Flight or Fight - Muscles, Hormones and Nerves working together

Structure of Skeletal Muscle

• Skeletal muscle is joined to bone by inelastic tendons
  • Muscle contraction / pulls on tendons / bone moves
• Each muscle is made of bundles of muscle fibres surrounded by connective tissue
• An individual muscle fibre
  • Has many nuclei → muscle fibre develops from fusion of many cells
  • Sarcoplasm (cytoplasm) filled by parallel myofibrils
  • Sarcolemma (surface membrane) forms deep tubes (T tubules) into the sarcoplasm along its length
  • Network of membranes called sarcoplasmic reticulum (ER)

The Sliding-Filament Theory

Striations in Skeletal Muscle

• Actin filament / thinner than myosin → lighter striations
• Myosin filament / thicker than actin filament → darker striation
• Distance between 2 adjacent Z lines: sarcomere / actin filament is attached to Z lines and extended into sarcomeres on either side
• Striation of actin alone → I band
• Striation of myosin alone → H zone
• Length of myosin → A band
• Central thickening of each myosin filament → M line

Actin and Myosin Filaments

• Actin filament: 2 actin strands twisted around each other
  • Troponin-tropomyosin-actin complex blocks binding site for myosin
• Myosin filament: bundles of myosin molecules
  • Bundle of myosin tails form a central stalk
  • Globular heads attach to specific sites on actin filaments
  • Myosin heads contain ATPase that hydrolyses ATP

Neuromuscular Junction

• Synapse between motor neurone and muscle fibre
• \ skeletal muscle fibres are stimulated by motor neurones
• IMPULSE REACHES NEUROMUSCULAR JUNCTION

1. Influx of Ca2+ / synaptic vesicles fuse with presynaptic membrane
2. Release of acetylcholine (ACh) into synaptic cleft by exocytosis
3. Neurotransmitter diffuses across cleft
4. Binds with receptors on motor end plate (→postsynaptic membrane of muscle fibre)
5. Depolarises sarcolemma
6. Threshold stimulates wave of depolarisation along muscle fibre
7. Changes permeability of sarcoplasmic reticulum to Ca2+
8. Ca2+ move into sarcoplasm / causes contraction of myofibril

Role of ATP and phosphocreatine

Stimulation Of Muscle Fibres By The Nervous System

• CONTRACTION → myosin heads attach to actin binding sites / form temporary cross bridges / bridges rapidly break and reform / new cross bridges form further along actin filament / causing shortening of each sarcomere
• WHEN STIMULATION STOPS → Ca2+actively taken up by sarcoplasmic reticulum / myosin head detaches from actin / cross bridges reform / muscle relaxes
• NO ATP AVAILABLE → cross bridges cannot detach / muscle becomes stiff / unable to relax / extreme form: rigor mortis / occurs after death

Cycle Of Events During Contraction Of A Myofibril

• Ca2+ ions enter sarcoplasm during wave of depolarisation
• Bind to troponin / changes shape of protein / removes block of tropomyosin / exposes actin binding sites
• ATP binds to myosin / stimulates ATPase / RELEASES ENERGY
• Allows myosin heads to form cross bridges with actin
• Allows POWER STROKE: myosin head changes angle / pulls on actin filaments
  • Width of I band, H zone decrease → filaments overlap increases
  • Z lines move closer together → length of sarcomere decreases
  • No change to A band → lengths of filaments stay constant
• Allows Ca2+ ions to be pumped back in by active transport
• New ATP binds to myosin / allows detachment from actin
• Myosin head changes to original position (cross bridges reform)
• Next attachment to actin filament and power stroke can occur
  • Ca2+ and ATP required for cycle to continue

Energy In Active Muscle Cells

• Breakdown of phosphocreatine / releases PI + energy / attach to ADP / forms ATP
  • PHOSPHOCREATINE + ADP → CREATINE + ATP
  • ATP is used faster than it can be supplied by respiration
  • Phosphocreatine allows regeneration of ATP without respiration
• Thus, Muscle cells continue exercise until slower pathways synthesis ATP
  • Breakdown of glycogen in muscle cells / aerobic respiration of glucose
  • Aerobic respiration of glucose, fatty acids from bloodstream / fatty acids last longer
• Prolonged exercise / not enough O2 for aerobic respiration
  • Anaerobic respiration continues
  • Lactate may cause cramps

Muscles As Effectors

• Motor neurones stimulate glands and muscles into action
• Respond to a stimulus → are effectors

Structure, location and general properties of slow and fast skeletal muscle fibres