PS120: Nerves 2

Question 1

What are the types of propagation of action potential?

Answer 1

1. local circuit mechanism in unmyelinated nerve fibre
2. saltatory conduction in myelinated nerve fibre

Question 2

What is the mechanism of propagation in unmyelinated nerve fibres?

and what is the charge of the segments

Answer 2

• 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)

Question 3

How do local circuits work?

Answer 3

• 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.

Question 4

What else is there to note in propagation in unmyelinate nerve fibres?

Answer 4

• 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.

Question 5

What is the term for the process of a depolarization wave?

Answer 5

It is called a nerve impulse.

Question 6

In which direction does a nerve impulse go in unmyelinated nerve fibres?

Answer 6

Since it affects segments adjacent to the initial depolarized segment, propagation occurs in both directions along the nerve fibre.

Question 7

What is the mechanism of propagation in myelinated nerve fibre?

Answer 7

Saltatory Conduction.

Question 8

How does it relate to propagation in unmyelinated nerve fibres?

Answer 8

It is similar conduction but differs due to the presence of the myelin sheath.

Question 9

Where do local circuits occur in myelinated fibre?

Answer 9

Local circuits are generated only between the nodes of Ranvier (gaps in the myelin sheath), as myelin act as an insulator.

Question 10

How does the wave of depolarization look like?

myelinated nerve fibres

Answer 10

The depolarization “jumps” from one node of Ranvier to the next, enabling faster transmission.

Question 11

How are myelinated axons different from unmyelinated axons?

Answer 11

1. Speed: saltatory conduction is 50 times faster.
2. Energy efficiency: requires less energy due to reduced ionic exchange along the axon.

Question 12

Why does saltatory conduction need less energy than in local circuit mechanism?

Answer 12

Because there is less ionic exchange along the axon.

Question 13

What are the 2 types of conduction based on transmitting of signals if action potential is iniated in the middle?

Answer 13

1. Orthodromic conduction
2. Antidromic conduction

Question 14

What is orthodromic conduction?

Answer 14

When impulses travel toward the axon termination

Question 15

What is antidromic conduction?

Answer 15

when impulses move toward the soma (cell body) but stop at the first synapse, as synapses allow conduction in only one direction.

Question 16

What stops antidromic conduction from progressing across to other nerves?

Answer 16

Because of the synapse as it allows conduction only in one direction.

Question 17

Define excitability

Answer 17

The ability of living tissues to respond to adequate stimulation

Question 18

What are the most excitable tissues of the body?

Answer 18

nerves and muscles

Question 19

How does excitability change during action potential?

Answer 19

1. during local response and up till the threshold/firing level, excitability increases.
2. during the absolute refractory period (ARP), excitability is zero, meaning the nerve fibre cannot respond to any stimulus
3. during the relative refractory period (RPR), excitability gradually recovers but remains below normal, meaning a stronger stimulus is required.

Question 20

What is the absolute refractory period?

Answer 20

coincides with the ascending limb and the first third of the descending limb of action potential.

Question 21

What are the voltage-gated sodium channels like during ARP?

Answer 21

• during the ascending limb: open & fully activated
• during the first third of the descending limb: closed & inactivated

Question 22

What is the relative refractory period?

Answer 22

coincides with the remainder of the descending limb.

Question 22

What causes RPR?

Answer 22

• partially inactivated sodium channels
• fully open K+ channels, increasing repolarizating forces

Question 23

When is the supernormal phase and what is its excitability?

Answer 23

supernormal phase is after-depolarization.
- excitability rises above normal.

Question 24

What is the subnormal phase and what is its excitability?

Answer 24

- coincides with after-hyperpolarization - excitability is slightly reduced but eventually returns to normal.

Question 25

What are the factors the affect nerve excitability?

Answer 25

1. Decreasing factors "membrane stabilizers": make it harder for a nerve to generate an action potential. 2. Increasing factors: lower the threshold for an action potential, making the nerve more excitable.

Question 26

What are chemical factors that decrease excitability?

Answer 26

1. high Ca2+ in ECF 2. low K+ in ECF 3. Low Na+ in ECF

Question 27

How does high Ca2+ level in ECF decrease excitability?

Answer 27

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.

Question 28

How does low K+ in ECF decrease excitability?

Answer 28

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.

Question 29

How does low Na+ in ECF decrease excitability?

Answer 29

- less Na+ is available for inflow during depolarization. - effect: it slows down/ inhibits action potential generation.

Question 30

What are other factors besides chemical factors that decrease excitability?

Answer 30

1. acidosis (low blood pH) 2. local anesthetics 3. hypoxia/ oxygen deficiency 4. thermal factors: cooling 5. mechanical factors: pressure

Question 31

How does acidosis decrease excitability?

Answer 31

increases ionized calcium in plasma, which stabilizes the membrane like high Ca2+ levels.

Question 32

How do local anesthetics decrease excitability?

Answer 32

- these drugs **block** Na+ permeability, preventing depolarization - effect: nerve conduction is inhibited, reducing excitability.

Question 33

How sensitive are different types of nerve fibres to local anesthetics?

Answer 33

from most to least sensitive to anesthesia: 1. C fibres (pain) 2. B fibres (autonomic) 3. A fibres (motor & sensory)

Question 34

How does hypoxia decrease excitability?

Answer 34

- reduced oxygen **impairs ATP-dependent ion pumps**, disrupting ionic gradients. - effect: **gradual** loss of excitability

Question 35

How sensitive are different nerve fibres to hypoxia?

Answer 35

1. B fibres (autonomic most sensitive to hypoxia) 2. A fibres 3. C fibres (pain least sensitive to hypoxia)

Question 36

How do thermal factors like cooling decrease excitability?

Answer 36

cooling slows down ion channel kinetics, reducing excitability.

Question 37

How do mechanical factors like pressure decrease excitability?

Answer 37

pressure compresses nerve fibres, disrupting ion channels and reducing conduction.

Question 38

How sensitive are different nerve fibres to pressure?

Answer 38

1. A fibres (motor & sensory are most sensitive) 2. B fibres 3. C fibres (pain fibres are least sensitive to pressure)

Question 39

What are the chemical factors that increase excitability?

Answer 39

1. Low Ca2+ in ECF 2. High K+ in ECF 3. High Na+ in ECF

Question 40

Describe each chemical factor in how they increase excitability.

Answer 40

1. low calcium in ECF reduces the voltage level required to open Na+ channels 2. high potassium in ECF raises the RMP, making it closer to the threshold, which makes depolarization easier. 3. high sodium in ECF means there is more Na available for inflow during depolarization,increasing excitability

Question 41

What are thermal factors that increase excitability?

Answer 41

Warming, as warmth speeds up ion channel kinetics, increasing excitability.

Question 42

How does alkalosis increase excitability?

Answer 42

alkalosis, high blood pH, decreases plasma-ionized calcium, reducing the membrane stabilization. effect: enhances excitability

Question 43

What is familial period paralysis (FPP)? | cause & effect

Answer 43

- cause: caused by a genetic disorder that results in low K+ in ECF - effect: leads to muscle paralysis due to hyperpolarization of nerve membranes

Question 44

How is familial periodic paralysis treated?

Answer 44

intravenous K+ administration. restores normal excitability.