Chapter Three
Communication Within the Body:
Synapses and Hormones
The Concept of the Synapse
The Properties of the Synapse
Speed of a reflex and delayed transmission at the synapse
Sherrington noticed that the overall speed of conduction
through the reflex arc was slower than the known speed
of conduction along an axon (slowed by the synapse)
Temporal summation
Repeated stimuli occurring within a brief time can have a
cumulative effect
Spatial summation
Several synaptic inputs originating from separate locations
can exert a cumulative effect on a neuron
Figure 3.2 Sherrington’s evidence for synaptic delay
An impulse traveling through a synapse in the spinal cord is slower
than one traveling a similar distance along an uninterrupted axon.
Figure 3.3 Recordings from a postsynaptic neuron during synaptic activation
Figure 3.4 Temporal and spatial summation
Postsynaptic and Action Potentials
Excitatory Postsynaptic Potentials
Graded depolarizations
Inhibitory Postsynaptic Potentials
Graded hyperpolarizations
Relationship between EPSP’s, IPSP’s and action potentials
All postsynaptic potentials are added together and if enough
EPSP’s occur to cause cell to cross threshold, an action
potential occurs
Chemical Events at the Synapse
The Discovery That Most Synaptic Transmission is Chemical
Otto Loewi
Chronically stimulated vagus nerve attached to frog’s heart
resulting in decreased heart rate
Collected fluid from around heart and transferred to
another frog’s heart
Second heart also slowed its rate
Figure 3.7 Loewi’s experiment demonstrating that
nerves send messages by releasing chemicals
Loewi stimulated the vagus nerve to one frog’s heart, decreasing
the heartbeat. Then he transferred fluid from that heart to another
frog’s heart and observed a decrease in its heartbeat.
Sequence of Events at a Synapse
Figure 3.8 Some of the major events in transmission at a synapse
Animation
Types of Neurotransmitters
Amino Acids-Acids containing an amine group
Peptides-long chains of amino acids
Acetylcholine-similar to amino acids but structurally different
Monoamines-nonacidic neurotransmitters containing an amine
group
Purines-category of chemical including adenosine and several of its
derivatives
Gases-specifically nitric oxide
Transmitters
Synthesis
Begins with substances in the diet
Transport
Some must be transported from cell body
Release and Diffusion
Release occurs through exocytosis
More than one transmitter can be released from each neuron
Diffusion of the transmitter occurs across the synapse
Transmitters
Multiple Transmitter Release
Most neurons release at least two neurotransmitters
Each neuron releases the same neurotransmitters from all
axon terminals
The release of multiple transmitters makes the message more
variable and complex
Figure 3.12 Neurotransmitter receptors
A neurotransmitter receptor as it would look if folded out.
Figure 3.13 Sequence of events at a metabolic synapse, using a second
messenger within the postsynaptic neuron
Activation of Receptors
Ionotropic
Immediately opens gates for ions
Ex: Glutamate, GABA
Ionotropic synapses are used for quick events like visual
stimulation, muscle movements, etc.
Metabotropic
Initiates a cascade of metabolic events that results in opening or
closing of ion channels, changes in protein production, or
activates chromosomes
Longer lasting than ionotropic effects
Uses G-protein and second messenger system
Activation of Receptors
Modulatory
An intermediate between neurotransmitter and hormone
Can have affects at all nearby cells
Modulate the effects of neurotransmitters
Inactivation and Reuptake
Inactivation
Breaking down a neurotransmitter with an enzyme
Ex: Acetylcholine
Reuptake
Once neurotransmitters detach from the receptor they are
taken back up by the presynaptic cell
Transporters are special membrane proteins that allow the
neurotransmitter to pass back through and into the
presynaptic cell
Ex: serotonin
Synapses, Abused Drugs, and Behavior
How Drugs Affect Synapses
Agonist-a drug that mimics or increases the effects of a
neurotransmitter
Antagonist-a drug that blocks or takes away from the effects of
the neurotransmitter
Affinity-ability of a drug to bind a receptor
Efficacy-the degree to which the drug activates the receptor
once bound
Synapses, Reinforcement, and Drug Use
Electrical Self-Stimulation of the Brain
Nucleus accumbens-subcortical area rich in dopamine
receptors
The role of the nucleus accumbens in addiction
Effects of Stimulant Drugs on Dopamine Synapses
Stimulants-increase behavioral arousal, increase activity at
dopamine receptors
Amphetamine
Cocaine
Ritalin
Figure 3.15 Events at a dopamine synapse
and how certain drugs affect the process
Drugs can alter any stage of processing at a
synapse, from synthesis of the neurotransmitter
through release and reuptake.
An Exploration of Drugs
Nicotine
Attaches to nicotinic receptors
Increases dopamine release in the nucleus accumbens
Opiates
Derived from opium poppy
Attach to opioid receptors in the brain
Result in increased dopamine release
PCP
Inhibits some glutamate receptors
Results in reinforcement in the nucleus accumbens
Marijuana/Cannabinoids
Bind to specific receptors in the brain
Leads to euphoria
An Exploration of Drugs
Hallucinogenic Drugs
Distort Perception
Many resemble activities of neurotransmitters
Caffeine
Constricts blood vessels in the brain
Interferes with the effects of the neurotransmitter adenosine
Alcohol
Inhibits flow of sodium across the membrane
Expands the surface of membranes
Decreases serotonin activity
Facilitates GABA responses
Blocks glutamate receptors
Increases dopamine activity
Video
Hormones and Behavior
Mechanisms of Hormone Actions
Attach receptors on cell membrane
Activate enzyme that produces cyclic AMP or some other
second messenger
Types of hormones
Protein-Long-chain amino acids
Peptide-shorter chain amino acids
Thyroid Hormones
Steroid Hormones-contain four carbon rings
Have action at membrane receptors and at receptors in the
cytoplasm and nucleus
Ex: cortisol, estrogen, progesterone, testosterone
Hormones and Behavior
Control of Hormone Release
Hypothalamus-controls the pituitary
Pituitary
Anterior
Adrenocorticotropic Hormone-secretions of the adrenal
cortex
Thyroid-Stimulating-Secretions of the thyroid
Prolactin-secretions of mammary glands
Somatotropin-promotes growth of the body
FSH and LH-controls secretions of the gonads
Posterior
Oxytocin-controls uterine contractions, milk release,
etc.
Vasopressin-constricts blood vessels and raises blood
pressure; decreases urine volume