2.1 Excitable tissue: Nerve Flashcards
What are the two major types of glial cells?
Outline with examples.
1) Microglia
- scavenger cells, arise from macrophages outside NS. Remove debris resulting from injury, infection + disease.
2) Macroglia
- oligodendrocytes - CNS myelin formation
- Schwann cells - PNS myelin formation
- astrocytes: fibrous in white matter, protoplasmic in gray matter. Both types send processes to vessels, inducing capillaries to form BBB.
Also produce tropic factors + help maintain appropriate K+ and NT concentrations.
What are the various parts of a typical neuron?
dendrites, soma, axon hillock, axon initial segment, axon divides into presynaptic terminals, terminal boutons
Based on number of processes that emanate from cell body, neurons can be classified as
unipolar (invertebrates)
bipolar e.g. retinal cell
multipolar e.g. motor neurons, pyramidal cells of hippocampus, purkinje cell of cerebellum
What is myelination?
Axons of many neurons are myelinated -
myelin sheath, protein-lipid complex -
PNS - Schwann cell wraps its membrane around the axon up to 100 times, layers condensed + locked in by “protein zero” P0.
Each Schwann cell forms myelin between 2 Nodes of Ranvier on ONE axon.
Compare to CNS -
Oligodendrocytes emit multiple processes that form myelin on many neighbouring axons in CNS.
What is wallerian degeneration?
If axon is cut, part distal to the cut degenerates (due to disruption of axoplasmic flow - cell body usually maintains functional integrity of the axon)
What types of transport occur along the axon?
Orthograde - from cell body toward axon terminals, microtubules + dynein + kinesin. Fast 400mm/day, Slow 10mm/day.
Retrograde - 200mm/day
Compare local/electrotonic potentials and action potentials
Local/synaptic potentials -
sub-threshold, non-propagated, graded, may be added - summation.
Action potential -
“all or none”, propagated along axon to its termination, “stereotypical” always same amplitude, not summated
Explain the resting membrane potential
RMP - at rest the inside of the cell is negative, relative to the outside of the cell, due to separation of positive and negative charges across the cell membrane.
Firstly, there is a concentration gradient established across the membrane by Na+ K+ ATPase, which concentrates K+ inside the cell and Na+ outside the cell.
This pump is electrogenic because it moves 3 + out and 2 + in, but this only accounts for about -5mV.
The RMP is then established by membrane permeability to K+, achieved by leaky K+ channels.
RMP ~ -70mV
(close to equilibrium/Nernst potential for K, -85mV)
Draw the action potential and describe each stage.
See diagram page 19 notes / page 89 Ganong’s
What effect does change in extracellular Na+ and K+ have on RMP and cell excitability?
Change in extracellular Na+ concentration has little effect on RMP, as at rest permeability of cell membrane to Na+ is very low.
Because of the resting permeability of membrane to K+, changes in K+ can have major effects.
Hypokalaemia - there is increased concentration gradient and K+ shifts out of cell, RMP is more negative, cell is hyper-polarised - less excitable.
Hyperkalaemia - decreased concentration gradient, K+ shifts into cell, RMP is less negative / “slightly depolarised” but this causes “accommodation” where Na+ channels are in inactivated state, membrane is less excitable.
What are the refractory periods during the action potential?
Absolute refractory period - from time of AP firing until repolarisation/hyperpolarisation is about one third complete - Na” channels are inactivated, no stimulus can excite nerve.
Relative refractory period - after absolute, as Na+ channels reset - excitation is possible, but stronger than normal stimuli are required, as K+ channels are still closing, membrane is still returning to RMP.
Discuss conduction of the action potential
Cell membrane is polarised at rest, + charges lined up outside and - charges on inside
During AP this polarity is reversed.
+ charges from adjacent area of membrane to AP flow into area of negativity = “current sink”
“Current sink” decreases polarity of adjacent membrane = electrotonic/local potential, which when reaches threshold, propagates AP.
In myelinated neurons:
myelin is an insulator (current flow through is negligible)
“Current sink” at one node of Ranvier electrotonically depolarises next node - so AP “jumps” from node to node = saltatory conduction
What is orthodromic/antidromic conduction
Orthodromic conduction of AP is in direction from synaptic junctions/receptors along axons to their termination.
Antidromic conduction also occurs along axon, but synapses permit conduction in one direction only, so they will fail to pass first synapse encountered and die out.
Briefly describe the structure/excitability of mixed/peripheral nerves
Peripheral nerves are made up of many axons bound together in fibrous epineurium
Thresholds of the individual axons vary
Axons with lower thresholds will fire at lower intensity stimulus.
As intensity of stimulating current increases, axons with higher thresholds are also discharged
“Maximal stimulus” produces excitation of all axons - application of supramaximal stimuli produces no further increase in size of observed potential.
What are the nerve fiber types, functions and susceptibilities?
See table in notes page 20 / Ganong 92
