albia dugger miami dade college cecie starr christine evers lisa starr chapter 29 neural control...

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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