7. transmission and saltatory conduction

Question 1

How is an AP transmitted in the neuron? 5

Answer 1

1. APs aren’t static events
2. they spread along neurons due to movement of na+ through cytosol and excitation of ion channels in nearby sections of the membrane
3. ability to relay signals within neurons along neuronal membranes or between them via synapses
4. Electrical signals within neurons
5. chemical signals - between neurons

Question 2

Describe neuronal electrical anatomy. 4

Answer 2

1. dendrites are the receiving end of the neuron
2. electrical stimulation bu injection of current of dendrite causes attenuated current
3. axons can be several meters in length
4. electrical stimulation of axons doesn’t result in attenuation, the current is not reduced but remains the same strength

Question 3

What are the causes of dendritic attenuation? 6

Answer 3

1. ions can pass out through walls of membrane
2. as signal passes down, capacitance must be charged up, reducing current that can pass through core
3. this can occur when trying to transmit electricity through any system
4. insulation weakens attenuation eg. rubber around transatlantic cables reduces attenuation
5. bigger diameter also reduces attenuation
6. V = Voexp^-x/lambda where v = voltage at any point
   Vo = starting voltage
   x = length of cable
   lambda = length constant, dependent on properties of cable

Question 4

What is cable theory? 3

Answer 4

1. more distance, smaller voltage
2. length constant is the distance over which the voltage drops to 37% of its original value
3. when x = length constant, voltage has dropped by 37%

Question 5

What was the transatlantic attenuation solution? 6

Answer 5

1. for efficient transmission, need cables with big length constants
2. length constant depends on Rm (leakiness) and Ri (conductivity)
3. to decrease Rm, better insulation
4. To increase Ri, better conducting cores
5. To increase d, fatter cables
6. nature does these except changing core

Question 6

Describe attenuation in dendrites. 5

Answer 6

1. not a big problem because short distances are involved, small compared to axon
2. there are also many inputs, big starting signal
3. dendrites can generate action potentials but can’t transmit them
4. dendritic transmission is passive, sodium diffusing through cytoplasm
5. doesn’t involve a wave of action potentials

Question 7

Describe attenuation in axons. 6

Answer 7

1. if axons behaved like dendrites, an axon capable of 1m of passive transmission would be 1cm in diameter
2. due to thick cone layer in skull, we’d need big heads
3. axons have a much higher density of sodium channels than dendrites
4. this allows an AP wave, key to non-attenuated transmission
5. axons can be myelinated or non-myelinated
6. the ap wave only moves in one direction due to inactivation of na+ channels

Question 8

How is axon conduction increased? 5

Answer 8

1. fatter cables not viable in complex organisms with complex nervous systems
2. some primitive invertebrates do this eg cephalopods like squid, octopus, squid neurones have been used as a model axon
3. axon myelinated but not all ove. 1nm long myelinated internodes spaced at regular intervals between very small nodes of ranvier
4. this decreases capacitance by bigger separation of charges
5. current travels very quickly through internodes, effectively jumping from node to node by saltatory conduction

Question 9

What is the difference between myelinated and unmyelinated axons? 4

Answer 9

1. unmyelinated neurons have slow APs as ions must move across membrane and depolarize to threshold
2. in a myelinated neuron, there is diffusion of ions through cytoplasm at myelinated internodes, faster than APs
3. APs at the nodes of ranvier act as booster stations and get AP back up to +40mV
4. diffusion at internodes causes slight attenuation

Question 10

What is rotary sheath migration? 4

Answer 10

1. during (and after) myelination formation, schwann cells and oligodendrocytes act as supporting cells that insulate and provide nutrients
2. the cell wraps itself around the neuron many times, up to 100, in a spiral
3. this happens gradually
4. myelination is specific to axons

Question 11

Describe the density of ion channels in axons. 4

Answer 11

1. at the axon under myelin ie internodal region, sodium channel density is very low, about 100/micron squared
2. in unmyelinated axons, sodium channel density is intermediate, about 1000microns squared
3. there is a high density of sodium channels at the nodes of ranvier, about 10000 per micron squared
4. this decreases rise time of AP so its more rapid

Question 12

What are the types of sensory nerve fiber? 3

Answer 12

1. A - alpha, beta and delta are all myelinated and have larger diameters.
2. fast conduction velocities
3. C is unmyelinated and has a smaller diameter and lower conduction velocity

Question 13

What are the advantages of myelination? 3

Answer 13

1. speed
2. compactness of neurons
3. energy efficiency, ion movements are smaller, sodium potassium pump being used less

Question 14

What diseases alter myelination? 6

Answer 14

1. MS- autoimmune mediated
2. oligodendrocytes are the target of the immune attack
3. demyelinating disease of the cns
4. guillain barre syndome is similar but attacks schwann cells of pns
5. in ms, plaques of scar tissue are left on the brain where the immune system has started to attack
6. progressive and degenerative with attacks and remission, can cause dramatic changes in brain structure over time and lost tissue doesn’t grow back

Question 15

what are the symptoms of ms? 4

Answer 15

1. almost all sufferers lose some sight, 49% initially complain of this
2. the occular nerve is particularly susceptible
3. incoordination in most as motor centre breaks down, more than just slowing down of neurones
4. paresis, parasthesia, genito-urinary/bowl and cerebral symptoms are also common

Question 16

describe conduction in multiple sclerosis.4

Answer 16

1, damaged myelin sheath leads to decreased conduction in axons

2. not the same as reverting to non-myelinated axon
3. sodium channels remain concentrated where nodes of ranvier should be
4. this leads to attenuation of current so transmission isn’t successful