Ch. 9 The Muscular System Flashcards
Functions of muscle
- Movement
- Maintaining posture
- Stabilizing the joints
- Temperature homeostasis
- Regulate movement of substances between compartments
Characteristics of Muscle Tissue
- Excitability - receives and responds to stimulus
- Contractility - ability to shorten
- Extensibility - ability to be stretched beyond resting length
- Elasticity - ability to resume original length after stretching
Skeletal
Muscle type
- attached to bones
- voluntary control
- striated
- limited regeneration
Cardiac
Muscle type
- in heart walls
- involuntary
- striated
- no regeneration
Smooth
Muscle type
- in the walls of the hollow organs
- involuntary
- not striated
- fast regeneration
Epimysium
Skeletal Muscle
- 1 of 3 connective tissue sheaths (all continuous with one another)
- Dense conn. tissue
- Surrounds whole muscle
Perimysium
Skeletal Muscle
- 1 of 3 connective tissue sheaths (all continuous with one another)
- Fibrous conn. tissue
- Surrounds fascicles
Endomysium
Skeletal Muscle
- 1 of 3 connective tissue sheaths (all continuous with one another)
- Sheath of fine areolar conn. tissue with reticular fibers
- Surrounds each muscle fiber (cell)
Origin
Attachment
attachment to bone that does not move during contraction
Insertion
Attachment
attachment to bone that moves during contraction
Direct
Attachment
epimysium of muscle fused to periosteum of bone or perichondrium of cartilage
Indirect
Attachment
- muscles connective tissue wrappings extend beyond muscle as a ropelike tendon or aponeurosis
- Indirect much more common due to durability & small size
General Anatomy of a skeletal muscle cell - fiber
- Long (up to 30 cm) cylinder shaped cells
- Cells are called “muscle fibers”
- Several important modifications related to function
Triad
Muscle fiber anatomy
- Formed where T-tubules and terminal cisterns meet
- Allows messages from the cell membrane to be transferred into the cell
T-tubule
Muscle fiber anatomy
- Invaginations from cell membrane
- Passes message deep into cell quickly
Terminal cisterns
Muscle fiber anatomy
- Flattened areas of the smER
- Closely associated with the T-tubules
Sacroplasmic reticulum
Muscle fiber anatomy
- Extensive smER
- Needed for Ca2+ storage
Sarcolemma
Muscle fiber anatomy
- Muscle cell membrane
- Forms T-Tubules
Sacroplasm
Muscle fiber anatomy
- Stores glycogen, myoglobin, and creatine phosphate
- All are related to energy needs
Nucleus
Muscle fiber anatomy
- Multinucleated because they originate from the fusion of several myoblast cells
- Nuclei are pushed to side
Mitochondrian
Muscle fiber anatomy
- Many large mitochondria
- For energy production
Myofibril
Muscle fiber anatomy
- Rod-like structures that fill the cell
- Contain bundles of proteins that compose the contractile units
Thin filaments
Muscle fiber anatomy
The proteins of the myofibrils
Sacromere
Muscle fiber anatomy
- The contractile units
- Formed by the thick and thin filaments of the myofibril
Skeletal musle
1st level of muscle organization
- surrounded by epimysium
- contains muscle fascicles
Muscle fascicle
2nd level of muscle organization
- Surrounded by perimysium
- contains muscle fibers
Muscle fiber
3rd level of muscle organization
- surrounded by endomysium
- contains myofibrils
Myofibrils
4th level of muscle organization
- surrounded by sacroplamsic reticulum
- consists of sacromeres (Z line to Z line)
Sacromere
5th level of muscle organization
contains thick and thin filaments
I band
Sacromere Anatomy
- thin actin filaments ONLY
- light region where Z disk is
H Zone
Sacromere Anatomy
- thick myosin filaments ONLY
- light region where M line is
M line
Sacromere Anatomy
thick myosin filaments linked by accesory proteins
A band
Sacromere Anatomy
- thick myosin and thin acitin filaments overlap
Thick filaments
- made out of myosin
- Each thick filament contains –> 300 myosin molecules
- Protein shaped like a double headed golf club
- Head “cross bridge”
Head contains: - ATPase (enzyme to split ATP)
- actin binding site
- ATP binding site
Thin filaments
- Made of Actin
- 2 thin strands of protein that wind around each other
Thin flaments contains:
1. myosin binding sites (that the myosin heads attach to)
2. troponin
3. tropomyosin
Troponin
Thin filament anatomy
- Has calcium binding sites
- Moves the tropomyosin off the myosin binding sites so that contraction can occur
Tropomyosin
Thin filament anatomy
- Reinforces the structure and covers the myosin binding sites when the cell is resting
Ion Channels
Muscle Physiology
Allow ions to diffuse across the cell membrane
Gated Channels
Muscle Physiology
These open and close, when closed do not allow any diffusion of ions
Voltage-Gated Channels
Type of gated channel
open when membrane
potential changes
Ligand-Gated Channels
Type of gated channel
open when chemicals (such as neurotransmitters) attach to them
Muscle contraction - relaxation steps
4 steps:
– Excitation
– Excitation-contraction coupling
– Contraction
– Relaxation
Nerve stimulation to the Skeletal Muscle (Excitation)
- Each muscle cell is stimulated by a nerve impulse
- Neuron cell body is in CNS, but sends axons out to muscles
- Axons branch so that each cell is in contact with a neuron
Excitation – change in Resting Membrane Potential
- Pumps move ions across the membrane against their concentration gradient (requires ATP)
- The Na/K pump creates and maintains a resting membrane potential (a voltage difference across the membrane)
- Muscle cells typically have a -90mV resting membrane potentia
Structure of the Neuromotor Junction
- Neuron’s axon end bulb filled with vesicles of acetylcholine (ACh)
- Synaptic cleft
- Muscle cell’s motor end plate with receptors for ACh
- Excitation
Muscle contraction - relaxation
Nerve impulse reaches axon end bulb
↓
ACh is released into cleft
↓
ACh attaches to receptors (ligand gated ion channels
↓
Na+ floods more quickly than K+ into the cell and the sarcolemma depolarizes
↓
Action Potential
Action Potential
- Membrane potential (Vm ) reaches threshold (-55mV)
- Fast Na+ channels open and Na+ rushes in causing the Vm to depolarize to +30mV.
- The depolarization stops when the Na+ channels become inactivated.
- Slow K+ channels have opened & K+ efflux occurs. This returns Vm back to its resting level. This is repolarization.
- Excitation – Contraction Coupling
Muscle contraction - relaxation
Neighboring voltage gated Na+ channels open and depolarization sweeps across sarcolemma and into T-Tubules
↓
Causes Ca2+ to be released from terminal
cisternae into cytoplasm
↓
Ca2+ attaches to troponin
↓
Troponin changes shape and pulls the tropomyosin off the myosin binding sites on
the actin
↓
Myosin heads bind to the exposed binding sites
Muscle Contraction - Power Stroke
When the cell is resting, Ca2+ levels in the cell are low and myosin binding sites on actin are physically blocked by Tropomyosin
1. When terminal Cisterns releases Ca2+ into cytoplasm
2. Calcium attaches to Troponin
3. Troponin/Tropomyosin complex moves which opens the Myosin binding sites on the Actin
4. Power Stokes occur
ATP hydrolysis
Contraction Cycle (sliding filament theory)
- The ATP binding site on myosin head contains ATPases
– ATP ADP + P (which are attached to myosin head)
– This energizes myosin head
Attachment of myosin to actin to form crossbridges
Contraction Cycle (sliding filament theory)
Energized myosin head attaches to myosin binding site on actin and releases P
Power Stroke
Contraction Cycle (sliding filament theory)
- Spot where ADP is bound on myosin head opens causing the head to rotate and release ADP
- Force is generated as the head moves near sarcomere center (actin filament slides past the myosin filament towards M line)
Detachment of Myosin from Actin
- Myosin head stays attached to actin until ATP binds to it at ATP binding site
- Myosin head detaches from actin
- Steps are repeated until contraction is complete
- tropomyosin blockage restored; contraction ends
For attachment of myosin to actin
Contraction Cycle (sliding filament theory)
P then ADP are released
For release of myosin from actin
Contraction Cycle (sliding filament theory)
ATP must reattach to myosin head
Muscle Contraction
- Contraction is the result of sarcomeres shortening
- When relaxed the thin and thick filaments overlap slightly
- During contraction myosin heads act as little ratchets to pull the thin filaments to the center of the sarcomere
- As a result thin and thick filaments overlap extensively - H zone disappears and Z lines are closer to each other
Contraction - Relaxation
- Cycle repeats to shorten Sarcomeres
(muscle contracts) - Cycle is broken when calcium is reabsorbed into Terminal Cisterns
- Tropinin/Tropomyosin move back to cover Myosin binding sites on Actin
- Sarcomeres return to original length (muscle relaxes)
Smooth Muscle
Gross Anatomy:
- Arranged in layers in walls of hollow organs
- Cells in adjacent layers have opposite orientation
(alternating contractions and relaxation causes peristalsis
Microscopic smooth muscle anatomy
- Small, spindle–shaped cells
- 1 nucleus
- Smooth ER extends to surface
- No T-tubules
- Gap junctions
- Few mitochondria
- Surrounded by thin endomysium
Myofibrils
- Uses Actin & Myosin
- No sarcomeres
- Intermediate filaments attach actin to plasma membrane
- Tropomyosin present
- No tropinin
- Calmodulin present
- Light chain kinases (LCK)
Physiology of Smooth Muscle Contraction
Stimulation for contraction may be:
- Autonomic innervation
- Hormonal
- Local conditions
Diffuse Junction
Physiology of Smooth Muscle Contraction
Varicosity = large bulb-like swelling in the neuron
Neurotransmitter is released into a wide cleft
Neurotransmitter attaches to many cells
Contraction of Smooth Muscle
Contractions are slow, synchronized because of gap junctions, cells are resistant to fatigue, and use little energy
Botulism
blocks release of Ach
Rigor Mortis
- 3 h after death, full rigor about 12 h
– After 12 h rigor slowly ceases, around 72 h rigor disappears. - some cells within their tissues continue to survive.
– After the circulation of blood ceases, surviving muscle cells resort to anaerobic glycolysis but eventually they become unable to make any more ATP. - no more ATP production after death so heads can not unattach
- Ca 2+ leaks into cytoplasm & myosin heads attach to actin – rigor sets in
- tissues decompose: proteolytic enzymes in the lysosomes of the muscle cells escape and begin to dissolve the myofilaments –> rigor disappears
Motor unit
- a motor neuron and all the cells it innervates
- As small as 1 neuron – 4 muscle cells
- As large as 1 neuron – several hundred muscle cells
- Average is 150 muscle cells
- The cells in the motor unit are spread throughout the muscle – stimulation of 1 motor unit causes a weak muscle contraction
The smaller the motor unit __
the finer and more delicate the movements
Extraocular muscles
typically have small motor units while the large postural muscles have large motor units
When this neuron is stimulated ___
all the muscle fibers it synapses upon will be stimulated and will contract as a unit
Cells have All-or-None contraction
when a muscle cell is stimulated to contract it contracts to its full extent
Muscle tissue
- has a graded response to stimulus
- Force, velocity, and duration of contraction is variable
- Depends on how many motor units are stimulated and the frequency of stimulation
Muscle Twitch
- stimulated isolated muscle
- response of motor unit to single action potential (this is not how we typically use our muscles)
Latent period
Excitation-Contraction
Coupling (muscle not changed shape)
Period of contraction
Power strokes are shortening the muscle
Period of relaxation
Calcium is returning to SR and muscle is relaxing
Myogram
- graphic record of muscle contraction
- Strength of contraction
- Altered by recruiting more motor units
- Small motor units in a muscle stimulated first and larger motor units later
Isotonic
- Muscle length changes, load moves
– Concentric: muscle shortens ex picking up book
– Eccentric: muscle lengthens ex walking uphill
Isometric
- tension builds to peak but no change in muscle length
- Ex: lifting a piano
ATP
- Molecule that directly attaches to cross bridges to them provide energy
- energy for muscle contraction
- Muscles only store enough ATP for 4-6 seconds –> must have mechanisms to regenerate it quickly
Three Mechanisms of energy for contraction
- Creatine-Phosphate
2.Anaerobic respiration (glycolysis)
3 Aerobic respiration
Creatine-Phosphate
Energy for Contraction
Provides enough ATP for an additional 15 seconds
Anaerobic respiration
Energy for contraction
- Makes ATP very quickly
- Only makes 2 ATP
- Makes Lactic acid (toxic)
- Can fuel muscles for up to 45 sec
Aerobic respiration
Energy for contraction
- Requires O2
- 2 sources Myoglobin and hemoglobin
- Makes CO2 (lungs remove)
- Makes a lot of ATP
- Can fuel muscles for hours
Muscle Fatigue
- Muscle fatigue: inability to contract, even when stimulated to do so
May be several factors: - Low ATP
- build up of lactic acid
- excess free phosphate binding calcium ions
- Na+/K+ imbalances
