YR1 Lecture 1H - Skeletal Muscle - the Neuromuscular Junction - Dr Yossi Buskila 2021.pdf

Neuromuscular junction & skeletal muscle Dr Yossi Buskila School of Medicine Email – [email protected] 1 Learning objectives: Learning objectives Outline the principles of synaptic transmission Axonal transmission What is a synapse? What define the synaptic function (Excitatory/Inhibito...

Neuromuscular junction & skeletal muscle Dr Yossi Buskila School of Medicine Email – [email protected] 1 Learning objectives: Learning objectives Outline the principles of synaptic transmission Axonal transmission What is a synapse? What define the synaptic function (Excitatory/Inhibitory)? The quantal nature of synaptic transmission Describe the process of muscle contraction Anatomical organization of the skeletal muscle Muscle fiber contraction (Excitation-contraction coupling) The “sliding filaments” hypothesis Describe the organization of the somatomotor system Organization of the motor unit Control of a skeletal muscle The size principle 2 Axonal 1. Axonaltransmission: transmission Earlier this week we have learned that… though communicate. electrochemical signals * neurons ① Dendrites > - Soma ↓ aNOU 3. Synapses What is a synapse? Synapse - A junction between a Neuron and another cell/muscle/gland that allows the neuron to transmit an electrical or chemical signal. of Synapses 2 Types Electrical synapses find between griab - such a a astrocyte , Chemical synapses cells - Common neurones synapes between. t Certain of recrones 4 Synapses Chemical synapse -information is transmitted via the release of chemical agent that we call neuro Presynaptic site – The axon terminal of the presynaptic neuron transmitter" Postsynaptic site – The dendrite (usually, but not always) of the postsynaptic cell Synaptic cleft – The gap between the presynaptic and postsynaptic membranes - which this encapsulate. synaptic region < astrocyte & Astrocidic ~ process) tripartite synapse" - PSD 95" Post or Synaptic - & density 95 Dark * Region due to, High density of receptors · 5 VESICLES : 00 (Dendrite) 5 Synapses - the neuromuscular junction The neuromuscular junction is the synapse between a motor neuron and the muscle fibers it innervates - motor navon - exon terminal It consist of a motor neuron axon terminal (boutons) as the presynaptic area and the sarcolemma (muscle fiber membrane) as the postsynaptic area The postsynaptic zone consist of many receptors to the neurotransmitter Acetylcholine (ACh). 6 Synapses - the neuromuscular junction: Acetylcholine * Regardless of the identity of the neurotransmitter in the postsynaptic > cell synaptic machinery itself , is fairly similar Acetylcholine (ACh) is the first substance identified as neurotransmitter (1915, Henry Dale). It mostly has an excitatory impact on the target cell and expressed in both CNS and PNS. ACh is synthesized from Acetyl CoA and Choline by the enzyme Choline Acetyl transferase (CAT), and loaded to vesicles by the vesicular ACh transporter (VAChT). ACh action on the postsynaptic receptors is terminated by the enzyme acetylcholine esterase (AChE), which hydrolyze ACh in the synaptic cleft. ACh activate both ionotropic receptors (nicotinic acetylcholine receptors; nAChR) and metabotropic receptors (muscarinic acetylcholine receptors; mAChR) 7 Neurotransmitters There are more than 100 neurotransmitters in the nervous system, which can be divided to two main categories: Small molecule neurotransmitters and neuropeptides Neuropeptides – large transmitter molecules (3-36 amino acids); synthesized in the Golgi apparatus and packaged into vesicles Small molecule neurotransmitters Usually an individual amino acid (Glutamate; GABA), synthesized and packaged locally at the synapse 8 Synaptic release – the vesicular cycle ① resides with Starts formed in golgi appartur the are ↓ to Transferred axon * the terminal Only large neuropiptides are synthesised and at the soma ↓ small packaged WHILE neurotransilters synthesized andpackaged. locally at the synapse are &one action a arrives to the the ↓ presynaptic potential synapse terminal If activates voltage calcium channels = in & gated Ca2 + postsynaptic potential/surrent propagates OrT - / - by simple diffusion - hydrolysis 9 What determine the synaptic nature? The nature of a synapse - excitatory or inhibitory - depends on the neurotransmitter as well as the receptor At most excitatory synapses the neurotransmitter (e.g. glutamate or ACh) causes an influx of Na+ into the cell and therfore depolarization of the postsynaptic membrane 'Bs At most inhibitory synapses the neurotransmitter (e.g. glycine or GABA) causes an influx of Cl- or efflux of K+ which result in hyperpolarization of the postsynaptic membrane de (potassium 10 What determine the synaptic nature? Can a certain neurotransmitter be both inhibitory and excitatory? yes !! 11 What determine the synaptic nature? At the neuromuscular junction ACh is excitatory, acting on the ionotropic nicotinic receptors to cause an influx of Na+, but at the sinus node of the heart, ACh is inhibitory, acting on metabotropic muscarinic receptors to slow down the heart rate at the neuromuscular. junction acetylcholine excitatory - Hyperpolarization Depolarization * Lonotropic , Receptor cer T / Acetylcholine Release Since mode the heart ↓ into the d Inhibition of of that neurone ↓ Activation of MUSCARINIC depolarisation. receptor L Open potassiu channel L of hyper polisation membrane cell the. 12 The quantal nature of synaptic transmission One way to study synaptic transmission is by recording the electrical activity at the postsynaptic side by inserting a recording electrode. These electrical signals called synaptic potentials (End plate potentials in the NMJ). Spontaneous activity - will occur > Spontaneous release. resides Miniature end plate potentials (mEPP’s) 13 of The quantal nature of synaptic transmission Using this technique, Bernard Katz found that ACh is released in packets of vesicles (quanta), which can be measured by the small voltages they produce (Miniature End Plate Potentials) in the post-synaptic membrane The amplitude multiplication of of the evoked response is either equal on their spontaneous synaptic potential amplitude of the Evoked Spontaneous The “Sydney gang” Stephen kuffler, John Eccles and Bernard Katz Hypothesised > - When Neurotransmitters essentially they are in. of the verside by applying stimuli is the size evoke the cell released quanta Release of , which vesicles several. ↳(multiplication factor 14 The quantal nature of synaptic transmission Katz also found that the release of ACh vesicles occurs in a probabilistic fashion (hence the failures). Therefore, not every action potential will lead to synaptic release and different synapses will have different release probability 15 Muscle contraction How neuronal signals leads to movement (muscle contraction)? 16 2. Anatomical organization of the skeletal muscle Skeletal muscle is composed of individual muscle fibers, each of which contains multiple myofibrils. The membrane of the muscle fiber is called sarcolemma A special feature of the sarcolemma is that it invade into the cytoplasm of the muscle cell, forming membrane tubules called transverse tubules (T-tubules), which increase the surface area of the membrane exposed to the synaptic cleft 17 Anatomical organization of the skeletal muscle On either side of the transverse tubules, there are Terminal cisterna, which are enlargements of smooth endoplasmic reticulum termed sarcoplasmic reticulum (SR) in muscle. The formation of T-tubules and SR is called Triad, and is essential for the transmission of the action potential to a muscle contraction. 18 Excitation-contraction coupling Excitation–contraction coupling is the process by which an action potential in the muscle fiber causes the myofibrils to contract. * Action potential Depolarization signal travels along the T-tubules Activation of Dihydropyridine receptors (DHPR) ↳ lead to Activation of Ryanodine receptors RYR on the SR conformational me region of this change &nd receptor change of the - essentially tem opens mechanically. Calcium release to the cytoplasm Activation of troponin C and muscle contraction DHP receptors Ophysically to their the are are connected region of receptors , which calcium release channels or the. SR & opens them 19 Muscle contraction Each myofibril is composed of repeating sections called sarcomeres, which appear under the microscope as dark and light bands. Sarcomere structure Each sarcomere is composed of long thick and thin filaments that slide past each other when a muscle contracts or relaxes. A sarcomere is defined as the segment between two neighboring Z-lines Microscopic view 20 Muscle contraction Thick filaments are composed of the protein myosin 10–12 nm diameter Thin filaments are composed of the protein actin 5–6 nm diameter Important structures: 3 bands; 2 lines filaments strand of activinholds the e Thick filaments contain ↓ myosin - in place - myosin andoverlapping t Both are of com e is e filaments - -Activ filaments I. ↳ Cross-connecting cytoskeleton elements of the & holds the myosin 21 filaments together. Muscle contraction Areas of overlap are responsible for the light and dark striations typical of skeletal muscle u & Dark - Tissue is desl very ↓ Due to both acting myosin filaments overlapping ach other. 22 Muscle contraction Thin and Thick Filaments Each thin filament consists of a twisted strand of several interacting proteins 5–6 nm in diameter and 1 μm in length. Troponin holds the tropomyosin strand in place. & Neyosin filament. Thick filaments are 10–12 nm in diameter and 1.6 μm in length, making them much larger 23 than thin filaments. Muscle contraction Sarcomeres contract according to the sliding filament hypothesis , which explains how muscle contracts through cross bridges between the actin and myosin filaments According to the sliding filament theory, the actin (thin) filaments of muscle fibers slide past the myosin (thick) filaments during muscle contraction, while the two groups of filaments remain at relatively constant length & the active site of the actin filaments to attach filaments * Myosin form *The across bridges. the of lengthremain filament constant. 24 Muscle contraction Following calcium release from the SR, calcium binds to the troponin. Troponin change its shape, alters the position of the tropomyosin strand and expose the active sites on the actin molecules Once s Calcium levels in the cytosol increase ↓ bind to They headtto ↓ of the Myosin Molecule binding Troponin exposed then active sitel of ↓ troponin the movement Strand troponin conformational mange of > - the the the active molecules binds to the active in site the myosin 25 Muscle contraction Once the myosin and actin filaments are attached, the myosin filaments push the actin filaments, using ATP hydrolysis and a power stroke ② conformational change "Power stroke" ⑦ conformational change 26 Muscle contraction - summary How contraction ends? When electrical stimulation ends: The SR will recapture the Ca2+ ions. The troponin–tropomyosin complex will cover the active sites. The contraction will end. 27 Muscle contraction - summary * Process starts with an action from the motor potential arriving. neurone (depolarisation) stops ↑ cate active Troponin Moves tropomyosh ↳. ↳ Expose the the What happens once an action potential arrives from the axon? active site on activcements. ↓ Contraction i Requires ATP d forming u power stroke. 28. - - musul & smooth cardiac steeletal muscle are inohatary voluntary are control binding site The entire E muscle filme will shortch in ↑ Skeletal muscle D muscle 08 Co G ② ⑳ d O / t ( / , ·muscle g filee mee (muscle Fiber) O T f/ > · , receive (signal they from me signals. system ⑳ LINE - THIN FILAMENT ACTIN (protein) - - Thin Anchored I line = - -. filuments stide past each other ⑳ the & the Causes the filament slidingene mechanism murde ofcontraction length whenatalong. bothsi filaments the myosin. read myosintend to - to erallsiteenga to activ , cross- - ach. bridge on = TO THE V cross binding site the on ACTIN. Filamens called O · - (Action). ⑳ mer i n · e -Troponi O ~ Troponin Troopomyocir ↑ (Tropomy. ↑ allowingmyosininament pull 'M toward the ↓ shortening ↓ ADPp ⑳ ; the = LINES Sacromere MYOSIN are released me n during stroke power. osin Muscle contractions controlled by the action of calciu are STROKE ↳ muscle blocks hethinaction associate remaine - attached Ho activ until new a molall ofAPo MYOSIN ANCHORED CENTRE OF ,. ↑ ⑬ ⑳ · , troponin a.) Basin Metichlem.. mus fie levels calciu ions which is relaxed , tropomyosin bridge · d a. shortens The sacromere filaments Thick filament appearance msilitriated contracts when The which activ the the from muscle its the ·. o - ⑳ present When contract. * ATP & · * e MYOSNEPulltit's. - - - - a --ge & errorobrage o Thick filament Actin) ( * P- calcium ion ↑ releasedfromthe begiadrolysed Morganic phosphate- A contraction exchange). containsContai f↑e. remous sanome · relax where or muul on ↓ MYOFIBRIL nuclei). neuro - muscle fibra · - (cel) (contain is bind to the = deplanein muscles will contract - " terminal &released inrespons O > are - receptor fiberbunder O -OC- au T o ·. C oo & & ATP. 4 - axon bridge When a - neurotransmitter Molecul forming displaced tropomyosin t = zu actin Calcium ions are stored reticulum the sarcoplasmic -. on I myosin binding attach to cross- L impulse The electricalTtubules & opens ene travell down. stores Calcium # new. ↓ This allows to - e the myosin ⑳..... Exposing siteonactin. AT THE THE SARCOMERE M LINE" 29. Organization of the somatomotor system: The basic unit of the somatomotor system called a motor unit, which consist of: the motoneurone (its axon and terminals) and the muscle fibers they innervate Motoneurones to a particular muscle are arranged in the ventral horn of the spinal cord in clusters - motoneuron pools the determined * ( Important > The size - the motor unit is activates # fibres that it of. O I Notor neurone pools. small motor units-↓ muscle fibres are innerated by a of by motor single neurore. > will In large neurones t in of Muscle Fibrei e motor single a motor large amount muscle - nee. fibres the same fibre. & Atype motor mit activated - > - is 30 all fibres. contract will Control of skeletal muscle: What determines the intensity of a muscle contraction? I order to I want control to control the H 2 skeleful of intensity of movement factors thataffect - its nuscle contraction. Interity -next page- > 31 Control of skeletal muscle: The intensity of a contraction is determined by: a) Recruitment of motoneurones according to the size principle (small motor units are recruited first; more motor units = more force) a) Firing rate, which impact on the time of the contraction 32 Control of skeletal muscle: During a voluntary contraction the smallest motor units, which produce the weakest force, are recruited first (The size principle) 20 instantaneous frequency firing rate of large motor unit FFBtia00 Hz 0 intramuscular EMG action potentials generated by single motor units, recorded from microelectrode inserted into the muscle small motor unit 5 µV surface EMGRMS TA surface EMG TA 10 mV large motor unit electromyographic (EMG) activity recorded from the skin overlying the muscle 33 Control of skeletal muscle: Muscle contraction A single action potential causes a brief muscle contraction - a twitch Increases in firing rate of active motor units result in summation of the twitch forces A tetanic contraction is one in which the contraction is continuous, sustained by a train of action potentials 34 Control of skeletal muscle: The contractile properties of single human motor units can be studied by intraneural microstimulation of single motor axons EMG response to single stimulus force response force responses to increasing stimulus rates 35 Fuglevand, Macefield & Bigland-Ritchie, J Neurophysiol 81: 1718-1729, 1999 Control of skeletal muscle: When we plot the results, it is clear that the force is linearly proportional to the firing rate, till the muscle reach saturation stimulus rate required to generate 50% of maximal force 36 Fuglevand, Macefield & Bigland-Ritchie, J Neurophysiol 81: 1718-1729, 1999 Control of skeletal muscle: Continuous stimulation of a motor unit can lead to a fall in force production - fatigue The force produced by the muscle decreasing due to fatigue single twitches are smaller and slower following fatigue 37 Fuglevand, Macefield & Bigland-Ritchie, J Neurophysiol 81: 1718-1729, 1999 Control of skeletal muscle: Force-frequency curves are shifted to the right following fatigue: higher stimulation rates are required to generate a given force higher stimulation rates are required following fatigue 38 Fuglevand, Macefield & Bigland-Ritchie, J Neurophysiol 81: 1718-1729, 1999 Control of skeletal muscle: Low-frequency fatigue: loss of twitch force, despite presence of EMG. High frequencies of stimulation can generate force - probably due to accumulation of Ca2+ loss of force fatigue at low stimulation rates force generation at stimulation rates >15 Hz 39 Fuglevand, Macefield & Bigland-Ritchie, J Neurophysiol 81: 1718-1729, 1999 Summary - control of skeletal muscle Small motoneurones are recruited first, with larger motor units being recruited progressively according to the size principle Small motor units produce weak forces Large motor units produce strong forces but fatigue readily 40 Summary Transmission of information across a synapse is chemical: the neurotransmitter at the neuromuscular junction is acetylcholine (ACh) Excitation-contraction coupling is Ca2+-dependent The motor unit consists of the motoneurone and the muscle fibres they innervate Small motoneurones are recruited first, with larger motoneurones being recruited progressively according to the size principle; large motor units produce strong forces but fatigue readily 41