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Chapter 48
Nervous Systems
Figure 48.2 Organization of some nervous systems
Eyespot
Nerve net
Brain
Brain
Nerve
cord
Transverse
nerve
Radial
nerve
Nerve
ring
Ventral
nerve
cord
Segmental
ganglion
(a) Hydra (cnidarian)
(b) Sea star (echinoderm)
(c) Planarian (flatworm)
(d) Leech (annelid)
Brain
Brain
Ventral
nerve
cord
Segmental
ganglia
(e) Insect (arthropod)
Anterior
nerve ring
Ganglia
Longitudinal
nerve cords
(f) Chiton (mollusc)
Brain
Ganglia
(g) Squid (mollusc)
Spinal
cord
(dorsal
nerve
cord)
Sensory
ganglion
(h) Salamander (chordate)
Figure 48.3 Overview of information processing by
nervous systems
Sensory input
Integration
Sensor
Motor output
Effector
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Figure 48.4 The knee-jerk reflex
2 Sensors detect
a sudden stretch in
the quadriceps.
3 Sensory neurons
convey the information
to the spinal cord.
Cell body of
sensory neuron
in dorsal
root ganglion
4 The sensory neurons communicate with
motor neurons that supply the quadriceps. The
motor neurons convey signals to the quadriceps,
causing it to contract and jerking the lower leg forward.
Gray matter
5 Sensory neurons
from the quadriceps
also communicate
with interneurons
in the spinal cord.
Quadriceps
muscle
White
matter
Hamstring
muscle
Spinal cord
(cross section)
Sensory neuron
Motor neuron
1 The reflex is
initiated by tapping
the tendon connected
to the quadriceps
(extensor) muscle.
Interneuron
6 The interneurons
inhibit motor neurons
that supply the
hamstring (flexor)
muscle. This inhibition
prevents the hamstring
from contracting,
which would resist
the action of
the quadriceps.
Figure 48.5 Structure of a vertebrate neuron
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Synaptic
terminals
Figure 48.6 Structural diversity of vertebrate neurons
Dendrites
Axon
Cell
body
(a) Sensory neuron
(b) Interneurons
(c) Motor neuron
Figure 48.8 Schwann cells and the myelin sheath
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
0.1 µm
Figure 8.15 The sodium-potassium pump: a specific case of active transport
Figure 48.12 Graded potentials and an action
potential in a neuron
Stimuli
0
Threshold
0
–50
0 1 2 3 4 5
Time (msec)
(a) Graded hyperpolarizations
produced by two stimuli that
increase membrane permeability
to K+. The larger stimulus produces
a larger hyperpolarization.
Threshold
Action
potential
0
–50
Resting Depolarizations
potential
Resting
potential Hyperpolarizations
–100
+50
Membrane potential (mV)
+50
Membrane potential (mV)
Membrane potential (mV)
+50
–50
Stronger depolarizing stimulus
Stimuli
–100
Threshold
Resting
potential
–100
0 1 2 3 4 5
Time (msec)
(b) Graded depolarizations produced
by two stimuli that increase
membrane permeability to Na+.
The larger stimulus produces a
larger depolarization.
0 1 2 3 4 5 6
Time (msec)
(c) Action potential triggered by a
depolarization that reaches the
threshold.
Figure 48.14 Conduction of an action potential
Axon
Action
potential
– –
+
+ ++
–
Na
–
+ +
– –
+
K+
+ +
– –
– –
+ +
K+
+
+
–
–
+
+
+
+
+
–
–
+
–
–
+
–
–
+
–
–
+
–
–
+
Action
potential
– –
+ ++
Na
+ +
– –
K+
+
–
–
+
+ +
– –
– –
+ +
K+
+
–
–
+
+
–
–
+
Action
potential
– –
+ ++
Na
+ +
–
–
+
–
–
+
–
+
+
–
1
An action potential is generated
as Na+ flows inward across the
membrane at one location.
2
The depolarization of the action
potential spreads to the neighboring
region of the membrane, re-initiating
the action potential there. To the left
of this region, the membrane is
repolarizing as K+ flows outward.
3
The depolarization-repolarization process is
repeated in the next region of the
membrane. In this way, local currents
of ions across the plasma membrane
cause the action potential to be propagated
along the length of the axon.
+
–
–
+
–
+
+
–
Figure 48.17 A chemical synapse
Postsynaptic cell
Presynaptic
cell
Synaptic vesicles
containing
neurotransmitter
5
Presynaptic
membrane
Neurotransmitter
Postsynaptic
membrane
Ligandgated
ion channel
Voltage-gated
Ca2+ channel
1 Ca2+
4
2
Synaptic cleft
Na+
K+
3
Ligand-gated
ion channels
Postsynaptic
membrane
6
Figure 48.19 The vertebrate nervous system
Central nervous
system (CNS)
Brain
Spinal cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Figure 48.21 Functional hierarchy of the vertebrate
peripheral nervous system
Peripheral
nervous system
Somatic
nervous
system
Autonomic
nervous
system
Sympathetic
division
Parasympathetic
division
Enteric
division
Figure 48.22 The parasympathetic and sympathetic
divisions of the autonomic nervous system
Parasympathetic division
Sympathetic division
Action on target organs:
Location of
preganglionic neurons:
brainstem and sacral
segments of spinal cord
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Location of
postganglionic neurons:
in ganglia close to or
within target organs
Action on target organs:
Dilates pupil
of eye
Constricts pupil
of eye
Inhibits salivary
gland secretion
Stimulates salivary
gland secretion
Constricts
bronchi in lungs
Sympathetic
ganglia
Cervical
Accelerates heart
Slows heart
Stimulates activity
of stomach and
intestines
Inhibits activity of
stomach and intestines
Thoracic
Inhibits activity
of pancreas
Stimulates activity
of pancreas
Neurotransmitter
released by
postganglionic neurons:
acetylcholine
Stimulates
gallbladder
Stimulates glucose
release from liver;
inhibits gallbladder
Lumbar
Stimulates
adrenal medulla
Promotes emptying
of bladder
Promotes erection
of genitalia
Relaxes bronchi
in lungs
Inhibits emptying
of bladder
Synapse
Sacral
Promotes ejaculation and
vaginal contractions
Location of
preganglionic neurons:
thoracic and lumbar
segments of spinal cord
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Location of
postganglionic neurons:
some in ganglia close to
target organs; others in
a chain of ganglia near
spinal cord
Neurotransmitter
released by
postganglionic neurons:
norepinephrine
Unnumbered figure page 1029
Unnumbered figure page 1030
Unnumbered figure page 1031
Figure 48.27 The human cerebral cortex
Frontal lobe
Parietal lobe
Speech
Frontal
association
area
Taste
Speech
Smell
Somatosensory
association
area
Reading
Hearing
Auditory
association
area
Visual
association
area
Vision
Temporal lobe
Occipital lobe