Local, Threshold and Action Potential Flashcards
What is a synapse?
A connection and communication point between a presynaptic neuron and a postsynaptic cell (eg. neuron, muscle, gland)
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)
True
How does a chemical signal get converted to an electrical signal?
- 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
What is the difference between leak channels and gated channels?
Leak channels are always open. Gated channels need a stimulus (‘key’) to open them
What are the three types of gated channels?
Chemically-gated ion channels, Voltage-gated ion channels, Mechanically-gated ion channels
What are the different components of the gated sensors?
The stimulus is the ‘key’ that opens the gate.
The channel is the ‘door’
The ions are the ‘people that walk through the door’
Describe the features of chemically-gated ion channels
- Stimulus (‘key’): chemical neurotransmitter that binds to the ion channel
- Channel changes shape (opens)
- Ions cross the membrane driven by their electrochemical gradient
- The neurotransmitter unbinds causing the channel to close
Describe voltage-gated ion channels
- Stimulus (key): membrane depolarises to threshold voltage (eg. -60mV)
- Channel changes shape/opens
- Ions cross the membrane driven by their electrochemical gradient
- Membrane threshold changes causing the channel to inactivate or close
(Note: K+ channels open/close more slowly than Na+ channels)
Describe mechanically-gated ion channels
- Stimulus (key): deformation of the membrane (eg. stretch or squish)
- Channel opens
- Ions cross the membrane driven by their electrochemical gradient
- When membrane returns to original shape the channel closes
Where are gated ion channels on a neuron?
The dendrites and cell body
Is the membrane polarised or depolarised at rest?
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
What is a local potential?
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)
How does a local potential occur?
- 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)
How do excitatory local potentials form (EPSP = excitatory post-synaptic potential)?
- 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)
How do inhibitory local potentials form (IPSP = inhibitory post-synaptic potentials)?
- 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)
What is the reality of neural comminucation?
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)
What are the two ways local potentials are summed?
Spatial summation - summed input from multiple pre-synaptic neurons
Temporal summation - summed input from repeated firing of one pre-synaptic neuron
Why are pre-synaptic inputs summed at the Axon Hillock?
- 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
What are the steps for an action potential?
- (VG) Voltage-gated (Na+) channels open when membrane depolarises to -60mV
- Massive influx of Na+ causes ‘rapid depolarisation phase’ of the action potential
- 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 - VG K+ channels begin to close, but slowly. This permits K+ excess to exit, causing ‘hyperpolarisation phase’ of the action potential
- When all VG K+ channels close, the membrane returns to its resting potential, -70mV
What is an action potential?
A series of sequential changes in membrane potential (it occurs very quickly ~2ms).
