L8-13: Excitable Cells Flashcards
What charge is the inside of the axon membrane, relative to the outside?
negative
How is the negative charge of the membrane maintained?
high permeability of K+
Active transport of Na+
How does the Na+ cause a slight charge on the outside?
Because it is electrogenic
What gradient do the ions move down?
Electrochemical
How is equilibrium potential determined?
Nernst equation
What are the 2 different types of refractory periods?
Absolute and relative
What happens during the absolute refractory period?
The membrane cant generate another action potential (AP) no matter how big the stimulus
Why cant another AP be stimulated during the absolute refractory period
Because the Na+ channels are inactivated
What happens during the relative refractory period?
Membrane can generate another AP but only if the stimulus is bigger than normal
How can another AP be stimulated during relative refractory period?
some K+ channels are still open and some Na+ are recovered
When does the relative refractory period occur?
From deploarisation to hyper-polarisation
Why is the refractory period useful?
Because it stops action potentials going backwards
What are the 5 stages of transmission of an action potential?
1) Resting state
2) Slow rising phase
3) Rapid rising phase
4) Early repolarisation
5) Hyperpolarisation
What factor makes the inactivation gate close?
Time
What factors makes the activation gates close?
Voltage and time
What is the approximate mV for threshold?
-55mV
What are the stages the Na+ gates go through?
Open, Inactivated and closed
What are myelin sheath made of?
Schwann cells
What is the term used when depolarisation jumps from node to node?
Saltatory conduction
What structural changes can be made to make depolarisation faster?
Increase diameter of axon and increase the membrane resistance (using myelin sheath)
What is a key feature of the nodes of ranvier?
There are a high density of Na+ channels
What are the 2 types of axons?
Myelinated and Unmyelinated
What happens when an AP invades a terminal?
Membrane is depolarised and Voltage Gated (VG) Ca2+ channels open
When Ca2+ channels are open where does Ca2+ diffuse?
Into the axon terminal
How is neurotransmitter (NT) released into the synaptic cleft?
Via exocytosis once vesicles fuse with the presynaptic membrane
What happens once the NT binds to the postsynaptic membrane?
Causes ligand-gated Na+ channels to open and Na+ diffuses into muscle endplate (at NMJ!)
How is End Plate Potential calculated?
(ENa+EK)/2
When do mini-EPPs occur?
When nerve muscles are at rest
What is the approximate amount of vesicles released in response to an AP and why?
~200-300 and the extra margin is a safety factor
What are the 5 zones, lines and bands of the sarcomere and what do they consist of?
M line - middle of myosin
Z line - End of actin
A band - actin and myosin
I band - actin only
H zone - myosin only
What happens to each zone, line and band during muscle contraction?
M line - stays same
Z line - distance decreases
A band - length stays the same
I band - length decreases
H zone - length decreases
What are thin filaments made up of?
Actin, tropomyosin and troponin
What molecules form F-actin strands?
G-actin molecules
What is the structure of the F-actin strands?
They are wrapped in a double helix
How are tropomyosin filaments arranged?
They are wound around the F-actin double helix
What are the 3 subunits of troponin?
T, I and C
Which subunits are bound to troponin and actin, blocking the myosin binding site?
T and I
How does the AP get into other parts of the muscle?
T-tubules
What is the structure of the T-tubule?
There are invaginations of the muscle membrane (sarcolemma) that penetrates deep into muscle fibre
What is the structure of the sarcoplasmic reticulum?
It is a tubular structure surrounding myofibrils that enlarges into terminal cisternae
How does muscle relaxation occur?
Removal of Ca2+ by SR so ATP stays bound to myosin so myosin remains bound therefore preventing muscle contraction
When does myosin bind to the binding site?
When myosin is in its high energy state (ATP is hydrolysed to ADP + Pi)
How does the power stroke take place?
Myosin heads rotate to the centre of the sarcomere
How does the actin-myosin cross-bridge break?
When ATP binds to myosin
Describe the structure of a myelinated motor neurone
Dendrites, cell body, myelinated axon, axon and axon terminal
What is the difference between bipolar and pseudo-polar neurones?
Pseudo-polar has myelinated axons but bipolar does not
What are the 2 different types of electrical impulses?
Action potential and graded potential
What is the main principle of an action potential?
All or nothing
Describe the action of a graded potential
It has variable strength, travels short distances and loses strength
Where do graded potentials take place?
In the dendrites, cell bodies and axon terminals
How do graded potentials have a variable strength during depolarisation?
Na+ diffuses through the membrane so concentration decreases causing depolarisation to get weaker
What is a depolarising graded potential known as?
An excitatory postsynaptic potential (EPSP)
What is a hyperpolarising graded potential?
An Inhibitory postsynaptic potential (IPSP)
Where in the neurone does the graded potential have to reach in order to stimulate an AP?
Axon Hillock
What is it known as if the threshold is not reached at the axon hillock?
Sub-threshold EPSP
What is it know as if the threshold is reached at the axon hillock?
Supra-threshold EPSP
What are the characteristics of the intracellular make-up? (Hint: there’s 6)
-Synaptic vesicle recycling
-Retrograde fast axonal transport
-Old membrane components digested in lysosomes
-peptides synthesised and packed by golgi
-fast axonal transport along microtubule network
-vesicle contents released by exocytosis
What changes when the strength of a stimulus is very big?
Frequency (stimulus intensity is frequency encoded)
Why do neurones require an abundant amount of glucose and oxygen?
As they have a high metabolic rate
When multiple pre-neurones connect to a singular post-neurone what is this known as?
Spatial summation (different points in space)
How is summation caused by 1:1 neurones?
From graded potentials, by them being close enough together to stimulate an AP
How can 2 EPSPs be diminished by summation? and what is this known as?
By an IPSP, postsynaptic inhibition
What is it called when graded potentials overlap in time?
Temporal summation
What is it called when the summation of graded potentials are demonstrated?
Postsynaptic modulation/integration
What happens during presynaptic modulation?
An inhibitory or excitatory neurone is found near the axon terminal causing either a release of NT or no release of NT
What are the presynaptic similarities?
During an action potential Ca2+ channels open, the increase causes exocytosis of NT that diffuses across the synaptic cleft
What are the postsynaptic differences?
NT identity differs and receptor identity differs (as they work differently)
What are the different types of NT? (Hint: there’s 6)
-Acetylcholine
-Amines
-Polypeptides
-Purines
-Gases
What are the main IPSP and EPSP NTs?
GABA (Gamma-aminobutyric acid)-IPSP
Glycine-IPSP
Glutamate-EPSP
What are the 2 different types of receptors?
Ligand-gated ion channels (ionotropic)
G-protein coupled receptors (metabotropic)
What are the main functions of the 2 receptors?
Ionotropic - fast synaptic potentials
Metabotropic - slow synaptic potentials
What are the NTs associated with ionotropic receptors?
Acetylcholine
Glutamate
GABA
What are the NTs associated with metabotropic receptors?
Adrenaline (both hormone and NT)
Histamine
Cholecystokinin
ATP
Acetylcholine (used in both receptors)
How do G-protein coupled receptors work?
2nd messenger pathway, modifys proteins and regulates synthesis of new proteins which coordinates an intracellular response
What is variation in electrical activity called?
Synaptic plasticity
How does synaptic plasticity work?
neuronal synapses change in response to past activity which causes variation in the electrical impulse
What is the process of long-term potentiation (LTP)
Repetitive stimulation at a synapse which increases the efficacy of transmission
How does LTP work?
Glutamate is released, binds to 2 ionotropic receptors, AMPA receptor is Na+ channel so EPSP (may or may not trigger AP), NMDA blocked by Mg2+, due to repetitive stimulation there’s greater depolarisation so Mg2+ is ejected, Ca2+ flows through NMDA receptor, makes post-neurone more sensitive to glutamate so more glutamate released
