M&R - Nerve Impulse Flashcards
What does a low membrane capacitance result in?
Increased conduction velocity as less work is required to depolarise adjacent regions of membrane to threshold for an action potential to be generated
How is an electrical signal propagated directionally down an axon?
After an action potential has fired in a region of nerve membrane, that part of the membrane becomes refractory to the firing of a second action potential because of the inactivation of the Na+ channels.
Therefore, a nerve impulse cannot re-enter a region of nerve axon that has just fired and so it is propagated directionally down a nerve axon
What are the two types of glia cell?
Oligodendrocytes - CNS
Schwaan cells - PNS
What are the unmyelinated regions between the internodes called?
Noes of Ranvier
What effect does myelination have on the electrical capacitance of the internodal length?
What happens?
Reduces it substantially
Electrical activity only occurs at nodes of Ranvier
Where are the Na+ channels located in a myelinated nerve?
Exclusively at the nodes of Ranvier
What happens to the electrical current in demyelination?
Excitation of successive nodes becomes progressively slowed because of the increased membrane capacitance.
This causes local current dissipation and conduction of the action potential ultimately fails.
What can restore some excitability after complete demyelination?
Insertion of new Na+ channels in internodal regions
Conduction velocity is slowed
Where does electrical stimulation occur?
What will happen to the excitability under an anode?
What is this used to stimulate?
Under a cathode (negatively charged)
Reduced - as anode is positively charged
An axon/group of axons to threshold, thus initiating an action potential
How is conduction velocity calculated?
Measuring the distance between the stimulating electrode and the recording electrode and the time gap between stimulus and action potential being registered by the recording electrode.
Conduction velocity = distance/time
How is action potential conducted along an axon?
Change in membrane potential in one part can spread to adjacent areas of the axon due to the local current spread.
Local current spread causes depolarisation of part of the axon to threshold then an action potential is initiated
What properties of the axon lead to a high conduction velocity?
High membrane resistance
Low membrane capacitance
Large axon diameter –> low cytoplasmic resistance
What does the membrane resistance depend on?
Number of ion channels open
What happens in low membrane resistance?
More ion channels are open
More loss of local current occurs across the membrane
Limiting the spread of local current effect
What is the effect of myelin?
Reduce the capacitance
Increase the resistance of the axonal membrane
Name a disease which causes demyelination of axons?
Multiple sclerosis - autoimmune attack of the myeline sheath
Can have dramatic effects on the ability of previously myelinated axons to conduct action potentials properly.
This leads to decreased conduction velocity, complete block or cases where only some action potentials are transmitted
What is electrical capacity (capacitance)?
Ability to store electrical charge
When during development does myelination begin to occur?
After 4 months of fetal development
Where in an myelinated nerve does electrical activity occur?
Node of Ranvier
How does the myeline sheath reduced the capcitance?
Reduces the dissipation of local current within the nerve fibre, thereby, permitting more distant regions of the fibre membrane to be brought to threshold for firing an action potential
Where are the Na+ and K+ channels located in a myelinated nerve fibre?
Na+ channels at the node of Ranvier
K+ channels more widely distributed
What may happen to the conduction of nerve impulses during nerve fibre demyelination?
Conduction may stop due to the dissipation of local currents because of the increase capacitance of the membrane
After a period of time, a demyelinated nerve may begin to conduct nerve impulses again due to the incorporation of ion channels in the internodal regions
Treatment of demyelinated nerves with K+ channel blockers can restore nerve impulse conduction to otherwise defective nerve.
