What factors determine the strength of muscle contraction generated by a single motoneurone supplying striated muscle? Does this apply to C.N.S. synapses?
Sanjay Manohar (Oxford 2015)
Somatic = voluntary = striated = skeletal muscle
Key experiments to describe:
Sherrington 1920
- definition motor unit = alpha motoneuron + fibre
- mn pool - neurones innervating 1 muscle
Sherrington 1906 - stroking foot of dog - temporal summation
Ramsay and Street 1939
- semitedinosus of frog; tetanic stimulus
- isometric force measure -> length-tension curve Eccles 1941 - endplate potentials
Fatt & Katz 1951
- minature EPP = fluctuations 100th size of response to AP
- smaller with curare, larger with pyridostigmine = cholinergic
- calcium depletion reduces EPP but not mEPP; Na depletion reduces both.
Eccles 1960 crossed innervation study
Gordon Huxley & Julian 1965
- sliding filament theory
- interference microscopy
- sarcomeres 2 microns long.
What affects force:
===================
motor unit size (number of fibres)
neuron's firing rate
length of fibre
fibre type
number of receptors
calcium release
receptor blockade
Myasthenia - nachr blocker
alpha bungarotoxin (blocker, venom)
botulinum toxin (prevents release)
fatigue; duration of stimulation
recruitment
number
order (smaller first, Henneman's size principle)
Muscle CNS
============================================
MEPP without input EPSP/IPSP with input
reliable AP unreliable AP
Ca/Mechanical summation EPSP summation
80 ns twitch, tetany, fusion probability of AP
"Motor unit" Many input cells
= no inputs to sum dendrites sum inputs
large complex endplate small dendritic spine
temporal integration by calcium shorter time
force is graded firing rate is graded
nAChR many receptors and transmittors
fast twitch slow twitch
============================================
type 2 type 1
larger motor unit smaller
adapting nonadapting
anaerobic aerobic
high frequ input 30/60Hz lower frq 10-20 hz
large diameter axon smaller
power posture
white red - myoglobin