Definition of Some Terms
- Resting membrane potential: term denoting that Em is about -70mV
- Depolarization: reversal of Em due to influx of sodium ions
- Sodium (Na+): major extracellular cation
- Threshold: minimal stimulus needed to cause an AP
- Absolute refractory period: cell membrane insensitive to stimuli.
- Potassium (K+): major intracellular cation
Answers to Questions
Describe a synapse, and include definitions of the pre- and post- synaptic neurons.
A synapse is an anatomically specialized junction between two neurons, and it is where one neuron alters the activity of another. A presynaptic neuron conducts signals toward a synapse, while a postsynaptic neuron conducts away from the synapse (Vander, Sherman, and Luciano, 2001; Campbell and Reece, 2008; Mader, 2004).
Describe the relationship between axon diameter and conduction velocity.
Conduction velocity is directly proportional to axon diameter. That is, the larger the axon diameter, the faster it can conduct signals. Specifically, conduction velocity increases approximately with the square root of fiber diameter (Harline and Colman, 2007).
What is meant when an axon is described as ‘myelinated’?
When an axon is myelinated, it is covered in myelin sheath, layers of lipid-rich multilamellar membrane. Myelin is formed and deposited on the axons by oligodendrocytes, a specialized type of neuroglial cells. These glial cells are called Schwann cells in the peripheral nervous system. The nodes of Ranvier are spaces between adjacent sections of myelin where the axon’s plasma membrane is exposed to extracellular fluid (Vander et al., 2001).
Explain how myelination affects nerve conduction velocity.
The myelin sheath primarily speeds up conduction of the electric signals, or nerve impulses, along the axon by acting as an “electrical insulator.” In unmyelinated axons, the conduction velocity is around 1.0 m/sec, while in myelinated fibers, it is more than 100 m/sec (Vander et al., 2001; Mader, 2004; Harline and Colman, 2007; Min et al., 2008).
What does it mean if a membrane is described as ‘polarized’?
A membrane is said to be polarized when there is an electrical difference across the cell membrane. That is, the electrical charge outside of the cell is different from the charge inside, and it can be caused by differing numbers of ions within and out of the cell (Norries and Siegfried, 2011).
Compare and contrast depolarization, repolarization, and hyperpolarization; and relate each to the action potential.
The terms “depolarization,” “repolarization,” and “hyperpolarization” are used to describe the changes in the membrane potential relative to the resting potential. A membrane is depolarized when the membrane potential is less negative (closer to zero) than the resting potential. A membrane is repolarizing when it has been depolarized and is returning towards the resting value. A membrane is hyperpolarized when the potential is more negative (farther to zero) than the resting level (Vander et al., 2001; Campbell and Reece, 2008).
The depolarizing phase of an action potential occurs when voltage-gated sodium channels open, which increases the permeability of the membrane to sodium ions and thereby allowing more sodium ions to move into the cell. Closure of the sodium channels and rapid increase in potassium permeability allows the membrane to repolarize its potential to the resting level. There is a brief period of hyperpolazation beyond the resting level (called afterhyperpolarization) in nerve cells, brought about by some of the voltage-gated potassium channels being left open momentarily while the sodium channels are still closed (Mader, 2004; Vander et al., 2001; Campbell and Reece, 2008).
Describe the absolute and relative refractory periods in a neuron. Include when each occurs, and why.
An absolute refractory period occurs during an action potential, wherein no new stimulus will produce a second impulse. This is to ensure that signal conduction occurs in one direction. At the peak of the action potential, the voltage-gated sodium channels enter a closed, or inactive, state. The membrane must repolarize first before the sodium channel proteins can be opened again by depolarization (Vander et al., 2001; Mader, 2004; Purves et al., 2001; Campbell and Reece, 2008).
The relative refractory period, which follows the absolute refractory period, is a brief period lasting about 10 to 15 milliseconds that coincides roughly with afterhyperpolarization. During this period, a considerably stronger stimulus can produce a second action potential, because there is lingering inactivation of the sodium channels and an increased number of open potassium channels (Vander et al., 2001; Mader, 2004; Purves et al., 2001).
Explain why resting membrane potential is important in the nervous system.
Changes in the membrane potential of neurons produce electric signals which are conducted along neurons to process and transmit information. The resting potential ensures that the signal or impulse is passed on to its destination. It also allows the nerves to rest. Without it the brain would be overstimulated, which could be detrimental to the body (Vander et al., 2001; Norries and Siegfried, 2011).
Explain how Em is established and how it is maintained.
Sodium and potassium ions hold the most important roles in generating the resting membrane potential. A potential is generated across the plasma membrane due to the movement of potassium through open potassium channels. The inside of the cell becomes negative because the decrease in potassium ions and the presence of negatively-charged molecules such as proteins and nucleic acids. Thus, there is net movement of ions through their respective channels: sodium moves into the cell and potassium out (Vander et al., 2001; Norries and Siegfried, 2011).
In a resting cell, the number of ions moved by the pump is equal to the number of ions that move in the opposite direction and the cell is in its resting state. The membrane potential will remain constant as long as the concentration gradients and membrane permeabilities of the ions remain unchanged (Vander et al., 2001; Norries and Siegfried, 2011)
Describe saltatory conduction.
Saltatory conduction is the conduction of an action potential or nerve impulse in myelinated fibers, wherein an action potential at one node of Ranvier causes an impulse at the next node. Saltatory conduction is much faster than in impulse conduction in unmyelinated fibers (Mader, 2004; Purves et al., 2001; Harline and Colman, 2007).
References:
Campbell, N.A., and Reece, J.B. (2008). Biology (8th Ed.) CA: Pearson Education, Inc.
Harline, D.K., and Colman, D.R. (2007). Rapid Conduction and the Evolution of Giant Axons and Myelinated Fibers. Current Biology 17: R29-R35.
Mader, S.S. (2004). Understanding Human Anatomy & Physiology (5th Ed.). New York: The McGraw-Hill Companies.
Min, Y., Kristiansen, K., Boggs, J.M., Husted, C., Zasadzinski, J.A., and Israelachvili, J. (2008). Interaction forces and adhesion of supported myelin lipid bilayers modulated by myelin basic protein. PNAS 106(9): 3154-3159.
Norries, M. and Siegfried, D.R. (2011). Anatomy and Physiology for Dummies (2nd Ed.). New Jersey: John Wiley & Sons, Inc.
Purves, D., Augustine, G.J., Fitzpatrick, D., Katz, L.C., LaMantia, A.S., McNamara, J.O. and Williams, S.M. (Eds.) (2001). Neuroscience (2nd Ed.). MA: Sinauer Associates.
Vander, A.J., Sherman, J.H., and Luciano, D.S. (2001). Human Physiology (8th Ed.) New York: The McGraw-Hill Companies.
