Download P215 - Basic Human Physiology

Muscle
Chapter 17
Movement
• Locomotion
– Movement of animal from one location to
another
• Repositioning
– Movement of animal appendages
• Internal movement
– Movement of gases, fluids and ingested solids
through the animal
Muscle Tissue
• Specially designed to physically shorten
(contract)
– Generates mechanical force
• Functions
– locomotion and external movements
– internal movement (circulation, digestion)
– heat generation
Muscle Types
• Skeletal Muscle
– large muscle fibers (cells)
– striated (banded)
– voluntary control
• Cardiac Muscle
– striated
– fibers linked by intercalated disks (electrical synapses)
– self-excitation
• Smooth Muscle
– small tapered fibers
– lack striations
Skeletal Muscle Organization
• Muscle fibers (cells)
– elongate cells, parallel arrangement
– sarcolemma – cell membrane
– sarcoplamic reticulum (SR)
• membraneous internal network
• Stores Ca2+
• linked to sarcolemma by transverse tubules
Skeletal Muscle Organization
• myofibrils - intracellular contractile elements
– Arranged in sarcomeres (repeated tandem units)
– Consist of myofilaments
• thick filaments (myosin)
• thin filaments (actin)
Thick Filament Structure
• Bundles of several hundred myosin molecules
– intertwining tails + globular heads
• heads contain
– actin binding sites
– ATP-hydrolyzing sites
• form crossbridges with actin
Thin Filament Structure
• Actin
– Primary structural protein
– Spherical protein subunits connected in long, double strand
– Contains myosin binding site
• Tropomyosin
– Threadlike proteins
– Normally cover myosin binding sites
• Troponin
– Ca2+ Binding Protein
– Holds tropomyosin in place
– Ca2+ binding induces shape change that repositions tropomyosin
Skeletal Muscle Contraction:
Sliding Filament Mechanism
• Movement of thin filaments over thick
– thick filaments are stationary; thin are dragged
across thick
– sarcomere shortening by increasing overlap of
thick and thin filaments
Crossbridge Cycling
• Myosin head binds to actin
• Cross bridge bends (Power Stroke)
– thin filaments pulled toward center of sarcomere
• Cross bridge link broken
• Cross bridge ‘unbends’ and binds to next actin
molecule
Neural Activation of Skeletal
Muscle Contraction
• Excitation-Contraction Coupling
– Events that link muscle excitation to muscle contraction
• Excitation = Action Potential
– Brief, rapid depolarization of cell membrane.
• Triggers release of Ca2+ into the cytosol of the
muscle fiber
Neural Input
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•
•
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Action potential travels down axon to terminal
Exocytosis of acetylcholine (ACh)
ACh diffuses across cleft
ACh opens ACh-gated Na+ channels
Action potential propagates down sarcolemma
Excitation-Contraction Coupling
• T-tubules conduct APs into the cell
• Triggers Ca2+ channels in SR to open
• Ca2+ released into the cytosol
Smooth Muscle Contraction
• No striations
– contractile proteins not arranged in sarcomeres
– arranged in fish-net network
– allows for extensive contraction, even when
stretched
Smooth Muscle ExcitationContraction Coupling
• Depolarization of sarcolemma opens Ca2+ channels
– Ca2+ enters the cell from the extracellular fluid
• Ca2+ binds with calmodulin in cytoplasm
• Ca2+-calmodulin binds to myosin light chain kinase
– activates MLCK
• MLCK phosphorylates myosin
– needed for myosin to bind actin
• Cross-bridge cycling
Whole Muscle Mechanics
Types of Contractions
• Isometric Contraction
– muscle is prevented from shortening
– contraction generates force on attachment points
• Isotonic Contraction
– muscle allowed to shorten upon contraction
– muscle moves and object of a given mass
Isometric Contractions
Twitch
– response to a single, rapid stimulus
Summation
– Application of stimuli in rapid succession
– Muscle responds to second before fully relaxed
from first
– Increased tension generated
Tetanus
– With repeated stimulation at high frequency
twitches fuse to form steady tension
Length-Tension Relationship
• Maximum force generated is associated with starting length
of muscle
• Related to the overlaps of thick and thin filaments
• Maximum tension generated at normal in vivo resting length
– Too long - pull thick and thin filaments apart
– Too short - thin filaments form opposite sides collide
Isotonic Contraction
• Muscle free to shorten with stimulation
–  shortening distance with load
–  shortening velocity with load
Isotonic Contraction
• Contraction Force (N)
– Anything that changes the state of
motion for an object
– Force  Cross-Sectional Area
• Work (J)
– Expresses forces applied to an object to
set it in motion
– Force × Distance
– With  load,  mass,  distance
– Max work obtained at ~40% max. load
– Work  Muscle Mass
Comparative Muscle Physiology:
Vertebrate Skeletal Fiber Types
• Fast (Twitch) Fibers
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Low myoglobin content (white)
used for rapid movements
large nerve fibers w/ high velocities
anaerobic activity
• Slow (Tonic) Fibers
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High myoglobin content (red)
used for low-force prolonged contractions
small nerve fibers w/ low velocities
aerobic activity
Comparative Muscle Physiology:
Fiber Types in Tuna
• Tonic Fibers
– located in lateral core areas
– used for “cruising”
• Twitch Fibers
– much of the remaining
muscle mass
– used for short bursts of high
activity
Comparative Muscle Physiology:
Molluscan Catch Muscle
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Used to seal shells of bivalves
Sustained contraction w/o fatigue
Little  in O2 consumption
Two groups of neurons innervate muscle
1.
Exciters - releases ACh
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2.
causes release of Ca2+
binds myosin w/o release
Relaxers - releases seratonin
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causes release of cAMP
causes Ca2+ release from myosin
Comparative Muscle Physiology:
Crustacean Chelipod Muscle
• Pinnate (angular) arrangement of fibers
– can have more, shorter fibers
– 2x increase in force
• Few or only single motor unit(s) per muscle
• Multiple innervation to each muscle fibers
– slow neuron - stimulates tonic activity
– fast neuron - stimulates twitch activity
– inhibitory neuron - restricts activity
Comparative Muscle Physiology:
Insect Flight Muscle
• Synchonous flight muscle
– moths, grasshoppers, dragonflies
– slow wingbeat frequencies
– each contraction is a response to a single nerve impulse
• Asynchronous flight muscle
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–
–
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flies, bees mosquitoes
high wingbeat frequencies (100-1000 bps)
too fast for response to individual nerve signals
multiple contractions per nerve signal
Comparative Muscle Physiology:
Asynchronous Muscle Function
• Fibrillar muscles
– attached to walls of thorax
• horizontal arrangement
• vertical arrangement
– not connected to wings
– distort shape of thorax
Comparative Muscle Physiology:
Asynchronous Muscle Function
• upstroke
– vertical muscles contract
– thorax distorted (click)
– stretch horizontal muscles
• induced them to contract
• downstroke
– horizontal muscles contract
– distort thorax
– stretch vertical muscles
• several wingbeats per nerve impulse
Comparative Muscle Physiology:
Electric Eels
• Electric organs – modified skeletal muscle
• Electrocytes – stacked in columns
– Respond to signals from motor neurons
– All electrocytes in column depolarize
spontaneously
– Up to 600 V discharge