Post on 13-Dec-2015
Albia Dugger • Miami Dade College
Cecie StarrChristine EversLisa Starr
www.cengage.com/biology/starr
Chapter 29Neural Control
(Sections 29.6 - 29.8)
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
Fig. 29.10.1, p. 474
Communication at a Synapse
Fig. 29.10.1, p. 474
Action potentials flow along the axon of a motor neuron to neuromuscular junctions, where an axon terminal forms a synapse with a muscle fiber.
neuromuscular junction
axon of a motor neuron
1
Communication at a Synapse
Fig. 29.10.2,3, p. 474
Communication at a Synapse
Fig. 29.10.2,3, p. 474
The axon terminal stores chemical signaling molecules (green) called neurotransmitter inside synaptic vesicles.
Arrival of an action potential causes exocytosis of synaptic vesicles, and neurotransmitter enters the synaptic cleft.
synaptic cleft
plama membrane of muscle fiber
axon terminal of motor neuron
synaptic vesicle
2
3
2
3
Communication at a Synapse
Fig. 29.10.4,5, p. 474
Communication at a Synapse
Fig. 29.10.4,5, p. 474
The plasma membrane of the muscle fiber has receptorsfor neurotransmitter.
ion channel closed
binding site for neurotransmitter (no neurotransmitter bound)
Binding of neurotrans-mitter opens a channel through the receptor. The opening allows ions to flow into the postsynaptic cell. ion flows through
now-open channel
neurotransmitter
4
5
Communication at a Synapse
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 dopamine-secreting neurons
• Former heavyweight boxer Muhammad Ali and actor Michael J. Fox are among those affected
Battling Parkinson’s Disease
Fig. 29.12b, p. 476
Battling Parkinson’s Disease
Fig. 29.12c, p. 476
Battling Parkinson’s Disease
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
Fig. 29.13a, p. 478
nerve fascicle (a number of axons bundled inside connective tissue)
blood vessel
axonmyelin sheath
A
the nerve’s outerwrapping
Nerve Structure
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
Fig. 29.13b-d, p. 478
B “Jellyrolled” Schwann cells of an axon’s myelin sheath
unsheathed node
resting potentialaction potential
Na+
resting potential restored
axon
action potential
resting potential
resting potential
Na+K+
++++
- - - -- - - -
++++ - - - -
++++
++++
- - - -
- - - -
- - - -
++++
++++
++++
- - - -- - - -
++++
++++
- - - -- - - -
++++
++++
- - - -- - - -
++++
Action Potential in a Myelinated Axon
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
Fig. 29.14, p. 479
(most ganglia near spinal cord)
midbrain
medulla oblonga
ta
(all gangli
a in walls
of organ
s)
pelvic nerve
sacral nerves (5 pairs)
lumbar nerves (5 pairs)
thoracic nerves (12 pairs)
cervical nerves (8 pairs)
Promotes erection, lubrication
Stimulates urination
Sympathetic Effects
Parasympathetic Effects
Constricts airways
Inhibits urination
Genitals
Bladder
Small intestine, large intestine
Adrenal gland
Increases heart rateHeart Decreases heart rate
Increases secretions and
movementsIncreases secretions to
digestive tract
Liver, pancreas
Widens airways
vagus nerve
Airways
Stomach
Decreases secretion
Salivary glandsIncreases salivation Decreases salivation
Slows secretions and movements
Slows secretions and movements
Increases secretion
Slows secretions to digestive tract
optic nerveNarrows pupilsWidens pupils Eyes
Organ
Increases secretions
and movements
Promotes ejaculation
Effects of Autonomic Nerves
ANIMATION : Autonomic nerves
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ANIMATION : Nerve structure
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