PSL201 Week 2

Afferent information is _______ (input or output?)

Input

Efferent information is _______ (input or output?)

Output

Which is more susceptible to nerve damage, the CNS or PNS? Why?

PNS, since it does not have much protection (the CNS has skull)

The PNS connects the CNS to ______

Organs

Efferent

Carries info from CNS to organs in the periphery (effector organs)

What are the excitable cells in the nervous system called?

Neurons

What are the support cells in the nervous system called?

Glial cells

90% of the cells in the NS are ____ cells?

Glial (“glue”)

Name the 4 types of glial cells

Schwann cells, Astrocytes, Oligodendrocytes, Microglia

Microglia (role)

Protect CNS from foreign matter

Astrocytes

Provide energy and guidance to neurons (where to go)
Help with NT communication

Oligodendrocytes (location + role)

Found in CNS
One can form many myelin sheaths for several axons

Schwann cells (location + role)

Found in PNS
One can form one myelin sheath for one section of an axon

Myelin

Insulation around the axon that is mainly made up of fat; appear white (white matter)

Myelin sheath increases ________ _________

Membrane resistance

Myelin sheath increases membrane resistance. What does this mean?

Reduces leakage of current out of the membrane

What would happen if our myelin sheath were damaged? Describe.

Multiple sclerosis (MS): NS disease that effects brain and spinal cord, resulting in slowing down or even blockage of signals within brain and between the brain and body

List some symptoms of multiple sclerosis

Symptoms: visual disturbances, muscle weakness, trouble with coordination and balance, problems with memory

What is a potential treatment for multiple sclerosis?

Potential treatment: reduce leakage of currents by blocking K+ channels

Leak channels (describe)

Always open, found in plasma membrane throughout neurons, involved in resting membrane potential

Ligand-gated channels (describe)

Open or close in response to ligand binding, found in dendrites and cell body, involved in synaptic potentials

What are the ligands in the nervous system?

NTs

Voltage-gated channels (describe)

Open or close in response to changes in membrane potential, in NS the main ones are Na, K, and Ca

Describe the location and role of Na and K voltage-gated channels

Found throughout neuron but clustered at axon hillock, involved in generating action potentials

Describe the location and role of Ca voltage-gated channels

Found at axon terminal, release of NTs

Describe membrane potential

Differences between charges inside a cell with respect to the inside (e.g. -20mV means the cell is -20mV more negative inside compared to outside)
All cells in the body generate a membrane potential (if 0, means cell is dead)
Usually negative

What does it mean if membrane potential is 0?

Cell is dead

What is the resting membrane potential of a neuron?

-70mV

What are the 2 factors that determine membrane potential?

1. Ion concentration gradients
2. Membrane permeability to these ions

__% of the resting membrane potential is directly due to the Na/K pump, __% is indirectly due to it

20; 80

What does it mean for the Na/K pump to have indirect effects on the resting membrane potential?

The Na/K pump establishes concentration gradients
Na is high outside and low inside, K is high inside and low outside

What is the direct effect of the Na/K pump on the resting membrane potential?

Net +1 out of cell, which makes inside of cell more negative relative to the outside

Describe potassium equilibrium potential (how/why it happens, what it is)

- Na/K pump established concentration gradient where there is more K+ inside, so K+ starts to leave cell by moving down its concentration gradient
- This K+ efflux occurs until there is enough buildup of K+ outside that K+ leaving is repelled (electrical force)
- Chemical driving force for K+ is directed outwards, electrical driving force is directed inwards, so these two forces are acting against each other
- The cell will eventually reach equilibrium, meaning there is no net force for K+ to move across the membrane (chemical force is equal and opposite in direction to the electrical force)
- The magnitude of the chemical driving force will be matched by the electrical driving force
- When membrane potential is -94mV, K+ is at equilibrium

What does it mean for an ion to be at equilibrium?

- There is no net force for the ion to move across the membrane
- Chemical force = negative electrical force OR electrochemical force = 0

Describe the Nernst equation

- Allows you to calculate an ion’s equilibrium potential when you know the ion’s concentration inside and outside the cell (there are also other constants/values that go into it)
- This equation assumes that only one ion is involved/moving/permeable

When the neuron is at rest, the membrane is most permeable to what ions?

K+ ions

Describe sodium equilibrium potential (how/why it happens, what it is)

- Chemical driving force: Na in
- Na accumulates inside cell, positive charges accumulate inside
- Electrical driving force: Na out
- +60mV

Given the equilibrium potentials of K and Na, why is the resting membrane potential of a neuron -70mV?

Membrane is 35x more permeable to K+, so resting MP leans more towards -ve
So, more K+ exits the cell than Na enters
When movement of K+ out is balanced with Na+ into the cell, cell is at resting MP (When one K+ leaves cell, one Na+ enters so that there is no net charge movement)

What equation is used to calculate resting membrane potential when considering multiple ions?

Goldman equation

Membrane potentials change due to what kinds of channels?

Voltage gated channels

Compare graded potentials to AP

Graded: small, short distances, magnitude varies (graded), initiated by a stimulus, decremental (decays as it travels)
AP: large, long distances

What is the threshold (what it means and value)?

-55mv
Level of depolarization needed to produce an AP

Subthreshold stimuli

Not strong enough to bring the membrane potential to the threshold level

Threshold stimulus

Strong enough to bring membrane potential to threshold level

Graded potentials are decremental. What does this mean, and why is this the case?

Means the magnitude decays as it spread; because the voltage spreads via passive charge movement called electrotonic conduction

What are the two types of summation for graded potentials?

Spatial and temporal

Describe temporal summation

Same stimulus repeated close together in time

Describe spatial summation

Different stimuli that overlap in time