5. Lectures 12, 13

Question 1

How do signals travel through neurons?

Answer 1

Signals (voltage changes) typically flow from dendrites (principle synaptic input site) to soma to axon and finally to synapses

Question 2

What types of channels are found at synaptic sites?

Answer 2

Ligand gated channels
These occur predominantly in the dendrites and somata, but synaptic inputs can also be found in axons

Na+ channels are located in the parts of the neuron that display action potentials, as would delay rectifying K+ channels

Question 3

How do different neuronal types respond to a continuous depol?

Answer 3

Neuron with fast Na currents and delayed rectifier K currents will repetitively spike (rate of firing regulated by presence of A type K current

Neuron with slow accumulating K current (Ca activated K currents) will display spike frequency adaptation

Neurons can also exhibit rhythmic bursting behaviour by exploiting the interplay between depolarizing and hyperpolarizing currents

Slide 6 lecture 12

Question 4

What are regenerative signals and non-regenerative signals?

Answer 4

Regenerative signals- action potentials (all or nothing)
Active responses do not decay with distance through axon

Non-regenerative signals (passive responses)- subthreshold potentials (graded potentials) that spread for short distances across cell membranes
Passive responses decay with distance through an axon

Slide 8 lecture 12

Question 5

What are receptor potentials and post synaptic synaptic potentials?

Answer 5

Receptor potentials- generated during the transduction of sensory stimuli

Postsynaptic synaptic potential- generated by opening of agonist-activates channels
Called graded potential
Response is proportional to stimulus intensity and decays with distance
Lose strength as they move through cell due to current leak and cytoplasmic resistance

Question 6

What does the spread of electrical correct depend on? (3 things)

Answer 6

Cell geometry
Electrical resistance of the aqueous solutions and cell membrane
Membrane capacitance

Question 7

What are the 2 types of postsynaptic potentials?

Answer 7

Excitatory postsynaptic potential (EPSP)- depolarization

Inhibitory postsynaptic potential (IPSP)- hyperpolarization

Question 8

What is signal (postsynaptic potential) summation?

Answer 8

A single neurons may receive inputs from tens of thousands of other neurons

Spatial summation- Excitatory postsynaptic potentials arriving from different dendrites combine

Temporal summation- excitatory postsynaptic potentials arrive rapidly in succession

Slide 10-12 lecture 12

Question 9

What is the space constant (l)?

Answer 9

Determined the spread of voltage changes in space

Greater specific membrane resistance (Rm) and cable radius (a), greater then length constant and less the loss of signal

Greater the resistance of the internal conductor (Ri) the smaller the length constant and the greater loss of signal

Slide 12-14 lecture 12

Question 10

What are the voltage gated channels in dendrites?

What are calcium spikes?

Answer 10

Low density of Nav and Kv channels

Some have V-gated Ca channels that boost the signal

Calcium spikes- purkinje cell (Ca-dendritic spikes)
Ca spikes can propagate into the soma (not doen axon)

Slide 15 lecture 12

Question 11

What are the 3 steps/facts about excitatory postsynaptic potential travelling?

Answer 11

1. EPSP attenuated in the soma and the initial segment, but the EPSP is large enough to trigger an action potential at the initial segment
2. The threshold is high in regions that have few Nav channels, but falls steeply in the hillock and initial segment
3. The density of Nav channels is high only at the initial segment and at each node of ranvier

Slide 16-17 lecture 12

Question 12

What are glial cells?

Answer 12

Constitute half the volume of the brain and outnumber neurons
Can proliferate throughout life (an injury to the nervous system is the usual stimulus for proliferation) whereas neurons do not
Neural stem cells can transform into neurons or glia

Question 13

What are the types of glia in CNS?

Answer 13

Central nervous system
Astrocytes- strength support, promote blood brain barrier
Oligodendrocytes- CNS myelin sheath
Microglia- phagocytes, immune cells
Ependymal cells- epithelia, line the ventricle and canal of spinal cord

Question 14

What are the types of glia in the PNS?

Answer 14

Peripheral nervous system
Schwann cells- PNS myelin sheath
Satellite cells- surround cell bodies of neurons, regulate exchange of materials)
Enteric glia- Schwann cell-like, no myelination

Question 15

What are the types of glial cells in the CNS?

4 of them

Answer 15

Astrocytes- strength support, promote blood brain barrier, regulate growth migration interconnection, scavengers of K+
Oligodendrocytes- CNS myelin sheath
Microglia- phagocytes, immune cells
Ependymal cells- epithelia, like ventricle and canal of spinal cord, production and circulation of cerebrospinal fluid

Question 16

What are the types of glial cells in the PNS?

3 of them

Answer 16

Schwann cells- PNS myelin sheath
Satellite cells- surround cell bodies of neurons, regulate exchange of materials
Enteric glia- Schwann cell-like, no myelination

Question 17

What is myelination?

Answer 17

Oligodendrocytes (CNS) and Schwann cells (PNS)
Leading edge of one of an oligodendrocyte/Schwann cell cytoplasm wraps around the axon many times

Compaction- cytoplasm is then squeezed out if the many cell layers surrounding axon
Myelin- layer on layer of tightly compressed membranes

In CNS, one oligodendrocyte myelinates many axons
In PNS, one schwann cell provides a single myelin segment to a single axon

Slides 3-4 lecture 13
Slide 9 lecture 13
Slide 14 lecture 13

Question 18

What is nerve degeneration and nerve regeneration?

Answer 18

Nerve degeneration (CNS and PNS)
7 steps remember on slide 5 lecture 12

Nerve regeneration (PNS)
Slide 5 lecture 13

Question 19

What affects signal conduction in axons?

Equation

Answer 19

Slide 6-7 lecture 13

Improve conduction by;
Increasing diameter (increase a, decrease Ri)
Myelin (increase Rm)- myelination speed transmission by increasing membrane resistance (Rm) and decreasing membrane capacitance (Cm)
Slide 11 lecture 13

Question 20

What are the 5 steps when a local current comes to a membrane?

Answer 20

1. Signal causes Vm change
2. Cytosol has slight more + charge compared to adjacent inactive regions of the slight - charge cytosol
3. Charge imbalance causes current to flow from excited region to adjacent region of cytoplasm
4. Current flows in complete circuit along pathways of least resistance and spreads:
   A) longitudinally from + to - regions along cyto
   B) across membrane conductance pathways (leak channels)
   C) along extracellular medium back to site of origin (closes current loop)
5. Cause of flow if current the region of membrane immediately adjacent to the active region becomes more depolarized

Question 21

What does intermittent myelination permit?

Answer 21

Saltatory transmission

Slide 9 lecture 13

Question 22

What is the time constant (τm)?

Answer 22

τm= Rm • Cm
Rm- membrane resistance
Cm- membrane capacitance

Influences the spread of voltage changes in time and this the velocity of signal propagation

Shorter the τm the quicker neighbouring regions if membrane will be brought the threshold
Shorter the τm, the faster the speed of impulse propagation

Slide 12 lecture 13

Question 23

Why is salutatory conduction faster?

Answer 23

For unmyelinated axons, conduction velocity increases roughly with the square root of the axons diameter, just as length Constance increase with square root of axon diameter or radius

For myelinated axons, conduction velocity is a linear function of diameter and increases ~6 m/s per 1μm increase in outer diameter

A mammalian myelinated axon with an outer diameter of 4μm has roughly the same impulse velocity as a squid giant axon with a diameter of 500 μm!!

Myelination speeds transmission by increasing membrane resistance and decreasing membrane capacitance

Slide 15 lecture 13

Question 24

What’s the difference between normal conduction and demyelinated conduction?

Answer 24

Normal- slide 16 lecture 13

Demyelinated- slide 17 lecture 13
Decreased conduction velocity
Frequency related block
Total conduction block
Ectopic impulse generation
Increase in mechanosensitivity 

MS is a result of demyelination, causes impaired conduction of action potentials

Question 25

What are astrocytes?

Answer 25

Control the immediate environment of neurons
Brain glycogen (energy storage), contain all enzymes required for glucose metabolism
Provide fuel to neurons in the form of lactate (from glycogen or glucose)
Astrocytes buffet excess extracellular K+
Synthesis of glutamate and GABA, provide glutamine to neurons
Removal of glutamate from synaptic cleft

Question 26

What is astrocyte K+ buffering?

Answer 26

Astrocytes take up K+ in response to elevated [K]o by 3 mechanisms:
Na-K pump
Na/K/Cl cotransporter
Uptake if K and Cl through channels

Astrocyte gap junctions coupling important for spatial buffering

Slide 21 lecture 13

Question 27

What are microglia?

Answer 27

Represent 20% of the total glial cells within the mature CNS
Rapidly activated by injury to the brain: proliferate, change shape, become phagocytic
Activated microglia releasing substances that are toxic to neurons (free radicals and NO)