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Chapter 44
Lecture 15
Neurons and Nervous System
Dr. Chris Faulkes
Neurons and Nervous Systems
Aims:
•  To examine the structure and function of the
cells involved in the nervous system
•  To understand the production of nervous
signals
Neurons and Nervous Systems
Aims:
•  To examine the structure and function of the cells involved in the nervous
system
•  To understand the production of nervous signals
These lecture aims form part of the knowledge
required for learning outcome 3:
Describe mechanisms for life processes (LOC3).
Neurons and Nervous Systems
Essential reading
•  Pages 942-955
All of this chapter is useful, but only the
contents of sections 44.1 and 44.2 will be
examined.
44 Neurons and Nervous Systems
• 44.1 What Cells Are Unique to the
Nervous System?
• 44.2 How Do Neurons Generate and
Conduct Signals?
44.1 What Cells Are Unique to the Nervous System?
Nervous systems have two categories
of cells:
Neurons generate and propagate
electrical signals, called action
potentials.
Glial cells provide support and
maintain extracellular environment.
44.1 What Cells Are Unique to the Nervous System?
Neurons are organized into networks.
Afferent neurons carry information into the
system.
Sensory neurons convert input into action
potentials.
Efferent neurons carry commands to
effectors.
Interneurons store information and help
with communication in the system.
44.1 What Cells Are Unique to the Nervous System?
Networks vary in complexity.
Nerve net: simple network of neurons.
Ganglia: neurons organized into
clusters, sometimes in pairs.
Brain: the largest pair of ganglia.
Figure 44.1 Nervous Systems Vary in Size and Complexity
44.1 What Cells Are Unique to the Nervous System?
Central nervous system (CNS) –
consists of cells found in brain and
spinal cord
Peripheral nervous system (PNS) –
neurons and support cells found
outside the CNS
Figure 44.2 Brains Vary in Size and Complexity
44.1 What Cells Are Unique to the Nervous System?
Neurons pass information at
synapses:
The presynaptic neuron sends the
message.
The postsynaptic neuron receives
the message.
44.1 What Cells Are Unique to the Nervous System?
Most neurons have four regions:
• Cell body: contains the nucleus and
organelles
• Dendrites: bring information to the
cell body
• Axon: carries information away from
the cell body
• Axon terminal: forms synapse at tip
of axon
Figure 44.3 Neurons
Neurons
Growing neurons
44.1 What Cells Are Unique to the Nervous System?
Glial cells, or glia, outnumber
neurons in the human brain.
Glia do not transmit electrical signals
but have several functions:
• Support during development
• Supply nutrients
• Maintain extracellular environment
• Insulate axons
44.1 What Cells Are Unique to the Nervous System?
Oligodendrocytes produce myelin
and insulate axons in the CNS.
Schwann cells insulate axons in the
PNS.
Astrocytes contribute to the blood–
brain barrier, which protects the
brain.
Figure 44.4 Wrapping Up an Axon
44.2 How Do Neurons Generate and Conduct Signals?
Action potentials are the result of ions
moving across the plasma membrane.
Ions move according to differences in
concentration gradients and electrical
charge.
Membrane potential is the electric potential
across the membrane.
Resting potential is the membrane
potential of a resting neuron.
44.2 How Do Neurons Generate and Conduct Signals?
Voltage causes electric current as
ions to move across cell membranes.
Major ions in neurons:
• Sodium (Na+)
• Potassium (K+)
• Calcium (Ca2+)
• Chloride (Cl–)
44.2 How Do Neurons Generate and Conduct Signals?
Membrane potentials are measured
with electrodes.
The resting potential of an axon is
–60 to –70 millivolts (mV).
The inside of the cell is negative at
rest. An action potential allows
positive ions to flow in briefly, making
the inside of the cell more positive.
Figure 44.5 Measuring the Resting Potential
44.2 How Do Neurons Generate and Conduct Signals?
The plasma membrane contains ion
channels and ion pumps that create
the resting and action potentials.
The sodium–potassium pump uses
ATP to move Na+ ions from inside the
cell and exchanges them for K+ from
outside the cell.
This establishes concentration
gradients for Na+ and K+.
Figure 44.6 Ion Pumps and Channels
44.2 How Do Neurons Generate and Conduct Signals?
Ion channels in the membrane are
selective and allow some ions to
pass more easily.
The direction and size of the
movement of ions depends on the
concentration gradient and the
voltage difference of the membrane.
These two forces acting on an ion are
its electrochemical gradient.
44.2 How Do Neurons Generate and Conduct Signals?
Potassium channels are open in the
resting membrane and are highly
permeable to K+ ions.
K+ ions diffuse out of the cell along the
concentration gradient and leave
behind negative charges within the
cell.
K+ ions diffuse back into the cell
because of the negative electrical
potential.
44.2 How Do Neurons Generate and Conduct Signals?
The potassium equilibrium potential
is the membrane potential at which
the net movement of K+ ceases.
The Nernst equation calculates the
value of the potassium equilibrium
potential by measuring the
concentrations of K+ on both sides of
the membrane.
Figure 44.7 Which Ion Channel Creates the Resting Potential? (Part 1)
Figure 44.7 Which Ion Channel Creates the Resting Potential? (Part 2)
Figure 44.7 Which Ion Channel Creates the Resting Potential? (Part 3)
44.2 How Do Neurons Generate and Conduct Signals?
Ion channels and their properties can
be studied by patch clamping.
A patch clamp electrode is placed
against the membrane and a seal
forms with applied suction.
Movement of ions and the opening
and closing of ion channels are
recorded as electric currents.
Figure 44.8 Patch Clamping
44.2 How Do Neurons Generate and Conduct Signals?
Some ion channels are gated, and open
and close under certain conditions.
•  Voltage-gated channels respond to a
change in the voltage across the
membrane.
•  Chemically-gated channels depend on
molecules that bind or alter the channel
protein.
•  Mechanically-gated channels respond to
force applied to the membrane.
44.2 How Do Neurons Generate and Conduct Signals?
Gated ion channels change the resting
potential when they open and close.
The membrane is depolarized when
Na+ enters the cell and the inside of
the neuron becomes less negative
than when at rest.
If gated K+ channels open and K+
leaves, the cell becomes more
negative inside and the membrane is
hyperpolarized.
Figure 44.9 Membranes Can Be Depolarized or Hyperpolarized
44.2 How Do Neurons Generate and Conduct Signals?
Action potentials are sudden, large
changes in membrane potential.
Voltage-gated Na+ and K+ channels
are responsible for action potentials.
If a cell body is depolarized, voltagegated Na+ channels open and Na+
rushes into the axon. The influx of
positive ions causes more
depolarization.
44.2 How Do Neurons Generate and Conduct Signals?
A threshold is reached at 5–10 mV above
resting potential.The influx of Na+ is not
offset by the outward movement of K+.
Many voltage-gated Na+ channels then
open, the membrane potential becomes
positive, and an action potential occurs.
The axon returns to resting potential as
voltage-gated Na+ channels close and
voltage-gated K+ channels open.
Figure 44.10 The Course of an Action Potential
44.2 How Do Neurons Generate and Conduct Signals?
Voltage-gated Na+ channels have a
refractory period during which they
cannot open.
Na+ channels have two gates:
•  An activation gate is closed at rest but
opens quickly at threshold.
•  An inactivation gate is open at rest and
closes at threshold but responds more
slowly. The gate reopens 1–2 milliseconds
later than the activation gate closes.
44.2 How Do Neurons Generate and Conduct Signals?
Voltage-gated K+ channels contribute
to the refractory period by remaining
open.
The efflux of K+ ions makes the
membrane potential less negative
than the resting potential for a brief
period.
The dip after an action potential is
called hyperpolarization or
undershoot.
44.2 How Do Neurons Generate and Conduct Signals?
An action potential is an all-or-none
event because voltage-gated Na+
channels have a positive feedback
mechanism that ensures the
maximum value of the action
potential.
An action potential is selfregenerating because it spreads to
adjacent membrane regions.
Figure 44.11 Action Potentials Travel along Axons
44.2 How Do Neurons Generate and Conduct Signals?
Myelination by glial cells increases the
conduction velocity of axons.
The nodes of Ranvier are regularly
spaced gaps where the axon is not
covered by myelin.
Action potentials are generated at the
nodes and the positive current flows
down the inside of the axon.
44.2 How Do Neurons Generate and Conduct Signals?
When the positive current reaches the
next node, the membrane is
depolarized and another axon
potential is generated.
Action potentials appear to jump from
node to node, a form of propagation
called saltatory conduction.
Figure 44.12 Saltatory Action Potentials
Neurons and Nervous Systems
Check out
•  44.1 RECAP, page 946
•  44.1 CHAPTER SUMMARY, page 962
•  44.2 RECAP, page 955, first 2 questions only
•  44.2 CHAPTER SUMMARY, page 962, See WEB/CD Activity 44.1
Self Quiz
page 962: Chapter 44, questions 1, 2, 4, 5 and 6
For Discussion
•  page 963: Chapter 44, question 1
Neurons and Nervous Systems
Key terms:
action potential, axon, brain, cell body, central
nervous system, depolarised, dendrites, efferent
neurons, electrical gradient, ganglion (pl. ganglia),
gated channel, glial cell (pl. glia), hyperpolarised,
interneuron, membrane potential, myelin, Nernst
equation, nerve cell, neuron, neurotransmitters,
oligodendrite, postsynaptic, presynaptic, refractory
period, resting potential, Schwann cells, sensory
neuron, synapase, voltage, voltage-gated channels