Bi 103 Notes: Chapter 35: Muscular/Skeletal System

 

I. Function of Muscle

            1) Produce movement

            2) Maintain posture

            3) Support soft tissue (e.g. abdominal wall)

            4) Guard entrance/Exit of body (lips/anus)

            5) Maintain body temperature

II. Types of muscle:

            A) Skeletal: Function = skeletal movement

                        a. Striated

                        b. multinucleate

                        c. variable contraction speed, voluntary control

            B) Cardiac: Function = pump blood

                        a. Irregular striations

                        b. 1 + nuclei per cell

                        c. Intermediate contraction speed: Generally involuntary control

           C) Smooth;  Function = move substances through hollow tubes

            a. not striated

            b. uninucleate

            c. slow contraction speed: Generally involuntary control

III. Skeletal Muscle: Human body has > 700 skeletal muscles

            Background:    Muscle structure and function closely linked, whole muscle consists of

                                                1) muscle fiber bundles

                                                2) blood vessels and nerves

                                                3) connective tissue wrap

            A. Microanatomy of muscle

                        1. Muscle fiber = muscle cell, each cell contains

                                    a) T-tubules

                                    b) Sarcoplasmic Reticulum : specialized ER

                                    c) Myofibrils: Structures with contractile fibers

                                                1. Actin: Thin filament

                                                2. Myosin: Thick filament

                                                3. Sarcomere = repeating units of actin and myosin.

                        2. Interactions between actin and myosin cause muscle contraction

                                    a) myosin heads on myosin filaments do the “pulling”

                                                1. form crossbridges binding myosin head to actin filament

                                                2. Sliding filament theory

            B. Muscle Contraction Events

                        1. Ach released from synaptic terminal on motor neuron

                        2. Ach binds to receptors on muscle fibers, generates AP in cell membrane

                        3. AP conducted along muscle fiber plasma membrane

                        4. AP descends into muscle fiber via T-tubules

                        5. AP triggers release of Ca++ from sarcoplasmic reticulum

                        6. Ca++ initiates actin-myosin (cross-bridging),  muscle contracts.

            C. Control of skeletal muscle contractions

                        1) Length of Muscle contraction determined by AP frequency

                                    a. low = twitch,  high = tetanus

                        2) Strength of Muscle contraction determined by # fibers stimulated

                                    a. motor unit = motor neuron + muscle fibers it innervates

                                                1. motor unit size dictates control