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Neurons and Nervous
Systems
Chapter 34 Neurons and Nervous Systems
Key Concepts
• 34.1 Nervous Systems Consist of Neurons
and Glia
• 34.2 Neurons Generate and Transmit
Electrical Signals
• 34.3 Neurons Communicate with Other
Cells at Synapses
Chapter 34 Neurons and Nervous Systems
Key Concepts
• 34.4 The Vertebrate Nervous System Has
Many Interacting Components
• 34.5 Specific Brain Areas Underlie the
Complex Abilities of Humans
Chapter 34 Opening Question
How can a small brain tumor so dramatically
affect personality and behavior?
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Nervous systems have two categories of
cells:
Neurons, or nerve cells, are excitable—they
generate and transmit electrical signals,
called action potentials.
Glia, or glial cells, provide support and
maintain extracellular environment.
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Most neurons have four regions:
• Cell body—contains nucleus and
organelles
• Dendrites— carries signals, called nerve
impulses or action potentials, to the cell
body
• Axon—generates action potentials and
conducts them away from the cell body
• Axon terminal—synapse at tip of axon;
releases neurotransmitters
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Neurons pass information at synapses:
• The presynaptic neuron sends the
message
• The postsynaptic neuron receives the
message
Figure 34.1 A Generalized Neuron
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Glial cells, or glia, outnumber neurons in the
human brain.
• Glia do not transmit electrical signals but
can release neurotransmitters.
• Glia also give support during development,
supply nutrients, remove debris, and
maintain extracellular environment.
• Important in neuroplasticity—synapse
modification
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Astrocytes are glia that contribute to the
blood–brain barrier, which protects the
brain.
The blood-brain barrier is permeable to fatsoluble compounds like alcohol and
anesthetics.
Microglia provide the brain with immune
defenses since antibodies cannot enter the
brain.
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Oligodendrocytes are glia that insulate
axons in the brain and spinal cord.
Schwann cells insulate axons in nerves
outside of these areas.
The glial membranes form a nonconductive
sheath—myelin.
Myelin-coated axons are white matter and
areas of cell bodies are gray matter.
Multiple sclerosis is a demyelinating
disease.
Figure 34.2 Wrapping Up an Axon (Part 1)
Figure 34.2 Wrapping Up an Axon (Part 2)
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Neurons are organized into neural
networks.
Afferent neurons carry sensory information
into the nervous system from sensory
cells that convert stimuli into action
potentials.
Efferent neurons carry commands to
effectors such as muscles, glands—motor
neurons are effectors that carry
commands to muscles.
Interneurons store information and
communicate between neurons.
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Networks vary in complexity.
Nerve net—simple network of neurons
Ganglia—neurons organized into clusters,
sometimes in pairs, in simple animals
Brain—the largest pair of ganglia, found in
animals with complex behavior requiring
more information-processing
Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 1)
Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 2)
Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 3)
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Neurons generate changes in membrane
potential—the difference in electrical
charge across the membrane.
These changes generate nerve impulses, or
action potentials.
An action potential is a rapid, large change
in membrane potential that travels along
an axon and causes release of chemical
signals.
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Voltage is a measure of the difference in
electrical charge between two points.
Electrical current in solution is carried by
ions. Major ions in neurons:
• Sodium (Na+)
• Potassium (K+)
• Calcium (Ca2+)
• Chloride (Cl–)
Different concentrations and charges inside
and out produce the membrane potential.
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Membrane potentials can be measured in all
cells with electrodes.
Resting potential is the membrane
potential of a resting, or inactive, neuron.
The resting potential of a membrane is
between –60 and –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 34.4 Measuring the Membrane Potential (Part 1)
Figure 34.4 Measuring the Membrane Potential (Part 2)
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Ion channels and ion transporters in the
membrane create the resting and action
potentials.
Sodium–potassium pump—moves Na+
ions from inside, exchanges for K+ from
outside—establishes concentration
gradients
The Na+–K+ pump is an antiporter, or
sodium–potassium ATPase, as it requires
ATP.
Figure 34.5 Ion Transporters and Channels (Part 1)
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Potassium channels are open in the resting
membrane and are highly permeable to K+
ions—allow leak currents
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.
These two forces acting on K+ are its
electrochemical gradient.
Figure 34.5 Ion Transporters and Channels (Part 2)
Concept 34.2 Neurons Generate and Transmit Electrical Signals
The equilibrium potential is the membrane
potential at which the net movement of an
ion ceases.
The Nernst equation calculates the value
of the equilibrium potential by measuring
the concentrations of an ion on both sides
of the membrane.
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Some ion channels are “gated”—open and
close under certain conditions:
• Voltage-gated channels respond to
change in voltage across membrane
• Chemically-gated channels depend on
molecules that bind or alter channel
protein
• Mechanically-gated channels respond to
force applied to membrane
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Gating provides a means for neurons to
change their membrane potentials in
response to a stimulus.
The membrane is depolarized when Na+
enters the cell and the inside of the neuron
becomes less negative.
If gated K+ channels open and K+ leaves,
the cell becomes more negative inside and
the membrane is hyperpolarized.
Figure 34.6 Membranes Can Be Depolarized or Hyperpolarized
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Graded membrane potentials are changes
from the resting potential.
Graded potentials are a means of
integrating input—the membrane can
respond proportionally to depolarization or
hyperpolarization.
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Voltage-gated Na+ and K+ channels are
responsible for action potentials—sudden,
large changes in membrane potential.
At rest most of these channels are closed.
Local depolarization by gated channels in
dendrites produces a graded potential.
It spreads to the axon hillock, where Na+
voltage-gated channels are concentrated.
Concept 34.2 Neurons Generate and Transmit Electrical Signals
The membrane in the axon hillock may
reach its threshold—5 to 10 mV above
resting potential.
Many voltage-gated Na+ channels
(activation gates) open quickly and Na+
rushes into the axon.
The influx of positive ions causes more
depolarization, the membrane potential is
briefly positive, and an action potential
occurs.
Concept 34.2 Neurons Generate and Transmit Electrical Signals
The axon quickly returns to resting potential
due to two things:
• Voltage-gated K+ channels open slowly
and stay open longer—K+ moves out
• Voltage-gated Na+ channels (inactivation
gates) close
Voltage-gated Na+ channels cannot open
again during the refractory period—a few
milliseconds.
Figure 34.7 The Course of an Action Potential (Part 1)
Figure 34.7 The Course of an Action Potential (Part 2)
Concept 34.2 Neurons Generate and Transmit Electrical Signals
An action potential is an all-or-none event—
positive feedback to voltage-gated Na+
channels ensures the maximum action
potential.
An action potential is self-regenerating
because it spreads to adjacent membrane
regions.
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Axon diameter and myelination by glial cells
increase the speed of action potentials in
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.
Concept 34.2 Neurons Generate and Transmit Electrical Signals
When positive current reaches the next
node, the membrane is depolarized—
another axon potential is generated.
Action potentials appear to jump from node
to node, a form of propagation called
saltatory conduction.
Figure 34.8 Saltatory Action Potentials (Part 1)
Figure 34.8 Saltatory Action Potentials (Part 2)
Concept 34.3 Neurons Communicate with Other Cells at
Synapses
Neurons communicate with other neurons or
target cells at synapses.
In a chemical synapse neurotransmitters
from a presynaptic cell bind to receptors in
a postsynaptic cell.
The synaptic cleft—about 25 nanometers
wide—separates the cells.
Concept 34.3 Neurons Communicate with Other Cells at
Synapses
In an electrical synapse, cells are joined
through gap junctions.
Gap junctions are made of proteins
(connexins) that create channels.
Ions flow through the channels—the action
potential spreads through the cytoplasm.
These action potentials are fast but do not
allow for complex integration of inputs.
Concept 34.3 Neurons Communicate with Other Cells at
Synapses
The neuromuscular junction is a chemical
synapse between motor neurons and
skeletal muscle cells.
An action potential causes voltage-gated
Ca+ channels to open in the presynaptic
membrane, allowing Ca+ to flow in.
The presynaptic neuron releases
acetylcholine (ACh) from its axon terminals
(boutons) when vesicles fuse with the
membrane.
Figure 34.9 Chemical Synaptic Transmission
Concept 34.3 Neurons Communicate with Other Cells at
Synapses
The postsynaptic membrane of the muscle
cell is the motor end plate.
ACh diffuses across the cleft and binds to
ACh receptors on the motor end plate.
These receptors allow Na+ and K+ to flow
through, and the increase in Na+
depolarizes the membrane.
If it reaches threshold, more Na+ voltagegated channels are activated and an
action potential is generated.
Figure 34.10 Chemically Gated Channels
Concept 34.3 Neurons Communicate with Other Cells at
Synapses
The postsynaptic cell must sum the
excitatory and inhibitory input.
Summation occurs at the axon hillock, the
part of the cell body at the base of the
axon.
Spatial summation adds up messages at
different synaptic sites.
Temporal summation adds up potentials
generated at the same site, over time.
Figure 34.11 The Postsynaptic Neuron Sums Information
Concept 34.3 Neurons Communicate with Other Cells at
Synapses
Neurotransmitters are cleared from the cleft
after release in order to stop their action in
several ways:
• Diffusion
• Reuptake by adjacent cells
• Enzymes present in the cleft may destroy
them
Example: Acetylcholinesterase acts on ACh.
Concept 34.3 Neurons Communicate with Other Cells at
Synapses
There are many types of neurotransmitters,
and each may have multiple receptor
subtypes.
For example, ACh has two:
• Nicotinic receptors are ionotropic and
mainly excitatory
• Muscarinic receptors are metabotropic and
mainly inhibitory
The action of a neurotransmitter depends on
the receptor to which it binds.
Concept 34.3 Neurons Communicate with Other Cells at
Synapses
Synapses can be fast or slow:
• Neurotransmitters binding to an
ionotropic receptor, or ion channel,
cause a change in ion movement—
response is fast and short-lived
• Metabotropic receptors induce signaling
cascades in the postsynaptic cell that lead
to changes in ion channels.
Cell responses are generally slower and
longer-lived.
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Vertebrate nervous systems:
Brain, spinal cord, and peripheral nerves
that extend throughout the body.
Central nervous system (CNS)—brain and
spinal cord
Peripheral nervous system (PNS)—
cranial and spinal nerves that extend or
reside outside of the brain and spinal cord,
and connect the CNS to all tissues
Figure 34.12 Organization of the Nervous System
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
The afferent part of the PNS carries sensory
information to the CNS.
The efferent part of the PNS carries
information from the CNS to muscles and
glands.
Efferent pathways can be divided into two
divisions:
• The voluntary division, which executes
conscious movements
• The involuntary, or autonomic, division,
which controls physiological functions
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Autonomic Nervous System (ANS)—the
output of the CNS that controls involuntary
functions
ANS has two divisions that work in
opposition—one will increase a function
and the other will decrease it
• Sympathetic division prepares the body
for emergencies—“fight or flight”
• Parasympathetic division slows the heart,
lowers blood pressure and increases
digestion—“rest and digest”
Figure 34.13 The Autonomic Nervous System
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Autonomic efferent pathways begin with
preganglionic neurons with cell bodies in
the CNS.
Axons of preganglionic neurons synapse on
a second neuron outside the CNS in a
collection of neurons called a ganglion.
The second neuron is postganglionic—its
axon leaves the ganglion and synapses in
the target organs.
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Sympathetic postganglionic neurons are
noradrenergic—use norepinephrine as
their neurotransmitter.
Postganglionic neurons of the
parasympathetic division are mostly
cholinergic—release acetylcholine.
Target cells respond in opposite ways to
acetylcholine and norepinephrine.
Example: Pacemaker cells in the heart.
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Sympathetic and parasympathetic divisions
have different anatomy.
The sacral region contains preganglionic
neurons of the parasympathetic region.
The thoracic and lumbar regions contain
sympathetic preganglionic neurons.
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Anatomy of the spinal cord:
• Gray matter is in the center and contains
cell bodies of spinal neurons
• White matter surrounds gray matter and
contains axons that conduct information up
and down the spinal cord
• Spinal nerves extend from the spinal cord
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Each spinal nerve has two roots.
• One spinal root connects to the dorsal
horn, the other to the ventral horn
• Afferent (sensory) axons enter through the
dorsal root
• Efferent (motor) axons leave through the
ventral root
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Spinal reflex—afferent information converts
to efferent activity without the brain.
The knee-jerk reflex is monosynaptic:
• Stretch receptors send axon potentials
through dorsal horn to ventral horn via
sensory axons
• At synapses with motor neurons in the
ventral horn, action potentials are sent to
leg muscles, causing contraction
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Most spinal circuits are more complex—limb
movement is controlled by antagonistic
muscle sets.
• Flexors bend or flex the limb
• Extensors straighten or extend the limb
Coordination of relaxation and contraction is
done by interneurons—they make
inhibitory synapses in a polysynaptic reflex
Figure 34.14 The Spinal Cord Coordinates the Knee-jerk Reflex
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
The embryonic neural tube develops into
the hindbrain, midbrain, and forebrain.
The hindbrain becomes the medulla, the
pons, and the cerebellum.
Together the pons, medulla, and midbrain
are known as the brainstem.
All information between the spinal cord and
the brain passes through the brainstem.
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Many sensory axons have branches in the
brainstem that form synapses with the
reticular system—a network of neurons in
the brainstem.
Low to mid-brainstem activity is involved
with balance, coordination.
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
The embryonic forebrain develops the
diencephalon and telencephalon.
The diencephalon consists of the:
• Thalamus—the final relay station for
sensory information
• Hypothalamus—regulates physiological
functions such as hunger and thirst
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Structures in primitive regions of the
telencephalon form the limbic system—
responsible for basic physiological drives.
• Amygdala—involved in fear and fear
memory
• Hippocampus—transfers short-term
memory to long-term memory
Figure 34.15 The Limbic System
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
The cerebrum is the dominant structure in
mammals, with left and right cerebral
hemispheres.
Cerebral cortex—a sheet of gray matter
covering each hemisphere, folded into
convolutions
Figure 34.16 The Human Cerebrum (Part 1)
Figure 34.16 The Human Cerebrum (Part 2)
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Regions of the cerebral cortex have specific
functions.
Association cortex is made up of areas
that integrate or associate sensory
information or memories.
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Each cerebral hemisphere consists of four
lobes:
• Temporal lobe
• Frontal lobe
• Parietal lobe
• Occipital lobe
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Temporal lobe:
Receives and processes auditory and visual
information
Association areas of the temporal lobe
involve:
• Identification
• Object naming
• Recognition
Agnosia: Disorder of the temporal lobe
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Frontal Lobe:
Primary motor cortex is anterior to the
parietal lobe and controls muscles in
specific body areas.
Association areas involve:
• Feeling, planning
• Personality
Figure 34.17 The Body Is Represented in Primary Motor and Primary Somatosensory Cortexes
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Parietal lobe:
Primary somatosensory motor cortex—
behind the primary motor cortex
• Receives touch and pressure
information
The entire body surface is mapped, more
area for fine discriminations in touch.
Concept 34.4 The Vertebrate Nervous System Has Many
Interacting Components
Occipital lobe:
Receives and processes visual information
Association areas involve:
• Making sense of the visual world
• Translating visual experience into
language
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
The lateralization of language functions
shows that 97 percent occurs in the left
brain hemisphere.
An aphasia is a deficit in the ability to use or
understand words; occurs after damage to
the left hemisphere.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Language areas:
Broca’s area—in frontal lobe; damage
results in slow or lost speech; still can read
and understand language
Wernicke’s area—in temporal lobe.
Damage results in inability to speak
sensibly; written or spoken language not
understood. Still can produce speech
Figure 34.18 Imaging Techniques Reveal Active Parts of the Brain
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Learning—modification of behavior by
experience.
Memory—what the nervous system retains.
Long-term potentiation (LTP) describes
how synapses become more responsive to
repeated stimuli.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Associative learning occurs when two
unrelated stimuli become linked to a
response.
A conditioned reflex is a type of
associative learning.
Example: Salivary reflex in Pavlov’s dog
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Complex, or observational learning in humans
has a pattern of three elements:
• We pay attention to another’s behavior
• We retain a memory of what we observe
• We try to copy or use that information
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Declarative memory is of people, places,
and things that can be recalled and
described.
Procedural memory is how to perform a
motor task and cannot be described.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Types of memory:
• Immediate—events happening now
• Short-term—lasts 10 to 15 minutes
• Long-term—lasts from days to a lifetime
Memories can be associated with specific
brain regions and neuronal properties.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Memories are transferred from short- to
long-term.
Hippocampal or limbic system damage may
prevent this transfer.
Example: H.M. was unable to transfer
memories to long-term storage after
removal of the hippocampus.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Sleep research uses electroencephalogram
(EEG):
Measures neuronal activity and records
changes in electrical potential in entire
brain regions
In birds and mammals, there are two main
sleep states:
• Rapid eye movement (REM) sleep
• Non-REM sleep
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
When awake, nuclei in the brainstem are
active and cells depolarize often.
Neurons in the thalamus and cortex are
near threshold and sensitive to input—
reflects a wakeful state.
At sleep onset, activity slows in the
brainstem—less neurotransmitter is
released, cells are less excitable.
Information processing slows and
consciousness is lost—the state of nonREM sleep
Figure 34.19 Stages of Sleep (Part 1)
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Cells in non-REM sleep fire in bursts, called
slow-wave sleep.
During non-REM and REM transition:
• Brainstem nuclei become active again and
firing bursts cease
• Cortex can process information as cells at
threshold can depolarize
• Sensory and motor pathways are still
inhibited; without this feedback the cortex
may produce bizarre dreams.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
After a period of REM sleep, we return to
non-REM sleep.
Repeated cycles occur in humans and other
mammals.
Hypotheses for sleep patterns include
immune function, maintenance of neural
connections, and for learning and memory.
Figure 34.19 Stages of Sleep (Part 2)
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities
of Humans
Consciousness refers to being aware of
yourself, your environment, and events
occurring around you.
Conscious experience requires a perception
of self, using integration of information
from the physical and social environment,
with information from past experience.
Answer to Opening Question
Charles Whitman’s brain tumor pressed on
the hypothalamus and parts of the limbic
system, including the amygdala.
When neurons in the amygdala are
activated, intense emotions such as fear
and rage may be felt.
These strong emotions may have led to his
actions.
Figure 34.20 Source of the Fear Response