Cell Bio- Lecture #5 (Muscle Tissues)

Epimysium

Connective tissue layer encasing an entire skeletal muscle (the organ)

Muscle fibers

Individual skeletal muscle cells

Fascicles

Groups/bundles of muscle fibers

Perimysium

Dense irregular connective tissue layer surrounding each muscle fascicle

Endomysium

Areolar loose connective tissue separating individual muscle fibers within a fascicle

Fascia

Collective term for the layers of connective tissue surrounding muscles and weaving between fascicles/fibers

Tendons

Formed where the fascia comes together at the ends of the muscle

Striations

Regular pattern in skeletal muscle created by A-bands and I-bands of sarcomeres

Myofibrils

Structures muscle fibers are packed with, running in parallel down the length of the cell

Thick filaments

Composed of two bundles of myosin motor proteins facing away from each other

Thin filaments

Formed of filamentous F-actin

Myosin

Motor protein composing thick filaments

F-actin

Filamentous actin composing thin filaments

Sarcomeres

Units myofibrils are divided into; extend from one Z-line to the next, with an M-line in the middle

Z-line

Boundary between sarcomeres; anchors I-band (thin filaments)

M-line

Middle of the sarcomere; anchors H-zone (thick filaments)

A-band

Full length of the thick filaments; darkest where thick and thin filaments overlap

I-band

Region with only thin filaments (anchored to Z-line)

H-zone

Region with only thick filaments (anchored to M-line)

Cross-bridges

Formed when myosin head groups attach thick filaments to actin of thin filaments

α-actinin

Attaches thin filaments (actin) to the Z-line disc

Myomesin

Attaches thick filaments (myosins) to each other at the M-line

Titin

Elastic protein connecting thick filaments to the Z-line disc; restores sarcomere to resting length, prevents excessive stretch

Muscular dystrophies

Disorders with muscular weakness and degeneration, caused by genetic defects in muscle proteins (e.g., dystrophin)

Powerstroke

Created when myosin pulls on actin filaments during contraction, increasing tension

Troponin C

Binds calcium ions, triggering a conformational change in the troponin-tropomyosin complex

Troponin-tropomyosin complex

Undergoes conformational change upon calcium binding, revealing myosin binding sites on actin

Tropomyosin

Obscures myosin binding sites on actin at rest

Rigor mortis

Condition of sustained muscular contraction after death, caused by loss of calcium pumping and ATP depletion

Excitation-contraction coupling

Process by which neural excitation leads to muscle fiber contraction

All-or-none principle

Muscle fibers are either fully contracted or relaxed, nothing in between

Motor unit

Composed of one α motor neuron and all the muscle fibers it innervates

α motor neurons

Responsible for exciting the sarcolemma, initiating the excitation-contraction sequence

Acetylcholine

Neurotransmitter released by α motor neurons, triggering an action potential in the muscle fiber

Action potential (AP)

Electrical excitation triggered in the muscle fiber by acetylcholine

Safety factor

Excess acetylcholine release beyond what's needed to trigger an action potential

Neuromuscular junction (NMJ)

Where motor neuron axons meet muscle fibers; has specific structural properties influencing function

T tubules

Membranous tunnels that encircle myofibrils and spread depolarization throughout the muscle fiber

Sarcoplasmic reticulum (SR)

Smooth ER that ensheaths myofibrils, associated with T tubules

Terminal cisternae

Swellings of the SR on either side of T tubules

Triad

Structure formed together by a T tubule and its two flanking terminal cisternae

SERCA (sarco/endoplasmic reticulum calcium ATPase)

Pumps that actively transport Ca2+ back into the SR, clearing the myoplasm and resetting the sarcomere

Myasthenia gravis

Autoimmune disorder with muscle weakness worsening with fatigue, caused by autoantibodies disrupting acetylcholine receptors at the NMJ

Slow units

Control thinner fibers creating less force, resist fatigue, have better blood supply and more myoglobin

Fast units

Control large diameter fibers that contract quickly/strongly, fatigue rapidly, have larger glycogen reserves

'White' muscle

Associated with fast units, specialized for rapid/strong contractions

'Red' muscle

Associated with slow units, specialized for long, low-intensity usage

'Pink' muscle

Most human muscles, having a varied mix of fiber types

Myoglobin

Protein providing oxygen reserves, plentiful in slow-unit muscle fibers

Hypertrophy

Increased diameter of muscle fibers, driven by training/repeated use

Atrophy

Decrease in size, tone, and contractile force from sustained lack of use

Muscle tone

Describes the basal activity in motor units

Intercalated discs

Specialized junctions where cardiac muscle cells connect to neighboring cardiac muscle cells