Lecture 9: Nerve conduction and neurotransmitter release Flashcards
What is Resting State?
The membrane is at the RMP (–65 mV)
What is Rising Phase?
a rapid depolarisation of Vm
What is Overshoot?
inside of the neuron is now positive relative to the outside.
What is Falling?
rapid repolarisation of Vm.
What is Undershoot?
inside of the neuron is now more negative than at rest (hyperpolarised)
Where do action potentials start?
Axon hillock due to the high proportion of voltage-gated Na+ and K+ channels there
What happens to the high proportion of Voltage gated channels down the axon?
They continue down the axon to the terminal
How is the rising phase initiated?
by the opening of hundreds or even thousands of Na+ channels simultaneously.
Where is an action potential initiated?
at the hillock propagates in one direction towards the terminal
How is diameter an important factor in axons?
The wider the diameter, the more ions flow through the axons
How can an action potential be regenerated?
- The first action potential brings some Na+ into the neuron.
- The segment of membrane immediately in front is still at rest (–65 mV).
- The Na+ is electrically attracted to this region and depolarises the membrane.
- Another action potential is then generated.
Can action potentials travel in both directions?
yes
What do Oligodendrocytes do?
help wrap myelin sheath around the axons, which speeds up the conduction of an action potential down the axon
What do glial cells provide in regard to myelin?
Myelin Sheath
What is action potential regeneration at each node called?
Saltatory Conduction
Does myelin sheath affect the rate of an action potential?
Saltatory conduction in a myelinated axon is significantly faster
Where are action potentials only generated?
node of Ranvier
What is the node of Ranvier?
spaces between divisions of myelin- exposed allow sodium and potassium to move
How is action potentials generated at the nodes?
Voltage-gated Na+ and K+ channels gather in high density at the nodes, to trigger a spike
Do the nodes have any leakage when action potentials are transmitted?
Very minimal leakage
The conduction of the action potential between any two nodes is completely passive
What does myelin provide Na and K+?
myelin provides the pumps with a rest
How does size affect conduction velocity?
Large diameter axons can pass more current
How does myelination affect conduction velocity?
The myelin sheath insulates axons.
How does temperature affect conduction velocity?
This affects the diffusion/ movement of ions in the axon.
What is the Golgi stain?
Golgi developed a method to impregnate nervous tissue with metal (silver).
In this way, a limited number of random cells were stained black.
How did Cajal relate to the Golgi stain?
Using Golgi’s stain, Cajal described the morphology of cells located within many different structures. He also produced exquisite drawings of neurons and their various connections
What is the Reticular Doctrine? (Golgi)
Neurons being hard-wired together to form a continuous network, similar to the arteries and veins of the circulatory system
What is the Neurones Doctrine? (Cajal)
The neurone being the functional unit in the brain
neurones communicate through chemicals and not continuity.
What is the process of information transfer at the synapse called?
Synaptic Transmission
What is an electrical synapse?
information can pass between cells in either direction. (Golgi)
Hard wiring between two physical thing sin the brain. Direction can go both ways
What is a chemical synapse?
information flow is unidirectional.
What are transmissions at electrical synapses like?
Fast
neurons are coupled together by gap junction
The movement of ions and small molecules can move across an electrical synapse is bidirectional
How do ions and molecules pass through a gap junction?
Gap junctions allow them to pass directly across the membrane
Is there a delay at an electrical synapse?
There is no measurable delay in signal propagation across an electrical synap
How does depolarisation spread across a gap junction?
Depolarisation can spread very quickly across gap junctions.
Which important reflexes do electrical synapses mediate?
coordination of heart muscle contractions and other reflexes
Where are electrical synapses rare?
between mammalian neurones
Where are electrical synapses common?
glia, epithelial, smooth cardiac muscle, liver and glandular cells
Which neurones are electrically coupled?
Pre- and post-synaptic
How wide is the synaptic cleft?
20-50 nm
What is the cleft filled with?
matrix of fibrous extracellular protein that anchors the pre- and postsynaptic regions together. The protein help maintain the synapses
How is information flow transmitted through chemical synapses?
Information flow at a chemical synapse is unidirectional and involves the conversion of an action potential into a chemical message
What does the presynaptic terminal consist of?
- Synaptic vesicles (spherical) are small (50 nm in diameter) and contain neurotransmitter.
- Secretory granules contain 2 larger peptide transmitters.
- The active zone is the site of neurotransmitter release.
- Mitochondria provide energy for synaptic transmission.
What are receptors in the postsynaptic density responsible for?
Receptors in the postsynaptic density are responsible for converting chemical messages back into electric or metabolic signals.
1. Postsynaptic receptors respond to transmitters by altering Vm or changing cell metabolism. 2. The postsynaptic membrane is studded with receptor proteins (postsynaptic density)
Why are chemical synapses more important?
Due to amplification
What can a brief pre-synaptic action potential trigger?
a large postsynaptic response
What is an axodendritic synapse?
this is where the axon makes contact on the dendrite of the next neuron
What is a axon somatic synapse?
this is where the axon makes contact on the soma of the next neurone
What is an axoaxonic synapse?
this is where the axon makes contact on the axon of the next neuron
What does Gray’s type 1 mean?
this is a synapse with asymmetrical membrane differentiation (excitatory)
What does Gray’s type 2 mean?
this is a synapse with symmetrical membrane differentiations (inhibitory)
What are the three major classes of neurotransmitters?
Amino acids: small and fats acting, found widely in the brain
Amine neurotransmitters - intermediate in side, both fast and modulatory
Peptide neurotransmitter - large, require protein synthesis to be produced, specialised in actions, slow and specialised.
Which NTs are made in the terminal bouton?
small- and intermediate-sized neurotransmitters are made directly in the axon terminal
1. Precursor molecule interacts with an enzyme in the cytosol at the terminal region and is converted into neurotransmitter. 2. Neurotransmitter is packaged and stored in synaptic vesicles.
How are neurotransmitters made in the soma?
- Precursor peptide is synthesised in the rough ER.
- In the Golgi apparatus, the new peptide is sorted, activated and packaged.
- Secretory granules bud away from the golgi apparatus
- The granules are transported to the terminal via axoplasmic transport.
Peptide neurotransmitters are made in the soma and require protein synthesis
What happens to the vesicle in producing an action terminal?
- Vesicle is docked along inside of presynaptic membrane.
- An action potential depolarises terminal opening voltage- gated Ca2+ channels.
- Ca2+ rapidly floods into the terminal (10,000 times greater concentration outside the neuron!).
- Ca2+ interacts with SNARE proteins causing the vesicle to fuse with the presynaptic membrane and spill its contents (exocytosis)
- The vesicle membrane is recycled (endocytosis).
How does calcium play in the part of releasing the synaptic vesicle?
Calcium binds to synaptotagmin (a calcium sensor) which then stimulates the v- and t-SNAREs to combine into an α-helical-shaped complex.
The SNARE complex forces the two membranes together (fusion) and then pulls them apart to spill the vesicle contents into the synaptic cleft (exocytosis).
What happens every time an action potential causes a neurotransmitter to be released?
more membrane is added to the presynaptic terminal
Unchecked, the terminal would expand to a very large size.
To prevent this, vesicle membrane is recycled via a process known as endocytosis.
