Neuromuscular Junction PDF

PHYSIOLOGY MR 0  Neuromuscular junction Definition area between branches of alpha motor neuron ending & skeletal muscle fiber, consists of 1. Terminal knobs (end feet) of alpha motoneuron: Contain acetyl choline vesicles....

PHYSIOLOGY MR 0  Neuromuscular junction Definition area between branches of alpha motor neuron ending & skeletal muscle fiber, consists of 1. Terminal knobs (end feet) of alpha motoneuron: Contain acetyl choline vesicles. Each skeletal muscle fibers receive only one axon terminal 2. Motor End Plate: Depression in skeletal muscle fiber (fit with terminal knob) Thickened and folded membrane Contains Acetylcholine receptors. 3. Synaptic cleft: extracellular space between nerve terminal& muscle membrane, Occupied by C.T. (basal lamina) to which acetyl choline esterase enzyme is bound Mechanism / events of Neuromuscular Transmission Definition Transmission of impulse from neuron to muscle fiber 1. Arrival of action potential: to nerve ending→ open voltage-gated Ca++ channels → Ca++ inflow →rupture of vesicles→ exocytosis of acetylcholine 2. Acetyl choline crosses synaptic cleft, bind ligand gated channels (receptor) on MEP→ open channel→ Na+ influx & depolarization of MEP (End plate potential) 3. End plate potential (EPP) graded, non propagated, acts as a stimulus → depolarizes adjacent muscle membrane to firing level (threshold) → produce action potentials → conducted in both directions → cause muscle contraction 4. Acetyl choline dissociates from receptor & hydrolyzed by acetyl choline esterase enzyme in synaptic cleft→ prevent multiple contractions. N.B.: Later, new vesicles are formed from invaginations of presynaptic membrane & refilled Miniature End-Plate Potential: At rest, few vesicles rupture spontaneously → release small amount of Ach → minute depolarization at MEP 1 Properties of Neuromuscular Transmission 1. Unidirectional: from nerve to muscle. 2. Delay: 0.5 msec →time for release of A Ch, Na inflow, depolarization to reach firing level. 3. Fatigue: exhaustion of synaptic vesicles- depletion of acetylcholine due to repeated stimulation 4. Effect of ions: ↑Ca++ entry→ rupture of vesicles →↑transmission ↑Mg++ (competes with Ca++) → ↓release. 5. Effect of drugs: Drugs stimulate neuromuscular transmission (NMT) Drugs block NMT By A.Ch likes action By inactivating (anti) choline-esterase Not destroyed Neostigmine Curariform (curare) → by acetylcholinesterase Physostigmine competes with A Ch for Action (minutes to hrs) Di isopropyl phlorophosphate its receptors on MEP Methacholine →Accumulation of ACH Carbachol Nicotine small dose. Myasthenia Gravis Cause Serious, fatal, autoimmune disease →antibodies against acetylcholine- receptors →cause weakness of skeletal muscles (inability of NMJ to transmit enough signals) Complication death from paralysis of respiratory muscles in severe cases Treatment neostigmine→ inactivating choline esterase→ ↑A Ch at neuromuscular junction 2  Muscle types Striated muscles (alternating light and dark bands) No distinguishing surface. Skeletal muscles cardiac muscles Smooth muscles 40 % 10 % of the body mass Skeletal Muscles: >400 voluntary skeletal muscles o Contraction depends on nerve supply. o Functions: 1. Locomotion and breathing. 2. Maintain posture and Stabilizing joints. 3. Heat production. 4. Help venous drainage. Structure Skeletal muscle Made up of Fibers (bundled together by C.T. (parallel): enveloped by cell membrane (sarcolemma) Made up of Myofibrils in sarcoplasm (parallel) Made up of Myofilaments (thick & thin) arranged longitudinally in sarcomeres (striated) 3 SARCOMERE functional unit of muscle, extends between two transverse protein (Z lines) 1-Thick (myosin) filaments: in middle Myosin molecule (2 heavy chains and 4 light chains) ✓ Helix: 2 heavy chains coil around each other ✓ 2 arms + 2 globular heads: formed of Terminal part of heavy chains + 4 light chains Head has 3 binding sites for (actin, ATP, ATPase). ✓ Cross- bridges: Head + arm o flexible at 2 points (hinges): between arm & body between heads & arm Titin: elastic protein, attach thick filaments to Z line → A. keep thick filaments centered B. helps muscle fiber to resist extreme stretch 2- Thin (actin) filaments on both sides of sarcomere, consist of 3 proteins Actin Tropomyosin Troponin Helix (2 chains)→ (has active site) cover active site on Globular protein One end attached to Z line actin during rest Attach tropomyosin to actin. Other end overlap myosin 3 subunits: Troponin I → affinity for actin Troponin T→ affinity for tropomyosin Troponin C → affinity for Ca++ 4  Sarcomere appears as cross (banding) striations on light microscopic. Light (I) band Dark (A) band On either side of Z disc In the center of sarcomere Contain thin filaments Contain thick & overlapping thin filaments except in middle (light H-zone) H – zone Area of A band (thick filaments) without actin During muscle contraction: 1. Sarcomere shortens 2. Z lines: closer. 3. A bands: not change 4. I bands and H zones: shorten (narrower) Tubular system 1- Transverse (T)- Tubules: Definition Invaginations of muscle membrane, contain ECF Function carry A.P. from surface of muscle fiber to inside contains voltage-sensitive dihydropyridine (DHP) receptor 2. Sarcoplasmic Reticulum (S.R.): endoplasmic reticulum of the skeletal muscle Definition site of Ca++ storage (high concentration) & release during contraction. Ends of S.R. expand (form terminal cistern) →make contact with T-Tubules Site Surround each myofibril and parallel with it Function SR membrane has ryanodine receptor = Ca++ release channel Foot process= small projection between SR & T tubule membranes N.B. excitation of T tubule by A.P.→ activate voltage - sensitive dihydropyridine (DHP) receptor →opens ryanodine Ca2+ channel on SR. → Ca++ release → contraction of myofibrils 5  Changes following skeletal muscle stimulation I- Electrical changes as nerve fiber with some differences. R.M.P. =-90 mv. A.P. lasts 2-4 msec & precedes muscle contraction by 2 msec. Conducted along muscle fiber at a rate of 5m/sec II-Excitability changes As nerve fiber, is refractory to re-stimulation during action potential. As muscle begins to contract→ it regains excitability→ respond to restimulation→ can be tetanized III- Mechanical changes- Molecular mechanism of muscle contraction / Excitation – Contraction Coupling: action potential initiate muscle contraction 1. Release of Ca++ A.P. from M.E.P to T tubules → open Ca++ channels on terminal cistern (T.C.) → Ca++ flows into cytoplasm. 2. Activation of muscle proteins Ca+2 bind troponin C. Troponin undergoes conformational change →cause tropomyosin to move away from its position→ uncovering binding sites on actin→ combine with cross bridges of myosin. 3. Generation of tension (force developed when muscle contract): by cross bridge cycling (4 steps) A- Binding of actin & myosin spontaneously B- Bending of cross bridges & Sliding of actin across myosin. Energy used to phosphorylate cross bridge from ATP hydrolysis. C- Detachment of cross bridges from actin. ADP+P must be removed from the cross bridge and new ATP is attached→ ↓affinity of cross bridges for active site. No ATP→ no separation (muscle contracture). D- Return of cross bridges to original upright position →To participate in another cycle. Cycling continues as long as Ca++ bind troponin and ATP is available. Tension transmitted through actin → Z line → sarcolemma→ tendon→bone 4. Relaxation: by removal of Ca++ from cytoplasm by Ca++ pump on S.R. →↓ intracellular Ca++ → troponin returns to its original position → tropomyosin moves back → cover active sites on actin → cross bridge cycling stops 6  Types of Skeletal Muscle Contraction Type Isometric Isotonic Isolated skeletal muscle is fixed to a holder Smaller load is attached to the muscle Too heavy load is attached at its lower end → stretch the muscle to certain degree ……………………………………………….. then support surface is placed Length Constant (no shortening) ↓ length= (shortening) Sarcomeres shorten (less sliding) More sliding Stretching of series elastic elements ( (present mainly in the tendons) Tension ↑ to maximum Constant (during shortening) Duration Lasts shorter Lasts longer Work No external work (load is not moved) External work (load is moved a distance) Mechanical % of energy input converted into 20- 25% Efficiency work) = zero Energy Less energy More energy since load is moved a distance E.g. Maintain posture against gravity. Movement of part of body or the whole body Tenses part of the body as during Contraction of biceps to lift object standing (tenses quadriceps Move smaller load muscles to tighten knee joints→ stiff leg) Muscle contraction starts isometric till it generates enough tension to overcome the load → then contraction continue as isotonic (muscle shortens). 1. With heavier loads ↑Duration of isometric contraction ↓rate and extent of shortening in isotonic contraction 2. With too heavy load: maximum tension developed by isometric contraction is not sufficient During running, contractions of leg muscles: mixture of isometric [when the legs hit the ground] and isotonic contractions [to move the limbs]. All or None Law: Single skeletal muscle fiber obeys all or none law (contracts maximally or not at all) Threshold stimulus → maximum contraction provided that experimental conditions remain the same Muscle Twitch: Single action potential → single brief contraction followed by relaxation. The twitch starts 2 msec after start of depolarization 7 8  Factors affecting Muscle Contraction 1. Type of muscle fiber: Most muscles contain mainly 2 types Slow (Red) muscle fiber (type I fiber) Fast (Pale) muscle fiber (type IIb fiber) Small fiber Large fiber innervated by small slowly conducting neuron innervated by large rapidly conducting neuron Red (high blood supply), extensive capillaries Pale (less blood supply), Rich in myoglobin (store O2) Less myoglobin (high mitochondrial volume) Fewer mitochondria Contain large number of oxidative enzymes Contain large number of glycolytic enzymes (rapid release of energy) Extensive SR (rapid release of Ca) Low ATPase activity High ATPase activity Slow contractile mechanism Rapid contractile mechanism Not easily (highly resistance to fatigue) Easily (less resistance to fatigue) (large capacity for aerobic metabolism) Muscles Adapted for long posture maintaining For fine skilled movements, e.g. contractions, e.g. soleus & back muscles hand, extraocular muscles Most muscles contain both types in different proportions Muscle with high percentage of fast fibers →exert more force & greater velocity Aging →loss of muscle mass + Loss of fast fibers + relative ↑in slow fibers. 9 2. Stimulus Factors (grading of muscle contraction) Motor unit= Motor neuron + its innervated muscle fibers Motor unit of muscle Motor unit of big muscle (Perform fine, graded, precise movements) (Perform gross movements) as hand muscles, ocular muscles as back, leg muscles A. Has 3-6 fibers Has 100-200 fibers A- Stimulus Strength: ↑strength → ↑ number of motor unit (recruitment) → gradual↑ force of contraction Maximal stimulus: activates all motor units. Supra maximal stimulus→ no further response (as each fiber responds maximally according to all or none law) B- Stimulus Frequency: ↑ frequency → more Ca2+ is released from SR → ↑ force of contraction Low frequency Medium frequency High frequency Separate twitches Clonus (incomplete tetanus) Tetanus (continuous contraction with (Contraction with no relaxation) incomplete relaxation) Tension developed is 4 times > developed by separate twitch (due to ↑ca2+) Treppe " Stair Case Phenomeno:” progressive ↑in magnitude of separate twitch contraction to plateau value during repetitive stimulation after a period of rest cause: due to (↑free Ca2+ release from SR) Grading of muscular activity A. With minimal voluntary activity: few motor units discharge B. With ↑ effort ✓ ↑strength → ↑ number of motor unit (recruitment) → ↑ force of contraction ✓ ↑ frequency of discharge During voluntary movement with moderate intensity, rate of discharge to motor units produce clonic contraction Motor units contract a synchronously → however, responses of various motor units merge into smooth contraction of the whole muscle 10 3. Length Tension Relationship (starling law): Preload: load before onset of contraction. Within limit: ↑ initial muscle length [preload]→ ↑ active tension developed during isometric contraction (direct proportional) At sarcomere length = 2.2 u. At sarcomere length At sarcomere length (Resting muscle length inside body). > 2.2 u skeletal muscle) MLCK become inactive Phosphatase removes phosphate from myosin → stop cycle. 4. Latch-bridges: dephosphorylated cross-bridges remain attached to actin → maintain tone with little energy (latch bridges do not cycle or slowly cycle) 5. Fatigue resistance: smooth muscle contraction uses less ATP than skeletal muscle 17  Control of Contractions of Smooth Muscle Smooth muscles are characterized by Spontaneous contractions Rhythmic or tetanic (Muscle tone= maintained partial contraction) in isolated muscle (no nerve supply) controlled by many stimulatory factors acting directly on smooth muscle & can initiate contraction via eliciting action potential or even without action potential 1. Stretch: ↑contraction Hollow organ automatically contracts & evacuate when distended 2. Local factors: relaxed by acids, ↑CO2, ↓oxygen. contracted by alkalis ,↑K+ 3. Cold: ↑contraction 4. Humoral Factors: binding of ligand to ….. 1) Excitatory receptors→ ↑cytoplasmic Ca++ → cause contraction 2) Inhibitory receptor→↓ cytoplasmic Ca++ → cause relaxation, mediated by a) Activation of K+ channels → inhibit Ca++ influx. b) ↑ active transport of calcium 5. Role of Nerve Supply Dual nerve supply from ANS Not initiate activity Modifies spontaneous activity & sensitivity to chemicals Plasticity (Length to Tension relation) ↑stretch→↑ tension at first Maintained stretch→↓ tension gradually. Cause: readjustment of position of the myosin cross-bridges on thin filaments. Benefit: urine can accumulate in urinary bladder without much rise of intra-vesical pressure. 18