What happens at the voltage-gated potassium channels?
- Depolarization of the membrane occurs (membrane potential becomes more positive/less negative)
- After a brief pause, K+ voltage-gated channels open - yet, it is still the same depolarization that opens the sodium channel which also causes a conformational change in the K+ channels (which are delayed)
- K+ flows out of the cell, down the electrical and chemical gradient
- Gate closes and channel returns to its resting configuration
- The channel is now ready to open again
K+ channels only contain one gate on the intracellular side of the channel and do not immediately open like the Na+ voltage-gated channels. In fact, they begin opening when the Na+ voltage-gated channels start to become inactivated. Unlike Na+ channels, the K+ channels do not have an inactivation period.
What is the absolute refractory period?
While the inactivation gate is closed on the Na+ channel, the channel will not open regardless of the strength of stimulation.
How does the sodium voltage-gated channel work?
- Depolarization of the membrane occurs (membrane potential becomes more positive/less negative)
- Activation gate opens immediately
- Na+ flows into the cell, down the concentration gradient
- Inactivation gate closes and Na+ can no longer flow into the cell
- Channel returns to the resting configuration inactivation gate open and activation gate closed) The channel is now ready to open again when depolarized sufficiently.
Both the activation and inactivation gate are on the intracellular side of the channel.
How is an action potential propagated down a myelinated nerve?
Myelinated axons are insulated with a fatty material called myelin, which is produced by special cells; Schwann cells in the PNS and oligodendrocytes in the CNS. The effect of this myelin is to insulate the axon so few ions can leak out through the membrane. Voltage gated channels only exist at the gapes between the myelin (the nodes of Ranvier)
- The positive charge from the existing action potential is attracted to and moves toward the adjacent node of Ranvier that is negative
- This node of Ranvier now deporalizes
- This depolarization triggers voltage-gated Na+ channels to open
- Na+ rushes into the cell and depolarizes the region to threshold, and a new action potential is generated
- By a repetition of this procedure, the action potential is propagated by saltatory conduction
What is the significance of the absolute refractory period in action potential propagation?
Since the sodium channels get inactivated and will not open regardless of the strength of depolarization once the inactivation gate is closed, this ensures that the action potential is unidirectional. By the time the sodium voltage-gated channels are ready to open again, the action potential has travelled to far down the axon to be able to affect them.
Because of the absolute refractory period of the Na+ channels, two action potentials cannot be fired one on top of the other. Therefore, action potentials will almost always have a fixed height or amplitude.
How is an action potential propagated down an unmyelinated nerve?
- Where an action potential exists on the axon, the inside of the membrane is positive (+35 mV)
- This positive charge is attracted to and moves towards an area of the membrane next to it that is at resting membrane potential (has a negative charge)
- This creates a local current of + to -
Because of this buildup of the positive charge, the adjacent area of the membrane now depolarizes - This depolarization triggers voltage-gated Na+ channels to open
- Na+ rushes into the cell and depolarizes the region to threshold, creating a new action potential
- By a repetition of this procedure, the action potential is propagated along the membrane
Why is the sodium potassium pump not required for repolarization?
The membrane potential is brought back to resting levels by the continued increased conductance of potassium when sodium permeability has returned to normal.
Why is there no appreciable change in the concentration gradients for the various ions after one action potential?
Very few ions move through the membrane during the action potential. Thousands of action potentials can be generated before the concentration gradients for sodium and potassium break down enough to prevent the generation of further action potentials.
Why does the action potential curve have a rounded peak?
The rounded peak of the action potential is a result of the Na+ voltage-gated channels beginning to close, while the K+ voltage-gated channels are simultaneously beginning to open. Consequently, a small number of Na+ ions are still entering the cell as the K+ begin to leave.
What is threshold potential?
Action potentials do not always occur because they require a strong depolarization at the axon hillock to open many Na+ channels.
If a small number of Na+ ions enter the cells, this will cause a small depolarization, but the cell will attempt to maintain its resting membrane potential at -70 mV. The buildup of positive charge will affect other ions inside and outside the cell; K+ will move out and Cl- will move in to repolarize the membrane potential back to normal.
In order to fire the action potential, the depolarizing force from the Na+ moving in must exceed the natural repolarizing forces from K+ moving out and Cl- coming in. This occurs when the membrane potential depolarizes to -55 mV. Once this value is reached, you will always have an action potential.
What is the relative refractory period?
This is the period during the action potential when the membrane is hyperpolarized to -90 mV because of the equilibrium potential of K+. This period is caused by the K+ channels, which are not only slow to open but are also slow to close. This allows K+ to continue to leave the cell even after it has repolarized to -70 mV. During this period of time, it is possible to fire another action potential since the inactivation gate on the sodium channels are open, but it would require a stronger stimulus to reach threshold.
What is the axon hillock?
The action potential begins at the spot on the axon called the axon hillock or initial segment. The axon hillock is the most electrically sensitive area of the nerve since it has the highest concentration of voltage-gated sodium channels.
What is an action potential?
An action potential is a rapid reversal of the resting membrane potential; it changes from -70 mV to +35 mV (depolarization) and then rapidly returns to -70 mV (repolarization). The membrane potential briefly becomes more negative, reaching -90 mV (hyperpolarization).
The following is a summary of the events:
- Strong depolarization at the axon hillock triggers the opening of most Na+ voltage-gated channels.=
- Na+ rushes into the neuron, down its electrochemical gradient
- Membrane depolarizes rapidly to roughly +35 mV
- Na+ Channels become inactivated while K+ channels begin opening
- K+ rushes out of the cell, down its electrochemical gradient
- Membrane begins repolarizing back to normal (+35 mV to -70 mV)
- K+ continues to rush out of the cell and the membrane hyperpolarizes (-90 mV)
- K+ channels begin to close and K+ no longer leaves the cell
- Membrane potential slowly returns to -70 mV by the passive movement of ions through leak channels
What is the structure of a multipolar neuron?
- Dendrites are thin branching processes of the cell body, whose function is to receive incoming signals. Dendrites increase the overall surface area of the neuron so that it can communicate with other neurons. The number of dendrites will vary depending on where the nerve cell is located.
- The cell body (soma) is the control center, containing the nucleus and organelles.
- An axon is a projection of the cell body that carries the outgoing signal to the target cell in the form of an action potential. The axon may or may not be myelinated.
- The myelin sheath is a layer of phospholipid membrane wrapped tightly around the axon, acting as an insulator. It forces the ionic changes that comprise the action potential to only take place at small regions of the axon called the nodes of Ranvier. The jumping of action potentials in saltatory conduction results in a faster speed of transmission.
- Collaterals are branches of the axon near the terminal end that increase the number of target cells a neuron can interact with.
- Terminal boutons are swellings at the end of the collaterals that contain mitochondria and vesicles with neurocrine molecules. The chemicals in the axon terminal facilitate the transmission of the signal across the synapse to the target cell.
Why are nerve and muscle cells considered excitable?
They can use the resting membrane potential to general an electrochemical impulse called an action potential.
What is multiple sclerosis?
MS is a disease in which the body’s natural immune system attacks and damages the myelin surrounding the axon of nerves. This damage can interrupt the natural flow of action potentials along the axon to the point where no transmission occurs. If the nerve that is damages is connected to a muscle, that muscle will not contract and the person can suffer paralysis.
What is synaptic transmission?
The contact between the neuron and another nerve cell, muscle cell, or an organ cells occurs at a chemical synapse. For example, at the neuromuscular junction (NMJ), an action potential from a nerve cell triggers an action potential on the muscle cell that eventually leads to contraction of that muscle.
What is the structure of the neuromuscular junction?
The neuron that contacts a muscle cell is called a motor nerve fiber. The membrane of the presynaptic axon terminal contains Ca++ voltage-gated channels that open when the cell depolarizes. The axon terminal of the motor cell/fiber also contains synaptic vesicles that contain the neurotransmitter acetylcholine. The basement membrane of the axon terminal contains the enzyme acetylcholinesterase. The muscle cell membrane (sarcolemma) is directly under the axon terminal and is thrown into folds called the end plate. The end plate contains receptors for acetylcholine, which are associated with ligand-gated ion channels. The gap between the motor fiber and the muscle cell is called the synaptic cleft.
What are the events at the neuromuscular junction?
- The action potential on the presynaptic motor nerve fiber triggers Ca++ voltage gated channels to open. Ca++ flows into the cell, down the concentration gradient
- Ca++ triggers the fusing of synaptic vesicles to the membrane and the release of ACh into the synaptic cleft by exocytosis
- ACh diffuses across the synaptic cleft and attaches to receptors on the muscle cell/fiber membrane
- Nonspecific Ligand-gated ion channels open. Lots of Na+ flows into the cell and a few K+ leave. This triggers a local depolarization called an end plate potential (EPP). There is still no action potential yet
- The depolarization of the EPP spreads to the adjacent cell membrane where voltage-gated channels are located. These channels, which are essential for the action potential, open. Large amounts of Na+ flows into the muscle cell and triggers the action potential
- The ACh is broken down into acetic acid and choline by the enzyme AChE. The choline is taken back into the axon terminal to be recycled
