NERVOUS
TISSUE
Chapter 44
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.
Neurons are organized into networks.
Afferent (Sensory) neurons carry
information into the system.
Efferent (Motor) neurons carry commands to
effectors.
Interneurons store information and help with
communication in the system.
Figure 44.1 Nervous Systems Vary in Size and Complexity (A,B)
Use Fig 44.1: A, B, and C
Figure 44.1 Nervous Systems Vary in Size and Complexity (C)
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 44.1 Nervous Systems Vary in Size and Complexity (D)
Figure 44.2 Brains Vary in Size and Complexity
Use Fig 44.2
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 (Part 1)
Use Fig 44.3: A
Figure 44.3 Neurons (Part 2)
Use Fig 44.3: B
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
Figure 44.4 Wrapping Up an Axon
Use Fig 44.4, could use just A
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.
Voltage causes electric current as ions to move
across cell membranes.
Major ions in neurons:
 Sodium (Na+)
 Potassium (K+)
 Calcium (Ca2+)
 Chloride (Cl–)
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.
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 (Part 1)
Figure 44.6 Ion Pumps and Channels (Part 2)
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.
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.
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.
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 (Part 1)
Figure 44.9 Membranes Can Be Depolarized or Hyperpolarized (Part 2)
Figure 44.9 Membranes Can Be Depolarized or Hyperpolarized (Part 3)
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, voltage-gated
Na+ channels open and Na+ rushes into
the axon. The influx of positive ions
causes more depolarization.
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 (Part 2)
Figure 44.10 The Course of an Action Potential (Part 3)
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 self-regenerating
because it spreads to adjacent
membrane regions.
Figure 44.11 Action Potentials Travel along Axons (Part 1)
Figure 44.11 Action Potentials Travel along Axons (Part 2)
Figure 44.11 Action Potentials Travel along Axons (Part 3)
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
How Do Neurons Communicate
with Other Cells?
Neurons communicate with other neurons
or target cells at synapses.
In a chemical synapse chemicals from a
presynaptic cell induce changes in a
postsynaptic cell.
In an electrical synapse the action
potential spreads directly to the
postsynaptic cell.
The neuromuscular junction is a
chemical synapse between motor
neurons and skeletal muscle cells.
The motor neuron releases acetylcholine
(ACh) from its axon terminals.
The postsynaptic membrane of the muscle
cell is the motor end plate.
Figure 44.13 Chemical Synaptic Transmission Begins with the Arrival of an Action Potential