Post on 22-Jan-2016
Neuromuscular Function: Neural Impulse and Neurotransmitter Release
Muscle Physiology
420:289
Agenda
Nerve impulse IntroductionChannels and pumpsThe neural impulse
Neurotransmitter release
Nerve Impulse - Introduction
What is a nerve impulse? A transmitted electrical charge that stimulates or inhibits
a physiological event What type of event?
Stimulate/inhibit another neural impulse Stimulate a gland Increase/decrease heart rate Activate skeletal muscle
What is an action potential? Synonym for impulse
Nerve Impulse - Introduction
Basic progression of events:
1. Disruption of the cell membrane’s electrical state
2. Restoration of the cell membrane’s electrical state
Nerve Impulse - Introduction In order to disrupt or restore a cell
membrane’s electrical state channels and pumps are needed
Agenda
Nerve impulse IntroductionChannels and pumps
General propertiesRegulatory channels pumps and other
The neural impulse Neurotransmitter release
Channels and Pumps – General Properties Purpose of channels and pumps: Maintenance of the cell membrane’s resting
electrical state (resting membrane potential – RMP) Both channels and pumps
Disruption of the cell membrane’s RMP Primarily channels
Restoration of the cell membrane’s RMP: Primarily pumps
Sarcolemma as well
Channels and Pumps – General Properties How is the RMP maintained, disrupted,
restored? Channels and pumps move charged ions into
and out of the cell Channels: Ions flow along electrochemical gradient Pumps: Move ions against electrochemical gradient
Terminology: Anion: Negatively charged ion Cation: Positively charged ion
Channels and Pumps – General Properties Speed and direction of transfer: Channels:
Can move several million ions / second Use diffusion (no energy required) Channels are less frequent Channels stay open for short periods of time
Pumps: Can move several hundred ions / second Require energy There are many more pumps than channels Pumps work constantly
Channels and Pumps – General Properties Selectivity: Channels and
pumps only allow certain molecules to pass
Mechanisms: Size: Water shell
Remember hydrophobic interior of membrane
Affinity: Specific proteins within the channels/pumps
Agenda
Nerve impulse IntroductionChannels and pumps
General propertiesRegulatory channels, pumps and other
The neural impulse Neurotransmitter release
Regulatory Channels, Pumps, Other Sodium channels Sodium-potassium pumps Potassium channels Calcium channels and pumps Anion channels
Na+ Channels
Structure: Two subunits
Alpha: Larger Acts as actual channel
Beta: Purpose unclear
Na+ Channels
Function: Disruption of RMP Voltage-gated
Change in electrical state of plasmalella activates channel
Sodium passes with concentration gradient
MacIntosh et al. 2006, Fig 9.7
Na+ Channels
Distribution: Na+ channels are found:
Axon hillock and nodes of Ranvier Sarcolemma Synaptic cleft T-tubules
Highest Na+ channel densities are observed at: Synaptic clefts Transverse tubules Nodes of Ranvier
Regulatory Channels, Pumps, Other Sodium channels Sodium-potassium pumps Potassium channels Calcium channels and pumps Anion channels
Na-K+ Pumps
Structure: Two subunits:
Alpha: Larger Contains Na+, K+
and ATP binding sites
Beta: Function not clear
Na-K+ Pumps
Function:
1. Maintain RMP at rest
2. Restore RMP after disruption
Na-K+ Pumps
Maintenance of RMP: Each cycle of the Na-K+ pump results in:
Removal of 3 Na+ Retrieval of 2 K+
Net removal of 1 cation intracellular negativity
Restoration of RMP after disruption
Na-K+ Pumps
Na-K+ pumps require ATP Enzyme Na-K+ ATPase Two states:
E1E2
MacIntosh et al, 2006, Fig 7.8
1. E2 releases Pi and picks up ATP
2. Energy from ATP releases 2 K+ and changes to E1
3. E1 releases ADP and picks up 3 Na+ and Pi
4. Pi changes back to E2 and releases 3 Na+ and picks up 2 K+
Regulatory Channels, Pumps, Other Sodium channels Sodium-potassium pumps Potassium channels Calcium channels and pumps Anion channels
K+ Channels
Structure: Several types of K+ channels with varying structures
Some allow K+ to leave the cell Some allow K+ to enter the cell
Different stimuli activate different K+ channels Increased intracellular [Na+] Decreased intracellular [ATP] Disruption of RMP Increased intracellular [Ca2+] Sarcoplasmic reticulum activation
K+ Channels
Function: Restoration of the RMP following disruption
Fast K+ channels allow outflow of K+ Activated via membrane disruption
Restoration of RMP during fatigue Several types of K+ channels inflow of K+ Activated via increased intracellular [Na+], [ATP],
[Ca2+]
Regulatory Channels, Pumps, Other Sodium channels Sodium-potassium pumps Potassium channels Calcium channels and pumps Anion channels
Ca2+ Channels and Pumps Structure: Ca2+ channels:
Several types: Voltage-gated Ca2+ channels:
Embedded within the axolemma of neuron Dihydropyridine (DHP) channels
Embedded within the sarcolemma of muscle Ryanodine (RYR) channels
Embedded within the SR membrane of muscle
Ca2+ pumps: Two types:
Ca2+ surface membrane pumps (SMP): Larger
Sarcoplasmic reticulum pumps (SERCA or Ca2+ ATPase) Occupies ~90% of the SR membrane
Ca2+ Channels and Pumps Function: Ca2+ channels:
Link disruption of cell membrane of neuron/muscle fiber to a molecular event
Neuron: Ach release Muscle fiber: Cross-bridge formation
Ca2+ SMPs: Maintain low intracellular [Ca2+]
SERCA or Ca2+ SR pumps: Remove Ca2+ from the sarcoplasm back into the SR Requires ATP (Ca2+ ATPase)
Regulatory Channels, Pumps, Other Sodium channels Sodium-potassium pumps Potassium channels Calcium channels and pumps Anion channels
Anion Channels
Structure: Similar to Na+ channels Most common is Cl- channel
Function: Maintain the RMP by flowing out
Distribution: Cl- channel is most common of all channels High permeability of Cl-
Anion Channels
The myotonic goat Genetic mutation results in decreased
permeability to Cl- Result: Inability of muscle fiber to restore RMP
following initial disruption Myotonic goat video
Agenda
Nerve impulse IntroductionChannels and pumpsThe neural impulse
Neurotransmitter release
Neural Impulse
The resting membrane potential Basic progression of events:1. Cell body of the neuron must receive adequate
stimulus-All-or-nothing fashion
2. RMP is disrupted (depolarized)3. RMP is rapidly restored (repolarized)4. Propagation
Neural Impulse - RMP
What is the Resting Membrane Potential (RMP)?
The difference in charge between the inside and the outside of the cell
Typical value -70 mVThe inside of the cell has a charge of –70 mV
relative to the outside of the cell
Neural Impulse
How is the RMP maintained? Several mechanisms:
1. Fixed anion structures increase negativity within cell
2. Na-K+ pump:-High extracellular [Na+], high intracellular [K+]
3. Permeability of membrane
Permeability of Membrane to Na+ Concentration gradient: Na+ into cell Electrical gradient: Na+ into cell Electrochemical gradient: Strong inward Channels: Few Effect: Low permeability of Na+ into the
cell
Permeability of Membrane to Cl-
Concentration gradient: Strong into cell Electrical gradient: Strong out of cell Electro chemical gradient: Weak outward Channels: Moderate Effect: Moderate permeability of Cl- out of
the cell
Permeability of Membrane to K+
Concentration gradient: Strong out of cell Electrical gradient: Strong into cell Electrochemical gradient: Weak outward Channels: Many Effect: High permeability of K+ out of cell
As K+ leaks out, they collect along the outer membrane due to negativity inside the cell
Bottom Line
The resting membrane potential is that “membrane potential” when the forces driving the influx/efflux of all ions are at equilibrium (no net movement of ions)
If membrane were permeable to only K+:RMP ~ -90 mV
Addition of Na+ and removal of Cl-RMP ~ -70 mV
Extracellular fluid
Intracellular fluid
Various fixed anionic structures
[Na+]
[Na+]
[Cl-]
[Cl-]
Least permeable. More permeable.
3 Na+
2 K+
[K+]
[K+]
Most permeable.
Neural Impulse
The resting membrane potential Basic progression of events:1. Cell body of the neuron must receive adequate
stimulus-All-or-nothing fashion
2. RMP is disrupted (depolarized)3. RMP is rapidly restored (repolarized)4. Propagation
Neural Impulse: Adequate Stimulus
Recall that the RMP is -70 mV The dendrites of a neuron will receive
multiple stimulus from multiple different neurons
Some of the neurons are excitatory and some are inhibitory
Neural Impulse: Adequate Stimulus
Excitatory neurons: Neurotransmitter: Acetylcholine Action: Activate sodium channels Na+ flows in which
increases the RMP (makes more positive) Inhibitory neurons:
Neurotransmitter: Gamma amino butyric acid (GABA) or glutatmate
Action: Open chloride channels Cl – flows in which decreases RMP
(more negative) Open potassium channels K+ flows out which decreases RMP
(more negative)
Neural Impulse: Adequate Stimulus
Excitatory neurons create EPSPs Excitatory postsynaptic potentials
Inhibitory neurons create IPSPs Inhibitory postsynaptic potentials
It is the sum of all EPSPs and IPSPs that determines the net stimulus
If the net stimulus exceeds ~15 mV, threshold is reached
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/ExcitableCells.html
Neural Impulse: Adequate Stimulus
All-or-nothing principle: The strength of an impulse is an intrinsic
property of that neuron Stronger stimuli do not increase the
strength of the impulse
Neural Impulse
The resting membrane potential Basic progression of events:1. Cell body of the neuron must receive adequate
stimulus-All-or-nothing fashion
2. RMP is disrupted (depolarized)3. RMP is rapidly restored (repolarized)4. Propagation
Neural Impulse - Depolarization
Depolarization: RMP -70 mV +30 mV Stimulus exceeds threshold Voltage-gated Na+ channels open
M gate Na+ flows into cell increasing RMP
H gate Change in charge closes second gate Depolarization activates adjacent voltage-gated Na+
channel Process continues along axolemma
MacIntosh et al. 2006, Fig 9.7
Neural Impulse - Repolarization
Repolarization: RMP +30 mV -70 mV H gate shuts Voltage gated K+ channels open
K+ flows out of cell Na-K+ pump assists Voltage gated K+ channels stay close
Overshoot of K+ outflow = hyperpolarization
Neural Impulse - Hyperpolarization
Also known as the “refractory period” Two parts:
Absolute refractory period: Due to the inactivation of the h gate Time: ~ 2.2 – 4.6 ms
Relative refractory period: Due to overshoot of K+ ion outflow past RMP Greater stimulus needed to create another action
potential
Neural Impulse - Propagation Propagation: The pattern of depolarization followed
by rapid repolarization along a membrane Differences between neurons and muscle fibers
Speed of transmission Muscle fiber: 3-6 m/s Neuron: 40-65 m/s
Saltatory conduction nodes of Ranvier High channel density
End result Muscle fiber: Muscle contraction Neuron: Neurotransmitter release onto neuron, gland, muscle
etc.
http://human.physiol.arizona.edu/sched/cv/wright/16action.htm
http://www.accessexcellence.org/RC/VL/GG/action_Potent.html
http://www.accessexcellence.org/RC/VL/GG/action_Potent.html
Agenda
Neural impulse Neurotransmitter release
Structural considerations of the neuromuscular junction (NMJ)
Basic progression of events
NMJ Structure
The NMJ includes:The distal neuron
Synaptic knobs/terminal endings/axon terminals Synaptic vesicles
The muscle fiber Motor end plate Primary and secondary synaptic clefts Acetylcholine receptors Sarcolemma
NMJ Structure – Distal Neuron
The distal neuron gradually loses its myelin as it approaches the muscle fiber
The neurons branch excessively and end in “boutons”Synaptic knobsTerminal endingsAxon terminals
NMJ Structure – Synaptic Knobs Lay in a semi-circle manner Do not make direct contact with muscle
fiber Function: Release neurotransmitter
AcetylcholineNEED FIGURE 3.1, MacIntosh
NMJ Structure – Synaptic Vesicles Small spheres located within
synaptic knobs Contain the neurotransmitter
Ach Formed when axolemma
becomes invaginated and “pinches off”
NMJ Structure
The NMJ includes:The distal neuron
Synaptic knobs/terminal endings/axon terminals Synaptic vesicles
The muscle fiber Motor end plate Primary and secondary synaptic clefts Acetylcholine receptors Sarcolemma
NMJ Structure – Muscle Fiber Motor end plate: The area of the muscle
fiber that makes “near” contact with the synaptic knob
NMJ Structure – Muscle Fiber Primary synaptic cleft: A small gap that separates the
membranes of the synaptic knobs and the muscle fiber ~70 nm
Secondary synaptic cleft: Regular repeated invaginations of the sarcolemma underneat the primary synaptic cleft Add Fig 3.1, MacIntosh
Acetylcholine receptors Embedded within plasmalella in junctional folds ~10,000/micrometer2 (2 binding sites/receptor) 5 subunits (2 bind Ach)
NMJ Structure – Muscle Fiber
Sarcolemma:Basement membrane lays over both synaptic
clefts Insulation
Contains acetylcholinesterase (AchE) Hydrolyzes Ach and stops synaptic transmission
Agenda
Neural impulse Neurotransmitter release
Structural considerations of the neuromuscular junction (NMJ)
Basic progression of events
Neurotransmitter Release
Basic Progression of Events: Action potential reaches synaptic knob Neurotransmitter release from synaptic vesicles Motor end plate depolarization Acetylcholinesterase hydrolyzes Ach Hydrolyzed Ach is resynthesized Resynthesized Ach is taken up by synaptic vesicles
Action Potential Reaches Synaptic Knobs Depolarization of synaptic knob activates
voltage-gated Ca2+ channels Ca2+ rushes into synaptic knob Role of Ca2+:
Assist with fusion of synaptic vesicles Assist with release of Ach from vesicles
Ca2+ mediated release of Ach is the rate limiting step (~0.2 ms)
Ach Release from Vesicles
Prior to Ca2+ influx, vesicles are “docked” Ca2+ assists with fusion with axolemma Ca2+ assists with Ach release from
vesicles via exocytosis New vesicles are created via endocytosis
Prevents build-up of tissue at synaptic knob
Ca2+
Marieb & Mallett, 2005, Fig 12.8
Motor End Plate Depolarization
2 Ach bind receptors opening center pore of receptorNa+ flows into cellK+ flows out less rapidly
Voltage-gated Na+ channels activated around the motor end plate
Sarcolemma depolarizes and action potentials propagate
Ach
Na+ K+
-Decreased negativity inside
-Increased RMP
-Motor end plate depolarized
AchE Hydrolyzes Ach
Upon receptor activation, Ach molecules dissociate
Ach molecules fall into secondary synaptic cleft
AchE hydrolyze acetate + choline molecules
Ach
AchE
A + Ch
Acetate Choline
AchE
Ach
Ach is Resynthesized
Choline molecules are absorbed into the synaptic knob
Choline acetyltransferase resynthesizes AchAcetyl CoA + choline acetylcholine
Acetyl CoA from mitochondria
New Ach Vesicles
Acetylcholine transporter assists with uptake of resynthesized Ach into vesicles
Filled vesicles dock near the axolemma