2 - Neuronal transport & membrane potential

Question 1

Dendrite

Answer 1

• Short, bristle-like, highly branched processes
• Receive nerve input (at synapses)
• Not myelinated

Question 2

Axon

Answer 2

• Long, thin process
• Propagates nerve impulse to 
another neuron, muscle fibre or 
gland
• Often myelinated
• Terminates at axon terminals or
synapses

Question 3

Soma (cell body)

Answer 3

• Contains the normal cell organelles
• Main site of protein synthesis and degradation
• Has pronounced rough ER = ‘Nissl’ substance

Question 4

Neuronal structure

Answer 4

Unique features of neurons:
• Can have very long axons, so nerve terminals are remote from the cell body, which is the main site of protein synthesis and degradation
• Have a well-defined cytoskeleton with a special type of intermediate filament (= neurofilament)

Question 5

Neuronal cytoskeleton

Answer 5

• Actin microfilaments – form a meshwork under the cell surface
• Microtubules - help maintain structure of axon
• Microtubule-associated proteins (e.g. tau)
• Neurofilaments - Involved in motility, structural support and axonal transport

Question 6

Anterograde (orthograde)

Answer 6

• Materials are transported from the soma to the axon terminals

Question 7

Retrograde transport

Answer 7

Transport of materials from the axon terminals to the soma

Question 8

Materials carried by Orthograde (fast)

Answer 8

200-400 nm/day: small vesicles, enzymes for transmitter metabolism
40 nm/day: mitochondria

Question 9

Materials carried by Orthograde (slow)

Answer 9

1-5 nm/day: tubulin, neurofilament proteins, Tau protein

Question 10

Materials carried by Retrograde

Answer 10

200 nm/day: Larger vesicles, nerve growth factor (NGF)

Can be hijacked by viruses and toxins (e.g. herpes, rabies, tetanus)

Question 11

Neuronal signalling

Answer 11

• Neurons receive information at dendrites (up to 100,000 synaptic inputs/neuron) and integrate in cell body
• Information is transmitted along the axon in the form of electrochemical signals or nerve impulses (= action potentials)
• Action potentials are due to the flow of ions (Na+, K+) through specific protein channels in the membrane
• The lipid bilayer of the membrane is impermeable to these charged ions

Question 12

2 types of forces that move ions across membranes

Answer 12

chemical + electrical

Question 13

chemical force

Answer 13

Differences in concentration: diffusion from a region of high concentration to a region of low concentration

Question 14

electrical force

Answer 14

Interior of cell is negatively charged so positively charged cations and retained and negative ions will be expelled
The electrochemical driving force is a combination of the chemical and electrical forces acting on any particular ion

Question 15

Movement of ions across cell membranes

Answer 15

2 categories of ion channels that facilitate ion movement into and out of neurons
1. Channels that are gated and require a stimulus to open
• ligands, mechanical force (pressure change in membrane) or voltage (+ or -)
• specific to particular ion(s)
2. Channels that are always open and allow free movement of ions

Question 16

Potassium movement

Answer 16

• There is a constant flow of K+ ions down their concentration gradient, from the inside of the neuron to the outside
• This movement occurs via open (or leaky) K+ channels that are situated in the membrane of the neuron

Question 17

Na+/K+ ATPase pump

Answer 17

• The ion gradient is maintained by the continuous operation of the Na+/K+ ATPase pump
• 3 Na+ ions move from the inside of the neuron to the outside of the cell
• At the same time, 2 K+ ions are moved from outside the neuron to the inside of the cell
• At each cycle of the Na+/K+ATPase pump, the cell loses one positively charged ion from the intracellular environment

Question 18

Resting membrane potential

Answer 18

• Because of the diffusion of K+ and the action of the Na+/K+ ATPase pump is a more positive charge outside the neuron compared to the inside of the neuron
• The difference in charge across the membrane of the neuron is referred to as polarisation
• The difference in voltage across the plasma membrane when the neuron is at rest (not firing or receiving a signal) is called the resting membrane potential
• For most neurons, the resting membrane potential is ~ -70mV

Question 19

Forces that drive ion movement

Answer 19

• When ion channels open, the chemical gradient drives ion movement
from high concentration to low concentration
• In the absence of polarisation, diffusion would occur until chemical equilibrium was reached
• However, this does not occur because of electric forces

Question 20

Electrochemical gradients of sodium

Answer 20

When Na+ channels open:
• chemical gradient drives ion movement into the cell (more Na+ on outside than inside)
• electrical force pulls + ions into the cell (attracted to the negative inside of cell)
• both act in the same direction = Na+ will enter the cell

Question 21

Equilibrium of sodium movement

Answer 21

• As Na+ moves into the neuron, the charge inside the cell starts to become positive and the electrical gradient decreases, along with the chemical gradient
• Eventually, the chemical and electrical forces will be exactly in balance and there will be no nett flow through any open channels

Question 22

Equilibrium potential

Answer 22

The equilibrium potential (E) is the membrane potential required to exactly counteract the chemical forces acting to move one particular ion across the membrane

Question 23

Electrochemical gradient of potassium

Answer 23

When K+ channels open:
• chemical gradient drives ion movement out of the cell (less K+ on outside)
• but electrical force pulls + ions into the cell
• two forces act in opposite directions
• chemical force > electrical force, so K+ moves out of the neuron

Question 24

Equilibrium of potassium movement

Answer 24

• As K+ moves out of the neuron, the charge inside the cell starts to become even more negative, so the electrical gradient becomes stronger
• Eventually, the chemical force that drives K+ out of the cell = the electrical force driving K+ back into the cell and there will be no nett flow of K+ ions

Question 25

Nernst equation

Answer 25

The equilibrium potential (E) can be calculated using the Nernst equation:
E = (61/z) log (C0/ Ci)
E = equilibrium potential in millivolts (mV)

Question 26

Equilibrium potential for Na+

Answer 26

E Na+ = +60 mV

No nett movement of Na+ ions

Question 27

Equilibrium potential for K+

Answer 27

E k+ = -94

No nett movement of K+ ions