lecture 7 - Action potentials Flashcards
(40 cards)
The action potential:
- the fundamental unit of information in the nervous system
- “an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls
action potentials Occurs in several types of cell (‘excitable’ cells), including:
Neurons
Muscle cell
Cardiac cells
Endocrine cells (such as pancreatic beta cells)
When we say “short-lasting”…
An action potential lasts 1-2 ms (depending on species, cell type, temperature etc)
types - excitable cells
Many cells have an electrical potential across the membrane.
This is most evident in ‘excitable’ cells. In these cells, the resting membrane potential is negative relative to extracellular space (usually somewhere between -50 and -90 mV)
The phases of an action potential
Membrane potential is dependent on …
leak K+ channels
Voltage-gated Na+ channel structure: an overview
- A large single alpha subunit -> ion-conducting pore
- 4 domains (I-IV), each containing 6 transmembrane segments
- Voltage-sensors on the 4th transmembrane segment
- One or more beta subunits –> regulation of gating, kinetics and expression
Voltage-gated Na+ channel structure: an overview
Voltage-gated Na+ channels open in response to
depolarisation of the membrane potential
Voltage-gated Na+ channel-mediated depolarisation triggers a ‘chain reaction’
During an action potential Na+ channels open, further depolarising the membrane potential, opening more Na+ channels
Voltage-gated Na+ channels rapidly inactivate after activation
Voltage-gated Na+ channels rapidly inactivate after activation
Voltage-gated K+ channels are also activated by depolarisation, but more slowly
Na+ channel inactivate.More slowly activated voltage-gated K+ channels are activated.
Ionic mechanisms of the action potential: a summary
Saltatory condution
Myelination
Myelin sheath insulates the axon from external –ve charge i.e. decreases membrane capacitance and increases resistance across the membrane ->
Faster transmission
However, the myelinated axon still has some capacity to store charge
So the signal (carried by +vely charged cations) degrades; cannot be amplified by additional Na+ channels
The solution: Nodes of Ranvier act as signal boosters
High density of voltage-gated Na+ and K+ channels at Nodes of Ranvier
When a propagating action potential reaches the Node a ‘new’ action potential is initiated.
Known as saltatory conduction (from the Latin saltare, to leap)
Action potential diversity
a )Purkinje neuron in the cerebellum (Equilibrium and fine movement). AP in this region are very brief only last 180 microsecond.
In contrast CA1 neurons of the hippocampus (b) tend to they tend to last more, around 800 microseconds and are followed by a slowly decaying after depolarization
Two examples from the hippocampus
- CA1 pyramidal neuron
- Oriens-Lacunosum moleculare interneuron
CA1 pyramidal neuron
Type: Glutamatergic principal cell
Afferents (Inputs):
CA3 pyramidal cells
Entorhinal cortex
Hippocampal GABAergic neurons
Efferents (Outputs):
Subiculum
Entorhinal cortex
Prefrontal cortex
GABAergic interneurons
Oriens-Lacunosum moleculare interneuron
GABAergic interneuron
Afferents:
Hippocampal pyramidal cells
Medial septum
Other interneurons
Efferents
Distal dendrites of CA1 pyramidal neurons
Other interneurons
Action potential waveforms
Pyramidal neurons – most likely to secrete Glu – Most abundant excitatory NT in the CNS
OLM interneuron – GABAergic – Inhibitory
