Chapter 48
Nervous Systems
Nerves
with giant axons
Ganglia
Brain
Arm
Eye
Nerve
Mantle
• Many animals have a complex nervous system
which consists of:
– A central nervous system (CNS) where
integration takes place; this includes the brain
and a nerve cord
– A peripheral nervous system (PNS), which
brings information into and out of the CNS
Sensory input
Integration
Sensor
Motor output
Effector
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Neuron Structure and Function
• Neurons- nerve cells that transfer
information within the body
• Cell body- location of most of a neuron’s
organelles
• Dendrites- highly branched extensions that
receive signals from other neurons
• Axon- much longer extension that transmits
signals to other cells at synapses
Neuron Structure and Function
• Synapse- is a junction between an axon
and another cell
• Synaptic terminal- one axon passes
information across the synapse in the form
of chemical messengers called
neurotransmitters
• Glia- cells that nourish or insulate neurons
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Presynaptic
cell
Axon
Synapse
Synaptic terminals
Postsynaptic cell
Neurotransmitter
Ion pumps and ion channels maintain the resting
potential of a neuron
• Every cell has a voltage (difference in electrical
charge) across its plasma membrane called a
membrane potential
• Messages are transmitted as changes in
membrane potential
Resting Potential- the membrane potential of a
neuron not sending signals
• Concentration of K+ is greater inside the cell, while
the concentration of Na+ is greater outside the cell
• Sodium-potassium pumps use the energy of ATP
to maintain these K+ and Na+ gradients
• Ion channels- converts chemical potential to
electrical potential
• A neuron at resting potential contains many open
K+ channels and fewer open Na+ channels; K+
diffuses out of the cell
Animation: Resting Potential
Key
Na+
K+
OUTSIDE
CELL
INSIDE
CELL
(b)
Sodiumpotassium
pump
Potassium
channel
Sodium
channel
Action potential
Membrane potential (mV)
+50
Falling
phase
0
Rising
phase
Threshold (–55)
–50
–100
Resting
potential
–70
Depolarization
Time (msec)
Undershoot
• An action potential occurs if a stimulus causes
the membrane voltage to cross a particular
threshold
• An action potential is a brief all-or-none
depolarization of a neuron’s plasma membrane
• Action potentials are signals that carry
information along axons
Generation of Action Potentials
•
At resting potential
–
•
•
1. Most voltage-gated Na+ and K+ channels are closed, but some
K+ channels (not voltage-gated) are open
When an action potential is generated
–
2. Voltage-gated Na+ channels open first and Na+ flows into the
cell
–
3. During the rising phase, the membrane potential increases
–
4. During the falling phase, voltage-gated Na+ channels become
inactivated; voltage-gated K+ channels open, and K+ flows out of
the cell
5. During the undershoot, membrane permeability to K+ is at first higher
than at rest, then voltage-gated K+ channels close; resting potential is
restored
Key
Na+
K+
3
4
Rising phase of the action potential
Falling phase of the action potential
Membrane potential
(mV)
+50
Action
potential
–50
2
2
4
Threshold
1
1
5
Resting potential
Depolarization
Extracellular fluid
3
0
–100
Sodium
channel
Time
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop
5
1
Resting state
Undershoot
• Refractory period after an action potential,
a second action potential cannot be
initiated.
• The refractory period is a result of a temporary
inactivation of the Na+ channels
BioFlix: How Neurons Work
Animation: Action Potential
Axon
Plasma
membrane
Action
potential
Cytosol
Na+
K+
Action
potential
Na+
K+
K+
Action
potential
Na+
K+
Conduction Speed
• Axons are insulated by a myelin sheath,
which causes an action potential’s speed to
increase
• Myelin sheaths are made by glia—
oligodendrocytes in the CNS and
Schwann cells in the PNS
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Nodes of
Myelin sheath Ranvier
Schwann
cell
Nucleus of
Schwann cell
Neurons communicate with other cells at synapses
• At electrical synapses, the electrical current
flows from one neuron to another
• At chemical synapses, a chemical
neurotransmitter carries information across the
gap junction
• Most synapses are chemical synapses
Animation: Synapse
5
Synaptic vesicles
containing
neurotransmitter
Voltage-gated
Ca2+ channel
Postsynaptic
membrane
1 Ca2+
4
2
Synaptic
cleft
Presynaptic
membrane
3
Ligand-gated
ion channels
6
K+
Na+
Neurotransmitters
• The same neurotransmitter can produce
different effects in different types of cells
• There are five major classes of
neurotransmitters: acetylcholine, biogenic
amines, amino acids, neuropeptides, and
gases
Acetylcholine- common neurotransmitter,
usually an excitatory transmitter
Biogenic Amines- include epinephrine,
norepinephrine, dopamine, and serotonin
Amino Acid- gamma-aminobutyric acid (GABA)
and glutamate
Neuropeptides- relatively short chains of amino
acids, include substance P and endorphins
Gases- nitric oxide and carbon monoxide are local
regulators in the PNS
• Central nervous system (CNS) where
integration takes place (brain and nerve
cord)
• Peripheral nervous system (PNS) brings
information into and out of the CNS
Nervous systems consist of circuits of neurons and
supporting cells
• Nerve net- a series of interconnected nerve
cells
• Nerves- bundles that consist of the axons of
multiple nerve cells
Radial
nerve
Nerve
ring
Nerve net
(a) Hydra (cnidarian)
(b) Sea star (echinoderm)
Eyespot
Brain
Nerve
cords
Transverse
nerve
Brain
Ventral
nerve
cord
Segmental
ganglia
(c) Planarian (flatworm)
(d) Leech (annelid)
Brain
Ventral
nerve cord
Anterior
nerve ring
Longitudinal
nerve cords
Segmental
ganglia
(e) Insect (arthropod)
(f) Chiton (mollusc)
Ganglia
Brain
Brain
Ganglia
(g) Squid (mollusc)
Spinal
cord
(dorsal
nerve
cord)
Sensory
ganglia
(h) Salamander (vertebrate)
Organization of the Vertebrate Nervous System
• Central nervous system (CNS) where
integration takes place (brain and nerve
cord)
• Peripheral nervous system (PNS) brings
information into and out of the CNS
Central nervous
system (CNS)
Brain
Spinal
cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
The brain and spinal cord contain
– Gray matter- neuron cell bodies, dendrites, and
unmyelinated axons
– White matter- bundles of myelinated axons
Gray matter
White
matter
Ventricles
Quadriceps
muscle
Cell body of
sensory neuron in
dorsal root
ganglion
Gray
matter
White
matter
Hamstring
muscle
Spinal cord
(cross section)
Sensory neuron
Motor neuron
Interneuron
The Peripheral Nervous System
• Afferent neurons transmit information to the CNS and
efferent neurons transmit information away from the CNS
• Motor system- carries signals to skeletal muscles and is
voluntary
• Autonomic nervous system- regulates the internal
environment involuntarily
• Sympathetic division- correlates with the “fight-or-flight”
response
• Parasympathetic division- promotes a return to “rest and
digest”
• Enteric division- controls activity of the digestive tract,
pancreas, and gallbladder
PNS
Afferent
(sensory) neurons
Efferent
neurons
Autonomic
nervous system
Motor
system
Locomotion
Sympathetic
division
Parasympathetic
division
Hormone
Gas exchange Circulation action
Hearing
Enteric
division
Digestion
Sympathetic division
Parasympathetic division
Action on target organs:
Action on target organs:
Constricts pupil
of eye
Dilates pupil
of eye
Stimulates salivary
gland secretion
Inhibits salivary
gland secretion
Constricts
bronchi in lungs
Cervical
Sympathetic
ganglia
Relaxes bronchi
in lungs
Slows heart
Accelerates heart
Stimulates activity
of stomach and
intestines
Inhibits activity
of stomach and
intestines
Thoracic
Stimulates activity
of pancreas
Inhibits activity
of pancreas
Stimulates
gallbladder
Stimulates glucose
release from liver;
inhibits gallbladder
Lumbar
Stimulates
adrenal medulla
Promotes emptying
of bladder
Promotes erection
of genitals
Inhibits emptying
of bladder
Sacral
Synapse
Promotes ejaculation and
vaginal contractions
Cerebrum (includes cerebral cortex, white matter,
basal nuclei)
Telencephalon
Forebrain
Diencephalon
Midbrain
Diencephalon (thalamus, hypothalamus, epithalamus)
Mesencephalon
Midbrain (part of brainstem)
Metencephalon
Pons (part of brainstem), cerebellum
Myelencephalon
Medulla oblongata (part of brainstem)
Hindbrain
Diencephalon:
Cerebrum
Mesencephalon
Hypothalamus
Metencephalon
Thalamus
Midbrain
Hindbrain
Forebrain
Diencephalon
Spinal cord
Telencephalon
Pineal gland
(part of epithalamus)
Myelencephalon
Brainstem:
Midbrain
Pons
Pituitary
gland
Medulla
oblongata
Spinal cord
Cerebellum
Central canal
(a) Embryo at 1 month
(b) Embryo at 5 weeks
(c) Adult
Cerebral
cortex
Cerebrum
Forebrain
Thalamus
Hypothalamus
Pituitary gland
Midbrain
Hindbrain
Pons
Medulla
oblongata
Cerebellum
Spinal
cord
The Brainstem
• Brainstem- coordinates and conducts information
between brain centers
– Midbrain- contains centers for receipt and
integration of sensory information
– Pons- regulates breathing centers in the
medulla
– Medulla oblongata- controls breathing,
cardiovascular activity, swallowing, vomiting,
and digestion
The brainstem and cerebrum control arousal and sleep
The Cerebellum
• Cerebellum- coordination and error
checking during motor, perceptual, and
cognitive functions
• It is also involved in learning and
remembering motor skills
The Cerebrum
• Corpus callosum- a thick band of axons provides
communication between the right and left cerebral
cortices
• The right half of the cerebral cortex controls the
left side of the body, and vice versa
• Left hemisphere- language, math, logic, and
processing of serial sequences
• Right hemisphere- pattern recognition, nonverbal
thinking, and emotional processing
Left cerebral
hemisphere
Right cerebral
hemisphere
Corpus
callosum
Thalamus
Cerebral
cortex
Basal
nuclei
Cerebral cortex- controls voluntary movement and
cognitive functions
• Each side of the cerebral cortex has four
lobes: frontal, temporal, occipital, and
parietal
Frontal lobe
Parietal lobe
Speech
Frontal
association
area
Somatosensory
association
area
Taste
Reading
Speech
Hearing
Smell
Auditory
association
area
Visual
association
area
Vision
Temporal lobe
Occipital lobe
Max
Hearing
words
Seeing
words
Min
Speaking
words
Generating
words
Memory and Learning
• Learning can occur when neurons make new
connections or when the strength of existing
neural connections changes
• Short-term memory is accessed via the
hippocampus
• The hippocampus also plays a role in forming
long-term memory, which is stored in the
cerebral cortex
Fig. 49-18
Thalamus
Hypothalamus
Prefrontal
cortex
Olfactory
bulb
Amygdala
Hippocampus
Stem Cell–Based Therapy
• The adult human brain contains stem cells
that can differentiate into mature neurons