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MBB 322 Week 5

Terms in this set (18)

what are the 3 types of neurons?
Sensory (or afferent) neurons

- carry information from receptors to the CNS from PNS


Motor (or efferent) neurons

- carry instructions from the CNS to muscles, glands and effector organs in the PNS


Interneurons

- largest group

- located within the CNS

- connect sensory and motor neurons
what are the parts of a mammalian neuron?

what is an action potential?

what is the resting membrane potential?
cell body, dendrites, axon and nerve terminal with terminal branches


Action potential: a rapid, transient, all-or-none electrical activity propagated in the plasma membrane of excitable cells


-70mV because of resting K channels that slow leak K+ to maintain membrane potential
how is signal transmitted from one neuron to the next?
action potential gets to nerve terminal in presynaptic cell and causes release of NT mlcs

the synaptic vesicle fuses with the membrane of the presynaptic and exocytosis of NT occurs

NT fuses through synaptic cleft

these NT bind and open transmitted gated ion channels in the plasma membrane of the postsynaptic cell
How is an action potential generated and propagated?
Opening of voltage gated Na+ channels results in depolarization of the membrane

in the resting state the cytosolic face of the membrane is neg and when depolarized the cytosolic face is positive

The inward movement of Na+ ions is energetically very favourable because Na+ will move along its [ ] gradient to go into the cell and it will repel the pos charge on the outside of the membrane

so electrochemical gradient makes this favourable
what are the 3 states of a voltage gated channel?
closed, nothing will move through them and membrane is at resting potential or below threshold

open, influx of ions will cause membrane pot to increase and channel to open so ions can move through (depolarization)

inactive (refractory period), repolarization occurs and nothing can move through, something called the channel inactivating segment will plug the channel so nothing can get through even though its open (slide 12)
How is an action potential propagated unidirectionally?
Action potentials are propagated unidirectionally and the different states allow for this propagation

look at slide 16

the channels will open while the channel behind it is closed, repolarized or depolarized

because of refractory period, open channels can only lead to opening of downstream channels after it
How is the original membrane potential restored?
Two mechanisms work in concert to repolarize the membrane back to its original resting state.

- Inactivation of the Na+ channels.

- Opening of voltage-gated K+ channels (also called delayed K+ channels).
Many vertebrate neurons are insulated by a myelin sheath

what is the purpose of myelin sheath?

where is myelin sheath made?
Many vertebrate neurons are insulated by a myelin sheath which increases the speed at which an axon can propagate an action potential.

Insulates the axonal membrane so that little current can leak across.

velocity of action potential in myelinated axon vs non-myelinated axon is around 10-100 times greater


Schwann cells in peripheral nerves

Oligodendrocytes in the CNS
what are nodes of ranvier and what is saltatory conduction?
Clusters of voltage gated Na+ channels and Na+/K+ ATPases are localized at nodes of Ranvier.

Action potentials jump from node to node: saltatory conduction.


look at diagram on slide 19
Synapses

what are Synapses?

what are most NT made of?
Neuronal signals are transmitted from cell to cell at synapses, specialized sites of contact.

Neurotransmitters are released by the presynaptic cell by exocytosis into the synaptic cleft.

Bind to either ligand gated ion channels or G-protein coupled receptors on the postsynaptic cell.

Neurotransmitter is rapidly removed after it has been secreted.

NT are taken back into membrane after AP stops


Most neurotransmitters are derived from amino acids.
how are NT imported into synaptic vesicles?

how do Synaptic vesicles dock at the plasma membrane?
using H+ antiporters

I think H+ moves out of vesicle along its [ ] gradient and then NT can move in using that energy


look at slide 25

number of vesicles are ready to be released and docked on PM

SNARE complex will dock the cell/ vesicle at the PM

botulinum stops the SNARE complex form helping the cell fuse
after the synaptic vesicle docks, what does the AP trigger?
The action potential opens voltage gated Ca2+ channels

slide 26

when Ca2+ moves into nerve terminal, this triggers the exocytosis of NTs into synaptic cleft

Ca2+ will interact wth the inhibitor (Botulinum toxin) of the SNARE complex and inhibits it, so SNARE can function to help the vesicle exocytose
after the NT is released, how is everything cleaned up?
signals are triggered quickly and need to be turned off quickly

The vesicles are recycled by endocytosis and the neurotransmitter must be cleared from the synaptic cleft


induction of clathrin-mediated endocytosis, proportion of membrane will be re endocytosed and this is how the endo of vesicle happens

clathrin forms coat along membrane and invaginates it

vesicles are now empty of NT


to reuptake NT they use sodium- NT transporter, Na moves in and so does NT

w/o NT reuptake we will have prolonged stimulation of post synaptic cell,


look at slide 27
Neurotransmitters can either be inhibitory or excitatory

explain how both classes work?
Excitatory neurotransmitters:

- Open cation channels

- Cause an influx of Na+ and Ca2+ depolarizing the postsynaptic membrane.


Inhibitory neurotransmitters:

- Open either K+ or Cl- channels making it harder for excitatory neurotransmitters to depolarize the postsynaptic membrane
now lets look at an example of muscles

explain what the acetylcholine receptor is like?
The acetylcholine receptor at the neuromuscular junction (NMJ) is a transmitter gated cation channel

it has neg AA in pore to attract pos ions, helps with Ca transport

it also has leucine (Hphobic), if these are close to each other, this closes gate of cation channel

acetylcholine will cause conformation change that separates leucine, to allow Ca flow
explain the 5 ion channels involved in causing the influx of Ca2+ in the muscle cell?
first acetylcholine is released from the presynaptic cell into synaptic cleft, Ca2+ channel cause this release

the it binds to acetylcholine receptors which causes a conformation change and allows Na+ to move into the muscle cell (so this is acetylcholine gated)

then this causes depolarization of membrane and causes Na voltage gated channels to open and more Na+ moves into the cell

this will trigger the voltage gated Ca2+ channels in the T-Tubules to open, something moves in to trigger next receptor but they don't say

this ligand binds to the Ca2+-gated channels (not voltage), and Ca2+ is released from the SR into the muscle cell


look at slide 32, ready yellow note
Structure of the skeletal muscle sarcomere

what are the components of sarcomere and muscle
linear actin is connected by Z disc and b/t filaments is myosin that forms multiple protein structures called the M line (slide 33)

sarcomere is made of actin and myosin filaments

rmr Ca2+ and ATP causes these to contraction and shorten because myosin pulls the actin in the pos direction

actin has pos and neg end


sarcomere connect to make myofibril which make the muscle cells, which make the muscle fibres which make muscles
What role does Ca2+ have in regulating contraction?
Binding of Ca2+ by troponin causes tropomyosin to shift position along the actin filament exposing myosin binding sites on actin.

actin filament is nvr naked its always covered by actin binding protiens

cover actin and prevent actin-myosin interaction
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