BIOL 2304
Muscles
Muscle Tissue Types
Skeletal muscle tissue
Packaged into skeletal muscles
Makes up 45% (nearly half) of body weight
Cells are striated
Cardiac muscle tissue – occurs only in the walls of the heart
Smooth muscle tissue – occupies the walls of hollow organs, cells lack striations
Functions Of Muscle Tissue
Movement
Body movement – Contract and pull on bones to move the body
Cardiovascular circulation – Help propel blood and lymph through vessels
Gastrointestinal movement – Churn food for digestion and propel food through alimentary canal
Elimination (urination) and evacuation (defecation) – Move wastes (urine, feces) out of the body
Uterine muscles birth babies
Sight – Move eyes and focus vision
Breathing – Inspiration and expiration (contraction and relaxation of thoracic muscles)
Vocalization – Move vocal folds for speaking
Posture and body position
Some continuously contract to maintain body posture
Heat generation to maintain body temperature
Contractions produce heat (shivering)
Joint Stabilization
Stabilize a joint so a desired movement can be performed by the same or a nearby joint
Functional Features of Muscles
Contractility – Long cells shorten and generate pulling force
Excitability – Electrical nerve impulse or hormone (i.e.: epinephrine) stimulates the muscle cell to
contract
Extensibility – Can be stretched back to its original length by contraction of an opposing muscle
Elasticity – Can recoil after being stretched
Similarities Among All Muscle Tissues
Muscle cells are called muscles fibers
Plasma membrane is called a sarcolemma
Cytoplasm is called sarcoplasm
Muscle contraction
Two types of myofilaments (contractile proteins) called actin and myosin
These two proteins generate contractile force
Skeletal Muscle
Each muscle is an individual organ
Consists mostly of muscle tissue
Skeletal muscle also contains:
Connective tissue
Blood vessels
Nerves
Each skeletal muscle supplied by branches of:
One nerve
One artery
One or more veins
Muscle Attachments
Most skeletal muscles run from one bone to another
One bone will move – other bone remains fixed
Origin – less movable attachment
Insertion – more movable attachment
Muscle Attachments
Muscles attach to origins and insertions by connective tissue (CT)
Tendon –
Cordlike, white non-elastic
Attaches muscles to bone (usually)
Sometimes muscle to skin, or cartilage
Most muscles attach to bone by tendons
Not to be confused with ligament which attach bone to
bone
Bone markings present where tendons meet bones:
i.e.: Tubercles, trochanters, and crests
Connective Tissue And Fascicles
CT sheaths bind a skeletal muscle and its fibers together
Fascicle – group of muscle fibers
Epimysium – dense irregular connective tissue surrounding entire muscle
Perimysium – surrounds each fascicle
(group of muscle fibers)
Endomysium – a fine sheath of connective tissue wrapping each muscle cell
Connective tissue sheaths are continuous with tendons
Histology of Skeletal Muscle Tissue
The skeletal muscle fiber
Fibers are long and cylindrical
Are huge cells – diameter is
10–100µm
Length – several centimeters
to dozens of centimeters;
often span the entire length of
a muscle.
Cells are multinucleate
Nuclei are peripherally located
Myofibrils and Sarcomeres
Striations result from internal structure of myofibrils
Myofibrils:
Long rods within cytoplasm
Are a specialized contractile organelle found in muscle tissue
Make up 80% of the cytoplasm
A long row of repeating segments called sarcomeres (the functional units of skeletal muscle
tissue)
Sarcomere
Sarcomere – the basic unit of contraction of skeletal muscle:
Z disc (Z line) – boundaries of each sarcomere
Actin (thin) filaments – extend from Z disc toward the center of the sarcomere
Myosin (thick) filaments – located in the center of the sarcomere
Overlap inner ends of the thin filaments
Contain ATPase enzymes
Sarcomere Structure
A bands – full length of the thick filament; includes inner end of thin filaments
H zone – center part of A band where no thin filaments occur
M line – thick dark line down center of H zone; contains tiny rods that hold thick filaments together
I band – region with only thin filaments; lies within two adjacent sarcomeres
Sliding Filament Model of Contraction
Contraction refers to the activation of myosin’s cross bridges – the sites that generate the force
In the relaxed state, actin and myosin filaments do not fully overlap
With the presence of calcium (Ca2+) from the SR, the myosin heads bind to actin and pull the thin
filaments
Actin filaments slide past the myosin filaments so that the actin and myosin filaments overlap to a
greater degree (the actin filaments are moved toward the center of the sarcomere, Z lines become
closer)
Changes in Striation During Contraction
Innervation of Skeletal Muscle
Motor neurons innervate skeletal muscle tissue
Neuromuscular junction (NMJ) is the point where nerve ending and muscle fiber meet
Motor Units
Motor Unit — A motor neuron and all the muscle cells it controls
Recruitment — To increase muscle tension by activating more motor units
Small motor units provide finer control
Muscles performing powerful, coarsely controlled movement have larger number of fibers per motor
unit
Smooth Muscle
Occurs within most organs
Walls of hollow visceral organs, such as the stomach
Urinary bladder
Respiratory passages
Arteries and veins
Helps substances move through internal body channels via peristalsis
No striations
Filaments do not form myofibrils
Not arranged in sarcomere pattern found in skeletal muscle
Is Involuntary
Single Nucleus
Smooth Muscle
Composed of spindle-shaped fibers with a diameter of 2-10 m and
lengths of several hundred m
Cells usually arranged in sheets within muscle
Organized into two layers (longitudinal and circular) of closely
apposed fibers
Have essentially the same contractile mechanisms as skeletal muscle
Smooth Muscle
Cell has three types of filaments
Thick myosin filaments
Longer than those in skeletal muscle
Thin actin filaments
Contain tropomyosin but lack troponin
Filaments of intermediate size
Do not directly participate in contraction
Form part of cytoskeletal framework that supports cell shape
Have dense bodies containing same protein found in Z lines
Cardiac Muscle Tissue
Occurs only in the heart
Is striated like skeletal muscle but has a branching pattern with
intercalated Discs
Usually one nucleus, but may have more
Is Involuntary
Contracts at a fairly steady rate set by the heart’s pacemaker
Neural controls allow the heart to respond to changes in bodily needs
Comparison of Muscle Tissues
Muscles of the Body
Skeletal muscles
Produce movements
Blinking of eye, standing on tiptoe, swallowing food, etc.
General principles of leverage
Muscles act with or against each other
Criteria used in naming muscles
Three Muscle Fiber Types (based on speed of contraction)
Skeletal muscle fibers are categorized according to
How they manufacture energy (ATP) and
How quickly they contract
Speed of contraction is determined by how fast their myosin ATPases split ATP
Oxidative fibers – use aerobic pathways
Glycolytic fibers – use anaerobic glycolysis
Based on these two criteria skeletal muscles may be classified as:
Slow oxidative fibers (Type I)
Red color due to abundant myoglobin (containing iron and oxygen)
Depend on aerobic pathways (oxygen dependent)
Contain many mitochondria
Richly supplied with capillaries
Contract slowly and resistant fatigue
Fibers are small in diameter
Example: Postural muscles of the neck and back
Fast oxidative fibers (Type IIa)
Have an intermediate diameter
Contract quickly like fast glycolytic fibers
More powerful than slow oxidative fibers
Depend on anaerobic pathways (oxygen dependent)
Have high myoglobin content and rich supply of capillaries
Somewhat fatigue-resistant
Example: Throughout body for jogging and swimming
Fast glycolytic fibers (Type IIb)
Contain little myoglobin and few mitochondria
About twice the diameter of slow-oxidative fibers
Contain more myofilaments and generate more power
Depend on anaerobic pathways
Contract rapidly and tire quickly
Example: Muscles of the thighs for sprinting
Interactions of Skeletal Muscles in the Body
A muscle cannot reverse the movement it produces
Another muscle must undo the action
Muscles with opposite actions lie on opposite sides of a joint
Action of Muscles
Prime mover or Agonist – the muscle that contacts to produce a
specified movement
Ex: biceps brachii which causes elbow flexion
Synergist – the muscles that prevent any unwanted movements
that might occur during contraction of the prime mover
Ex: Adjacent arm muscles, such as the brachialis, prevent
twisting of the arm during elbow flexion
Antagonist – the muscle on the opposite side of the prime mover.
This muscle must relax to allow the prime mover to contact.
Ex: triceps brachii during elbow flexion
Fixator – a synergist or stabilizer that immobilizes the bone of the
prime mover’s origin, providing a stable base for the prime mover
Ex: deltoid muscle during elbow flexion
Arrangement of Fascicles in Muscles
Skeletal muscles – consist of fascicles
Fascicles – arranged in different patterns
Fascicle arrangement – tells about action of a muscle
Types of Fascicle Arrangement
Parallel – fascicles run parallel to the long axis of the muscle
Strap-like – sternocleidomastoid
Fusiform – tapering at each end
Example: biceps brachii
Convergent
Origin of the muscle is broad
Fascicles converge toward the tendon of insertion
Example – pectoralis major
Circular -fascicles are arranged in concentric rings
Surround external body openings
Sphincter – general name for a circular muscle
Pennate
Unipennate – fascicles insert into one side of the tendon
Bipennate – fascicles insert into the tendon from both sides
Multipennate – fascicles insert into one large tendon from all sides
Naming the Skeletal Muscles
Number of origins
Two, three, or four origins
Indicated by the words biceps, triceps, and quadriceps
Action - the action is part of the muscle’s name
Indicates type of muscle movement
Flexor, extensor, adductor, or abductor
Location
Example: the brachialis is located on the arm
Shape
Example: the deltoid is triangular; the trapezius muscle is trapezoid-shaped
Relative size:
Maximus, minimus, longus and brevis indicate size
Example: gluteus maximus and gluteus minimus
Direction of fascicles and muscle fibers
Name tells direction in which fibers run
Example: rectus abdominis and transversus abdominis
Location of attachments – name reveals point of origin and insertion
Example: brachioradialis
Muscle Movements
Flexion
Extension
Hyperextension
Abduction
Adduction
Circumduction
Rotation
Pronation, supination
Angular Movements
Rotational Movements
Special Movements
Foot and ankle
Inversion
Eversion
Dorsiflexion
Plantar flexion
Hand
Opposition of thumb, palm
Head
Protraction, retraction
Depression, elevation (jaw)