Muscle Flashcards
(39 cards)
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
