BIOLOGY
Twelfth Edition
Raven
Chapter 42
Lecture Outline
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or further distribution permitted without the prior written consent of McGraw-Hill Education.
Chapter 42: The Nervous System
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Nervous System Organization
All animals must be able to respond to environmental
stimuli
In most invertebrate phyla and in all vertebrate classes,
animals use:
•
•
•
Sensory receptors – detect stimulus.
Motor effectors – respond to it.
Nervous system links the two.
•
Consists of neurons and supporting cells.
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Central Nervous System
CNS
Brain and spinal cord
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Types of Neurons
Vertebrates have three types of neurons
1. Sensory neurons (afferent neurons) carry impulses
to central nervous system (CNS)
2. Motor neurons (efferent neurons) carry impulses
from CNS to effectors (muscles and glands)
3. Interneurons (association neurons) provide more
complex reflexes and associative functions
(learning and memory)
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Peripheral Nervous System
PNS
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•
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Sensory and motor neurons.
Somatic NS stimulates skeletal muscles.
Autonomic NS stimulates smooth and cardiac
muscles, as well as glands.
•
Sympathetic and parasympathetic NS.
•
Counterbalance each other.
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Figure 42.1
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Figure 42.2
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Components of a Neuron
Neurons have the same basic structure
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Cell body.
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Dendrites.
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Enlarged part containing nucleus.
Short, cytoplasmic extensions that receive stimuli.
Axon.
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Single, long extension that conducts impulses away
from cell body.
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Figure 42.3a
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Supportive Cells
Neuroglia
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Support neurons both structurally and functionally.
Schwann cells and oligodendrocytes produce
myelin sheaths surrounding axons.
In the CNS, myelinated axons form white matter.
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Dendrites/cell bodies form gray matter.
In the PNS, myelinated axons are bundled to form
nerves.
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Figure 42.3b
The formation of the myelin sheath around a peripheral axon.
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Electrical Difference Across the Plasma
Membrane
A potential difference exists across every cell’s plasma
membrane
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Negative pole − cytoplasmic side.
Positive pole − extracellular fluid side.
When a neuron is not being stimulated, it maintains a
resting potential
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Ranges from −40 to −90 millivolts (mV).
Average about −70 mV.
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Interior of the Cell
The inside of the cell is more negatively charged than
the outside
1. Sodium–potassium pump
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Brings two K+ into cell for every three Na+ it pumps
out.
2. Ion leakage channels
•
Allow more K+ to diffuse out than Na+ to diffuse in.
3. Differential distribution of other ions and molecules
across the pm
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Especially negatively charged proteins.
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Nerve Impulse Transmission
Sodium–potassium pump creates significant concentration
gradient
Concentration of K+ is much higher inside the cell
Membrane not permeable to negative ions
Leads to buildup of positive charges outside and negative
charges inside cell
Attractive force to bring K+ back inside cell
Equilibrium potential – balance between diffusional force
and electrical force
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Table 42.1
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Figure 42.5
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Uniqueness of Neurons
Uniqueness of neurons compared with other cells is
not the production and maintenance of the resting
membrane potential
Rather the sudden temporary disruptions to the resting
membrane potential that occur in response to stimuli
2 types of changes.
•
•
Graded potentials.
Action potentials.
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Overview of Potentials
Graded potentials
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Small consistent changes in membrane potential
due to activation of gated ion channels.
Action potentials
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Transient disruptions triggered by a threshold
change in potential.
The actual signals that move along an axon.
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Gated Channels
Chemically-gated or ligandgated channels
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Ligands are chemical
signals.
•
•
hormones or
neurotransmitters.
Induce opening and
cause changes in cell
membrane permeability.
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Depolarization
Depolarization makes the membrane potential more
positive
Hyperpolarization makes it more negative
These small changes result in graded potentials
Size depends on either the strength of the stimulus or the
amount of ligand available to bind with their receptors
Can reinforce or negate each other
Summation is the ability of graded potentials to combine
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Figure 42.7
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Action Potentials
Result when depolarization reaches the threshold
potential (−55 mV)
Depolarizations bring a neuron closer to the threshold
Hyperpolarizations move the neuron further from the
threshold
Caused by voltage-gated ion channels
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Voltage-gated Na+ channels.
Voltage-gated K+ channels.
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Voltage-Gated Channels
Voltage-gated Na+ channels
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Activation gate and inactivation gate.
At rest, activation gate closed, inactivation gate open.
Transient influx of Na+ causes the membrane to
depolarize.
Voltage-gated K+ channels
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•
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Single activation gate that is closed in the resting state.
K+ channel opens slowly.
Efflux of K+ repolarizes the membrane.
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Phases of an Action Potential
The action potential has three phases
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Rising, falling, and undershoot.
Action potentials are always separate, all-or-none
events with the same amplitude
Do not add up or interfere with each other
Intensity of a stimulus is coded by the frequency, not
amplitude, of action potentials
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Figure 42.8
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Nerve Impulse Propagation
Propagation of action potentials
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Each action potential, in its rising phase, reflects a
reversal in membrane polarity.
Positive charges due to influx of Na+ can depolarize
the adjacent region to threshold.
And so the next region produces its own action
potential.
Meanwhile, the previous region repolarizes back to
the resting membrane potential.
•
Signal does not go back toward cell body.
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Figure 42.9
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Velocity of Conduction
Two ways to increase velocity of conduction
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Axon has a large diameter.
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Less resistance to current flow.
Found primarily in invertebrates.
Axon is myelinated.
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Action potential is only produced at the nodes of
Ranvier.
Impulse jumps from node to node.
Saltatory conduction.
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Figure 42.10
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Synapses
Specialized intercellular junctions with the other
neurons, with muscle cells, or with gland cells
Presynaptic cell transmits action potential
Postsynaptic cell receives it
Two basic types: electrical and chemical
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Electrical and Chemical Synapses
Electrical synapses
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Involve direct cytoplasmic connections between the
two cells formed by gap junctions.
Relatively rare in vertebrates.
Chemical synapses
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Have a synaptic cleft between the two cells.
End of presynaptic cell contains synaptic vesicles
packed with neurotransmitters.
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Figure 42.12
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Chemical Synapses
Action potential triggers influx of Ca2+
Synaptic vesicles fuse with cell membrane
Neurotransmitter is released by exocytosis
Diffuses to other side of cleft and binds to chemical- or
ligand-gated receptor proteins
Produces graded potentials in the postsynaptic
membrane
Neurotransmitter action is terminated by enzymatic
digestion or cellular uptake
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Figure 42.13
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Neurotransmitters: Acetylcholine
Acetylcholine (ACh)
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•
Crosses the synapse
between a motor neuron
and a muscle fiber.
Neuromuscular junction.
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Acetylcholine
Binds to receptor in the postsynaptic membrane
Causes ligand-gated ion channels to open
Produces a depolarization called an excitatory
postsynaptic potential (EPSP)
Stimulates muscle contraction
Acetylcholinesterase (AChE) degrades ACh
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Causes muscle relaxation.
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Neurotransmitters: Amino Acids
Glutamate
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Major excitatory neurotransmitter in the vertebrate
CNS.
Glycine and GABA (γ-aminobutyric acid) are
inhibitory neurotransmitters.
•
•
Open ligand-gated channels for Cl−.
Produce a hyperpolarization called an inhibitory
postsynaptic potential (IPSP).
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Figure 42.15
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Neurotransmitters: Biogenic Amines
Epinephrine (adrenaline) and norepinephrine are
responsible for the “fight or flight” response
Dopamine is used in some areas of the brain that
control body movements
Serotonin is involved in the regulation of sleep
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Neurotransmitters: Neuropeptides
Neuropeptides
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Substance P is released from sensory neurons
activated by painful stimuli.
Intensity of pain perception depends on enkephalins
and endorphins.
Nitric oxide (NO).
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A gas – produced as needed from arginine.
Causes smooth muscle relaxation.
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Synaptic Integration
Integration of EPSPs (depolarization) and ISPSs
(hyperpolarization) occurs on the neuronal cell body
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•
Small EPSPs add together to bring the membrane
potential closer to the threshold.
IPSPs subtract from the depolarizing effect of
EPSPs.
•
Deter the membrane potential from reaching
threshold.
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Figure 42.16
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Synaptic Integration and Threshold Voltage
There are two ways that the membrane can reach the
threshold voltage
1. Spatial summation
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Many different dendrites produce EPSPs.
2. Temporal summation
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One dendrite produces repeated EPSPs.
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Drug Addiction
Habituation
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Prolonged exposure to a stimulus may cause cells
to lose the ability to respond to it.
Cell decreases the number of receptors because
there is an abundance of neurotransmitters.
In long-term drug use, means that more of the drug
is needed to obtain the same effect.
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Drug Addiction: Cocaine
Cocaine
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•
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Affects neurons in the brain’s “pleasure pathways”
(limbic system).
Binds dopamine transporters and prevents the
reuptake of dopamine.
Dopamine survives longer in the synapse and fires
pleasure pathways more and more.
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Figure 42.17
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Drug Addiction: Nicotine
Nicotine
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Binds directly to a specific receptor on postsynaptic
neurons of the brain.
Binds to a receptor for acetylcholine.
Brain adjusts to prolonged exposure by “turning
down the volume” by.
•
•
Making fewer receptors to which nicotine binds.
Altering the pattern of activation of the nicotine
receptors.
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Evolution of the Central Nervous System
Sponges are only major phylum without nerves
Cnidarians have the simplest nervous system
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Neurons linked to each other in a nerve net.
No associative activity.
Free-living flatworms (phylum Platyhelminthes) are simplest
animals with associative activity
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Two nerve cords run down the body.
Permit complex muscle control.
All of the subsequent evolutionary changes in nervous systems
can be viewed as a series of elaborations on the characteristics
already present in flatworms
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Figure 42.18
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Vertebrate Brains
All vertebrate brains have three basic divisions:
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Hindbrain or rhombencephalon.
Midbrain or mesencephalon.
Forebrain or prosencephalon.
In fishes,
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Hindbrain – largest portion.
Midbrain – processes visual information.
Forebrain – processes olfactory information.
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Figure 42.19
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Table 42.4
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TABLE 42.4 Subdivisions of the Central Nervous System
Major Subdivision
Function
SPINAL CORD
Spinal reflexes; relays sensory and motor information
BRAIN
Hindbrain (rhombencephalon)
Medulla oblongata
Sensory nuclei; reticular-activating system; autonomic functions
Pons
Reticular-activating system; autonomic functions
Cerebellum
Coordination of movements; balance
Midbrain (mesencephalon)
Reflexes involving eyes and ears
Forebrain (prosencephalon)
Diencephalon
Thalamus
Relay station for ascending sensory and descending motor tracts; autonomic functions
Hypothalamus
Autonomic functions; neuroendocrine control
Telencephalon (cerebrum)
Basal nuclei
Motor control
Corpus callosum
Connects and relays information between the two hemispheres
Hippocampus
Memory; emotion
(limbic system) Cerebral cortex
Higher cognitive functions; integrates and interprets sensory information; organizes motor output
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Relative Sizes of Vertebrate Brains
Relative sizes of different brain regions have changed as
vertebrates evolved
Forebrain became the dominant feature
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Forebrain
Forebrain is composed of two elements
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Diencephalon.
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Thalamus – integration and relay center.
Hypothalamus – participates in basic drives and
emotions, controls pituitary gland.
Telencephalon (“end brain”).
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Devoted largely to associative activity.
Called the cerebrum in mammals.
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Cerebrum
The increase in brain size in mammals reflects the
great enlargement of the cerebrum
Split into right and left cerebral hemispheres, which are
connected by a tract called the corpus callosum
Each hemisphere receives sensory input from the
opposite side
Hemispheres are divided into: frontal, parietal,
temporal, and occipital lobes
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Figure 42.21
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Cerebrum: Cerebral Cortex
Cerebral cortex
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Outer layer of the cerebrum.
Contains about 10% of all neurons in brain.
Highly convoluted surface.
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Increases threefold the surface area of the human
brain.
Activities are motor, sensory, or associative.
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Cerebral Cortex
Primary motor cortex – movement control
Primary somatosensory cortex – sensory control
Association cortex – higher mental functions
Basal nuclei
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Aggregates of neuron cell bodies – gray matter.
Participate in the control of body movements.
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Figure 42.22
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Figure 42.23
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Other Brain Structures
Thalamus
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Integrates visual, auditory, and somatosensory
information.
Hypothalamus
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Integrates visceral activities.
Controls pituitary gland.
Limbic system
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Hypothalamus, hippocampus, and amygdala.
Responsible for emotional responses.
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Complex Functions of the Brain
Sleep and arousal
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One section of reticular formation is the reticularactivating system.
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Controls consciousness and alertness.
Brain state can be monitored by means of an
electroencephalogram (EEG).
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Records electrical activity.
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Complex Functions of the Brain: Language
Language
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Left hemisphere is “dominant” hemisphere.
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Different regions control various language
activities.
Adept at sequential reasoning.
Right hemisphere is adept at spatial reasoning.
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Primarily involved in musical ability.
Nondominant hemisphere is also important for the
consolidation of memories of nonverbal
experiences.
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Complex Functions of the Brain: Memory
Appears dispersed across the brain
Short-term memory is stored in the form of transient
neural excitations
Long-term memory appears to involve structural
changes in neural connections
Two parts of the temporal lobes, the hippocampus and
the amygdala, are involved in both short-term memory
and its consolidation into long-term memory
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Synaptic Plasticity
Cellular basis of learning and memory
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Long-term changes in the strength of synaptic
connection.
Two examples of synaptic plasticity
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Long-term potentiation (LTP).
Long-term depression (LTD).
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Figure 42.24
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Alzheimer Disease
Condition where memory and thought become
dysfunctional
Two causes have been proposed
1. Nerve cells are killed from the outside in.
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External protein: β-amyloid.
2. Nerve cells are killed from the inside out.
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Internal proteins: tau (τ).
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Spinal Cord
Cable of neurons extending from the brain down through
the backbone
Enclosed and protected by the vertebral column and the
meninges
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Composition of the Spinal Cord
2 zones
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Inner zone is gray matter.
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•
Primarily consists of the cell bodies of interneurons,
motor neurons, and neuroglia.
Outer zone is white matter.
•
Contains cables of sensory axons in the dorsal
columns and motor axons in the ventral columns.
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Role of the Spinal Cord
It serves as the body’s “information highway”
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Relays messages between the body and the brain.
It also functions in reflexes
•
•
The knee-jerk reflex is monosynaptic.
However, most reflexes in vertebrates involve a
single interneuron.
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Figure 42.26
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Figure 42.27
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Composition of the Peripheral Nervous
System
Consists of nerves and ganglia
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•
Nerves are bundles of axons bound by connective
tissue.
Ganglia are aggregates of neuron cell bodies.
Function is to receive info from the environment,
convey it to the CNS, and to carry responses to
effectors such as muscle cells
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Figure 42.28
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Neurons of the Peripheral Nervous System
Sensory neurons
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•
Axons enter the dorsal surface of the spinal cord
and form dorsal root of spinal nerve.
Cell bodies are grouped outside the spinal cord in
dorsal root ganglia.
Motor neurons
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•
Axons leave from the ventral surface and form
ventral root of spinal nerve.
Cell bodies are located in the spinal cord.
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The Somatic Nervous System
Somatic motor neurons stimulate the skeletal muscles
to contract
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In response to conscious command or reflex
actions.
Antagonist of the muscle is inhibited by.
hyperpolarization (IPSPs) of spinal motor neurons.
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The Autonomic Nervous System
Composed of the sympathetic and parasympathetic
divisions, plus the medulla oblongata
In both, efferent motor pathway has 2 neurons
•
•
Preganglionic neuron – exits the CNS and synapses
at an autonomic ganglion.
Postganglionic neuron – exits the ganglion and
regulates visceral effectors.
•
Smooth or cardiac muscle or glands.
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Table 42.5
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Figure 42.29
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Divisions of the Autonomic Nervous System
Sympathetic division
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•
Preganglionic neurons originate in the thoracic and
lumbar regions of spinal cord.
Most axons synapse in two parallel chains of ganglia
right outside the spinal cord.
Parasympathetic division
•
•
Preganglionic neurons originate in the brain and sacral
regions of spinal cord.
Axons terminate in ganglia near or even within internal
organs.
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Figure 42.30
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Table 42.6
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G Proteins
Mediate cell responses to autonomic signals
activate target cells
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Cranial Nerves
Twelve pairs of cranial nerves arise from the underside
of the brain.
They carry sensory neurons for the special and general
senses as well as somatic and autonomic motor
neurons.
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Table 42.7
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TABLE 42.7 Cranial Nerves and Their Functions
Number
Name
Function
I
Olfactory
Sense of smell
II
Optic
Vision
III
Oculomoter
Motor control of some eye muscles and eyelid
VI
Trochlear
Motor control of some eye muscles
V
Trigeminal
Chewing muscles and some facial sensation
VI
Abducent
Motor control of some eye muscles
VII
Facial
Motor control of facial muscles, salivation Taste and cutaneous
sensations.
VII
Acoustic
Equilibration, static sense and hearing
IX
Glossopharyngeal
Salivation, sensations of skin, taste and viscera
X
Vagus
Motor control of the heart and viscera, sensation from the
thorax, pharynx and abdominal viscera
XI
Accessory
Motor impulses to the pharynx and shoulder
XII
Hypoglossal
Motor control of the tongue, some skeletal muscles, some
viscera, sensation from skin and viscera
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