Question

1a. Describe how the permeability of the axon membrane to Na 1 and K 1 is regulated and how changes in permeability to these ions affect the membrane potential.

1b. Describe how gating of Na 1 and K 1 in the axon membrane results in the production of an action potential.

1c. Explain the all-or-none law of action potentials, and describe the effect of increased stimulus strength on action potential production. How do the refractory periods affect the frequency of action potential production?

1d. Describe how action potentials are conducted by unmyelinated nerve fibers. Why is saltatory conduction in myelinated fibers more rapid?

1e. Describe the location of neurotransmitters within an axon and explain the relationship between presynaptic axon activity and the amount of neurotransmitters released.

1f. Describe the sequence of events by which action potentials stimulate the release of neurotransmitters from presynaptic axons.

Answer 1

1 a)

Depolarization - makes the cell less polar (membrane potential gets smaller as ions quickly begin to equalize the concentration gradients) . Voltage-gated sodium channels at the part of the axon closest to the cell body activate, thanks to the recently depolarized cell body. This lets positively charged sodium ions flow into the negatively charged axon, and depolarize the surrounding axon. We can think of the channels opening like dominoes falling down - once one channel opens and lets positive ions in, it sets the stage for the channels down the axon to do the same thing. Though this stage is known as depolarization, the neuron actually swings past equilibrium and becomes positively charged as the action potential passes through

Repolarization - brings the cell back to resting potential. The inactivation gates of the sodium channels close, stopping the inward rush of positive ions. At the same time, the potassium channels open. There is much more potassium inside the cell than out, so when these channels open, more potassium exits than comes in. This means the cell loses positively charged ions, and returns back toward its resting state.

Hyperpolarization - makes the cell more negative than its typical resting membrane potential. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell temporarily hyperpolarizes, or gets even more negative than its resting state. As the potassium channels close, the sodium-potassium pump works to reestablish the resting state.

C) All or None principle :

The amplitude of an action potential is independent of the amount of current that produced it. In other words, larger currents do not create larger action potentials. Therefore, action potentials are said to be all-or-none signals, since either they occur fully or they do not occur at all.This is in contrast to receptor potentials, whose amplitudes are dependent on the intensity of a stimulus.In both cases, the frequency of action potentials is correlated with the intensity of a stimulus.

During the absolute refractory period, the neuron cannot be excited to generate a second action potential (no matter how intense the stimulus). During the relative refractory period, the neuron can be excited with stimuli stronger than that needed to bring a resting neuron to threshold

D) In unmyelinated axons, the Na⁺ and K⁺ channels taking part in action potential generation are distributed along the axon, and the action potential propagates along the length of the axon through local depolarization of each neighboring patch of membrane, causing that patch of membrane to also generate an action

Myelin greatly speeds up action potential conduction because of exactly that reason: myelin acts as an electrical insulator! Myelin sheath reduces membrane capacitance and increases membrane resistance in the inter-node intervals, thus allowing a fast, saltatory movement of action potentials from node to node.

E) Neurotransmitters are stored in synaptic vesicles, clustered close to the cell membrane at the axon terminal of the presynaptic neuron. Neurotransmitters are released into and diffuse across the synaptic cleft, where they bind to specific receptors on the membrane of the postsynaptic neuron.

Calcium is a key ion involved in the release of chemical transmitter substances. to eject a small amount of calcium in the vicinity of the presynaptic terminal.