Action Potential and Synaptic Transmission

Question 1

Action potential phases

Answer 1

Question 2

Action potential properties

Answer 2

• Generator potential: causes a depolarisation of the membrane
• There is a threshold for generation of action potential; i.e. small random fluctuations in the membrane potential are not interpreted as useful information.
• The all-or-none law ensures full size of action potential.
• There is frequency coding of the strength and latency of the initial stimulus

Question 3

Absolute refractory period

Answer 3

Period in which is impossible to initiate another AP (~ 1msec)

Question 4

Relative refractory period

Answer 4

The amount of current required to depolarize the neuron to AP threshold is elevated above normal

Question 5

Action potential process

Answer 5

• Depolorisation = reduction in membrane potential
• Threshold = -40mV

Question 6

Voltage gated sodium channel properties

Answer 6

• They open with little delay – making the rising phase occur quickly
• They stay open for about 1 msec and then close (inactivate) – partly explaining why the AP is so brief
• They cannot be opened again by depolarization until the action potential returns to a negative value near threshold (refractory period)

Question 7

Propagation of action potential

Answer 7

• APs can propagate at different speeds - 0.1 m/sec to 100 m/sec
• Over long distances
• Without loss
• The thicker an axon, the more ions can move down axon inside membrane → AP adjacent Na-channels can be opened earlier → makes AP propagation faster
• Myelinated axons are much faster than non-myelinated ones → myelinated: up to 100m/sec; unmyelinated: up to 1m/sec

Question 8

Saltatory conduction

Answer 8

• Propagation of action potentials along myelinated axons from one node of Ranvier to the next

Question 9

Synaptic cleft

Answer 9

Small gap between the axon terminal of the presynaptic neuron and the membrane of the postsynaptic cell

Question 10

Synaptic vesicles

Answer 10

Membrane-bound spheres filled with neurotransmitter molecules

Question 11

Receptor

Answer 11

• Protein which are configured to receive a particular endogenous chemical
• Located on postsynanptic cell

Question 12

Synaptic transmission

Answer 12

• Step 1: Neurotransmitter is synthesized in the presynaptic neuron
• Step 2: Neurotransmitter is stored in a synaptic vesicle
  
  • Requires the help of transporters – proteins in the vesicle membrane
• Step 3: Neurotransmitter release
  
  • Action potential → depolarizes the membrane → voltage-gated Ca2+ channels open increased [Ca2+] → Vesicles release neurotransmitters by exocytosis
  • The vesicle membrane is then recovered by endocytosis– vesicle is recycled and filled with neurotransmitters
• Step 4: Neurotransmitter binds to receptor in postsynaptic membrane
• Step 5: Removal of neurotransmitters from the synaptic cleft

Question 13

Excitatory/inhibitory postsynaptic potentials

Answer 13

When a neurotransmitter binds to its receptor on a receiving cell, it causes ion channels to open or close. This can produce a localized change in the membrane potential—voltage across the membrane—of the receiving cell.

• In some cases, the change makes the target cell more likely to fire its own action potential. In this case, the shift in membrane potential is called an excitatory postsynaptic potential, or EPSP. → depolarising
• In other cases, the change makes the target cell less likely to fire an action potential and is called an inhibitory post-synaptic potential, or IPSP.

If the depolarisation caused by the EPSP goes beyond the threshold, this leads to an action potential in the postsynaptic neuron. Typically one presynaptic AP does not lead to postsynaptic AP.

Question 14

Principles of chemical synapses

Answer 14

• A chemical synapse can either be excitatory or inhibitory – never both at same time
• Unidirectional → information flows from pre- to postsynaptic cell (not back)
• Different synapses can be differently effective; depends on
  
  • Site of synapse
  • Neurotransmitter used
  • Ion channels activated by neurotransmitter

Question 15

Spatial summation

Answer 15

Integration of postsynaptic potentials that occur in different locations but at about the same time.

Question 16

Temporal summation

Answer 16

The integration of postsynaptic potentials that occur in the same place but at slightly different times

Question 17

Electrical synapse

Answer 17

At electrical synapses, unlike chemical synapses, there is a direct physical connection between the presynaptic neuron and the postsynaptic neuron. This connection takes the form of a channel called a gap junction, which allows current—ions—to flow directly from one cell into another.

→ Bidirectional

→ Very fast

→ Relatively unspecific for different ions