1. The Muscular System
Muscles are organs composed of specialized cells that use the chemical energy stored in nutrients to contract.
Muscular actions also:
Provide muscle tone
Propel body fluids and food
Generate the heartbeat
Distribute heat
Three types of muscle tissue:
Skeletal muscle
Smooth muscle
Cardiac muscle

2. Tissues in Skeletal Muscle
A skeletal muscle is an organ of the muscular system
Tissues in a skeletal muscle include:
Skeletal muscle tissue
Nervous tissue
Blood
Other connective tissues

3. Connective Tissue Coverings

4. Fascia Layers of fibrous connective tissue
Separate individual skeletal muscles from adjacent muscles
Hold muscles in position
Surround each muscle and may project beyond its end to form a cordlike tendon
Tendon fibers may intertwine with the bone’s periosteum, attaching the muscle to the bone

5. Aponeurosis Connective tissue that forms broad fibrous sheets
May attach muscle to bone
May attach muscle to the coverings of adjacent muscles.
Examples:
Epicranial aponeurosis (p. 194)
Aponeurosis of external obliques (p. 197)

6. Tendons
Tendonitis
Tendons may become painfully inflamed and swollen following injury or repeated stress of athletic activities
Tendons most commonly affected include the joint capsules of the:
Shoulder
Elbow
Hip
Muscles that move the hand, thigh, and foot

7. Other Connective Tissues
Epimysium
Layer of connective tissue that closely surrounds a skeletal muscle
Perimysium
Layer of connective tissue that extends inward from the epimysium
Separates the muscle tissue into bundles of skeletal muscle called fascicles
Endomysium
Thin layer of connective tissue that encloses each muscle fiber within a fasicle
These layers of connective tissue enclose and separate all parts of a skeletal muscle, allowing parts to move somewhat independently.
Blood vessels and nerves travel through these layers.

8. Other Connective Tissues

9. Skeletal Muscle Fibers
A skeletal muscle fiber
Is a single cell that contracts in response to stimulation and relaxes when the stimulation ends
Is a thin, elongated cylinder with rounded ends
May extend the full length of the muscle
Has a cell membrane called the sarcolemma
Has cytoplasm called the sarcoplasm
Is multinucleated
Contains many threadlike myofibrils that lie parallel to one another
Contain two types of protein filaments in a myofibril:
Myosin – thick filaments
Actin – thin filaments

10. Skeletal Muscle Fibers (cont.)
Within the sarcoplasm of a muscle fiber
Is a network of membranous channels that surrounds each myofibril and runs parallel to it.
Are a network of membranes called the sarcoplasmic reticulum (SR) which corresponds to the endoplasmic reticulum of other cells.
Is another set of membranous channels called transverse tubules (or T tubules) which extend inward as invaginations from the fiber’s membrane and passes all the way through the fiber.
The T tubules lie between two enlarged portions of the SR called cisternae, near the region where the actin and myosin filaments overlap
The SR and T tubules activate the muscle contraction mechanism when the fiber is stimulated.

11. Skeletal Muscle Fibers

12. Striations in Skeletal Muscle
The organization of myosin and actin filament produces a characteristic pattern of alternating light and dark bands or striations of a skeletal muscle fiber.
The striations form a repeating pattern of units called a sarcomere along each muscle fiber.

13. Striation Patterns – 2 main parts
I bands (light bands)
Composed of actin filaments
Directly attached to structures called Z lines
A bands (dark bands)
Composed of myosin (thick) filaments overlapping with actin (thin) filaments
H zone consists of only myosin filaments
M line consists of proteins that hold myosin in place
A sarcomere extends from one Z line to the next.

14. Striation Patterns

15. Muscle Strains
Occur when muscle fibers and their connective tissues are overstretched and tear.
Mild strain:
few muscle fibers injured,
fascia remains intact,
minimal loss of function
Severe strain:
many muscle fibers as well as the fascia tear,
muscle function may be completely lost,
results in pain, discoloration and swelling.

16. Neuromuscular Junctions
Neurons that control effectors (including skeletal muscles) are called motor neurons.
Each skeletal muscle fiber is functionally connected to the axon of a motor neuron that passes outward from the brain or spinal cord
This functional connection is called a synapse.
Neurons communicate with the cells they control by releasing chemicals called neurotransmitters at synapses.
The connection between the motor neuron and the muscle fiber is called a neuromuscular junction.
The muscle fiber membrane is specialized to form a motor end plate where the nuclei and mitochondria are abundant and the sarcolemma is folded.

17. Neuromuscular Junctions

18. Neuromuscular Junctions
The end of the motor neuron branches and projects into recesses of the muscle fiber membrane
When a nerve impulse traveling from the brain or spinal cord reaches the end of a motor neuron axon some of the vesicles release neurotransmitter molecules into the synaptic gap between the neuron and the motor end plate of the muscle fiber
This action stimulates the muscle fiber to contract.

19. Motor Units A muscle fiber usually has only one motor end plate
A motor neuron is highly branched and may connect to many muscle fibers
When a motor neuron transmits an impulse, all of the muscle fibers that are connected will be stimulated simultaneously
A motor unit consists of a motor neuron and all the muscle fibers that are controlled by the motor neuron.

20. Skeletal Muscle Contraction
Myosin molecule
Thick filament
Composed of two twisted protein strands with globular parts called cross-bridges that project outward along the length of the myosin.
Actin molecule
Globular structure with a binding site to which the myosin cross-bridges can attach
Many actin molecules twist into a double strand or helix, forming a thin filament
Proteins troponin and tropomyosin are part of the actin filament

21. Skeletal Muscle Contraction
The sarcomere is the functional unit of skeletal muscles
The contraction of an entire skeletal muscle can be described in terms of the shortening of the sarcomere.
The force that shortens the sarcomere comes from the cross-bridges pulling on the actin filaments
Myosin cross-bridge can attach to an actin binding site and bend slightly, pulling on the actin filament
This can release and repeat

22. Sliding Filament Model
Includes all the events of myosin cross-bridges pulling on the actin filaments
Gets its name from the way the sarcomeres shorten
Thick and thin filaments do not change length, but slide past each other.
Thin filaments slide toward the center of the sarcomere from both ends.
The enzyme ATPase catalyzes the breakdown of ATP to ADP and phosphate to provide energy to put the myosin cross-bridges in a “cocked” position
When a “cocked” cross-bridge binds to actin, it pulls on the actin
After the cross-bridge pulls, another ATP causes the cross-bridge to be released from actin even before ATP splits
This cycle can repeat as long as ATP is available and the muscle is stimulated

23. Sliding Filament Model

24. Sliding Filament Model

25. Stimulus for Contraction
Skeletal muscle fibers normally do not contract unless stimulated by a neurotransmitter.
Main neurotransmitter for skeletal muscles is: acetylcholine
Made in the cytoplasm of the motor neuron
Stored in vesicles at the distal end of the motor neuron axons
When a nerve impulse reaches the end of a motor neuron axon, some of the vesicles release acetylcholine into the space between the motor neuron axon and the motor end plate.

26. Stimulus for Contraction
Acetylcholine diffuses rapidly across the synaptic cleft and binds to specific receptors in the muscle fiber membrane
Permeability to sodium ions (Na+) then increases
The resulting muscle impulse passes in all directions over the surface of the muscle fiber membrane and travels through the T tubules until it reaches the SR
The SR contains a high concentration of calcium (Ca++) ions which diffuses into the sarcoplasm of the muscle fiber.

27. Stimulus for Contraction
When the muscle impulse reaches the SR, the membranes of the cisternae become more permeable to calcium ions
Calcium ions diffuse into the sarcoplasm of the muscle fiber
When there is a high concentration of calcium ions in the sarcoplasm, the troponin and tropomyosin move to expose the binding or active sites on actin
Linkages form between actin and the myosin filaments
The muscle fiber contracts

28. Stimulus for Relaxation
ATP is required for muscle contraction
The contraction lasts as long as acetylcholine is released
Two events lead to muscle relaxation:
Acetylcholine is decomposed by the enzyme acetylcholinesterase
Calcium ions are transported back into the SR

29. Stimulus for Contraction
Two things can happen to the acetylcholine that is released but not used during a muscle impulse:
It can be taken back into the motor neuron and reused (reuptake)
It can be decomposed by the enzyme acetylcholinesterase

30. Botulism Type of food poisoning
Caused by a bacteria called Clostridium botulinum
Anaerobic bacteria found in soil
Produces a neurotoxin that prevents release of acetylcholine at neuromuscular junctions
Usually caused by eating canned or home processed foods that haven’t been heated long enough to kill the bacteria or to inactivate the neurotoxin
Paralyzes muscles including respiratory muscles
Found in “botox” injected into muscles

31. Energy Sources for Contraction
ATP (adenosine triphosphate) supplies the energy for muscle fiber contraction
Muscles only have enough ATP for a short time
When muscle fiber is active, ATP must be regenerated by combining ADP (adenosine diphosphate) and a phosphate group.

32. Energy Sources for Contraction
Creatine phosphate is
A molecule that contains high-energy phosphate bonds
4 – 6 times more abundant in muscle fibers than ATP
Not able to supply energy directly to cells
Able to store excess energy released from mitochondria
Able to transform ADP back into ATP with its energy
Exhausted quickly when muscles are active, so cells must use cellular respiration to synthesize ATP

33. Oxygen Supply & Cellular Respiration
Cellular respiration occurs in the mitochondria and requires oxygen
Blood carries oxygen to body cells bound to hemoglobin
Pigment responsible for the red color of blood
Myoglobin is another pigment
Made in muscle cells
Responsible for reddish-brown color of skeletal muscle tissue
Can combine loosely with oxygen
Reduces muscle’s requirement for a continuous oxygen supply during muscle contraction

34. Oxygen Debt During strenuous exercise:
When oxygen is low, pyruvic acid produces lactic acid that may accumulate in muscles and liver
Available oxygen is used to synthesize ATP instead of converting lactic acid to glucose in the liver
Oxygen debt is the amount of oxygen that liver cells need to convert lactic acid into glucose, plus the amount muscle cells need to restore ATP and creatine phosphate to their pre-exercise concentrations (may take hours).

35. Muscle Fatigue
The loss of ability of a muscle to contract after exercising strenuously for a prolonged period of time
3 main causes:
Interruption of blood supply to the muscle
Lack of acetylcholine in the motor neuron
Accumulation of lactic acid as a result of anaerobic respiration
Most common cause
Lowers muscle pH so that muscle fibers don’t respond to stimulation

36. Cramp
Painful condition in which a muscle undergoes a sustained involuntary contraction
Thought to occur when changes in the fluid surrounding muscle fibers and their motor neurons trigger uncontrolled stimulation of the muscle

37. Rigor Mortis
Partial muscle contraction that occurs several hours after death
May continue for 72 hours or more
Results from an increase in membrane permeability to calcium ions and a decrease in ATP in muscle fibers
Prevents relaxation of muscles
Actin and myosin filaments remain linked until the muscles begin to decompose

38. Steroids Help to increase muscular strength
Hasten adulthood, stunting height and causing early hair loss
In males may lead to smaller genitalia and larger breasts
In females may lead to deepened voice, hairiness, and a male physique
May damage kidneys, liver, & heart and lead to infertility
May cause psychiatric symptoms including delusions, depression, and violence
Used now in medicine to treat wasting associated with AIDS and to reduce inflammation.

39. Heat Production
Less than half the energy released from cellular respiration is used in metabolic processes
Most of the energy is released as heat
Muscle tissue is a major heat source because muscle is such a large proportion of the body’s mass
Blood transports the heat generated in muscles to other tissues to help maintain body temperature

40. Muscular Responses Threshold stimulus
Minimum stimulation needed to cause a muscle fiber to contract
Twitch
Contraction of a single muscle fiber in response to stimulus
Consists of:
Period of contraction in which the fiber pulls at its attachments
Period of relaxation in which the pulling force declines

41. Muscular Responses Latent period
Brief delay between the time of stimulation and the beginning of contraction
In human muscles, may be less than 2 milliseconds
All-or-none response
When a muscle fiber is brought to threshold stimulus, the fiber tends to contract completely
Misleading because in normal muscle use, the force generated by muscles varies

42. Muscular Responses Summation
Increased force of contraction by a skeletal muscle fiber when a twitch occurs before the previous twitch relaxes
Muscle fiber does not completely relax before the next stimulus arrives
Tetanic contraction
Sustained contraction that lacks even partial relaxation

43. Muscular Responses Recruitment
Muscle fibers in a muscle are organized into motor units controlled by a single neuron
Each motor unit is a functional unit, meaning the motor neuron will cause all its muscle fibers to contract at the same time
A whole muscle is made up of many motor units
When a high intensity stimulation causes more motor neurons to respond and more motor units to be activated, it is called recruitment

44. Muscular Responses Sustained contractions
Can be produced as a result of summation and recruitment together
Are a result of responses to a rapid series of impulses transmitted from the brain or spinal cord to motor neurons
Can occur even when a muscle appears to be at rest
Muscle tone or tonus is the result of a sustained muscle contraction and is important in maintaining posture
If a person faints or collapses, muscle tone is lost

45. Use and Disuse of Skeletal Muscles
Hypertrophy
Enlarged, forcefully exercised muscles
Not inherited, developed by individuals
Atrophy
Decrease in muscle size and strength
Results from not using muscles either from choice or due to injury or disease

46. Slow-twitch vs. Fast-twitch Fibers
Muscles contract with lower intensity
Develop more mitochondria
Fatigue resistant
More extensive capillary networks
Activities that develop slow-twitch muscles:
Swimming
Running
Also called red-fibers due to higher myoglobin levels in the cells
Do not twitch when fatigued
Fast-twitch
Muscles are used forcefully
Fatigable
Produce new actin and myosin filaments
Increase diameter of muscle resulting in enlargement of the entire muscle
Activities that develop fast-twitch muscles:
Weight-lifting
Forceful exercise
Also called white fibers due to lower myoglobin levels
Twitch when fatigued

47. Different Types of Muscle Fibers

48. Smooth Muscle (see Table 8.3, p. 190)
Fibers are elongated, with tapering ends
Fibers contain actin and myosin organized differently than in skeletal muscle fibers
Lack striations
Involuntary
Single nucleus per cell
Sarcoplasmic reticulum not well developed

49. Two Types of Smooth Muscle
Multiunit smooth muscle
Muscle fibers separate rather than organized into sheets
Found in the irises of the eyes and in the walls of blood vessels
Usually contract only in response to stimulation by motor nerve impulses or hormones

50. Two Types of Smooth Muscle
Visceral smooth muscle
Composed of sheets of spindle-shaped cells in close contact with one another
Found in the walls of hollow tubular organs like the stomach, intestines, urinary bladder, and uterus
Have rhythmicity, a pattern of repeated contractions, due to self-exciting fibers
Contractions cause a wave-like motion called peristalsis
Occurs in tubular organs
Forces contents to move along their lengths

51. Smooth vs. Skeletal Muscle Contraction
Similarities:
Include reactions of actin and myosin
Triggered by membrane impulses and an increase calcium ions
Use energy from ATP
Differences:
Two neurotransmitters in smooth muscle:
Acetylcholine
Norepinephrine
Is affected by hormones
Is slower than skeletal muscle
Can maintain a forceful contraction longer with a given amount of ATP
Can change length without changing tautness, allowing the stomach to stretch as it fills, but still maintain pressure

52. Cardiac Muscle (see Table 8.3, p. 190)
Found only in the heart
Involuntary
Striated
Branching cells that form a 3-D network
Single nucleus per cell
Have intercalated discs that form junctions between cardiac cells and allow cardiac cells to communicate
Self-exciting
Have rhythmicity

53. Cardiac vs. Skeletal Muscle Contraction
Similarities
Essentially the same mechanism as in skeletal muscle and smooth muscle
Has a system of sarcoplasmic reticulum, many mitochondria, and a system of transverse tubules
Differences
Cisternae less developed but transvers tubules larger
Store less calcium ions but release large numbers of calcium ions into the sarcoplasm
Twitches are longer than in skeletal muscle
Intercalated discs allow muscle impulses to pass freely so that they can travel rapidly from cell to cell
Muscle impulses pass to all networked cells and the whole structure contracts as a functional unit
Self-exciting and rhythmic

54. Inherited Muscle Diseases
Muscular dystrophy
Results from missing proteins called dystrophins that attach actin to the extracellular matrix
Cause muscles to weaken and degenerate
Fat and connective tissue replace muscle tissue
Duchenne’s muscular dystrophy
Affects only males
Symptoms begin by age 5
Usually can’t walk by teenage years
Usually fatal by early adulthood, due to respiratory failure

55. Inherited Muscle Diseases
Charcot-Marie-Tooth Disease
Causes a slowly progressing weakness in the muscles of the hands and feet
Causes a decrease in tendon reflexes in the hands and feet
Caused by an extra gene that impairs the insulating sheath around nerve cells so that they cannot stimulate muscles correctly
Diagnosed by electromyography and nerve conduction velocities, or a test for gene mutation

56. Inherited Muscle Diseases
Myotonic Dystrophy
Delays muscle relaxation following contraction
Causes facial and limb weakness, cataracts, and irregular heartbeat
Caused by inheritance of two “expanding genes” that grow with each generation
As the gene enlarges, symptoms increase in severity or start at an earlier age
So a grandfather may have mild weakness, but his children and grandchildren would have increasingly severe symptoms

57. Inherited Muscle Diseases
Hereditary Idiopathic Dilated Cardiomyopathy
Rare inherited form of heart failure
Usually begins in a person’s forties and is lethal in 50% of cases within 5 years of diagnosis
Treatment: heart transplant
Caused by a tiny genetic error in a form of actin found only in cardiac muscle
Disturbs the actin’s ability to anchor to Z lines in heart muscle cells
Eventually causes heart chambers to enlarge and eventually fail

58. Skeletal Muscle Actions
Variety of body movements possible with bones and muscles depend on:
Type of joint
How the muscle attaches on the other side of the joint
Muscles always work in groups where one group is contracting and the other group is relaxing

59. Origins and Insertions
Origin – the end of a muscle that attaches to the relatively immovable or fixed part of a moveable joint
Insertion - the end of a muscle that is attached to the movable end of a muscle
Muscles may have more than one origin or insertion
For example: biceps brachii
2 origins
Biceps means 2 heads
The head of a muscle is always closest to the origin.
What does triceps mean?

60. Interactions of Skeletal Muscles
Prime mover or agonist
Muscle that provides most of the movement in a muscle
Ex: biceps brachii muscle is prime mover when one flexes the forearm
Synergist
Muscle(s) that contract and assist the prime mover
Ex: brachialis is synergistic to biceps brachii
Antagonists
Muscle(s) that resist a prime mover’s action
Cause movement in the opposite direction
Responsible for smooth body movements as they give way to prime movers
Ex: triceps brachii is antagonistic to biceps brachii
If both the agonist and antagonist contract simultaneously, the part they act upon remains rigid and motionless

61. Interactions of Skeletal Muscles