Chapter 9 Flashcards
Muscle Tissue
Nearly 1/2 the body’s mass
Function:
-Chemical energy to movement
Types of Muscle Tissue (3)
Skeletal
Cardiac
Smooth
Prefixes of Muscle Involvement (3)
Myo, Mys, Sarco
Skeletal Muscle Tissue
- Organs attached to bones and skin
- Muscle Fibers
- Striated
- Voluntary
- Contract rapidly, tire easily, powerful
- Require nervous system stimulation
Cardiac Muscle Tissue
- Only in heart, bulk in heart walls
- Striated
- Can contract without nervous system stimulation
- Involuntary
Smooth Muscle Tissue
- In walls of hollow organs
- No striations
- Can contract without nervous system stimulation
- Involuntary
Special Characteristics of Muscle Tissue
-Excitability (responsiveness)
-Contractility (contract)
-Extensibility (Stretchable)
Elasticity (Recoil)
Muscle Functions (8)
-Movement
-Posture
-Stabilization
-Heat
Can also
-Protect Organs
-Forms Valves
-Controls Pupil Size
-Causes “Goosebumps”
Skeletal Muscle
Each muscle served by one artery, one nerve, and one or more veins
- Run together in connective tissue sheaths
- Every skeletal muscle fiber supplied by nerve ending that controls its activity
- Huge nutrient and oxygen need, generates large amount of waste
Connective Tissue Sheaths of Skeletal Muscle
Supports cells, reinforce whole muscle External to internal -Epimysium -Perimysium -Endomysium
Epimysium
Dense irregular connective tissue surrounding entire muscle
Perimysium
Fibrous connective tissue surrounding fascicles
Endomysium
Fine areolar connective tissue surrounding each muscle fiber
Fascicles
Groups of muscle fibers
Skeletal Muscle Attachment
Two Places
- Insertion
- Origin
Insertion
Movable bone
Origin
Immovable/less movable bone
Microscopic Anatomy of Skeletal Muscle Fiber
Long, cylindrical cell -up to 30cm long Sarcoplasm=cytoplasm -Glycosomes for glycogen storage -Myoglobin for oxygen storage Modified structures: myofibrils, sacroplasmic reticulum, and T tubules
Myofibrils
Densely packed, rodlike elements -about 80% of cell volume Contains sarcomeres contractile units -Sarcomeres contain myofilaments Exhibit striations- perfectly aligned repeating series of dark bands and light bands
Sarcomere
- Smallest contractile unit
- Align along myofibril like boxcars of train
- Composed of thick and thin myofilaments made of contractile proteins
Myofibril Banding Pattern
Orderly arrangement of actin and myosin myofilaments within sacromere
- Actin
- Myosin
Actin Myofilaments
Thin Filaments
Myosin Filaments
Thick Filaments
Structure of Thick Filament
Composed of protein myosin
Each of 2 heavy and four light polypeptide chains
-Myosin tails contain 2 heavy polypeptide chains
-Myosin heads contain 2 light polypeptide chains
–Act as cross bridges during contraction
–Binding sites for actin of thin filaments
–Binding sites for ATP
Structure of Thin Filament
Double strand of fibrous protein
Bears active sites for myosin head attachment during contraction
Tropomyosin and Troponin- regulatory proteins bound to actin
Sacroplasmic Reticulum
Network of smooth endoplasmic reticulum surrounding each myofibril
-Run longitudinally
Pairs of terminal cisterns form perpendicular cross channels
Functions in regulation of intracellular Ca2 levels
-Stores and releases Ca2
Sliding Filament Model of Contraction
Generation of force
Does not necessarily cause shortening of fiber
Sliding Filament Model of Contraction
In relaxed state, thin and thick filaments overlap only at ends of A band
Sliding filament model of contraction
-During contraction, thin filaments slide past thick filaments > Actin and myosin overlap more
-Occurs when myosin heads bind to actin > cross bridges
Sliding Filament Model of Contraction
Myosin heads bind to actin, sliding begins
Cross bridges form and break several times, ratcheting thin filaments toward center of sacromere
-Causes shortening of muscle fiber
-Pulls Z discs toward M line
Skeletal Muscle to Contract
Activation (at neuromuscular junction)
-Nervous system stimulation
-Must generate action potential in sarcolemma
Excitation-Contraction Coupling
-Action potential propagated along sarcolemma
-Intracellular Ca2+ levels must rise briefly
Nerve Stimulus and Events at Neuromuscular Junction
- Skeletal muscles stimulated by somatic motor neurons
- Axons of motor neurons travel from central nervous system via nerves to skeletal muscle
- Each axon forms several branches as it enters muscle
- Each axon ending forms neuromuscular junction with single muscle fiber
- -Usually only one per muscle fiber
Neuromuscular Junction (NMJ)
- Stimulated midway along length of muscle fiber
- Axon terminal and muscle fiber separated by gel-filled space called synaptic cleft
- NMJ includes
- -Axon Terminals
- -Synaptic Cleft
- -Junctional Folds
Events at Neuromuscular Junction
- Nerve impulse arrives at axon terminal
- ACh released into synaptic cleft
- ACh diffuses across cleft and binds with receptors on sarcolemma
- Electrical events
- Generation of action potential
Destruction of Acetylcholine
ACh effects quickly terminated by enzyme acetylcholinesterase in synaptic cleft
- Breaks down ACh to acetic acid and choline
- Prevents continued muscle fiber contraction in absence of additional stimulation
Generation of an Action Potential
Resting Sarcolemma polarized
-Voltage across membrane
Action potential caused by changes in electrical charges
Action Potential by changes in Electrical Charges (3 Steps)
End Plate Potential
Depolarization
Repolarization
End Plate Potential
- ACh binding opens ion channels
- Simultaneous diffusion Na+ inward and K+ outward
- More Na+ diffuses in, so interior of sarcolemma becomes less negative
- Local depolarization = end plate potential
Depolarization
Generation and propagation of an action potential (AP)
- End plate potential spreads to adjacent membrane areas
- Na+ channels open
- Na+ influx decreases membrane voltage toward critical voltage called threshold
- If threshold is reached, AP initiated
- Once initiated, is unstoppable- muscle fiber contraction
Events in Generation of an Action Potential
AP spreads across sarcolemma
Voltage-gated Na+ channels open in adjacent patch, causing it to depolarize to threshold
Repolarization
Restoring electrical conditions of RMP
- Na+ channels close and voltage-gated K+ channels open
- K+ efflux rapidly restores resting polarity
- Fiber cannot be stimulated- in refractory period until repolarization complete
- Ionic conditions of resting state restored by Na+ -K+ pump
Channels Involved in Initiating Muscle Contraction
Review of Muscle Contraction
- Nerve impulse reaches axon terminal
- Voltage-gated calcium channels open
- ACh is released to synaptic cleft
- ACh binds to its receptors on sarcolemma
- Opens ligand-gated Na+ and K+ channels
- End Plate Potential
- Opens voltage-gated Na+ channels
- AP propagation
- Voltage-sensitive proteins in T tubules change shape
- SR releases Ca2+ to cytosol
Role Calcium in Muscle Contraction
Low Calcium= relaxed muscle
High Calcium= contractions start
Cross Bridge Cycle
Continues as long as Ca2+ signal and adequate ATP present
Cross bridge formation- high-energy myosin head attaches to thin filament
Rigor Mortis
Cross bridge detachment requires ATP
3-4 hours after death muscles begin to stiffen with weak rigidity at 12 hours post mortem
-Dying cells take in calcium- cross bridge formation
-No ATP generated to break cross bridges
Muscle Tension
Force exerted on load or object to be moved
Iso
Same as
Metric
Length
Tonic
Amount of contraction shorten or lengthen
Isometric Contraction
No shortening, muscle tension increases but does not exceed load
Isotonic Contraction
Muscle shortens because muscle tension exceeds load
Motor Unit
Motor Neuron and all muscle fibers it supplies
Each muscle is served by at least one motor nerve
Muscle fibers from motor unit spread throughout muscle so single motor unit causes weak contraction of entire muscle
Motor units in muscle usually contract asynshronously
Motor Nerve
Contains axons of up to hundreds of motor neurons
Asynchronously
Take turns, not all at the same time
Muscle Twitch
3 Phases -Latent Period -Period of Contraction -Period of Relaxation Contracts faster than it relaxes
Latent Period
Events of excitation-contraction coupling, no muscle tension
Period of Contraction
Cross bridge formation, tension increases
Period of Relaxation
Calcium leaves cells
Muscle Twitch Comparisons
Different strength and duration of twitches due to variations in metabolic properties and enzymes between muscles
-Normal muscle contractions are smooth
Graded Muscle Response
Varying Strength of contraction for different demands
Required for control of skeletal movement
Responses graded by
-Changing frequency of stimulation
-Changing strength of stimulation
Stimulus results
Single stimulus results in single contractile response- muscle twitch
Response to Change in Stimulus Frequency
If stimuli are given quickly enough, muscle reaches maximal tension- fused complete, tetany results
- Smooth sustained contraction
- No muscle relaxation- muscle fatigue
- -Muscle cannot contract; zero tension
Recruitment
Controls force of contraction
Subthreshold
Not much muscle contraction
Threshold
Muscle contraction as much as needed
Maximal Stimulus
All the strength activates everything
Recruitment and size principle
- Motor units with smallest muscle fibers recruited first
- Motor units with larger fibers recruited as stimulus intensity increases
- Largest motor units activated only for most powerful contractions
Isotonic Contractions
Concentric- shortens
Eccentric- lengthens
Isometric Contractions
Tension increases to muscle’s capacity but muscle neither shortens nor lengthens
Muscle Tone
Constant, slightly contracted state of all muscles
Keeps muscles firm, healthy, and ready to respond
Anaerobic
No air, lots of lactic acid waste
- Glycolysis- does not require oxygen
- Lactic acid diffuses into bloodstream
Aerobic
Uses air
