Section 5: Muscles
Muscles
Module 5
- Biological machines that utilize chemical energy from breakdown and metabolism of food to perform useful work
- 600+ muscles
- Perform 3 main functions
- Movement
- Heat production
- Body support, posture
Skeletal Muscle
- Whole muscles made up of bundles of fasciculi
- Each fascicle made up of groups cells or fibres
- Each muscle cell --> bundles myofibrils
- Each myofibril contains thick and thin myofilaments
- Thin <-- protein actin with troponin and tropomyosin
- Thick <-- myosin protein
--> interaction thin and thick = muscle contraction
--> fasciculi surrounded by white connective tissue called perimysium
Structure Skeletal Muscle
- Muscle cells surrounded by sarcolemma (muscle cell membrane)
--> action potential transmitted on
- Sarcolemma has small tube-like projections
--> called transverse tubules
--> extend down into cell
--> conduct action potential deep into cell where contractile proteins are located
- Within muscle cell are long cylindrical myofibrils that contain contractile proteins (thick and thin)
- Myofibrils surrounded by sarcoplasmic reticulum (SR)
--> mesh-like network of tubes containing calcium ions (Ca2+) (essential for contraction)
- Terminal cisternae (membranous enlargement of SR)
--> either end and continuous SR
--> its close to T-tubule (action potential travels)
Thin Myofilament
- Composed of predominately of globular protein --> actin
- Each actin has special binding site for myosin
- Actins strung together like beads on a necklace and then twisted to form a backbone of thin myofilaments
- Tropomyosin <-- long protein strands on thin myofilaments
--> muscle at rest, proteins cover binding sites of myosin
- Troponin <-- regulatory protein
A --> binds to actin
B --> binds to tropomyosin
C --> binds with Ca2+
- at rest troponin complex holds tropomyosin over myosin binding sites
Thick Myofilament
- Made of protein myosin
- Protein has long, bendable tail and two heads that can each attach to myosin binding sites on actin
- Heads have site that can bind and split ATP --> splitting of ATP that releases energy to myosin that powers contraction of the muscle
- Many myosin molecules arranged form one thick filament
Actin/Myosin Relationship
- Arranged in repeating pattern
- Each group thin myofilaments extends outward in opposite directions from central Z disc --> where they are anchored
- Groups thick myofilaments extend outward from central M line --> where they are attached
- Myofilament parallel to length of myofibril and muscle cell
- Sarcomere : region from one Z disc to another --> smallest functional contractile unit
- Under microscope gives striated appearance
A bands <-- thick filaments as dark bands
I bands <-- thin filaments with light bands
Muscle Contraction
- Interaction between actin and myosin leads to contraction
- Head myosin attaches binding site of actin --> forms cross-bridge, myosin undergoes change in shape
- Change in shape cause myosin head to swing --> producing power stroke
- Power stroke --> slides actin past myosin
*Thick/thin do NOT shorten during contraction*
Excitation Contraction Coupling & Muscle Contraction
- Process action potential excites muscle cell to produce muscle contraction
- Action potential at NMJ spread over sarcolemma and down T-tubules into core of muscle cell --> produce muscle contraction
- Action potential travels close to SR and opens Ca2+ channels --> releases Ca2+ from terminal cisternae of SR
- Ca2+ will bind to troponin C on thin myofilaments causing tropomyosin to uncover myosin binding sites on actin
- Myosin attaches to actin and power stroke occurs
Relaxation of Muscle
- Action potential stop, Ca2+ no longer diffuse out of SR
- Special Ca pumps, pumps Ca2+ back into SR (against concentration grad) --> requires ATP
- Tropomyosin covers myosin binding sites
- Myosin unable binds --> relax muscle no power strokes
Actin-Myosin and ATP cycle
- Splitting ATP to adenosine diphosphate (ADP) and inorganic phosphate releases energy to myosin and prepares myosin head for activity
- Formation cross-bridges occur when Ca2+:
- have been released from SR by Action Potential, binds Troponin C
- rolls tropomyosin off myosin binding sites
- Power stroke occurs when myosin heads bend and slides the thin myofilaments of actin over thick myofilaments of myosin
- ADP and P molecules released from head
- New molecule ATP binds to myosin heads
Two ways Muscles Alter Force Contraction
- Recruit Motor Units
- Summation Twitch Contractions
Motor Unit
- Is a motor neuron and all muscle cell/fibres causes contact
- One motor neuron contact several muscle cells --> ever muscle cell only one motor neuron
Large --> 200 cells
Small --> Few cells
Motor Unit Recruitment
--> Progressive activation of motor units resulting in more forceful contraction
Muscle Twitch
- Simplest and smallest contraction
- Result of an action potential in motor neuron
- Will cause excite cell and release Ca2+ from SR --> very small contraction
- Varies 10-100ms --> AP = 2ms
- Latent period due to all events at NMJ
- Can increase force contraction by increasing # action potential/second that travel down nerve (frequencies)
- High frequencies
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