Neuromuscular anatomy physiology

1. DR TR SHRESTHA, KMCTH

2. Motor Neuron
▪ Nerve that controls skeletal muscle activity

3. Motor Neuron
▪ Originate in the ventral horn of the spinal cord
▪ Each motor neuron connects to several skeletal muscle
fibres/cells to form a motor unit (functional group)
▪ Most human muscles have only one NMJ per cell
▪ Some cells in extraocular muscles innervated with
several NMJs
▪ As the motor neuron enters a muscle, the axon divides
into telodendria, the ends of which, the terminal buttons,
synapse with the motor endplate

5. Nerve terminal
▪ As the terminal reaches muscle fiber, it loses its myelin,
forms a spray of terminal branches against the muscle
surface, covered by Schwann cells.
▪ Vesicles clustered about the membrane thickenings, (active
zones), toward synaptic side and mitochondria and
microtubules toward other side
▪ Synaptic cleft: nerve terminal separated from the surface of
the muscle ~approximately 20 nm
▪ Nerve and muscle are held in tight alignment by protein
filaments: basal lamina

7. Motor Endplate
▪ Muscle surface is heavily corrugated, with deep
invaginations of the junctional cleft—the primary and
secondary clefts
▪ Oval in shape, covering an area -3000μm2
▪ Sodium channels located in the depths of the folds
▪ Shoulders of the folds populated with Ach receptors ~5
million in each junction
▪ Clefts contain AchE

8. ▪ Perijunctional zone: area of muscle immediately beyond
the junctional area, critical to function of NMJ
▪ Mixture of receptors, including a smaller density of
AChRs and a high density of sodium channels
▪ Responds to the depolarization produced by the AChRs
and initiate muscle contraction

9. Ach synthesis
▪ Choline+Acetyl CoA Ach
▪ Ach stored in cytoplasm until it is transported and incorporated
into vesicles
▪ Acetyl-coA from pyruvate in mitochondria
▪ 50% of choline from ECF by active transport
▪ 50% from Ach breakdown at NMJ
▪ Choline from diet or hepatic synthesis
▪ Choline acetyltransferase produced on ribosomes in cell body of
motor neurone→transported distally by axoplasmic flow to the
terminal buoton
Choline acetyltransferase

10. Storage
▪ Quanta of Ach + ATP stored in vesicles within terminal buoton
▪ Each vesicle: approximately 10,000 Ach molecules
▪ Vesicles are loaded with Ach via a Mg++ dependent active
transport system in exchange for a H+
▪ Readily releasable pool, VP2: Responsible for the maintenance of
transmitter release under conditions of low nerve activity,1% of
vesicles.
▪ Reserve pool, VP1: Released in response to nerve impulses. 80%
of vesicles.
▪ Stationary store: The remainder of the vesicles.

12. Storage
▪ Majority of the synaptic vesicles (VP1) are sequestered in
the reserve pool
▪ Tethered to cytoplasmic skeleton in a filamentous network
made up of primarily actin, synapsin, synaptotagmin and
spectrin
▪ May be moved to the readily releasable store to replace
worn-out vesicles or to participate in transmission when
the nerve is stimulated at very high frequencies or for a
very long time

13. Release
▪ Spontaneous release of single vesicles of Ach occurs randomly
and results in miniature endplate potentials of 0.5-1mV
(unknown function)
▪ Arrival of a nerve AP, Na+ from outside flows across membrane
→ resulting depolarizing voltage opens large numbers of Ca++
channels in the terminal membrane → Ca++ entry into the cell
▪ Number of quanta released by a stimulated nerve greatly
influenced by concentration of ionized calcium in ECF
▪ Calcium current persists until the membrane potential is
returned to normal by outward fluxes of potassium from inside
the nerve cell

14. Release
▪ Calcium channels: P channels and the slower L channels
▪ P channels immediately adjacent to the active zones
▪ Lambert Eaton myasthenic syndrome: autoimmune disease
in which antibodies are directed against Ca channels
• Higher-than-normal concentrations of bivalent inorganic
cations (e.g., Mg, Cd, Mn) can block the entry of Ca via P
channels
• L channels affected by Ca channel blocking drugs

15. Release
▪ Depolarization of presynaptic terminal and influx of Ca triggers
Ach vesicles to fuse with the presynaptic membrane at specific
release sites and release Ach into the synaptic cleft
▪ 200 quanta of approximately 5000 Ach molecules each released
▪ →Brief depolarization in the muscle that triggers a muscle AP
▪ The depleted vesicles are rapidly replaced with vesicles from the
readily releasable store and empty vesicles are recycled.
▪ At rest the free Ca concentration is kept below 10 –6M by a low
membrane permeability to Ca, active Na+/Ca++exchange pump
and mitochondrial sequestration.

16. Exocytosis

17. Acetylcholine Receptors
▪ Post-junctional membrane receptors of motor endplate: nicotinic
AchRs
➢ Junctional or mature receptor
➢ Extrajunctional or immature (fetal) receptor
➢ Neuronal α7 receptor
▪ Synthesized in muscle cells and are anchored to the end-plate
membrane by a special 43-kd protein: rapsyn
▪ 5 million AchRs on a normal endplate, situated mainly on the crests of
the junctional folds
▪ Each receptor is a protein comprised of 5 polypeptide subunits, form a
ring structure around a central ion channel

18. Receptor protein
▪ Molecular mass of approximately 250,000 daltons
▪ Mature receptor consists of 2 α, β, δ, and ε
subunits
▪ Fetal (immature, extrajunctional) receptor
consists 2α, β, δ, and γ-subunits
▪ Neuronal α7 AChR consists of five α7-subunits
▪ Receptor subunits consists of approximately
400 to 500 amino acids

19. Junctional Receptor
▪ Present in the post junctional membrane of the motor end
plate
▪ Consists of 2 α, β, δ, and ε subunits joined to form a channel
that penetrates through and projects on each side of the
membrane
▪ Each receptor has central funnel shaped core: ion channel,
4 nm in diameter at entrance narrowing to less than 0.7nm
within the membrane.
▪ 11 nm in length and extends 2nm into the cytoplasm of the
muscle cell

20. Junctional Receptor
▪ 2 gates, an upper voltage
dependent and a lower
time-dependent
▪ Ach binds to both α-
subunits→ conformation
change→opens channel
▪ For ions to pass through
the channel both the
gates should be open.

21. ▪ Ach bind to specific sites on the α subunits, when both are
occupied a conformational change occurs, opening the ion
channel for just 1 msec
▪ The channel allows movement of all cations, esp sodium that
predominates in terms of both quantity and effect.
▪ →Depolarisation, the cell becomes less negative
▪ When a threshold of –50mV is achieved (from a resting potential
of –80mV), sodium channels open → increase rate of
depolarization→ result in end plate potential of 50-100mV
▪ Triggers the muscle AP → muscle contraction

22. Extrajunctional Receptors
▪ Extra-junctional receptors found in their greatest
concentration around the endplate in the peri-junctional
zone
▪ Denervation injuries and burns are associated with large
increases in the number of extra-junctional receptors on
the muscle membrane
▪ Affects receptor: increased sensitivity to depolarising
muscle relaxants & reduced sensitivity to non-depolarising
relaxants.

23. Neuronal α7 receptors
▪ Consists of five α7 subunits
▪ When three subunits are bound by an antagonist, the two
other subunits are still available for binding by agonist and
cause depolarization
▪ Resistance to muscle relaxants when α7 AChRs are
expressed in muscle and in other tissues during pathologic
states like sepsis, denervation , immobilisation , burns

24. Prejunctional Receptors
▪ Present on the terminal bulb, have a positive feedback role
▪ Control ion channel specific for Ca++ which is essential for
synthesis and mobilization of Ach
▪ Composed of α and β subunits only
▪ In very active neuromuscular junctions Ach binds to these
receptors→increase in transmitter production via a second
messenger system
▪ Protein subunits that are blocked by non depolarising
muscle relaxants resulting in tetanic fade

25. Acetylcholinesterase
▪ Type B carboxylesterase enzyme, secreted by the muscle cell,
remains attached to it by thin collagen threads linking it to the
basement membrane.
▪ Found in junctional clefts, breaks down Ach to choline & acetate
▪ A molecule of Ach reacts with only one receptor before it is
hydrolyzed.
▪ Ach is a potent messenger, but its actions are very short lived
because it is destroyed in less than 1 msec after it is released.
▪ Inward current through the Ach receptor is transient and
followed by rapid repolarization to the resting state.

26. Acetylcholinesterase
▪ Active site in the AchE-- two regions: an ionic site with a
glutamate residue and an esteratic site with a serine
residue.
▪ Hydrolysis: Transfer of the acetyl group to the serine
residue → an acetylated enzyme and free choline
▪ Acetylated serine group→ rapid, spontaneous hydrolysis→
acetate and enzyme ready to repeat the process.
▪ The speed at which this occurs can be gauged by the fact
that approximately 10,000 molecules of Ach can be
hydrolysed per second by a single site.

27. Ach R Agonists
▪ Depolarizing muscle relaxants or nicotine and carbachol
▪ Act on these receptors to mimic the effect of Ach
▪ Cause depolarization of the end plate
▪ As SCh is not metabolized by AChE→persistently
depolarizes the motor endplate→ inactivation of voltage-
gated sodium channels with continuing depolarization

28. ▪ Reversible competitive antagonism of ACh at the α-
subunits of the AChRs
▪ Incapable of inducing the conformational change necessary
for ion channel opening
▪ Prevent Ach from binding to the receptor
▪ Prevent depolarization by agonists (acetylcholine,
carbachol, succinylcholine)
Ach R Antagonists

29. ▪ Reversal drugs or antagonists of neuromuscular paralysis
(neostigmine), inhibit acetylcholinesterase
▪ Impair the hydrolysis of Ach
▪ Increased accumulation of undegraded Ach can effectively
compete with NDMRs
▪ Thereby displace the NDMRs from the receptor (i.e. law of
mass action) and antagonize the effects of NDMRs
AChE Inhibitors

30. NEWBORN
▪ Just before birth, AChRs are clustered around junctional
area, and minimal extrajunctional AChRs are present.
▪ Newborn postsynaptic membrane: no synaptic folds, a
widened synaptic space, and a reduced number of AChRs
▪ Early postnatal: AChR clusters appears as an oval plaque
▪ Within a few days simplified folds appear.
NMJ at extremes of Age

31. ▪ With continued maturation, the plaque is transformed to a
multiperforated pretzel-like structure.
▪ Polyinnervated end plate converted to a singly innervated
juntion due to retraction of all but one terminal.
NMJ at extremes of Age

32. OLD AGE
▪ Anatomic changes: increased preterminal and axonal
branching within the individual NMJ, either with or
without an increase in the junctional size.
▪ The points of contact between the junctional and post-
junctional membrane decrease→ decline in trophic
interactions between nerve and muscle and stimulus
transmission→ age-associated functional denervation,
muscle wasting and weakness
NMJ at extremes of Age

33. ▪ Thomas Willis (1621-1675), English physician,
published a book, De anima brutorum (1672)
▪ a woman who temporarily lost her power of speech
and became 'mute as a fish’
▪ NMJ Disorders
▪ Immune-mediated
▪ Toxic or metabolic
▪ Congenital syndromes
NMJ Disorders

34. ▪ IgG directed attack on the NMJ
▪ Binding of AB to AchR→ direct block to binding of Ach
▪ Complement-directed attack, with destruction of AChR and
post junctional folds
▪ Antibody binding→ increase in the normal removal of
AChR receptors from the postsynaptic membrane
▪ Affects voluntary muscle, extra ocular muscles, muscles of
facial expression, and swallowing
Myasthenia Gravis

35. ▪ Degree of muscle weakness varies greatly among patients
▪ Localized form, limited to eye muscles (ocular myasthenia)
▪ Severe or generalized form affecting muscles that control
breathing
▪ Symptoms: Ptosis, diplopia, waddling gait, weakness in
arms, difficulty swallowing, SOB, dysarthia

36. ▪ Selectively digests one or
all of the SNARE proteins
▪ Blocks exocytosis of
vesicles
▪ Results in muscle
weakness/ profound
muscle paralysis
▪ May produce a partial or
complete chemical
denervation
Botulism

37. ▪ IgG directed at the presynaptic voltage-gated Ca channel
▪ Interfere with Ca dependent release of Ach quanta
▪ Subsequent reduced endplate potential → NMJ
transmission failure
▪ Weakness and fatigability primarily affect the lower limbs,
particularly the pelvic girdle and thigh muscles.
▪ Difficulty in climbing stairs, arising from a chair
▪ May involve shoulders and upper limbs, sparing the neck,
bulbar, and extraocular musculature.
Lambert-Eaton Myasthenic Syndrome

38. References
▪ Henderson J. Miller RD. Miller's Anaesthesia,
8th ed. Churchill Livingstone: Philadelphia.
2015
▪ Pino RM. Handbook of Clinical Anesthesia
Procedures of the Massachusetts General
Hospital.9th ed.Boston:Wolters Kluwer.2016
▪ Gwinnutt C. Physiology of the
Neuromuscular Junction. Salford. 2006