NEUROMUSCULAR
JUNCTION
AND
EFFECT OF ELECTRICAL
STIMULATION ON NERVES
SYED MASOOD

NEUROMUSCULAR JUNCTION
 It is a junction between the terminal branch
of the nerve fiber and muscle fiber.
 The membrane of nerve is called presynaptic
membrane.
 The membrane of muscle fiber is called
postsynaptic membrane.
 Space between two is called synaptic cleft.

THE NEUROMUSCULAR JUNCTION

THE NEUROMUSCULAR JUNCTION

THE NEUROMUSCULAR JUNCTION
• Arrival of an action potential at the terminus of a presynaptic motor
neuron induces opening of voltage-gated Ca2+ channels
• subsequent release of acetylcholine, which triggers opening of the
ligand-gated nicotinic receptors in the muscle plasma membrane
• The resulting influx of Na+ produces a localized depolarization of the
membrane
• leading to opening of voltage-gated Na+ channels and generation of an
action potential
*
*
*

ACETYLCHOLINE – GENERAL INFO
• Motor neuron transmitter at the
neuromusccular junction (NMJ) in
vertebrates
• Present in brain (10% of synapses)
• Packaged in high numbers in vesicles
1,000 to 10,000 molecules per vesicle at
the NMJ
• Like all small chemical transmitters Ach is
synthesized and packaged into vesicles in
the synapse
• The NMJ pre-synaptic side is packed full of
vesicles in the axon terminal
• Many vesicles are released per action
potential to ensure a large safety margin so
that the muscle fiber (i.e. the postsynaptic
cell) will depolarize to beyond threshold.

ACETYLCHOLINE – RECEPTOR
• Officially called the nicotinic ACH receptor
(nAChR) because nicotine binds to this
receptor and activates it
• ligand gated ion channel
• has a depolarizing effect because Na+ is the
dominant ion through these channels

ACETYLCHOLINE – RECEPTOR
• generates an excitatory postsynaptic potential which at the NMJ
(motor end plate) is often called an "end plate potential“

EPP - END PLATE POTENTIAL
 Aka Excitatory Junctional Potential (EJP)
End plate potentials (EPPs) evoked by stimulation of a motor neuron are
normally above threshold and therefore produce an action potential
in the postsynaptic muscle cell.

POSTSYNAPTIC MEMBRANES AND ION CHANNELS
Ligand gated ion channels – a
review
a. Resting K+ channels: responsible for generating the resting potential
across the membrane
b. Voltage- gated channels: responsible for propagating action potentials
along the axonal membrane
Two types of ion channels in dendrites and cell bodies are responsible for
generating electric signals in postsynaptic cells.
(c) Has a site for binding a specific extracellular neurotransmitter
(d) Coupled to a neurotransmitter receptor via a G protein.

NEUROMUSCULAR BLOCKING AGENTS
 Ultra-Short: Succinylcholine chloride
 Short: Mivacurium chloride
 Intermediate: Rocuronium bromide,
Vecuronium bromide, Atracurium
besylate
 Long: Pancuronium bromide, curare,
metocurine, Pipecuronium
bromide, Doxacurium chloride

ELECTRICAL STIMULATION TECHNIQUES

CURRENT FLOW
 Electron Flow
(shown in red)
 Between the generators and
electrodes
 To and from the generator
 Ion Flow
(shown in yellow)
 Occurs within the tissues
 Negative ions flow towards the
anode and away from the
cathode
 Positive ions flow towards the
cathode and away from the
anode
+
+
-
-

ELECTRODES
 Purpose
 Completes the circuit between the generator and body
 Interface between electron and ion flow
 Primary site of resistance to current
 Materials
 Metallic (uses sponges)
 Silver
 Carbon rubber
 Self-adhesive

ELECTRODE SIZE
 Determines the Current Density
 Equal size
 Bipolar arrangement
 Approximately equal effects under exach

ELECTRODE ARRANGEMENTS
 Based on:
Current Density
Proximity to Each Other
Anatomic Location (Stimulation Points)

CURRENT DENSITY
 Bipolar Technique
 Equal current densities
 Equal effects under each electrode
(all other factors being equal)
 Monopolar Technique
 Unequal current densities
 At least 4:1 difference
 Effects are concentrated under the smaller electrode
 “Active” electrode(s)
 No effects under larger electrode
 “Dispersive” electrode
 Quadripolar Technique
 Two bipolar electrode arrangements
 Two independent electrical channels
 TENS is a common example
“Active” “Dispersive”

ELECTRODE PROXIMITY
 Determines the number of
parallel paths
 The farther apart the
electrodes the more parallel
paths are formed
 More current is required to
produce effects as the
number of paths increases

STIMULATION POINTS
 Motor Points
 Superficial location of motor nerve
 Predictably located
 Motor nerve charts
 Trigger Points
 Localized, hypersensitive muscle spasm
 Trigger referred pain
 Arise secondary to pathology
 Acupuncture Points
 Areas of skin having decreased electrical resistance
 May result in pain reduction
 Traumatized Areas
 Decreased electrical resistance (increased current flow)

PATH OF LEAST RESISTANCE
 Ion flow will follow the path of
least resistance
 Nerves
 Blood vessels
 The current usually does not
flow from electrode-to-
electrode (the shortest path)
 The path of least resistance
is not necessarily the
shortest path

SELECTIVE STIMULATION OF NERVES
 Nerves always depolarize in the same order
 Sensory nerves
 Motor nerves
 Pain nerves
 Muscle fiber
 Based on the cross-sectional diameter
 Large-diameter nerves depolarize first
 Location of the nerve
 Superficial nerves depolarize first

PHASE DURATION AND
NERVE DEPOLARIZATION
 Phase duration selectively depolarizes
tissues
Phase Duration Tissue
Short Sensory nerves
Medium Motor nerves
Long Pain nerves
DC Muscle fiber

ADAPTATIONS
 Patients “get used” to the treatment
 More intense output needed
 Habituation
 Central nervous system
 Brain filters out nonmeaningful, repetitive information
 Accommodation
 Peripheral nervous system
 Depolarization threshold increases
 Preventing Adaptation
 Vary output (output modulation) to prevent
 The longer the current is flowing, the more the current must be
modulated.

ELECTRICAL STIMULATION GOALS

MOTOR-LEVEL STIMULATION
COMPARISON OF VOLUNTARY AND ELECTRICALLY-INDUCED CONTRACTIONS
Voluntary
 Type I fibers recruited
first
 Asynchronous
 Decreases fatigue
 GTO protect muscles
Electrically-induced
 Type II fibers recruited
first
 Synchronous
recruitment
 Based on PPS
 GTOs do not limit
contraction

MOTOR-LEVEL STIMULATION
 Parameters:
Amplitude: Contraction strength increases as
amplitude increases
Phase duration: 300 to 500 µsec targets motor
nerves:
 The shorter the phase duration, the more amplitude
required
 Longer durations will also depolarize pain nerves
 Pain often limits quality and quantity of the contraction
Pulse frequency: Determines the type of
contraction

PULSE FREQUENCY
 Frequency determines the time for mechanical
adaptation
 Lower pps allows more time (longer interpulse
interverals)
Label Range Result
Low < 15 pps* Twitch: Individual contractions
Medium 15-40 pps* Summation: Contractions blend
High >40 pps* Tonic: Constant contraction
* Approximate values. The actual range varies from person-to-person
and between muscle groups

EFFECT OF PULSE FREQUENCY ON MUSCLE
CONTRACTIONS
1 pulse per second
Twitch Contraction
The amount of time
between pulses – the
interpulse interval – is
long enough to allow the
muscle fibers to return to
their original position
20 pulses per second
Summation
The amount of time
between pulses allows
some elongation of the
fibers, but not to their
starting point.
40 pulses per second
Tonic Contraction
The current is flowing so
rapidly that there is not
sufficient time to allow the
fibers to elongate

ELECTRICAL STIMULATION GOALS
Pain Control

PAIN CONTROL
Sensory-level Motor-Level Noxious Level
Target A-beta fibers Motor nerves A-delta
Tissue C fibers
Phase < 60 µsec 120 to 250 µsec 1 msec
Duration
Pulse 60 to 100 pps 2 to 4 pps Variable
Frequency 80 to 120 pps
Intensity Submotor Moderate to To tolerance
Strong contraction

ELECTRICAL STIMULATION GOALS
Edema Control and Reduction

EDEMA CONTROL
 Cathode placed over injured
tissues
 High pulse frequency
 Submotor intensity
 Thought to decrease capillary
permeability
 Do not use if edema has already
formed

EDEMA REDUCTION
 Muscle contractions “milk”
edema from extremity
 Electrodes follow the vein’s
path
 Alternating rate targets
muscle groups
 Elevate during treatment

ELECTRICAL STIMULATION GOALS
Fracture Healing

FRACTURE HEALING
 Electrical current triggers bone
growth
 Piezoelectric effect within the
collagen matrix
 Alternating current
 Applied transcutaneously
 Similar to diathermy units (no heat
production)
 Direct current
 Implanted electrodes

CONTRAINDICATIONS AND PRECAUTIONS
 Areas of sensitivity
 Carotid sinus
 Esophagus
 Larynx
 Pharynx
 Around the eyes
 Temporal region
 Upper thorax
 Severe obesity
 Epilepsy
 In the presence of
electronic monitoring
equipment
 Cardiac disability
 Demand-type pacemakers
 Pregnancy (over lumbar
and abdominal area)
 Menstruation (over lumbar
and abdominal area)
 Cancerous lesions (over
area)
 Sites of infection (over area)
 Exposed metal implants