Biological Psychology: Synapses
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Transcript of Biological Psychology: Synapses
© Cengage Learning 2016 © Cengage Learning 2016
Chapter 2
Synapses
© Cengage Learning 2016
2.1 The Concept of the Synapse
• Neurons communicate by transmitting chemicals at junctions, called “synapses”– The term was coined by Charles Scott
Sherrington in 1906 to describe the specialized gap that existed between neurons
– Sherrington’s discovery was a major feat of scientific reasoning
© Cengage Learning 2016
The Properties of Synapses
• Sherrington – Investigated how neurons communicate with
each other by studying reflexes (automatic muscular responses to stimuli) in a process known as a reflex arc
• Example – Leg flexion reflex: a sensory neuron excites a
second neuron, which excites a motor neuron, which excites a muscle
© Cengage Learning 2016
The Relationship Among a Sensory Neuron, Intrinsic Neuron, and Motor Neuron
© Cengage Learning 2016
Three Important Points About Reflexes
• Sherrington’s observations– Reflexes are slower than conduction along an
axon
– Several weak stimuli present at slightly different times or slightly different locations produce a stronger reflex than a single stimulus
– As one set of muscles becomes excited, another set relaxes
© Cengage Learning 2016
Difference in the Speed of Conduction
• Sherrington found a difference in the speed of conduction in a reflex arc from previously measured action potentials– He believed the difference must be accounted
for by the time it took for communication between neurons
– Evidence validated the idea of the synapse
© Cengage Learning 2016
Sherrington’s Evidence for Synaptic Delay
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Temporal Summation
• Sherrington observed that repeated stimuli over a short period of time produced a stronger response
• Thus, the idea of temporal summation – Repeated stimuli can have a cumulative effect
and can produce a nerve impulse when a single stimuli is too weak
© Cengage Learning 2016
Excitatory Postsynaptic Potential (EPSP)
• Presynaptic neuron: neuron that delivers the synaptic transmission
• Postsynaptic neuron: neuron that receives the message
• Excitatory postsynaptic potential (EPSP): graded potential that decays over time and space
• The cumulative effect of EPSPs are the basis for temporal and spatial summation
© Cengage Learning 2016
Spatial Summation, Part 1
• Sherrington also noticed that several small stimuli in a similar location produced a reflex when a single stimuli did not
• Thus, idea of spatial summation – Synaptic input from several locations can have
a cumulative effect and trigger a nerve impulse
© Cengage Learning 2016
Recordings From a Postsynaptic Neuron During Synaptic Activation
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Spatial Summation, Part 2
• Spatial summation is critical to brain functioning
• Each neuron receives many incoming axons that frequently produce synchronized responses
• Temporal summation and spatial summation ordinarily occur together
• The order of a series of axons influences the results
© Cengage Learning 2016
Temporal and Spatial Summation
© Cengage Learning 2016
The Effects of Summation
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Inhibitory Synapses
• Sherrington noticed that during the reflex that occurred, the leg of a dog that was pinched retracted while the other three legs were extended– Suggested that an interneuron in the spinal
cord sent an excitatory message to the flexor muscles of one leg and an inhibitory message was sent to the other three legs
© Cengage Learning 2016
Antagonistic Muscles
© Cengage Learning 2016
Inhibitory Postsynaptic Potential (ISPS)
• Thus, the idea of inhibitory postsynaptic potential (ISPS) – the temporary hyperpolarization of a membrane– Occurs when synaptic input selectively opens
the gates for positively charged potassium ions to leave the cell, or negatively charged chloride ions to enter the cells
– Serves as an active “brake” that suppresses excitation
© Cengage Learning 2016
Sherrington’s Inference of Inhibitory Synapses
© Cengage Learning 2016
Relationship Among EPSP, IPSP, and Action Potentials
• Sherrington assumed that synapses produce on and off responses
• Synapses vary enormously in their duration of effects– The effect of two synapses at the same time
can be more than double the effect of either one, or less than double
© Cengage Learning 2016
A Possible Wiring Diagram for Synapses
© Cengage Learning 2016
Wiring Diagram for an “A or B” Response
© Cengage Learning 2016
Wiring Diagram for an “A and B” Response
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Wiring Diagram for an “A and B if not C” Response
© Cengage Learning 2016
Spontaneous Firing Rate
• The periodic production of action potentials despite synaptic input– EPSPs increase the number of action
potentials above the spontaneous firing rate
– IPSPs decrease the number of action potentials below the spontaneous firing rate
© Cengage Learning 2016
The Discovery of Chemical Transmission at Synapses
• German physiologist Otto Loewi– The first to convincingly demonstrate that
communication across the synapse occurs via chemical means
• Neurotransmitters: chemicals that travel across the synapse and allow communication between neurons– Chemical transmission predominates
throughout the nervous system
© Cengage Learning 2016
Module 2.2 Chemical Events at the Synapse
• The great majority of synapses rely on chemical processes
• Otto Loewi’s experiment– Found that stimulating one nerve released
something that inhibited heart rate, and stimulating a different nerve released something that increased heart rate
– Realized that he was collecting and transferring chemicals, not loose electricity
© Cengage Learning 2016
Nerves Send Messages by Releasing Chemicals
© Cengage Learning 2016
The Sequence of Chemical Events at the Synapse, Part 1
• The major sequence of events allowing communication between neurons across the synapse– The neuron synthesizes chemicals that serve
as neurotransmitters
– Action potentials travel down the axon
– Released molecules diffuse across the cleft, attach to receptors, and alter the activity of the postsynaptic neuron
© Cengage Learning 2016
The Sequence of Chemical Events at the Synapse, Part 2
– The neurotransmitter molecules separate from their receptors
– The neurotransmitters may be taken back into the presynaptic neuron for recycling or diffuse away
– Some postsynaptic cells may send reverse messages to slow the release of further neurotransmitters by presynaptic cells
© Cengage Learning 2016
Some Major Events in Transmission At a Synapse
© Cengage Learning 2016
Table 2.1 Neurotransmitters
Types of Neurotransmitters, Part 1
Amino acids Glutamate, GABA, glycine, asparate, maybe others
A modified amino acid Acetylcholine
Monoamines (also modified from amino acids)
indoleamines: serotoninCatecholamines: dopamine, norepinephrine, epinephrine
Neuropeptides (chains of amino acids)
Endorphins, substance P, neuropeptide Y, many others
Purines ATP, adenosine, maybe others
Gases NO (nitric oxide), maybe others
© Cengage Learning 2016
Types of Neurotransmitters, Part 2
• Neurons synthesize neurotransmitters and other chemicals from substances provided by the diet– Acetylcholine synthesized from choline found
in milk, eggs, and nuts
– Tryptophan serves as a precursor for serotonin
• Catecholamines contain a catechol group and an amine group (epinephrine, norepinephrine, and dopamine)
© Cengage Learning 2016
Pathways in the Synthesis of Transmitters
© Cengage Learning 2016
Storage of Transmitters
• Vesicles: tiny spherical packets located in the presynaptic terminal where neurotransmitters are held for release
• MAO (monoamine oxidase): breaks down excess levels of some neurotransmitters
• Exocytosis: bursts of release of neurotransmitter from the presynaptic terminal into the synaptic cleft– Triggered by an action potential
© Cengage Learning 2016
Anatomy of a Synapse
© Cengage Learning 2016
Release and Diffusion of Transmitters
• Transmission across the synaptic cleft by a neurotransmitter takes fewer than 0.01 microseconds
• Most individual neurons release at least two or more different kinds of neurotransmitters
• Neurons may also respond to more types of neurotransmitters than they release
© Cengage Learning 2016
Activating Receptors of the Postsynaptic Cell
• The effect of a neurotransmitter depends on its receptor on the postsynaptic cell
• Transmitter-gated or ligand-gated channels are controlled by a neurotransmitter
© Cengage Learning 2016
Ionotropic Effects
• Occurs when a neurotransmitter attaches to receptors and immediately opens ion channels
• Most effects: – Occur very quickly (sometimes less than a
millisecond after attaching) and are very short lasting
– Rely on glutamate or GABA
© Cengage Learning 2016
The Acetylcholine Receptor
© Cengage Learning 2016
Metabotropic Effects and Second Messenger Systems, Part 1
• Occur when neurotransmitters attach to a receptor and initiate a sequence of slower and longer lasting metabolic reactions
• Metabotropic synapses use many neurotransmitters such as dopamine, norepinephrine, serotonin, and sometimes glutamate and GABA
© Cengage Learning 2016
Metabotropic Effects and Second Messenger Systems, Part 2
• When neurotransmitters attach to a metabotropic receptor, it bends the receptor protein that goes through the membrane of the cell – Bending allows a portion of the protein inside
the neuron to react with other molecules
• Metabotropic events include such behaviors as taste, smell, and pain
© Cengage Learning 2016
Sequence of Events at a Metabotropic Synapse
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G-Proteins
• G-protein activation: coupled to guanosine triphosphate (GTP), an energy storing molecule– Increases the concentration of a “second-
messenger”
– The second messenger communicates to areas within the cell
– May open or close ion channels, alter production of activating proteins, or activate chromosomes
© Cengage Learning 2016
Neuropeptides
• Metabotropic effects utilize a number of different neurotransmitters
• Neuropeptides are often called neuromodulators – Release requires repeated stimulation
– Released peptides trigger other neurons to release same neuropeptide
– Diffuse widely and affect many neurons via metabotropic receptors
© Cengage Learning 2016
Distinctive Features of Neuropeptides
Neuropeptides Other Neurotransmitters
Place Synthesized Cell body Presynaptic terminal
Place released Mostly from dendrites, also cell body and sides of axon
Axon terminal
Released by Repeated depolarization
Single action potential
Spread of effects Diffuse to wide area Effect mostly on receptors of the adjacent postsynaptic cell
Duration of effects Many minutes Less than a second to a few seconds
© Cengage Learning 2016
Drugs that Act by Binding to Receptors
• Many hallucinogenic drugs distort perception– Chemically resemble serotonin in their
molecular shape
– Stimulate serotonin type 2A receptors (5-HT2A) at inappropriate times or for longer duration than usual, thus causing their subjective effect
• Nicotine stimulates acetylcholine receptors
© Cengage Learning 2016
Opiate Drugs and Endorphins
• Opiates attach to specific receptors in the brain
• The brain produces certain neuropeptides now known as endorphins—a contraction of endogenous morphines
• Opiate drugs exert their effects by binding to the same receptors as endorphins
© Cengage Learning 2016
Inactivation and Reuptake of Neurotransmitters, Part 1
• Neurotransmitters released into the synapse do not remain and are subject to either inactivation or reuptake
• During reuptake, the presynaptic neuron takes up most of the neurotransmitter molecules intact and reuses them
• Transporters are special membrane proteins that facilitate reuptake
© Cengage Learning 2016
Inactivation and Reuptake of Neurotransmitters, Part 2
• Examples of inactivation and reuptake– Serotonin is taken back up into the presynaptic
terminal
– Acetylcholine is broken down by acetylcholinesterase into acetate and choline
– Excess dopamine is converted into inactive chemicals
• COMT: enzymes that convert the excess into inactive chemicals
© Cengage Learning 2016
Stimulant Drugs
• Amphetamine and cocaine – Stimulate dopamine synapses by increasing
the release of dopamine from the presynaptic terminal
• Methylphenidate (Ritalin)– Also blocks the reuptake of dopamine but in a
more gradual and more controlled rate
– Often prescribed for people with ADD; unclear whether Ritalin use in childhood makes one more likely to abuse drugs as an adult
© Cengage Learning 2016
Negative Feedback from the Postsynaptic Cell
• Negative feedback in the brain is accomplished in two ways– Autoreceptors: receptors that detect the
amount of transmitter released and inhibit further synthesis and release
– Postsynaptic neurons: respond to stimulation by releasing chemicals that travel back to the presynaptic terminal where they inhibit further release
© Cengage Learning 2016
Cannabinoids
• The active chemicals in marijuana that bind to anandamide or 2-AG receptors on presynaptic neurons or GABA
• When cannabinoids attach to these receptors, the presynaptic cell stops sending
• In this way, the chemicals in marijuana decrease both excitatory and inhibitory messages from many neurons
© Cengage Learning 2016
Effects of Some Drugs at Dopamine Synapses
© Cengage Learning 2016
Electrical Synapses
• A few special-purpose synapses operate electrically
• Faster than all chemical transmissions
• Gap junction: the direct contact of the membrane of one neuron with the membrane of another
• Depolarization occurs in both cells, resulting in the two neurons acting as if they were one
© Cengage Learning 2016
A Gap Junction for an Electrical Synapse
© Cengage Learning 2016
Hormones
• Chemicals secreted by a gland or other cells that is transported to other organs by the blood where it alters activity
• Produced by endocrine glands
• Important for triggering long-lasting changes in multiple parts of the body
© Cengage Learning 2016
Location of Some Major Endocrine Glands
© Cengage Learning 2016
A Selective List of Hormones
Organ Hormone Hormone Functions (Partial)Hypothalamus Various releasing hormone Promote/inhibit release of hormones from pituitary
Anterior pituitary Thyroid-stimulating hormoneLuteinizing hormoneFollicle-stimulating hormoneACTHProlactinGrowth hormone
Stimulates thyroid glandStimulates ovulationPromotes ovum maturation (female), sperm production (male)Increases Steroid hormone production by adrenal glandIncreases milk productionIncreases body growth
Posterior pituitary OxytocinVasopressin
Uterine contractions, milk release, sexual pleasureRaises blood pressure, decreases urine volume
Pineal Melatonin Sleepiness; also role in puberty
Adrenal cortex AldosteroneCortisol
Reduces release of salt in the urineElevated blood sugar and metabolism
Adrenal medulla Epinephrine, norepinephrine Similar to actions of sympathetic nervous system
Pancreas InsulinGlucagon
Helps glucose enter cellsHelps convert stored fats into blood glucose
Ovary Estrogens and progesterone Female sexual characteristics and pregnancy
Testis Testosterone Male sexual characteristics and pubic hair
Kidney Renin Regulates blood pressure, contributes to hypovolemic thirst
Fat cells Leptin Decreases appetite
© Cengage Learning 2016
Proteins and Peptides
• Composed of chains of amino acids
• Attaches to membrane receptors where they activate second messenger systems
© Cengage Learning 2016
The Pituitary Gland and the Hypothalamus
• Attached to the hypothalamus and consists of two distinct glands – Anterior pituitary: composed of glandular tissue
• Hypothalamus secretes releasing and inhibiting hormones that control anterior pituitary
– Posterior pituitary: composed of neural tissue• Hypothalamus produces oxytocin and vasopressin,
which the posterior pituitary releases in response to neural signals
© Cengage Learning 2016
Location of the Hypothalamus and Pituitary Gland in the Human Brain
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Pituitary Hormones
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Negative Feedback in the Control of Thyroid Hormones
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Maintaining Hormonal Levels
• The hypothalamus maintains a fairly constant circulating level of hormones through a negative-feedback system– Example: TSH-releasing hormone and thyroid
hormone levels