The structure of a muscle fiber Sarcolemma T-tubule Cisternae Sarcoplasmic reticulum Lecture 4: Skeletal Muscle

Structure of a Myofibril The lower figure shows the sequence of dark and light bands. The upper figure shows the typical configuration of actin and myosin molecules within a myofibril. Z-line Actin Myosin A-bandI-band A-band Cross bridges

Structure of the Thin Filament (Actin) Note long tropomyosin molecules in parallel with the actin strands. Troponin attaches to tropomyosin at regular intervals. Actin TroponinTropomyosin

Neuromuscular Synapse A presynaptic nerve action potential induces movement of vesicles with acetylcholine (ACh) to the presynaptic membrane, their fusion, and release of ACh into the cleft. ACh diffuses to the postsynaptic muscle membrane, depolarizes it, and induces an action potential. Presynaptic membrane Synaptic vesicles ACh Nerve action potential Motor end plate AChesterase

Neuromuscular Synapse  Neurotransmitter: acetylcholine (ACh)  Always excitatory  Obligatory  No multiple innervation  AChesterase quickly destroys ACh in the synaptic cleft

Muscle AP Miniature excitatory postsynaptic potentials (end plate potentials, MEPPs) spontaneously occur on the postsynaptic muscle membrane. A presynaptic nerve action potential always reaches the depolarization threshold and induces a muscle action potential. Nerve action potential V Time Spontaneous MEPPs

Direct Effects of the Muscle AP Muscle action potential travels along the sarcolemma, enters T-tubules, and leads to a release of Ca ++ ions from the sarcoplasmic reticulum. Sarcolemma Sarcoplasmic reticulum Ca ++ Ca ++

Sliding Filament Theory Ca ++ ions remove tropomyosin and free a site for myosin to bind to troponin (this process uses the energy from ATP). A ratchet motion occurs, moving the filaments with respect to each other. Ca ++ Troponin Cross bridge ATP Myosin Actin Tropomyosin

Muscle Twitch A typical twitch contraction of a muscle in response to a single stimulus. Time Force Latent period 100 ms

Temporal Summation of Muscle Twitches Two action potentials come at a short interval and induce two twitch contractions. Their mechanical effects are superimposed, leading to a higher level of muscle force. Force Time

Tetanus A sequence of action potentials may lead to a tetanus (a sustained contraction). At a high frequency of action potentials, individual contractions may fuse, leading to a smooth tetanus. Force Time Smooth tetanus Action potentials

A Bit of Basic Mechanics  Stiffness: a property of a spring, a structure that deforms and accumulates potential energy under the influence of an external force F = −k*∆x  Damping (“viscosity”): a property of an object to generate force against a velocity vector F = −b*V  Inertia: a coefficient of proportionality between force and acceleration F = m*a

A Simple Hill-Type Muscle Model A simple mechanical model of a muscle. It contains a force generator (F), a viscous element (B), and two elastic elements: a parallel spring (K 1 ) and a series spring (K 2 ). Force F B K2K2 K1K1

Spring Properties of an Isolated Muscle Force-length curves measured in a muscle for different levels of external stimulation (S 1, S 2, and S 3 ). The muscle behaves like a nonlinear spring. Changing the strength of the stimulation modifies the zero length of the spring. Force Length S1S1 S2S2 S3S3

Force-Velocity Muscle Properties A typical force-velocity curve for a whole muscle. According to tradition, the Y- axis represents the velocity of muscle shortening. The muscle produces higher forces when it is lengthening (negative velocity) than when it is shortening (positive velocity). Compare this figure with the Hill equation. Force Velocity F 0 0

External Loads A muscle always works against a load. Three types of loads are illustrated: an isometric load prevents changes in “muscle plus tendon” length; an isotonic load does not change; and an elastic load acts like a spring. A typical muscle characteristic (the thin curve) is shown for comparison. Length Load Isotonic Isometric Elastic Muscle characteristic

Regimes of Muscle Contraction  Concentric: A muscle develops force while shortening.  Eccentric: A muscle develops force while lengthening.  Isometric: The “muscle plus tendon” length does not change.  Isotonic: The apparent external load does not change.  Elastic: The load is a spring. External loads: