Q.Define a motor unit.

A triad = one T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum; it lets a nerve impulse trigger calcium release throughout the muscle fibre almost instantly.

Each skeletal muscle fibre contains numerous parallel contractile units called myofibrils, which run through the sarcoplasm. The sarcoplasm also contains an extensive membranous network called the sarcoplasmic reticulum (SR), which encircles each myofibril and acts as a store of Ca²⁺ ions.

At the junction of the A-band and I-band of each sarcomere, the sarcoplasmic reticulum forms enlarged sac-like chambers known as terminal cisternae. Running between two such terminal cisternae (belonging to adjacent segments of the SR) is a tubular inward extension of the plasma membrane (sarcolemma) called a transverse tubule or T-tubule.

The structural unit formed by one T-tubule sandwiched between two terminal cisternae, one on either side, is called a triad. When a nerve impulse depolarises the sarcolemma, the depolarisation spreads rapidly along the T-tubule deep into the fibre, and this triggers the terminal cisternae to release stored Ca²⁺ ions into the sarcoplasm almost simultaneously throughout the fibre. This calcium influx initiates the sliding of actin and myosin filaments, i.e., muscle contraction.

The triad system links the muscle fibre's surface excitation to the release of calcium ions from the sarcoplasmic reticulum, coupling nerve stimulation to contraction.

A skeletal muscle fibre's sarcolemma (plasma membrane) forms tubular invaginations called transverse tubules (T-tubules) that run deep into the fibre, close to the junction of the A and I bands of each myofibril.

Running parallel to the myofibrils, and surrounding them, is the sarcoplasmic reticulum (SR), a modified endoplasmic reticulum that stores Ca2+ ions. The SR is enlarged at intervals into sac-like terminal cisternae.

At each such interval, one T-tubule is flanked on both sides by a terminal cisterna of the SR - this three-part structure (cisterna-tubule-cisterna) is called a triad.

Functional significance: When a motor neuron fires, the action potential travels along the sarcolemma and down the T-tubule. This depolarisation is sensed by voltage-sensor proteins in the T-tubule membrane, which mechanically trigger Ca2+-release channels in the adjacent terminal cisternae. Ca2+ floods into the sarcoplasm, binds troponin, and initiates the sliding filament mechanism of contraction. The triad is therefore the structural basis of excitation-contraction coupling.

A triad is the set of three membranous elements in a muscle fibre - one T-tubule between two terminal cisternae of the sarcoplasmic reticulum - that couples the surface impulse to Ca2+ release for contraction.

What it is: In a skeletal muscle fibre, the plasma membrane (sarcolemma) dips inward as fine tubes called transverse tubules (T-tubules). The endoplasmic reticulum of the fibre, called the sarcoplasmic reticulum (SR), stores calcium ions and forms swollen sacs called terminal cisternae.

The triad: At the junction of the A-band and I-band, each T-tubule is closely bordered on both sides by two terminal cisternae of the SR. This grouping of two cisternae + one T-tubule = a triad.

Function: When a nerve impulse reaches the sarcolemma, it spreads inward along the T-tubules. At the triad this signal makes the terminal cisternae release stored Ca2+ into the sarcoplasm. Calcium then binds troponin, exposing the actin binding sites and initiating the sliding of filaments (contraction). Thus the triad system ensures the impulse reaches all myofibrils almost simultaneously.

The triad system is the structural unit (T-tubule + two terminal cisternae) that couples membrane excitation to Ca2+-triggered muscle contraction.

In a skeletal muscle fibre, the sarcolemma (cell membrane) sends tubular invaginations, called transverse (T) tubules, deep into the fibre at regular intervals. The sarcoplasmic reticulum (SR), the muscle's internal Ca2+ store, surrounds each myofibril and expands into sac-like terminal cisternae on either side of a T-tubule.

The combination of one T-tubule sandwiched between two terminal cisternae of the SR is called a triad.

Significance: when a nerve impulse depolarises the sarcolemma, the depolarisation travels rapidly inward along the T-tubules. At the triad, this signal is transmitted to the terminal cisternae, which release stored Ca2+ into the sarcoplasm. This Ca2+ binds troponin, moving tropomyosin off the actin binding sites and initiating cross-bridge formation — i.e., muscle contraction (excitation–contraction coupling).