Chaper 2 Communication at Synapses Flashcards
Properties of Synapses [placeholder]
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In the 1800s Ramon y Caljal showed that
a synapse sperates one neuron from another
Synapse
Seperates one neuron from another as well as links one neuron from another
In 1906 Sherrington inferred
inferred several properties of synapses through experiments on the reflex arc:
Delayed Transmission
speed of conduction through the reflex arc was slower than the known speed of conduction along an axon
Temporal Summation
Several impulses from one neuron over time produce a reflex
Spatial Summation
several synaptic inputs (impulses) originating from several neurons at the same time produce and (fire) a reflex
Presynaptic Neuron
the neuron that sends the message
Postsynaptic Neuron
the neuron that receives the neurotransmitter message
Postsynaptic Action Potentials [placeholder]
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In 1960 Eccles confirmed Sherrington’s findings using microelectrodes to measure postsynaptic potentials, what did he find?
- stimuli produced excitatory postsynaptic potentials (EPSPs), on membrane
- repeated stimuli generated EPSPs sufficient to cross the threshold and produce an action potential
- several stimuli at different locations also generated EPSPs sufficient to produce action potential
Postsynaptic Action Potentials [Info Dump]
Action potentials are always depolarizations.
- Graded potentials are changes from rest (-70) that don’t produce an action potential - can be either excitatory (depolarization) or inhibitory (hyperpolarization)
- These are called excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs)
- An EPSP results from Na entering the cell (just like an action potential, but smaller).
Inhibitory Senses [placeholder]
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Inhibitory Senses [Info Dump]
- Sherrington: when flexor muscles of stimulated leg contracted, the extensor muscles relaxed, inferring an inhibitory synapse
- Eccles: when flexor muscles contract, an interneuron in spinal cord generates hyperpolarizations, or inhibitory postsynaptic potentials (IPSPs), on extensor muscles of same leg
- IPSPs increase negative charge on membrane, decreasing the probability of an action potential
- It does so by opening the gates for K+ ions to leave the cell or for Cl- ions to enter the cell
Relationship Among EPSPs, IPSPs and Action Potential
- Neurons may have thousands of synapses along their membranes, some generating EPSPs others IPSPs
- A neuron has a periodic rate of firing (action potentials) even without synaptic input is a rate of the combined excitatory and inhibitory potentials
- rate of firing is determined by the amount of ESPS or ISPS
- The rate of firing is increases with EPSPs and decreases with IPSPs
- But, any behavior is a function of many, many neurons
Chemical Events at the Synapse [placeholder]
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Elliot (1905)
- found that the hormone adrenaline (a chemical) applied to surface of heart, stomach and pupils mimics the sympathetic nervous system
- suggested that synapses use chemicals to link neurons
When Otto Loewi stimulated the vagus nerve attached to frog’s heart, it decreased heart rate
- collected fluid from around heart and transferred to another frog’s heart
- second heart also slowed its rate
- concluded that nerves send messages by releasing chemicals (neurotransmitters) at synapse
The sequence of the 6 chemical events of a synapse [placeholder]
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Sequence 1
The neuron makes chemicals called neurotransmitters (NTs) and places them in little sacs called vessicles. Smaller ones are made in the terminal button and larger ones are made in the soma
Sequence 2
The ones in the soma get carried down the axon to the TB
Sequence 3
At the presynaptic TB, an action potential causes calcium to enter the cell, which causes the NT to be released from the TB into the synapse (or synaptic cleft)
Sequence 4
The released NTs attach to receptors on the postsynaptic neuron and cause things to happen on that neuron
Sequence 5
Then the NTs release from the receptors and sometimes are converted into inactive chemicals
Sequence 6
As many of the NT molecules as possible are taken back into the presynaptic TB for recycling. This is called reuptake.
Categories of Neurotransmitters [placeholder]
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Amino acids
which contain an amine group (examples = glutamate, GABA)
Peptides
long chains of amino acids, including polypeptides and proteins (ex = endorphins)
Acetylcholine
similar to amino acids but a little different structurally (Ach)
Monoamines
nonacidic neurotransmitters containing an amine group (serotonin, dopamine, norepinephrine, epinephrine) =
Purines
category of chemical including adenosine and several of its derivatives (ATP, adenosine)
Gases
nitric oxide and maybe others
Synthesis of Neurotransmitters [Info Dump]
How do neurotransmitters combine?
- Neurons synthesize neurotransmitters from substances in the diet. Ex:
—> Actycholine is synthesized from choline which is abundant in cauliflower, milk, and beef
—> Tryptophan, a precursor to serotonin which is important in the brain, increases after meal rich in protein (turkey, soy)
- Smaller neurotransmitters (like Ach) are synthesized in presynaptic terminals, close to release point, larger ones (like peptides) are made in the soma and transported down the axon.
Transport and Release of Neurotransmitters cont. [placeholder]
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Release and Diffusion
- When the AP reaches the end of an axon, the depolarization opens voltage dependent calcium channels and Ca flows into the TB
- This causes the release of neurotransmitter through membrane (exocytosis, lasting 1-2 ms)
- Diffuses across the synapse (a very tiny distance) in about 10 µs (microseconds)
- Usually two or even three transmitters are released from each neuron
- They then attach to postsynaptic receptors
Activation of Receptors
- The meaning of a NT depends on its receptor…Ach may excite one neuron, inhibit a 2nd and have no effect on another
Ionotropic effects
Definition
When the NT attaches to a receptor and immediately opens some type of ion gate
Function
- quick start (10 ms)
- Immediately opens gates for ions; e.g., acetylcholine
- Most abundant: glutamate (excitatory), GABA (inhibitory)
- Ionotropic synapses are used for quick events like visual stimulation, muscle movements, etc.
Metabotropic effects
Definition
When the NT hits the receptor and initiates a sequence of reactions that are slower and longer lasting than ionotropic effects.
Function
- late start (30 ms)
- NT attaching to a metabotropic receptor activates a G-protein inside membrane
- G-Protein is called the “second messenger” (NT is the first messenger), which communicates with the rest of the cell to do things:
- opens or closes ion channels, changes protein production, or activates chromosomes in cell
Neuromodulators
- these neurotransmitters are mainly peptides
- chemicals released in small quantities that diffuse farther away in the extracellular fluid than neurotransmitters
- Often bind with metabotropic receptors
Hormones
- chemicals released in larger quantities in blood stream
- Also travel farther
- Often bind with metabotropic receptors
Epinephrine, norepinephrine, insulin, oxytocin and some others serve both as neurotransmitters and hormones
Inactivation and Reuptake [placeholder and def]
Normal functioning neurotransmitters cause a EPSP or IPSP for limited time and then are inactivated or taken back
Inactivation
- some neurotransmitters are broken down by an enzyme
- Ex: Acetylcholine is broken down by Acetylcholinesterase (Ache)
- If not enough Ache is present, Ach can linger at the synapse and continue to excite it. Some drugs work by blocking Ache to cause this to happen
Reuptake
- some neurotransmitters detach from the receptor and are taken back up by the presynaptic cell through special membrane proteins called transporters
- Ex: serotonin and the catecholamines
- Again, if the reuptake is blocked, the effects of these NTs are prolonged. SSRIs work this way (selective serotonin reuptake inhibitors)
Synapses And Drug Effects [placeholder and def]
- Neurotransmitters act on receptors to mediate behavior such as learning, anxiety, and depression
- All drugs that affect behavior act on synaptic receptors
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 – like a lock and key
efficacy
the degree to which the drug activates the receptor once bound
Synapses and Personality [Info Dump]
Drugs, e.g., tranquilizers, antidepressants, affect people differently
—> drugs affect behavior through several kinds of receptors
—> people vary in number of each kind of receptors, e.g., dopamine D1, D2, D3, D4 and D5
- D2 receptor related to alcohol consumption, recreational drug use, overeating and habitual gambling
- Research on alternative form of D4 receptor mixed:
—> related to being impulsive, exploratory and quick-tempered
—> related to schizophrenia
—> no link to personality
