CH.8

What is the neurotransmitter used at the neuromuscular junction?

ACh is the neurotransmitter used at every neuromuscular junction.

Explain briefly how an IPSP can summate with an EPSP. What is the result of such a sumation. In other words, how does the postsynaptic response caused by summation of the EPSP and IPSP differ from the synaptic response that would have been evoked only by the excitatory input or the inhibitory input to the cell?

If one of the inputs is excitatory and the other inhibitory, the charges from the inhibitory inputs subtract the charges from the excitatory input. Since the positive charges entering the neuron are what cause the membrane potential to depolarize, the subtraction of positive charges by the negative charges would cause the membrane potential to depolarize less than it would have if the only synaptic input was from the excitatory input. Similarly, the positive charges injected into the cell from the excitatory synapse make the negative charges from the inhibitory synapse less effective. That is, if only the inhibitory inputs were activated, the result would be a hyperpolarization but if the excitatory inputs were also activated, the result would either be a much smaller hyperpolarization or even a depolarization.

What is the difference between spatial and temporal summation?

Spatial summation refers to the summation of the influx of charges from two or more inputs located on different parts of the dendrite or cell body. Temporal summation refers to the effects of sequential inputs from the same input fiber.

What is meant by the summation of synaptic events?

The addition of charges that enter a postsynaptic cell due to the release of transmitter from one or more presynaptic neurons. In other words, the positive charges that enter through ligand gated channels sum and cause a larger response than that evoked by one input alone

What factors determine whether a synapse is excitatory or inhibitory?

The critical factor is the response of the ligand-gated channel to the release of transmitter. If the channel is permeable to chloride, it allows the influx of negative charges, which causes the membrane potential to hyperpolarize and is inhibitory. If the channel is permeable to sodium and potassium, the influx of sodium and efflux of potassium drives the membrane potential to the reversal potential, which is depolarizing and typically ranges from about -17-0 mV. Usually, inhibitory presynaptic fibers release either glycine or GABA while excitatory fibers release glutamate or acetylcholine. However, there are synapses in the brain in which a particular transmitter is inhibitory at one synapse but excitatory at another because the receptors at each synapse are different. The classic example is ACh. ACh is excitatory at all neuromuscular junctions, because the receptors at the neuromuscular junction are all nicotinic ACh receptors, but ACh is inhibitory at the heart because the heart has muscarinic ACh receptors.

What is an excitatory postsynaptic potential (EPSP)?

The depolarization of the postsynaptic cell in response to the release of transmitter a the presynaptic neuron at a synaptic site. It is called excitatory because the depolarization of the membrane tends to drive the membrane potential closer or even to threshold for an action potential.

In testing the effects of a drug on synaptic transmission at a synapse, you find that the drug lowers the amplitude of the EPSP but that the drug does not alter the amplitude of the miniature endplate potentials. What can you conclude from this finding about where and how the drug is acting?

The drug must be acting on the presynaptic terminal to limit the number of vesicles that release transmitter in response to an action potential. The smaller number of quanta released (i.e., the smaller amount of transmitter released) results in a smaller EPSP. The receptors on the postsynaptic cell are unaffected by the drug, as shown by normal mepps that are recorded. In other words, the spontaneous release of a quantum of transmitter causes the same mepp when the drug is present or when it is absent.

What is an end-plate potential and how does it differ from an EPSP (or does it differ)?

The end-plate potential, or epp, is the depolarization at the endplate caused by the release of ACh from the axonal terminals of the motor neuron that innervates the muscle. It is exactly the same thing as and excitatory postsynaptic potential (EPSP), except that the depolarization is called an epp in muscle while the depolarization in postsynaptic neurons is called an EPSP.

What is an inhibitory postsynaptic potential (IPSP)?

The hyperpolization (increase in membane negativity) of the postsynaptic cell in response to the release of transmitter from a presynaptic neuron at a synaptic site. It is called inhibitory because the hyperpolariztion of the membrane tends to drive the membrane potential farther from threshold and tends to prevent the genertion of an action potential.

What exactly is the neuromuscular junction and what are its major parts?

The neuromuscular junction is the synapse that a motor neuron makes with a muscle fiber. The major parts are the synaptic boutons, the axonal endings of the motor neuron, the endplate, which is the specialized region of the muscle fiber at the junction. The enplate has numerous junctional folds, the infolding of the muscle membrane at the endplate that contains the ACh receptors. In addition, there is the basal lamina, a basement membrane located between the bouton endings and the endplate. The basal lamina contains acetylcholine esterase, the enzyme that hydrolyzes ACh into acetate and choline.

Consider two synapses located at different sites on the dendritic tree of a neuron. If both synapses cause identical potential changes at the site at which they are located, which synapse is most effective in determining the generation of action potentials of the neuron: the synapse closest to or furthest from the axon hillock? Why?

The synapse closest to the axon hillock will be the most effective synapse. The reason is that some of the positive charges that entered the cell from the synapse farther from the axon hillock will leak out of the cell through potassium channels as it travels from the synaptic site to the hillock. The closer the synapse, the less current will leak out of the cell and thus there will be more current to depolarize the hillock, the site at which action potentials are initiated.