Transcript Excitable tissue

Excitable tissue
The fascinating world of muscles,
nerves and little sparkly things.
Dr Hannah Rose
EDGH 2005
The Nerve Cell
• Dendrites-afferent,
numerous,
excitatory/inhibitory
• Cell body
• Axon hillock
• Axon – efferent, single,
ends on axon/dendrite/cell
body/muscle
• +/- myelin and Nodes of
Ranvier
Types of nerves
Type
Myelin
Velocity Function
(m/s)
Thickness
Aa
+
120
Motor
Ab
+
60
Touch/
Pressure
Ag
+
30
Muscle
spindle
Ad
+
15
Pain/T
b
+
<15
ANS
pregang
C
-
<2
Pain/ANS <1mm
postgang
1-20mm
1-3mm
The Action Potential
• -70mV RMP (Nernst)
• Stimulus-ligand/elec/local
currents
• Quanta-MEPP (IPSP/EPSP)
• Summation (temporal/spatial)
• -55mV TP
• ALL or NOTHING
• Depolarisation - Na
• Repolarisation - K
• After- and Hyperpolarisation
• Na/K/ATPase
The Neuromuscular Junction
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Chemical Bridge
Axon buttons into m fibre
En plaque / En grappe
50nm wide
ECF
Folded m membrane
Shoulders – receptors
Clefts – enzyme
Presynaptic receptors
Getting excitable
• Wave of depolarisation
• Ca influx
• ACh from 200 vesicles
=10 000 molecules
• ACh into cleft
• Bind to Nicotinic
receptors
• Broken down AChE
Ach / Nicotinic receptor
• From Choline (diet and
recycling) + CoEnzyme A enzyme=choline
acetyltransferase
• Hydrolysed by ACHesterase
into acetate and choline in 1ms
• Receptor = Pentameric
Nicotinic transmembranal
ligand gated ion channel
(Na/Ca)
• a,a,b,e,d subunits
• Bind a – increased affinity for
2nd a- opening of channel when
2nd bound
• Resultant AP – m contraction
and +ve feedback
Show us your muscles
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AP flows down T-tubule
Ca released from SR via Voltage
gated Ryanodine receptor
(remember Matt’s talk?)
Ca binds to Troponin
Tropomysin forced to reveal the
myosin binding site on Actin
Myosin and actin bind releasing
ATP
Fuels the SLIDING FILAMENT
HYPOTHESIS
Contraction
Ca pumped into SR
Relaxation
EXCITATION CONTRACTION
COUPLING
Muscle
Contraction
Muscle Types
• Skeletal – Type 1 - Slow Red Oxidative
Type 2 –Fast White Glycolytic
Type 3 – Fast Oxidative
• Smooth
• Cardiac
• Love
Smooth Muscle
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a and b adrenergic autonomic receptors, hormones, 2nd messengers,
local factors
Involuntary
Spindle shaped, single nucleus
Less mitochondria-glycolysis dependant
No troponin and poorly developed SR
1/3rd Myosin, but 2x Actin
May function as a syncitium due to gap junctions
+/- spontaneous action potentials (unstable RMP) with Ca influx
causing depolarisation
Ca released from SR and Ca channels-bind to Calmodulin-myosin light
chain kinase-cross bridging
May have graded contractions (no all or none phenomenon)
Cardiac
No
Smooth
No
Few
Sarcomere Yes
SR
+++
Synctium No
Pacemaker No
Response All or
none
Many
Yes
++
Yes
Yes fast
All or
none
Few
No
+
Yes
Yes slow
Graded
Tetanic
No
Yes
Motor
endplate
Mito
Skeletal
Yes
Yes