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Neural and Sensory
• Efferent innervation– Neuromuscular junction– -, -motorneurons
• Afferent innervation– Muscle spindle– Golgi tendon organ
Overview
• Motor Unit– Motorneuron + many muscle fibers– Cholinergic synapse
• Spindle– Enapsulated, specialized muscle-like cells– Combined length-velocity proprioception
• Tendon organ– Nerve endings woven through collagen– Active tension mechanorecption
Efferent Innervation• a-motorneuron
– 100 um cell body/soma• Ventral horn of spinal cord• Columnar organization by muscle
– 8-18 um diameter axon• 60-120 m/s conduction velocity• Terminal branches within target muscle
– Dendritic tree• 10,000+ inputs per MN• Excitatory glutamic synapses• Inhibitory glycinergic/GABAergic synapses
Dorsal/posterior
Ventral/anteriorMN
densityCervical
Thoracic
Lumbar
Sacral
Arm/hand
Leg/foot
Neuromuscular junction
• Motor endings– Synaptic vessicles– Mitochondria
• Synaptic cleft– Primary– Secondary
invaginations• Muscle
– Post-synaptic density– Mitochondria
deHarven & Coers, 1959
By SEM
Sohn & al., 1999
Synaptic vessicle anchoring
• SV anchored by “pegs and ladders”
Harlow & al., 2000
Synaptic structure
• Complementary– Presynaptic active zone– Postsynaptic density/receptor
• ECM coordination– NerveagrinMuSK– MuscleLam 421/521VGCC
• Electrical coordination– Nerve: ubiquitinylation preserves synaptotagmin– Muscle: depolarization degrades AChR
Presynaptic side
• Docking site– SNAP-25– Syntaxin (T-SNARE)
• Synaptic vesicle– Synaptobrevin (V-SNARE)– Synaptotagmin
• Docking trimer: t/v-SNARE+SNAP• Calcium sensitivity: synaptotagmin
Presynaptic active zone
De Wit & al., 2009
Presynaptic active zone
Garner 2002
Postsynaptic side
• Post-synaptic density– MuSK/LRP4 receptor– Rapsyn/Dok-7 adapter– Src kinase– Acetylcholine receptor
• Junctional folds– Sodium channels– Acetylcholinesterase
Post-synaptic density
Shi & al., 2012
Afferent innervation
• Afferent– Sensory– Dorsal root ganglion– Synapses in SC
• Spindle– Muscle-like
cells– Mechanosensitive
nerve ending
Muscle Spindle
• Capsule• Polar regions
– -motorneurons– Striated contractile proteins
• Equatorial region– Primary afferent (Ia)– Secondary afferent (II)– Non-contractile– Nuclear bag & chain
Barker, 1948
Spindle afferents
Barker, 1948
Spindle efferents
• Dynamic bag/bag1– Usually only influences the primary ending– Increases velocity gain
• Static bag/bag2– Offset both primary & secondary– Reduces length sensitivity of primary
• Chain– Take up slack– Increase length sensitivity of secondary
Spindle Mechanics
• Equatorial stretch induces depolarization– Windings amplify length change– Stretch imposed from outside– Stretch imposed by contraction of poles
• Accommodation– Viscoelastic creep/relaxation– Primary fast: velocity– Secondary slow: length
Stretch responsePrimary ending
Houk & al, 1981
Secondary ending
Afferent firing rate
Muscle length
Ian Boyd video clip
Stretch Reflex
• Ia afferent“homonymous” motorneuron– Servo-like length control– Tendon tap– Vibration illusions
• IaIa interneuron– Inhibit antagonist(s)– Mechanical equivalence– Myotatic unit
• IIdorsal horn interneurons– Distributed, slower excitation
Golgi tendon organ
• Free nerve endings embedded in capsule at muscle-tendon interface
• Group Ib
Nitatori, 1988
Goldfinger & Fukami 1982
GTO response
• MTJ tension depolarizes afferent
Muscle length
Muscle force
GTO discharge rate
Crago & al., 1982
GTO Reflex
• IbIb interneuron– Delayed homonymous inhibition– Distributed excitation/inhibition– “Interaction” torques– Locomotor pattern integration
• Polysynaptic pathways are subject to extra modulation
Summary
• -motorneuron efferent forms coordinated NMJ– Presynaptic active zone for ACh release– Postsynaptic densities for ACh transduction
• Monosynaptic stretch reflex– Combined length and velocity signal– Intrafusal fiber mechanics mimic extrafusal fibers
• Polysynaptic GTO reflex– Force feedback– Distributed