the muscular system (rev 3-10)

Muscle Lecture-BIO 006 Handout 1

The Muscular System (rev 3-10)

Muscular tissue enables the body and its parts to move

• Movement is caused by ability of muscle cells (called fibers) to shorten or contract

– Function of muscle tissue is to contract (or shorten)

• Muscle cells use energy to produce movement

• We have 3 types of muscle tissue in our bodies


Skeletal muscle —also called striated or voluntary muscle– This type muscle has stripes or striations– Contractions can be voluntarily controlled


• Cardiac muscle —composes bulk of heart– Striated muscle; involuntary (contractions

not under voluntary control)– Characterized by unique dark bands

called intercalated disks– Cells branch frequently– Interconnected nature of cardiac muscle

cells allows heart to contract efficiently as a unit


• Smooth or visceral muscle– Nonstriated, involuntary muscle– Lacks cross stripes or striations when seen

under a microscope; appears smooth– Found in walls of hollow visceral structures

such as digestive tract, blood vessels, and ureters

– Contractions not under voluntary control; movement caused by contractions is involuntary


Structure of Skeletal Muscle• Each skeletal muscle is an organ composed

of skeletal muscle cells and connective tissue– Most skeletal muscles extend from one

bone across a joint to another bone• Parts of a skeletal muscle

– Origin—attachment to the bone that remains relatively stationary or fixed when movement at the joint occurs

– Insertion—point of attachment to the bone that moves when a muscle contracts

– Body or Belly—main part of the muscle


– Muscles attach to the bone by tendons which are made of strong fibrous connective tissue

– Some tendons are enclosed in synovial-lined tubes, called tendon sheaths, and are lubricated by synovial fluid

--Bursae—small synovial-lined sacs containing a small amount of synovial fluid; located between some tendons and underlying bones

– Both tendon sheaths and bursae make it easier for a tendon to slide over bone


Microscopic structure• Specialized contractile cells called fibers are

grouped into bundles– Each skeletal muscle fiber contains thick

myofilaments (formed from the protein myosin) and thin myofilaments (formed from the protein actin)

– These myofilaments make the striations or stripes we can see in skeletal muscle fibers

– The basic functional (contractile) unit of the muscle cell is called a sarcomere. Sarcomeres are separated from each other by dark bands called Z lines


• Sliding filament model explains mechanism of contraction– Thick and thin myofilaments slide toward

each other as a muscle contracts and shorten the muscle. This is called the sliding filament model.• Contraction requires calcium and ATP

Functions of Skeletal Muscle:1.Movement2.Posture or muscle tone3.Heat production


• Movement– Muscles produce movement; as a muscle

contracts, it pulls the insertion bone closer to the origin bone

– Movement occurs at the joint between the origin and the insertion


– Groups of muscles usually contract to produce a single movement• Prime mover —muscle whose

contraction is mainly responsible for producing a movement

• Synergist —muscle whose contractions help the prime mover produce a movement

• Antagonist —muscle whose actions oppose the action of a prime mover in a movement


Types of Movements Produced by Skeletal Muscle Contractions

• Flexion—movement that decreases the angle between two bones at their joint: bending

• Extension—movement that increases the angle between two bones at their joint: straightening

• Abduction--movement of a part away from the midline of the body

• Adduction--movement of a part toward the midline of the body

• Rotation—movement around a longitudinal axis


• Supination--hand position with the palm turned to the anterior position

• Pronation—hand position with the palm turned to the posterior position

• Dorsiflexion--elevation of the dorsum or top of the foot and

• Plantar flexion--the bottom of the foot is directed downward


Posture– A specialized type of muscle contraction,

called tonic contraction, enables us to maintain body position

• In a tonic contraction, only a few of a muscle’s fibers shorten at one time

• Tonic contractions produce no movement of body parts but DO hold muscles in position

– Tonic contractions maintain muscle tone called posture


Heat production– Survival depends on the body’s ability to maintain

a constant body temperature• Fever—an elevated body temperature; often a

sign of illness• Hypothermia—a reduced body temperature

– Contraction of muscle fibers produces most of the heat required to maintain normal body temperature

• Energy for muscle contraction is obtained from ATP. Some of the energy is lost as heat and this is what maintains our body temperature


Fatigue• Causes reduced strength of muscle

contraction due to lack of sufficient rest• Caused by repeated muscle stimulation• Repeated muscular contraction uses up

the cellular ATP store and exceeds the ability of the blood supply to replenish oxygen and nutrients


• Contraction in the absence of adequate oxygen produces lactic acid, which contributes to muscle soreness

• Oxygen debt —term used to describe the metabolic effort to remove the excess lactic acid that may accumulate during prolonged periods of exercise; the body is attempting to return the cells’ energy and oxygen reserves to pre-exercise levels

• Heavy breathing after exercises helps to return level of oxygen in blood to normal levels


Motor Unit• Stimulation of a muscle by a nerve impulse is

required before a muscle can produce movement

• A motor neuron is the nerve that transmits an impulse to a muscle, causing contraction

• A neuromuscular junction is the specialized point of contact between a nerve ending and the muscle fiber it innervates

• A motor unit is the combination of a motor neuron with the muscle cell or cells it innervates


Muscle Stimulus• A muscle will contract only if the

stimulus reaches a certain level of intensity– A threshold stimulus is the minimal level

of stimulation required to cause a muscle fiber to contract

• Once stimulated by a threshold stimulus, a muscle fiber will contract completely; this is called an all or none response


• Different muscle fibers in a muscle are controlled by different motor units having different threshold-stimulus levels– Although individual muscle fibers always

respond all or none to a threshold stimulus, the muscle as a whole does not

– Different motor units responding to different threshold stimuli permit a muscle as a whole to execute contractions of graded force


Types of Skeletal Muscle Contraction• Isotonic contractions

Contraction of a muscle that produces movement at a joint

During isotonic contractions, the muscle changes length, causing the insertion end of the muscle to move relative to the point of origin

Most types of body movements such as walking and running are caused by isotonic contractions


Types of Skeletal Muscle Contraction

• Isometric contractions Isometric contractions are muscle contractions that

do not produce movement Although no movement occurs, tension within the

muscle increases


Selected Skeletal Muscle GroupsMuscles of the head and neckMuscles of the head and neck Facial muscles

Orbicularis oculi—closes your eye Orbicularis oris—puckers your lips (kissing muscle) Zygomaticus—raises the corners of the mouth

(smiling muscle) Temporalis—chewing muscle; closes jaw Masseter—chewing muscle; closes jaw

Neck and Shoulder muscles Sternocleidomastoid—flexes head Trapezius—elevates shoulders and extends head


• Muscles that move the upper extremities Pectoralis major—flexes upper arm; hugging

muscle Latissimus dorsi—extends upper arm Deltoid—abducts upper arm Biceps brachii—flexes forearm Triceps brachii—extends forearm


Muscles of the trunk Abdominal muscles

Rectus abdominis—flexes trunk• Respiratory muscles

Intercostal musclesDiaphragm


Muscles that move the lower extremities Iliopsoas—flexes thigh Gluteus maximus—extends thigh Hamstring muscles—flex lower leg

SemimembranosusSemitendinosusBiceps femoris


• Muscles that move the lower extremities Quadriceps femoris group—extend lower

legRectus femoris3 Vastus muscles

Tibialis anterior—dorsiflexes foot Gastrocnemius—plantar flexes foot

the muscular system (rev 3-10)

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