Nervous System: Action Potential Generation and Transmission part 2 Flashcards
Unmyelinated axon diameter
1μm
Description of Unmyelinated axon AP transmission
Slow transmission of action potentials, continuous
Mylinated axon diameter
5-10μm
Description of Myelinated axon AP transmission
Fast transmission of action potentials, “saltatory” (in large steps)
The two stages of action potential transmission in both types of axons
- Passive spread of current
2. Generation of action potentials
Stage 1 of the passive spread of current
there is a depolarisation att one segment of the cell membrane. This depolarisation has a higher potential leading to current to flow from + to -
Stage 2 of the passive spread of current
Current flows passively (inside/outside axon). It goes in both directions and flows across the membrane through leak channels and then flows back through ‘circuit’
Stage 3 of the passive spread of current
The passive flow of current causes the depolarization of adjacent parts of the membrane (rule #2) causing electric signals to spread across the axon
How far can passive current spread and why
Only short distances (less than 1mm) because current goes through the path of least resistance so will dissipate over time
What occurs under a anode
the membrane potential becomes hyperpolarized
What occurs under a cathode
the membrane potential becomes depolarised
Process of Action potential transmission in unmyelinated axons
- an action potential is generated under a cathode, causing the passive spread of current in both directions
- current flows passively in both directions
- adjacent parts of the cell membrane depolarise to threshold
- Voltage gated Na+ channels in adjacent parts of the membrane open
- New full size AP is generated in adjacent parts of the membrane
Speed of AP transmission in unmyelinated axons
~1m/sec AP (slow)
Why is the passive spread of current in unmylinated axons slow?
Because AP must be regenerated at every point in the membrane
What cells form myelin sheaths in the CNS
oligodendrocytes
What cells form myeline sheaths in the PNS
Schwann cells
Gaps in myelination
Nodes of Ranvier
Length of Nodes of Ranvier
1mm
Spead of AP transmission in myelinated axons
20-100m/sec
How does myelination increase speed of AP transmission?
It acts as an insulator so it reduces current dissipation at it flows along the axon and AP ONLY need to be regeenrated at nodes of ranvier (not every other point)
Where are AP generated in myelinated axons and why
Nodes of Ranvier - this is the point of least resistance in myelinated axons
Name of current flow in myelinated axons
Saltatory Conduction
Why are unmyelinated axons used in the brain
Despite taking longer, they have a small diameter so more can pack into nervous tissue
Why do AP’s only flow in one direction in physiological conditions
The abolsute refractory period (1-2ms) means AP’s cannot flow backwards as it won’t be able to reactivate voltage gated Na channels
What is a muscle spindle?
the sensory part of a muscle
How does information travel from the muscle spindle to the CNS
sensory unipolar neurons
Structural features of muscle spindle and its function
It contains stretch sensistive ion channels that sense the displacement of the cell membrane and causes channels to open, causing the depolarisation of the most distal part of the axon (AP).
Where is the synaptic terminal located?
in the CNS
What is a receptor potential
the graded depolarisation of the muscle spindle (small depolarisations due to small stimuli do not cause an AP, the bigger the stimuli = bigger receptor potential)
What codes for the strength of the stimulus in the muscle spindle
the amplitude of the receptor potential and the frequency of the action potential
