Synapses and Neurotransmitters

Question 1

What is the neuromuscular junction?

Answer 1

Chemical synapse between a motoneuron innervating a muscle fiber.

Question 2

What are the basic events that occur at a neuromuscular junction?

Answer 2

1. Action potential depolarizes motoneuron’s synaptic terminal.
2. Depolarization causes voltage-gated Ca2+ channels to open and Ca2+ enters synaptic terminal.
3. Ca2+ entry leads to exocytosis of vesicles filled with Ach
4. Ach diffuses across synaptic cleft to the muscle, binds with nicotinic receptor, depolarizes end-plate of muscle cell causing action potential
5. Ach removed form synaptic cleft by acetylcholinesterase (which degrades it) and diffusion

Question 3

What is happening at the NMJ in Myasthenia gravis?

Answer 3

Failure of end-plate depolarization to create an action potential in the muscle cell - Muscle weakness.

Question 4

What are two features shared by all synapses?

Answer 4

1. The chemical transmitter is stored in vesicles

2. Ca2+ is the trigger for neurotransmitter release

Question 5

What proteins are Ca2+ acting on to produce release of neurotransmitters? How?

Answer 5

SNARE proteins – Ca2+ changes their conformation, which leads to fusion of synaptic vesicles with presynaptic cell membrane.

Question 6

How does Botulinum cause flaccid muscle paralysis?

Answer 6

Cleaves SNARE proteins, making transmitter release impossible.

Question 7

equilibrium potentials for ions involved in synaptic potentials?

Answer 7

E(Cl) = -90mV

E(k) = -100mV

E(Na) = +70mV

Question 8

Excitatory postsynaptic potentials (EPSPs)

Answer 8

depolarize the cell via opening of non-selective cation channels that equalize at 0mV

Question 9

Inhibitory postsynaptic potentials (IPSPs)

Answer 9

hyperpolarize the cell by increasing K+ or Cl- permeability

Question 10

What is the role of the interferon in a reflex arc?

Answer 10

creates inhibitory postsynaptic potentials (IPSPs) in the motoneurons innervating the flexor muscles (creating relaxation of the muscle opposing the muscle receiving stretch stimulus

Question 11

ionotropic receptors

Answer 11

receptors for transmitters as well as ion channels - create fast/brief synaptic potentials b/c there are no intermediate steps between binding of receptor and change in conductance

Question 12

What are the proteins that clear released neurotransmitter at synaptic membrane (not at NMJ)?

Answer 12

transporters at the glial cells near the synapse the reuptake the transmitter

Question 13

What is receptor desensitization?

Answer 13

closure of associated channel despite continued presence of neurotransmitter

Question 14

Metabotropic receptors

Answer 14

conductance via GCPR cascade that alters ion channel activity

These create slow synaptic potentials b/c of the steps required for G-protein cascade.

conductance can persist well after the transmitter has been removed

Question 15

What is the difference in generation of action potential between NMJ and CNS neurons?

Answer 15

NMJ: single presynaptic action potential can create an action potential on a muscle

CNS: requires many inputs to be integrated to create an action potential

Question 16

Temporal integration

Answer 16

integration from repetitive activation of one synaptic input - pile up over time to create a larger response

Question 17

Spatial integration

Answer 17

integration from different synaptic inputs - when produced at a similar time, stimuli will be combined in dendrites/soma and create a larger response

Question 18

Presynaptic inhibition

Answer 18

action potential arrives at inhibitory neuron, which then inactivates Ca2+ channels on the presynaptic membrane so that when an action potential arrives at the presynaptic membrane less calcium enters, less neurotransmitter is released, and the effect on the postsynaptic membrane is reduced

Question 19

Postsynaptic inhibition

Answer 19

combining of IPSPs from inhibitory inputs and EPSPs from excitatory inputs - if IPSPs outweigh EPSPs, the firing rate will decrease/inhibition occurs

Question 20

Where are electrical synapses found?

Answer 20

sensory system like the retina, non-neural systems like the liver and heart

Question 21

How is a signal transmitted at an electrical synapse?

Answer 21

signals are transmitted from one cell to another by direct flow through gap junctions

Question 22

What do gap junctions allow to move between cells?

Answer 22

ions and second messengers

Question 23

What is the reversal potential for gap junctions and what does this mean for movement of voltage between cells?

Answer 23

reversal potential is 0mV - means that voltage between two cells connected by a gap junction will equalize - if one has a higher voltage, ions will flow into the opposite