Muscular system

Purpose of muscles Stabilize joints

Work with tendons to reinforce and stabilize joints that have poorly fitting articulating surfaces

Posture Skeletal muscles

Fibers shorten at different times Movement

Moves bones by pulling on them Primary –muscle that provides the most movement Synergist- muscles that contract and assist the prime mover Antagonist-resist a prime mover’s action and cause

movement in the opposite direction Heat production

Maintain homeostasis By product of muscle activity

Muscle introduction

Three types of musclesskeletalSmoothcardiac

A skeletal muscle is an organ of the muscular systemComposed of skeletal muscle tissue, nervous

tissue, blood, and connective tissue

Smooth muscle

Elongated with tapering ends Lack striations Sarcoplasmic reticulum in not well

developed Two major types of smooth muscle

Multiunit smooth muscleVisceral smooth muscle

Multiunit smooth muscle

Muscle fibers are separate rather than organized into sheets

Found in the irises of the eyes and the walls of the blood vessels

Contracts only in response to stimulation by motor nerve impulses and certain hormones

Visceral smooth muscle Composed of sheets of spindle shaped cells in close

contact with one another More common type of smooth muscle Found in walls of hollow organs, such as the stomach,

intestines, urinary bladder, and uterus Fibers can stimulate each other

When one fiber is stimulated, the impulse moving over its surface may excite adjacent fibers, which in turn stimulate others

Display rhythmicity- a pattern of repeated contractions This is due to self-exciting fibers that deliver spontaneous

impulses periodically into surrounding muscle tissue These two features are largely responsible for

peristalsis- the wave like motion that occurs in certain tubular organs such as the intestines

Smooth muscle contractions

Two neurotransmitters affect smooth muscle Acetylcholine Norepinephrine

Can also be affected by hormones Slower to contract and relax than skeletal

muscle Can maintain forceful contraction longer

Cardiac muscle

Found only in the heart Opposing ends are connected by intercalated

disks Elaborate junctions between cell membranes Help to join cells and to transmit the force of

contraction from cell to cell Allow muscle impulses to pass freely so that they travel

rapidly from cell to cell Responds in an all or none manner

Self-exciting and rhythmic

Skeletal muscle

Voluntary Structure:

Long and cylindricalMultinucleatedStriations

Contract rapidly but tire easily

Structure of skeletal muscle Endomysium- connective tissue sheath that

encloses each muscle fiber Fascicle- bundle of fibers Perimysium- connective tissue that wraps a

fascicle Epimysium- connective tissue that covers the

entire muscle Tendon- strong cordlike structure formed from

epimysia that attaches muscles to bones Aponeuroses- sheet-like structure that connects

muscles indirectly to bones, cartilage, or connective coverings

Microscopic anatomy of skeletal muscle Alternating light (I) bands and dark (A)

bandsThe light I band has a midline

interruption which is a darker area called the z disc

The dark band A has a lighter central area called the H zone

The M line is the center of the H zone Contains tiny protein rods that hold thick

filaments together

Microscopic anatomy cont.

Two types of myofilaments Thick filaments (myosin filaments)

Made mostly of myosin Extend the entire length of the A band Cross bridges- projections off the ends of the myofilaments

that link the thick and thin filaments during contractions Thin filaments (actin filaments)

Composed of actin Anchored to the Z disc Makes up the I band

Microscopic anatomy cont.

When a contraction occurs actin containing filaments slide toward each other into the center of the sarcomere the H zones disappear because myosin and actin filaments are completely overlapped

Sliding filament theory that the head of a myosin cross bridge can

attach to an actin binding site and bend slightlyMyosin molecule is composed of two twisted

protein strands with globular parts called cross-bridges projecting outward along their lengths

This pulls the actin filament with it Then the head can release, straighten, and

combine with another binding site farther down the actin filament and pull again

Skeletal muscle activity

Irritability- the ability to receive and respond to stimulus

Contractibility- the ability to shorten when adequate stimulus is received

Must be stimulated by nerve impulses to contract

One motor neuron may stimulate a few muscle cells to hundreds

One neuron and all the skeletal muscle cells it stimulates are a motor unit

Nerve stimulus and action potential

The extension of the neuron (axon) reaches the muscle it branches into a number of axon terminals

Each forms junctions with the sarcolemma of a different muscle cell Junctions are neuromuscular junctions

The gap between nerve endings and muscle cell membrane is the synaptic cleft

Nerve stimulus and action potential Nerve impulse reaches axon terminals neurotransmitter

is released Acetylcholine (Ach) stimulates skeletal muscles

Ach diffuses across the synaptic cleft and attaches to receptors that are part of the sarcolemma

Sarcolemma becomes temporarily more permeable to Na+ and they rush into the cell while K diffuse out of the cell

The electrical conditions of the membrane are reversed and opens more channels to allow Na+ entry only

This “upset” generates an electrical current called an action potential

Leads to contraction of the muscle cell

Oxygen debt Oxygen is used to synthesize ATP When oxygen is low muscles produce lactic acid

anaerobic respiration As lactic acid accumulates, an oxygen debt is

produced Oxygen debt equals the amount of oxygen required

to convert lactic acid into glucose, plus the amount muscle cells require to restore ATP and creatine phosphate to their original concentrations

The conversion is slow and may take several hours to pay back

Muscle fatigue

Fatigue- the ability to contract after strenuous exercise

Interruption in the muscle’s blood supply or lack of acetylcholine in motor nerve fibers

Usually from accumulation of lactic acidLowers pH causing muscles not to respond to

stimulation

Muscle cramp

Painful condition in which a muscle undergoes a sustained involuntary contraction

Thought to occur when changes in extracellular fluid surrounding the muscle fibers and their motor neurons somehow trigger uncontrolled stimulation of the muscle

Muscle responses

Threshold stimulus- the minimal strength of stimulation to cause a contraction

All-or-none response- a skeletal muscle exposed to stimulus of threshold strength or above responds to its fullest extentDoes not contract partially Increasing the strength of the stimulus does not

affect the fiber’s degree of contraction

Recording muscle contractions Myogram- recording of muscle that is stimulated electrically Twitch- a single contraction that only last a fraction of a

second Muscle is exposed to a single stimulus of sufficient

strength to activate motor units

Latent period- the delay between the time the stimulus is applied and the time the muscle respondsFollowed by period of contraction- when the

muscle pulls at its attachmentsPeriod of relaxation- when it returns to former

length

Summation

A muscle fiber exposed to a series of stimuli of increasing frequency reaches a point when it is unable to completely relax before the next stimulus in the series arrives

Tetanic contraction- When the resulting forceful sustained contraction lacks even partial relaxation

Muscle tone

Certain amounts of sustained contraction occur in the fibers

Is a response to nerve impulses that originate repeatedly from the spinal cord and stimulate a few muscle fibers

Important in maintaining posture If suddenly lost the body collapses

Skeletal muscle actions

Origin- the immovable end of the muscle Insertion- the movable end of the muscle

When a muscle contracts the insertion is pulled towards the origin

Some muscles have more than one origin or insertion

Naming skeletal muscles Direction of the muscle

Named for reference to some imaginary line usually the midline Rectus- fibers run straight Oblique- at an slant

Relative size of the muscleEx. Maximus- largestMinimus- smallestLongus- liong

Location of the musclesNamed with bone associated with

Temporalis and frontalis overlie the frontal and temporal bones

Number of origins Location of muscle’s origin and insertion Shape of the muscle Action of the muscle

Naming skeletal muscles Number of origins

Biceps- two origins, triceps-three etc. Location of muscle’s origin and insertion

Named for attachment sightsEx. Sternocleidomastoid- origins at sternum and

clavicle and inserts on mastoid process Shape of the muscle

Have distinctive shape Deltoid is roughly triangular

Action of the muscleNamed for their actions

Flexor, extensor, and adductor