1 Physiology and Pathophysiology of Neuromuscular Transmission “The neuromuscular junction... [is] an experimentally favourable object whose study could throw considerable light on synaptic mechanisms elsewhere” Sir Bernard Katz, Fenn Lecture, IUPS Glasgow, 1993 http://www.ricercaitaliana.it/prin/dettaglio_completo_prin_en-2004053317.htm
29
Embed
Physiology and Pathophysiology of Neuromuscular Transmission€¦ · 1 Physiology and Pathophysiology of Neuromuscular Transmission “The neuromuscular junction... [is] an experimentally
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Physiology and Pathophysiology ofNeuromuscular Transmission
“The neuromuscularjunction... [is] anexperimentally favourableobject whose study couldthrow considerable lighton synaptic mechanismselsewhere”
‘Bassoon’ immunostaining localises to active zones
Pseudocoloured
Surface Plot
Ca imaging during high frequency stimulation
Greg Lnenicka
OH
N N
O
OHN
OH
O-
N N
O
OHN
OH
- H+ + H+
λabs = 397 nm
λabs = 475 nm
chromophore
Measuring exocytosis with “synaptopHluorin”
4
Recycling vesicles take up fluorescent styryl (“FM”) dyes
hνHydrophobic Hydrophilic
Bewick
Desaki & Uehara, 1981
5
Ch.2
10 mV
5.00 ms
Latency
(1-2 ms)
Amplitude
(1-40 mV)
Rise Time(1-2 ms) Half-decay Time
(2-3 ms)
Typically-measured characteristics of the EPP (or MEPP)
Ch0
-5 mV 5.00 ms
1
2
3
4
0
Action potentials are “all-or-nothing” signals...… but EPPs are variable responses
Quantal size = Effect of one vesicle released
Quantal content = Number of vesicles released
6
Gillingwater D. Thomson
5 ms
Facilitation
300 ms
10 mV
Short-term Depression
EPP’s show short-term plasticity
Physiology and Pathophysiology ofNeuromuscular Transmission
1. Botulism and Myasthenias
2. Characteristics of MEPPS and EPPS
3. Quantal analysis
4. Safety factor and size-strength relationships
Physiology and Pathophysiology ofNeuromuscular Transmission
1. Botulism and Myasthenias
2. Characteristics of MEPPS and EPPS
3. Quantal analysis
4. Safety factor and size-strength relationships
7
Botulinum toxins cleave SNARE proteins
MG: AChR antibodies
X X
Myasthenia gravis and LEMS are autoimmune diseases
LEMS: Ca channelantibodies
X X
EPP’s teeter on the brink of the muscle fibreaction potential firing threshold in myasthenias
8
• Weight loss • Slurred speech• Weak shoulder abduction and hip flexion• Reflexes absent, but re-appear after exercise• Sensation normal
Lambert-Eaton Myasthenic Syndrome
Complete recording from one LEMS patient demonstrating an initial small restingcompound muscle action potential(CMAP), followed by a 10-second period of maximal
voluntary contraction and subsequently 30 CMAPs illustrating augmentation andexponential decay.
Maddison P et al. Neurology 1998;50:1083-1087
0 Ca +Ca +4AP TTXDirectDirect +Mg +4AP
9
Myasthenia GravisBefore
After edrophonium(Tensilon Test)
• Bilateral ptosis• Double vision in all directions• Fatiguable weakness• Reflexes disappear after exercise• Sensation normal
dTC dTC neo sux sux sux neo direct
Pre- and post-synaptic abnormalities have distinctive effects on EPPs
- Normal presynaptic function Normal quantal content (impaired postsynaptic function)
- Impaired presynaptic function Low quantal content (normal postsynaptic function)
Synaptic Depression
Synaptic Facilitation
10
Lambert-Eaton Myasthenic Syndrome
EMG
EPPs have low quantal contentand show facilitation
EPP
Normal
LEMS
Myasthenia gravisEMG
Intracellular recording - NMJ
Summary of electrophysiological changes inMyasthenia Gravis and Myasthenic Syndrome
(NI=Normal Individual)
50
11
C h.0
5 m V
5 .0 0 m s
C h .0
5 m V
5 .0 0 m s
C h .0
5 m V
5 .0 0 m s
C h .0
5 m V
5 .0 0 m s
Neostigmine (5 µM)
Control
Kosala Dissanayake
Anticholinesterases increase EPP amplitude and prolong EPP decay time
Physiology and Pathophysiology ofNeuromuscular Transmission
1. Botulism and Myasthenias
2. Characteristics of MEPPS and EPPS
3. Quantal analysis
4. Synaptic strength and safety factor
Desaki & Uehara, 1981, J Neurocytol 10,101
12
MEPPs (aka ‘minis’) are independent eventsthat occur with low release probability. Thisgenerates and exponential, Poisson distributionof intervals between events.
If the mean frequency is m (s-1), then thefrequency of a given number of MEPPs, x, ineach one second raster sweep is given by:
P(x) = exp(!m).m
x
x!
Mini analysis
Fatt & Katz, 1952, JPhysiol
Amplitude
Interval
y = exp(!(x ! µ)2 / 2" 2) / (" 2# )
MEPP
EPP
13
Physiology and Pathophysiology ofNeuromuscular Transmission
Endplate area, fibre diameter and MEPP amplitude, frequency are correlated
Quantal size (q) = response to a single vesicular release (i.e. the amplitude of the spontaneous MEPP)
Abnormalities in quantal size indicate a postsynaptic problem
Quantal content (m) = amount of transmitter released (i.e. the number of synaptic vesicles producing an EPP)
Abnormalities in quantal content indicate a presynaptic problem
http://neuromuscular.wustl.edu/musdist/dag2.htm
Neuromuscular Junction: postsynaptic
29
Congenital Myasthenic Syndromes
Palace & Beeson (2008) J Neuroimmunol
SUMMARY
1. Variation in MEPP interval and EPP amplitudeconforms to a Poisson Distribution
2. Quantal content of EPP’s can be estimated byDirect, ‘Failures’, and Variance Methods.Remember to make allowance, if necessary, fornon-linear summation of synaptic potentials
3. Quantal content at rodent NMJ’s is about 50 andthe ‘safety factor’ is about 3.
4. Determinants of synaptic strength and safety-factor at the NMJ include Ca sensitivity (LEMS),ACh receptor density (MG), endplate size(CMS), and muscle fibre size (input resistance),