Chapter 8: Muscular
System
Muscle Overview
The three types of muscle tissue are skeletal, cardiac, and
smooth
These types differ in structure, location, function, and means
of activation
(Skeletal – voluntary & striated, Cardiac – involuntary &
striated, Smooth – involuntary & non-striated)
Muscle Similarities
Skeletal and smooth muscle cells are elongated and are called
muscle fibers (muscle cell = muscle fiber)
Muscle contraction depends on two kinds of myofilaments –
actin & myosin (proteins)
Muscle terminology is similar
Sarcolemma – muscle plasma membrane
Sarcoplasm – cytoplasm of a muscle cell
Prefixes – myo, mys, and sarco all refer to muscles (Ex
sarcoplasmic reticulum – ER in muscle cell)
Skeletal Muscle Tissue
Packaged in skeletal muscles that attach to and cover the
bony skeleton
Has obvious stripes called striations
Is controlled voluntarily (i.e., by conscious control)
Contracts rapidly but tires easily
Is responsible for overall body motility
Is extremely adaptable and can exert forces ranging from a
fraction of an ounce to over 70 pounds
Cardiac Muscle Tissue
Occurs only in the heart
Is striated like skeletal muscle but is not voluntary
Contracts at a fairly steady rate set by the heart’s pacemaker
Neural controls allow the heart to respond to changes in
bodily needs
Smooth Muscle Tissue
Found in the walls of hollow visceral organs, such as the
stomach, urinary bladder, and digestive organs
Forces food and other substances through internal body
channels
It is not striated and is involuntary
Functional Characteristics of
Muscle Tissue
Excitability or irritability – the ability to receive and respond
to stimuli
Contractility – the ability to shorten forcibly
Extensibility – the ability to be stretched or extended
Elasticity – the ability to recoil and resume the original
resting length
Muscle Function
Skeletal muscles are responsible for all locomotion
Cardiac muscle is responsible for coursing the blood through
the body
Smooth muscle helps maintain blood pressure and squeezes
or propels substances (food & feces) through organs
Muscles also maintain posture, stabilize joints, and generate
heat
Skeletal Muscle
Each muscle is a discrete organ composed of muscle tissue,
blood vessels, nerve fibers, and connective tissue
Skeletal Muscle
The three connective tissue sheaths are:
Endomysium – fine sheath of connective tissue composed of
reticular fibers surrounding each muscle fiber
Perimysium – fibrous connective tissue that surrounds groups of
muscle fibers called fascicles (bundle of muscle cells)
Epimysium – an overcoat of dense regular connective tissue that
surrounds the entire muscle
These three membranes merge to make up a tendon
Skeletal Muscle: Nerve & Blood
Supply
Each muscle is served by one nerve, an artery, and one or
more veins
Each skeletal muscle fiber is supplied with a nerve ending
that controls contraction
Contracting fibers require continuous delivery of oxygen and
nutrients via arteries
Wastes must be removed via veins
Skeletal Muscle: Attachments
Most skeletal muscles span joints and are attached to bone in
at least two places
When muscles contract the movable bone, the muscle’s
insertion moves toward the immovable bone, the muscle’s
origin
Skeletal Muscle: Attachments
Muscles attach:
Directly – epimysium of the muscle is fused to the periosteum
of a bone
Indirectly – connective tissue wrappings extend beyond the
muscle as a tendon or aponeurosis
Microscopic Anatomy of a
Skeletal Muscle Fiber
Each fiber is a long, cylindrical cell with multiple nuclei just
beneath the sarcolemma
Fibers are 10 to 100 micrometers in diameter and up to
hundreds of centimeters long
Each cell is a syncytium produced by fusion of embryonic
cells
Skeletal Muscle Contraction
In order to contract, a skeletal muscle must:
Be stimulated by a nerve ending
Propagate an electrical current or action potential along its
sarcolemma
Have a rise in intracellular Ca2+ levels, the final trigger for
contraction
Linking the electrical signal to the contraction is excitationcontraction coupling
Nerve Stimulus of Skeletal
Muscle
Skeletal muscles are stimulated by motor neurons of the
somatic nervous system
Axons of these neurons travel in nerves to muscle cells
Axons of motor neurons branch profusely as they enter
muscles
Each axonal branch forms a neuromuscular junction with a
single muscle fiber
Neuromuscular Junction
The neuromuscular junction is formed from:
Axonal endings which have small membranous sacs (synaptic
vesicles) that contain the neurotransmitter acetylcholine (ACh)
The motor end plate of a muscle, which is a specific part of the
sarcolemma that contains ACh receptors and helps form the
neuromuscular junction
Neuromuscular Junction
Though exceedingly close, axonal ends and muscle fibers are
always separated by a space called the synaptic cleft
Energy Sources for Muscle
Contraction
ATP molecules supply the energy for muscle fiber
contraction
Initial source of energy available to a contracting muscle
comes from existing ATP molecules in the cell
When initial ATP is gone:
Creatine phosphate is used to transform ADP to ATP
Muscle fibers depend on cellular respiration of glucose as an
energy source for synthesizing ATP
Oxygen Debt
Muscle fibers must depend increasingly on anaerobic
respiration for energy
In anaerobic respiration:
Glucose molecules are broken down by glycolysis to yield
pyruvic acid
When oxygen supply is low, the pyruvic acid reacts to produce
lactic acid which can accumulate in the muscles
Lactic acid enters blood stream and travels to the liver where
glucose is made from the lactic acid
Oxygen Debt
During strenuous exercise:
Available oxygen is used primarily to synthesize ATP the muscle
fiber requires to contract
This causes the lactic acid to accumulate in the muscles which
causes an oxygen debt
Equals the amount of oxygen liver cells require to convert the
accumulated lactic acid into glucose plus the amount muscle
cells require to restore ATP & creatine phosphate to their
original concentrations
Muscle Fatigue
A muscle exercised strenuously for a prolonged period may
lose its ability to contract - fatigue
Can happen from an interruption in the muscle’s blood
supply or lack of acetylcholine in motor nerve fibers (rare)
Most likely to arise from the accumulation of lactic acid in
the muscle as a result of anaerobic respiration
Lactic acid buildup lowers pH levels which causes the muscle
fibers to no longer respond to stimulation
Muscle Fatigue
Muscles can cramp which happens when the muscle
undergoes a sustained involuntary contraction
Cramps are thought to occur when changes in the
extracellular fluid surrounding the muscle fibers and their
motor neurons trigger an uncontrolled stimulation
Muscular Responses
Threshold stimulus – the minimal strength required to cause a
contraction
All-or-none response – if muscles contract, it contracts completely;
muscles do not contract partially
Twitch – when a muscle is exposed to a single stimulus that causes
it to contract and relax; last a fraction of a second; produces a
myogram (recording of a muscle contraction)
Latent period – the delay between the time the stimulus was applied
and the time the muscle responded
Period of contraction – when the muscle pulls at its attachments
Period of relaxation – when muscle returns to its former length
Muscular Responses
Summation – increased force of contraction by a skeletal
muscle fiber when twitches occur so rapidly that the next
twitch occurs before the previous twitch relaxes
Tetanic contraction – continuous, forceful muscular
contraction without relaxation
Recruitment – increase in the number of motor units
activated as stimulation intensity increases
Muscular Responses
Sustained contraction – the combination of summation and
recruitment; Ex) lifting weights or walking; response to a
rapid series of stimuli transmitted from the brain and spinal
cord on motor neurons
Muscle tone (tonus) – a response to nerve impulses that
originate repeatedly from the spinal cord and stimulate a few
muscle fibers
Important in maintaining posture
If muscle tone is suddenly lost the body collapses (when a
person loses consciousness)
Smooth Muscle
2 major types:
Multiunit smooth muscle – muscle fibers are separate rather than
organized into sheets; found in irises of the eyes & in walls of blood
vessels
Visceral smooth muscle – composed of sheets of spindle-shaped cells in
close contact with one another; more common; found in walls of hollow
organs (stomach, intestines, bladder,& uterus)
Contraction:
Resembles skeletal muscle contraction in a # of ways
Both include reactions of actin & myosin
Both triggered by membrane impulses
Both use ATP for energy
Smooth Muscle
Contraction differences:
Two neurotransmitters affect smooth muscle: acetylcholine &
norepinephrine
Hormones affect smooth muscle  stimulates contractions and
alters the degree of response to neurotransmitters in some cases
Smooth muscle is slower to contract & relax than skeletal
muscle
Can maintain a forceful contraction longer with a given amount
of ATP
Fibers can change length without changing tautness
Cardiac Muscle
Only found in the heart
Main function is the pumping action of the heart
Well-developed transverse tubule system
Has intercalated disks that separate adjacent cells
Moves involuntarily and has striations
Contraction characteristics:
Network of fibers contracts as a unit
Self-exciting
Rhythmic
General Overview
3 Major Muscle Functions:
Heat production
Movement
Posture
Four Major Actions of Muscles:
Flexor
Extensor
Abductor
Adductor
General Overview
Muscle Fibers Run 3 Ways:
Rectus – parallel
Transverse – perpendicular (across)
Oblique - diagonal
There are 5 Shapes/Sizes of Muscles:
Maximus
Minimus
Longus
Deltoid
Trapezius
Muscle Diseases
Fibrosis – degenerative disease in which fibrous connective
tissue replaces skeletal muscle tissue
Fibrositis – inflammation of fibrous connective tissues,
especially in the muscle fascia (muscle rheumatism)
Muscular dystrophies – group of inherited disorders in which
deficiency of cytoskeletal protein (glycoprotein) collapses
muscle cells, leading to progressive loss of function
Myalgia – pain from any muscular disease or disorder
Torticollis – condition where the neck muscles contract
involuntarily (wryneck)
Muscle Diseases
Myasthenia gravis – chronic disease in which muscles are
weak and easily fatigued because of malfunctioning
neuromuscular junctions
Myokymia – persistent quivering of a muscle
Myoma – tumor composed of muscle tissue
Myopathy – any muscular disease
Myositis – inflammation of skeletal muscle tissue
Myotonia – prolonged muscular spasm
Shin splints – soreness on the front of the leg due to straining
the flexor digitorum longus, often results from walking up
and down hills