Question

Describe fertilization and nerve conduction in terms of the properties of membranes and membrane polarization. Include in your answer: a. the development of the "resting potential" in the egg and the neuron and the events occurring during fertilization and an "action potential" b. the events that lead to slow and fast blocks to polyspermy c. a description of the differences between the “wave of depolarization” that passes over the egg and the “action potential” that propagates along a neuron

Answer 1

a)As the depolarization gets over, the cell “resets” its membrane voltage back to the resting potential. The Na+ channels close, which leads to start of the neuron’s refractory period. At the same time, voltage-gated K+ channels open, which leads K+ to leave the cell. As K+ ions leave the cell, the membrane potential once again becomes negative. The diffusion of K+ out of the cell hyperpolarizes the cell, making the membrane potential more negative than the cell’s normal resting potential. At this point, the sodium channels return to their resting state, ready to open again if the membrane potential again exceeds the threshold potential. Hence, the extra K+ ions diffuse out of the cell through the potassium leakage channels, which brings the cell from its hyperpolarized state back to its resting membrane potential.

A neuron can receive input from other neurons via a chemical called a neurotransmitter. If this input is strong enough, the neuron will pass the signal to downstream neurons. Transmission of a signal within a neuron (in one direction only, from dendrite to axon terminal) is carried out by the opening and closing of voltage-gated ion channels, which cause a brief reversal of the resting membrane potential to create an action potential. As an action potential travels down the axon, the polarity changes across the membrane. Once the signal reaches the axon terminal, it stimulates other neurons.

b)The two primary pathways known as the fast block and the slow block to polyspermy,

Fast block-After the sperms receptors come in contact with the egg's jelly layer and the acrosomal enzyme are released and that leads to the breaking of jelly layer.The sperm head comes in contact with the viteline and plasma membrane of the egg.when the two plasma membrane come in conact wih each other , signals in the eggs are related.

First, Na+ channels in the egg open, allowing Na+ to flood into the egg. This causes a depolarization of the egg from it’s normal resting potential of -70 mV.

While depolarization is occurring, the remainder of the jelly layer is dissolving. With the dissolution of the jelly layer and the depolarization of the plasma membrane, the first block to preventing fertilization by multiple sperm is put into place.

Within 1/10th of a second of contact, the fast block to polyspermy is initiated.

Slow block-

he slow block, takes place 10-60 seconds after the initial penetration of the egg’s plasma membrane by the sperm. In this pathway, the physical layout of the membranes change.

Once the sperm has made its way through the jelly layer, receptors in the vitelline layer confirm that it is indeed a sea urchin sperm. As fertilization happens entirely externally in sea urchin species, this step is to confirm that another organism’s sperm is not fertilizing the sea urchin egg. Once the sperm is confirmed to be compatible, the plasma membranes of the sperm and egg fuse.

Fusion of the two membranes causes a release of inosital triphosphate. The release of inosital triphosphate, or IP3, causes Ca2+ to be released into the egg’s cytoplasm.

Waiting in the cytoplasm are cortical vescicles, which are allowed to fuse with the egg’s plasma membrane and release their contents when the appropriate signal is sent. Ca2+ released is that necessary signal, and the vesicles fuse with the membrane. The cortical vescicles contain enzymes which cause a number of changes to occur in the egg.

1. The egg’s sperm receptors are removed

2. H2O is absorbed, causing the membrane to swell and push the vitelline membrane away from the plasma membrane.

3. The vitelline membrane hardens.

These changes ensure that fertilization of the egg by additional sperm is prevented. They are part of the slow block to polyspermy. The slow block is initiated within 10 seconds of the fusion of the plasma membranes of the two gametes.

c)During the fast block, when the channels for Na+ gets opened,Na+ flows into the egg cell, depolarizing the membrane. This depolarization wave prevents additional sperm from fusing to the egg plasma membrane.

A traveling nerve impulse is the movement of an action potential, through connected neurons,which leads to transient change in electrical potential on the membrane surface of a neuron or effector cell. This momentary change occurs when the cell is stimulated, resulting in the transmission of an electrical (nerve) impulse through some part of the nervous system.