Muscle Physiology

ganglion

a structure containing a number of nerve cell bodies, typically linked by synapses, and often forming a swelling on a nerve fiber.

striations

• Formed by repeating arrangements of thin filaments

• Thick filament portions bind and pull thin filament past to shorten

myofibril

bundles of protein filaments

pennate muscle

skeletal muscles with fibers that attach obliquely (at an angle) to a central tendon

• more individual fibers of pennate muscles = greater total cellular cross sectional area and greater potential for force generation

• fusiform muscle fibers are oriented along the long axis of the muscle, while pennate fibers converge at angles

sarcomere

a structural unit of a myofibril in striated muscle, consisting of a dark band and the nearer half of each adjacent pale band.

how do regulatory proteins inhibit contractile protein interactions?

• Myosin binding sites on actin are blocked by tropomyosin in resting fibers

• Binding sites are exposed (permitting myosin head biinding and cross-bridge formation) after Ca2+ binds to troponin

role of structural proteins

orient the contractile proteins and link sarcomeres to extracellular structures

titin

huge protein that steadies thick filaments and contributes to elastic recoil of sarcomeres collowing contraction

Nebulin

Binds to actin and acts as a ruler, helping determine overall thin filament length

Myosin crossbridges

Myosin crossbridges are the projections (heads) of myosin molecules that physically bind to actin filaments during muscle contraction.

• provides force that pushes the actin filament during contraction

costameres

A costamere is a protein complex that anchors muscle fibers’ sarcomeres to the cell membrane, transmitting contractile force to the extracellular matrix.

• costamere proteins link Z-disks to the sarcolemma and ECM

• convey internsal tension generated by sliding filaments to tendons and bones

Z-disc

A Z-disc is the structural boundary of a sarcomere where actin filaments are anchored, maintaining alignment and transmitting force during muscle contraction.

what does a motor unit consist of

consists of an alpha motor neuron and all of the fibers it innervates

• the dispersion of fibers in motor units assures uniform generation of tension across the muscle, regardless of the number and frequency of motor units activated

neuromuscular junction (NMJ)

The neuromuscular junction is the specialized synapse where a motor neuron communicates with a skeletal muscle fiber, releasing acetylcholine to trigger muscle contraction.

• this is where muscle fiber stimulation occurs

mechanotransduction

the biological process by which cells convert mechanical stimuli—such as pressure, shear stress, or stretching—into chemical signals

• depends on the regulation of calcium

motor end plate

region of the sarcolemma where the neurons synapse

T-tubule

A T-tubule is an inward extension of the muscle cell membrane that rapidly conducts action potentials deep into the muscle fiber to coordinate contraction.

• allow deep penetration of the depolarizing stimulus to the cell’s interior, where many contractile elements are found

what does membrane action potential activate?

Activates L-type calcium channels (DHPR) that link to the Ryanodine receptor (RyR) calcium channels

• as t-tubules depolarize, dihydropyridine receptors change shape, ryanodine receptors are activated, and Ca2+ floods out of the SR into the sarcoplasm

• calcium binds to troponin, moving topomyosin away from myosin head binding sites

• crossbridges form, inducing filament sliding

what happens after the sarcolemma and T-tubules repolarize?

• the two receptors return to their resting states

• Calcium release stops, inward pumping via SERCA decreases sarcoplasmic levels, causing Ca2+ release by troponin

• crossbridges separate

what are contraction and relaxation determined by?

• determined by rates of cross-bridge turnover and Ca2+ removal rates

Fast glycolytic units (FGs)

contract very fast, generate high force, rely mainly on anaerobic glycolysis, and fatigue quickly

Fast oxidative glycolytic units (FOGs)

contract fast, produce moderate force, use both aerobic and anaerobic metabolism, and have moderate fatigue resistance (e.g., sustained running).

Slow Oxidative Units (SOs)

fibers contract slowly, generate low force, rely on aerobic metabolism, and are highly fatigue resistant

Hill Model

The Hill muscle model explains how muscle generates force using three mechanical elements

Contractile element (CE)

• Represents the active force produced by actin–myosin crossbridge cycling.

• Force depends on neural activation, muscle length, and shortening velocity.

series elastic element (SE)

• Represents elastic components in series with the contractile element, mainly tendons and crossbridges.

• Stores and releases elastic energy and transmits force from muscle to bone.

Parallel Elastic element (PE)

• Represents passive elastic tissues within the muscle (e.g., connective tissue, titin).

• Generates passive tension when the muscle is stretched.