014 + 015 an introduction to neurons and action potentials

Question 1

what are the 4 main structures of neurons?

Answer 1

• soma
• axon
• dendrite
• pre-synaptic terminals

Question 2

what is the soma of a neuron?

Answer 2

• the cell body
• contains nucleus and other organelles involved in metabolism and making proteins
• also receives some signals

Question 3

what is the dendrite of a neuron?

Answer 3

-receives input from many synapses
- tend to taper and covered with spines
- on the soma

Question 4

what is the axon of a neuron?

Answer 4

• transmits signals from the soma/body to the presynaptic terminals
• usually uniform in diameter and can be myelinated or unmyelinated

Question 5

what are the presynaptic terminals of a neuron?

Answer 5

• little terminals louded with synaptic vesicles at the end of an axon
• gives the output of a neuron = release a neurotransmitter to a receptor or another neuron

Question 6

what are the 4 main types of neurons?

Answer 6

• bipolar/interneuron
• unipolar/sensory neuron
• multipolar/motorneuron
• pyramidal cell

Question 7

what transmitters do excitatory/projection neurons use?

Answer 7

• glutamate, acetylcholine, noradrenaline…

Question 8

what transmitters do inhibitory neurons use?

Answer 8

• GABA, serotonin, peptides…

Question 9

what is the neuron pathway in a reflex?

Answer 9

sensory neuron –> interneuron –> motor neuron
or sometimes sensory neuron –> motor neuron

Question 10

what are the differences in action potentials within a neuron vs between neurons?

Answer 10

• within neuron = all or nothing action potential
• between cells = synapse potential varies due to variations in size of terminals

Question 11

what is the resting membrane potential of a neuron?

Answer 11

-70mV

Question 12

what maintains the resting potential of a neuron?

Answer 12

• Na/K ion ATPase pump
• 3 Na out for 2 K in
• so more positive leaving cell so inside cell stays negative

Question 13

describe the cell membrane of a neuron

Answer 13

• phospholipid bilayer
• has ion channels that allows ions to flow down their electrochemical gradienr
• ion pumps that control the resting potential of the cell (Na/K ATPase pump)

Question 14

what are the 2 ion forces opposing each other in a neuron membrane?

Answer 14

• concentration of ions gradient
• membrane potential (electrical charges, + to -) gradient

Question 15

what is the Nernst equation used for?

Answer 15

use the temperature, charges of ions, concentration of ions and constants to work out the membrane potential of a cell

Question 16

what is the Nernst equation?

Answer 16

E =RT/zF . ln Co/Ci

E = membrane potential
R= gas constant
T = temperature (Kelvin)
z = charge of ion
F = Faraday’s constant
ln = natural log
Co = concentration of ion outside cell
Ci = concentration of ion inside cell

Question 17

if you are at 37 degrees, what does the Nernst equation simplify to?

Answer 17

E = 62/z . LogCo/Ci

so for Na, if Co = 100 and Ci = 10

E = 62/1 . 1
= +62mV

Question 18

what is the equilibrium potential and resting concentrations for Cl- in a neuron?

Answer 18

-70mV
(7mM in, 120mM out = channels shut, some leak in)

Question 19

what is the equilibrium potential and resting concentration for K+ in a neuron?

Answer 19

-90mV
(135mM in, 2mM out = channels open, flow out of cell)

Question 20

what is the equilibrium potential and resting concentration for Na+ in a neuron?

Answer 20

+60mV
(18mM in, 150mM out = channels shut)

Question 21

what is the equilibrium potential and resting concentration for Ca2+ in a neuron?

Answer 21

+130mV
(0.1mM in, 120mM out, channels shut)

Question 22

what causes an action potential in a neuron?

Answer 22

• depolarisation from the axon hillock (the connection between soma and axon)
• if the threshold if greater than -50mV then it causes Na+ channels to open so Na moves into the cell, causing further depolarisation as it is positive, producing an action potential

Question 23

what are the 5 phases of an action potential?

Answer 23

1 = resting potential at -70mV with Na/K ATPase pump maintaining the balance (3 Na out, 2 K in), some K leaking out and some Na leaking in
2 = stimulus increases potential to -50mV, depolarisation near axon hillock due to neurotransmitter binding
3 = threshold is reached causing Na channels to open causing more depolarisation up to +40mV producing an action potential
4 = the potential of Na becomes greater than K, so the Na channels shut and the K channels open causing repolarisation down to -70mV
5 = hyperpolarisation of K channels still open letting K out of the cell down to -90mV
then back to 1 when the Na/K ATPase pump reactivate during the refractory period and balance to resting potential at -70mV

Question 24

what can mutations of sodium channels in neurons cause?

Answer 24

• decreased Na channels in interneurons decrease inhibition which can cause severe epilepsy (Dravet syndrome)
• increased Na channels in excitatory neurons can lead to too much excitation = also cause epilepsy

Question 25

what can mutations in potassium channels of neurons cause?

Answer 25

- some can lead to hyper excitable motor neurons and thus over active muscles (myotonia) - mutations in the CNS can cause neurons to be too excitable and lead to epilepsy

Question 26

describe the phases of an action potential in the heart ventricle

Answer 26

phase 4 = resting membrane potential, K channels are open so K is leaking out so resting potential at about -90mV phase 0 = stimulus causing depolarisation cause Na channels to open causing increase in depolarisation, the Na channels inactivate almost as soon as they open, up to +50mV phase 1 = notch, partial repolarisation as Na channels inactivate, K channels rapidly open allowing efflux of K out phase 2 = plateau, K channels remain open, L type Ca channels now open, allowing influx of Ca, causing a plateau of positive in and out of cell (at around +40mV) phase 3 = repolarisation, Ca channels shut and K remain open continuing efflux of K down to -90mV

Question 27

describe the phases of an action potential in the heart SA node

Answer 27

phase 4 = unstable slow depolarisation due to opening of HCN (Hyperpolarised-activated cyclic nucleotide-gated channels) channels (If = funny current) allowing Na influx, T type Ca channels open towards end of phase allowing Ca influx ( from -60mV to -40mV) phase 0 = reach threshold of about -40mV and L type Ca channels open causing depolarisation up to +10mV (HCN channels shut) phase 3 = repolarisation due to inactivation of Ca channels and opening of K channels causing K efflux down to -60mV

Question 28

what current/channel controls heart rate via the SA node?

Answer 28

If = funny current HCN (hyperpolarised-acivated cyclic nucleotide-gated) channel - Na channel = activated by hyperpolarisation (-60mV)

Question 29

why are heart action potentials slower than neuron action potentials?

Answer 29

- heart action potentials rely on Ca channels which are much slower release than Na channels in neurons

Question 30

why does an action potential in a neuron only travel in 1 direction?

Answer 30

- Na channels open locally in response to stimulus/depolarisation - this causes Na to move into the cell causing further depolarisation which causes neighbouring Na channels to open - upstream channels inactivate after they have opened and K channels open and it repolarises upstream as the depolarisation continues downstream

Question 31

what does the rate of propagation of action potentials depend on?

Answer 31

diameter of axon ( the larger the faster / less resistance) myelination

Question 32

how does myelination increase the speed of propagation of action potentials down an axon?

Answer 32

- Schwann cells wrap around the axon leaving gaps in between called nodes of Ranvier - the depolarisation jumps between the nodes of Ranvier which is faster than going through the whole axon - ion channels are also not open where it is myelinated, only at the node of Ranvier so less ATP and time is wasted opening and closing channels

Question 33

what happens to the action potential at the end of the axon?

Answer 33

- the depolarisation is converted to chemical signals/release of neurotransmitters at the pre-synaptic terminals across the synapse to another neuron or a target tissue

Question 34

describe the steps of action potential causing neurotransmitter release at the synapse

Answer 34

1. action potential causes depolarisation at the presynaptic terminals 2. this causes voltage-gated Ca channels to open 3. Ca enters the presynaptic terminal which triggers a conformational change in the docking and SNARE proteins causing the vesicles containing neurotransmitters to fuse to the plasma membrane and release the neurotransmitter into the synapse

Question 35

describe the recycling process of vesicles in the presynaptic terminal

Answer 35

1. vesicle fuses with plasma membrane and exocytose neurotransmitters 2. vesicle endocytoses back into the terminal 3. vesicle translocates, and fuses into an endosome 4. budding off the endosome forms individual vesicles 5. vesicle uptakes neurotransmitter 6. vesicle moves to plasma membrane and docks 7. vesicle fuses with the plasma membrane...

Question 36

what happens if you block K channels in a neuron?

Answer 36

- keeps K stuck in the cell, causing depolarisation, cannot repolarise

Question 37

what diseases are associated with the presynaptic terminal?

Answer 37

-congenital myasthenic syndromes cause impaired endocytosis of vesicles - LEMS (Lambert-Eaton myasthenic syndrome) attacks presynaptic Ca channels (autoantibodies) - Botulinum and tetanus toxins affect SNARE proteins involved in vesicle fusion