The Neuromuscular Junction

• Motor unit = motor neuron and all the muscle fibers it innervates.
• Motor neuron = the nerve originates with a single cell body in the ventral horn of the spinal cord; the axon extends peripherally to the muscle fibers that it innervates.
• Neuromuscular junction is the point of contact between a motor neuron and a skeletal muscle cell.
• Neurotransmitter at the neuromuscular junction is acetylcholine.
• Acetylcholine is synthesized from choline and acetyl coenzyme A (acetyl CoA) in the axon terminal and then filled into synaptic vesicles.
• Synthesis requires the enzyme choline acetyltransferase (CAT).
• Once released into the cleft, acetylcholine is rapidly broken down by the enzyme acetylcholinesterase (AChE).
• Choline is transported back into the axon terminal and reused to make acetylcholine.
• Amount of neurotransmitter contained within 1 vesicle is called a quantum.
• Each vesicle contains ~10,000 molecules of acetylcholine, which interact with ~2,000 postsynaptic nicotinic acetylcholine receptors.
• The change in the membrane potential of a muscle cell produced by a single quantum is called a miniature end-plate potential, or mEPP.
• A mEPP is about 1/100 the size of the end-plate potential (EPP) produced by electrical stimulation of the entire nerve innervating the muscle fibre.
• Normal neurotransmission results from the release of many vesicles simultaneously.
• The receptor to which acetylcholine binds is the nicotinic receptor.
• Each endplate has up to 5 million receptors.
• Each open channel allows the entry of Na+ into the cell and the exit of K+ thereby generating an endplate potential (EPP).
• Summation leads to the firing of an action potential (all or nothing).
• 250,000 to 500,000 channels (5-10%) of a given NMJ must be open to reach receptor reserve.
• Nicotinic receptor consists of 5 subunits
• 2 alpha, 1 beta, 1 delta and 1 epsilon
• arranged in a rosette to form an ion channel
• The two alpha subunits are occupied by Ach or exogenous agonist for an otherwise all or none opening of a given ion channel to occur

Myasthenia Gravis (MG)

• In MG, antibodies are directed toward the acetylcholine receptor at the neuromuscular junction of skeletal muscles
• Results in:
  • Decreased number of nicotinic acetylcholine receptors at the motor end-plate
  • Reduced postsynaptic membrane folds
  • Widened synaptic cleft
• Clinical Presentation is
  • Fluctuating weakness increased by exertion
  • Weakness increases during the day and improves with rest
  • Extraocular muscle weakness
  • Ptosis is present initially in 50% of patients and during the course of disease in 90% of patients
  • Head extension and flexion weakness
  • Weakness may be worse in proximal muscles
• Edrophonium (Tensilon test) is used as a diagnostic test
• Patients with MG have low numbers of ACHR at the NMJ
• Ach released from the motor nerve terminal is metabolized by Acetylcholine esterase
• Edrophonium is a short acting Acetylcholine Esterase Inhibitor that improves muscle weakness
• Evaluates weakness (i.e. ptosis and opthalmoplegia) before and after administration

Lambert-Eaton Myasthenic Syndrome (LEMS)

• Rare autoimmune disorder where the immune system mistakenly attacks the body's own tissues.
• LEMS is an immune system attack at the neuromuscular junction, which is the connection between nerve and muscle cells.
• Two Groups of LEMS Patients can be Autoimmune disorder diagnosed after age 40, sometimes diagnosed with small cell lung cancer
• Can also occur in children < age 10
• Prevalence is about 3,000 patients

Neuromuscular Junction Blocking Agents

• Neuromuscular function is influenced by factors affecting the site of action at the neuromuscular endplate, the release of transmitter, and the transmission of nerve impulses.
• Drugs can block neuromuscular transmission by:
  • acting presynaptically by inhibiting ACh synthesis or release
  • acting postsynaptically by inhibiting the binding of ACh to the NM receptor
• Neuromuscular blockade = compounds that prevent normal functioning of the motor end plate.
• Two groups exist as a result:
  • Non-depolarizing blockade
  • Depolarizing blockade.

Why Study Neuromuscular Function?

Many toxins act by producing neuromuscular blockade Clinically:

• Used as an adjunct to anesthesia
• Facilitates endotracheal intubations during diagnostic procedures
• Some neurological conditions
• This use requires artificial respiration of patient
• Does not cause sedation or relief of pain
• Use requires knowledge of Hazards of Deep Anaesthesia
• In order to achieve muscle relaxation and abolition of reflexes sufficient for deep surgery, dangerous levels of anesthesia are required.
• This often led to patient death on the operating table.

Depolarizing vs Non-depolarizing Blockade

• Non-depolarizing blocking agents
  • Act by blocking NM receptors (antagonist)
  • The nerve membrane is not depolarized
  • Prototype is tubocurarine
• Non-depolarizing Blockade has active ingredient tubocurarine – prototype neuromuscular blocking drug
• Curare was a blowdart and arrow poison by indigenous forest peoples of South America to paralyze their prey
• Compete with acetylcholine to prevent EPP being large enough to activate muscle contraction.
• Paralysis is flaccid in all muscles.
• Since action potentials are all or none, the dose response curves are very steep.
• Not every junction fails at the same point, so there is a period of declining force for the muscle as a whole. -There's a percentage of receptors that may be blocked before there is any effect on contraction called the receptor reserve.
• Effects are not seen until 75% of receptors are occupied in some muscles
• It is greatest in the muscles of respiration and large muscles of coarse movement
• Least in the muscles of the face and eyes
• Lack of facial expression and ptosis are diagnostic of neuromuscular failure in disease states
• Also blocks ganglia – loss of control of blood pressure
• Affects muscarinic receptors – loss of para-sympathetic control of heart rate-tachycardia
• Releases histamine → hypotension, broncho-constriction and anaphylaxis
• Its duration of action is over 30 minutes – too long for short procedures such as intubation
• Depolarizing blocking agents
• Act as agonists at the NM receptors
• Depolarize skeletal muscle
• Longer lasting than ACh
• Prototype is succinylcholine
• Depolarizing Blockade is achieved by succinylcholine, the only drug still in clinical use
• Initial depolarization of muscle seen as fasciculation and twitches at onset.
• Muscle weakness followed by total flaccid paralysis – small rapidly moving muscles are affected first, then muscles of the limbs, neck and trunk and then diaphragm – respiratory depression

Depolarizing Blockade (Succinylcholine)

• Transient muscle fasciculations followed by paralysis
• Bradycardia
• Post-operative pain
• Resistant to acetylcholinesterase

Phase I Block

• Membrane remains depolarized
• Unresponsive to additional impulses
• Flaccid paralysis results
  • Excitation coupling requires repetitive firing
• Potentiated by cholinesterase inhibitors

Phase II Block

• With continued exposure end plate repolarizes
• Despite repolarization end plate cannot depolarize as long as succinylcholine is present
• Mechanism unknown
• Desensitization?

Comparison of blockade types

• Competitive (Curare)
• Initial effect at receptor - Inhibition
• Effect of cholinesterase inhibition - Reversal
• Duration of action - Variable by drug
• Phase II Block? - No
• Depolarizing (Succinylcholine)
• Initial effect at receptor - Activation
• Effect of cholinesterase inhibition - Enhancement
• Duration of action - Very short
• Phase II Block? - Yes

Malignant Hyperthermia

• Rare disorder, triggered by general anesthetics and neuromuscular blocking agents
• Patients have a hereditary impairment in ability of SR to sequester calcium.
• Condition caused by a defect in the molecule linking muscle membrane t-tubules to the sarcoplasmic reticulum (ryanodine receptor).
• the SR releases Ca2+ unchecked
• This leads to contracture and a rise in body core temperature.
• Dantrolene blocks this inappropriate response of the ryanodine receptor and prevents Ca2+ loss