Local, Threshold and Action Potential

Question 1

What is a synapse?

Answer 1

A connection and communication point between a presynaptic neuron and a postsynaptic cell (eg. neuron, muscle, gland)

Question 2

Any neuron can be pre-synaptic and post-synaptic at the same time, the labels just depend on what synapse you focus on (T/F)

Answer 2

True

Question 3

How does a chemical signal get converted to an electrical signal?

Answer 3

• A chemical signal (neurotransmitter) binds to and opens chemically gated ion channels
• Ions (K+ or Na+) flow in or out, changing the voltage at a localised area of the membrane
• If the membrane voltage reaches -60mV at the Axon Hillock…
• … an electrical signal (action potential) begins

Question 4

What is the difference between leak channels and gated channels?

Answer 4

Leak channels are always open. Gated channels need a stimulus (‘key’) to open them

Question 5

What are the three types of gated channels?

Answer 5

Chemically-gated ion channels, Voltage-gated ion channels, Mechanically-gated ion channels

Question 6

What are the different components of the gated sensors?

Answer 6

The stimulus is the ‘key’ that opens the gate.
The channel is the ‘door’
The ions are the ‘people that walk through the door’

Question 7

Describe the features of chemically-gated ion channels

Answer 7

1. Stimulus (‘key’): chemical neurotransmitter that binds to the ion channel
2. Channel changes shape (opens)
3. Ions cross the membrane driven by their electrochemical gradient
4. The neurotransmitter unbinds causing the channel to close

Question 8

Describe voltage-gated ion channels

Answer 8

1. Stimulus (key): membrane depolarises to threshold voltage (eg. -60mV)
2. Channel changes shape/opens
3. Ions cross the membrane driven by their electrochemical gradient
4. Membrane threshold changes causing the channel to inactivate or close
   (Note: K+ channels open/close more slowly than Na+ channels)

Question 9

Describe mechanically-gated ion channels

Answer 9

1. Stimulus (key): deformation of the membrane (eg. stretch or squish)
2. Channel opens
3. Ions cross the membrane driven by their electrochemical gradient
4. When membrane returns to original shape the channel closes

Question 10

Where are gated ion channels on a neuron?

Answer 10

The dendrites and cell body

Question 11

Is the membrane polarised or depolarised at rest?

Answer 11

Polarised!
Because there are lots of negatively charged proteins inside the cell (organelles) and the Na+/K+ pumps move 3Na+ out for every 2K+ in so it is a negative net charge overall

Question 12

What is a local potential?

Answer 12

a change in membrane potential voltage at a localised area of the dendrite or cell body membrane. (These can also be described as graded potentials)

Question 13

How does a local potential occur?

Answer 13

• neurotransmitter binds to and opens chemically gated ion channels on dendrites and/or cell body
• allowing (Na+) ions to move in or allowing (K+) ions to move out
• Thus, local potentials can either be inhibitory (IPSP) or excitatory (EPSP)

Question 14

How do excitatory local potentials form (EPSP = excitatory post-synaptic potential)?

Answer 14

• A presynaptic neuron releases an excitatory neurotransmitter (eg. ACh or NE)
• When neurotransmitter binds it opens chemically-gated Na+ channels
• Na+ enters post-synaptic cell, causing depolarisation (membrane becomes more +ve)

Question 15

How do inhibitory local potentials form (IPSP = inhibitory post-synaptic potentials)?

Answer 15

• A presynaptic neuron releases an inhibatory neurotransmitter (eg. GABA)
• When neurotransmitter binds, it opens chemically-gated K+ channels
• K+ exits post-synaptic cell, causing hyper polarisation (membrane becomes more -ve)

Question 16

What is the reality of neural comminucation?

Answer 16

Usually a post-synaptic neuron receives input from multiple pre-synaptic neurons. The summed effect of all EPSP’s and IPSP’s determines if the post-synaptic neuron activates (fires an action potential)

Question 17

What are the two ways local potentials are summed?

Answer 17

Spatial summation - summed input from multiple pre-synaptic neurons
Temporal summation - summed input from repeated firing of one pre-synaptic neuron

Question 18

Why are pre-synaptic inputs summed at the Axon Hillock?

Answer 18

• The Axon hillock has a high density of voltage gated channels
• Threshold potential (-60mV) is the key that opens voltage-gated sensors)
• Thus, if summation occurs to or above -60mV, voltage gated Na+ channels open at the axon hillock

Question 19

What are the steps for an action potential?

Answer 19

1. (VG) Voltage-gated (Na+) channels open when membrane depolarises to -60mV
2. Massive influx of Na+ causes ‘rapid depolarisation phase’ of the action potential
3. VG channel inactivate (get blocked) and Na+ entry stops. VG K+ channels activate, K+ exits, causing the ‘repolarisation phase’ of the action potential
   3.5. VG Na+ channels begin to close
4. VG K+ channels begin to close, but slowly. This permits K+ excess to exit, causing ‘hyperpolarisation phase’ of the action potential
5. When all VG K+ channels close, the membrane returns to its resting potential, -70mV

Question 20

What is an action potential?

Answer 20

A series of sequential changes in membrane potential (it occurs very quickly ~2ms).