The Muscular System


Functions and Types of Muscles

Contractile cells, called fibers (well, really skeletal muscle normally)  





Spindle shaped cells, usually found in parallel lines forming sheets

Found in walls of hollow organs (also called visceral)

Slow contractions but can sustain – doesn’t fatigue easily





Auto-rhythmic – doesn’t require nervous system stimulation to contract

Cylindrical and branched cells (fibers)

Joined by intercalated disks – allow passage of electrical activity throughout muscle

Relax completely between contractions, avoids fatigue





Cylindrical fibers are long – run the length of the muscle

Can contract multiple times between relaxations, can fatigue (use up all the available ATP)

Connective Tissue Coverings

Epimysium – outer covering of muscle (covers bundles of fasicles), extends beyond muscle to become tendon

Perimysium – surrounds bundles of muscle fibers called fasicles

Endomysium – covers individual fibers (cells)

Functions of Skeletal Muscles

Stabilize joints – tendons may extend across joints

Maintains posture, which also takes strain off bones and joints

“Opposes the force of gravity and keeps us upright”

“Muscle contraction refinements allow us to assume different positions”


Contraction produces heat


Microscopic Anatomy and Contraction of Skeletal Muscle Structure

Muscle Fiber

Myofibrils and Sarcomeres

Muscle fibers contain bundles of myofibrils, the contractile elements of the cell, which run the length of the fiber

Myofibrils contain actin and myosin myofilaments arranged in sarcomeres (sarcomeres can be said to be the contractile elements of the myofibrils).

Myosin filaments are thick, consist of two molecules with entertwined tails and two heads that bind to actin.

Actin filaments consist of two entertwined chains of actin monomers with myosin head binding sites. The myosin binding sites are blocked by a linear molecule of tropomyosin, which runs the length of the thin filament in the groove between the two chains. A regulatory molecule, troponin, is bound to the actin and the tropomyosin.

The myofilaments are anchored to Z-discs at each end of the sarcomere.

The sarcolemma (plasma membrane) forms tubes that dip into cell –T-tubules (transverse) - over each z-disc.

T-tubules come in contact with sarcoplasmic reticulum (ER) at sites of calcium storage (calcium storage sacs, or terminal cisternae of the SR)

Skeletal Muscle Contraction

Neuromuscular junction – site of interaction between neuron and muscle fiber

A nerve impulse travels from neuromuscular junction, along sarcolemma, down T-tubules, stimulates calcium release from sarcoplasmic reticulum, which allows the thin filaments to slide to the center of the sarcomere, causing it to shorten and the muscle to contract.




The Role of Actin and Myosin

Sliding Filament Mechanism of Contraction:

Calcium binds to troponin.

Troponin changes shape and pulls tropomyosin out of the myosin head binding sites.

The interaction between myosin and actin pulls the actin filaments toward the center of the sarcomere.

The myosin head has an ATP binding site. When ATP binds the myosin head hydrolyzes ATP to ADP + P and "cocks" into its high energy conformation, pointing away from the center of the sarcomere and toward the z-discs.

If calcium is present and the myosin head binding sites are unblocked the myosin heads form crossbridges with the actin filaments.

The myosin heads relax into their low energy conformation, pulling the actin filaments toward the center of the sarcomere (causing all the sarcomeres along the myofibril to shorten, causing the myofibril to shorten, causing the muscle to shorten - or contract).

At this point ADP and P are released and ATP can bind to the myosin head. ATP binding causes the myosin head to relase from the actin filament and the cycle starts all over.


Contraction of Smooth Muscle

Thick and thin filaments are present but there are no myofibrils, no sarcomeres, no striations.

Thin filaments are anchored to proteins in the sarcoplasm called dense bodies or directly to the sarcolemma.

Contraction cause fibers to shorten in all directions, cells go from spindle shaped to more round.

Contraction of smooth muscle is much slower than skeletal muscle but smooth muscle is more fatigue resistant than skeletal muscle and can maintain contractions for far longer.

Energy for Muscle Contration

ATP required for contraction and is generated by 3 mechanisms.

Under anaerobic conditions only about 4-6 seconds worth of ATP present.

Creatine Phosphate Breakdown

Creatine phosphate quickly regenerates ATP by directly donating a phosphate (good for about 15 seconds, first ATP regeneration mechanism to kick in under anaerobic conditions)

"Fermentation" (glycolysis)

Anaerobic production of ATP by glycolysis occurs during strenuous exercise after CP stores are exhausted.

Very inefficient, good for only 1-2 minutes before muscle fatigue occurs.

The end product of glycolysis, pyruvic acid, is converted to lactic acid if it isn't metabolized by cellular respiration fast enough to meet ATP needs of the muscle. Lactic acid builds up, the pH drops, muscles fatigue and ache.

Cellular Respiration

ATP is made in mitochondria by aerobic means - requires oxygen. Pyruvic acid is transported into mitochondria and oxidized to carbon dioxide and water.

As long as demand is below anaerobic threshold ATP made by this mechanism will fuel muscle activity - much more efficient than glycolysis alone.



Oxygen Debt

Lactic acid is transported to the liver where it is converted back to pyruvic acid, which is then metabolized by cellular respiration, which of course requires oxygen.

The amount of oxygen required to finish metabolism of lactic acid is the oxygen debt - that’s the wind-sucking you do after going anaerobic.

Muscle Responses

In the Laboratory

All-or-None Law

When stimulated enough to contract a muscle fiber (cell) completely contracts

A muscle (a bunch of fasicles, which is a bunch of fibers) doesn’t follow the All-or-None Law; how much the muscle contracts depends on how many fibers are stimulated to contract

Muscle Twitch, Summation, and Tetanus

Single stimulus, muscle contracts and relaxes

Stimulation in succession before the muscle can relax results in summation of tension – tension gets greater with same level of stimulation because it doesn’t get back to “0” tension before stimulated to contract again

Tetanus is maximal sustained contraction – muscle can’t shorten anymore despite continued stimulation

Fatigue – muscle runs out of ATP, can’t maintain contraction in spite of continued stimulation, lactic acid builds up and pH drops, motor nerves run out of neurotransmitter. Usually the voice in your head talks you into quitting before you actually experience fatigue.

In the Body

Motor unit – motor neuron from spinal cord branches and innervates several muscle fibers.

Each motor unit obeys the all-or-none law, so the amount of force a muscle contracts with depends on recruitment of motor units in that muscle.

Muscles that perform fine, delicate tasks and require great precision have motor units with few fibers per motor neuron (as few as 4), muscles that do the heavy lifting have motor units with many fibers per motor neuron (may be thousands).

Muscle Tone – partial contraction of muscles, maintains posture

Muscle spindles are sensors that send information to the CNS to allow partial contraction or tone to be maintained

Isotonic Vs. Isometric Contraction

Isotonic – muscle contracts, shortens, movement occurs (tension remains constant)

Isometric - muscle contracts, can’t move load, tension increases but length of muscle remains constant

Exercise and Size of Muscles

Hypertrophy – increase in size due to repeated forceful contraction (exercise)

Requires 75% maximum effort to stimulate hypertrophy

Number of muscle fibers may also increase (but this is due to splitting of fibers; basically ripping them into shreds and allowing them to heal, not hyperplasia)

Atrophy – disuse or very low intensity use causes muscles to shrink – fibers shorten and are replaced with fat and CT

Slow Twitch and Fast-Twitch Muscle Fibers

Slow-twitch fibers

Intermediate-twitch fibers

Fast-twitch fibers

Skeletal Muscles of the Body

Basic Principles

Muscles connect two bones, when they contract one bone moves and one remains stationary

Origin – where the muscle attaches to the stationary bone

Insertion – attachment site on the bone that moves

Prime mover – muscle that does most of the work

Synergists – assisting muscles

Antagonist – muscle that opposes the action of the prime mover

Muscles can only move bones in one direction since they shorten when they contract –don’t cause movement by lengthening

Have to have an opposing muscle to move the bone back in the opposite direction

Naming Muscles

Size – gluteus maximus, the largest buttock muscle

Shape – deltoid, shaped like a delta

Direction of Fibers – rectus abdominus – longitudinal abdominal muscle (rectus means straight)

Location – frontalis overlies the frontal bone

Number of Attachments – biceps, triceps

Action – extensor digitorum, flexors, adductors

Skeletal Muscle Groups

Superficial Skeletal Muscles - Anterior View


Superficial Skeletal Muscles - Posterior View

Muscles of the Head

Muscles of Facial Expression

Frontalis – corn row muscle

Orbicularis oculi – blinking muscle, crow’s feet

Orbicularis oris – pucker muscle

Buccinator – in cheek, compresses cheek

Zygomaticus – cheekbone to corners of mouth, smiley muscle

Muscles of Mastication

Masseter – chewing muscle


Muscles of the Neck

Muscles That Move the Head

Sternocleidomastoids (2) – sternum to mastoid process, both: head to chest (flex neck); one: head turns

Trapezius – shoulder shrug, neck extension

Muscles of the Trunk

Muscles of the Thoracic Wall

External intercostals – ribs up and out

Internal intercostals – ribs down and in

Diaphragm - separates thoracic cavity from abdominal cavity, assists inspiration

Muscles of the Abdominal Wall

External oblique – abdominal wall, lateral rotation

Internal oblique – same

Transversus abdominis – abdominal wall

Rectus abdominis – flexes vertebral column

Muscles of the Shoulder

Muscles That Move the Scapula

Trapezius – shoulder shrug, neck extension

Serratus anterior – pulls scapula down and forward (pushing)

Muscles That Move the Arm

Deltoid – abducts the arm

Pectoralis major – flexes and adducts the arm (pulls across chest)

Latissimus dorsi – extends and adducts the arm

Muscles of the Arm

Biceps brachii – flexes forearm and supinates hand

Triceps brachii – extends forearm

Brachialis – flexes forearem

Muscles of the Forearm

Extensor and flexor carpi – move wrist and hand

Extensor and flexor digitorum – move fingers

Muscles of the Hip and Lower Limb (Leg)

Muscles That Move the Thigh

Iliopsoas – flexes thigh

Gluteus maximus – extends thigh

Gluteus medius – abducts thigh

Adductor group - adducts thigh

Muscles That Move the Lower Limb (Leg)

Quadriceps femoris group – extends lower leg

Hamstring group – flexes lower leg and extends hip

Sartorius – flexes, abducts, and rotates leg

Muscles That Move the Ankle and Foot

Gastrocnemius – plantar flexion and eversion of foot

Tibialis anterior – dorsiflexion and inversion of foot

Peroneus group – plantar flexion and eversion of foot

Soleus – plantar flexes foot

Flexor and extensor digitorum longus – moves toes

Effects of Aging

Mass and Strength  - tend to decrease

Endurance – tends to decrease

Exercise – anti-aging effect, combats loss in mass, strength and endurance


Medical Focus - Muscular Dystrophy

A group of muscular disorders characterized by muscle degeneration.

Symptoms result from progressive skeletal muscle weakness

Due to degeneration of the fibers

Several types, all inherited, cause unknown in most

Duchenne muscular dystropy

Passed from the mother and nearly always only seen in male offspring

Muscle weakness, difficulty in walking, curvature of the spine

Gradual wasting of muscle, replacement with fat and CT

Confinement to a wheelchair by about 12 an death by 20 or so

Recently discovered that the cause is lack of a protein that maintains the integrity of the sarcolemma (dystrophin)

Gene therapy:

Myoblast transfer therapy – inject with health myoblast cells that fuse with health ones, this provides the normal gene and allows the fibers to produce normal dystrophin

Insertion of plasmids with the correct genes also being tried

Myotonic muscular dystrophy

Failure of muscles to relax after contraction

Inherited from either parent

Face and neck affected first


Lifting and turning head



Later problems:

Abnormal heart rhythms


Abdominal cramps

Sometimes leads to confinement to bed or wheelchair

MedAlert – Benefits of Exercise

Improves strength

The force a muscle can exert against resistance in one maximal effort.

The size of the muscle and the number of myofibrils and even fibers will increase as the strength increases

Capillaries and CT will increase also, including CT of tendons and ligaments

Strength training (resistance) benefits all adults (remember it also increases bone density)

Improves endurance

Ability of the muscle to contract repeatedly or to sustain a contraction.

Improves flexibility

Range of motion around a joint

Any level of exercise can improve health:

Lowers risk of heart attack

Increases HDL levels

Lowers resting heart rate, lowers blood pressure

Reduces pain and swelling in arthritis patients

Also reduces fatigue and depression associated with the disease

May be beneficial in a variety of other long-term diseases that include fatigue and depression as part of their secondary symptoms

Stimulates osteoblast activity, helps prevent osteoporosis

Cancer prevention:

Linked to decreases in liklihood of developing colon, breast, cervical, uterine, and ovarian cancers

Increases intestinal motility and fat mobilization (and utilization)