Chapter 7 Nerve Cells and Electrical Signaling
7.1. Overview of the Nervous System (Figure 7.1)
7.2. Cells of the Nervous System
o Neurons are excitable cells which can generate action potentials
o Glial cells provide support to neurons.
o 90% cells in the nervous system
• A Typical Neuron (Figure 7.2)
• Components of a Neuron
a.
b.
c.
d.
e.
cell body (soma)
dendrites
axon
axon terminal
axon hillock
1) Where axon originates from the cell body, for initiation of action potential
______________
2) ________ branch from the cell body, receive input signals and convey information to
cell body .
3) ________________ contain nucleus and organelles, carry out cellular functions such as
protein synthesis and cell respiration.
4) ________________________specialized to releases neurotransmitters upon arrival of an
action potential.
5) _____________ nerve fiber, send out electrical information action potential.
•
Location of Ion Channels in Neuron Plasma Membrane
o The different functions of a neural component may be contributed by the
different channels located in the plasma membrane
 Leak channels
 Voltage-gated channels
 Ligand-gated channels
o Leak channels or called non gated channels
 Are always open
 Found throughout plasma membrane of neuron
 Are responsible for ___________________________________
o Voltage-gated Channels


Open or close in response to __________________________
Example: Sodium and potassium channels
 Throughout, but more in axon (especially axon hillock)
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 Responsible for generate or conduct ________________
Example: Calcium Channels
 Most located in axon terminal
 Trigger the release of neurotransmitter
o Ligand-gated channels


reversibly with anther chemical.

Open or close in response to a _________________________________________ binding

neurotransmitters

•
Ligand is a substance (atom, ion or a molecule) that binds specifically and
Found in most in the dendrites and cell body, regions that receive
Responsible for generating _______________potentials
Functional Classes of Neurons (Figure 7.6)
o ____________: originated in PNS, input signals from receptors
o _____________: located in CNS, integrate signals
o _____________: output signals to effectors
7.3. Establish of the Resting Membrane Potential
•
Table 7.1 Types of Electrical Potentials
•
Membrane Potential
•
•
o Due to unequal distribution of anions and cations across cell membrane strength
of force
o Usually measured in millivolts
Distribution of Ion across the Membrane
o Potassium ions and fixed anions are more abundant outside or inside of the cell.
o More sodium, chloride and calcium ions are located in outside or inside of the
cell
Equilibrium Potential (Ex) (Figure 7.6)
o Ex is the membrane potential (Vm) when electrochemical driving force = ____
o Computation of El: Nernst equation (page 99)
E1 = (61 mV/Z) log [I]o/ [I]i
Where
 El = equilibrium potential of ion l
 Z = valence of ion l
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

•
o EK+ = -90mV
o ENa+ = +60mV
Resting Membrane Potential
o
Resting member potential is the membrane potential when a cell is ________
o
•
[I]o = ECF concentration of ion l
[I]i = ICF concentration of ion l
The resting membrane potential is closer to ____(Ek or ENa)
Factors Critical in Establishing Resting Vm
Differential membrane permeability to ions
 Membrane is more permeable to potassium ions to sodium ions, not
permeable to fixed anions
o Sodium/Potassium pumps
 Prevent the dissipating of sodium and potassium gradient across the
membrane
o
Difference in ion concentration gradients across plasma membrane
Establishing Resting Membrane Potential (Figure 7.8)
o
•
•
Na+/K+ Pump and Resting Vm
o 20% of resting membrane potential directly due to Na/K-ATPase

Electrogenic: 3 Na+ out, 2 K+ in;
Net +1 out
o 80% of resting membrane potential indirectly due to Na/K-ATPase
o Produces concentration gradients


_____: high outside, low inside
_____: low outside, high inside
7.4. Electrical Signaling Through Changes in Membrane Potential
•
Changes in Membrane Potential
o Resting potential: reference point
o _________________: Vm is less negative than resting
Vm (less polarized)
o ________________: Vm returns to resting Vm
following a depolarization
o _________________: Vm is more negative (more
polarized) than resting Vm
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•
•
Changes in Membrane Potential :( Figure 7.11)
o Graded potentials are small changes in Vm in response to a stimulus that
triggers the opening or the closing of ions channels; magnitude proportional
(graded) to the strength of a stimulus
o Action potentials are large changes in Vm in response to a graded potential that
has reached the threshold.
Graded Potentials (Figure 7.12)
o Location:
 Sensory receptors: Receptor potentials can be produced in response to
a stimulus on a sensory receptor
 Dendrites and cell body: Synaptic potentials are produced when
neurotransmitter molecules binding to the receptors at the post synaptic
cells
o Strength
 Relatively weak
 Proportional to the strength of stimulus (graded)
o Spread by electronic conduction (Figure 7.13)
o
Decremental (Figure 7.13)
 Travel for a short distance
 ____________ decays as it spreads
o Hyperpolarization or Depolarization Figure 7.14
 Which stimulus is excitatory? _____
•
 Which stimulus is inhibitory? ______
Graded Potentials Summation
o
o
A stimulus that repeated close together in time can be summed in ______________
summation
_____________ summation: different stimuli from different sources overlap in time
are summed
•
o Does an action potential occur (Figure 5.15 d)? ______
Action Potentials (APs)
o When graded potentials reach a_________________, a large, rapid depolarization
occur in membranes of excitable cells (such as neuron and muscle cells) = action
potentials
o _______________ is the minimum depolarization necessary to induce the
regenerative mechanism for the opening of sodium channels
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o ________________________
principle: Action potentials from threshold and
supra-threshold stimulus have the __________ magnitude; if the membrane is not
depolarized to threshold, no action potential occurs.
•
Threshold Stimulus (Figure 7.19)
•
Properties of Action Potentials (APs)
o Rapid large depolarization from about -70 mV to +30 mV
o Strength: Communicate over long distances without decrease in strength (100
mV)
o All –or- none
o Cannot be summed
Three Phases of an Action Potential (Figure 7.16)
o Depolarization
o Repolarization
o After-hyperpolarization
Ionic Base of an Action Potential (Figure 7.16 Table 6.3)
o Depolarization: permeability of sodium (PNa) increases, sodium ions moves into
cell rapidly.
o Repolarization: Permeability of potassium increases (Pk) , potassium ions
moves out cell
o After-hyperpolarization: Continued movement of K+ out of the cell.
Voltage-Gated Sodium Channel (Figure 1.17)
o Two gates associated with channel
o Activation gate
 Voltage dependent
 Positive feedback
o Inactivation gate
 Voltage and time dependent
Block Action Potentials Formation
o Local anesthetics such as lidocaine (Xylocaine)
o Block the voltage-gated sodium channels in neurons
o No action potentials will be produced therefore brain will not receive the pain
stimuli.
Refractory Periods (figure 7.20)
o Period of time following an action potential marked by decreased excitability
o Contribute to
 All-or-none principle of action potentials
 Frequency coding of the stimulus intensity
 Unidirectional propagation along an axon
o Phases
 Absolute
 Relative
o Absolute refractory period
 Spans all of depolarization and most of the repolarization phase
•
•
•
•
•
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Second action potential can_______ be generated regardless the strength of
the stimulus
 Sodium gates are inactivated
o Relative refractive period
 Spans last part of repolarization phase and hyperpolarization
 Second action potential can be generated—with a stronger stimulus
 Some sodium gates closed, some are inactivated, and more potassium gates
are open.
How APs Convey the Intensity of a Stimulus (Figure 7.21)?

•
o By _________________ coding.
•
•
o The stronger of the supra threshold stimuli, the higher frequency of APs
Propagation of Action Potentials
o Action potentials are propagated along the length of the axon to axon terminal
o Mechanisms depend on presence or absence of myelin sheath
Factors Affecting Propagation (Table 7.4)
o Refractory period contributes to unidirectional propagation
o Axon diameter
 Large: Less resistance, ______

o
Smaller: More resistance, slower
Myelination
•
Myelinated Fibers Conduction
•
o ______________ conduction: Leaping depolarization at the node of Ranvier
Unmyelinated Axon Conduction
o Action potentials are propagated by __________________conduction.
7.5. Maintaining Neural Stability
• Graded potentials and action potentials tend to dissipate Na+ and K+ concentration
gradients
• Na+ /K+ pump prevents dissipation
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