muscular system. purpose of muscles stabilize joints work with tendons to reinforce and stabilize...
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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
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