Myology- study of muscles
ANATOMY
Skeletal muscles- contract to carry out functions of generating body movements, producing heat,
supporting body, maintaining posture
~700 skeletal muscles Make up 40% of body mass
Each skeletal muscle is an organ- made of muscle tissue, connective tissue, epithelial tissue &
nervous tissue
Characteristics of all muscle tissue
Excitability (irritability)
Sensitive to stimuli from nerve impulse
Can receive or respond to an electrical impulse
Contractility
Responds to stimuli by contracting/shortening
Ability to become shorter/thicker when contracting & thus producing movement
Extensibility
Stretching beyond resting length by contractions of an opposing muscle
Preparing for another contraction
Elasticity
Tendency to recoil to original resting length
Ability to return to original length when relaxing
Skeletal muscles attach to bone on each end by tendon (dense regular connective tissue that binds to
periosteum of bone)
As muscle contracts & bones move
Insertion- more movable bony attachment of muscle; distal end; narrower end or end with
single attachment
Origin- less movable bony attachment of muscle; undergoes little change in position; proximal
attachment; broad end or end with multiple attachments
Belly- fleshy thickened part of muscle
Loose connective tissue holds bundles of fibers to bundles of fibers & is also there to protect/strengthen
Endomysium- innermost, fine sheath of connective tissue surrounding individual muscle fibers &
binding adjacent fibers together
Perimysium- binds groups of muscle fibers together into fascicule; contains blood vessels &
nerves
Epimysium- outermost connective tissue covering; is continuous with tendon; covers entire
muscle
Fascia- fibrous connective tissue that covers muscle & attaches to skin
Types of muscle contractions
Isotonic
Contraction that results in visible muscle shortening & load movement
Force of contraction remains relatively constant throughout shortening process
Isometric
Occurs when too few muscle fibers are activated to shorten the muscle
Movement does not occur because load is too great
Muscle functional groups
Synergistic muscles- contract together to accomplish a particular movement; ex. hamstrings
Antagonistic muscles- perform opposite functions; ex. Biceps & triceps
Muscles have high rate of metabolic activity & need lots of blood vessels to move nutrients, oxygen &
wastes
Small muscles may have 1 artery & 2 veins
Large muscles may have several arteries/veins
Muscles also need nerves to stimulate them to contract
Sensory neurons conduct nerve impulses away from muscle fiber
Motor neurons stimulate muscles to contract; begins with an electrical impulse that then causes
a chemical response
Myoneural or neuromuscular junction- where motor neuron branch connects with muscle fiber;
surface of nerve doesn’t touch cell membrane (there is a gap)
Motor unit- single motor neuron & the many muscle fibers it innervates; size of motor unit
determines delicacy of muscle movement
Ratio of about 1 motor neuron for each 100-150 fibers
May be as low as 1:10 for muscles needed for precise movement (ex. In hand)
May be as high as 1:500 in muscles with large scale movements (ex. Thigh)
When nerve impulse travels through motor unit, all fibers served by it contract at the same time
to their maximum ability
Magnitude of task determines how many motor units will be activated
Power of muscle determined by total number of cells/fibers available to contract
Atrophy-decrease of muscle tone if not periodically stimulated; wasting of tissue resulting in reduction
in muscle size, tone & power
Exercise causes muscle fibers to increase in diameter but not to multiply
Hypertrophy- increase in muscle fiber size but not number
Skeletal muscle fibers are like other cells except that they are multi-nucleated & striated
Muscle fiber
Sarcolemma- cell membrane surrounding it
Sarcoplasm- cytoplasm in muscle cell
Sarcoplasmic reticulum- similar to smooth endoplasmic reticulum of cell; network of
membranous channels extending through sarcoplasm; stores calcium ions for initiating
contraction
Transverse tubules- run perpendicular to sarcoplasmic reticulum; opens to outside of cell;
carries impulse into cell
Myofibril- threadlike structures embedded in sarcolemma extending from one end of muscle
fiber to other; made of protein myofilaments
Myofilaments- thin one are made of actin; thick ones are made of myosin
Striations due to arrangement of myofilaments
Dark bands = A bands
Light bands = I bands = thin filaments
Z lines are thin dark lines in middle of I bands
From Z line to next Z line is subunit called sarcomere
Z bands are darker because thin/thick filaments overlap
H zone is central lighter region of A bands; contains only myosin fibers
Sarcomere is the contracting unit of muscle fiber
Muscle fiber organization (patterns of fascicle arrangement)
Circular
Concentrically arranged around an opening or recess
Can be called sphincter if it closes an opening
Located at entrances/exits of internal passageways
Ex. Orbicularis oris, orbicularis oculi, anal sphincter
Parallel
Fascicles run parallel to long axis of muscle
High endurance muscle but not as strong as some
Ex. Rectus abdominis, biceps brachii, masseter
Convergent
Widespread & converge to common attachment site
Often triangular in shape
Direction of pull can be modified by activating different groups of fibers
Do not pull as hard as parallel muscles
Ex. Pectoralis major
Pennate
Resemble feather with tendon in middle & fascicles arranged on both sides at
oblique angle
Do not move tendon as far but is stronger & creates more tension
Unipennate- all fibers on the same side of tendon; ex. Extensor digitorum
Bipennate- fibers on both sides of tendon; most common; ex. Rectus femoris
Multipennate- branches of tendon within muscle; ex. deltoid
Skeletal muscles are named on the basis of shape, location, attachment, orientation of fibers,
relative position or function
Action- indicates primary function or movement; ex. Flexor, extensor, pronator, adductor,
abductor
Body region- ex. Oris, oculi
Muscle attachment- identify origin, insertion or other prominent attachment; origin is first
part of name & insertion is second part; ex. Sternocleidomastoid
Orientation of fibers- rectus means straight; oblique means angled
Shape or size- brevis means short; longus means long; magnus means big or major; minor
means small; deltoid, trapezius, orbicularis
Heads/tendons of origin- indicates how many; ex. Triceps
Muscles of the axial skeleton
Responsible for facial expression, mastication, eye movement, tongue movement, neck
movement & respiration; also includes those of abdominal wall, pelvic outlet &
vertebral column
Temporalis- elevates mandible; more up on side of head
Masseter- elevates manbidle; clench your teeth to feel
Rectus muscles- 4 that move eyeball up/down and side to side
Oblique muscles- 2 that rotate eyeball on axis
Orbicularis oculi- closes eye
Orbicularis oris- closes lips
Zygomaticus- elevates corner of mouth (smile)
Sternocleidomastoid- turns head to side & flexes neck
Diaphragm, external/internal intercostals- used in breathing
External/internal abdominal oblique- compresses abdomen, rotates lumbar region, draws
thorax down
Muscles of appendicular skeleton
Pectoral girdle, arm, forearm, wrist, hand, fingers; Pelvic girdle, thigh, leg, ankle, foot, toes
Pectoralis minor- pulls scapula forward & down
Trapezius- braces shoulder, moves scapula & hyperextends neck
Pectoralis major- flexes, adducts, rotates shoulder
Latissimus dorsi- extends, adducts, rotates shoulder
Deltoid- abducts, extends flexes shoulder
Biceps brachii, brachialis, brachioradialis- flexes elbow
Triceps brachii- extends elbow
Flexor carpi radialis- flexes/abducts hand at wrist
Flexor carpi ulnaris- flexes/adducts hand at wrist
Supinator- turns forearm & hand
Extensor carpi radialis longus- extends & abducts wrist
Extensor carpi ulnaris- extends & adducts wrist
Gluteus maximus- extends & rotates thigh laterally
Gluteus minimus- abducts & rotates thigh
Gracilis- adducts thigh at hip; flexes/rotates leg at knee
Sartorius- flexes knee/hip; flexes/rotates leg at knee; longest muscle in body
Quadriceps femoris- extends leg at knee
Biceps femoris- flexes knee joint; extends/laterally rotates thigh
Hamstrings include biceps femoris, semimembranous & semitendinosus
Tibilais anterior- flexes ankle; inverts foot at ankle
Gastrocnemius- plantar flexes foot at ankle; flexes knee joint
Soleus- plantar flexes foot at ankle
Achilles tendon- attaches gastrocnemius & soleus to calcanus of heel
Plantaris- plantar flexes foot at ankle
Tibialis posterior- plantar flexes & inverts foot at ankle; supports arches
PHYSIOLOGY
Benefits of muscle tissue
Maintain erect posture- supports body structures
Move body
Movement to secure food & flee danger
Communication
Vital life functions
Maintain normal body temperature- heat production
Review of muscle fiber structure
Muscle is made of fascicle which are made of muscle fibers which are made of myofibrils
Myofibrils are divided into sarcomeres (functional unit of muscle contraction) which is where
stripes form due to I bands & A bands; sarcomere is unit from Z line to Z line
In center of each I band is a Z line & in center of each A band is H zone
Myofibril is made of two types of myofilaments
Thin myofilaments are made of actin (globular proteins strung together like pearls)
Thick myofilaments are made of dozens of myosin proteins (long/slender)
Non-contracting muscle
Calcium ions are concentrated in small extensions of the sarcoplasmic reticulum
Before contraction
Nerve impulse gets to neuromuscular junction
Calcium ions are released from synaptic knob of motor neuron at neuromuscular junction
Tells muscle to contract as it causes acetylcholine (neurotransmitter chemical) to be released
into the gap
Neurotransmitter chemical then diffuses to receptor sites on membranes of muscle fibers
Acetylcholine initiates electrochemical reaction that results in muscle contraction- it is the
electrical impulse that causes the calcium ions to move & the filament to slide
Contracting muscle
Occurs when the thick dark & thin light myofilaments of myofibril slide over one another
H zones almost disappear, I bands shorten & become narrow, Z lines draw close together
Filament sliding begins when calcium ions are released into the sarcoplasm when the
sarcoplasmic reticulum is stimulated & interactions cause the actin filaments to pull
toward the center of the myosin filament (myosin pulls itself along actin fiber)
Actin filaments slide toward each other past the myosin filaments making/breaking
crossbridges as they slid
ATP cuases the sliding to occur since it supplies the energy
Filament lengths don’t change but the length of the overlap does
Shortening causes the whole muscle to contract
Contraction will continue until there is no acetylcholine present- muscle can’t relax until its gone
Acetylcholinesterase- enzyme produced at neuromuscular junction to breakdown
acetylcholine; acetylcholine is broken down into acetate & choline which are
then reabsorbed by nerve ending & used to make acetylcholine for next time
Once acetylcholine is removed the muscle relaxes & the filaments return to their original
position
When muscles contract, they pull on tendons attached to bones & produce movement
Muscle can contract about 1/3 of resting length
Muscles characteristically remain in a state of partial contraction
Muscle tone
Continual contraction which maintains posture without undue fatigue
Sustained partial contraction
Muscle fatigue
Occurs because ATP available for muscle contraction has been used up
No energy = no contractions
Temperature can affect muscles
Speed of contraction increases as temperature increases
All or none principle- individual muscle fibers either contract all the way or not at all
There are no partial contractions of a muscle fiber
Threshold stimulus- weakest electrical stimuli that will cause a muscle fiber to contract
Subthreshold stimulus- stimulus so weak that contractions won’t take place
Twitch- simplest type of muscle response due to a single brief electrical impulse; not common
Latent period- short delay before contraction occurs after a stimulus is applied; due to time for
biochemical things to take place
Summation- if muscle is stimulated twice in succession with second one before it can relax from first;
the stimuli add together to produce a bigger stimulus/contraction
Refractory period- short amount of time before a muscle can respond again to contract; time between
two contractions; muscle temporarily can’t respond to stimuli
Some disorders of Muscular System
Muscle spasm- involuntary contraction of muscle; causes weakness/pain; prolonged cases may be due
to pinched spinal nerve
Muscle cramp- tectonic contraction of muscle; caused by extreme cold over prolonged period of time or
by severe physical exertion; stretching cramp usually relieves pain & relaxes muscle
Pulled muscle- fibers of muscle tear due to sudden violent contraction of muscle; tendon may separate
from muscle; accompanied by sharp pain & loss of muscle function
Flexibility/stretching can help prevent injuries during sports as well as reduce risk of back pain, prepare
muscles of workout, ease muscle soreness, relax muscles, relieve stress, enhance athletic
performance & improve posture/balance; also increases efficiency of chemical reactions
Hiccup- sudden spasmodic contraction of diaphragm which produces a sharp intake of breath; the
breath is abruptly cut off when vocal cords involuntarily pull together & produce the HIC sound;
may be caused by low level of carbon dioxide in blood & inhibited by high levels; some
studies claim men hiccup more than women
Trichinosis- infection by parasitic roundworm; symptoms include diarrhea, weakness & muscle pain
when larval worms invade/encyst in perimysium & endomysium; get it by eating undercooked
pork; can die from it but estimated that 1.5 million Americans carry it
Fibromyalgia- most common musculoskeletal disorder affecting women under 40; symptoms include
chronic fatigue, chronic aches/pains, stiffness, tender points (below kneecap, distal end of
humerus, between 2nd rib & sternum), men can also get this
Duchenne’s Muscular Dystrophy- appears between ages 3 & 7; generally a male disease since it is
sex-linked inherited; progressive muscular weakness & usually dies by age 20 due to respiratory
paralysis; mostly affects skeletal muscles & fibers are structurally different from normal
individuals; muscles waste away/shrink; eventually muscles are replaced by fatty tissue; fibers
degenerate/atrophy; no cure
Polio- viral disease of nervous system that causes muscle paralysis because virus attacks somatic motor
neurons in CNS; individual develop fever 7-14 days after infection & about a week later there is
muscle pain, cramping/paralysis of one or more limbs; respiratory paralysis may occur;
survivors of this type of polio may develop progressive muscular weakness 20-30 years after
initial infection
Carpal tunnel syndrome- inflammation of tendon sheath surrounding flexor tendons of palms which
compresses the median nerve; symptoms include pain, tingling or numbness, weakness; fairly
common & is linked to repetitive hand movements; treated with anti-inflammatory drugs,
injection with steroids & use of a splint to keep wrist stabilized; surgery may be necessary