Download Neurons Part 1

PowerPoint® Lecture Slides prepared by Vince Austin, University of Kentucky
Fundamentals of the
Nervous System and
Nervous Tissue
Part B
Human Anatomy & Physiology, Sixth Edition
Elaine N. Marieb
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
11
Electrical Current and the Body
 Potential energy generated by separated charges is
called voltage.
 Reflects the flow of ions rather than electrons
 There is a potential on either side of membranes
when the number of ions is different across the
membrane
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Role of Ion Channels
 Types of plasma membrane ion channels:
 Passive, or leakage, channels – always open
 Chemically gated channels – open with binding of a
specific neurotransmitter
 Voltage-gated channels – open and close in
response to membrane potential (change in charge)
 Mechanically gated channels – open and close in
response to physical deformation of receptors
PLAY
InterActive Physiology®: Nervous System I: Ion Channels
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Operation of chemical Gated Channel
Figure 11.6a
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Operation of a Voltage-Gated Channel
Figure 11.6b
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Gated Channels
 When gated channels are open:
 Ions move along chemical gradients, diffusion from
high concentration to low concentration.
 Ions move along electrical gradients, towards the
opposite charge.
Together they are called the Electrochemical Gradient
 An electrical current and Voltage changes are
created across the membrane
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Electrochemical Gradient
 The EG is the foundation of all electrical
phenomena in neurons.
 It is also what starts the Action Potential.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Resting Membrane Potential (Vr)
 The potential difference (–70 mV) across the
membrane of a resting neuron
 It is generated by different concentrations of Na+,
K+, Cl, and protein anions (A)
 The cytoplam inside a cell is negative and the
outside of the cell is positive. (Polarized)
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Membrane Potentials: Signals
 Used to integrate, send, and receive information
 Membrane potential changes are produced by:
 Changes in membrane permeability to ions
 Alterations of ion concentrations across the
membrane
 Types of signals – graded potentials and action
potentials
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Changes in Membrane Potential
 Changes are caused by three events
 Depolarization – the inside of the membrane
becomes less negative
 Repolarization – the membrane returns to its resting
membrane potential
 Hyperpolarization – the inside of the membrane
becomes more negative than the resting potential
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Changes in Membrane Potential
Figure 11.9
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Graded Potentials
 Short-lived, local changes in membrane potential
 Decrease in intensity with distance
 Their magnitude varies directly with the strength of
the stimulus
 Sufficiently strong graded potentials can initiate
action potentials
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Graded Potentials
 A stimuli from sensory input causes the gated ion
channels to open for a short period of time.
 Positive Cations flow into the cell and move
towards negative locations around the stimuli.
 Alternately the now negative area on the outside of
the cell will flow towards the positive areas.
 However, this spread of depolarization is short lived
because the lipid membrane is not a good conductor
and is very leaky, so charges quickly balance out.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Graded Potentials
Figure 11.10
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Graded Potentials
Figure 11.11
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Action Potentials (APs)
 A brief change in membrane potential from 70mV(resting) to +30mV (hyperpolarization)
 Action potentials are only generated by muscle cells
and neurons
 They do not decrease in strength over distance
 An action potential in the axon of a neuron is a
nerve impulse
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potential: Step 1 Resting State
 Na+ and K+ channels are closed
 Leakage accounts for small movements of Na+ and
K+
 Each Na+ channel has two voltage-regulated gates
 Activation gates –
closed in the resting
state
 Inactivation gates –
open in the resting
state
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.12.1
Action Potential: Step 2 Depolarization Phase
 The local depolarization current flips open the
sodium gate and Na+ rushes in.
 Threshold: when enough Na+ is inside to reach a
critical level of depolarization (-55 to -50 mV)
threshold, depolarization becomes self-generating.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.12.2
Action Potential: Step 2 Cont.
 Na + will continue to rush in making the inside less
and less negative and actually overshoots the 0mV
(balanced) mark to about +30mV.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potential: Step 3 Repolarization Phase
 After 1 ms enough Na+ has entered that positive
charges resist entering the cell.
 Sodium inactivation gates close and membrane
permeability to Na+ declines to resting levels
 As sodium gates close, voltage-sensitive K+ gates
open
 K+ exits the cell and
internal negativity
of the resting neuron
is restored
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.12.3
Action Potential: Step 4 Hyperpolarization
 Potassium gates are slow and remain open, causing
an excessive efflux of K+
 This efflux causes hyperpolarization of the
membrane (undershoot).
 The neuron is
insensitive to
stimulus and
depolarization
during this time
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Figure 11.12.4
Action Potential:
Role of the Sodium-Potassium Pump
 Repolarization
 Restores the resting electrical conditions of the
neuron
 Does not restore the resting ionic conditions
 Ionic redistribution back to resting conditions is
restored by the sodium-potassium pump
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Phases of the Action Potential
 1 – resting state
 2 – depolarization
phase
 3 – repolarization
phase
 4–
hyperpolarization
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Propagation of an Action Potential
 When one area of the cell membrane has begun to
return to resting the positivity has opened the Na+
gates of the next area of the neuron and the whole
process starts over.
 A current is created that depolarizes the adjacent
membrane in a forward direction
 The impulse propagates away from its point of
origin
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Propagation of an Action Potential (Time = 0ms)
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Figure 11.13a
Propagation of an Action Potential (Time = 1ms)
Figure 11.13b
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Propagation of an Action Potential (Time = 2ms)
Figure 11.13c
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Coding for Stimulus Intensity
 All action potentials are alike and are independent of
stimulus intensity
 Strong stimuli can generate an action potential more
often than weaker stimuli
 The CNS determines stimulus intensity by the
frequency of impulse transmission
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Conduction Velocities of Axons
 Conduction velocities vary widely among neurons
 Rate of impulse propagation is determined by:
 Axon diameter – the larger the diameter, the faster
the impulse
 Presence of a myelin sheath – myelination
dramatically increases impulse speed
PLAY
InterActive Physiology®:
Nervous System I: Action Potential
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Saltatory Conduction
 Current passes through a myelinated axon only at
the nodes of Ranvier
 Voltage-gated Na+ channels are concentrated at
these nodes and Action potentials jump from one
node to the next
 Much faster than conduction along unmyelinated
axons where the entire axon has continuous
conduction.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Saltatory Conduction
Figure 11.16
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings