CHAPTER 48 Nervous System
Nervous System
Function: coordinates and controls bodily functions with
nerves and electrical impulses
The human brain
Contains an estimated 100 billion nerve cells, or neurons
Each neuron
May communicate with thousands of other neurons
Communication between neurons can be long-distance
electrical signals or short-distance chemical signals
Nervous System
In all vertebrates, the nervous
system shows a high degree of
cephalization and distinct CNS
and PNS components
The brain provides the integrative power that
underlies the complex behavior of vertebrates
The spinal cord integrates simple responses to
certain kinds of stimuli and conveys
information to and from the brain
Figure 48.19
Central nervous
system (CNS) Peripheral nervous
system (PNS)
Brain
Spinal cord
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Information Processing
Nervous systems process information in three stages
Sensory input, integration, and motor output
Figure 48.3
Sensor
Effector
Motor output
Integration
Sensory input
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Anatomy
The central canal of the spinal cord and the four
ventricles of the brain are hollow, since they are
derived from the dorsal embryonic nerve cord
4 characteristics of chordates: notochord, dorsal hollow
nerve cord, pharyngeal slits, and post-anal tail
Gray matter
White
matter
Ventricles
Figure 49.5
Anatomy
Gray matter – no myelin sheath
Located on outside in brain and inside in spinal cord
White matter – has myelin sheath
Located on outside in spinal cord and inside in brain
Peripheral Nervous System
The PNS transmits information to and from the CNS
Plays a large role in regulating a vertebrate’s movement and internal environment
Divisions of PNS:
Sensory and Motor division – sends signals to and from the CNS
Motor divides into the Somatic nervous system and the Autonomic nervous system
Autonomic nervous system divides into Parasympathetic, Sympathetic, and Enteric divisions
Peripheral Nervous System
Somatic nervous system
Carries signals to skeletal muscles
Autonomic nervous system
Regulates the internal environment, in an involuntary manner
Carries signals to cardiac muscle, smooth muscle, and
glands
Sympathetic and parasympathetic divisions
Antagonistic effects on target organs
Parasympathetic division Sympathetic division
Action on target organs: 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
Neurotransmitter
released by
postganglionic neurons:
acetylcholine
Constricts pupil
of eye
Stimulates salivary
gland secretion
Constricts
bronchi in lungs
Slows heart
Stimulates activity
of stomach and
intestines
Stimulates activity
of pancreas
Stimulates
gallbladder
Promotes emptying
of bladder
Promotes erection
of genitalia
Cervical
Thoracic
Lumbar
Synapse
Sympathetic
ganglia
Dilates pupil
of eye
Inhibits salivary
gland secretion
Relaxes bronchi
in lungs
Accelerates heart
Inhibits activity of
stomach and intestines
Inhibits activity
of pancreas
Stimulates glucose
release from liver;
inhibits gallbladder
Stimulates
adrenal medulla
Inhibits emptying
of bladder
Promotes ejaculation and
vaginal contractions Sacral
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
Figure 49.8
Autonomic Nervous System
The sympathetic division
Correlates with the “fight-or-flight” response
The parasympathetic division
Promotes a return to self-maintenance functions
Resting and digesting
The enteric division
Controls the activity of the digestive tract, pancreas, and gallbladder
Controlled by other 2 divisions
Neuron Structure
Most of a neuron’s organelles are located in the cell
body with cytoplasmic projections off the cell body
Figure 48.4
Dendrites
Cell body
Nucleus
Axon hillock
Axon Signal
direction
Synapse
Myelin sheath
Synaptic
terminals
Presynaptic cell Postsynaptic cell
Neuron Structure
Most neurons have dendrites
Highly branched extensions that receive signals from other
neurons
The axon is typically a much longer extension
Transmits signals to other cells at synapses
May be covered with a myelin sheath – fatty cell that
wraps around the axon
Types of Neurons
Neurons have a wide variety of shapes that reflect
their input and output interactions
Figure 48.5
Axon
Cell
body
Dendrites
(a) Sensory neuron (b) Interneurons (c) Motor neuron
Types of Neurons
Sensory neurons transmit information from sensory
receptors to the CNS
Detects external stimuli and internal conditions
Interneurons integrate the information in the CNS
Motor output leaves the CNS via motor neurons and
travels to the PNS
Neuron communicates with effector cells (muscles and
glands)
Three stages of information processing
Reflex – body’s automatic responses to certain stimuli
Figure 49.3
Sensory neurons
from the quadriceps
also communicate
with interneurons
in the spinal cord.
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.
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.
4
5
6
The reflex is
initiated by tapping
the tendon connected
to the quadriceps
(extensor) muscle.
1
Sensors detect
a sudden stretch in
the quadriceps.
2 Sensory neurons
convey the information
to the spinal cord.
3
Quadriceps
muscle
Hamstring
muscle
Spinal cord
(cross section)
Gray matter
White
matter
Cell body of
sensory neuron
in dorsal
root ganglion
Sensory neuron
Motor neuron
Interneuron
Supporting Cells (Glia)
Essential for the structural integrity of the nervous
system and for the normal functioning of neurons
CNS
Astrocytes – nutrients to neurons in CNS
Oligodendrocytes – protection
Ependymal cells – lines ventricles and has cilia to move
cerebrospinal fluid
Microglial cells – protection against microorganisms
PNS
Schwann cells – protection
Astrocytes
Provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters
Helps create the blood-brain barrier that regulates substances entering the brain tissue
Figure 49.6 50 µ
m
Oligodendrocytes and Schwann Cells
Glia that form the myelin sheaths around the axons of
many vertebrate neurons
Helps to transmit signals faster down the axon
Myelin sheath Nodes of
Ranvier
Schwann
cell Schwann
cell
Nucleus of
Schwann cell
Axon
Layers of myelin
Node of Ranvier
0.1 µm
Axon
Figure 48.12
Nerve Physiology
Across the plasma membrane, every cell has a voltage
called a membrane potential
The inside of a cell is negative relative to the outside and
is measured using a voltmeter
The resting potential is the membrane potential of a
neuron that is not transmitting signals
Resting membrane potential = - 70mV
Resting Membrane Potential
In all neurons, the resting potential depends on the
ionic gradients that exist across the plasma membrane
Ion pumps and ion channels maintain the resting potential
of a neuron
CYTOSOL EXTRACELLULAR
FLUID
[Na+]
15 mM
[K+]
150 mM
[Cl–]
10 mM
[A–]
100 mM
[Na+]
150 mM
[K+]
5 mM
[Cl–]
120 mM
–
–
–
–
–
+
+
+
+
+
Plasma
membrane
Figure 48.6
Resting Membrane Potential
The concentration of Na+ is higher in the extracellular
fluid than in the cytosol while the opposite is true for K+
A neuron that is not transmitting signals contains many
open K+ channels and fewer open Na+ channels in its
plasma membrane
The diffusion of K+ and Na+ through these channels
leads to a separation of charges across the membrane,
producing the resting potential
Why is charge -70 mV?
Figure 48.7
Inner
chamber Outer
chamber Inner
chamber
Outer
chamber –90 mV +62 mV
Artificial
membrane
Potassium
channel
K+ Cl–
150 mM
KCL
150 mM
NaCl 15 mM
NaCl
5 mM
KCL
Cl–
Na+
Sodium
channel
+ –
+ –
+ –
+ –
+ –
+ –
(a) Membrane selectively permeable to K+ (b) Membrane selectively permeable to Na+
Action Potential
Gated ion channels open or close in response to the
binding of a specific ligand or a voltage change
The response is a change in the membrane potential
When ion channels open, two different responses can
occur: hyperpolarization or depolarization
Both are called graded potentials because the magnitude
of the change in membrane potential varies with the
strength of the stimulus
Cell Responses
Some stimuli trigger a
hyperpolarization
An increase in the magnitude of
the membrane potential
Figure 48.9
+50
0
–50
–100
Time (msec) 0 1 2 3 4 5
Threshold
Resting
potential Hyperpolarizations
Me
mb
ran
e p
ote
ntial (m
V)
Stimuli
(a) Graded hyperpolarizations
produced by two stimuli that
increase membrane permeability
to K+. The larger stimulus produces
a larger hyperpolarization.
Cell Responses
Other stimuli trigger a
depolarization
A reduction in the magnitude of
the membrane potential
Figure 48.9
+50
0
–50
–100
Time (msec)
0 1 2 3 4 5
Threshold
Resting
potential Depolarizations
Me
mb
ran
e p
ote
ntial (m
V)
Stimuli
(b) Graded depolarizations produced
by two stimuli that increase
membrane permeability to Na+.
The larger stimulus produces a
larger depolarization.
Cell Responses
A stimulus strong enough to
produce a depolarization that
reaches the threshold triggers
a different type of response,
called an action potential
Threshold = membrane
voltage amount that causes an
action potential
- 55 mV Figure 48.9
+50
0
–50
–100
Time (msec)
0 1 2 3 4 5 6
Threshold
Resting
potential
Me
mb
ran
e p
ote
ntial (m
V)
Stronger depolarizing stimulus
Action
potential
(c) Action potential triggered by a
depolarization that reaches the
threshold.
Action Potential
An action potential is a brief all-or-none
depolarization of a neuron’s plasma membrane that
carries information along axons
Both voltage-gated Na+ channels and voltage-gated
K+ channels are involved in the production of an action
potential
Action Potential
When a stimulus depolarizes the membrane Na+
channels open, allowing Na+ to diffuse into the cell
As the action potential subsides K+ channels open, and
K+ flows out of the cell
A refractory period follows the action potential during
which a second action potential cannot be initiated
Conduction of Action Potentials
An action potential can travel long distances by
regenerating itself along the axon
At the site where the action potential is generated,
usually the axon hillock an electrical current
depolarizes the neighboring region of the axon
membrane
The speed of an action potential increases with the
diameter of an axon
In vertebrates, axons are myelinated causes the speed of
an action potential to increase
Conduction of Action Potentials
Action potentials in myelinated axons jump between
the nodes of Ranvier in a process called saltatory
conduction
Cell body
Schwann cell
Myelin
sheath
Axon
Depolarized region
(node of Ranvier)
+ + +
+ + +
+ + +
+ +
– �–
– �–
– �–
– – –
–
–
–
Figure 48.13
Synapse
Synapse connects one cell to another
Space between the two cells is called the synaptic cleft
In an electrical synapse electrical current flows directly
from one cell to another via a gap junction
The vast majority of synapses are chemical synapses
Synapse
In a chemical synapse, a presynaptic neuron releases
chemical neurotransmitters, which are stored in the
synaptic terminal
Figure 48.14
Postsynaptic
neuron
Synaptic
terminal
of presynaptic
neurons
5 µ
m
Synapse
When an action potential reaches a terminal the final
result is the release of neurotransmitters into the
synaptic cleft
Figure 48.15
Presynaptic
cell
Postsynaptic cell
Synaptic vesicles
containing
neurotransmitter Presynaptic
membrane
Postsynaptic
membrane
Voltage-gated
Ca2+ channel
Synaptic cleft
Ligand-gated
ion channels
Na+ K+
Ligand-
gated
ion channel
Postsynaptic
membrane
Neuro-
transmitter
1 Ca2+
2
3
4
5
6
Direct Synaptic Transmission
The process of direct synaptic transmission involves the
binding of neurotransmitters to ligand-gated ion
channels
Neurotransmitter binding causes the ion channels to
open, generating a postsynaptic potential
Postsynaptic potentials fall into two categories:
Excitatory or Inhibitory
Direct Synaptic Transmission
After its release, the neurotransmitter diffuses out of
the synaptic cleft
May be taken up by surrounding cells and degraded by
enzymes
Neurotransmitters
Chemical messengers that act on cells to create a
response
The same neurotransmitter can produce different
effects in different types of cells
Types:
Acetylcholine, biogenic amines, various amino acids and
peptides, and certain gases
Neurotransmitters
Acetylcholine is one of the most common
neurotransmitters in both vertebrates and invertebrates
Can be inhibitory or excitatory
Used in muscle contraction
Biogenic amines: include epinephrine, norepinephrine,
dopamine, and serotonin
Are active in the CNS and PNS
Neurotransmitters
Various amino acids and peptides are active in the
brain
Gases such as nitric oxide and carbon monoxide are
local regulators in the PNS
Animal Nervous Diversity
All animals except sponges have some type of nervous
system
What distinguishes the nervous systems of different
animal groups is how the neurons are organized into
circuits
Animal Nervous Diversity
The simplest animals with nervous systems, the cnidarians have neurons arranged in nerve nets
Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring
Figure 49.2
Nerve net
(a) Hydra (cnidarian)
Nerve
ring
Radial
nerve
(b) Sea star (echinoderm)
Animal Nervous Diversity
In relatively simple cephalized animals, such as flatworms a central nervous system (CNS) is evident
Annelids and arthropods have segmentally arranged clusters of neurons called ganglia
These ganglia connect to the CNS and make up a peripheral nervous system (PNS)
Figure 49.2
Eyespot
Brain
Nerve
cord
Transverse
nerve
(c) Planarian (flatworm)
Brain
Ventral
nerve
cord
Segmental
ganglion
Brain
Ventral
nerve cord
Segmental
ganglia
(d) Leech (annelid) (e) Insect (arthropod)
Anterior
nerve ring
Longitudinal
nerve cords
Ganglia
Brain
Ganglia
Figure 49.2 (f) Chiton (mollusc) (g) Squid (mollusc)
Animal Nervous Diversity
Nervous systems in molluscs correlate with the animals’
lifestyles
Sessile molluscs have simple systems while more complex
molluscs have more sophisticated systems
Animal Nervous Diversity
In vertebrates, the central nervous system consists of a
brain and dorsal spinal cord
The PNS connects to the CNS
Figure 49.2
Brain
Spinal
cord
(dorsal
nerve
cord)
Sensory
ganglion
(h) Salamander (chordate)
Embryonic Development of the Brain
In all vertebrates, the brain develops from three
embryonic regions: the forebrain, the midbrain, and
the hindbrain
By the fifth week of human embryonic development,
five brain regions have formed from the three
embryonic regions
As a human brain develops further the most profound
change occurs in the forebrain, which gives rise to the
cerebrum
Functional magnetic resonance imaging
Technology that can reconstruct a three-dimensional map of brain activity
The results of brain imaging and other research methods reveal that groups of neurons function in specialized circuits dedicated to different tasks
Figure 48.1
Brainstem
The brainstem consists of three parts:
medulla oblongata, pons, and midbrain
The medulla oblongata contains centers that control
heart rate, blood pressure, breathing, swallowing, and
vomiting
The pons controls breathing
The midbrain contains centers for the passing
ascending and descending signals
Arousal and Sleep
A diffuse network of neurons called the reticular formation is present in the core of the brainstem
A part of the reticular formation, the reticular activating system (RAS) regulates sleep and arousal
Figure 49.10
Eye
Reticular formation
Input from touch,
pain, and temperature
receptors
Input from ears
Cerebellum
The cerebellum is important for coordination and
balance
Also involved in learning and remembering motor skills
Diencephalon
The embryonic diencephalon develops into three adult brain regions:
epithalamus, thalamus, and hypothalamus
The epithalamus includes the pineal gland (releases melatonin) and the choroid plexus (capillaries that produce cerebrospinal fluid)
The thalamus sends sensory information to the cerebrum and sends motor information from the cerebrum
Diencephalon
The hypothalamus regulates homeostasis
Basic survival behaviors such as feeding, fighting, fleeing,
and reproducing
Also known as the limbic center
The hypothalamus regulates circadian rhythms
Such as the sleep/wake cycle (biological clock)
Biological clocks usually require external cues to remain
synchronized with environmental cycles
Cerebrum
The cerebrum contains right and left cerebral
hemispheres
Each consist of cerebral cortex overlying white matter and
basal nuclei (regions of gray matter inside brain) – centers
for planning and learning movement sequences
Left cerebral
hemisphere
Corpus
callosum
Right cerebral
hemisphere
Basal
nuclei
Figure 49.13
Cerebrum
A thick band of axons, the corpus callosum provides
communication between the right and left cerebral
cortices
In humans, the largest and most complex part of the
brain is the cerebral cortex, where sensory information
is analyzed, motor commands are issued, and
language is generated
Cerebrum
Each side of the cerebral cortex has four lobes
Frontal, parietal, temporal, and occipital
Frontal lobe
Temporal lobe Occipital lobe
Parietal lobe
Frontal
association
area
Speech
Smell
Hearing
Auditory
association
area Vision
Visual
association
area
Somatosensory
association
area
Reading
Speech
Taste
Figure 48.27
Cerebrum
In the somatosensory cortex and motor cortex neurons
are distributed according to the part of the body that
generates sensory input or receives motor input
Figure 48.28
Tongue
Jaw Lips
Primary
motor cortex Abdominal
organs
Pharynx
Tongue
Genitalia
Primary
somatosensory
cortex
Toes
Parietal lobe Frontal lobe
Lateralization
The left hemisphere becomes more adept at language,
math, logical operations, and the processing of serial
sequences
The right hemisphere is stronger at pattern recognition,
nonverbal thinking, and emotional processing
Language and Speech
Studies of brain activity have mapped specific areas
of the brain responsible for language and speech
Figure 48.29
Hearing
words
Seeing
words
Speaking
words
Generating
words
Max
Min
Emotions
The limbic system is a ring of structures around the
brainstem
Figure 48.30
Hypothalamus Thalamus
Prefrontal cortex
Olfactory
bulb Amygdala Hippocampus
Emotions
This limbic system includes three parts of the cerebral
cortex: amygdala, hippocampus, and olfactory bulb
These structures attach emotional “feelings” to
survival-related functions
Structures of the limbic system form in early
development and provide a foundation for emotional
memory, associating emotions with particular events or
experiences
Memory and Learning
The frontal lobes are a site of short-term memory
Interact with the hippocampus and amygdala to
consolidate long-term memory
Many sensory and motor association areas of the
cerebral cortex are involved in storing and retrieving
words and images
Neural Stem Cells
The adult human brain contains stem cells that can
differentiate into mature neurons
The induction of stem cell differentiation and the
transplantation of cultured stem cells are potential
methods for replacing neurons lost to trauma or
disease
Figure 49.24
Nervous Disorders
Unlike the PNS, the mammalian CNS cannot repair itself
when damaged or assaulted by disease
Current research on nerve cell development and stem
cells may one day make it possible for physicians to
repair or replace damaged neurons
Mental illnesses and neurological disorders take an
enormous toll on society, in both the patient’s loss of a
productive life and the high cost of long-term health care
Schizophrenia
About 1% of the world’s population suffers from
schizophrenia
Schizophrenia is characterized by hallucinations,
delusions, blunted emotions, and many other symptoms
Available treatments have focused on brain pathways
that use dopamine as a neurotransmitter
Depression
Two broad forms of depressive illness are known
Bipolar disorder and major depression
Bipolar disorder is characterized by manic (high-mood) and
depressive (low-mood) phases
In major depression patients have a persistent low mood
Treatments for these types of depression include a
variety of drugs such as Prozac and lithium
Alzheimer’s Disease
Alzheimer’s disease (AD) is a mental deterioration
characterized by confusion, memory loss, and other
symptoms
AD is caused by the formation of neurofibrillary
tangles and senile plaques in the brain
A successful treatment for AD in humans may hinge on
early detection of senile plaques
Parkinson’s Disease
Parkinson’s disease is a motor disorder caused by the
death of dopamine-secreting neurons in the substantia
nigra
Characterized by difficulty in initiating movements,
slowness of movement, and rigidity
There is no cure for Parkinson’s disease although
various approaches are used to manage the symptoms