SKELETAL MUSCLE

• The 3 types of muscle (= muscle tissue):

• Smooth (non-striated; involuntary)
• Skeletal (striated; voluntary)
• Cardiac (striated; involuntary!)

• The 4 properties of muscle:

• Excitability - the ability to respond to stimulation.
• Contractility - the ability to shorten.
• Extensibility - the ability to continue to contract over a range of resting lengths.
• Elasticity - the ability of a muscle to rebound toward its original length after a contraction.

• The 5 functions of skeletal muscle:
• Produce skeletal movement.
• Maintain posture.
• Support and protect viscera.
• Provide sphincteric control of orifices.
• Maintain body temperature (heat loss during contraction).



• The 4 connective tissues of muscle:
• Endomysium - dense irregular CT.
• Perimysium - dense irregular CT.
• Epimysium - dense irregular CT = deep fascia.
• Tendon - dense regular CT.

(5.) Aponeurosis = flattened tendon.




• 3 important Nerve-Muscle relations:
• neuromuscular junction - synaptic terminal between a neuron and a muscle cell (synaptic knob + synaptic cleft + motor end plate).
• motor end plate - the specialized segment of the sarcolemma at the neuromuscular junction that contains ACh receptor sites.
• motor unit - all of the muscle cells controlled by a single motor neuron.



• 3 general features of skeletal muscle cells (= fibers):
• They arise by the fusion of myoblasts and thus are multinucleated; the nuclei are eccentric (just beneath the sarcolemma); satellite cells are unfused myoblasts ("back-up") cells that may aid in muscle repair.
• The cell membrane = the sarcolemma.
• Cytoplasm = sarcoplasm.



• 3 constituents of the sarcoplasm:
• Mitochondria.
• Myofibrils.
• Internal conducting system (stimulates and coordinates muscle contractions).



• 2 components of the internal conducting system:
• Transverse tubules - invaginations of the sarcolemma which surround myofibrils.
• Sarcoplasmic reticulum - another membrane complex similar to smooth endoplasmic reticulum; on either side of the circular T-tubes, the SR is expanded into chambers called terminal cristae (the combination of a pair of terminal cristae plus a T-tubule = triad; although the membranes of the triad are in close contact and tightly bound together, there is no direct connection between them).



• 2 types of myofilaments:
• Thin filaments (actin chains of twisted strands and globular clusters with active sites).
• Thick filaments (myosin chains with a core of titin; filaments have a tail, 2 hinges and a head).



• 8 fun facts to know about sarcomeres:
• They are the smallest functional units of the muscle cell.
• Each myofibril contains approximately 10,000.
• The "base plate" for the myosin filament = the M-line.
• The "base plate" for the actin filament = the Z-line; the Z-lines delineate the ends of the sarcomere.
• Where thin and thick filaments overlap is known as the Zone of Overlap.
• The I-band = adjacent zones of thin filaments (with an Z-line in the middle).
• The A-band = adjacent overlapping zones (with an M-line in the middle).
• The H-zone is the non-overlapping area just around the M-line.



• The mystery of muscle contraction explained in 6 easy steps:
• ACh is released at the neuromuscular junction which binds to receptors on the sarcolemma....
• ...which sets up an action potential across its surface and within the T-tubules ...
• ...which (somehow?) causes calcium ions to be released that have been stored in the terminal cisterns of the SR...
• ...which bind to the actin filaments (somehow?) exposing the active sites...
• ...for the myosin filaments to attach forming cross-bridges which (somehow?) pull the actin filaments toward the M-line; this is known as the Sliding Filament Theory.
• Contraction ceases when ACh-esterase breaks down ACh (terminating the action potential) and calcium ions are reabsorbed by the SR.






• 3 types of skeletal muscle cells:
• Fast fibers - white and large (anerobic).
• Slow fibers (3 x slower) - red (myoglobin) and smaller (by ½).
• Intermediate fibers - faster than slow, but slower than fast.

Notes: A. most muscles are mixed.

• all fibers within 1 motor unit are of the same type.
• there are no slow fibers in muscles of the eye and hand.
• the percentage of fast vs. slow fibers/muscle is genetically determined.
• fast fibers may be converted to intermediate fibers through training.



• 2 more types of skeletal muscle cells:
• Extrafusal = regular muscle fibers; innervated by alpha motor neurons (SE).
• Intrafusal = muscle spindles = stretch receptors; innervated by gamma motor neurons (SE); two types (both with non-contractile, multinucleated central regions):
• nuclear bag with annulospiral sensory endings (SA).
• nuclear chain with flower-spray endings (SA).



• 4 things to know about a simple stretch reflex (e.g. knee jerk reflex):
• Tapping the patella tendon stretches the muscle spindle stimulating its sensory endings.
• SA (proprioceptive) impulses travel through the dorsal root synapsing with alpha motor neurons in the ventral horn.
• SE impulses travel to the quadriceps muscle (via the femoral nerve) causing contraction.
• Gamma motor neurons are needed to keep the muscle spindles under proper tension while the muscles are relaxed thus maintaining proper tone.