RESEARCH FOCUS 5-1 THE BASIS OF NEURAL COMMUNICATION IN A HEARTBEAT

5-1 A CHEMICAL MESSAGE

EXPERIMENT 5-1 QUESTION: HOW DOES A NEURON PASS ON A MESSAGE?

CLINICAL FOCUS 5-2 PARKINSON DISEASE

STRUCTURE OF SYNAPSES

NEUROTRANSMISSION IN FOUR STEPS

VARIETIES OF SYNAPSES

EXCITATORY AND INHIBITORY MESSAGES

EVOLUTION OF COMPLEX NEUROTRANSMISSION SYSTEMS

5-2 VARIETIES OF NEUROTRANSMITTERS AND RECEPTORS

FOUR CRITERIA FOR IDENTIFYING NEUROTRANSMITTERS

FOUR CLASSES OF NEUROTRANSMITTERS

CLINICAL FOCUS 5-3 AWAKENING WITH l-DOPA

VARIETIES OF RECEPTORS

5-3 NEUROTRANSMITTER SYSTEMS AND BEHAVIOR

NEUROTRANSMISSION IN THE SOMATIC NERVOUS SYSTEM

DUAL ACTIVATING SYSTEMS OF THE AUTONOMIC NERVOUS SYSTEM

ENTERIC NERVOUS SYSTEM AUTONOMY

FOUR ACTIVATING SYSTEMS IN THE CENTRAL NERVOUS SYSTEM

CLINICAL FOCUS 5-4 THE CASE OF THE FROZEN ADDICT

5-4 ADAPTIVE ROLE OF SYNAPSES IN LEARNING AND MEMORY

HABITUATION RESPONSE

EXPERIMENT 5-2 QUESTION: WHAT HAPPENS TO THE GILL RESPONSE AFTER REPEATED STIMULATION?

SENSITIZATION RESPONSE

EXPERIMENT 5-3 QUESTION: WHAT HAPPENS TO THE GILL RESPONSE IN SENSITIZATION?

LEARNING AS A CHANGE IN SYNAPSE NUMBER

RESEARCH FOCUS 5-5 DENDRITIC SPINES: SMALL BUT MIGHTY

The Basis of Neural Communication in a Heartbeat

Discoveries about how neurons communicate stem from experiments designed to study what controls an animal’s heart rate. As with any animal, your heartbeat quickens if you are excited or exercising; if you are resting, it slows. Heart rate changes to match energy expenditure—that is, to meet the body’s nutrient and oxygen needs.

Heartbeat undergoes a most dramatic change when you dive beneath water: it slows almost to stopping. This drastic slowing, called diving bradycardia, conserves the body’s oxygen when you are not breathing. Bradycardia (brady-, meaning slow, and -cardia, meaning heart) is a useful survival strategy. This energy-conserving response under water is common to many animals. But what controls your heartbeat?

Otto Loewi, a great storyteller, recounted that his classic experiment, which earned him a Nobel Prize in 1936, came to him in a dream. As shown in the Procedure section of Experiment 5-1. Loewi first maintained a frog’s heart in a salt bath, then electrically stimulated the vagus nerve—the cranial nerve that leads from the brain to the heart. At the same time, he channeled some of the fluid bath from the vessel containing the stimulated heart through a tube to another vessel in which a second heart was immersed but not electrically stimulated.

Loewi recorded both heart rates. His findings are represented in the Results section of Experiment 5-1. The electrical stimulation decreased the rate of the first heart, but more important, the second heartbeat also slowed. This result indicated that the fluid transferred from the first to the second container carried instructions to slow down.

Where did the message come from originally? Loewi reasoned that a chemical released from the stimulated vagus nerve must have diffused into the fluid bath to influence the second heart. His experiment therefore demonstrated that the vagus nerve contains a chemical that tells the heart to slow its rate.

Loewi subsequently identified the messenger chemical. Later, he also identified a chemical that tells the heart to speed up. The heart adjusts its rate in response to at least two different messages: an excitatory message that says speed up and an inhibitory message that says slow down.