Chapter 3 - Synapses Flashcards
What is a reflex arc?
The circuit from sensory neuron to muscle neuron. Eg. In a leg reflex, a sensory neuron excites a second neuron, which in turn excites a motor neuron, which excites a muscle.
What are reflexes?
Automatic muscular responses to stimuli. This was studied by Sherrington who also named the synapse
What were 3 of Sherrington’s observations about reflexes?
- Reflexes are slower than conduction along an axon(an impulse traveling through a synapse is slower than an impulse travelling the same distance along an uninterrupted axon. Thus the synapse slows down the process)
- Several weak stimuli presented at slightly different times or locations produce a stronger reflex than a single stimuli does (temporal summation and spatial summation)
- When one set of muscles become excited, a different set becomes relaxed
Sherrington discovered temporal summation. What is it?
Repeated stimuli within a brief time have a cumulative effect. If they occur in quick succession then they will add up to a threshold before they decay. Note that this partial depolarisation is a graded potential. It can be either depolarisation (excitatory) or hyperpolarisation (inhibitory)
What is a presynaptic neuron?
The neuron that delivers the transmittion
What is a postsynaptic neuron?
The neuron that receives the transmittion
Explain excitatory postsynaptic potential (EPSP).
It is a graded depolarisation. It results from a flow of sodium ions into the neuron. If an EPSP does not cause the cell to reach its threshold, the depolarisation decays quickly.
Sherrington found that synapses have the property of spatial summation. What does this mean?
It is the summation over space. Synaptic input from separate locations combine their effects on a neuron. If a combined excitation exceeds the threshold an action potential will occur.
Why is spatial summation critical to brain functioning?
Sensory input to the brain arrives at synapses that individually produce weak effects. However, each neuron receives many incoming axons. Spatial summation ensures that a sensory stimulus stimulates neurons enough to activate them.
Sherrington found that when one muscles become excited, a different set becomes relaxed. What causes the muscles to relax?
At the synapses, input from an axon hyperpolarises the postsynaptic cell. This increase in negative charge moves it further from the threshold and decreases the probability of an action potential. This happens by allowing potassium to leave the cell or chloride to enter the cell. (IPSP)
Explain the inhibitory postsynaptic potential (IPSP).
An IPSP occurs when synaptic input selectively opens the gates for potassium ions to leave the cell or for the chloride ions to enter the cell
What is a spontaneous firing rate?
Most neurons have a spontaneous firing rate, a periodic production of action potentials even without synaptic input.
Outline the chemical event that occur at the synapse.
- The neuron synthesises chemicals that serve as neurotransmitters. It synthesises the smaller neurotransmitter in the axon terminals and synthesises 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 tminals sand into the synaptic clef.
- The released molecules diffuse across the clef, attach to receptors, and alter the activity of the postsynaptic neuron
- 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 neurotransmitters by presynaptic cells.
What are neurotransmitters?
A chemical released by one neuron that affects another neuron. There are suspected to be about 100.
List the main categories of neurotransmitters.
Amino acids, mono amines, acetylcholine, neuropeptides, purines, gases, nitric oxide
What are amino acids?
They are neurotransmitters. Acids containing a amine group. They include glutamate, GABA, glycine, asparate
What are monoamines?
Neurotransmitters. Chemicals formed by a change in certain amino acids. They include serotonin, dopamine, norepinephrine, epinephrine, insoles mines, catecholamines
What is acetylcholine?
It is a neurotransmitter. It is a modified amino acid.
What are neuropeptides?
Neurotransmitters. They are chains of amino acids. They include endorphins, substance P, neuropeptides Y
What are purines?
Neurotransmitters.a category of chemicals including adenosine and several of its derivatives. They include ATP, adenosine
What gases act as neurotransmitters?
Nitric oxide and possibly others. this gas is released by many small local neurons. In large quantities NO is poisonous. The nitric oxide relates to blood flow. It tells the blood which part of the brain is active and needs a greater supply.
Where do neurotransmitters come from?
Neurons synthesise nearly all neurotransmitters from amino acids, which the body obtains from proteins in the diet. Eg, acetylcholine is synthesised from choline found in milk, eggs, and peanuts. The amount of tryptophan in the diet controls the amount of serotonin in the brain. Serotonin levels rise after you eat food like soy and falls after maize.
What are catecholamines?
They are compounds. They contain a catecholamines group and a amine group. Compounds that are known as catecholamines are epinephrine, norepinephrine and dopamine.
How is serotonin created?
The amino acid tryptophan crosses the blood brain barrier by a special transport system that it shares with other large amino acids. To decrease the competition for use of the system and ensure that tryptophan gets across you could eat less phenylalanine, or eat more carbohydrates. Carbohydrates release insulin which takes several competing amino acids out of the bloodstream and into body cells. Once in the neuron tryptophan creates serotonin.
What are vesicles?
Most neurotransmitters are synthesised in the presynaptic terminal, near the point of release. They are stored in vesicles which are tiny nearly spherical packets. Not all neurotransmitters a kept in vesicles. And nitric oxide is released immediately (it is not stored)
What is MAO (mono amine oxidase)?
Sometimes neurons accumulate excess levels of neurotransmitters. Neurons that release serotonin, dopamine, or norepinephrine contain an enzyme, MAO that breaks down these transmitters into inactive chemicals.
How do neurotransmitters get released?
The action potential itself doesn’t release the neurotransmitters. The depolarisation opens voltage dependent calcium gates in the presynaptic terminal. After the calcium enters the presynaptic terminals, the neurotransmitters are released in bursts into the synaptic clef. It diffuses across the synaptic clef and attaches to a receptor.
What is exocytosis?
The release of the neurotransmitter in bursts from the presynaptic terminal. This is caused by the entry of the calcium into the presynaptic terminal.
How many types of neurotransmitters does each neuron have?
Many neurons release a combination of 2 or more neurotransmitters. Eg motor neurons in the spinal cord have one branch to the muscles where they release acetylcholine, and another branch to other spinal cord neurons where they release acetylcholine and glutamate. The combination makes the message more complex
What determines the effect of the neurotransmitter?
It depends on the receptor on the postsynaptic cell. The receptor may open a channel (exerting an ionotropic effect) or it may produce a slower but longer effect (a metabotropic effect)
Describe ionotropic effects.
- It has brief on/off effects
- When the neurotransmitter attaches to the receptor it opens a central channel, which is shaped to let a particular type so ion pass through (in contrast to the sodium and potassium channels which are voltage gated). these are transmitter gated or ligand gated channels
- The effects to begin quickly
What are transmitter gated or ligand gated channels?
They are channels controlled by neurotransmitters. A ligand is a chemical that binds to another chemical.
When is the ionotropic effect useful?
Well suited to conveying visual information and anything else that needs to be updated as quickly as possible.
Which neurotransmitters are used during the ionotropic effect?
Most of the Brian’s excitatory ionotropic synapses use the neurotransmitter glutamate. Most of the inhibitory ionotropic synapses use GABA (which opens chloride gates, enabling chloride ions jwith their negative charge to cross the membrane into the cells more quickly than usual) or glycine (mostly found in the spinal cord). Acetylcholine is also used, it is mostly excitatory.
