PS120: Nerves 2 Flashcards
What are the types of propagation of action potential?
- local circuit mechanism in unmyelinated nerve fibre
- saltatory conduction in myelinated nerve fibre
What is the mechanism of propagation in unmyelinated nerve fibres?
and what is the charge of the segments
- action potentials are spread by the local circuit mechanism
- the resting segment has a positive charge outside and negative inside, while the activated segment is reversed (negative outside, positive inside)
How do local circuits work?
- current flows between the depolarized activated segment and adjaacent resting segments
- this flow decreases the resting membrane potential (RMP) in adjacent segments, leading to depolarization once the firing threshold is reached.
What else is there to note in propagation in unmyelinate nerve fibres?
- Na channels opening: sodium channels open due to decreased RMP, increasing ion permeability and causing further depolarization.
- wave of depolarization: the depolarized segment stimulates adjacent areas to depolarize, transmiting the wave of depolarization (action potential) along the fibre.
- repolarization: occurs first at the site of initial stimulation and spreads along the membrane, restoring the resting potential.
What is the term for the process of a depolarization wave?
It is called a nerve impulse.
In which direction does a nerve impulse go in unmyelinated nerve fibres?
Since it affects segments adjacent to the initial depolarized segment, propagation occurs in both directions along the nerve fibre.
What is the mechanism of propagation in myelinated nerve fibre?
Saltatory Conduction.
How does it relate to propagation in unmyelinated nerve fibres?
It is similar conduction but differs due to the presence of the myelin sheath.
Where do local circuits occur in myelinated fibre?
Local circuits are generated only between the nodes of Ranvier (gaps in the myelin sheath), as myelin act as an insulator.
How does the wave of depolarization look like?
myelinated nerve fibres
The depolarization “jumps” from one node of Ranvier to the next, enabling faster transmission.
How are myelinated axons different from unmyelinated axons?
- Speed: saltatory conduction is 50 times faster.
- Energy efficiency: requires less energy due to reduced ionic exchange along the axon.
Why does saltatory conduction need less energy than in local circuit mechanism?
Because there is less ionic exchange along the axon.
What are the 2 types of conduction based on transmitting of signals if action potential is iniated in the middle?
- Orthodromic conduction
- Antidromic conduction
What is orthodromic conduction?
When impulses travel toward the axon termination
What is antidromic conduction?
when impulses move toward the soma (cell body) but stop at the first synapse, as synapses allow conduction in only one direction.
What stops antidromic conduction from progressing across to other nerves?
Because of the synapse as it allows conduction only in one direction.
Define excitability
The ability of living tissues to respond to adequate stimulation
What are the most excitable tissues of the body?
nerves and muscles
How does excitability change during action potential?
- during local response and up till the threshold/firing level, excitability increases.
- during the absolute refractory period (ARP), excitability is zero, meaning the nerve fibre cannot respond to any stimulus
- during the relative refractory period (RPR), excitability gradually recovers but remains below normal, meaning a stronger stimulus is required.
What is the absolute refractory period?
coincides with the ascending limb and the first third of the descending limb of action potential.
What are the voltage-gated sodium channels like during ARP?
- during the ascending limb: open & fully activated
- during the first third of the descending limb: closed & inactivated
What is the relative refractory period?
coincides with the remainder of the descending limb.
What causes RPR?
- partially inactivated sodium channels
- fully open K+ channels, increasing repolarizating forces
When is the supernormal phase and what is its excitability?
supernormal phase is after-depolarization.
- excitability rises above normal.
What is the subnormal phase and what is its excitability?
- coincides with after-hyperpolarization
- excitability is slightly reduced but eventually returns to normal.
What are the factors the affect nerve excitability?
- Decreasing factors “membrane stabilizers”: make it harder for a nerve to generate an action potential.
- Increasing factors: lower the threshold for an action potential, making the nerve more excitable.
What are chemical factors that decrease excitability?
- high Ca2+ in ECF
- low K+ in ECF
- Low Na+ in ECF
How does high Ca2+ level in ECF decrease excitability?
effect: it reduces sodium inflow, leading to decreased excitability.
- calcium binds to voltage-gated sodium channel proteins, altering their electrical state
- this increases the voltage threshold required to open Na+ channels, reducing their permeability.
How does low K+ in ECF decrease excitability?
it causes hyperpolarization because more K+ diffuses out of the cell due to the increased concentration gradient.
- hyperpolarization makes it harder to reach the threshold for an action potential.
How does low Na+ in ECF decrease excitability?
- less Na+ is available for inflow during depolarization.
- effect: it slows down/ inhibits action potential generation.
What are other factors besides chemical factors that decrease excitability?
- acidosis (low blood pH)
- local anesthetics
- hypoxia/ oxygen deficiency
- thermal factors: cooling
- mechanical factors: pressure
How does acidosis decrease excitability?
increases ionized calcium in plasma, which stabilizes the membrane like high Ca2+ levels.
How do local anesthetics decrease excitability?
- these drugs block Na+ permeability, preventing depolarization
- effect: nerve conduction is inhibited, reducing excitability.
How sensitive are different types of nerve fibres to local anesthetics?
from most to least sensitive to anesthesia:
1. C fibres (pain)
2. B fibres (autonomic)
3. A fibres (motor & sensory)
How does hypoxia decrease excitability?
- reduced oxygen impairs ATP-dependent ion pumps, disrupting ionic gradients.
- effect: gradual loss of excitability
How sensitive are different nerve fibres to hypoxia?
- B fibres (autonomic most sensitive to hypoxia)
- A fibres
- C fibres (pain least sensitive to hypoxia)
How do thermal factors like cooling decrease excitability?
cooling slows down ion channel kinetics, reducing excitability.
How do mechanical factors like pressure decrease excitability?
pressure compresses nerve fibres, disrupting ion channels and reducing conduction.
How sensitive are different nerve fibres to pressure?
- A fibres (motor & sensory are most sensitive)
- B fibres
- C fibres (pain fibres are least sensitive to pressure)
What are the chemical factors that increase excitability?
- Low Ca2+ in ECF
- High K+ in ECF
- High Na+ in ECF
Describe each chemical factor in how they increase excitability.
- low calcium in ECF reduces the voltage level required to open Na+ channels
- high potassium in ECF raises the RMP, making it closer to the threshold, which makes depolarization easier.
- high sodium in ECF means there is more Na available for inflow during depolarization,increasing excitability
What are thermal factors that increase excitability?
Warming, as warmth speeds up ion channel kinetics, increasing excitability.
How does alkalosis increase excitability?
alkalosis, high blood pH, decreases plasma-ionized calcium, reducing the membrane stabilization.
effect: enhances excitability
What is familial period paralysis (FPP)?
cause & effect
- cause: caused by a genetic disorder that results in low K+ in ECF
- effect: leads to muscle paralysis due to hyperpolarization of nerve membranes
How is familial periodic paralysis treated?
intravenous K+ administration.
restores normal excitability.
