Chapter 3- Synapses Flashcards
Automatic muscular responses to stimuli
Reflexes
A circuit from sensory neuron to muscle response
Reflex arc
For example: in a leg flexion reflex, a sensory neuron excites a second neuron, which in turn excites a motor neuron, which excites a muscle.
A cumulative effect of repeated stimuli within a brief time
Temporal summation
Sherrington found that a light pinch of the dogs foot did not evoke a reflex, but a few rapidly repeated pinches did. Surmised that a single pinch did not reach the threshold of excitation for the next neuron.
Neuron that delivers transmission to another neuron
Presynaptic neuron
Neuron that receives transmission from another neuron
Postsynaptic neuron
Graded depolarization
Excitatory postsynaptic potential EPSP
Unlike action potentials, which are always depolarizations, graded potentials maybe either depolarizations (excitatory) or hyperpolarizations (inhibitory). A graded depolarization is known as an excitatory postsynaptic 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
Eccles I found that when he stimulated an axon twice, if the delay between EPSPs was short enough, the second EPSP added to what was left of the first one, producing temporal summation.
Combination of effects of activity from two or more synapses onto a single neuron
Spatial summation
Sherrington began with a pinch too weak to elicit a reflex. He then pinched two points at once. Although neither pension loan produced a reflex, together they did. Concluded that pinching two points activated separate sensory neurons, whose axons converged onto a neuron in the spinal cord. Excitation from either sensory axon excited that spinal neuron on their own, but not enough to reach the threshold. A combination of excitations exceeded the threshold and produced an action potential.
Temporary hyperpolarization of a membrane
Inhibitory postsynaptic potential IPSP
When Sherrington vigourously pinched a dogs foot, the flexor muscles of that leg contracted, and so did the extensor muscles of the other three legs. Also, the dog relaxed the extensor muscles of the stimulated leg and the flexor muscles of the other lakes. Sherrington’s explanation assumed certain connections in the spinal cord: a pinch on the foot sends a message along a sensory neuron to an interneuron in the spinal cord, which in turn excites the motor neurons connected to the flexor muscles of that leg. Surmised that the enter neuron also sends a message to block activity of motor neurons to the extensor muscles in the same leg and the flexor muscles of the three other legs.
At the synapse ease, input from an axon hyperpolarizes the postsynaptic cell. That is, it increases the negative charge within the cell, moving it further from the threshold and decreasing the probability of an action potential. This is called an inhibitory postsynaptic potential, or 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
A periodic production of action potentials even without synaptic input
Spontaneous firing rate
When neurons have a spontaneous firing rate, the EPSPs increase the frequency of action potential’s above the spontaneous rate, whereas IPSPs decrease it
A specialized gap as a point of communication between two neurons
Synapse
How did Sherrington measure or study reflexes?
Sherrington strapped a dog into a harness above the ground and pinched one of the dogs feet. After a fraction of a second, the dog flexed or raised the pinched leg and extended the other legs. Sherrington found the same reflexive movements after he made a cut that disconnected the spinal cord from the brain
What are three properties Sherrington observed of reflexes suggesting special processes at the junctions between neurons?
- Reflexes are slower than conduction along an axon
- Several weak stimuli presented at slightly different times or locations produce a stronger reflex then a single stimulus does
- When one set of muscles becomes excited, a different set becomes relaxed
What did Sherrington find when he measured the total distance that the impulse travels from skin receptor to spinal cord to muscle and calculated the speed at which the impulse must travel to produce the response?
He found that the speed of conjunction through the reflex arc buried but was never more than about 15 m/s. In contrast, previous research had measured action potential velocities along sensory or motor nerves at about 40 m/s. Concluded that some process was slowing conduction through the reflex, and he inferred that the delay must occur where one neuron communicates with another. This established the existence of synapses.
What is the difference between temporal summation and spatial summation?
Temporal summation is the combination effects of quickly repeated stimulation at a single synapse. Spatial summation is the combined effects of several nearly simultaneous stimulations at several synapses onto one neuron
What evidence lead Sherrington to conclude that transmission at a synapse is different from transmission along an axon?
Sherrington found that the velocity of conduction through a reflex arc was significantly slower than the velocity of an action potential along an axon. Therefore, some delay must occur at the junction between one neuron and the next.
What was Sherrington as evidence for inhibition in the nervous system?
Sherrington found that a reflex that stimulates a flexor muscle prevents contraction of the extensor muscles of the same limb. He therefore inferred that an axon sending an excitatory message for the flexor muscle also sent an inhibitory message for the extensor muscle
What ion gates in the membrane open during an EPSP? What the gates open during an IPSP?
During an EPSP, sodium gates open. During an IPSP, potassium or chloride gates open
Can an inhibitory message flow along an axon?
No. Only action potential’s propagate along an axon. Inhibitory messages – IPSPs – decay over time and distance
Describe the contributions of TR Elliot and O Loewi to the question of whether most synaptic transmission is electrically or chemically mediated
Sherrington originally inferred that synaptic transmission must be electrical.
In 1905, TR Elliott reported that applying the hormone adrenaline directly to the surface of the heart, the stomach, and the pupils produces the same affects as those of the sympathetic nervous system. He therefore suggested that the sympathetic nerves stimulate muscles by releasing adrenaline or a similar chemical. Sherrington’s prestige, however, was so great that his findings continued to be ignored.
O Loewi came up with an idea in the middle of the night and performed an experiment. He repeatedly stimulated a frogs Vegus nerve, thereby decreasing the heart rate. He then collected fluid from that heart, transferred it to a second frogs heart, and found that the second heart also decreased it’s rate of beating. He then stimulated the Excelerator nerve to the first frogs heart, increasing the heart rate. When he collected fluid from that hard and transferred it to the second from his heart, it’s heart rate increased. Therefore, stimulating one nerve released something that inhibited heart rate, and stimulating a different nerve release to something that increased heart rate. He then concluded that nerves send messages by releasing chemicals
What was Loewi’s evidence that neurotransmission depends on the release of chemicals?
When he stimulated a nerve that increased or decreased a frogs heart rate, he could withdraw some fluid from the area around the heart, transfer it to another frogs heart, and thereby increase or decrease its rate also.
What are six major events at a synapse?
- The neuron synthesizes chemicals that serve as neurotransmitters. It synthesizes the smaller neurotransmitters in the axon terminals and neuropeptides in the cell body.
- Action potentials travel down the axon. At the presynaptic 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 cleft, attach to receptors, and alter the activity of the postsynaptic neuron
- The neurotransmitter molecules separate from their receptors
- The neurotransmitter molecules maybe taking back into the presynaptic neuron for recycling or they may diffuse away
- Some postsynaptic cells send reverse messages to control the for the release of neurotransmitter by presynaptic cells
List the six major types of neurotransmitters
Amino acid’s, monoamine’s, acetylcholine, neuropeptides, purines, gases
Chemicals released by neurons that affect other neurons
Neurotransmitters
Acids containing an amine group NH2
Amino acids
A chemical similar to an amino acid, except that it includes an N(CH3)3 group instead of an NH2 group
Acetylcholine
Chemicals formed by a change in certain amino acids
Monoamines
Chains of amino acids
Neuropeptides
A category of chemicals including adenosine and several of its derivatives
Purines
One of the categories of neurotransmitters, including nitric oxide and possibly others
Gases
A gas released by many small local neurons (NO)
Nitric oxide
What does a highly active brain area due to increase its blood supply?
In a highly active brain area, many stimulated neurons release nitric oxide, which dilates the blood vessels in the area and thereby increases blood flow to the area
Describe the role of diet in the synthesis of neurotransmitters
Neurons synthesize nearly all transmitters from amino acids, which the body obtains from proteins in the diet.
Acetylcholine is synthesized from choline, which is abundant in milk, eggs, and peanuts.
The amino acids phenylalanine and tyro scene, present in proteins, are precursors of dopamine, norepinephrine, and epinephrin
The amino acid tryptophan, the precursor to serotonin, crosses the blood-brain barrier buy a special transport system that shares with other large amino acids. The amount of tryptophan in the diet controls the amount of serotonin in the brain, so your serotonin levels rise after you eat foods rich are in tryptophan, such as soy, and fall after something low and tryptophan such as maize or American corn. However, tryptophan has to compete with other, more abundant large amino acids, such as phenylalanine, that share the same transport system. One way to increase tryptophan entry to the brain is to decrease consumption of phenylalanine. Another is to eat carbohydrates which increase the release of the hormone insulin, which takes several competing amino acids out of the bloodstream and into the body, thus decreasing the competition against tryptophan.
Compounds that contain a catechol and an amine group
Catecholamines
Epinephrine, norepinephrine, and dopamine are compounds known as catecholamines because they contain a catechol group and an amine group
Name the three catecholamine neurotransmitters
Epinephrine, norepinephrine, and dopamine
Tiny nearly spherical packets filled with neurotransmitter molecules
Vesicles
Most neurotransmitters are synthesized in the presynaptic terminal, near the point of release. The presynaptic terminal stores high concentrations of neurotransmitter molecules in vesicles. Nitric oxide is an exception to this rule.
Enzyme that converts catecholamines and serotonin into synaptically inactive forms
Monoamine oxidase MAO
Describe the processes of transport, release, and diffusion of neurotransmitters
At the end of an axon, the action potential itself does not release the neurotransmitter. Rather, the Depol arise Asian opens voltage-independent calcium gates in the presynaptic terminal. After calcium enters the presynaptic terminal, it causes exocytosis – release of neurotransmitter in bursts from the presynaptic neuron into the synaptic cleft of that separates one neuron from another.
After its release from the presynaptic cell, the neurotransmitter diffuses across the synaptic left to the postsynaptic membrane, where it attaches to a receptor.
Although the brain as a whole uses many neurotransmitters, no single neuron releases them all. Although a neuron releases only a limited number of neurotransmitters, it may receive and respond to many neurotransmitters at different synapses.
When the action potential reaches the presynaptic terminal, which ion must enter the presynaptic terminal to evoke release of the neurotransmitter?
Calcium
Synaptic effects that depend on the rapid opening of some kind of gate in the membrane.
Ionotropic effects
Begin quickly, sometimes within less than a millisecond after the transmitter attaches. The effects decay with a half-life of about 5 ms. They are well-suited to conveying visual information, auditory information, and anything else that needs to be updated as quickly as possible.
Most of the brains excitatory ionotropic synapses use the neurotransmitter glutamate. Glutamate is the most abundant neurotransmitter in the nervous system. Most of the inhibitory ionotropic synapses use the neurotransmitter GABA or gamma-amino-butyric acid which opens chloride gates, enabling chloride ion’s, with their negative charge, to cross the membrane into the cell more rapidly than usual. Glycine is another common inhibitory transmitter, found mostly in the spinal cord. Acetylcholine is excitatory in most cases.
Ion channel that opens temporarily when a neurotransmitter binds to it
Transmitter-gated channel or ligand-gated channels
A ligand is a chemical that binds to another chemical
A sequence of metabolic reactions that produce slow and long-lasting effects at a synapse
Matabotropic effects
Emerge 30 ms or more after the release of the neurotransmitter. Typically lasts up to a few seconds, but sometimes longer.
Use many neurotransmitters, including dopamine, norepinephrine, and serotonin. Sometimes glutamate and GABA too.
Better suited for more enduring effects such as taste, smell, and pain where the exact timing isn’t important anyway. Also important for many aspects of arousal, attention, pleasure, and emotion.
A protein coupled to guanosine triphosphate GDP, an energy-storing molecule
G protein
When a neurotransmitter attaches to a Matab tropic receptor, 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. Bending the receptor protein detaches that G protein, which is then free to take its energy elsewhere in the cell. The result of the G protein is increased concentration of a second messenger, such as cyclic adenosine monophosphate inside the cell. Just as the first messenger, the neurotransmitter, carries information to the postsynaptic cell, the second messenger communicates too many areas within the cell. It may open or close ion channels in the membrane or activate a portion of a chromosome.
A chemical that, when activated by a neurotransmitter, initiates communication to many areas within the neuron
Second messenger
Researchers often refer to neuropeptides as ________, because they have several properties that set them apart from other transmitters.
What are five things that make neuropeptides different from other neurotransmitters?
Neuromodulators
- Whereas the neuron synthesizes most other neurotransmitters in the presynaptic terminal, it’s synthesizes neuropeptides in the cell body and then slowly transports them to other parts of the cell.
- Whereas other neurotransmitters are released at the axon terminal, neuropeptides are released mainly by dendrites, and also by the cell body and the sides of the axon.
- Whereas a single action potential can release other neurotransmitters, neuropeptide release requires repeated stimulation. However, after a few dendrites release a neuropeptide, the released chemical primes other nearby dendrites to release the same neuropeptide also, including dendrites of other cells.
- Unlike other transmitters that are released immediately adjacent to their receptors, neuropeptides diffuse widely, affecting many neurons in the region of the brain. In that way they resemble hormones.
- Whereas the duration of effects other neurotransmitters is generally less than a second to a few seconds, neuropeptides effects are long-lasting, in the range of 20 minutes or more.
Neuropeptides are important for hunger, thirst, intense pain, and other long-term changes in behaviour and experience.
A kind of neuron that releases huge amounts of GABA all at once, producing widespread inhibition
Neurogliaform cell
An exception to the rule that a neuron delivers neuropeptides that diffuse to receptors throughout a wide area, but it delivers other transmitters only in small amounts directly adjacent to their receptors. The neuroglia form cell releases huge amounts of GABA all at once, forming a “cloud” that spreads to a large number of neurons in the area, producing widespread inhibition
How do ionotropic and Matabotropic synapses differ in speed and duration of effects?
Ionotropic synapses act more quickly and more briefly
What are second messengers, and which type of synapse relies on them?
At metabotropic synapses, the neurotransmitter attaches to its receptor and thereby releases a chemical, the second messenger, within the postsynaptic cell, which alters metabolism or gene expression of the postsynaptic cell
