The nervous system is characterized by electrical signals that are sent from one area to another. Whether those areas are close or very far apart, the signal must travel along an axon. The basis of the electrical signal is the controlled distribution of ions across the membrane. Transmembrane ion channels regulate when ions can move in or out of the cell, so that a precise signal is generated. This signal is the action potential which has a very characteristic shape based on voltage changes across the membrane in a given time period.
The membrane is normally at rest with established Na+ and K+ concentrations on either side. A stimulus will start the depolarization of the membrane, and voltage-gated channels will result in further depolarization followed by repolarization of the membrane. A slight overshoot of hyperpolarization marks the end of the action potential. While an action potential is in progress, another cannot be generated under the same conditions. While the voltage-gated Na+ channel is inactivated, absolutely no action potentials can be generated. Once that channel has returned to its resting state, a new action potential is possible, but it must be started by a relatively stronger stimulus to overcome the K+ leaving the cell.
The action potential travels down the axon as voltage-gated ion channels are opened by the spreading depolarization. In unmyelinated axons, this happens in a continuous fashion because there are voltage-gated channels throughout the membrane. In myelinated axons, propagation is described as saltatory because voltage-gated channels are only found at the nodes of Ranvier and the electrical events seem to “jump” from one node to the next. Saltatory conduction is faster than continuous conduction, meaning that myelinated axons propagate their signals faster. The diameter of the axon also makes a difference as ions diffusing within the cell have less resistance in a wider space.
absolute refractory period
time during an action period when another action potential cannot be generated because the voltage-gated Na+ channel is inactivated
activation gate
part of the voltage-gated Na+ channel that opens when the membrane voltage reaches threshold
continuous conduction
slow propagation of an action potential along an unmyelinated axon owing to voltage-gated Na+ channels located along the entire length of the cell membrane
depolarization
change in a cell membrane potential from rest toward zero
electrochemical exclusion
principle of selectively allowing ions through a channel on the basis of their charge
excitable membrane
cell membrane that regulates the movement of ions so that an electrical signal can be generated
gated
property of a channel that determines how it opens under specific conditions, such as voltage change or physical deformation
inactivation gate
part of a voltage-gated Na+ channel that closes when the membrane potential reaches +30 mV
ionotropic receptor
neurotransmitter receptor that acts as an ion channel gate, and opens by the binding of the neurotransmitter
leakage channel
ion channel that opens randomly and is not gated to a specific event, also known as a non-gated channel
ligand-gated channels
another name for an ionotropic receptor for which a neurotransmitter is the ligand
mechanically gated channel
ion channel that opens when a physical event directly affects the structure of the protein
membrane potential
distribution of charge across the cell membrane, based on the charges of ions
nonspecific channel
channel that is not specific to one ion over another, such as a nonspecific cation channel that allows any positively charged ion across the membrane
refractory period
time after the initiation of an action potential when another action potential cannot be generated
relative refractory period
time during the refractory period when a new action potential can only be initiated by a stronger stimulus than the current action potential because voltage-gated K+ channels are not closed
repolarization
return of the membrane potential to its normally negative voltage at the end of the action potential
resistance
property of an axon that relates to the ability of particles to diffuse through the cytoplasm; this is inversely proportional to the fiber diameter
resting membrane potential
the difference in voltage measured across a cell membrane under steady-state conditions, typically -70 mV
saltatory conduction
quick propagation of the action potential along a myelinated axon owing to voltage-gated Na+ channels being present only at the nodes of Ranvier
size exclusion
principle of selectively allowing ions through a channel on the basis of their relative size
voltage-gated channel
ion channel that opens because of a change in the charge distributed across the membrane where it is located
What happens across the membrane of an electrically active cell is a dynamic process that is hard to visualize with static images or through text descriptions. View this animation to really understand the process. What is the difference between the driving force for Na+ and K+? And what is similar about the movement of these two ions?
Sodium is moving into the cell because of the immense concentration gradient, whereas potassium is moving out because of the depolarization that sodium causes. However, they both move down their respective gradients, toward equilibrium.
Visit this site to see a virtual neurophysiology lab, and to observe electrophysiological processes in the nervous system, where scientists directly measure the electrical signals produced by neurons. Often, the action potentials occur so rapidly that watching a screen to see them occur is not helpful. A speaker is powered by the signals recorded from a neuron and it “pops” each time the neuron fires an action potential. These action potentials are firing so fast that it sounds like static on the radio. Electrophysiologists can recognize the patterns within that static to understand what is happening. Why is the leech model used for measuring the electrical activity of neurons instead of using humans?
The properties of electrophysiology are common to all animals, so using the leech is an easier, more humane approach to studying the properties of these cells. There are differences between the nervous systems of invertebrates (such as a leech) and vertebrates, but not for the sake of what these experiments study.
1. What ion enters a neuron causing depolarization of the cell membrane?
A) sodium
B) chloride
C) potassium
D) phosphate
A
2. Voltage-gated Na+ channels open upon reaching what state?
A) resting potential
B) threshold
C) repolarization
D) overshoot
B
3. What does a ligand-gated channel require in order to open?
A) increase in concentration of Na+ ions
B) binding of a neurotransmitter
C) increase in concentration of K+ ions
D) depolarization of the membrane
B
4. What does a mechanically gated channel respond to?
A) physical stimulus
B) chemical stimulus
C) increase in resistance
D) decrease in resistance
A
5. Which of the following voltages would most likely be measured during the relative refractory period?
A) +30 mV
B) 0 mV
C) -45 mV
D) -80 mv
D
6. Which of the following is probably going to propagate an action potential fastest?
A) a thin, unmyelinated axon
B) a thin, myelinated axon
C) a thick, unmyelinated axon
D) a thick, myelinated axon
D
1. What does it mean for an action potential to be an “all or none” event?
The cell membrane must reach threshold before voltage-gated Na+ channels open. If threshold is not reached, those channels do not open, and the depolarizing phase of the action potential does not occur, the cell membrane will just go back to its resting state.
2. The conscious perception of pain is often delayed because of the time it takes for the sensations to reach the cerebral cortex. Why would this be the case based on propagation of the axon potential?
Axons of pain sensing sensory neurons are thin and unmyelinated so that it takes longer for that sensation to reach the brain than other sensations.