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Chapter 45 - Sadava
Neurons and Nervous Systems
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Nervous systems have two categories of cells:
• Neurons, or nerve cells, are excitable—
they generate and transmit electrical
signals, called action potentials.
• Afferent neurons carry sensory information into the nervous
system from sensory neurons that convert stimuli into
action potentials.
• Efferent neurons carry commands to effectors such as
muscles, glands.
• Glia, or glial cells, provide support and
maintain extracellular environment.
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Networks in animals vary in complexity.
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Nerve net—simple network of neurons.
• Ganglia—neurons organized into clusters,
sometimes in pairs.
• Brain—the largest pair of ganglia.
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Neurons pass information at synapses:
• The presynaptic neuron sends the message
• The postsynaptic neuron receives the message
Process information in the form of action potentials
• Shifts in membrane potential
• Membrane potential is the electrical potential,
the charge difference, across the membrane.
• All animal cells have more K inside and more
Na outside the cell
• Remember diffusion – these ions want to
move to equilibrium but cannot because
they need to cross membrane through
channels that are closed
• Action potentials are generated by opening
and closing ion channels
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Voltage causes electrically charged particles, ions, to move across
cell membranes.
Major ions in neurons:
• Sodium (Na+)
• Potassium (K+)
• Calcium
(Ca2+)
• Chloride (Cl–)
Membrane potentials are measured with electrodes.
• The resting potential of an axon is
–60 to –70
millivolts (mV).
• The inside of the cell is negative at rest. An action
potential allows positive ions to flow in briefly, making the
inside of the cell more positive.
Figure 45.6 Ion Transporters and Channels (Part 1)
Figure 45.6 Ion Transporters and Channels (Part 2)
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An action potential is an all-or-none event—positive feedback
to voltage-gated Na+ channels ensures the maximum action
potential.
An action potential is self-regenerating because it spreads to
adjacent membrane regions.
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Neurons communicate with other neurons or target
cells at synapses.
• In a chemical synapse chemicals from a presynaptic
cell induce changes in a postsynaptic cell.
• The neuromuscular junction is a chemical
synapse between motor neurons and skeletal
muscle cells.
• The motor neuron releases acetylcholine (ACh)
from its axon terminals.
• The postsynaptic membrane of the muscle cell is
the motor end plate.
• In an electrical synapse the action potential spreads
directly to the postsynaptic cell.
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The synaptic cleft is the space
between the presynaptic and
postsynaptic membranes.
An action potential causes release of
the neurotransmitter ACh when
voltage-gated Ca2+ channels open
and Ca2+ enters the axon terminal.
Vesicles release ACh into the
synaptic cleft.
The postsynaptic membrane
responds to ACh.
ACh diffuses across the cleft and
binds to ACh receptors on the
motor end plate.
These receptors allow Na+ and K+ to
flow through and the increase in
Na+ depolarizes the membrane.
Neurotransmitters are cleared from the cleft
after release in order to stop their action in
several ways:
 Diffusion
 Reuptake by adjacent cells
 Enzymes present in the cleft may destroy
them
Example: Acetylcholinesterase acts on ACh.
Drugs treat the nervous system by
modulating synaptic interactions.
• Agonists mimic or potentiate the effect
of a neurotransmitter.
• Antagonists block the actions of a
neurotransmitter.
• Example: Morphine is an agonist at the
endorphin receptor, therefore blocks
pain.
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45
The Mammalian Nervous
System: Structure and Higher
Function
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Vertebrate nervous systems: Brain, spinal
cord, and peripheral nerves that extend
throughout the body.
• Central nervous system (CNS): Brain and
spinal cord.
• Peripheral nervous system (PNS): Cranial and
spinal nerves that connect the CNS to all
tissues.
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Efferent pathways can be divided into
two divisions:
• The voluntary division, which executes
conscious movements
• The involuntary, or autonomic, division,
which controls physiological functions
• Sympathetic
• Parasympathetic
Figure 47.2 Development of
the Central Nervous System
(Part 3)
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Brainstem
• Medulla and pons
• Breathing, heartrate
• Cerebellum
• Muscle control
Diencephalon
• Thalamus – relay station for sensory information
• Hypothalamus
• Regulates physiological aspects like temp, hunger,
thirst
Telencephalon – Cerebrum
• 2 hemispheres
• Evolutionary trend is for this to get larger
Anatomy of the spinal cord:
 Gray matter is in the center, and
contains cell bodies of spinal neurons
 White matter surrounds gray matter
and contains axons that conduct
information up and down the spinal
cord
 Spinal nerves extend from the spinal
cord
Structures in primitive regions of the telencephalon
form the limbic system—responsible for basic
physiological drives.
 Amygdala—involved in fear and fear memory
 Hippocampus—transfers short-term memory to
long-term memory
Figure 47.5 The Human
Cerebrum (Part 2)
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Autonomic Nervous System (ANS)
• the output of the CNS that controls
involuntary functions.
• 2 divisions that work in opposition—one will
increase a function and the other will decrease
it.
• Sympathetic – increase heart rate, blood pressure, and
cardiac output
• Parasympathetic – slows heart, lowers
blood pressure
Parasympathetic and Sympathetic
• Every efferent pathway has:
• Cholinergic neuron
• Preganglionic neuron – it’s cell body is in the
CNS
• Uses acetylchoine as neurotransmitter
• Postganglionic neuron
• Outside the CNS in ganglion
• Axon extends out from ganglion
• Sympathetic – noadrenergic (use
norepinephrine as neurotransmitter)
Figure 47.10 The Autonomic
Nervous System