Discussion Paper

Chapter 4

The Nervous System

Chapter Objectives

After reading this chapter, students should be able to:

· Understand how psychoactive drugs alter communication among the billions of cells in the human brain.

· Explain the concept of homeostasis.

· Know the general properties of glia and neurons.

· Understand and describe the action potential.

· Describe the roles of the sympathetic and parasympathetic branches of the autonomic nervous system and associated neurotransmitters.

· Be able to associate important neurotransmitters with key brain structures and chemical pathways, and describe the major functions of the neurotransmitters.

· Describe the life cycle of a neurotransmitter molecule.

· Understand the importance of receptor subtypes in determining the action of a neurotransmitter at a particular site in the brain.

· Give examples of a drug that alters neurotransmitter availability and of a drug that interacts with neurotransmitter receptors.

Chapter Vocabulary (in order of appearance in chapter) – double check order bc a lot changed in this chapter…

4-8

psychoactive

homeostasis

neurons

cell body

dendrites

receptors

axon

glial cells (glia)

multiple sclerosis

blood-brain barrier

semipermeable

lipophilic

vesicles

neurotransmission

action potential

neurotransmitters

resting potential

hyperpolarized

depolarized

somatic nervous system

acetylcholine

autonomic nervous system

sympathetic branch

parasympathetic branch

central nervous system

cortex

motor cortex

basal ganglia

Parkinson’s disease

hypothalamus

limbic system

brain stem

dopamine

ventral tegmental area

mesolimbic dopamine pathway

schizophrenia

nigrostriatal dopamine pathway

precursor

acetylcholine

nucleus basalis

Alzheimer’s disease

norepinephrine

locus ceruleus

serotonin

raphe nuclei

GABA (γ-amino butyric acid)

glutamate

endorphin

uptake

synthesis

enzymes

synapse

transporter

metabolize

availability

agonist

antagonist

humors

yin

yang

monoamine

monamine theory of mood

positron emission tomography (PET)

magnetic resonance imaging (MRI)

functional magnetic resonance imaging (fMRI)

Major Structures of the Brain:

cerebellum

cerebrum

frontal lobe of cerebrum

hypothalamus

medial forebrain bundle

medulla oblongata

midbrain

pituitary gland

pons

reticular activating system

Major Subdivisions of the Human Cerebral Cortex:

central sulcus

cerebellum

frontal lobe

occipital lobe

parietal lobe

postcentral gyrus

precentral gyrus

temporal lobe

Chapter Outline – check the order bc a lot changed in this chapter.

I. Chemical Messengers

A. Homeostasis—humans must maintain their internal environment within certain limits

1. Temperature

2. Acidity

3. Water content

4. Sodium content

5. Glucose concentrations

6. Other physical and chemical factors

II. Components of the Nervous System

A. Nerve cells (neurons)

1. Analyze and transmit information

2. More than 100 billion in nervous system

3. Four defined regions

a. Cell body

b. Dendrites (contain receptors)

c. Axon

d. Presynaptic terminals

4. Stimulation of receptors by psychoactive drugs can activate or inhibit a neuron

B. Glial cells (glia)

1. About 90% of the cells in the human brain are glia

2. Glia serve important functions:

a. Provide firmness and structure to the brain

b. Get nutrients into the system

c. Eliminate waste

d. Form myelin

i. Multiple sclerosis is a neurodegenerative autoimmune disorder resulting from damage to or loss of myelin.

e. Create the blood-brain barrier

3. Our understanding of glia cells has been “neuro-centric,” and glia may serve other functions as well.

a. Communication capabilities.

b. Key role in CNS disorders, including neuropathic pain and depression.

III. Neurotransmission

A. Action Potential—a brief electrical signal transmitted along the axon

1. Neurotransmitters: the messengers

2. Resting action potential caused by uneven distribution of ions

3. Action potential occurs when sodium ions move across channels

4. Blocking channels prevents the action potential and disrupts communication between neurons

IV. The Nervous System(s)

A. Somatic Nervous System (SNS)

1. Carries sensory information into the central nervous system

2. Carries motor (movement) information back out to the peripheral nerves

3. Controls voluntary actions

4. Acetylcholine is the neurotransmitter at neuromuscular junctions

5. The same neurotransmitter can have different effects depending on which receptor it activates

B. Autonomic Nervous System (ANS)

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

2. Many psychoactive drugs affect the brain and the ANS

3. Two branches act in opposition

a. Sympathetic branch

b. Parasympathetic branch

C. Central Nervous System (CNS)

1. Consists of the brain and the spinal cord

2. Integration of information, learning and memory, and coordination of activity

V. The Brain

A. Chemical Pathways Implicated in Reward

1. Dopamine

a. Found in basal ganglia and other regions

b. Mesolimbic dopamine pathway

i. Related to psychotic behavior

ii. Possible component of the “reward” properties of drugs, though there is a growing body of contradictory evidence

c. Nigrostriatal dopamine pathway

i. Substantial loss of cells along this pathway leads to Parkinson’s disease

2. Acetylcholine

a. Found in the cerebral cortex

b. Involved in Alzheimer’s disease and initiation of REM sleep

3. Norephinephrine

a. Regulates level of arousal and attentiveness

b. May play a role in initiation of food intake (appetite)

4. Serotonin

a. Found in the brain stem raphe nuclei

b. May have a role in impulsivity, aggression, depression, control of food intake

c. Hallucinogenic drugs influence serotonin pathways

5. GABA (γ-amino butyric acid)

a. Found in most regions of the brain

b. Inhibitory neurotransmitter

c. Interference can lead to seizures resembling those seen in epilepsy

6. Glutamate

a. Found in most regions of the brain

b. Major excitatory neurotransmitter

7. Endorphins

a. Opioid-like chemical occurring naturally in the brain

B. Major Structures

1. Cerebral cortex

a. Processing of visual, auditory, somatosensory information

b. Control of muscles

c. Higher mental processes

2. Basal ganglia

a. Maintenance of proper muscle tone

b. Critical in specific aspects of learning

3. Hypothalamus—involved in feeding, drinking, temperature regulation, sexual behavior

4. Limbic system—involved in emotion, memory for location, level of physical activity

5. Midbrain, pons, and medulla—involved in sensory and motor reflexes, coordinated control of complex movements, production of neurotransmitters

6. Brain stem—involved in regulation of respiration and other functions

VI. Drugs and the Brain

A. Drugs travel throughout the body and reach the brain via the bloodstream.

B. Life Cycle of a Neurotransmitter

1. Neurotransmitter precursors are found circulating in the blood supply.

2. Selected precursors are taken up by cells, a process requiring energy.

3. Precursors are changed (synthesized) into neurotransmitters through the action of enzymes.

4. Neurotransmitters are stored in small vesicles.

5. When the action potential arrives, neurotransmitters are released into the synapse.

6. Released neurotransmitters bind with receptors on the membrane of the next neuron.

7. Neurotransmitters may have excitatory or inhibitory effects.

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

C. Examples of Drug Actions

1. Drug effects on neurotransmitter availability

2. Drug effects directy on receptors

a. agonist

b. antagonist

VII. Chemical Theories of Behavior

A. Attempts to explain normal variations in behavior in terms of changes in brain chemistry

1. Greek physician Hippocrates and the four humors

2. Chinese philosophy—yin and yang

B. No single biochemical theory of drug dependence has achieved sufficient experimental support.

C. Monoamine theory of mood—too little activity in monoamine systems can cause depression, and vice-versa.

1. Drug treatments for the majority of psychopathologies are symptom-relievers, not cures.

2. No single neurochemical theory of depression has garnered sufficient experimental support to be considered an explanation.

3. This theory was proposed in the 1960s, when only about five neurotransmitters were known. Now there are more than 160, but the theory has not changed substantially.

VIII. Brain Imaging Techniques

A. Positron emission tomography (PET)

1. Lack of radioactively labeled chemicals that bind to receptors for neurotransmitters other than dopamine

B. Magnetic resonance imaging (MRI)

1. Non-invasive

C. Functional magnetic resonance imaging (fMRI)

1. Real-time information about changes in blood flow, reflecting changes in brain activity.

D. In doing imaging studies, we must be careful to examine the behavior of interest so as not to draw inappropriate conclusions about the neural basis of cognition.

Key Points

· Homeostasis is mediated by the release of endogenous regulatory chemicals such as neurotransmitters and hormones. Many drugs affect these substances and change the function of the nervous or endocrine system.

· There are two main types of cells in the nervous system: glia and neurons.

· Glial cells make up the blood-brain barrier that protects the brain from toxic chemicals in the blood.

· All nervous systems consist of neurons, axons, and receptors.

· Activation of receptors by neurotransmitters cause a change in activity of the target cell, and many of the effects of psychoactive drugs are due to the ability to alter neurotransmitters.

· Neurons are the basic structural unit of the nervous system that is responsible for analyzing and transmitting information. There are more the 100 billion neurons in the nervous system.

· The gap between neurons is called the synapse.

· The receiving region is called the dendrite.

· The receptors are proteins that help regulate activity of cells in the nervous system and throughout the body.

· Some specific drugs and natural neurotransmitters can activate the same receptors.

· Communication is accomplished through a precise, rapid method. The message is transmitted along a neuron’s axon. Neurotransmitters are released so the communication can happen from one neuron to the other.

· There are agonistic and antagonistic effects on receptors.

· Agonistic drugs interact with the receptor and produce a response, whereas antagonistic drugs interact with the receptor but prevent a response.

· Selective blocking of some channels prevents the communication between the neurons. An example of this would be from using cocaine or other local anesthetics.

· Neurotransmitters most likely altered by drug abuse include acetylcholine (ACh), epinephrine, dopamine, serotonin, and the endorphins.

· The somatic nervous system carries sensory information from outside the body into the CNS and carries motor information out to the periphery.

· Voluntary movements come from large cells with long axons. The seven senses are produced from this system.

· The autonomic nervous system (ANS) cell bodies are located within the brain or spinal cord but their axons project outside the CNS to involuntary muscles.

· The ANS is divided into two components that oppose each other: the sympathetic branch and the parasympathetic branch.

· Like amphetamines and other sympathomimetic drugs, the sympathetic branch speeds up both heart and breathing rates and acts as a vasoconstrictor.

· The parasympathetic branch releases the neurotransmitter ACh and has essentially the opposite effects of sympathetic branch.

· The central nervous system comprises the brain and spinal cord.

· The cerebral cortex receives sensory input, interprets visual information, and processes auditory information. Reasoning and language also occur in the cerebral cortex.

· The basal ganglia are the primary centers for involuntary movement and are hidden from external view underneath the cerebral cortex.

· The hypothalamus integrates information from many sources and is the control center for the autonomic nervous system.

· The hypothalamus is a small structure near the base of the brain that is involved in sex drive, hunger, body temperature, and others functions.

· The limbic system regulates emotional activities, memory, and modulation of basic hypothalamic functions, mating, procreation, and caring for young.

· The neurotransmitters most associated with psychoactive drugs are dopamine, acetylcholine, norepinephrine, serotonin, GABA, glutamate, and endorphins. There can be positive or negative effects from drugs on these neurotransmitters.

· Some drugs help misfiring neurotransmitters or help to increase the amount of a neurotransmitter when the body is naturally low. Other drugs, however, do harm by doing the same action as a neurotransmitter in a user who already has a natural level of the neurotransmitter.

· Precursors are the building blocks of neurotransmitters; they are found circulating in the blood. Figure 4.6 shows the process of synthesis from precursors to neurotransmitter.

· After they are synthesized, neurotransmitters are stored in vesicles waiting to be released. The release happens in microseconds in less than 1/10,000th of an inch of space to several thousand neurotransmitters.

· GABA is called an inhibitory neurotransmitter. Many sedatives are dependent upon their binding to the GABA receptors.

· One way neurotransmitters molecules are removed from the synapse is that some molecules have specific transporters built into their terminals; this brings the neurotransmitter molecules back into the releasing neuron. Other neurotransmitters have enzymes in the synapse that metabolize molecules.

· In a positron emission tomography (PET) scan, a radioactively labeled chemical is injected in the bloodstream and then a computer tracts it as it flows through the brain.

· Magnetic resonance imaging (MRI) uses strong magnetic fields and measures the energy coming from molecules as the field is collapsed.

· fMRI can be used to evaluate brain activity in real time, e.g., while a person is performing a cognitive task.

· In doing imaging studies, we must be careful to examine the behavior of interest so as not to draw inappropriate conclusions about the neural basis of cognition.