Muscles
Focus: Skeletal Muscles
Functions
- Produce skeletal movement
- Maintain posture and body position
- Support soft tissues
- Guard entrances and exits
- Maintain body temperature
- Store nutrient reserves
There are over 600 muscles!
Cardiac
Skeletal
Skeletal Muscles
Allow us to move the muscular system
Includes only skeletal muscles
What makes up muscle?
Muscle tissue (muscle cells or fibers)
Nerves
Blood vessels
Connective tissues:
Connective Tissues
Epimysium
Exterior collagen layer
- Connected to deep fascia
- Separates muscle from surrounding tissues
Perimysium
- Surrounds muscle fiber bundles (fascicles)
- Contains blood vessel and nerve supply to fascicles
Endomysium
- Surrounds individual muscle cells (muscle fibers)
- Contains capillaries and nerve fibers contacting muscle cells
- Contains myosatellite cells (stem cells) that repair damage
Tendons are formed from connective tissues.
Muscle Attachments
Endomysium, perimysium, and epimysium come together at the ends of muscles to form connective tissue attachment to bone matrix
i.e., tendon (bundle) or aponeurosis (sheet)
Blood vessels and nerves
Muscles have extensive vascular systems that:
Supply large amounts of oxygen
Supply nutrients
Carry away wastes
Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord)
Skeletal muscle cells (muscle fibers)
Are very long and develop through fusion of mesodermal cells (myoblasts)
These become very large and contain hundreds of nuclei
Characteristics of skeletal muscle fibers
The Sarcolemma and Transverse Tubules
The sarcolemma = the cell membrane of a muscle fiber (cell)
Surrounds the sarcoplasm (cytoplasm of muscle fiber)
A change in transmembrane potential begins contractions
Transverse tubules (T tubules) = tubules that transmit action potential through cell
Allow entire muscle fiber to contract simultaneously
Have same properties as sarcolemma
Myofibrils
Lengthwise subdivisions within muscle fiber
Made up of bundles of protein filaments (myofilaments)
Myofilaments are responsible for muscle contraction
Types of myofilaments:
Thin filaments
Made of the protein actin
Thick filaments
Made of the protein myosin
The Sarcoplasmic Reticulum (SR) (Endoplasmic reticulum)
A membranous structure surrounding each myofibril
Helps transmit action potential to myofibril
Similar in structure to smooth endoplasmic reticulum
Forms chambers (terminal cisternae) attached to T tubules
Triad of the sarcoplasmic reticulum
Is formed by one T tubule and two terminal cisternae
Cisternae
Concentrate Ca2+ (via ion pumps)
Release Ca2+ into sarcomeres to begin muscle contraction
Sarcomeres
Structural units of myofibrils
Form visible patterns within myofibrils
A striped or striated pattern within myofibrils
Alternating dark, thick filaments (A bands) and light, thin filaments (I bands)
Sarcomere components
The A Band contains:
M line - located in the center of the A band located at the midline of the sarcomere
The H Band - in the area around the M line, it has thick filaments but no thin filaments
Zone of overlap - The densest, darkest area on a light micrograph, where thick and thin filaments overlap
The I Band contains:
Z lines - located in the center of the I band at the ends of the sarcomere
Titin - strands of protein that reach from tips of thick filaments to the Z line, these stabilize the filaments
Thin filaments
F-actin (filamentous actin)
Is two twisted rows of globular G-actin
The active sites on G-actin strands bind to myosin
Nebulin
Holds F-actin strands together
Tropomyosin
Is a double strand
Prevents actin–myosin interaction
Troponin
A globular protein
Binds tropomyosin to G-actin
Controlled by Ca2+
Thick filaments
Contain about 300 twisted myosin subunits
Contain titin strands that recoil after stretching the mysosin molecule
Tail - Binds to other myosin molecules
Head - Made of two globular protein subunits
Reaches the nearest thin filament
Initiating Contraction
Ca2+ binds to receptor on troponin molecule
Troponin–tropomyosin complex changes
Exposes active site of F-actin
Myosin Action
During contraction, myosin heads:
Interact with actin filaments, forming cross-bridges
Pivot, producing motion
Sliding filament theory
Thin filaments of sarcomere slide toward M line, alongside thick filaments
The width of A zone stays the same
Z lines move closer together
Muscle contractions: Putting it all together
- Ach is released from the axon terminal into the synaptic cleft
- ACh binds to ACh receptors located in the muscle fiber causing cation channels to open
- Sodium rushes in and stimulates an action potential down the sarcolemma
- ACH is immediately broken down by the enzyme acetylcholinesterase
- As the action potential moves down the sarcolemma of the muscle fiber, CA2+ ions are released from the sarcoplasmic reticulum
- Ca2+ ions bind to tbe troponin changing its configuration so that tropomyosin moves off of the myosin-binding site on actin allowing myosind to bind to actin and beginning the contraction
Contraction occurs as follows:
- The myosin heads hydrolyze ATP so that they reorient and become energized.
- The myosin heads bind to the exposed actin sites creating cross-bridges
- The myosin cross-bridges rotate toward the middle of the sarcomere
- The myosin heads, while still attached to the actin bind with ATP, which allows the myosin heads to detach from the actin
- This continues as long as CA2+ and ATP are available
Contraction Cycle Begins
Active-Site Exposure
Cross-Bridge Formation
Myosin Head Pivoting
Cross-Bridge Detachment
Myosin Reactivation
Topic video
Contraction duration
Duration of neural stimulus
Number of free calcium ions in sarcoplasm
Availability of ATP
Muscle relaxation
- The motor neuron cease firing and the neurotransmitter ACh is removed from the synapse
- The calcium release channels of the sarcoplasmic reticulum close and pumps bring back the level of CA2+ ions in the SR to levels prior to muscle contraction
- The tropomyosin returns to its configuration of blocking myosin binding sites in actin
Rigor mortis
A fixed muscular contraction after death
Caused when: Ion pumps cease to function; ran out of ATP
Calcium builds up in the sarcoplasm
Tension and Contraction
As a whole, a muscle fiber is either contracted or relaxed
Depends on:
The number of pivoting cross-bridges
The fiber’s resting length at the time of stimulation
The frequency of stimulation
Length–Tension Relationships
Number of pivoting cross-bridges depends on:
Amount of overlap between thick and thin fibers
Optimum overlap produces greatest amount of tension
Too much or too little reduces efficiency
Normal resting sarcomere length is 75 to 130 percent of optimal length
The Frequency of Stimulation
A single neural stimulation produces:
A single contraction or twitch which lasts about 7–100 msec.
Sustained muscular contractions require many repeated stimuli
Twitches
Latent period - The action potential moves through sarcolemma causing Ca2+ release
Contraction phase - Calcium ions bind and tension builds to peak
Relaxation phase- Ca2+ levels fall and active sites are covered and tension falls to resting levels
Treppe
A stair-step increase in twitch tension
Repeated stimulations immediately after relaxation phase
Stimulus frequency <50/second
Causes a series of contractions with increasing tension
Wave summation
Wave summation
Increasing tension or summation of twitches
Repeated stimulations before the end of relaxation phase
Stimulus frequency >50/second
Causes increasing tension or summation of twitches
Incomplete tetanus
Twitches reach maximum tension
If rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension
Complete tentanus
If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction