Lion Den logo


© 1988-2016
Kevin Patton
Lion Den ®
ALL rights reserved


| More

Hot Tips

Free email newsletter for A&P students.

Subscribe to RSS headline updates from:
Powered by FeedBurner


 Website security

Lion Track icon Lion Den » A&P » AP1 Lec » Outlines » Muscle

Learning Outline

Muscular System

A&P 1

In the lecture course we will focus on the general anatomy and physiology of muscle organs. In the lab course we will focus on the names, locations and actions of specific muscles. Therefore, in the lecture course I will not quiz you on specific muscles, except as they are used as examples for the more general principles.

For an optional overview of muscles and their actions click here.

Muscular System Overview

Includes organs comprised mainly of skeletal muscle tissue and connective tissue

General functions


Heat production


Body shape

Vesalius's muscular body



Human Musculature. This woodcut from Vesalius's medical textbook shows the muscular system. A quick glance reveals the importance of the skeletal muscle organs in shaping the contours of the human form.

Click the image to enlarge it



Skeletal muscle organs


Made up of muscle fibers, nerves, blood vessels and connective tissues

Muscle cells are usually called muscle fibers

Functional characteristics of muscle fibers slide

General principles of functional anatomy of the muscle organ

Skeletal muscle organs contract (not expand) with force

Skeletal muscle organs act in teams slide

Skeletal muscles attach to the skeleton

Fibrous connective tissue wraps and compartmentalizes muscle fibers slide

Hernia — "rupture" or protrusion (sticking out) through a wall

lion trackRequired: read article from A&P Connect called Hernias in Chapter 11 Resources at Evolve website

Neck and shoulder muscles

Neck and shoulder muscles. From Leonard da Vinci's notebooks. This sketch from Leonardo da Vinci's notebook shows the structure of the skeletal muscle organs of the neck and shoulder.

(Click the image to enlarge it)

Notice the "mirror writing" that Leonardo used in his notes. I don't recommend that as a study tip.


Muscle fiber structure

Basic structures

Sarcolemma = plasma membrane

Sarcoplasm = intracellular material

Sarcoplasmic reticulum (SR) = special form of smooth ER with Ca++ ion pumps that allow SR to store Ca++


Cylindrical units of the cytoskeleton made up of microfilaments (= myofilaments)

Sarcomeres — repeating, overlapping pattern of thin and thick [myo]filaments in the myofibril

human skeletal muscle micrograph Muscle micrograph. The striated pattern of the human sarcomeres is clearly visible in this specimen (click to enlarge) from Blue Histology

Thick filaments

Thin filaments

Muscle fiber contraction

A love story?

Myosin and actin want each other (can't get together because of tropomyosin and troponin)

Troponin wants Ca++ but Ca++ is unavailable

Scenes of the unfolding love story

sliding filament animation
Sliding Filament Model animation
From used by permission

Myosin and ATP

ATP is the energy source slide

Power stroke animation
Power stroke of myosin head.
From used by permission

sequence of events in muscle fiber contraction
From used by permis

Summary of the story

  1. Excitation

  2. Contraction

  3. Relaxation

Excitation-contraction coupling refers to the fact that these two processes (excitation and contraction) are linked [coupled]

lion trackNot required — See p. 166 in Survival Guide For Anatomy And Physiology: Tips, Techniques And Shortcuts for a summary of the "muscle love story"

For an interesting alternate hypothesis about how muscle fibers work, I highly recommend the book Cells, Gels and the Engines of Life by Gerald Pollack


Energy for contraction

Energy sources

ATP — from aerobic (slow) and anaerobic (fast) respiration slide

Creatine phosphate (CP) — "backup battery" for quick recharging of ATP

More on this (a little) later

Fiber types


Specific types

quick muscle action
Fast fibers are useful for brief powerful contractions.
walking animation
Slow fibers are useful for endurance.

Muscle organs are mix of types slide

Cellular Respiration


The key here is focusing on "what's really happening" without getting bogged down in the details of the biochemistry

lion trackYou should also print out the Getting Energy review outline and use it to help you understand these concepts.

Basic definitions

Metabolism — body chemistry

Respiration — literally "re-breathing" and refers to bringing in oxygen (O2) and releasing carbon dioxide (CO2) slide

Coenzyme — coenzymes "help" enzymes

Summary of chemical changes

C6H12O6 + O2chemical yield arrow H2O + CO2 + energy (in ATP)
[this equation is not balanced]

Step 1 — Glycolysis

Breaks glucose (C6) into two pyruvic acids (2 C3) and releases energy

Enough energy is released for 2 ATP molecules


Occurs in cytosol outside of mitochondria

Step 2 — Transition reaction

If pyruvic acid is to continue, it enters the mitochondrion and one carbon is removed —forming Acetyl (C2)

Coenzyme A (CoA) temporarily binds to acetyl and escorts it into the citric acid cycle

This begins the aerobic process (although O2 will not actually be used until later, the molecule will not enter this pathway until and unless O2 is there at the end of the line)

Step 3 — Citric Acid Cycle

Citric Acid Cycle also known as Krebs Cycle or TCA Cycle video

Acetyl rides this "ferris wheel" where it is broken apart, releasing its energy

The Cs and Os simply fall away, forming the waste CO2

Most of the energy released in the form of energized electrons from H (the H+ proton also tags along for the trip)

Hans Adolf KrebsHans Adolph Krebs shared the 1953 Nobel prize in Physiology or Medicine for his discovery of the citric acid cycle.  At the prize ceremony, the presenter stated, 

"It was Krebs who discovered how these individual reactions are linked to each other in a cyclic process. He brought us a clear understanding of the essential principle of how the released energy is used for the building up processes which take place within the cell."

(The prize was shared with Fritz Albert Lipmann for his discovery of the role of Coenzyme A.)

Ferris wheel animationThe ferris wheel is one model for how the citric acid cycle (Krebs cycle) works. 

Coenzyme A escorts a molecule of acetyl (obtained from glycolysis) into the cycle, just as the ride operator escorts riders onto the ferris wheel.  Instead of getting into a car, the acetyl molecule is transferred to a "carrier" molecule to form citric acid.  As this combined molecule completes the cycle, the "passenger" is broken apart and pieces "fly off" and away from the cycle.  The "car" returns empty, ready to pick up another "passenger." Some of the "flying pieces" are high-energy electrons that are escorted to the ETS by coenzymes and their energy eventually transferred to ATP.

Step 4 — Electron Transport System (ETS)

Also known as Electron Transport Chain (ETC)

High-energy electrons (and H+) are dropped off at molecules in the cristae

The electrons are shuttled from molecule to molecule losing their energy as they go (passed like a hot potato, eventually "cooling off")

The energy lost by electrons is used to pump the protons (H+) into the intermembrane space, like water behind a dam

As the protons flow back through the dam (down their concentration gradient), this powers the "phosphorylation of" or "adding phosphate to" ATP (oxidative phosphorylation)

The electrons unite with their protons, forming H2 which is explosive

Hindenburg zeppelin burningThe Hindenburg zeppelin, a blimp filled with hydrogen gas (H2), exploded as it attempted to land at Lakehurst NJ near New York City in May, 1937.

Although the explosion is now thought to have been caused by lightning that set off an explosive powdered aluminum coating on the skin of the balloon, this disaster was originally thought to be primarily an explosion of hydrogen gas—thus effectively ending the age of the H2 zeppelins. 

In aerobic cellular respiration, H2 produced in the mitochondria (ETS) is oxidized immediately before it can build up to an explosive amount.  Thus the need for O2.

(Click here to see more on the Hindenburg disaster, including video of the disaster.)

Total Energy Yield

A total of 36-38 ATPs are available from aerobic respiration

See the ATP Yield Table for clarification

Anaerobic Option

Lactic acid

Formation of lactic acid means that the molecules have followed the "anaerobic pathway"

Fuel sources slide

Glucose primarily (or anything that can be coverted to glucose)

Glycogen (stored in muscle fibers, not most other cells)

Other fuels converted to some form of glucose (lipids, proteins)

lion trackNot required click here for the full chart of the Krebs Cycle —if you dare!

Muscle organ contraction

Motor unit

Motor unit — group of muscle fibers all connected to the same motor neuron, thus acting as a unit slide

Different levels of contraction in a muscle organ can result from recruitment of different numbers of motor units


Myography produces a wave-like picture of muscle organ contraction called a myogram

Twitch contraction — single, brief contraction in response to a single stimulus

Treppe (staircase effect)

Wave summation (tetanus) — sustained contraction

Muscle tone

Muscle tone — continuous, low-level sustained (tetanic) contraction of any (or all) muscle organ(s) —the starting point for stronger contractions

Flaccidity — abnormally low tone, as in paralysis or immobility (cast)

Spasticity — abnormally high tone, as in CP or Parkinsonism


Fibrillation — asynchronous, uncoordinated contractions within a muscle organ

Abnormal; occurs with fatigue, chemical imbalance

Spectrum of muscle organ contractions

Isotonic contraction — same tension; shorter length

Isometric contraction — increased tension; same length

Most muscle contractions are somewhere between two ends of spectrum

Graded strength principle

Recruitment of more or less motor units


Metabolic condition

Stretch reflexes

Skeletal muscles contract only if stimulated

Smooth and cardiac muscles, however, may contract rhythmically without external stimulation

Muscle size

Atrophy — disuse atrophy is a reduction in muscle size resulting from lack of use

Hypertrophy — increase in muscle size resulting from maximal use, especially heavy-load-bearing contractions

Stretching muscles can cause a rapid increase in the length of myofibrils

Muscle from Leonardo da Vinci's notebook

This sketch from Leonardo da Vinci's notebook shows contraction of the biceps brachii muscle and flexion (bending) of the elbow. How do we know whether it's isometric or isotonic contraction? Could it be both?

(Click the image to enlarge it)

To see an animated cartoon of the muscle's contraction click here.


This is a Learning Outline page.
Did you notice the EXTRA menu bar at the top of each Learning Outline page with extra helps?

This page updated on 23-sep-16