Physiology · MU Pleven

Anatomy of Skeletal Muscle

Covers the anatomy of skeletal muscle from fascicles and fibres down to myofilaments and sarcomeres, then the role of the sarcoplasmic reticulum and calcium in switching contraction on and off. It explains the sliding filament theory, the role of troponin and tropomyosin, relaxation, and fatigue. Also outlines the neuromuscular junction, muscle action potential, motor units and excitation-contraction coupling.

Inside: 7 sections · 14 labelled pictures · 25 flashcards · 10 min

Topic 8 of 17 in the MU Pleven syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 7 sections, about 1,439 words (10 min of reading), with 25 flashcards.

In this note

  1. TOPIC 8. PHYSIOLOGIC ANATOMY OF SKELETAL MUSCLE. GENERAL MECHANISM AND MOLECULAR MECHANISM OF MUSCLE CONTRACTION. NEUROMUSCULAR JUNCTION. MUSCLE ACTION POTENTIAL. MOTOR UNIT. EXCITATION – CONTRACTION COUPLING.
  2. PHYSIOLOGIC ANATOMY OF SKELETAL MUSCLE
  3. GENERAL MECHANISM AND MOLECULAR MECHANISM OF MUSCLE CONTRACTION
  4. NEUROMUSCULAR JUNCTION
  5. MUSCLE ACTION POTENTIAL
  6. MOTOR UNIT
  7. EXCITATION – CONTRACTION COUPLING

PHYSIOLOGIC ANATOMY OF SKELETAL MUSCLE

  • Attached to bones & moves skeleton
  • Also, called striated muscle (because of its appearance under the microscope)
  • Voluntary muscle

CHARACTERISTICS

  • EXCITABILITY → responds to stimuli (e.g. nervous impulses)
  • CONTRACTILITY → able to shorten in length
  • EXTENSIBILITY → stretches when pulled
  • ELASTICITY → tends to return to original shape & length after contraction or extension

FUNCTIONS

  • Motion
  • Maintenance of posture
  • Heat production

STRUCTURE

Consists of numerous subunits or bundles called FASCICLES → surrounded by connective tissue (PERIMYSIUM). Each fascicle is composed of numerous muscle FIBERS (muscle cells fibres). Muscle cells, consist of many fibrils (MYOFIBRILS) → made up of long proteins (MYOFILAMENTS: thick or thin)

MUSCLE FIBRE of MUSCLE CELL consist of:

  • Multinucleated long cylindrical cell
  • Sarcolemma
  • Sarcotulular system - transverse tubules; sarcoplasmic reticulum
  • Myofibrils

The cell membrane of a muscle cell is called the SARCOLEM → maintains a membrane potential.

  • Impulses travel along muscle cell membranes just as they do along nerve cell membranes.
  • The 'function' of impulses in muscle cells is to cause contraction.

The SARCOLEMMA has a unique feature: it has holes in it.

  • These "holes" lead into tubes called TRANSVERSE TUBULES (or T-TUBULES for short).
  • These tubules pass down into the muscle cell and go around the MYOFIBRILS.

However, these tubules DO NOT open into the interior of the muscle cell; they pass completely through and open somewhere else on the sarcolemma.

The function of T-TUBULES is to conduct impulses from the surface of the cell (SARCOLEMMA) down into the cell and, specifically, to another structure in the cell called the SARCOPLASMIC RETICULUM.

The SARCOPLASMIC RETICULUM (SR) is a bit like the endoplasmic reticulum of other cells, e.g., it's hollow.

  • Its primary function is to STORE CALCIUM IONS.
  • It is very abundant in skeletal muscle cells and is closely associated with the MYOFIBRILS (and, therefore, the MYOFILAMENTS).
  • The membrane of the SR is well equipped to handle calcium: there are "pumps" (active transport) for calcium so that calcium is constantly being "pumped" into the SR from the cytoplasm of the muscle cell (called the SARCOPLASM).

IN A RELAXED MUSCLE → there is a very high concentration of calcium in the SR and a very low concentration in the sarcoplasm (and, therefore, among the myofibrils & myofilaments).

In addition, the membrane has special openings, or "gates", for calcium.

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