge and
generate an electrical potential difference
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oToo much positive charge K ions repelled
Nernst Equation
oCalculates potential required to balance concentration gradient
oEquation in notes
Membrane Potential
oFactors contributing to membrane potential:
oDistribution of ions across membrane
oRelative permeability of the ions
oCharges of the ions
oGoldman Equation for membrane potential (Em) take into
consideration Na, K, and Cl
The Goldman Equation
oEm membrane potential
oR gas constant
oT temperature (Kelvin)
oF Faradays constant
oPx relative permeability of ion
oDistribution of Na is opposite of K
oDepends on which channel is open, permeability etc.
oOther ions (Ca, Mg etc.) are ignored in this simplified form of
the equation because their permeabilities are very low
Gated Ion Channels
oNeurons depolarize or hyperpolarize by selectively altering
permeability
oGated ion channels open or close in response to a stimulus
Ex. Neurotransmitter
oChannels allow only specific ions to pass through the membrane
Ion moves down its electrochemical gradient
Only relatively small numbers of ion move across
concentration gradient stays the same and is not affected
oAs permeability to a specific ion increases, membrane potential
will approach that ions equilibrium potential (Nernst equation)
Changes in Membrane Potential
oHyperpolarization K channel, flows inside outside (more
negative membrane potential)
oWhen Na channels open, Na flows from outside inside,
making the membrane less polar, bringing positive charges
inside
o**Resting membrane potential of a neuron is usually about -70
mV; during depolarization, the membrane potential becomes
less negative; during hyperpolarization, membrane potential
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