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The Nervous System
Chapter 45
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Outline
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Neuron Organization
Resting Membrane Potential
Action Potentials
Structure of Synapses
Neurotransmitters and their Functions
Evolution of the Vertebrate Brain
Human Forebrain
The Spinal Cord
Peripheral Nervous System
Autonomic Nervous System
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Neuron Organization
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Sensory neurons carry impulses from
sensory receptors to the central nervous
system (CNS).
Motor neurons carry impulses from the CNS
to effectors.
Interneurons help provide more complex
reflexes and higher associative learning.
Sensory and motor neurons constitute the
peripheral nervous system (PNS).
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Neuron Types
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Neuron Organization
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Somatic motor neurons stimulate skeletal
muscles’ contraction.
Autonomic motor neurons regulate activity of
smooth muscles, cardiac muscles, and
glands.
– sympathetic
– parasympathetic
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Neuron Organization
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Cell body integrates the information that
arrives at its dendrites.
– triggers impulses that are conducted away
from the cell body along an axon
Neurons are supported structurally and
functionally by supporting cells (neuroglia).
– Schwann cells
– oligodendrocytes
 produce myelin sheath
 interrupted by nodes of Ranvier
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Neuron Structure
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Resting Membrane Potential
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Potential difference exists across every cell’s
plasma membrane.
– cytoplasm side is negative pole, and
extracellular fluid side is positive pole
Inside of cell negatively charged because:
– large, negatively charged molecules are
more abundant inside the cell
– sodium potassium pump
– voltage-gated ion channels
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Sodium-Potassium Pump
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Resting Membrane Potential
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When a neuron is not being stimulated, it
maintains a resting membrane potential.
– cations outside the cell are attracted to
anions inside the cell
 Resting plasma membrane is more
permeable to K+ than other cations, so
K+ enters the cell but the sodiumpotassium pump is driving K+ out of the
cell.
 equilibrium potential
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Resting Membrane Potential
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When a nerve or muscle cell is stimulated,
sodium channels become more permeable,
and Na+ rushes into the cell.
– sudden influx of positive charges causes
the cell to depolarize
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 K flows out of cell and the inside of the
cell again hyperpolarizes
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Resting Membrane Potential
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Action Potentials
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Graded potentials are caused by the
activation of gated ion channels.
– closed in normal resting cells
– chemical- or ligand-gated channels
Summation is the ability of graded potentials
to combine.
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Binding of Acetylcholine
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Action Potentials
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Generation of action potentials
– Once a particular level of depolarization is
reached, a nerve impulse (action potential)
is produced.
 threshold
 A depolarization that reaches or
exceeds the threshold opens both the
Na+ and K+ voltage-gated ion
channels.
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Action Potential
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Action Potentials
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Propagation of action potentials
– events are reproduced at different points
along the axon membrane
 positive charges can depolarize the next
region of the membrane to threshold
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Action Potential Propagation
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Action Potentials
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Saltatory conduction
– one node of Ranvier depolarizes the next,
so that action potentials can skip between
nodes
 saltatory conductions in myelinated
axon more rapid than conduction in an
unmyelinated axon
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Saltatory Conduction
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Structure of Synapses
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Synapses are intercellular junctions.
– The neuron transmitting an action
potential to the synapse is the presynaptic
cell, while the receiving cell on the other
side of the synapse is the postsynaptic
cell.
 synaptic cleft - narrow space separating
two cells
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Structure of Synapses
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End of presynaptic axon contains synaptic
vesicles, each packed with neurotransmitters.
– diffuse rapidly to the other side of the cleft,
and bind to receptor proteins in the
membrane of postsynaptic cell
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Neurotransmitter Release
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Neurotransmitters and Their Functions
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Acetylcholine
– binds to its receptor proteins in the
postsynaptic membrane and thereby
causes ion channels within the proteins to
open
 produces an excitatory postsynaptic
potential (EPSP)
 acetycholine eliminated from the
synaptic cleft by acetylcholinesterase
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Neurotransmitters and Their Functions
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Glutamate, glycine, and GABA
– Glutamate is the major excitatory
neurotransmitter in the vertebrate CNS.
– Glycine and GABA are inhibitory
neurotransmitters.
 produces inhibitory postsynaptic
potential (IPSP)
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Neurotransmitters and Their Functions
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Biogenic amines
– dopamine
– norepinephrine
– serotonin
Other neurotransmitters
– neuropeptides
 substance P - activated by painful stimuli
 intensity of pain perception depends on
enkephalins and endorphins
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 nitric oxide
Neurotransmitters and Their Functions
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Synaptic integration
– Small EPSPs add together to bring the
membrane potential closer to threshold,
while IPSPs subtract from the depolarizing
effect, keeping the membrane potential
below the threshold.
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Neurotransmitters and Their Functions
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Neurotransmitters and drug addiction
– If receptor proteins within synapses are
exposed to high levels of neurotransmitter
molecules for prolonged periods, that nerve
cell often responds by inserting fewer
receptor proteins into the membrane.
 may lose ability to respond to stimulus habituation
 cocaine
 nicotine
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Drug Addiction
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Evolution of the Vertebrate Brain
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All of the subsequent evolutionary changes
in nervous systems can be viewed as a
series of elaborations on the characteristics
already present in flatworms.
– hindbrain was the principal component of
the brain of early vertebrates
 devoted to control of motor activity
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Basic Vertebrate Brain
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Evolution of the Vertebrate Brain
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Dominant forebrain
– Forebrain in reptiles, amphibians, birds,
and mammals is composed of two
elements:
 thalamus - integration and relay center
between incoming sensory information
and the cerebrum
 hypothalamus - participates in basic
drives and emotions
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Evolution of the Vertebrate Brain
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Telencephalon (endbrain) is located at the
front of the forebrain.
– called cerebrum in mammals
 mammals have brains particularly large
relative to their body mass
 largely reflects enlargement of
cerebrum
 center for correlation, association,
and learning in mammals
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Human Forebrain
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Cerebral cortex
– much of neural activity of the cerebellum
occurs within the cerebral cortex
 contains 10% of all neurons in the brain
 activities fall into three categories:
motor, sensory, and associative
 portion not occupied by one of these
is the association cortex, and is the
site of higher mental activities
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Cerebrum
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Human Forebrain
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Basal ganglia
– aggregates of neuron cell bodies
 receive sensory information from
ascending tracts and motor commands
from the cerebral cortex and cerebellum
Thalamus
– primary site of sensory integration
Hypothalamus
– integrates visceral activities
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Language and Other Functions
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Arousal and sleep
– one section of reticular formation controls
consciousness and alertness
 reticular activating system controls both
sleep and waking state
 sleep not a loss of consciousness
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Language and Other Functions
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Language and spatial recognition
– left hemisphere dominant hemisphere for
language
 sequential reasoning
– right hemisphere usually adept at spatial
reasoning
 musical ability
Memory and learning
– fundamental differences between short
and long-term memory
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Brain Regions and Language Activities
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The Spinal Cord
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Spinal cord is a cable of neurons extending
from the brain down through the backbone.
– protected by vertebral column and layers
of membranes (meninges)
 relays messages, and functions in
reflexes
 knee-jerk reflex is monosynaptic
 very fast
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Knee-Jerk Reflex
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Components of the Peripheral Nervous System
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Axons of sensory neurons enter the dorsal
surface of the spinal cord and form the
dorsal root of the spinal nerve.
Motor axons leave from the ventral surface
and form the ventral root of the spinal cord.
– Cell bodies of sensory neurons are
grouped together outside each level of the
spinal cord in dorsal root ganglia.
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Autonomic Nervous System
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Autonomic nervous system is composed of
the sympathetic and parasympathetic
divisions and the medulla oblongata of the
hindbrain, which coordinates the system.
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Autonomic Nervous System
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Sympathetic division of the autonomic
system, together with the adrenal medulla,
activates the body for fight or flight
responses.
– produced by norepinephrine
Parasympathetic division generally has
antagonistic effects.
– produced by ACh
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G Proteins
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Indirectly produce many parasympathetic
effects of ACh
– regulated by guanosine diphosphate and
triphosphate
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Summary
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Neuron Organization
Resting Membrane Potential
Action Potentials
Structure of Synapses
Neurotransmitters and their Functions
Evolution of the Vertebrate Brain
Human Forebrain
The Spinal Cord
Peripheral Nervous System
Autonomic Nervous System
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