MNSR 14 - Neurophysiology: Chemical synapses

Question 1

What is membrane potential?

Answer 1

Membrane potential he electrical difference across the plasma membrane of a cell, which is maintained by the difference in ions between the axoplasm and the surrounding fluid.

Question 2

What is the resting potential difference?

Answer 2

-70 to -80 millivolts

Question 3

Explain membrane potential in terms of concentration of ions.

Answer 3

In the axoplasm, there is a high concentration of K+ ions, and a low concentration of Na+ ions.
In the surrounding fluid, it is the opposite.

Question 4

How is membrane potential maintained?

Answer 4

There are two ways membrane potential is maintained:

1. Na+/K+ -ATPase is a sodium/potassium pump located in the axon membrane that hydrolizes 1 ATP to pump 3 Na+ ions into the surrounding fluid, and 2 K+ ions into the axoplasm.
2. Ion channels that are 50-100 more permeable to K+ ions than Na+ ions. Na+ ions can only diffuse inward with difficulty, anions cannot diffuse outward so there is a high concentration of sulphate and phosphate ions in the axoplasm, but K+ ions and cations can diffuse in and out with ease. This makes the axoplasm more electronegative, and the surrounding fluid more electropositive.

Question 5

What is action potential?

Answer 5

Action potential is a self-propagating wave of discharge that travels along the membrane of a cell, causing the membrane to become more permeable to Na+ ions and the membrane potential to change to +45 millivolts. As action potential moves across a membrane, repolarization occurs behind it, and depolarization occurs infront of it.

Question 6

What are the four phases of action potential?

Answer 6

1. Depolarization (allows Na+ ions to move into axoplasm, along its concentration gradient
2. Repolarization (causes K+ ions to move into the surrounding fluid after the influx of Na+ ions into the axoplasm)
3. Hyperpolarization (exceeds the resting potential difference for a few seconds, closing the K+ channels, and preventing them from leaving, while the Na+/K+ ATPase recharges the membrane)
4. Refractory Period (the time in which action potential cannot occur. There is the Absolute Refractory Period and the Relative Refractory Period)

Question 7

What is the threshold?

Answer 7

• 50 millivolts

- insufficient stimulation that does not reach the threshold will not trigger action potential

Question 8

Describe action potential speeds for myelinated and non-myelinated fibers.

Answer 8

In general, the speed of the impulse depends on the diameter of the fiber.

1. Non-myelinated fibers have velocities of 0.5m/sec
2. Myelinated fibers have velocities of 130m/sec, however since the nodes of Ranvier insulate the axons, action potential jumps from node to node, called SALTATORY CONDUCTION. The farther apart the nodes, the faster the conduction.

Question 9

What is the synapse?

Answer 9

Synapse is a region of contact where the PRESYNAPTIC NEURON comes close to the POSTSYNAPTIC NEURON to continue transmission. There are two types of synapse:

1. Electrical synapse
2. Chemical synapse

Question 10

Explain chemical synapse.

Answer 10

1. The presynaptic neuron breaks up into many synaptic knobs or boutons, which consist of the presynaptic terminal, synaptic cleft, and postsynaptic process.
2. Once the action potential reaches the presynaptic terminal, Ca2+ ions flow through the Voltage-Gated Ca2+ channels into the membrane due to the difference in the concentration gradient.
3. The Ca2+ ions cause the synaptic vesicles to fuse with the presynaptic terminal membrane and release chemicals into the synaptic cleft through a process called excitation-secretion coupling.
4. The chemicals diffuse to the postsynaptic membrane and bind to the Transmitter Gated Ion channel receptors, converting the chemical signals into electrical signals.
5. The electrical signals initiate the start of depolarization in the postsynaptic neuron.