1. The Muscular System
Chapter 8

3. Structure of a Skeletal Muscle
Layers of fibrous connective tissue called fascia separate an individual skeletal muscle from adjacent muscles and hold it in position
Fascia may project beyond the end of the muscle forming a tendon

4. Cont.
Epimysium – layer of con. tissue that surrounds a skeletal muscle
Perimysium – extends inward from the epimysium and separates the muscle tissue into small compartments
Fascicles – bundles of skeletal fibers found in compartments
Endomysium – con. tissue that surrounds individual skeletal fibers within fascicles

5. Cont. Skeletal muscle fiber Thin, elongated cylinder with rounded ends
The cytoplasm (sarcoplasm) contains many small, oval nuclei, mitochondria, and myofibrils
Myofibrils contain two types of protein filaments
Myosin (thick)
Actin (thin)

7. Striation pattern
I bands (the light bands) are composed of thin actin filaments directly attached to Z lines
A bands (the dark bands) are composed of thick myosin filaments overlapping thin actin filaments
Sarcomere – segment of a myofibril that extends from one Z line to the next Z line

9. Sarcoplasmic reticulum – membranous channels that surrounds each myofibrl and runs parallel to it
Transverse tubules – extend inward as invaginations from the fiber’s membrane and passes all the way through the fiber; contains extracellular fluid
Both activate the muscle contraction mechanism when the fiber is stimulated

10. Neuromuscular Junction
Each skeletal muscle fiber connects to an axon from a nerve cell, called a motor neuron
The axon extends outward from the brain or spinal cord
The muscle fiber contracts only when the motor neuron stimulates it
The connection between the motor neuron and the muscle fiber is called a neuromuscular junction
Muscle fiber membrane forms a motor end plate

11. The end of the motor neuron branches and projects into recesses of the muscle fiber membrane
Cytoplasm at the distal ends of these motor neuron axons contains synaptic vesicles that store chemicals called neurotransmitters

13. Summary of Muscle Contraction
Nerve impulse travels from brain to the end of a motor neuron axon; synaptic vesicles release a neurotransmitter (acetylcholine) into the synaptic cleft between the neuron and the motor end plate of the muscle fiber stimulating the muscle fiber to contract.

14. Role of Myosin and Actin
Myosin – composed of two twisted protein strands with globular parts called cross-bridges projecting outward along their lengths
Actin – a globular structure with a binding site to which the myosin cross-bridges can attach
Composed of troponin and tropomyosin

15. Sliding filament model
Sarcomeres shorten as myosin cross-bridge attaches to an actin binding site that is exposed by the release of calcium ions and bends slightly, pulling the actin filament
Then the head can release, straighten, combine with another binding site further down the actin filament, and pull again

17. Globular portions of the myosin filaments contain an enzyme, ATPase
ATPase catalyzes the breakdown of ATP to ADP and phosphate, releasing energy
This energy puts the myosin cross-bridge in a “cocked” position
When a cocked cross-bridge binds to actin, it pulls on the thin filament
Filament moves toward the center of the sarcomere, and the sarcomere shortens
Cycle repeats as long as ATP is available and muscle fiber is stimulated to contract

18. Vocabulary Actin Myosin Cross bridges Binding sites Sarcomere
Tropomysium
Troponin
Calcium
Sarcoplasmic reticulum
Propagation
Action potential
Permeability
Channels
Use the above vocabulary and pictures to explain the sliding filament model.

19. Stimulus for Contraction
Acetylcholine – neurotransmitter that stimulates skeletal muscle fibers
When nerve impulses cease, two events lead to muscle relaxation
Acetylcholine is decomposed by acetylcholinesterase
Calcium ions transported back into the sarcoplasmic reticulum

20. Energy Sources for Contraction
Existing ATP molecules
Once used up cells must regenerate ATP from ADP and phosphate
Creatine phosphate contains high energy phosphate bonds
An enzyme in the mitochondria catalyzes the synthesis of creatine phosphate
Stores excess energy released from the mitochondria
Cellular respiration of glucose

21. Oxygen Supply and Cellular Respiration
Glycolysis can take place in the absence of oxygen
More complete breakdown of glucose (cellular respiration) requires oxygen
Oxygen comes from lungs and is carried by rbcs and are attached to hemoglobin
Myoglobin is synthesized in muscle cells and imparts the reddish-brown color of skeletal muscle tissue; temporarily stores oxygen

22. Oxygen Debt
When skeletal muscles are used even for a minute or two, the muscle fibers must increasingly use anaerobic respiration to obtain energy
Pyruvic acid used to regenerate glycolysis, but produces lactic acid
Lactic acid accumulates in the muscles, diffuses into the bloodstream, and reaches the liver
In the liver, reactions occur requiring ATP, synthesize glucose

23. Oxygen debt has to be repaid
May take several hours
Metabolic capacity may change with training (Table 8.2)

24. Muscle Fatigue
A muscle exercised strenuously for a prolonged period may lose its ability to contract, a condition called fatigue
Interruption in the muscle’s blood supply or lack of acetylcholine in motor neuron axons may also cause fatigue
Most often caused from the accumulation of lactic acid which lowers pH and fibers no longer respond to stimulation
Cramps occur when a muscle undergoes a sustained involuntary contraction

25. Heat Production
More than half of the energy released in cellular respiration is heat
Blood transports heat to other tissues, which helps maintain body temperature

26. Muscular Responses
When an isolated muscle fiber is exposed to a series of stimuli of increasing strength, the fiber remains unresponsive until a certain strength of stimulation is applied
Minimal strength is called the threshold stimulus
Skeletal muscle fiber does not respond partially – all-or-none response
Extent of shortening depends on resistance

27. Skeletal muscle removed from an animal can be used to show how a whole muscle responds to stimulation
Muscle is mounted in a device and then stimulated electrically
When the muscle contracts, it pulls on a lever, and its movement is recorded
Resulting pattern is a myogram

28. Twitch - single contraction that lasts only a fraction of a second
Latent period – delay between the time the stimulus was applied and the time the muscle fiber responded
Shorter than 0.01 sec. in humans
Period of contraction – muscle pulls at attachment
Period of relaxation – muscle returns to its former length

30. Summation – process that combines the force of individual twitches
When the resulting forceful, sustained contraction lacks even partial relaxation, it is called a tetanic contraction, or tetanus

32. Electromyography
EMG measurements record the electrical activity associated with muscle contraction, so it is not a direct measure of force

33. Muscle Tone
Def. – muscle undergoes some sustained contraction when it appears to be at rest
Response to nerve impulses that originate repeatedly from the spinal cord and stimulate a few muscle fibers
Important in maintaining posture

35. Smooth Muscle Lack striations
Can change length without changing tautness
Can maintain contraction longer with a given amt. of ATP
Multiunit smooth muscle
Muscle fibers are separate
Found in the irises of the eyes and in walls of blood vessels
Contracts only in response to stimulation by motor nerve impulses or certain hormones

36. Visceral smooth muscle
Composed of sheets of spindle-shaped cells in close contact with one another
Found in the walls of hollow organs
Fibers can stimulate each other
Display rhythmic pattern of repeated contractions
Peristalsis – wavelike motion that occurs in tubular organs and helps force the contents of these organs along their lengths

37. Cardiac Muscle Found only in the heart
Composed of branching, striated cells interconnected
Twitches are longer than skeletal twitches
Intercalated discs allow muscle impulses to pass freely from cell to cell
Self-exciting and rhythmic

38. Skeletal Muscle Actions
Origin – immovable end of a muscle
Insertion – movable end of a muscle
When a muscle contracts, its insertion is moved toward its origin

39. Interaction of Skeletal Muscles
Function in groups
Prime mover (agonist) – muscle responsible for a particular body movement
Synergists – muscles that contract and assist the prime mover
Antagonists – muscles that resist a prime mover’s action and cause movement in the opposite direction

40. Muscle disorders
Muscular dystrophies – inherited diseases; progressive weakness and deterioration; caused by mutation of gene which codes for dystrophin
Charcot-Marie-Tooth disease - comprises a group of disorders that affect peripheral nerves which lie outside the brain and spinal cord and supply the muscles in the limbs
Myotonic dystrophy – form of muscular dystrophy; gradual reduction in strength and control
Hereditary idiopathic dilated cardiomyopathy - the heart becomes weakened and enlarged, and cannot pump blood efficiently

41. Major Skeletal Muscles

42. Doc, what’s wrong with me?
Create a doctor/patient interview about a given muscular disorder
Include symptoms, description, cause, how diagnosis is made, risk factors, treatment, and alternate names
Must have two copies of interview and one prop