Synapse Flashcards
Charles Scott Sherrington
physiologically demonstrated that communication between one neuron and the next differs from communication along a single axon
tested reflex of dog and found flexion and extension reflexes
based on timing of reflex he figured out that the reflexes are slower than an action potential on the length of the axon
he inferred that there were gaps between axons that were slowing things down
synapse
a specialized gap between neurons
flexion reflex
sensory neuron excites a second neuron, which in turn excites a motor neuron, which excites a muscle
reflex arc
The circuit from sensory neuron to muscle response
temporal summation
repeated stimuli within a brief time have a cumulative effect
although the subthreshold excitation in the postsynaptic neuron decays over time, it can combine with a second excitation that follows it quickly
presynaptic neuron
neuron that delivers transmission
postsynaptic neuron
the one that receives the transmission
excitatory postsynaptic potential (EPSP)
graded depolarization
partial depolarization is a graded potential
results from a flow of sodium ions into the neuron
if an EPSP does not cause the cell to reach its threshold, the depolarization decays quickly.
spatial summation
summation over space
Synaptic inputs from separate locations combine their effects on a neuron.
Inhibitory Synapses
A pinch on the foot sends a message along a sensory neuron to an interneuron (an intermediate neuron) that excites the motor neurons connected to the flexor muscles of that leg and the extensor muscles of the other legs
the interneuron sends messages to inhibit the extensor muscles in that leg and the flexor muscles of the three other legs
input from an axon hyperpolarizes the postsynaptic cell.
it increases the negative charge within the cell, moving it farther from the threshold and decreasing the probability of an action potential
inhibitory postsynaptic potential (IPSP)
occurs when synaptic input selectively opens the gates for potassium ions to leave the cell (carrying a positive charge with them) or for chloride ions to enter the cell (carrying a negative charge)
spontaneous firing rate
a periodic production of action potentials even without synaptic input
EPSPs increase the frequency of action potentials above the spontaneous rate, whereas IPSPs decrease it
discovery of chemical transmission at synapse
Otto Loewi
repeatedly stimulated the vagus nerve, thereby decreasing a frog’s heart rate
collected fluid from around that heart, transferred it to a second frog’s heart, and found that the second heart also decreased its rate of beating
did the same to increase the heart rate and did the same
Sequence of Chemical Events
at a Synapse
- The neuron synthesizes chemicals that serve as neu- rotransmitters. It synthesizes the smaller neurotransmit- ters in the axon terminals and synthesizes neuropeptides in the cell body.
- Action potentials travel down the axon. At the presynap- tic terminal, an action potential enables calcium to enter the cell. Calcium releases neurotransmitters from the terminals and into the synaptic cleft, the space between the presynaptic and postsynaptic neurons.
- The released molecules diffuse across the narrow cleft, attach to receptors, and alter the activity of the postsynap- tic neuron. Mechanisms vary for altering that activity.
- The neurotransmitter molecules separate from their receptors.
- The neurotransmitter molecules may be taken back into the presynaptic neuron for recycling or they may diffuse away.
- Some postsynaptic cells send reverse messages to control the further release of neurotransmitter by presynaptic cells.
neurotransmitters
chemicals that affect another neuron
hundred or so kinds known or suspected
includes amino acids, modified amino acids, monoamines, neuropeptides, purines, gases
nitric oxide
gas released by many small local neurons
neurons release nitric oxide when they are stimulated
influencing other neurons, nitric oxide dilates the nearby blood vessels, thereby increasing blood flow to that brain area
tryptophan
precursor to serotonin
serotonin levels rise after you eat foods richer in tryptophan, such as soy, and fall after something low in tryptophan, such as maize
Serotonin
Involved in many functions, including mood, appetite, sleep, and aggression.
Low levels of serotonin are associated with depression, and some drugs designed to treat depression (known as selective serotonin reuptake inhibitors, or SSRIs) serve to prevent their reuptake.
likely matabotropic
Glutamate
The most common neurotransmitter, it’s released in more than 90% of the brain’s synapses. Glutamate is found in the food additive MSG
Excess glutamate can cause overstimulation, migraines and seizures.
Usually ionotropic
GABA (gamma-aminobutyric acid)
The major inhibitory neurotransmitter in the brain.
Alcohol stimulates the release of GABA, which inhibits the nervous system and makes us feel drunk. Low levels of GABA can produce anxiety, and GABA agonists (tranquilizers) are used to reduce anxiety
mostly ionotropic
Endorphins
Released in response to behaviors such as vigorous exercise, orgasm, and eating spicy foods.
Dopamine
Involved in movement, motivation, and emotion, Dopamine produces feelings of pleasure when released by the brain’s reward system, and it’s also involved in learning
likely metabotropic
Acetylcholine
A common excitatory neurotransmitter used in the spinal cord and motor neurons to stimulate muscle contractions. It’s also used in the brain to regulate memory, sleeping, and dreaming.
acetylcholine attaches as in Figure 2.15b, the receptor folds outward, widen- ing the sodium channel
synthesis of neurotrnsmitters
Most neurotransmitters are synthesized in the presynaptic terminal, near the point of release
vesicles
presynaptic terminal stores high concentrations of neurotransmitter molecules in vesicles, tiny nearly spherical packets
MAO (monoamine oxidase)
Neurons that release serotonin, dopamine, or norepinephrine contain an enzyme, MAO (monoamine oxidase), that breaks down these transmitters into inactive chemicals, thereby preventing the transmitters to accumulate to harmful levels.
exocytosis
bursts of release of neurotransmitter from the presynaptic neuron
depolarization opens voltage- dependent calcium gates in the presynaptic terminal, 1 or 2 milliseconds (ms) after calcium enters the terminal, it causes exocytosis
spreads across the cleft to the postsynaptic receptor
neurons release a combination of two or more transmitters at a time
ionotropic effects
on-off effect of neurotransmitters on a receptor site; brief on off effect;
like a paper bag twisted opening and closing to allow a particular type of ion through
transmitter-gated or ligand-gated
(A ligand is a chemical that binds to something.)
compared to voltage gated
when the neurotransmitter attaches, it opens a channel. Ionotropic effects begin quickly, sometimes within less than a millisecond after the transmitter attaches
effect decays very quickly
Glycine
another common inhibitory transmitter
inhibitory ionotropic neurotransmitters
opens chloride gates, enabling chloride ions, with their negative charge, to cross the membrane into the cell more rapidly than usual
Metabotropic Effects and Second
Messenger Systems
initiating a sequence of metabolic reactions that start slowly but last longer than ionotropic effects
effects emerge 30 ms or more after the release of the transmitter
it bends the receptor protein that goes through the membrane of the cell. The other side of that receptor is attached to a G protein—that is, a protein coupled to guanosine triphosphate (GTP), an energy- storing molecule
Bending the receptor protein detaches that G protein, which is then free to take its energy elsewhere in the cell
the result of that G protein is increased concentration of a second messenger, such as cyclic adenosine monophosphate (cyclic AMP), inside the cell. Just as the “first messenger” (the neurotransmitter) carries information to the postsynaptic cell, the second messenger communicates to areas within the cell
by way of its second messenger, influences activity in much or all of the cell and over a longer time.
Ionotropic vs metabotropic synapses
vision and hearing, the brain needs rapid, up-to-date information, the kind that ionotropic synapses bring
metabotropic synapses are better suited for more enduring effects such as taste; also important for many aspects of arousal, attention, pleasure, and emotion
neuropeptides
neuromodulators, because they have properties that set them apart from other transmitters
synthesized in the cell body and then slowly transports them to other parts of the cell
neuropeptides are released mainly by dendrites, and also by the cell body and by the sides of the axon
neuropeptide release requires repeated stimulation
they resemble hormones diffuse widely, slowly affecting many neurons in their region of the brain
important for hunger, thirst, and other long-term changes in behavior and experience
acetylcholine inactivation
broken down by a specific enzyme into acetate and choline
choline diffuses back to the presynaptic neuron and reconnects it with acetate already in the cell
catecholamines
serotonin, dopamine, norepinephrine, and epinephrine
reuptake
presynaptic neuron takes up much or most of the released neurotransmitter molecules intact and reuses them
occurs through special membrane proteins called transporters
one not taken up are broken down by another enzyme and excreted through blood and urine
amphetamine and cocaine
inhibit the transporters for dopamine, serotonin, and norepinephrine, thus decreasing reuptake and prolonging the effects of the neurotransmitters
autoreceptors
many presynaptic terminals have receptors sensitive to the same transmitter they release
receptors that respond to the released transmitter by inhibiting further synthesis and release
Other Forms of Negative Synaptic Feedback
some postsynaptic neurons respond to stimu- lation by releasing chemicals that travel back to the pre- synaptic terminal to inhibit further release of transmitter
nitric oxide
hydrogen ions
Cannabinoids
the chemicals in marijuana decrease both excitatory and inhibitory messages from neurons that release glutamate, GABA, and other transmitters. In various cases the result can be either brief or more long-lasting suppression of release
resulting in decreased anxiety
Electrical Synapses
faster than even the fastest chemical trans- mission, electrical synapses have evolved in cases where ex- act synchrony between two cells is important
eg, rhythmic breathing
gap junction
the membrane of one neuron comes into direct contact with the membrane of another, in electrical synapses
large pores of the membrane of one neuron line up precisely with similar pores in the membrane of the other cell where sodium and other ions pass readily
act is if a single neuron
Hormones
a chemical secreted by cells in one part of the body and conveyed by the blood to influence other cells
function like a radio instead of a telephone
useful for coordinating long-lasting changes in multiple parts of the body
Two types of hormones
protein hormones and peptide hormones
proteins are longer and peptides are shorter
attach to membrane receptors, where they activate a second messenger within the cell—exactly like a metabotropic synapse
pituitary gland
responsible for producing hormones
consists of anterior pituitary and the posterior pituitary
posterior pituitary
composed of neural tissue, can be considered an extension of the hypothalamus. Neurons in the hypothalamus synthesize the hormones oxytocin and vasopressin
migrate down axons to the posterior pituitary, where they are released into the blood
anterior pituitary
composed of glandular tissue, synthesizes six hormones, although the hypothalamus controls their release
hypothalamus secretes releasing hormones, which flow through the blood to the anterior pituitary. There they stimulate or inhibit the release of other hormones.
hypothalamus
maintains fairly constant circulating levels of certain hormones through a negative feedback system
eg,
when the level of thyroid hormone is low, the hypothalamus releases TSH-releasing hormone, which stimulates the anterior pituitary to release TSH, which in turn causes the thyroid gland to secrete more thyroid hormones
COMT
(catechol-o-methyltransferase) any transmitter molecules that the transporters do not take will instead break down by an enzyme called COMT
amphetamine and cocaine,
Stimulant drugs, including amphetamine and cocaine, inhibit the transporters for dopamine, serotonin, and norepinephrine, thus decreasing reuptake and prolonging the effects of the neurotransmitters
LSD/Hallucinogenic drugs
attach to serotonin type 2A (5-HT2A) receptors and provide stimulation at inappropriate times or for longer-than-usual durations. LSD increases the connections among brain areas that ordinarily do not communicate much with one another.
Nicotine
stimulates a family of acetylcholine receptors, conveniently known as nicotinic receptors. Because nicotinic receptors are abundant on neurons that release dopamine, nicotine increases dopa- mine release
Opiate drugs
opiates relieve pain by act- ing on receptors in the brain as well as in the skin.
opiates attach to specific receptors in the brain
bind to same receptors as endorphons?
Cannabinoids
bind to anandamide or 2-AG receptors on presynaptic neurons, indi- cating, “The cell got your message. Stop sending it.
anandamide and 2-AG
like nitric oxide, other reverse transmitters
respond to stimu- lation by releasing chemicals that travel back to the pre- synaptic terminal to inhibit further release of transmitter
