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Neuromuscular Physiology Chapter 5 Review
Terms in this set (55)
Potential difference (V)
a measure of the potential energy that must be used to move a positive charge from one location to another.
the rate at which positive charges move between the two locations that represent a potential difference.
something through which current flows
ability to conduct electric current
capacity to conduct electric current
(with respect to: a) resistance and b) conductance)
I = gV
Current (I)= conductance (g) x potential difference (V)
I = V / R
Current (I)= voltage / resistance
Relationship between conductance and resistance
Resistance (R) is the inverse of conductance (g)
For an excitable membrane, describe the process of events when an experimenter applies a current across the membrane and simultaneously measures the resulting membrane potential.
The membrane potential will increase (depolarization), voltage gated Na+2 channels open (influx of Na+2 due to chemical and electrical gradient), Efflux of K+ due to concentration gradient
Electrical circuit that models the behavior of an excitable membrane
(battery, conductor, capacitor, current generator)
Battery: concentration gradients across membrane
Conductor: ion channels that allow ion flow
Capacitor: membrane ability to separate charged particles
Current generator: sodium potassium pump to maintain gradients
Structure of the cell membrane
How does this structure contribute to the development of the resting membrane potential?
Outer membrane (extracellular side):
Inner membrane (intracellular side/ cytoplasm): negative (K+)
Passive ion flux through "resting channels" creates the resting membrane potential: inside of membrane is negative relative to outside of membrane.
Phospholipid bilayer: impermeable to ions (due to its hydrophobic outer layer)
Ion channels allow transport of ions
Only permeable to K+ (channels always open)
change in membrane potential opens or closes channel
chemical binding to receptor (e.g. neurotransmitter)
due to potential difference across membrane
Explain how the chemical and electrical forces act on the following ions under typical conditions of an excitable membrane:
Na+, K+, Cl-, Ca++
chemical force driving Na+ inward
electrical force driving Na+ inward
Net force: IN
chemical force driving K+ outward
electrical force driving K+ inward
Net force: OUT
chemical force driving Cl- inward
electrical force driving Cl- outward
Net force: 0
Both the chemical force and electrical force are trying to drive Ca++ from the outside to the inside of the cell
What does the equilibrium potential calculated from the Nernst equation represent?
chemical and electrical forces are balanced
How does the cell membrane maintain ionic gradients when ions leak across the membrane at rest and when generating an action potential?
There is a larger net driving force for Na+ than for K+. But, there are more resting channels for K+ than for Na+, therefore, there is greater conductance for K+ than for Na+. On net, there are similar currents for K+ and Na+ (leak currents of potassium).
When potassium "leaks" out of the cell (due to the concentration gradient), the anion is left alone which generates a negative charge. When the inside of the cell is negative (membrane potential), the potassium is driven back into the cell.
During intense exercise, why does plasma K+ concentration go up?
The sodium potassium pump cannot keep up
membrane potential becomes less negative
opening of voltage gated Na+ channels causes an increase in conductance of Na+ which causes the inside of the cell to become more positive= membrane depolarization and overshoot
membrane potential becomes more negative
gradual opening of voltage gated K+ channels causes an increase in conductance of K+ which leads to net K+ efflux=
K+ channels open for a long time which causes afterpolarization; membrane is refractory
Explain how the input conductance of a neuron affects the amount of current needed to change the membrane potential a given amount (hint - use Ohms law).
Reflects how much a change there will be for the amount of current injected; there needs to be
enough to make the membrane potential exceed threshold in order to propagate an action potential.
How does neuron size affect input conductance of a neuron? How does the relationship between neuron size and input conductance affect the ease of changing the membrane potential? What is the importance of this in terms of generating action potentials?
Input conductance (g in) is directly proportional to the neuron size (smaller neurons have smaller g in)
Because the voltage threshold is similar in neurons of all sizes, the smallest neurons will achieve the required Vm to reach threshold with the smallest Im. This means that when a current is applied to a group of neurons increases gradually, the smallest neuron will be the first to reach threshold and the discharge an action potential.
How does neuron size affect the input capacitance of a neuron? How does the relationship between neuron size affect the ease of changing the membrane potential? What is the importance of this in terms of generating action potentials?
The shift from a capacitive current to an ionic current depends on the size of the capacitor that has to be charged. Because large neurons require a greater amount of capacitive current to charge the capacitor, it takes them longer to realize a significant amount of ionic current and the corresponding membrane potential.
Define lambda (λ) in terms of electrotonic conduction. What factors affect the size of λ?
Length constant (λ)- distance the current will spread until the change in membrane potential (delta Vm) is 37% that at current source
The better the axial conductance (horizontal), the bigger lambda will be
The bigger the membrane conductance, the smaller lambda will be
Define an action potential.
Synapse activated --> input current causes local depolarization of about 15 mV --> activation of
voltage gated Na+2 and then K+ channels
"a transient reversal in the potential difference across the membrane that is transmitted rapidly
along an excitable membrane"
Describe the flow of Na+ and K+ ions in the process of an action potential (include depolarization, repolarization, and afterhyperpolarization).
-Opening of voltage gated Na+ channels --> increase Na+ conductance --> Na+ driven into
cell --> increase (+) inside cell moves voltage towards battery --> membrane depolarization and overshoot
-Na+ channels remain open for only a few msec.
-Gradual opening of voltage gated K+ channels --> increase K+ conductance --> net K+ efflux --> repolarizes membrane
-K+ channels open for a long time --> after hyperpolarization; membrane is refractory
Explain the role of electrotonic conduction in the transmission of an action potential from one node of Ranvier to the next.
Na+ current discharges membrane capacitance transmitted via electrotonic conduction along membrane; if myelinated, transmitted to next node of Ranvier --> AP at next node; repeat
(Nodes of Ranvier have high density of ion channels)
Refractory period prevents backward AP propagation
Explain how neuron diameter affects the velocity of electrotonic conduction.
Increase diameter --> increase velocity
-Direct flow of current between cells via gap junctions
-Allow bidirectional ion flow
-Rare in neuron-neuron communication in mammals
-Neurotransmitter release by presynaptic cell binds to receptors associated with ligand-gated channels on postsynaptic membrane
-Slow relative to electrical synapses
-Small molecule neurotransmitters
(acetylcholine, glycine, glutamate)
-Neuroactive peptides (amino acid polymers)
Describe the role of calcium in the process of neurotransmitter release from a presynaptic membrane.
AP on presynaptic cell → (+) voltage gated Ca++ channels → Ca++ influx into presynapatic cell → (+) exocytosis of vesicles containing neurotransmitter (vesicles fuse with presynaptic membrane) → neurotransmitter diffuses across synaptic cleft → binds to receptors on postsynapatic membrane → activated receptors (ligand gated) (+) opening of ion channels → ∆ Vm.
vesicle release sites; focus neurotransmitter release on proper area
amount of neurotransmitter released from one vesicle
Explain the role of ligand gated vs. voltage gated ion channels in the process of action potential generation at the neuromuscular junction.
Depolarization via ligand (ligand=ACh) gated ion channels stimulates voltage gated ion channels --> action potential
Ligand gated channels are stimulated by alpha neuron --> depolarization
Voltage gated channels propagate action potential down the muscle cell
Under what conditions is neuromuscular transmission from the alpha motor neuron to the muscle cell potentially impaired? Include pharmacological, poisonous, and disease possibilities.
Can be impaired by pharmacological agents (diseases that affect the neuromuscular junction) or prolonged exercise
-Myasthenia Gravis: an autoimmune disease that targets ACh receptors (decrease in autoimmune receptors decreases muscle activation)
-Lambert Eaton myasthenic syndrome
a group of transmembrane ion channels that are opened or closed in response to the binding of a chemical messenger.
ACh receptors at neuromuscular junction are ionotropic receptors; receptor that contains ACh binding site AND ion channel
a type of membrane receptor of eukaryotic cells that acts through a secondary messenger. It may be located at the surface of the cell or in vesicles.
Neurotransmitter binds to receptor --> (+) 2nd messenger system --> can affect ion channel gating, but slower than with ionotropic receptors
Explain the role of EPSPs and IPSPs in affecting membrane potential, and how their information is integrated to determine whether an action potential will occur.
Neurons receive both excitatory (depolarizing) and inhibitory (hyperpolarizing) inputs.
-excitatory postsynaptic potential (EPSP) - response to a depolarizing current
-inhibitory postsynaptic potential (IPSP) - response to a hyperpolarizing current
EPSP vs IPSP depends on the ion channel opened (ionotropic receptors), not the neurotransmitter per se.
Explain how EPSPs and IPSPs information is integrated to determine whether an action potential will occur.
Axon hillock Vm is the integration site for AP generation - trigger zone
EPSP/IPSP effects on Vm at axon hillock depend on:
-Spatial Summation - sum of inputs from multiple synapses
-Temporal Summation - sum of multiple inputs at same synapse; "overlap of
EPSP/IPSPs conducted via electrotonic transmission; therefore distance matters (λ).
postsynaptic potentials produced by consecutive inputs from the presynaptic neuron".
Trigger zone has high Na+ channel density; lower threshold for activation
Differentiate between presynaptic inhibition and presynaptic facilitation, and describe how metabotropic neurons can influence these processes.
Presynaptic Inhibition- interneuron decreases amount of neurotransmitter released by innervated neuron; types:
-Metabotropic neuron decreases Ca++ influx into presynaptic terminal --> decrease neurotransmitter exocytosis
-Metabotropic neuron directly inhibits neurotransmitter release
Presynaptic facilitation: mechanisms increase Ca++ influx
Products from cell body have to be moved to neurotransmitter release sites, and also move substances back to cell body.
-Orthograde transport: movement of substances away from cell body
-Retrograde transport: movement of substances toward cell body
Accompanied with cytoskeletal structures and molecular motors.
Axonal transport changes with physical activity (run training increases rate of ACh-esterase transport from cell body to neuromuscular junction) and aging (older rats have slower axonal transport)
Discuss how axonal transport can influence nerve and muscle characteristics.
Axonal transport effects on nerve and muscle cells
-Nerve-muscle trophism (neurogenic) - e.g., changes in muscle cells when motor neuron connection is lost
-Muscle-nerve trophism (myogenic) - e.g., changes in motor neuron when connection with muscle is lost
Neurogenic trophism - muscle denervation
-Muscle membrane depolarization (within 2 hr)
-↑ membrane resistance
-Changes in Na+ channel structure
-"synthesis of extrajunctional ACh receptors"
-Axonal sprouting by surviving neurons
records muscle action potentials
Describe how a bipolar electrode system records muscle electrical activity.
EMG uses extracellular recordings; detect extracellular filed potentials associated with the currents that underlie muscle fiber action potentials.
Bipolar recordings: two electrodes used to record the EMG
-signal represents the difference in voltage between the two electrodes
-overlapping action potentials from many muscle cells
limits the frequency content of the EMG signal
Low pass filter
allows low frequencies to pass
High pass filter
allows high frequencies to pass
Band pass filter
allows frequencies in a specific range to pass
rejects frequencies between two set values
Describe how electrode location relative to the innervation zone affects the amplitude of the EMG signal.
-Position relative to innervation zone
The greatest amplitude is recorded when the interelectrode distance is relatively large and the electrodes are placed on the same side of the innervation zone.
If placed on the innervation zone the reading is 0.
Describe factors that can influence the recording of EMG signals.
Crosstalk- signal from nearby muscle contaminates recording from muscle of interest
-Mainly due to extinction phase of action potential
-Can't filter out since both signals have information in same frequencies
-2 pairs of electrodes (2 bipolar recordings) --> send the two differentiated signals into another differential amplifier
-Differentiation attenuates information common in both electrodes. If double and single differentiated signals have similar amplitude, little cross talk.
Isometric constant force contractions
Linear or curvilinear relation between EMG amplitude and force
Interpretation of contraction intensity complicated by
-Non-stationarity of signal
-Position of electrode over muscle tissue
-Changes in tissue conductivity
What is mechanomyography (MMG)?
Muscle contraction causes vibration of muscle fibers (surface MMG measures this effect)
-Mainly use accelerometers to record
Appears to provide information regarding motor unit recruitment and firing rate.
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