Muscle Flashcards
Three Major Function of Muscles
- movement of body
- movement of materials throughout the body
- maintenance of body temp through heat production (homeostasis)
Three different Types of Muscle:
- Skeletal: attaches to skeleton
- cardiac: heart
- smooth: walls of hollow structures
Structure of skeletal muscle fiber:
- elongated cylindrical shape
- multi-nucleated
- 10-100 microns in diameter
- up to 20cm long
What is the plasma membrane in muscle cells called
Sarcolemma
What is the cytoplasm of skeletal muscle Fibers called
Sarcoplasm
Contents of Sarcoplasm of Skeletal Muscle Fibers
- high conc. glycogen for energy for contraction
- myoglobin: high O2 levels that can be used for energy production
- myofibrils
- sarcoplasmic reticulum
- T-Tubules
Myofibrils in Skeletal Muscle Cells:
- cylindrical bundles of filaments
- 1-2 microns
- 80% vol of sarcoplasm
- contain myofilaments which comprise contractile elements of muscle fibre
Sarcoplasmic Reticulum in Skeletal Muscle Fibers:
- interconnected series of segments that surround microfibrils
- has lateral sacs
high conc of Ca2+ essential for muscle contraction
Transverse (or T-) Tubules in Skeletal Muscle Fibers:
- continuous with extracellular space
- runs across thickness of muscle fiber
- closely associated with lateral sacs of sarcoplasmic reticulum
- initiate muscle contraction by conducting electrical signals from sarcolemma into muscle fiber
What subdivision of the nervous system are motor-neurons apart of?
Somatic Nervous System (of the Efferent (motor) division
Structural Features of Motor-neurons:
- nerves cells whose axons innervate skeletal muscle fiber
- cell bodies (somata) located in brain-steam or anterior horns of spinal cord
- large myelinated axon (rapid conduction)
- axon branches to innervate multiple muscle fibers
- each muscle fiber on innervated by one motor-neuron
Structural Features of Motor Unit:
a motor-neuron + muscle fiber it innervates.
unit size determine muscle function:
muscles producing large forces = large motor unit
muscles producing small forces = small motor unit
Structural Significance of motor-neurons having myelinated axons
Allows conducting electrical signals (APs) at high velocities from CNS to skeletal muscle fibers, with minimum delay
Structural Components of Neuromuscular Junction:
- Axon Terminals
- Motor Endplate
Function of Neuromuscular Junction
Synaptic site for the transmission of action potential from nerve to the muscle
Axon Terminal of Neuromuscular Junction:
- loose myelinsheath as they approach muscle fiber but Schwan cells maintain insulation.
- terminals contain ACh (neurotransmitter) & mitochondria
- membrane of terminal has large no. of voltage-gated ca2+ channels
- terminal is separated from sarcolemma * contains enzymes (acetylcholinesterase) responsible for degradation of ACh
- choline molecules released are transported back to terminal & reused to form ACh. choline recycling allows neuromuscular transmission of reaction potentials to continue
Motor Endplate of Neuromuscular Junction:
- is opposite axon terminal
- highly folded to increase SA
- large no. of nicotinic acetylcholine receptors
- acetylcholinesterase enzyme metabolises acetylcholine
Effect of ACh is limited by Two Mechanisms:
- diffusion of ACh from endplate
2. ACh degradation by acetylcholinesterase in cleft
Describe the mechanism of excitation-contraction coupling
efers to AP produced at the end-plate which travels along through muscle fibre length in both directions causing contraction
Muscle Fiber Contraction:
- AP travels along muscle fiber -> meets T-Tubule -> flows down T-Tubule in middle of fiber
- presence of AP in T-Tubule triggers opening of Ca2+ channels is sarcoplasmic reticulum
- Ca2+ runs down conc gradient in sarcoplasm
- increases sarcoplasmic Ca2+ triggers myofibrils to shorten -> contraction of whole muscle fiber
Relaxation of Muscle Fiber:
- when AP stops flowing down T-Tubule, Ca2+ is pumped out of sarcoplasm back to sarcoplasmic reticulum
- ca2+ conc in sarcoplasmic reticulum returns to resting levels & myofibrils return to normal length - relaxation
Structure of Thin Myofilaments:
5-8nm in diameter & 1 micron long has three constituent proteins: 1. actin 2. tropomyosin 3. troponin complex
Myofilaments - Actin Structure:
- two chain globular protein molecules twisted around each other forming double helix
- high affinity binding site for myosin
- forms backbone of thin myofilaments
Myofilaments - Tropomyosin Structure:
- 2 chains wrapped around actin helix. positioned on top so that myosin binding sites are blocked
Myofilaments - Troponin Complex Structure:
- 3 polypeptides with high binding affinity
- 1st polypeptide: holds tropomyosin threads over myosin binding sits of actin
- 2nd polypeptide: binds to actin
- 3rd polypeptide: binds to ca2+, triggering contraction of myofibrils
Structure of Thick Myofilaments:
- 12-18nm diameter
- 1.6 microns long
- made up of ~200 myosin molecules
- 1 end of myosin is made up of 2 folded protein strands
- heads form cross-bridges each have an actin binding site and an ATPase
Thick & Thin Myofilament Arrangement:
- highly ordered
- striated appearance
- thick located in between thin & overlap
- thick surrounded by 6 thin in hexagonal arrangement
What is a Sarcomere?
Contractile unit of a muscle fiber.
represented by distance between Z-lines
how the sarcomere length changes during muscle contraction and relaxation
changes in sarcomere reflect changes in whole myofibril
What is Rigor Mortis
- stiffness of skeletal muscles (starts 4hrs after death)
- reaches peak after 12 hrs
- subsides after 2-3 days
- stiffness due to leakage of Ca2+ into sarcoplasm from extracellular fluid & sarcoplasmic reticulum
- cells die & are unable to maintain large ion conc
- Some of this calcium binds to troponin & displaces troponin filament. Allows binding of some cross-bridges and initiates power stroke. Results in muscle fiber shortening
- There is no ATP available to detach the cross-bridges
Muscle fibers remain contracted until filaments decompose after one’s death
Isotonic Contraction of Skeletal Muscles:
tension of muscles remain constant as muscle shortens. Results in movement.
E.g. Length of biceps brachii muscle fibers decreases
Isometric Contraction of Skeletal Muscles:
Tension of muscle increases, but no change in muscle length.
E.g. Maintenance of posture
Motor Unit Recruitments:
- weak contraction -> less motor units recruited
- strong contraction -> more motor units recruited
- sustained contraction -> different motor neurons recruited
- coordinated by CNS
Action Potential Frequency:
increased AP freq -> increases force generated
Twitch Contraction:
smallest contractile response to a single electrical stimulus
Tetanic Contraction
largest contractile response to a single electrical stimulus
Slow Twitch (Type I) Fibers:
- develop at slow rate
- small in diameter
- rich capillary network
- high conc of myoglobin in sarcoplasm
- suited to movements that require low force but sustained contractions
e. g. distance running
Electromyography:
- recording of electrical activity in skeletal muscle during contraction
- recorded by two electrodes on skin surface
- connected to voltmeter & measures potential difference
Plot: changes in potential diff over time
electromyography in sport and clinical sciences.
- used to analyse muscle dysfunction in athletes, detects inappropriate muscle activation patters, assists in establishing & assessing treatment outcomes in conditions
