Neurophysiology and NMJ Flashcards
Neuron
What is the input zone?
dendrites and cell body
part where incoming signals from other neurons are received
Neuron
What is the trigger zone
axon hillock
part where APs are initiated
Neuron
What is the conducting zone?
axon (1 mm to > 1 m long)
part that conducts APs in undiminishing fashion, often over long distances
Neuron
What is the output zone?
axon terminals
part that releases neurotransmitter that influences other cells
What cell types use electrical activity to perform their physiological roles?
neurons
cardiac myocytes
skeletal muscle cells
some secretory cells (ie. pancreatic 𝜷-cells)
What are excitable cells?
electrically active cells
How do excitable cells perform their physiological role?
harness difference in electrical charge between inside and outside of their cell membrane to function
Describe the different in charge between the inside and outside of excitable cells.
more negatively charged on inside than outside (0 mV outside)
electrical potential difference located immediately adjacent to cell membrane
What are the 2 initial conditions required for electrical activity?
- selectively permeable cell membrane
- differential distribution across membrane of electrically charged ions in solution (Na+ and K+)
What is diffusion?
movement of solute (ie. ion) from area of high concentration to lower concentration by random thermal movement (no added energy needed)
How do ions (charged particles) diffuse through the membrane?
FACILITATED DIFFUSION
require path through the bilayer
ion channels in membrane provide path for Na+ and K+, etc.
Why can’t ions (charged particles) diffuse directly through the membrane?
not lipid soluble
What type of process is facilitated diffusion?
passive
Describe facilitated diffusion by means of conformation change.
- molecule to be transported binds to carrier (on binding sites exposed to ECF)
- carrier changes its conformation
- binding sites are now exposed to ICF, and transported molecule detaches from carrier
What is electrical activity in excitable cells important for?
neurons
cardiac and skeletal muscle
What is the distribution of K+ and Na+ inside and outside of the membrane?
K+ inside: 150 mM
K+ outside: 5 mM
Na+ inside: 15 mM
Na+ outside: 150 mM
How do cells concentrate K+ inside, and Na+ outside the cell membrane?
Na+ - K+ ATPase
- pumps 3 Na+ out
- pumps 2 K+ in
(uses ATP –> ADP)
What is the resting membrane potential (RMP)?
-70 mV
What is E_K+ and how is it established?
-90 mV
established by relatively large net diffusion of K+ outward
What is E_Na+ and how is it established?
+60 mV
relatively small net diffusion of Na+ inward neutralizes some of the potential created by K+ alone
Do large intracellular anionic proteins diffuse across the membrane?
no
What is permeability (or conductance)?
ease with which an ion can travel across membrane
How does permeability (P) include membrane voltage (Vm)?
greater P = greater Vm
What is the relative permeability of K+ : Na+ in most cells? Why?
~ 50 : 1
greater # of open K+ channels at rest (K+ leak channels)
What is depolarization?
Vm becomes more positive
What happens if you increase permeability of the membrane to Na+?
membrane potential shifts toward ENa (+60 mV) = depolarization
How can you depolarize the membrane? (3)
- increase membrane permeability to Na+
- decrease membrane permeability to K+
- (theoretically) changing chemical gradient of Na+ or K+
What is hyperpolarization?
Vm becomes more negative
What happens if you increase permeability of the membrane to K+?
membrane potential shifts toward EK (-90 mV) = hyperpolarization
How can you hyperpolarize the membrane? (3)
- decrease membrane permeability to Na+
- increase membrane permeability to K+
- (theoretically) changing chemical gradient of Na+ or K+
What determines membrane potential (Vm)? (2)
- relative permeabilities of ions
- electrochemical gradient
What is an action potential?
large, all-or-nothing electrical event triggered when membrane potential reaches threshold
What occurs during an action potential?
- rapid membrane depolarization (due to increased Na+ permeability)
- rapid return toward resting membrane potential (due to increased K+ permeability)
Why is AP a regenerative event?
AP in one part of membrane will initiate AP in a more distant part of cell
What occurs at an AP subthreshold?
stimuli will not elicit AP → elicits graded potentials
What occurs at an AP suprathreshold?
stimuli will elicit AP of the same size, regardless of the magnitude of the stimulus
Components of Axonal AP
What is the initial depolarization?
How does it occur?
initial depolarization to AP threshold (not part of AP)
occurs in many ways, including EPSP, generator potential, and in lab by external stimulation
Components of Axonal AP
What occurs at the peak of AP?
Why?
Vm approaches ENa
far greater Na+ conductance (gNa) from open Na+ channels than K+ conductance (gK) resulting from open K+ channels
Components of Axonal AP
What occurs after hyperpolarization (AHP)?
Why?
Vm is closer to EK than at rest
K+ channels are open, and gK is greater than at rest
Where are generator potentials (GP) produced?
at sensory endings in periphery
Does a larger stimulus result in larger depolarization?
yes
GP is graded in amplitude - proportional to strength of the stimulus
When are APs produced?
if GP reaches threshold
What is the m-gate?
activation gate for Na+ channel
What is the h-gate?
inactivation gate for Na+ channel
What is the n-gate?
activation gate for K+ channel
Does K+ have an inactivation gate?
no
When do voltage-gated channels move their activation gate?
they have voltage sensor that moves in response to changes in membrane voltage
this movement is coupled to activation gate
How does depolarization influence the probability that the activation gate or inactivation gate is open?
- increases probability that ACTIVATION gate is open
- decreases probability that INACTIVATION gate is open
What happens if either gate (activation or inactivation) is closed?
VG channel will not conduct
In what state are most VG channels in at rest?
available state
- activation gate closed
- inactivation gate open
- channels are non-conducting, but ready to be activated by depolarization
What does a greater depolarization result in?
greater probability of getting all ions
Describe the positive feedback loop initiated by AP threshold.
- initial depolarization (ie. GP) opens some available VG Na+ channels in membrane
- Na+ influx (conductance) results in further membrane depolarization
- more depolarized membrane = increases probability that activation gate of available (but presently non-conducting) VG Na+ channels will open
loop repeats rapidly, opening all available VG Na+ channels
What are the 4 different phases of AP?
- resting state
- rising phase
- falling phase
- afterhyperpolarization (AHP)
Describe channel gating during resting state.
Na+ activation gate: closed (high prob.)
Na+ inactivation gate: closed (high prob.)
K+ activation gate: open (likely)
Describe channel gating during rising phase.
Na+ activation gate: open
Na+ inactivation gate: open
- recovery from inactivation
- channel is now in available state again
- takes time
K+ activation gate: closed (likely)
- results in membrane returning to resting voltage
What happens when K+ activation gate closes?
results in membrane returning to resting voltage
What happens when Na+ inactivation gate opens?
- recovery from inactivation
- channel is now in available state again
- takes time
Describe channel gating during falling phase.
Na+ activation gate: remains open
Na+ inactivation gate: closed
K+ activation gate: open
Describe channel gating during afterhyperpolarization (AHP).
Na+ activation gate: closed
Na+ inactivation gate: remains closed
K+ activation gate: remains open
Describe the rising and falling phases in the change in conductance of Na+ ions (gNa).
- rising phase: Na+ channel opening
- falling phase: Na+ channels inactivating
Describe the rising and falling phases in the change in conductance of K+ ions (gK).
- rising phase: K+ channel opening
- falling phase: K+ channels closing
What happens to gNa and gK during AHP?
- gNa has returned to resting value
- gK is still elevated over resting values because K+ channels are still open
What is gNa and gK at rest?
not 0, but gK is significantly greater than gNa due to leak channels
What is the absolute refractory period?
period of time during which a second AP absolutely cannot be initiated, no matter how large the stimulus is
What is the relative refractory period?
interval immediately following the absolute refractory period during which initiation of a second AP is inhibited, but not impossible
(requires a greater stimulus intensity than the previous stimulus to generate AP)
Why does an AP fail to evoke during the absolute refractory period?
- some Na+ channels are inactivated and cannot conduct current
- still gK+ from open voltage-gated K+ channels (hyperpolarizing)
What are the 2 types of AP propagation?
AP propagation combines both types:
- passive (electrotonus)
- active
What is electrotonus?
passive process by which electrical events propagate
What is current?
flow of + charge
Where does current flow after entering neuron?
- enters axon (or dendrite) of neuron through ion channels
- current will travel axially along resistive pathways of axon
Describe the steps of electrotonus.
- triggering event opens Na+ channels → current enters axon (or dendrite) through ion channels in region of membrane and depolarizes it
inactive regions are at resting potential
- intracellular + charge is attracted to adjacent negatively charged regions of membrane → local current flow occurs between active and adjacent inactive areas (depolarizes inactive areas)
What is electrotonic decay?
during electrotonus, charge leaks outward across the membrane as current travels along axon
What does the length constant determine?
distance a passive electrical event (ie. depolarization) can propagate along a neuronal process
How is the length constant calculated?
length constant = (Rm/Ra)^1/2
What is the length constant?
ratio of axial resistance (Ra) and membrane resistance (Rm)
How do you get a longer propagation distance.
larger length constant = longer propagation distance
increase length constant by:
- increasing Rm
- decreasing Ra
How do you decrease Ra?
increase diameter of axon
APs must travel along axons that are many times longer than their length constant (~0.1-1.0 mm). Electrotonus cannot do this. What is the solution?
active propagation
APs activate VG channels along axonal membrane to regenerate depolarization
Describe active propagation in unmyelinated axon.
‘boosting’ with VG channels:
- regenerates inward current
- counteracts outward current leak
What are the two variables that primarily influence rate of AP regeneration?
- diameter of fibre
- amount of membrane capacitance
How does the diameter of the fibre influence AP?
increase axon diameter = decrease axial resistance (Ra)
greater axial current flow provides more charge per unit time to neighbouring membrane segments
neighbouring membrane segments reach AP threshold more rapidly
What is a capacitor
two conductive plates separated by insulating layer, that has ability to store electrical charge
How does a capacitor work?
Vm gradually changes as charge separation across capacitor changes
changing charge on membrane surfaces (conductive plates of capacitor) during propagation of electrical potential takes time
How does the amount of membrane capacitance influence AP?
less membrane capacitance = less time to charge membrane to threshold = greater rate of AP propagation
What is the amount of capacitance proportional to?
- proportional to surface area of membrane
- inversely proportional to distance between parallel plates
specific capacitance of membrane is fixed (~ 1µF/cm2)
How do axons reduce membrane capacitance?
by increasing effective thickness of membrane (distance between plates of capacitor) with a substance called myelin
What is myelin formed from?
- Schwann cells in PNS
- oligodendrocytes in CNS
How does myelin affect AP?
APs can travel up to 50x times faster in myelinated fibres than in unmyelinated fibres
Describe AP propagation between nodes of Ranvier.
- AP travels electrotonically (passively)
- membrane capacitance is lower
- charge time of membrane is short
- Rm is increased = longer length constant
- depolarization propagates rapidly
Describe AP propagation at nodes of Ranvier.
- activation propagation
- membrane capacitance is greater
- charge time of membrane is longer
- action potential slows
What is a myelin sheath?
can have up to 300 layers of membrane
Example: How is the stretch reflex activated?
- AP travels along axon, into spinal cord
- AP reaches axon terminal
- AP depolarizes terminal
- depolarization of terminus is converted into a release of chemical messenger, initiating synaptic transmission
Where do synapses occur?
on cell body and dendrite of postsynaptic neuron
Describe the steps of synaptic transmission.
- AP propagation in presynaptic neuron
- Ca2+ entry into synaptic knob
- release of neurotransmitter by exocytosis
- binding of neurotransmitter to postsynpatic receptor
- opening of specific ion channels in subsynaptic membrane
Postsynaptic Membrane Effects
What does binding of a neurotransmitter to an extracellular receptor on a postsynaptic ion channel do?
induces conformation change of of channel, opening the channel pore
this allows ions to move across the membrane
Postsynaptic Membrane Effects
What is generated as a result of ion movement through the pore in the postsynaptic cell membrane?
synaptic current (Isyn) is generated, which generates postsynaptic potential (PSP) - which is a change in Vm
Postsynaptic Membrane Effects
What is excitatory postsynaptic potential (EPSP)?
change in membrane voltage of a postsynaptic cell following the influx of positively charged ions into a cell
Postsynaptic Membrane Effects
How do EPSPs occur?
excitatory neurotransmitters bind to receptors that generate depolarizing PSPs, bringing Vm closer to AP threshold (EPSP)
How many ions occupy a pore?
only one ion occupies the pore at any given moment
Why can’t EPSPs fire in the postsynaptic neuron?
- EPSP’s decrease in amplitude while travelling toward soma
- single EPSP will not bring neuron to AP threshold
What do EPSPs need to do to cause AP firing in postsynaptic neuron?
EPSPs must summate
What are the 2 types of summation?
temporal
spatial
What is temporal summation?
involves repetitive activation of a single synapse
- frequency is important – EPSPs must add together
What is the result of temporal summation?
large compound EPSP results
- compound EPSP may reach AP threshold at soma/axon hillock
What is spatial summation?
involves simultaneous activation of multiple synapses
What is the result of spatial summation?
large compound EPSP results
- compound EPSP may reach AP threshold at soma/axon hillock
What neurotransmitter do most excitatory synapses in CNS use to generate EPSPs?
glutamate
What does glutamate (a neurotransmitter) act on?
may act on many receptor subtypes – two most common are AMPA and NMDA receptor-gated channels
Glutamate and AMPA-gated EPSPs
What do AMPA-gated channels do?
allow both Na+ and K+ ions through open pore
- this generates EPSP with equilibrium potential of ~0 mV
- brings postsynaptic neuron closer to AP threshold
Glutamate and AMPA-gated EPSPs
What happens to AMPA-gated and NMDA-gated channels when dendrites are at RMP?
- AMPA-gated channel activation dominates EPSP generation
- NMDA-gated channels blocked
- fast EPSP
Glutamate and AMPA-gated EPSPs
Example: What happens at relatively negative values of postsynaptic Vm?
glutamate binding activates AMPA receptor, which depolarizes the cell
BUT, Mg2+ blocks NMDA receptor
What is inhibitory postsynaptic potential (IPSP)?
type of pSP that keeps Vm from reaching AP threshold (and therefore generating AP)
How are IPSPs generated?
inhibitory neurotransmitters bind to receptors
What is 𝜸-aminobutyric acid (GABA)?
common inhibitory neurotransmitter in CNS
- there are several classes of GABA receptors
What do GABA-A type GABA receptor-gated channels do?
allow entry of Cl- ions through open pore
- inward movement of negative charge generates IPSP with equilibrium potential = ECl = -70 mV
- note: in some cells ECl = RMP
What physiological processes are skeletal muscles involved in?
movement breathing GI tract activity (swallowing/defecating) temperature regulation speaking venous/lymphatic fluid movement protection of organs
What does skeletal muscle activation require?
reliable and rapid transmission of signaling between nervous system and muscle cell
What is the neuromuscular junction (NMJ)?
specialized synaptic contact between alpha motor neuron (a-MN) and muscle cell
What does a motor neuron innervate?
one set of muscle fibres
What is a motor unit?
functional unit consisting of a-MN and all muscle fibres (skeletal muscle cells) it innervates
What is the motor unit for?
force generation
Describe the structure of a motor unit, and explain how APs contract the unit.
- a-MN axons can branch many times
- each branch terminus innervates a single muscle cell (fibre) at NMJ
- APs will travel down all branches of axon
- a-MN APs simultaneously initiate excitation and contraction of each muscle fibre within motor unit
What is a motor unit pool?
all of the motor units innervating a skeletal muscle
- consists of many motor neurons, each of which innervates a motor unit within muscle
What does the NMJ ultrastructure consist of?
- presynaptic terminal
- synaptic cleft
- postsynaptic membrane
Where is the NMJ formed?
at terminus of each branch of a-MN
Where does NMJ usually contact muscle?
at mid-point along its length
Describe how vesicles and peptide neurotransmitters travel from cell body to nerve terminal.
???
- vesicle and peptide neurotransmitter precursors and enzymes are synthesized in cell and are released from Golgi
- vesicles travel through axon on microtubule tracks via fast axonal transport - peptide neurotransmitters are already in some vesicles
- non-peptide neurotransmitters are synthesized and transported into vesicles in nerve terminal
NMJ Ultrastructure - Presynaptic Terminal
What is the presynaptic termina?
distal part of a-MN and supportive Schwann cell
NMJ Ultrastructure - Presynaptic Terminal
What is synthesized in the terminal?
ACh
NMJ Ultrastructure - Presynaptic Terminal
Where is ACh stored?
in vesicles
NMJ Ultrastructure - Presynaptic Terminal
How are vesicles organized?
into active zones close to synaptic membrane
NMJ Ultrastructure - Presynaptic Terminal
How is ACh released?
via exocytosis by specialized machinery - SNARE proteins and Ca2+ sensor associated with each vesicle and underlying synaptic membrane
NMJ Ultrastructure - Presynaptic Terminal
Where are ACh autoreceptors and when do they function?
present on terminal membrane
function during high frequency NMJ activation
Presynaptic Terminal - a-MN and their Terminal Bouton
What does the soma contain?
organelles and cellular machinery required to manufacture empty vesicles
Presynaptic Terminal - a-MN and their Terminal Bouton
What happens if soma dies?
results in denervation
Presynaptic Terminal - a-MN and their Terminal Bouton
What happens to severed axons?
may regenerate and re-establish functional NMJ
Presynaptic Terminal - a-MN and their Terminal Bouton
What is the soma required for?
health of axon
Presynaptic Terminal - a-MN and their Terminal Bouton
Where is ACh synthesized?
in presynaptic terminal
Presynaptic Terminal - a-MN and their Terminal Bouton
What is ACh synthesized from?
acetyl CoA and choline
Presynaptic Terminal - a-MN and their Terminal Bouton
Where is ACh stored and transported?
stored in vesicles, which are then transported (fast axonal transport) to each axon terminal
NMJ Ultrastructure - Synaptic Cleft
What is the synpatic cleft?
50 nm space between neuron and muscle cell
contains basal lamina (extracellular matrix)
??? adhesion and alignment of axon terminal active zones and muscle junctional folds
NMJ Ultrastructure - Synaptic Cleft
Where is acetylcholinesterase (AChE)?
anchored within matrix, in close proximity to postsynaptic AChRs (ACh receptors)
NMJ Ultrastructure - Postsynaptic Membrane
What do longitudinal junctional folds do?
provides large surface area for ACh receptor (AChR) activation
NMJ Ultrastructure - Postsynaptic Membrane
What is at the ‘shoulders’ of each longitudinal junctional fold?
junctional AChRs are anchored, physically positioned opposite presynaptic active zones
NMJ Ultrastructure - Postsynaptic Membrane
What is the perijunctional zone?
site of muscle AP initiation
NMJ Ultrastructure - Postsynaptic Membrane
What does the perijunctional membrane have?
high density of VG Na+ channels
NMJ Ultrastructure - Postsynaptic Membrane
What is found in non-junctional muscle membrane?
a second AChR type
more common during fetal development, during inflammation (ie. burn patients), and following denervation of muscle
NMJ Chemical Transmission
Describe the sequence of transmission.
(generally same sequence as synaptic transmission)
- each terminal branch of a-MN is simultaneously activated by axonal AP
- AP activates VG Ca2+ channels in terminal bouton
- vesicles containing ACh dock with synaptic membrane and exocytose ACh into synaptic cleft
- ACh diffuses across cleft and binds to AChRs at postjunctional folds
- AChRs open, allowing Na+ and K+ ions through pore, resulting in muscle membrane depolarization
- AChE within basal lamina hydrolyses ACh into acetate and choline, terminating NM transmission
Postsynaptic Junction Physiology
Where do EPPs propagate?
EPPs only need to propagate a short distance from postjunctional folds
Postsynaptic Junction Physiology
What does the perijunctional region have a high density of? Why?
VG Na+ channels in membrane
ensures sarcolemma reaches AP threshold
Postsynaptic Junction Physiology
Do EPPs normally reach AP threshold?
EPP is normally always large enough to reach AP threshold at perijunctional membrane (~40 mV in amplitude)
Postsynaptic Junction Physiology
NMJ has a high safety factor. What does this mean?
every a-MN AP will result in muscle AP and subsequent contraction
Presynaptic Effects
What are the two effects?
- motor neuron cell body destruction
- demyelination
Presynaptic Effects - Motor Neuron Cell Body Destruction
What does death of a-MN cell body cause?
- paralysis, as axon degeneres and NM transmission blocked
- loss of trophic factors released by presynaptic terminal – muscle atrophy
characteristic of polio viral infection
Presynaptic Effects - Demyelination
What does destruction of a-MN axonal myelin do?
impairs AP propagation
- can slow or block axonal APs
Presynaptic Effects - Demyelination
What does AP slowing result in?
weakness, as motor units fire dis-synchronously
Presynaptic Effects - Demyelination
What does AP blockade result in?
paralysis
What are neuromuscular blocking agents (NMBAs)?
pharmacological agents used to block muscle contractions
What are NMBAs used in?
surgical anesthesia
NMBAs
What are the two types of muscle blockers?
- depolarizing muscle blockers
- non-depolarizing muscle blockers
NMBAs
How do depolarizing muscle blockers work?
- activate AChRs and keep them open
- membrane depolarizes and muscle contracts
- prolonged AChR activation keeps muscle membrane depolarized
- VG Na+ channels remain in inactive state
- muscle APs are blocked and muscle cannot contract
ie. succinylcholine
NMBAs
How do non-depolarizing muscle blockers work?
- binds to AChR without opening channel
- blocks ACh binding and prevents EPP production
- no muscle AP = no muscle contraction
ie. rocuronium, tubocurarin
NMBAs
What are non-depolarizing muscle blockers?
competitive antagonist of AChR
