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
Contract and pull on bones to move the body
Help propel blood and lymph through vessels
Move food through alimentary canal
Move wastes (urine, feces) out of the body
Uterine muscles birth babies
Move eyes
Move air in and out of lungs
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 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 of Muscle Tissue
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 - a cordlike attachment of muscles to bone (usually), or sometimes to skin, cartilage, or
sheets of fascia. Most muscles attach by tendons.
Aponeuroses - flat sheet that attaches muscles to bones (usually) or sometimes to skin,
cartilage, or sheets of fascia.
Bone markings present where tendons meet bones:
Tubercles, trochanters, and crests
Connective Tissue And Fascicles
CT sheaths bind a skeletal muscle and its fibers together
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
Make up 80% of the cytoplasm
Are a specialized contractile organelle found in muscle tissue
A long row of repeating segments called sarcomeres (functional unit of skeletal muscle tissue)
Sarcomere
Basic unit of contraction of skeletal muscle:
Z disc (Z line) – boundaries of each sarcomere
Thin (actin) filaments – extend from Z disc toward the center of the sarcomere
Thick (myosin) 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 – in center of H zone, contains tiny rods that hold thick filaments together
I band – region with only thin filaments, lies within two adjacent sarcomeres
Sarcoplasmic Reticulum and T Tubules
The Sarcoplasmic Reticulum (SR) is a specialized smooth ER
The T Tubules are interconnecting tubules surrounding each myofibril
Some tubules form cross-channels called terminal cisternae
Cisternae occur in pairs on either side of a t-tubule
SR Contains calcium ions (Ca2+) – released when muscle is stimulated to contract
The Contraction Process
Muscle contraction is controlled by nerve-generated impulse (action potentials)
The nerve impulse is propagated along (across) the sarcolemma and travels down through the T tubules causing
calcium channels in the SR to open and releasing calcium into the sarcoplasm (where the myofilaments are)
Ca2+ ions diffuse through cytoplasm, triggering the sliding filament mechanism
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 form 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
Types of Skeletal Muscle Fibers
Skeletal muscle fibers are categorized according to
How they manufacture energy (ATP)
How quickly they contract
Speed of contraction – determined by how fast their myosin ATPases
split ATP
Oxidative fibers – use aerobic pathways
Glycolytic fibers – use anaerobic glycolysis
Divided into 3 classes:
Slow oxidative fibers (Type I)
Red Slow twitch
Fast oxidative fibers (Type IIa)
Intermediate fibers
Fast glycolytic fibers (Type IIb)
White fast-twitch
Types of Skeletal Muscle Fibers
Slow oxidative fibers (Type I)
Red color due to abundant myoglobin (containing iron and oxygen)
Depend on anaerobic pathways (oxygen dependent)
Contain many mitochondria
Richly supplied with capillaries
Contract slowly and resistant fatigue
Fibers are small in diameter
Types of Skeletal Muscle Fibers
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
Types of Skeletal Muscle Fibers
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
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
Lever Systems:
Bone-Muscle Relationships
Levers allow a given effort to
Move a heavier load, moves a large
load over small distances
Move a load farther, allows a load
to be moved over a large distance
Movement of skeletal muscles involves
leverage
Lever – a rigid bar that moves
(bone)
Fulcrum – a fixed point (joint)
Effort – applied force (muscle contraction)
Load – resistance (anything lifted, and weight of tissues)
Lever Systems:
Bone-Muscle Relationships
First-class lever
Effort applied at one end
Load is at the opposite end
Fulcrum is located between load and effort
Lever Systems:
Bone-Muscle Relationships
Second-class lever
Effort applied at one end
Fulcrum is at the opposite end
Load is between the effort and fulcrum
Third-class lever
Effort is applied between the load
and the fulcrum
Work speedily
Always at a mechanical
disadvantage
Lever Systems:
Bone-Muscle Relationships
Most skeletal muscles are third-class levers
Example – biceps brachii
Fulcrum – the elbow joint
Force – exerted on the proximal region of the radius
Load – the distal part of the forearm
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
Muscles Classified into Several Functional Groups
Prime mover (agonist)
Has major responsibility for a certain movement
Antagonist
Opposes or reverses a movement
Synergist – assists the prime mover
By adding extra force
By reducing undesirable movements
Fixator - a type of synergist that holds a bone firmly in place
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)