Muscular Tissue and Skeletal Muscles Flashcards
Muscle Types
o Skeletal
o Cardiac
o Smooth
- Skeletal and smooth muscle cells are elongated
- Contraction and shortening of muscles are due to the movement of microfilaments
Skeletal Muscle
- Most skeletal muscle fibres are attached by tendons to bones
- Skeletal muscle cells are large, cigar shaped and multinucleate
- Striated Muscle (due to stripes)
- Voluntary Muscle (under conscious control, stimulated by nerve activity)
Skeletal Muscle Layers
- Endomysium (enclose a single muscle fibre)
- Perimysium (wraps around a fascicle of muscle fibres)
- Epimysium (covers the entire skeletal muscle, composed of fibrous tissue)
- Fascia (on the outside of the epimysium)
- Sarcolemma (cell membrane of muscle fibre)
Epimysium
- Blends into a connective tissue attachment
Tendons (cordlike structures)
Mostly collagen fibers
Often cross a joint because of their toughness and small size
Aponeuroses (sheetlike structures)
Attach muscles indirectly to bones, cartilage, or connective tissue coverings
Skeletal Muscle Fibre Types
Slow Twitch (type I) fibres
Fast Twitch (type II) fibres (produce more force, fatigue more rapidly) o Type IIa fibres (intermediate contraction rate) o Type IIb fibres (fastest contraction)
Smooth Muscle
- No striations
- Involuntary (no conscious control)
- Spindle-shaped fibres that are uninucleate
- Contractions are slow and sustained (without muscle becoming fatigued)
- Found main in the walls of hollow visceral organs, bladder and respiratory passages (visceral muscle)
- Arranged in layers
- Moving food through the digestive system, emptying the bladder, changing diameter of blood vessels
o Food is propelled through a process called peristalsis
Cardiac Muscle
- Striations
- Involuntary (under control of ANS)
- Found only in the walls of the heart
- Uninucleate
- Branching cells joined by gap junctions called intercalated discs
o Arrangement allows simultaneous contraction of neighbouring cells - Contracts at a steady rate set by pacemaker cells
Skeletal Muscle Functions
- Maintain posture and body position (postural muscles are continually contracting)
- Stabilise joints
- Generate heat
Types of Muscle Contractions
Isotonic Contractions
o Myofilaments can slide past each other during contractions
o Muscle shortens, and movement occurs
o Example: bending the knee, lifting weights, smiling
o Concentric Isotonic = shortening of the muscle (muscle tension is greater than the resistance)
Isometric Contractions
o Muscle filaments are trying to slide, but the muscle is pitted against an immovable object
o Tension increases, but muscles do not shorten
o Example: pushing your palms together in front of you
Organisation of the Sarcomere
Myofilaments produce banding (striped) pattern
Thick filaments = myosin filaments
Thin filaments = actin filaments
Thick Filaments = Myosin Filaments
o Composed of the protein myosin
o Contain ATPase enzymes to split ATP (ADP and P) to release energy for muscle contractions
o Possess projections known as myosin heads (binding site for ATP)
o Myosin heads are known as cross bridges when they link thick and thin filaments during contraction
Thin Filaments = Actin Filaments
o Composed of the contractile protein actin
o Actin is anchored to the Z-disc
Small spherical molecules bind together to form a fibrous strand
Some action molecules contain an actin binding site myosin head attaches to these sites during contraction to form a cross bridge
Nerve Stimulus and Action Potential
Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract
o Otherwise cannot develop tension/n
- Motor Unit (one motor neuron and all the skeletal muscle cells stimulated by that neuron)
- Motor Neuron – nerve that stimulates skeletal muscle (under voluntary control)
Neuromuscular Junction
- Association site of axon terminal of the motor neuron and sarcolemma of muscle
- Size of motor unit determines degree of control (inverse relationship)
Events at the Neuromuscular Junction
- Axon connects the motor neuron of a cell body with the muscle fibres
- Axon then branches into the axon terminals
o Axon terminals than branch out to separate muscle fibers - Nerve impulse reaches axon terminal -> neurotransmitter released and diffused across the synaptic cleft
o Neurotransmitters (ACh) than attach to receptors on the muscle fiber sarcolemma
o ACh make sarcolemma temporarily permeable to ions - Ion channels open
o Positive sodium ions rapidly enter the fibre
o Positive potassium ions rapidly exit the fibre
Net Effect = Positive Charge in the fibre (more sodium enter than potassium leave) - Change in charge triggers the opening of more channels on the membrane
o Only allow entry of additional sodium ions (temporarily more permeable to sodium)
Generates electrical charge (action potential)
Action potential spreads from one end of the fiber to another - Enzyme acetylcholinesterase breaks down ACh to acetic acid and choline
- Ion channels close and muscle contraction ends
- Muscle fibers return to resting state
o When potassium ions diffuse out of the cell
o Sodium-potassium exchange pump moves ions back to original positions
Neurotransmitter
- Chemical released by nerve upon arrival of nerve impulse in the axon terminal
- Acetylcholine (Ach) is the neurotransmitter that stimulates skeletal muscle
Synaptic Cleft
- Gap between nerve and muscle filled with interstitial fluid
- Although very close, the nerve and muscle do not make contact
Sliding Filament Theory
o Calcium ions bind to regulatory proteins on thin filaments and expose myosin-binding sites
Allows the myosin heads on the thick filaments to attach
o Each cross bridge pivots
Causes the thin filaments to slide toward the center of the sarcomere
o Contraction occurs, and the cell shortens
o During a contraction
Cross bridge attaches and detaches several times
Calcium ions released allow the myosin heads to attach to the actin filaments
o ATP provides the energy for the sliding process (continues as along as calcium ions are present)
o Variation in force and speed (regulated by variation in frequency and amount of action potentials)
Cross Bridge Cycle
- Initiated when calcium ions released from sarcoplasmic reticulum bind to troponin
o Binding causes troponin to change shape - Tropomyosin moves away from the myosin binding sites on actin
o Allows myosin head to bind actin -> form a cross bridge
o Myosin head must be activated before a cross bridge cycle can begin - Cycle will repeat if actin remain exposed
o Through repetitions -> thin myofilaments are pulled towards each other -> sarcomere shortens - Cycle will end when calcium ions are actively transported back into the sarcoplasmic reticulum
o Troponin returns to its original shape
o Allows tropomyosin to cover the myosin binding site on actin
Four Main Stages of Cross Bridge Cycle
Step 1: Cross Bridge Formation
- Activated myosin head binds to actin forms cross bridge
- Inorganic phosphate is released
- Bond between myosin and actin becomes stronger
Step 2: Power Stroke
- ADP is released and activated myosin head pivots
• Slides thin myofilament toward the center of the sarcomere
Step 3: Cross Bridge Detachment
- Another ATP binds to myosin head
• Link between myosin head and actin weakens
• Myosin head detaches
Step 4: Reactivation of Myosin Head
- ATP is hydrolysed to ADP and inorganic phosphate e
• Energy reactivates the myosin head -> returning it to the cocked position
Special Functional Properties of Skeletal Muscles
Special functional properties of skeletal muscles
o Extensibility (ability of muscle cells to be stretched)
o Elasticity (ability to recoil and resume resting length after stretching)
o Irritability / responsiveness (ability to receive and respond to a stimulus)
o Contractility (ability to forcibly shorten when an adequate stimulus is received)
Unique to muscle tissue
Graded Responses
- Increase in voltage -> more motor units contract
o Tension within each contraction increases till point of maximum muscle contraction is reached
All motor units are stimulated
Known as recruitment - Muscles fiber contraction ‘all-or-none’ meaning it will contact to its fullest when stimulated adequately
o If stimulus is not sufficient, muscle will not contract - Within a whole skeletal muscle
o Not all fibers may be stimulated during the same interval - Different combinations of muscle fiber contractions may give differing responses
- Graded responses – different degrees of skeletal muscle shortening
Production of Graded Responses
Graded responses can be produced in two ways:
o By changing the frequency of muscle stimulation
o By changing the number of muscle cells being stimulated at one time
Muscle Response to Increasingly Rapid Stimulation
Muscle Twitch
o Single, brief, jerky contraction
o Not a normal muscle function
- In most types of muscle activity, nerve impulses are delivered at a rapid rate
- As a result, contractions are ‘summed’ (added) together, and one contraction is immediately followed by another
- When stimulations become more frequent, muscle contractions get stronger and smoother
- Fused (complete) tetanus is achieved when the muscle is stimulated so rapidly that no evidence of relaxation is seen (tetanic contraction)
- Contractions are smooth and sustained
