Discussion 2

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Chapter 4 The Nervous System

Homeostasis

Humans maintain their internal environment within certain limits

Examples: body temperature, water content, and glucose concentrations

Psychoactive drugs influence homeostasis

Alcohol inhibits the release of antidiuretic hormone vasopressin

Causes an increase in the excretion of urine

Compared with light drinkers, heavy drinkers produce less urine after a drink

During alcohol withdrawal, heavy drinkers exhibit an increased vasopressin release

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Components of the Nervous System

Two major types of cells in the nervous system

Neurons or nerve cells

Glia or glial cells

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Neurons

primary elements of the nervous system that analyze and transmit information

Four defined regions

Cell body

Contains the processes that maintain the life of the neuron, including the nucleus

Dendrite

Contains receptors that respond to chemical signals

Axon

Specializes in transmitting signals to other neurons

Axon terminal

Contains synaptic vesicles that store neurotransmitters

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Figure 4.1: Every Neuron Has Four Regions: Cell Body, Dendrites, Axon, and Axon Terminals

Jump to Figure 4.1: Every Neuron Has Four Regions: Cell Body, Dendrites, Axon, and Axon Terminals, Appendix

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Glia

Major functions

Providing firmness and structure to the brain

Getting nutrients into the system

Eliminating waste

Forming myelin

Communicating with other glia and neurons

Creating the blood-brain barrier

A drug molecule must be able to pass the barrier to be psychoactive

Protecting the brain from toxic chemicals

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Neurotransmission

Transferring information from one neuron to another at a synapse

Brief chain of events

Resting potential is caused by an uneven distribution of ions, resulting in the hyperpolarization of a neuron

Ion channels open, which allows ions to move inside the cell making the cell depolarized

If the cell is depolarized to the threshold of excitation, “all-or-none” action potential occurs

Action potential is a brief electrical signal transmitted along the axon when a neuron fires

Note: Blocking ion channels prevents the action potential and disrupts neuronal communication

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Somatic Nervous System

Nerve cells on the “front lines,” interacting with the external environment

Controls voluntary actions

Carries sensory information into the central nervous system, or C N S

Carries motor or movement information back out to the peripheral nerves

Has the neurotransmitter acetylcholine at neuromuscular junctions

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Autonomic Nervous System

Monitors and controls the body’s internal environment and involuntary functions

Examples: heart rate and blood pressure

Many psychoactive drugs affect the brain and the autonomic nervous system simultaneously

Two branches often act in opposition

Sympathetic branch

Example: norepinephrine is involved in increasing heart rate

“Fight-or-flight” response

Parasympathetic branch

Example: acetylcholine is involved in decreasing heart rate

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Central Nervous System

Consists of the brain and spinal cord

Functions

Integration of information

Learning and memory

Coordination of activity

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Chemical Pathways in the Brain, 1

Dopamine

Mesolimbic dopamine pathway

From the ventral tegmental area in the midbrain to the nucleus accumbens

Proposed to mediate some types of psychotic behavior

Possible component of the “rewarding” properties of drugs

Nigrostriatal dopamine pathway

From the substantia nigra to the striatum, past the hypothalamus

Substantial loss of cells leads to Parkinson’s Disease

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Figure 4.3: Mesolimbic and Nigrostriatal Dopamine Pathways

Jump to Figure 4.3: Mesolimbic and Nigrostriatal Dopamine Pathways, Appendix

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Chemical Pathways in the Brain, 2

Acetylcholine

Pathways arise from cell bodies in the nucleus basalis in the lower part of the brain and project widely throughout the cerebral cortex

Involved in Alzheimer’s disease

Acetylcholine blockers impair memory

Norepinephrine pathways

Arise from the locus ceruleus in the brain-stem and project both up and down in the brain

Influence the level of arousal and attentiveness

Play a role in the initiation of food intake

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Chemical Pathways in the Brain, 3

Serotonin pathways

Arise from the brain-stem raphe nuclei and project both upward into the brain and downward into the spinal cord

May have a role in impulsivity and aggression, depression, food intake and weight control, and alcohol use

Serotonin receptors play a role in the actions of some hallucinogenic drugs

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Chemical Pathways in the Brain, 4

GABA or Gamma-Amino Butyric Acid

Not neatly organized into discrete pathways

Found in most regions of the C N S

Inhibitory neurotransmitter, which exerts generalized inhibitory functions

Glutamate

Found in most regions of the brain

Excitatory neurotransmitter

Evidence indicates that specific glutamate pathways may be important for the expression of some psychoactive drug effects

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Chemical Pathways in the Brain, 5

Endorphins

Found throughout the brain

Naturally occurring morphinelike chemicals

Play a role in pain relief and other functions

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Common Neurotransmitters

Neurotransmitter	Type of effect	C N S changes	Drugs of abuse
Dopamine	Inhibitory or excitatory	euphoria agitation paranoia	amphetamines cocaine
GABA	Inhibitory	sedation relaxation drowsiness depression	alcohol barbiturates
Serotonin	Excitatory or inhibitory	sleep relaxation sedation	L S D
Acetylcholine	Excitatory or inhibitory	mild euphoria excitation insomnia	nicotine
Endorphins	Inhibitory	mild euphoria block pain slow respiration	opioids

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Figure 4.5: Cross Section of the Brain: Major Structures

Jump to Figure 4.5: Cross Section of the Brain: Major Structures, Appendix

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Life Cycle of a Neurotransmitter, 1

Step 1: Neurotransmitter precursors are found circulating in the blood supply

Step 2: Uptake

Selected precursors are taken up by cells

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Figure 4.6: Neurons Use Enzymes to Synthesize the Neurotransmitters Dopamine and Norepinephrine

Jump to Figure 4.6: Neurons Use Enzymes to Synthesize the Neurotransmitters Dopamine and Norepinephrine, Appendix

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Life Cycle of a Neurotransmitter, 2

Step 3: Synthesis

Precursors are synthesized into

neurotransmitters through the actions

of enzymes

Step 4: Storage

Neurotransmitters are stored in

synaptic vesicles near the terminal from which they will be released

Jump to Life Cycle of a Neurotransmitter, 2, Appendix

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Life Cycle of a Neurotransmitter, 3

Step 5: Release

When the action potential arrives,

neurotransmitters are released into

the synapse

Step 6: Binding

Released neurotransmitters bind

with receptors on the membrane of

the postsynaptic cell

Neurotransmitters may have

excitatory or inhibitory effects depending on

the type of receptor

Jump to Life Cycle of a Neurotransmitter, 3, Appendix

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Life Cycle of a Neurotransmitter, 4

Step 7: Metabolism

Once a signal has been sent, neurotransmitters are removed from the synapse

Jump to Life Cycle of a Neurotransmitter, 4, Appendix

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Examples of Drug Actions

Altering neurotransmitter availability in the synapse through actions on synthesis, storage, release, uptake, or metabolism

Example: Many antidepressants block the reuptake of dopamine, serotonin, and norepinephrine

Direct action on the receptor

Drug as an agonist mimics the action of neurotransmitters by activating the receptor

Drug as an antagonist occupies the receptor and prevents the neurotransmitter from activating it

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Chemical Theories of Behavior, 1

Many attempts have been made to explain normal variations in behavior in terms of changes in brain chemistry

Historical precedents

Greek physician Hippocrates and the four humors: blood, phlegm, yellow bile, and black bile

Chinese philosophy of yin and yang

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Chemical Theories of Behavior, 2

The major theory guiding the treatment of clinical depression proposes that too little activity of the monoamine neurotransmitters can cause depression and too much activity can cause a manic state

Drug treatments for the vast majority of psychopathologies are not cures

they only provide relief from disease-related symptoms

No single neurochemical theory of depression has obtained sufficient experimental support to be considered an explanation

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Brain Imaging Techniques: M R I

Magnetic Resonance Imaging or M R I

Technique that uses powerful magnets to determine the amount of hydrogen atoms at different locations in the body

Benefits

Provides a high-resolution image of the brain’s anatomy

Is noninvasive

Limitation

Provides no information about brain functioning

Source: National Cancer Institute Visuals Online

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Image Source: National Cancer Institute Visuals Online

Brain Imaging Techniques: P E T

Positron Emission Tomography or P E T

A radioactively labeled chemical is injected into the bloodstream, and a computerized scanning device then maps out the relative amounts of the chemical in various brain regions

Benefit

Provides a direct measure of brain activity and an indirect measure of potential toxicity to specific neurons

Limitations

Requires the injection of radioactive materials

Does not provide information about brain structures

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Brain Imaging Techniques: f M R I

Functional Magnetic Resonance Imaging or f M R I

Provides real-time information about changes in brain blood flow as an individual speaks about his or her mood or performs behavioral or cognitive tasks

Benefits

Gives real-time information about changes in brain blood flow

Noninvasive

Limitation

Does not provide any information about the anatomy of the brain

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appendices

Figure 4.1: Every Neuron Has Four Regions: Cell Body, Dendrites, Axon, and Axon Terminals, Appendix

The cell body of the neuron contains the nucleus. One end of the cell body has branchlike structures that are labeled dendrites. The other end is connected to an axon, which is a long, tubular structure. The axon is wrapped by five myelin sheaths. Each node between two myelin sheaths is labeled node of Ranvier. The end of the axon away from the cell body has the axon terminals, which are branchlike structures. There is a an arrow along the axon that point from the cell body to the axon terminals. This arrow is labeled movement of electrical impulse.

Jump back to Figure 4.1: Every Neuron Has Four Regions: Cell Body, Dendrites, Axon, and Axon Terminals

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Figure 4.3: Mesolimbic and Nigrostriatal Dopamine Pathways, Appendix

Frontal cortex, cerebral cortex, striatum, substantia nigra, hippocampus, ventral tegmental area, hypothalamus, and nucleus accumbens are labeled in the figure. Mesolimbic dopamine pathway that extends from the ventral tegmental area to the nucleus accumbens and the nigrostriatal dopamine pathway that extends from the substantia nigra to the striatum are shown in the figure.

Jump back to Figure 4.3: Mesolimbic and Nigrostriatal Dopamine Pathways

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Figure 4.5: Cross Section of the Brain: Major Structures, Appendix

The structures that are labeled are frontal lobe of cerebrum, hypothalamus, pituitary gland, pons, reticular activating system, cerebrum, medial forebrain bundle, midbrain, cerebellum, and medulla oblongata.

Jump back to Figure 4.5: Cross Section of the Brain: Major Structures

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Figure 4.6: Neurons Use Enzymes to Synthesize the Neurotransmitters Dopamine and Norepinephrine, Appendix

There are four molecular structures in the figure. Each structure consists of carbon, oxygen, hydrogen, and nitrogen molecules. Tyrosine reacts with tyrosine hydroxylase to form DOPA, which reacts with Dopa decarboxylase to form dopamine, which reacts with dopamine beta oxidase to form norepinephrine.

Tyrosine consists of 9 carbon molecules, 3 oxygen molecules, 11 hydrogen molecules, and 1 nitrogen molecule. The DOPA molecule consists of 9 carbon molecules, 4 oxygen molecules, 11 hydrogen molecules, and 1 nitrogen molecule. Dopamine consists of 8 carbon molecules, 2 oxygen molecules, 11 hydrogen molecules, and 1 nitrogen molecule. Norepinephrine consists of 8 carbon molecules, 3 oxygen molecules, 11 hydrogen molecules, and 1 nitrogen molecule.

Jump back to Figure 4.6: Neurons Use Enzymes to Synthesize the Neurotransmitters Dopamine and Norepinephrine

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Life Cycle of a Neurotransmitter, 2, Appendix

The first image shows a precursor molecule, a fragment of the precursor molecule, and the synthetic enzyme. The precursor molecule and the fragment are away from each other and shown above the synthetic enzyme. There is one arrow each extending from the precursor molecule and the fragment that points to the synthetic enzyme. The second image shows the precursor molecule and the fragment being attached to the synthetic enzyme. The third image shows the precursor molecule getting attached to the fragment, which is represented by an arrow. These two attached molecules are bound to the synthetic enzyme. The fourth image shows the resulting neurotransmitter molecule being released from the synthetic enzyme. The release is represented by an arrow.

Jump back to Figure 4.7: Schematic Representation of the Action of a Synthetic Enzyme

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Life Cycle of a Neurotransmitter, 3, Appendix

There are two neurons shown in the figure. The presynaptic cell of one neuron is shown on the top, and the postsynaptic cell of the other neuron is shown at the bottom. There is a gap between the two neurons, which is labeled synapse. The top part of the presynaptic cell is labeled axon terminal. In the presynaptic cell, round structures called vesicles are shown. Ball-like structures inside the vesicles are labeled neurotransmitters. There are five vesicles shown in the presynaptic cell, with two of the vesicles near the base that is near the synapse. An arrow extends toward one of these vesicles from a vesicle above it. The end of the postsynaptic cell that is near the synapse is labeled membrane of neuron and has tiny projections above it, which are labeled receptor site. The neurotransmitters from the vesicles in the presynaptic cell are shown separating from the vesicles and attaching themselves to the receptor site. There is an arrow shown from one such neurotransmitter pointing toward a receptor on the receptor site to illustrate this attachment process.

Jump back to Life Cycle of a Neurotransmitter, 3, Appendix

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Life Cycle of a Neurotransmitter, 4, Appendix

In the first image, the neurotransmitter is shown above the metabolic enzyme. There is an arrow extending from the neurotransmitter toward the metabolic enzyme. In the second image, the neurotransmitter is shown attached to the metabolic enzyme. In the third image, a fragment of the neurotransmitter is shown to have detached itself from the neurotransmitter, both of which are still connected to the metabolic enzyme. An arrow from the neurotransmitter points toward the detached fragment. In the fourth image, the resulting metabolite and the fragment are shown away and detached from the metabolic enzyme. Arrows from the metabolic enzyme point toward the metabolite and the fragment.

Jump back to Life Cycle of a Neurotransmitter, 4

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