Neuromuscular Function:
Neural Impulse and
Neurotransmitter Release
Muscle Physiology
420:289
Agenda

Nerve impulse
 Introduction
 Channels
and pumps
 The 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
 Introduction
 Channels
and pumps
General properties
Regulatory 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
 Introduction
 Channels
and pumps
General properties
Regulatory 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
 Sarcolemma
 Synaptic cleft
nodes of Ranvier
 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

1.
2.
Function:
Maintain RMP at rest
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:

 E1
 E2
4. Pi changes
back to E2 and
releases 3 Na+
and picks up 2
K+
3. E1 releases
ADP and picks
up 3 Na+ and Pi
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
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:


Dihydropyridine (DHP) channels


Embedded within the sarcolemma of muscle
Ryanodine (RYR) channels


Embedded within the axolemma of neuron
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 ClResult: Inability of muscle fiber to restore RMP
following initial disruption
Myotonic goat video
Agenda

Nerve impulse
 Introduction
 Channels
and pumps
 The neural impulse

Neurotransmitter release
Neural Impulse


1.
The resting membrane potential
Basic progression of events:
Cell body of the neuron must receive adequate
stimulus
-All-or-nothing fashion
2.
3.
4.
RMP is disrupted (depolarized)
RMP is rapidly restored (repolarized)
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 mV

inside of the cell has a charge of –70 mV
relative to the outside of the cell
 The
Neural Impulse

1.
2.
How is the RMP maintained? Several
mechanisms:
Fixed anion structures increase negativity
within cell
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 ClConcentration 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
3 Na+
Least permeable.
More permeable.
[Na+]
[Cl-]
[K+]
[Cl-]
[K+]
[Na+]
2 K+
Intracellular fluid
Most permeable.
Various fixed
anionic structures
Neural Impulse


1.
The resting membrane potential
Basic progression of events:
Cell body of the neuron must receive adequate
stimulus
-All-or-nothing fashion
2.
3.
4.
RMP is disrupted (depolarized)
RMP is rapidly restored (repolarized)
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

Inhibitory neurons create IPSPs
 Inhibitory


postsynaptic potentials
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/Bi
ologyPages/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


1.
The resting membrane potential
Basic progression of events:
Cell body of the neuron must receive adequate
stimulus
-All-or-nothing fashion
2.
3.
4.
RMP is disrupted (depolarized)
RMP is rapidly restored (repolarized)
Propagation
Neural Impulse - Depolarization



Depolarization: RMP -70 mV  +30 mV
Stimulus exceeds threshold
Voltage-gated Na+ channels open


Na+ flows into cell increasing RMP




M gate
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


Muscle fiber: 3-6 m/s
Neuron: 40-65 m/s


 End


of transmission
Saltatory conduction  nodes of Ranvier
High channel density
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
knobs
 Terminal endings
 Axon terminals
NMJ Structure – Synaptic
Knobs
Lay in a semi-circle manner
 Do not make direct contact with muscle
fiber
 Function: Release neurotransmitter

 Acetylcholine
 NEED
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

Secondary synaptic cleft: Regular repeated
invaginations of the sarcolemma underneat the
primary synaptic cleft
 Add

nm
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
Marieb & Mallett, 2005, Fig 12.8
Ca2+
Motor End Plate Depolarization

2 Ach bind receptors opening center pore
of receptor
 Na+
flows into cell
 K+ 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
AchE
Ach
A
Acetate
+
Ch
Choline
Ach is Resynthesized
Choline molecules are absorbed into the
synaptic knob
 Choline acetyltransferase resynthesizes
Ach

 Acetyl

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
