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Neurophysiology Part One

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Title: Neurophysiology Part One


1
Neurophysiology Part One
  • Anatomy Organization
  • Membrane Excitation
  • Electrical Electro Chemical Properties
  • Action Resting Potentials

2
Neuronal Structure, Function Organization
  • Neuron are highly specialized cells to receive,
    process and transmit information with high
    fidelity without loss of signal strength over
    distance
  • Basic structure soma (cell body) thin fibers
    emanating from soma (nerve processes) 2 types
  • Multiple dendrites a single axon
  • Dendrites (many branches the more branches, the
    more input from many other neurons) receive
    information axon conducts signal from soma
    (tend to be longer) ending in axon terminals to
    other neurons, glands or muscles myelin sheath

3
Signal TransmissionMotor-neuron as an Example
  • plasma membrane of dendrites soma receive
    signal from (innervation) terminals of other
    neurons
  • Spike-initiating zone (specific region of
    membrane) integrates signal thus determines if
    neuron will generate its own signal (action
    potential or AP)
  • In AP the voltage across membrane rapidly rises
    falls spike/nerve impulse
  • Axon carries AP from point of origin to axon
    terminals to skeletal muscle fibers
  • Some spike-initiating zones axon hillocks near
    soma

4
Physiological Behavior
  • Depends on passive electrical properties e.g.
    capacitance resistance (like other electrical
    conductors discussed later) active electrical
    properties allow conduction without decrement
    (no loss of signal strength)
  • Active properties depend on presence of
    voltage-gated ion channels in plasma membrane
    (specific proteins) allowing ions to move across
    membrane in regulated fashion ion channels
    localized to special regions having specialized
    signaling functions e.g. axon membrane is
    specialized for conduction of APs by having
    fast-acting, voltage-gated ion channels
    selectively allowing Na K to cross membrane

5
Transmission between Neurons
  • Sensory neurons collect info externally/internally
    send it to other neurons sensory axons called
    afferent fibers (carry signal inward toward
    higher processing centers)
  • Interneurons most numerous type lie entirely
    within CNS carry info between other neurons
    info exchanged at special locations (synapses)
  • To respond to info, neurons at effector organs
    (e.g. muscle, gland) must be activated
    neurons/fibers carrying info from effector organs
    efferents
  • Afferents interneurons efferents the
    synapses in between neural circuit

6
Some terminology
  • Presynaptic and post-synaptic neurons
  • Neurotransmitters specific molecules released
    from pre-synaptic axon terminals in response to
    the APs in its axon ( carries transmission to
    the next neuron)
  • Plasma membrane of post-synaptic neuronal soma
    dendrites contain ligand-gated ion channels
    bind neurotransmitters cause postsynaptic cell
    to respond to presence of the chemical signal
  • s signals must be integrated to produce change
    in membrane potential at spike-initiating zone
  • All-or-none signals (amplitude of signal
    invariant) vs. gradient signals (amplitude of
    signal varies with stimulus strength or other
    variable)

7
Organization
  • Neurons glia cells
  • Neurons sensory, interneurons and motor
  • Sensory transform energy of stimulus into
    electrical signals
  • Interneurons exchange info perform complex
    computations producing thought behavior
  • Motor output, carrying specific instructions to
    muscles or glands
  • Gathered in clusters somata of most in CNS
    brain nerve cord (invertebrates brain
    ganglia vertebrates have ganglia outside CNS
    spinal cord)

8
Organization cont
  • Glia cells (neuroglia) fill space between
    neurons (except for thin fluid-filled space)
  • More complex animals more neuroglia
  • 10x-50x more neuroglia than neurons vertebrates
    occupy volume of CNS
  • Generally dont produce APs

9
Glia Cells Function ?
  • Possibilities
  • structural metabolic support for neurons some
    glia oligodentrocytes (CNS) Schwann cells
    (periphery) wrap axons in an insulting myelin
    sheath contributes to reliable rapid
    transmission of APs and/or
  • help regulate concentration of K pH of
    fluid-filled spaces - highly perm to K
    build-up of K deleterious for neuron and/or
  • remove neurotransmitters from extracellular space
    limits time of neurotransmitters action)

10
Membrane Excitation
  • Stable voltage (electric potential difference)
    exists across plasma membrane of all animal cells
    however, only membranes of electrically excitable
    cells (neurons/muscle fibers) can respond to
    changes in transmembrane potential differences by
    generating APs
  • Membrane potential (difference across the
    membrane) measured in volts

11
More Terminology
  • Hyperpolarization increase in potential
    difference across plasma membrane when current
    pulse causes positive charge to exit i.e.
    interior of cell becomes more negative - cell
    membrane responds passively
  • Depolarization decrease in potential difference
    across plasma membrane if current pulse causes
    addition of positive charge to interior of cell
    some voltage-gated channels selectively perm to
    Na to open if sufficiently depolarized, AP is
    triggered) - Threshold potential the value of
    the membrane potential where an AP is triggered
    50 of time

12
Role/Types of Ion Channels
  • ion selectivity (electrochemical gradient
    specific to certain ions e.g. Na, K)
  • voltage-gated (produce APs open when plasma
    membrane depolarizes)
  • leak (maintain resting potential where passive
    change occurs during hyperpolarization mostly
    K)
  • ligand-gated (bind to messenger molecules e.g.
    neurotransmitters)

13
Passive Electrical Properties
  • Lipid bilayer impermeable to ions acts as
    insulator forming an electrical capacitor (stores
    energy in form of separated electric charges)
    channel proteins allow ions to pass across
    membrane give it its electrical conductance
  • Resistance (R) Conductance (g) R of membrane
    measure of impermeability to ions g
    permeability R 1/g i.e. the lower the
    resistance, the greater the conductance more
    ionic charges cross open ion channels/time
    Ohms law pertains voltage drop produced across
    membrane by a current that passes through it is
    directly proportional current x resistance of the
    membrane (Vm I x R) total R encountered by
    current flowing into out of cell input R

14
Passive Electrical Properties cont
  • Capacitance ability of membrane to store
    electrical charge movement of ions up to one
    side of membrane away from the other ionic
    current interactions between oppositely charged
    ions accumulating on both sides of membrane is
    strong because membrane is thin
  • Dielectric constant property reflects inherent
    ability of a particular capacitor to store charge

15
Electrochemical Potentials
  • Defn voltage difference across plasma membrane
    that depends on
  • Concentration of various ions is different on
    either side of membrane concentration gradient
    maintained at the expense of metabolic energy
  • Ion channels are selectively perm to ions
  • 1 2 result in membrane potential membrane
    potential passive electrical properties basis
    of signaling by neurons

16
Electrochemical Potentials cont
  • Equilibrium potential voltage difference across
    a semi-permeable at which an ion can diffuse
    across is in electrochemical equilibrium (depends
    on concentration gradient) if energy is
    required to maintain a potential, it is not an
    equilibrium state but a steady state (e.g.
    dynamic equilibrium) membrane resting potential
    is a steady-state potential
  • Various laws of physics/electrical formulas
    apply to membranes, cells conduction of signals

17
Resting Potential
  • Every cell in its non-excited or resting state
    has a potential difference (Vrest) across
    membrane governed by 2 factors
  • Presence of open selective ion channels Resting
    Potentials of muscle, nerve and other cells found
    to be far more sensitive to changes in K than
    other ion consistent with high perm of plasma
    membranes to K and this depends on K selective
    leak channels which remain open in the resting
    membrane
  • Unequal distribution of inorganic ions between
    cells interior exterior (maintained by active
    transport esp. Na/K pump see Fig 5-15 p. 132
    Donnan equilibrium)

18
Action Potential
  • Most neurons use the AP to send info along the
    axon and APs in nervous system basis of
    sensation, memory?, thought ...
  • AP large, brief change in Vm propagated along
    axon ( over great distances) without decrement
    production of AP depends on 3 key elements

19
Action Potential cont
  • Active transport of ions by specific proteins in
    plasma membrane generates asymmetric
    concentrations of ions across membrane
  • Unequal distribution of ions generates
    electrochemical gradient across membrane
    providing a reservoir of potential energy
  • Electrochemical gradient drives ions across
    membrane when ion-selective channels open, making
    mem. Perm. to certain ions this ionic current
    dramatically changes Vm

20
General Properties of APs
  • Threshold current intensity of stimulating
    current sufficient to bring membrane to its
    threshold potential elicit an AP not usually
    measurable in an absolute amount
  • once threshold potential reached, AP becomes
    regenerative event becomes self-perpetuating
    Vm continues to become more inside-positive
    without further stimulation reaching a peak
    (brief time called overshoot - neurons
    millisecond cardiac ½ second) after peak, Vm
    drops to ( goes beyond) its more negative state
    also transient called hyperpolarization or
    undershoot)

21
General Properties of APs cont
  • Refractory brief time after AP when another AP
    cannot be initiated (absolute refractory period)
    if stim. delivered slightly later, AP may be
    initiated but the amplitude may be smaller than
    usual generally the threshold potential is
    higher during this time relative refractory
    period (all-or-nothing doesnt apply here) toilet
    flush analogy
  • Accommodation temporary increase in threshold
    that develops during the course of a stimulus

22
Accommodation cont
  • Determines how individual neurons respond to
    input whether they are continuously active or
    produce only bursts of APs
  • Phasic response when stimulated continuously by
    a current of constant intensity, some neurons
    accommodate rapidly generate only 1 or 2 APs at
    beginning of stim.
  • Tonic response when accommodating more slowly
    fire repeatedly although with gradually
    decreasing frequency in response to a prolonged
    constant stim

23
Accommodation cont
  • reduction in frequency of APs typically seen in
    neurons responding tonically during a sustained
    stimulus, visible as increasing distance between
    APs, is adaptation

24
Review
  • at rest the membrane most perm to K, but in
    early phase of AP, it becomes more perm to Na
    than to K at rest
  • inc. perm strong force pushing Na into cell
    spurt of charge enters cell less
    inside-negative
  • when voltage-gated Na channels close perm to
    Na drops, mem. more perm to K than is at rest,
    because voltage-gated K channels are still open
  • later, perm to K drops back to resting value
    with only passive leak channels remaining open

25
A Closer Look at Ion Channels
  • 4 key features of ion channels
  • Distribution of ion channels in neuronal
    membranes
  • Nature of current flow through a single channel
  • Mechanism by which depolarization of membrane
    opens a voltage-gated channel
  • Physical basis for channel selectivity
  • (Molecular structure has been determined only
    recently seems homologous in structure across
    many species and yet it heterogeneity shows many
    genes are expressed)

26
Voltage-gated Na Channels
  • Na channels occupy only small fraction of total
    surface area however, each channel can pass a
    very large amount of Na per second provides
    sufficient Na to account for macroscopic
    currents
  • 1 activated Na channel carries Na ions at a
    rate of 10,000 ions per millisecond driving
    force as an AP gets underway summed activity
    (I.e. opening closings) of 1000s of Na
    channels, each allowing a minute unitary current
    (i.e. current through a single channel) to cross
    membrane gives rise to macroscopic current

27
Voltage-gated Na Channels cont
  • Gating occurs in distinct steps (Fig 5-22 p. 142)
  • Activation inactivation of the channel are
    coupled processes
  • Selectivity filter area of channel that
    determines its selectivity (based on size,
    charge, level of hydration, etc.)

28
Voltage-gated Na Channels overview of activity
  • During AP, Na channels respond to an initial
    depolarization by opening allows Na to enter
    cell this further depolarizes the membrane
    which cause more channels to open allowing more
    Na to enter creates an explosive, regenerative
    event once AP starts, needs no more stimulus
  • Relationship between membrane potential Na
    conductance (Hodgkin cycle) is e.g. of feedback
  • As membrane potential reaches equilibrium
    potential, driving force on Na is reduced, so
    less Na is driven into the cell open Na
    channels become inactivated after time

29
Voltage-gated K Channels
  • Respond more slowly to voltage changes
  • Membrane conductance increases very little until
    AP near its peak remains elevated during
    falling phase
  • Properties vary more than Na channels
  • Rapid membrane repolarization produced by current
    through K channels does shorten the AP, allowing
    neurons to generate APs at a higher frequency
    than they otherwise could
  • Falling phase of AP depends on inactivation of
    Na channels continued activation of K channels
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