Propagation And Tranimssion Of Action Potentials Flashcards
What happens to depolarization as distance from stimulus increases?
Decay rapidly
Distance between axon hillock and synapse
Long
- can be meters long
- AP has to maintain strength to elicit response
Single APs
Usually not sufficient to carry info along a neuron
What kind of feed back is AP propagation?
Positive
AP propagation
Initial AP begets new APs in adjacent membrane
-bi direction if able, physiologically in one direction
Each new AP in propagation
Is full strength and follows the same steps and propagates itself
What allows for full strength APs to travel along length of neuron?
Each new AP is full strength and follows the same steps and propagates itself
Speed at which APs are conducted away from initiation site
Conduction velocity
What is conduction velocity dependent upon
Time and length constants
-how quickly you change charge across membrane
How fast membrane can depolarize to 63% of max AP voltage
How far depolarization travels before falling below 63% of max
Smaller time constant (t)
Faster depolarization, faster conduction
What does the time constant depend on (Rm)
Number of open channels (resistance)
-more open channels, lower resistance, more ion flow, faster depolarization
What is the second thing that the time constant depends on? (Cm)
How well the membrane holds charge (capacitance)
-less charge in membrane, more is transferred inside, faster depolarization
What allows depolarization to travel further?
High resistance, less open channels fewer ions leaking out, depolarization can travel further
How easy current can flow inside the cell
Internal resistance (Ri)
What is a better conductor than the membrane?
Cytoplasm
_____ nerves have more cytoplasm than thinner nerves (better conduction)
Thicker
Increasing diameter does what to Ri
Lowers by increasing area of cytoplasm to membrane
-higher length constant means further/faster conduction
____ nerves conduct impulses faster than smaller nerves
Thick
What limits the size of nerves?
Anatomical constraints
Which of the following set of conditions would give you the most rapid conduction?
- low time constant
- high length constant
Layers of glial cells wrapped around axon
Myelination
What are the cells that myelinated in the CNS
Oligodendrocytes
What are the cells that myelinated in the PNS
Schwann
Myelin distribution in axons
Evenly distributed along axon with small spaces between
What are the small spaces between the myelination
Node of ranvier
What does myelin do for the axons?
Insulates the axon to make AP ‘jump’ between nodes
What is it called when the AC jumps between the nodes of ranvier
Saltatory conduction
Myelin wraps around axon and doesn’t allow any ______ in the area it surrounds
Ion flux
What does myelin do the Rm (membrane resistance) under the sheath?
Increases it to increase the length constant
Rm (membrane resistance at the nodes
Allows for ion channels to be enriched, so decreases Rm at node to decrease time constant
Myelin essential does what to the nerve
Shortens it
Which nerve is likely the most heavily myelinated?
Motor neuron because its long distance and needs to be quick (reflexes)
What are some examples of demyelination diseases?
- MS (central)
- Guilin-Barre syndrome (peripheral)
- Diabetic Neuropathy (peripheral)
Usually autoimmune
Because myelin makes nerves fast, they control __________- receptors
Skeletal muscle and pain/touch receptors
Los of myelin decreases ____ to a point where APs might not be propagated
Length constant
-low number of ion channels under sheath. When destroyed are unable to depolarize
What does loss of myelin cause?
Muscle weakness, loss of coordination, loss of sensation, paralysis, death
Site of communication fro one cell (usually a neuron) to another
Synapse
Electrical synapse
Gap junctions (not that common)
Chemical synapse
Most common
Direct connections between cells which allows ions to flow between (easiest)
Electrical synapse
What type of synapse creates a syncytium?
Electrical synapses
What is the purpose of the gap junctions in electrical synapses?
Connexons make holes in neighboring membranes
Examples of electrical synapses
Heart, smooth muscle, bladder, where coordinated contraction is required
Is there integration in electrical synapse?
No
Speed of electrical synapses
Fast
Information is transported via chemicals across the synaptic cleft in a unidirectional flow
Chemical synapse
What alters the membrane potential on the post synaptic cell
neurotransmitters released from presynaptic cell by the AP
What do neurotransmitters do once released
Bind to receptors on post synapse
Speed of chemical synapse
Slow
Is there integration in a chemical synapse?
Yes
Synaptic transmission
- propagation of AP
- release of neurotransmitter
- binding of neurotransmitter to receptors
- effect on post synaptic cell
Propagation of AP
AP is propgated down the axon of the pre synaptic nerve
Release of neurotransmitter
- When AP reaches the terminal, voltage gated calcium channels *VGCC) open
- Ca2+ enters, causes the release of NT into the synaptic cleft
What happens when the AP reaches the terminal?
Voltage gated calcium channels (VGCC) open
Binding of neurotransmitter to receptors
- NT diffuse across the cleft from pre to post synaptic cell
- bind to receptors on post synaptic cell membrane
Effect on post synaptic cell of NT
- binding of NT to receptor alters membrane potential
- depolarize - excitatory- more likely for AP to fire (Na+, Ca+)
- hyperpolarize - inhibitory - less likely for AP to fire (K+, Cl)
Predict the effect of calcium ionosphere on NT release
More NT is released because voltage gated calcium channels open in the terminal bouton
Predict the effect of hypocalcemia on NT release
Less NT release
Action of NT at the neuromuscular junction
- AP travels down motor neuron, open VGCC, allow influx of Ca+
- Ca2+ causes release of Ach
- Ach binds to nicotinic receptors (nAchr) on post synaptic cell
- nAchr are Na+ channels-leads to EPP(Na goes in, hypopolarization, AP)
- if enough open, depolarization reaches threshold. New AP in the muscle causes contraction
What is Ach broken down by?
Acetylcholine esterase (AchE)
What does acetylcholine get broken down into?
Choline and acetate
_____ is taken up by pre and used to regenerate Ach for next transmission
Choline
Once Ach is no longer bound
EPPs stop, APs stop and contraction stops
Altering the NMJ
Done therapeutically for anesthesia or treating diseases. Can be fatal
Predict the mechanism of action of neostigmine
Inhibits acetylcholine esterase (allows Ach to accumulate in synapse)
What are some chemicals that affect NMJ?
- Botulinum toxin-Botox
- Curare-arrow poison
- Neostigmine -AchE inhibitor
- hemicholinium-blocks choline reabsoroption - no Ach
How does botulinum toxin (Botox) affect NMJ?
Blocks release of Ach- paralysis
- locally, no wrinkles on your forehead.
- systemically, paralyzed respiratory muscle, dead
How does curare (poison arrow) affect NMJ?
Blocks Ach receptors-paralysis
-no medicinal use, paralyzed respiratory muscle, dead
How does neostigmine affect NMJ?
AchE inhibitor-prolongs time Ach stays in cleft, increases contractions
-used in myasthenia gravis and to reverse paralytic anesthesia
What are some uses of disruption of the NMJ?
- potent chemical ware fare
- pesticides (organophosphates)
