1. Muscular system

2. Purpose of muscles Stabilize joints Posture Movement Heat production
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

3. Muscle introduction Three types of muscles
skeletal
Smooth
cardiac
A skeletal muscle is an organ of the muscular system
Composed of skeletal muscle tissue, nervous tissue, blood, and connective tissue

4. Smooth muscle Elongated with tapering ends Lack striations
Sarcoplasmic reticulum in not well developed
Two major types of smooth muscle
Multiunit smooth muscle
Visceral smooth muscle
Sarcoplasmic reticulum (SR)
Specialized smooth ER
Surrounds each and every myofibril
Store calcium and release it on demand when muscle fiber is stimulated

5. 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

6. 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

7. 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

8. 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

9. Skeletal muscle Voluntary Structure: Contract rapidly but tire easily
Long and cylindrical
Multinucleated
Striations
Contract rapidly but tire easily

10. 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

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

14. 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

15. 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

16. Sliding filament theory
that the head of a myosin cross bridge can attach to an actin binding site and bend slightly
Myosin 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

17. 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

18. 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

20. 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

21. 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

22. 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 acid
Lowers pH causing muscles not to respond to stimulation

23. 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

24. 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 extent
Does not contract partially
Increasing the strength of the stimulus does not affect the fiber’s degree of contraction

25. 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 responds
Followed by period of contraction- when the muscle pulls at its attachments
Period of relaxation- when it returns to former length

26. 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

27. 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

28. 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

29. 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 muscle
Ex. Maximus- largest
Minimus- smallest
Longus- liong
Location of the muscles
Named 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

30. Naming skeletal muscles
Number of origins
Biceps- two origins, triceps-three etc.
Location of muscle’s origin and insertion
Named for attachment sights
Ex. 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 muscle
Named for their actions
Flexor, extensor, and adductor