Chapter Four: Neurons, Hormones, and the Brain

Communication in the Nervous System

INTRODUCTION

How does the nervous system communicate? Through neurons, neurotransmitters, and hormones.

WHAT'S AHEAD

• Which nervous-system cells are "communication specialists"---and how do they "talk" to each other?
• How do learning and experience alter the brain's circuits?
• Why do neural impulses travel more slowly in babies than in adults?
• What happens when levels of brain chemicals called neurotransmitters are too low or too high?
• Which substances in the brain mimic the effects of morphine by dulling pain and promoting pleasure?
• Which hormones can improve your memory?

KEY CONCEPTS

Introduction
The Structure of the Neuron
How Neurons Communicate
Chemical Messengers in the Nervous System

Neurotransmitters
Endorphins
Hormones

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Introduction/Overview (p.117)

INTRODUCTION

Neurons are the basic building blocks of the nervous system.

KEY CONCEPTS EXPLAINED

• Neurons are the communication specialists, transmitting signals from neuron to neuron. There are about 100 billion in your brain -- that is why your head often feels so full.
  
  
• Glial cells insulate and hold the neurons in place while providing nutrients.

GLOSSARY

		neuron
		glial cell

The Structure of the Neuron (p.118)

INTRODUCTION

An explanation of the structure of a neuron.

Labels and structures are linked to text

click for an enlarged view

KEY CONCEPTS EXPLAINED

• Dendrites receive messages.
  
  
• The cell body determines whether or not the cell will fire.
  
  
• The axon transmits messages.
  
  
  • Axon terminals are branches.
    
    
• Myelin sheath insulates axons from each other and can speed up the transmission.
  
  
• Nerves in PNS are actually bundles of neurons.
  
  
• Can neurons regenerate?

	Old view	=	NO
	Recent view	=	some spinal cord cells can reproduce, and "precursor" cells can also transform into neurons.

GLOSSARY

	dendrites
	cell body
	axon
	myelin sheath
	nerve

LINKS About the Structure of the Neuron

• www.link: Multiple Sclerosis - myelin sheath problem.

How Neurons Communicate [p.120]

INTRODUCTION

Neurons communicate by sending an electrical charge down the axon and across the synapse to the next neuron. Because the neurons are not physically connected, chemical messengers called neurotransmitters cross the synaptic gap to get the message to the next neuron. Communication is both electrical and chemical.

KEY CONCEPTS EXPLAINED

..The Synapse

click for enlarged figure

• The synapse is where the communication between neurons occurs.
  
  
• Synapses and new synaptic connections continue to develop through life. New learning leads to new synaptic connections and unused synaptic connections are lost.
  
  
• Plasticity is the flexibility of the brain's circuits. They are not fixed and can change in response to changes in the environment.

..Electrical Communication: The Action Potential

• Brief changes in electric voltage produces an electric impulse.
  
  

  Animation of an action potential in action! By John Krantz Hanover College

  
  
  
• Myelin sheath allows action potential to hop between the nodes for faster transmission.
  
  
  • Myelin sheaths are not as fully developed in babies as in adults. As a result, nerve impulses travel more slowly for them.

• The chemistry of the action potential

The action potential is actually produced by a chemical process, a sudden, momentary inflow of positively charged sodium ions.

		A very good animation of the chemical process resulting in an action potential in action by April Lange of WWNorton Publishing on a web site to support another introductory psychology text. One of the nicest the Web Wizard has seen.
		Another animation of the propagation of the action potential by following the changes in positive and negative charges along the axon. April Lange at WWNorton again.

..Chemical Communication:

When neural impulses get to the axon terminal, it stimulates synaptic vesicles to release chemical neurotransmitters which cross the synaptic cleft and fit into receptor sites that will either increase or decrease the likelihood that the receiving neuron will produce its own electrical impulse.

	Animation of the neurotransmitters crossing the synaptic gap by April Lange of WWNorton web site.
	Enlarged figure of the synapse and neurotransmitters crossing the gap.
	A dendrite is usually receiving neurotransmitters from more than one neuron.

..
All or None Effect

• The receiving neuron averages all the messages it is getting from the other neurons.
  
  
• When a certain threshold is reached, the neuron will fire.
  
  
• The process is like a light switch. It is turned on or off-- the "all or none effect"

GLOSSARY

	synapse
	neurotransmitter

Chemical Messengers in the Nervous System [p.122]

INTRODUCTION

Neurotransmitters, endorphins and hormones are important chemical messengers.

Neurotransmitters: Versatile Couriers [p.122]

INTRODUCTION

Neurotransmitters can affect mood, memory and well-being.

KEY CONCEPTS EXPLAINED

Important Neurotransmitters:

• Serotonin: involved in sleep, appetite, sensory perception, temperature regulation, pain suppression and mood.
  
  
• Dopamine: voluntary movement, learning, memory, and emotion.
  
  
• Acetylcholine increased heart rate and slowing intestinal activity during stress, and neurons involved in learning, memory, dreaming, waking from sleep, and emotional
  
  
• GABA (gamma-aminobutyric acid) is the major inhibitory neurotransmitter in the brain.

Effects of Low and High Levels of Neurotransmitters

• Depression associated with low levels of serotonin and norepinepherine.
  
  
• Abnormal GABA level
  
  
  • sleep disorders
  • Eating disorders
  • Convulsive disorders
    
    
• Childhood autism associated with elevated levels of serotonin.
  
  
• Alzheimer's disease
  
  
  • Memory loss, personality changes, and eventual disintegration of physical and mental abilities.
    
    
  • Associated with loss of brain cells that produce acetylcholine.
    
    
• Parkinson's Disease
  
  
  • Associated with degeneration of cells that produce dopamine.
    
    
  • Tremors, muscular spasms, and increasing muscular rigidity.
    
    
  • L-dopa, a precursor to dopamine, sometimes helps.
    
    
  • Dopamine producing brain tissue from aborted fetuses has been promising.

Endorphins: The Brain's Natural Opiates [p.123]

INTRODUCTION

Endorphins have effects similar to natural opiates

KEY CONCEPTS EXPLAINED

• Like opiates, endorphins reduce pain and promote pleasure.
  
  
• Act as neuromodulaters which alter the effects of neurotransmitters.
  
  
• Endorphins are endogenous.
  
  
• Endorphin level rise during stress or fear.
  
  
  • Make pain bearable.
    
    
• Link between endorphins and pleasure of social contact.
  
  
  • Addicted to love?

Hormones: Long-Distance Messengers [p.124]

INTRODUCTION

Substances that are produced in one part of the body and affect another. Hormones originate in endocrine glands and released directly into the blood stream.

KEY CONCEPTS EXPLAINED

Melatonin

• Secreted by the pineal gland.
  
  
• Promotes sleep and regulate bodily rhythms.

Adrenal hormones

• Produced by the adrenal glands.
  
  
• Involved in emotion and stress.
  
  
• Cortisol increased blood-sugar levels and boosts energy.
  
  
• Epinephrine (adrenaline) and Norepinephrine activate the autonomic nervous system ( to autonomic nervous system)
  
  
• Moderate levels (i.e. moderate arousal) is best for learning and encoding events into memory.

Sex Hormones

Secreted in the gonads, occurs in both sexes, but differs in proportion in men and women.

• Androgens
  
  
  • "Masculinizing" hormones
    
    
  • Testosterone
    
    
  • Physical changes in puberty
    
    
  • Testosterone also influences sexual arousal in both sexes.
    
    
• Estrogen are hormones.
  
  
• Progesterone contributes to the growth and maintenance of the uterine lining in preparation for the fertilized egg.

GLOSSARY

	endorphins
	neuromodulators
	hormones
	endocrine glands
	melatonin
	adrenal hormones
	sex hormones

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