Information Flow
and the Neuron
Chapter 37
Learning Objectives
List the 4 functions of neural cells
 Describe the two types of neuronal cells
 Diagram the basic nerve cell and
myelination
 Explain a basic neuronal circuit (SAINE)

Learning Objectives



Describe the flow of an action potential across
the axon of a neuron, including the action of
Na+ and K+ gated ion channels, and understand
how myelination adds to the transmission of the
action potential
Diagram the action of neurotransmission at
synapses
List the types of medications that can affect
neurotransmission in humans
Organ systems:
Nervous System
37.1 Neurons and the Nervous
System
Neurons are cells specialized for the
reception and transmission of
informational signals
 Neurons are supported structurally and
functionally by glial cells
 Neurons communicate via synapses

4 Functions of the
Animal Nervous System
1. Reception

Receives information about conditions in
internal and external environment
2. Transmission

Transmits message along neurons
4 Functions of the
Animal Nervous System
3. Integration

Integrates information to formulate
appropriate response
4. Response

Sends out signals to muscles or glands
Neuron Structure

Dendrites
 Receive
information
 Conduct signals toward the cell body

Axons
 Conduct
signals away from the cell body to
another neuron or an effector
Dendrites
Cell
body
Axon
Nucleus
Axon
hillock
Axon terminals
Fig. 37.3, p. 849
SAINE
Sensory
 Afferent
 Inter Neuron
 Efferent

A Basic Neuron Circuit
An afferent neuron, an interneuron, and an
efferent neuron make up a basic circuit
 Circuits combine into networks that
interconnect the peripheral and central
nervous systems
 The Sensory, afferent, and efferent are the
PNS
 The Interneurons are the CNS
Stimulus
Reception
Peripheral
nervous
system
(PNS)
Afferent
(sensory)
neurons
Transmission
Central
nervous
system
(CNS) Interneurons
Integration
External, for example, light, movement;
or internal, for example, change in blood
pressure, change in body temperature
Sensory receptors of afferent neurons
in external or internal organs detect
stimulus.
Message travels along neuron.
Neural messages sorted and
interpreted.
Interneurons
Transmission
Message travels along neuron.
Efferent
neurons
Response
Neural messages from efferent
neurons transmitted to effectors.
Effectors
Action
Fig. 37.2, p. 848
Node of Ranvier
Myelin sheath of
Schwann cell
Cytoplasm of
axon
Myelin sheath of
Schwann cell
Axon of neuron
Plasma membrane
of axon
Fig. 37.5, p. 850
How do neurons “work”
Step 1- Resting potential
 Step 2- Threshold potential
 Step 3- Action potential
 Step 4- Refractory period
(hyperpolarization)

Membrane Potential of a Cell

Unequal distribution of positive and
negative charges on either side of the
membrane

Establishes a potential difference across
the membrane
37.2 Signal Conduction by
Neurons

Resting potential is the unchanging membrane potential
of an unstimulated neuron – it is approx -70 mVolts

Membrane potential changes from negative to positive
during an action potential

Action potential is produced by ion movements through
the plasma membrane (Na+ and K+)
Resting Potential of Neurons
(1) Na+/K+ active transport pump

Sets up concentration gradients of Na+ ions
(higher outside) and K+ ions (higher inside)
(2) Open channel (passive) allows K+ to flow
out freely
(3) Negatively charged molecules (proteins)
inside cell can’t pass through membrane
(semipermeable)
Na+/K+ pump
3 Na+ out
Voltage-gated Voltage-gated Axon plasma
K+ channel
Na+ channel membrane
(closed)
(closed)
Na+
K+
A–
A–
A–
2 K+ in
A–
A–
A–
A–
Anions (negatively charged
proteins, nucleic acids, and
other large molecules) that
cannot pass through
membrane
K+
Na+
Axon
A–
A–
A–
Charged Particle Concentrations (mM)
Inside
Outside
Na+
15
150
K+
150
5
A–
100
0
Fig. 37-8, p. 853
Action Potential

Generated when stimulus pushes resting
potential to threshold value
Na+ and K+ channels open in the
plasma membrane
 Voltage-gated

Inward flow of Na+ changes membrane
potential from negative to positive peak

Potential falls to resting value as gated K+
channels allow ion to flow out
Refractory period
Membrane potential (mV)
Peak of
action potential
Threshold
potential
Resting
potential
Stimulus
Hyperpolarization
Time (msec)
Fig. 37.9, p. 853
Outside cell
Na+ channel
Activation
Na+
gate
K+ channel
Activation gate
Na+
K+
Membrane potential (mV)
Inactivation gate
Cytoplasm
Threshold potential
Resting potential
Time (msec)
1. A stimulus raises the membrane
potential to threshold. The activation
gate of the Na+ channel opens.
Fig. 37-10a (1), p. 854
Na+
K+
Membrane potential (mV)
Na+
Time (msec)
2. Above the threshold, more Na+ channels open
and Na+ flows inward along its concentration
gradient, raising the membrane potential toward
the peak of the action potential.
Fig. 37-10a (2), p. 854
Na+
K+
Membrane potential (mV)
Na+
Time (msec)
3. As the action potential reaches its peak, the
inactivation gate of the Na+ channel closes and
the K+ channel activation gate opens, allowing
K+ ions to flow outward.
Fig. 37-10a (3), p. 854
Membrane potential (mV)
Time (msec)
4 The outward flow of K+ along its
concentration gradient causes the
membrane potential to begin to fall.
Fig. 37.10b (1), p. 855
Membrane potential (mV)
Time (msec)
6 Closure of the K+ activation gate stabilizes
the membrane potential at the resting value.
Fig. 37.10b (3), p. 855
Summary





Rest- active pump Na+ out, passive pump K+ in;
more positive outside
Stimulation causes active gated channel to open
and Na+ goes in until…
Threshold- all Na+ open quickly rises to peak
Peak- Na+ gated close, K+ gated open to let K+
out
Hyperpolarization- gates close and the pumps
take over to reach resting potential again.
Propagation of Action Potential
Action potentials are initiated by dendrite,
and build in the axon hillock.
 Action potentials move along an axon as
the ion flows generated in one segment
depolarize the potential in the next
segment

Adjacent
area that
was brought
Previous
active area to threshold
New adjacent
returning to by local
inactive area
current
resting
into which
flow;
now
potential; no
depolarization
longer active active at
is spreading;
because of peak of
will soon reach Remainder of axon still
action
refractory
threshold
at resting potential
potential
period
Time = 1
Membrane potential
(mV)
––––––
Fig. 37.11b, p. 856
Refractory Period

Action potentials are prevented from
reversing direction by a brief refractory
period

A segment of membrane that has just
generated an action potential can’t
produce another for a few milliseconds
Saltatory Conduction

In myelinated axons, ions can flow across
the plasma membrane only at nodes
where the myelin sheath is interrupted

Action potentials skip rapidly from node to
node
Fig. 37.12a, p. 857
Fig. 37.12c, p. 857
Synapses

Site where a neuron communicates with
another neuron or effector

Presynaptic cell
 Neuron

that sends a signal
Postsynaptic cell
 Neuron
that receives a signal
2 Types of Synapses

Electrical synapse
 Impulses
pass directly from sending cell to
receiving cell

Chemical synapse
 Neurotransmitter
released by presynaptic cell
diffuses across synaptic cleft
 Binds to receptors in the plasma membrane of
postsynaptic cell
Neurotransmitters

Released into synaptic cleft

Bind to receptors in plasma membrane of
postsynaptic cell

Alter flow of ions across plasma
membrane of postsynaptic cell
 Push
membrane potential toward or away
from threshold potential
Types of Neurotransmitters

Direct neurotransmitter
 Binds
to receptor associated with ligand-gated
ion channel in postsynaptic membrane
 Binding opens or closes the channel

Indirect neurotransmitter
 Binds
to receptor in postsynaptic membrane
 Triggers second messenger (leads to opening
or closing of gated channel)
Neurotransmitter Release (1)

Neurotransmitters are released from
synaptic vesicles into the synaptic cleft by
exocytosis

Exocytosis
by entry of Ca2+ ions into cytoplasm
of axon terminal (through voltage-gated Ca2+
channels opened by arrival of action potential)
 Triggered
Neurotransmitter Release (2)

Neurotransmitter release stops when
action potentials cease arriving at axon
terminal

Neurotransmitters removed from synaptic
cleft
 Broken
down by enzymes
 Taken up by axon terminal or glial cells
Types of Neurotransmitters

Acetylcholine, amino acids, biogenic
amines, neuropeptides, gases such as NO
and CO

Many biogenic amines and neuropeptides
are also released into the general body
circulation as hormones
a. Electrical synapse
Axon terminal of
presynaptic cell
Plasma membrane
of axon terminal
Gap junctions
Plasma
membrane of
postsynaptic cell
In an electrical synapse, the plasma membranes of the presynaptic and
postsynaptic cells make direct contact. Ions flow through gap junctions
that connect the two membranes, allowing impulses to pass directly to
the postsynaptic cell.
Fig. 37.6a, p. 851
b. Chemical synapse
Axon terminal of
presynaptic cell
Vesicle releasing
neurotransmitter
molecules
Synaptic
cleft
Receptors
that bind neurotransmitter
molecules
Plasma
membrane of
postsynaptic cell
In a chemical synapse, the plasma membranes of the presynaptic and
postsynaptic cells are separated by a narrow synaptic cleft. Neurotransmitter
molecules diffuse across the cleft and bind to receptors in the plasma
membrane of the postsynaptic cell. The binding opens channels to ion flow
that may generate an impulse in the postsynaptic cell.
Fig. 37.6b, p. 851
Acetylcholine
Biogenic amines
Serotonin
Amino acids
Aspartate
Glutamate
GABA
(gamma aminobutyric acid)
Glycine
Norepinephrine
Neuropeptides
Met-enkephalin
Epinephrine
Substance P
Dopamine
Fig. 37.14, p. 861
Chemicals that affect
neurotransmission






Specific Serotonin Reuptake Inhibitors (SSRIs)—
Fluoxetine
Specific Norepinephrine Reuptake Inhibitors (SNRIs)—
Reverse inward bound serotonin reuptake (MDMA)
MAO inhibitors- block the breakdown of
neurotransmitters (selegiline)
Dopamine and serotonin antagonizers (risperidone,
olanzapine)
GABA enhancers- (topiramate)