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
