Week 2: AP II & Cell-to-Cell Signalling Flashcards
What triggers neurotransmitter release at the axon terminal?
An action potential opening voltage-gated calcium channels.
What does calcium influx cause in the presynaptic cell?
Fusion of neurotransmitter vesicles with the membrane, releasing neurotransmitters.
What kind of ion channels do neurotransmitters activate on the postsynaptic cell?
Ligand-gated ion channels.
What happens if the local depolarization in the postsynaptic cell reaches threshold?
Voltage-gated Na+ channels open and an action potential is initiated.
What defines a graded potential?
A depolarization that does not reach threshold and fades away.
What defines the ‘all-or-nothing’ principle of action potentials?
If threshold is reached, the AP always reaches the same size regardless of stimulus strength.
What aspect of action potentials encodes stimulus intensity?
Frequency of firing, not amplitude.
What is the absolute refractory period?
The period during which no new AP can be initiated due to inactivated Na+ channels.
When does the absolute refractory period occur?
During the peak of depolarization and repolarization (1–2 ms).
What is the relative refractory period?
A period when another AP is possible but requires a stronger stimulus.
Why does the relative refractory period require a stronger stimulus?
Because some Na+ channels are still recovering and the membrane is hyperpolarized.
What is electrotonic flow?
Passive current spread that is fast but inefficient due to ion leakage.
Why is electrotonic flow insufficient alone for AP transmission?
It degrades over distance and cannot reach axon terminals.
How is AP propagation sustained along the axon?
By generating new APs at adjacent regions along the membrane.
Why does AP only move in one direction?
Because the area behind is in a refractory state and cannot fire again immediately.
What are two main factors affecting conduction velocity?
Axon diameter and myelination.
How does increased axon diameter affect AP speed?
Reduces resistance, allowing faster ion flow and AP propagation.
What is the role of myelin in AP conduction?
Acts as an insulator, enabling saltatory conduction between nodes of Ranvier.
Where are voltage-gated Na+ channels located in myelinated neurons?
At nodes of Ranvier.
What is saltatory conduction?
AP jumps from node to node, increasing speed and efficiency.
What are A-alpha fibers used for?
Proprioception in skeletal muscle; very fast (80–120 m/s).
What are A-beta fibers used for?
Touch and vibration; moderately fast (35–75 m/s).
What are A-delta fibers used for?
Fast pain and temperature; slower (5–30 m/s).
What are C fibers used for?
Dull pain and itch; slowest (0.5–2 m/s) and unmyelinated.
What happens in demyelinating diseases like MS?
Loss of myelin disrupts AP propagation, causing signal failure.
What are the two general modes of cell-to-cell signalling?
Electrical and chemical signalling.
What is the key feature of electrical synapses?
Bidirectional ion flow via gap junctions; very fast and synchronous.
Which tissues commonly use electrical synapses?
Heart, gut, liver, and some brain regions.
What is chemical synaptic signalling?
Use of neurotransmitters to convey signals between neurons.
How do neurotransmitters initiate a cellular response?
By binding to receptors that trigger a conformational change.
What determines receptor specificity?
Structure and shape of the receptor’s binding site.
What are the two broad classes of receptor location?
Membrane-bound and intracellular.
What type of chemicals use intracellular receptors?
Lipid-soluble agents like steroids and thyroid hormones.
What are ionotropic receptors?
Receptors that directly form ion channels, producing rapid responses.
What are metabotropic receptors?
Receptors linked to G-proteins; produce slower, second-messenger responses.
What activates G-protein coupled receptors (GPCRs)?
Binding of a ligand causes GDP to be replaced by GTP on the alpha subunit.
What happens when a G-protein is activated?
Alpha subunit dissociates and activates effector proteins to generate second messengers.
Name three common second messengers.
cAMP, IP3, and DAG.
What is the role of calcium as a second messenger?
Triggers cellular responses like neurotransmitter release or gene expression.
What is receptor desensitization?
A reduction in response to a chemical signal despite its presence.
What is receptor sequestration?
Temporary removal of receptors into vesicles inside the cell.
What is receptor downregulation?
Long-term decrease in receptor synthesis.
What is an agonist?
A molecule that binds to a receptor and activates it.
What is an antagonist?
A molecule that binds to a receptor and blocks activation.
What determines a ligand’s affinity for a receptor?
How strongly it binds to the receptor’s binding site.
What allows multiple effects from the same neurotransmitter?
Different receptor subtypes (e.g., muscarinic vs nicotinic for acetylcholine).
What is the role of G-protein alpha subunit?
Activates effector proteins when bound to GTP.
What is a key feature of second messenger systems?
Signal amplification and longer-lasting effects.
What can intracellular receptors do once activated?
Bind DNA and alter gene transcription.
