Transcript chapter29_Sections 6

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 29
Neural Control
(Sections 29.6 - 29.8)
Albia Dugger • Miami Dade College
29.6 Chemical Communication at
Synapses
• Action potentials cannot pass directly from a neuron to
another cell
• Chemicals relay signals from a neurons (presynaptic cell) to
another neuron, muscle or gland (postsynaptic cell) across a
fluid-filled synaptic cleft
• synapse
• Region where a neuron’s axon terminals transmit signals
to another cell
Sending Signals at Synapses
• When an action potential arrives at the presynaptic cell’s axon
terminals, it triggers release of a neurotransmitter
• Neurotransmitter molecules diffuse across the synaptic cleft
and bind to receptors on the postsynaptic cell
• Example: At a neuromuscular junction, a motor neuron
releases acetylcholine, which binds to receptors on a muscle
fiber
Key Terms
• neurotransmitter
• Chemical signal released by axon terminals
• neuromuscular junction
• Synapse between a neuron and a muscle
• acetylcholine (ACh)
• Neurotransmitter released at neuromuscular junctions,
and at synapses in the heart and brain
Communication at a Synapse
Communication at a Synapse
Fig. 29.10.1, p. 474
Communication at a Synapse
Action potentials flow
along the axon of a motor
neuron to neuromuscular
junctions, where an axon
terminal forms a synapse
with a muscle fiber.
1
axon of
a motor
neuron
neuromuscular
junction
Fig. 29.10.1, p. 474
Communication at a Synapse
Fig. 29.10.2,3, p. 474
Communication at a Synapse
axon terminal of
motor neuron
The axon terminal
stores chemical signaling
molecules (green) called
neurotransmitter inside
synaptic vesicles.
2
plama membrane
of muscle fiber
synaptic
vesicle
2
Arrival of an action
potential causes
exocytosis of
synaptic vesicles,
and neurotransmitter
enters the synaptic
cleft.
3
3
synaptic cleft
Fig. 29.10.2,3, p. 474
Communication at a Synapse
Fig. 29.10.4,5, p. 474
Communication at a Synapse
The plasma membrane of
the muscle fiber has receptors
for neurotransmitter.
4
binding site for
neurotransmitter
(no neurotransmitter
bound)
ion channel closed
Binding of neurotransmitter opens a channel
through the receptor. The
opening allows ions to flow
into the postsynaptic cell.
5
neurotransmitter
ion flows through
now-open channel
Fig. 29.10.4,5, p. 474
Animation: Synaptic Structure and Function
Cleaning the Cleft
• After neurotransmitter molecules do their work, they must be
removed from synaptic clefts
• Membrane pumps transport some neurotransmitter back into
presynaptic cells or into nearby neuroglial cells
• Postsynaptic cells have enzymes that break down
neurotransmitter (e.g. acetylcholinesterase)
Synaptic Integration
• Neurotransmitter can have an inhibitory or excitatory effect on
a postsynaptic cell
• The postsynaptic cell’s response is determined by synaptic
integration of messages arriving at the same time
• synaptic integration
• The summation of excitatory and inhibitory signals by a
postsynaptic cell
Synaptic Density
• A typical neuron or
effector cell receives
messages from many
neurons
• An interneuron in the
brain can have
thousands of incoming
synapses
Neurotransmitter and
Receptor Diversity
• Different kinds of neurons release different neurotransmitters
• Examples: norepinephrine, epinephrine, dopamine,
serotonin, glutamate, GABA
• Different kinds of postsynaptic cells have receptors that
respond differently to the same neurotransmitter
• Receptors may be stimulating or inhibiting
Effects of Some Neurotransmitters
Key Concepts
• How Neurons Work
• Messages flow along a neuron’s plasma membrane, from
input to output zones
• The messages are brief, self-propagating reversals in the
distribution of electric charge across the membrane
• At an output zone, chemical signals are sent to other
neurons, muscles, or glands
ANIMATION: Chemical synapse
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ANIMATION: Neurotransmitters
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ANIMATION: Events at a
neuromuscular junction
BBC Video: Exploring Neurotransmitters
Animation: Synapse Function
29.7 Disrupted Signaling:
Disorders and Drugs
• Disorders of the nervous system often involve disruption of
signaling at synapses
• Symptoms of neurological disorders may arise from lowered
levels of neurotransmitter; treatment with drugs raises the
level of the appropriate neurotransmitter
• Psychoactive drugs mimic neurotransmitters or disrupt their
release or uptake
Parkinson’s Disease
• Damage to dopamine-secreting neurons in the part of the
brain that governs motor control results in tremors, loss of
balance, and involuntary movement
• PET scans show high metabolic activity in dopaminesecreting neurons
• Former heavyweight boxer Muhammad Ali and actor Michael
J. Fox are among those affected
Battling Parkinson’s Disease
Battling Parkinson’s Disease
Fig. 29.12b, p. 476
Battling Parkinson’s Disease
Fig. 29.12c, p. 476
Attention Deficit Hyperactivity Disorder
• A lower than normal dopamine level also plays a role in
attention deficit hyperactivity disorder (ADHD)
• Affected people have trouble concentrating, are unusually
impulsive, and tend to fidget when required to remain seated
• Drugs used to treat ADHD are stimulants that increase
dopamine availability in the brain
Alzheimer’s Disease
• Alzheimer’s disease is the leading cause of dementia (loss of
ability to think)
• An affected person becomes increasingly confused, cannot
communicate, and eventually is incapable of living
independently
• Affected people have a lower than normal level of ACh in the
brain
Mood Disorders
• Interactions among several neurotransmitters, including
serotonin, dopamine, and norepinephrine, affect mood
• Antidepressants, including Prozac and Paxil, increase the
level of serotonin by preventing its reuptake
• Depression has a genetic component, and families
predisposed to depression may be prone to anxiety disorders
Effects of Psychoactive Drugs
• People take psychoactive drugs, both legal and illegal, to
alleviate pain, relieve stress, or feel pleasure
• All major addictive drugs stimulate the release of dopamine
• Habituation and tolerance can lead to drug addiction
Warning Signs Of Drug Addiction
Stimulants
• Stimulants make users feel alert but also anxious, and they
can interfere with fine motor control
• Nicotine blocks brain receptors for ACh
• Caffeine blocks receptors for adenosine
• Cocaine prevents reuptake of dopamine, serotonin, and
norepinephrine from synaptic clefts
• Amphetamines increase secretion of serotonin,
norepinephrine, and dopamine in the brain
Analgesics
• Narcotic analgesics, including morphine, codeine, heroin,
fentanyl, and oxycodone, mimic the effects of endorphins
• Ketamine and PCP (phencyclidine) numb the extremities by
slowing the clearing of synapses
• endorphins
• Natural painkillers produced by the central nervous system
• Promote feelings of pleasure
Depressants
• Depressants such as alcohol (ethyl alcohol) and barbiturates
slow motor responses by inhibiting ACh output
• Alcohol also stimulates the release of endorphins and GABA,
so users typically experience a brief euphoria followed by
depression
Hallucinogens
• Hallucinogens distort sensory perception and bring on a
dreamlike state
• LSD resembles serotonin and binds to receptors for it
• Mescaline and psilocybin have weaker effects
• THC in marijuana alters levels of dopamine, serotonin,
norepinephrine, and GABA
Key Concepts
• Disrupted Signaling
• Some common neurological disorders cause symptoms by
interfering with the flow of signals through the nervous
system
• Psychoactive drugs also affect nervous system activity by
raising or lowering the amount of signaling chemicals in
the brain
BBC Video: A New Genetic Link to Alzheimer’s
Disease
BBC Video: Targeting Alzheimer’s Disease
BBC Video: Unnecessary Antidepressant
Therapy
29.8 Peripheral Nervous System
• Peripheral nerves are bundles of axons that run through your
body, carrying signals to and from the spinal cord and brain
• Myelin sheaths formed by neuroglial cells (Schwann cells)
wrap around axons of most peripheral nerves
• myelin
• Insulating material that wraps most axons and increases
the speed of signal transmission
Nerve Structure
Nerve Structure
myelin
sheath
axon
blood vessel
nerve fascicle (a
number of axons
bundled inside
connective tissue)
A
the nerve’s outer
wrapping
Fig. 29.13a, p. 478
Axons Bundled as Nerves
• Action potentials occur only at nodes, where there are gated
ion channels and no myelin
• After an action potential occurs at a node, positive ions diffuse
quickly through the cytoplasm to the next node because
myelin prevents them from leaking out across the membrane
• Arrival of positive ions at the next node pushes the region to
threshold, and an action potential occurs
• Jumping from node to node increases signal speed in
myelinated axons
Action Potential in a Myelinated Axon
Action Potential in a Myelinated Axon
unsheathed node
axon
B “Jellyrolled” Schwann cells
of an axon’s myelin sheath
Na+
----
++++
++++
++++
++++
-------
-------
----
++++
++++
action potential
K+
resting potential
resting potential
Na+
++++
----
++++
-------
++++
++++
-------
++++
----
++++
resting potential restored
action potential
resting potential
Fig. 29.13b-d, p. 478
ANIMATION: Ion flow in myelinated axons
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Somatic and Autonomic Divisions
• The vertebrate peripheral nervous system has two divisions:
somatic and autonomic
• somatic nervous system
• Set of nerves that control skeletal muscle and relay
signals from joints and skin
• autonomic nervous system
• Set of nerves that relay signals to and from internal organs
(smooth and cardiac muscle) and to glands
Sympathetic and
Parasympathetic Nerves
• Sympathetic and parasympathetic nerves work
antagonistically in most organs – signals from one division
oppose signals from the other
• Sympathetic neurons of the autonomic system increase
their output in times of stress or danger
• During less stressful times, signals from parasympathetic
neurons dominate
Key Terms
• sympathetic neurons
• Neurons of the autonomic system that prepare the body
for danger or excitement (“fight-or-flight”)
• Sympathetic ganglia are close to the spinal cord
• parasympathetic neurons
• Neurons of the autonomic system that encourage
housekeeping tasks
• Parasympathetic ganglia are in or near the organs they
affect
Effects of Autonomic Nerves
Effects of Autonomic Nerves
Sympathetic
Effects
Widens pupils
Organ
Eyes
Parasympathetic
Effects
optic
nerve
Narrows pupils
Increases salivation
Salivary glands
Decreases
salivation
Increases heart rate Heart Decreases heart rate
vagus
nerve
Widens airways
Airways Constricts airways
Increases
Stomach secretions
and
movements
Liver,
Increases
pancreas
secretions to
digestive tract
Increases secretion
Adrenal gland Decreases
secretion
Slows
secretions and
movements
Slows
secretions to
digestive tract
(most
ganglia near
spinal cord)
thoraci
c
nerves
(12
pairs)
(all
gangli
Increases
Slows
Small intestine,
a in
secretions
secretions andlarge intestine
walls
and
movements
of
movements
organ
Inhibits urination Bladder Stimulates urination s)
Promotes
ejaculation
Promotes erection,
Genitals
lubrication
midbrai
n
medulla
oblonga
cervica
ta
l
nerves
(8
pairs)
pelvic
nerve
lumbar
nerves
(5
pairs)
sacral
nerves
(5
pairs)
Fig. 29.14, p. 479
ANIMATION : Autonomic nerves
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ANIMATION : Nerve structure
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