Lecture 7.1: Muscle Flashcards
What is Myalgia?
Muscle Pain
What is Myasthenia?
Weakness of the muscles
What is the Myocardium?
It is the muscular component of the heart
What is Myopathy?
Any disease of the muscles
What is a Myoclonus?
A sudden spasm of the muscles
Sarcolemma
The outer membrane of a muscle cell
Sarcoplasm
The cytoplasm of a muscle cell
Sarcoplasmic Reticulum
The smooth endoplasmic reticulum of a muscle cell
Types of Muscle Tissues (3)
Skeletal Muscle (Striated)
Cardiac Muscle (Striated)
Smooth Muscle (Non-striated)
How much of total body mass is muscle?
c.40%
What is the function of muscle?
Muscles generate motile forces through contraction to allow movement to support bodily functions
How do muscles work- key aspects?
Assemblies of contractile muscle cells are ‘machines’ that convert chemical energy to power mechanical work
Actin and myosin filaments interact to facilitate the contraction of whole muscle cells in each case
Smooth Muscle [Str, Contra, Nuc, Sha, Ctrl, Mech]
Striations: Absent
Contractions: Slow, sustained or rhythmic
Nuclei: Single central cigar shaped nucleus
Shape: Spindle shaped/Fusiform (up to 10 x 100 µm)
Control: Intrinsic, hormonal, autonomic, local factors (NO)
Mechanism: Contractile filaments rotate dense bodies causing “corkscrewing” in response to Ca2+ release
Cardiac Muscle [Str, Contra, Nuc, Sha, Ctrl, Mech]
Striations: Present
Contractions: Variable, rhythmic
Nuclei: One to two centrally located nuclei per cell
Shape: Branched cylindrical cells (up to 20 x 100 µm) joining at intercalated discs, branching facilitates synchronous contraction
Control: Intrinsic, but with autonomic influence
Mechanism: Contractile myofilaments shorten sarcomeres in response to Ca2+ release from SR
Skeletal Muscle [Str, Contra, Nuc, Sha, Ctrl, Mech]
Striations: Present (discrete A and I bands)
Contractions: Rapid, forceful
Nuclei: Multinucleated, peripheral nuclei
Shape: Large cylindrical fibres (up to 0.1 x 20 cm) arranged into fascicles
Control: Somatic innervation (voluntary)
Mechanism: Contractile myofilaments shorten sarcomeres in response to Ca2+ release from SR
How does Skeletal Muscle Develop? (4)
Mesodermally-derived, multipotent myogenic stem cells give rise to myoblasts
Nearsynchronous fusion of myoblasts forms a primary myotube with a chain of multiple central nuclei
These fuse to form myofibres in which nuclei are gradually displaced to the periphery by newly-synthesised actin and myosin myofilaments
Skeletal muscle retains a stem cell population (satellite cells) that allow for hypertrophy and repair
Red Fibres in Muscle
Muscles that work to resist gravity tend to contain red oxidative fibres, with abundant myoglobin
ATP is generated by aerobic respiration
Muscles that resist gravity, but also require bursts of movement are rich in intermediate (type IIa) fibres
These are capable of rapid contraction, but also appear red as they contain abundant myoglobin
White Fibres in Muscle
Muscles required for darting movements have less myoglobin
Hence fast glycolytic fibres appear white
They depend on anaerobic respiration for short bursts
Organisation of Skeletal Muscle
[look up image]
What does the form of Skeletal Muscle depend on?
Form depends on the orientation of constituent fibres
Skeletal Muscle Forms
Sphincter Muscles
Convergent Muscle
Pennate
Parallel Muscles
Skeletal Muscle: Sphincter Muscles
Organised into circles
Skeletal Muscle: Convergent Muscle
The origin is wider than the point of insertion
Skeletal Muscle: Pennate
In pennate (feather-like) muscles, fibres lie in a different plane to the point of insertion
Skeletal Muscle: Parallel Muscles
Fibres in parallel muscles lie in the same plane as their tendons, and are termed fusiform if they are wider at the belly
Skeletal Muscle Functions (4)
Movement
Posture
Stability of joints (e.g. muscles of the rotator cuff of the shoulder)
Thermoregulation: 40% of body mass and generates considerable heat
Myotendinous Junctions
These are the points of force transmission from myofilaments to tendons
Tendon fibrils interdigitate with folds in the sarcolemma, and are continuous with the connective tissue layers of muscle (e.g. endomysium)
These are the weakest part of the muscle/ tendon complex and are susceptible to tears
Intrinsic Muscles of the Tongue
Those within the tongue
Originate in fibrous connective tissue rather than bone
They allow the tongue to change shape, but not position
Extrinsic Muscles of the Tongue
The extrinsic muscles protrude, retract and move the tongue from side to side
These muscle originate from bone (mandible or hyoid)
What is the smallest functional unit of skeletal muscle?
A sarcomere
Skeletal Muscle Contraction: Neuromuscular Junction
A motor nerve impulse (action potential) causes the release of the neurotransmitter acetylcholine (ACh) at axon terminals supplying muscle fibres at synaptic cleft
Activation of nicotinic ACh receptors on muscle fibre membranes opens ligand-gated Na+ channels allowing an influx of Na+ in the sarcolemma
APs are propagated deep into muscle fibres through invaginations in the
sarcolemma (called T-tubules) as voltage-gated Na+ channels open
The activation of a single motor neurone leads to weak, but distributed contraction
The activation of more motor neurons will activate more muscle fibres, increasing the force of contraction
Raised intracellular Ca2+ stimulates skeletal muscle contraction
All things at the neuromuscular junction occur
T-tubule depolarisation activates ryanodine receptors in the membrane of the SR, releasing stored Ca2+ into the cytoplasm
Ca2+ stimulates muscle contraction by binding to troponin subunit C (TnC)
This allows myosin binding sites to be revealed
Ca2+ is returned to the SR via ATPase pumps
Sliding Filament Mechanism of Contraction
Upon stimulation, thin (actin) filaments are pulled towards the M-line
This shortens the sarcomere
When the impulse to contract ceases, a titin “spring” allows sarcomeres to passively return to their original size
Sarcomere Structure
[look up image]
What is contraction the result of?
Contraction results from the interaction of thick and thin myofilaments
Thick Myofilaments
Myosin is the main component of thick filaments:
• Each myosin molecule is shaped like two golf clubs twisted together
• Myosin tails point toward the M line in centre of the sarcomere
• Myosin heads project outward from shaft in spiralling fashion
• Myosin hydrolyses ATP to provide energy to adopt a “cocked” position
Thin Myofilaments
Actin is main component of thin filaments:
• Globular actin (G actin) polymerises to form helical actin filaments
• Each actin subunit has a binding site for myosin heads, but these are hidden by tropomyosin
• Ca2+ binding to troponin C (TnC) in thin filaments causes a conformational shift in tropomyosin, revealing a myosin binding site
Skeletal Muscle Contraction (5)
When a nerve impulse stimulates a myofibre to contract, Ca2+ released from the SR binds to troponin C, promoting a conformational shift in tropomyosin
This reveals a myosin binding site on actin
The myosin head hydrolyses ATP to adopt a “cocked” configuration
When bound to actin, the head flexes, pulling actin towards the M-line
ADP and Pi are then released, and the cycle begins again
What in skeletal muscle form a triad?
T-tubules
Sarcoplasmic Reticulum
In skeletal muscle, triads run along the junction between the A- and I-bands
Mechanism of Cardiac Muscle Contraction
The mechanism of cardiac muscle contraction is similar to skeletal muscle,
Although APs are generated by pacemaker cells located within the heart, such as the sino-atrial (SA) node
Heart rate and force of contraction can be modulated by the ANS
What do the SR and T-tubules form in cardiac muscle?
A diad
Where do T-tubules lie in cardiac muscle in comparison to skeletal muscle?
In contrast to skeletal muscle, the T tubules of cardiac muscle lie in register with the Z bands and not with the A-I band junction
Purkinje Fibres: What are they? Structure? Function?
Purkinje fibres are specialised cardiac myocytes that lie adjacent to the endocardium
They are large cells with abundant glycogen, sparse myofilaments, and extensive gap junctions
Purkinje fibres conduct action potentials more rapidly than cardiac muscle fibres, allowing the ventricles to contract in a synchronous manner
Smooth Muscle: Locations
Found in the walls of passageways and cavities
Smooth Muscle: Function
Smooth muscle supports vascular structures, as well as the gut, respiratory tract and genitourinary system
Smooth Muscle: Mass Structure
Smooth muscle forms sheets, bundles or layers containing thousands of cells, supported by endomysium
Not striated: no sarcomeres or T-tubules
Control of Smooth Muscle Contraction
Smooth muscle is not under voluntary control, and responds to stimuli in form of innervation, hormones, drugs, or local concentrations of blood gases
Smooth muscle from different sites may respond differently to the same stimulus
It requires less ATP, but still relies on actin-myosin interactions
Mechanism of Smooth Muscle Contraction
Thick (myosin) and thin (actin and tropomyosin) filaments are arranged diagonally within smooth muscle cells, and are anchored at dense bodies
Filaments spiral down the long axis like stripes on a barbers pole, so the smooth muscle cell twists as it contracts
Ca2+ release stimulates contraction via sliding filaments
Smooth muscle lacks troponin, and myosin is activated by phosphorylation rather than by hydrolysing ATP directly
Time taken for smooth muscle to contract as compared to skeletal muscle
Smooth muscle takes 30 times longer to contract than skeletal muscle, but can maintain tension for long periods (using 1% of the energy)
Types of Modified Smooth Muscle Cells (3)
Myoepithelial Cells
Myofibroblasts
Myoid Cells
Myoepithelial Cells
Stellate cells forming a basketwork around the secretory units of some exocrine glands
Contraction assists secretion into secretory ducts
Myoepithelial cells in the ocular iris contract to dilate the pupil
Myofibroblasts
Produce collagenous matrix, but also contract at sites of wound healing
Also important in tooth eruption
Myoid Cells
Surround seminiferous tubules
Their contraction helps move immature sperm towards the efferent ducts (which are surrounded by smooth muscle)
