NB1-4 - Stimulated Excitable Cells

Question 1

Answer 1

E

Question 2

What is a neurons length constant? What symbol is used to denote the length constant? What two factors most greatly influence a neurons length constant? Describe why this constant is important

Answer 2

A neurons length constant (γ) is the distance over which a graded electrical potential decays to 37% of its maximal amplitude. It is most heavily influenced by the neurons intracellular resistance (lower the better) and its membrane resistance (higher the better). This number is important for describing how “easy” it is for an AP to be triggered in a neuron.

Question 3

Where are the AP initiation zones located on the different types of neurons?

Answer 3

Multipolar Neurons - at the axon hillock

Bipolar and Pseudounipolar - where the dendrites meet the axon

Refer to image

Question 4

Answer 4

D

Question 5

Describe the distribution of voltage gate channels in myelinated and unmyelinated axons.

Answer 5

• In unmyelinated axons the v-gated Na+ channels are most concentrated at the impulse initiation zone. Then v-gated Na+ and K+ channels are evenly distributed throughout the length of the axon and its terminal. v-gated Ca++ channels are present at the axon terminal
• In myelinated axons, v-gated Na+ channels are concentrated at the IIZ, each node of ranvier, and the axon terminal. v-gated Ka+ channels are present in the paranodal and juxtaparanodal regions and the axon terminal. v-gated Ca++ channels are present in the axon terminal

Refer to image

Question 6

Explain why a myelinated nerve is able to transmit an AP faster than an unmyelinated one.

Answer 6

The myelinated sections of axon have very high membrane resistance, which increases the length constant of these regions. The increased concentration of v-gated Na+ channels at the nodes of ranvier also allows these regions to respond to a voltage change much more quickly. These two changes combined allow a neuron to conduct an AP more quickly.

Question 7

Discuss the differences between lidocaine and phenytoin and when it is appropriate to use each.

Answer 7

Lidocaine is a local anesthetic that blocks v-gated Na+ channels. It will prevent any neuron it comes into contact with from firing APs. Most useful for temporary numbing to perform medical procedures

Phenytoin is an anti-seizure medication that binds to already inactivated v-gated Na+ channels. This allows it to preferentially block hyperexcitable neurons and prevent pain but not block other sensations

Question 8

Answer 8

C

Question 9

Answer 9

A

Question 10

Answer 10

E