NEUR3001 Lecture Notes - Lecture 1: Skeletal Muscle, Vapa, Neuromuscular Junction
Neurotransmission: Methods for Measuring &
Role of Ca2+ Ions
1. Method Used to Measure Neurotransmission
• Fatt & Katz
Intracellular recordings (+ve signals) Miniature end-plate potentials (MEPPs) & end-plate potentials (EPPs)
• Gopfert & Schaefer
Extracellular recordings (-ve signals) Miniature end-plate currents (MEPCs) & end-plate currents (EPCs)
MEPPs & MEPCs
EPPs & EPCs
Spontaneous release of transmitter
Evoked by nerve AP
Low frequency
Synchronised release from many release sites
Uni-modal gaussian distribution, sometimes multi-
modal
(not all NT released, some are pulled back)
Skewed distribution low release rate
Gaussian distribution high release rate
QUANTAL SIZE
Multi QUANTA
No. of observations vs EPP/EPC amplitude = quantal
nature
2. Quantal Nature of Transmitter Release
• Low extracellular Ca2+ concentration, size of EPP fluctuates
• Release in discrete packets of quanta
• Quantal size determined by MEPP amplitude
• EPPs made up of multiple quanta units
2.1. Ways to determine quantal content (Avg no. of quanta released per nerve stimulation)
I. Method of failures
• Used when no. of stimulations failed to register an EPP >30%
• Poisson distribution (skewed)
•
II. Binomial method
• Used when no. of stimulations failed to register EPP <30%
•
•
3. Intermittent Nature of Transmitter Release
• Most release sites (>66%) have p <0.005, small number (<10%) have p >0.2
o Variable Ca2+ entry
o Variable arrangement og vesicular associated proteins
o Variable delivery of vesicles
o Variable levels of phosphorylation
4. Latency between Terminal Stimulation & Post Synaptic Potential
Events contributing to latency (~200μs):
I. VGCCs activated by depolarisation
II. Ca entry to form microdomain
III. Activation of vesicular associated proteins
IV. Fusion of vesicle and exocytosis of NT
V. NT diffuses across synaptic cleft
VI. Activation of postsynaptic ligand gated channels
VII. Ion flow through channels and depolarisation of postsynaptic cell
5. Calcium Movement
I. Diffusion
- Flood of Ca into region = microdomain
- Ca diffused in low concentration, delays apoptosis
II. Ca-binding proteins
- Removes Ca changes phosphorylation state of terminal Activates vesicle movement into active
zone
- Inflow of Ca activates vesicular associated proteins funnel towards active zone vesicular
membrane + presynaptic membrane fuse exocytosis induced
III. Endoplasmic reticulum
- Puffs Ca into ER from cytoplasm
IV. Mitochondria
- Stored Ca and removal of Ca, entry of Na = ATP required
V. Ca/Na-exchanger
- Active zone holds up to 20 vesicles in 2 rows parallel to each
other
Presynatpic terminal branches
Postsynaptic Ach receptors
Each terminal branch bar aligned to
an Ach bar to allow fast transmission
(misalignment = disorder) Allows
skeletal muscle movement
Document Summary
Role of ca2+ io(cid:374)s: method used to measure neurotransmission, fatt & katz. Intracellular recordings (+ve signals) miniature end-plate potentials (mepps) & end-plate potentials (epps: gopfert & schaefer. Extracellular recordings (-ve signals) miniature end-plate currents (mepcs) & end-plate currents (epcs) Uni-modal gaussian distribution, sometimes multi- modal (not all nt released, some are pulled back) No. of observations vs epp/epc amplitude = quantal nature: quantal nature of transmitter release. Low extracellular ca2+ concentration, size of epp fluctuates: release in discrete packets of quanta, quantal size determined by mepp amplitude, epps made up of multiple quanta units. Ways to determine quantal content (avg no. of quanta released per nerve stimulation: method of failures, used when no. of stimulations failed to register an epp >30, poisson distribution (skewed) Ion flow through channels and depolarisation of postsynaptic cell: calcium movement. Ca diffused in low concentration, delays apoptosis. Removes ca changes phosphorylation state of terminal activates vesicle movement into active zone.
