Muscle Physiology L02 - Summer 24 PDF

MUSCLE PHYSIOLOGY 2. Neuromuscular transmission Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected] Learning objectives for this lecture Describe the components of a neuromuscular junction Understand the concept of motor unit Describe the role of ac...

MUSCLE PHYSIOLOGY 2. Neuromuscular transmission Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected] Learning objectives for this lecture Describe the components of a neuromuscular junction Understand the concept of motor unit Describe the role of acetylcholine in the neuromuscular transmission Describe how neuromuscular transmission occurs Describe how Botulinic toxin, Organophosphate/Carbamate pesticides, and Myasthenia Gravis interact with neuromuscular transmission Motor neuron Skeletal muscle fibers are controlled by a MOTOR NEURON Motor neuron is a neuron whose cell body is located within the CNS (spinal cord or brainstem) and whose axon travel within peripheral nerves and synapse with effector organs (muscle fibers) Neuromuscular junction Each motor nerve ending makes a specialized synapse with the muscle, called NEUROMUSCULAR JUNCTION There is usually only one junction per muscle fiber The transmission of the electric impulses (action potentials) is unidirectional Motor endplate The motor endplate is a modified area of the muscle fiber membrane at which a synapse occurs. A motor nerve axon ending may have up to 50 synaptic knobs/boutons (motor unit), but a single muscle fiber has only one endplate. Motor unit A motor neuron can have several terminal branches, with each one ending on a separate muscle fiber MOTOR UNIT – consists of a motor neuron and all the muscle fibers that it innervates Motor unit Functional characteristics of motor units: Size of Motor Units: Motor units vary in size; smaller units control fine movements with few muscle fibers, while larger units control powerful movements with many fibers. Extraocular muscle function requires fine movements to move the globe smoothly. These muscles have very small motor units, with only a small number of myofibers (1 to 4) innervated by each axon. In contrast, the quadriceps muscle is designed for the generation of force, and motor units are pretty large, with many myofibers (100 to 150 or more) innervated by a single axon. All-or-None Response: Each motor unit responds completely when its motor neuron is activated, causing all innervated muscle fibers to contract simultaneously. Recruitment and Gradation of Force: Muscle contraction strength is controlled by the number and activation frequency of motor units, allowing for precise adjustments in force. The neuromuscular junction Like most chemical synapses, the NMJ is composed by: Presynaptic side (presynaptic terminal) Synaptic cleft Post synaptic side (terminal endplate) The neuromuscular junction PRESYNAPTIC TERMINAL Is the terminal portion of the motor neuron Has a button-like shape (synaptic button) Contains a large number of synaptic vesicles Storage chemical neurotransmitters – Acetylcholine Localized in the inner surface of the terminal membrane ACTIVE ZONE Specific proteins hold the vesicles in the right place Contains high numbers of mitochondria Mitochondria products like ATP and Acetyl-CoA play a role in the production and release of Acetylcholine Acetylcholine (ACh) Neurotransmitter of the neuromuscular junction Chemical released by a nerve cell to send signals to other cells Is synthesized in the cytoplasm of the terminal From AcetylCoA + Choline Stored into synaptic vesicles Lined up in rows in the active zone Released in the synaptic cleft ACh release ACh release When the action potential reaches the neuromuscular junction, ACh is released from the terminals into the synaptic space The wave of depolarization opens VOLTAGE-GATED CALCIUM CHANNELS Calcium enters the presynaptic terminal The rise in Ca activates SYNAPTOTAGMIN (an enzyme in the vesicle membrane) SYNAPTOTAGMIN activate SNARE proteins SNARE COMPLEX promotes docking, fuse, and release of ACh by exocytosis The neuromuscular junction SYNAPTIC CLEFT Is the narrow space between the presynaptic and postsynaptic membranes Numerous small folds of the muscle membrane greatly increase the surface area – SUBNEURAL CLEFTS Action of synaptic neurotransmitters Contains extracellular fluid and matrix molecules Matrix molecules help in the neuro-muscle adhesion The neuromuscular junction POSTSYNAPTIC MEMBRANE In the motor endplate The muscle cell membrane has specialized features there that facilitate synaptic transmission ACETYLCHOLINE RECEPTORS JUNCTIONAL FOLDS Receptors at the mouth of the folds Folds aligned with the active zones of the presynaptic terminals ACh action ACh diffuses across the synaptic cleft and binds with transmitter-specific receptors in the postsynaptic membrane NICOTINIC ACETYLCHOLINE RECEPTORS Named because also bind to nicotine Is a ligand-gated ion channel, with 2 binding sites for Ach The binding opens the channel for specific ions Na flows inside of the cell Locally depolarizes the muscle fiber membrane END PLATE POTENTIAL (increases in positive direction 50 – 75 mV) Action potential in the muscle fibers will occur in the same way described for nerve cells Muscle fibers have resting membrane potential and can be depolarized by synaptic transmission Action potential on muscle cell ACh degradation ACh is rapidly removed from the synaptic cleft Few milliseconds after release Most of the ACh is destroyed by the enzyme Acetylcholinesterase Acetic acid + Choline Choline is transported back into the presynaptic terminal - recycled Small amounts diffuses out of the cleft and is no longer available Prevents continuous muscle re-excitation Neuromuscular transmission overview Clinical correlation #1 Videos https://www.youtube.com/watch?v=OrmJF0E5D_s Organophosphate toxicosis Major cause of animal poisoning https://www.youtube.com/watch?v=IKnUV2d880c used as insecticide, pesticide, and antiparasitic Irreversibly inactivate acetylcholinesterase leading to an excess of ACh and overstimulation of ACh receptors NICOTINIC – muscle spasms and twitching (fasciculations) MUSCARINIC – hypersalivation, miosis, frequent urination, diarrhea, vomiting, colic, and dyspnea (bronchoconstriction and increased bronchial secretions) CENTRAL – Nervousness, ataxia, hyperreactivity, and seizures Prolonged stimulation of the receptors can lead to receptor desensitization Eventually, the affected muscles become unable to contract, resulting in flaccid paralysis (chronic cases) Carbamate also inactivate acetylcholinesterase, but reversibly Carbamate toxicosis produce the same clinical signs, but with shorter duration Clinical correlation #2 Botulism Ingestion of the botulinum toxin (neurotoxin) Produced by a Gram-positive, rod shape, anaerobic bacteria called Clostridium botulinum Usually ingested in uncooked and spoiled food – decaying carcasses, decaying grass, hay, grain or spoiled silage Toxin destroys SNARE proteins 7 different types – A, B, C, D, E, F, G Toxin targets and cleaves different SNARE proteins, depending on the specific serotype of the toxin Prevents release of ACh from the vesicles (skeletal and autonomic synapses) Clinical correlation #2 Video https://www.youtube.com/watch?v=vBYQ01gV58A Botulism More common in chickens and birds than in cattle and horses Dogs and cats are comparatively resistant to all types of botulinum toxin, but cases are eventually seen Fishes are also susceptible Clinical signs of botulism typically develop hours to days after ingesting contaminated food Vomiting/regurgitation, progressive motor paralysis (loss of muscle function), disturbed vision (dilated pupils), inability to blink, difficulty in chewing and swallowing, constipation/reduced peristalsis, atonic bladder Death occurs due to respiratory paralysis Clinical correlation #3 Myasthenia Gravis Disease caused by the abnormal reduction in the number of Ach receptors on the neuromuscular endplate Results in clinical signs of exercise-induced weakness Congenital form - present from birth Recurrent and progressive muscle fatigue usually becomes apparent between 6-9 weeks of age Can be linked with problems in Ach synthesis as well Acquired form – an autoimmune disease – IgG against Ach receptors ABs may bind directly to the Ach receptor blocking ion channel opening ABs may increase the degradation rate of Ach receptors, resulting in a decreased concentration of receptors at the postsynaptic membrane Complement-mediated lysis of the muscle endplate may take place Neuromuscular transmission problems Organophosphate irreversibly inactivate acetylcholinesterase leading to an excess of ACh and overstimulation of nicotinic receptors at the neuromuscular junctions. Carbamate does a reversible inactivation of AChE Botulinum toxin destroys the binding proteins involved in vesicle docking (SNARE), preventing ACh release Myasthenia gravis: Insufficient production or immune mediated destruction of AChR Additional reading https://www.vetspecialists.co.uk/fact-sheets-post/myasthenia-gravis-fact-sheet/ https://www.vet.cornell.edu/departments/riney-canine-health-center/canine-healthinformation/myasthenia-gravis Neuromuscular transmission video https://www.youtube.com/watch?v=eTYe1CtjJRE Questions?