1. Muscular System
Unit 5

2. Introduction
The muscular system works with two other systems to generate movement. The nervous system provides the electrical impulse that stimulates the movement and the skeletal system provides a framework for muscles to attach to and pull against. When all three systems work together, we gain the ability to move.
The essential function of muscle is contraction, or shortening.

3. Muscle Types Three types:
Skeletal
Cardiac
Smooth
Differ in cell structure, body location, and how they are stimulated to contract
All muscle cells are elongated  all muscles are called muscle fibers
All muscle cells use microfilaments to contract

4. Skeletal Muscles
Skeletal muscle fibers are cigar-shaped, multinucleate cells and the largest of the muscle fiber types – some can be over 1 ft in length
It is striated muscle because its fibers appear to be striped.
It is voluntary muscle because it is the only muscle type subject to conscious control, although the muscles can also be activated by reflexes.
Individual muscle fibers are soft and fragile on their own, but strong and powerful when they are bundled together.

5. Skeletal Muscles
Each muscle fiber is enclosed in a delicate connective tissue sheath called an endomysium.
Several sheathed muscle fibers are then wrapped by a coarser fibrous membrane called a perimysium to form a bundle of fibers called a fascicle.
Multiple fascicles are bound together by an even tougher sheath called epimysium, which covers the entire muscle.

6. Skeletal Muscles
Epymysia blend into the tendons (connect muscle to bone) or into aponeuroses, which attach muscles indirectly to bones, cartilages, or connective tissue coverings of each other.
Tendons are mostly tough collagenic fibers and are smaller in size than muscles, which mean more can pass over a joint.

7. Smooth Muscles No striations Involuntary control
Found mainly in the walls of hollow visceral organs such as the stomach, urinary bladder, and respiratory passages, among others.
Propels substances along a definite tract (pathway) within the body.
Smooth muscle cells are spindle shaped with single nucleus, arranged into sheets or layers.
There are usually two layers of smooth muscle, one running circularly and one running longitudinally, that alternately contract and relax, changing the size and shape of the organ.

8. Cardiac Muscles Striated Involuntary control
Found in only one place in the body – the heart
Cushioned by small amounts of soft connective tissue and arranged in spiral or figure-8 shaped bundles
Cardiac muscle fibers are branching cells joined by intercalated discs

9. Muscle Functions 1. Produces movement 2. Maintains posture
3. Stabilizes joints
4. Generates heat

10. Producing Movement Result of muscle contraction
Mobility of body reflects the action of skeletal muscles
Enable us to respond to environmental changes quickly
Emotional expression  facial muscles
Substances are moved through the body by smooth and cardiac muscles

11. Maintaining Posture Muscles function continuously
Make small adjustments one after the other
We can maintain a standing or sitting posture despite the downward pull of gravity

12. Stabilizing Joints Help keep bones in place
Especially important for poorly fitting articulating surfaces, such as the shoulder joint

13. Generating Heat By-product of muscle activity
ATP is used to power muscle contractions, but nearly ¾ of its energy escapes as heat
Skeletal muscle produces most heat

14. Microscopic Anatomy of Skeletal Muscle
Multinucleate
Sarcolemma = plasma membrane (“muscle husk”)
Myofibrils = ribbonlike organelles that push the nuclei aside, have alternating light (I) and dark (A) bands
I band has a midline (darker area) called the Z disc
A band has a midline (lighter area) called the H zone
Banding pattern reveals the working structure of myofibrils
Myofibrils are actually chains of tiny contractile units called sarcomeres lined up end to end like cars on a train along the length of the myofibrils

15. Microscopic Anatomy of Skeletal Muscle
Within the sarcomeres are myofilaments  produce the banding pattern
Two types of myofilaments:
Thick filaments = myosin filaments, split ATP to get energy, extend entire length of dark A band, have small projections called myosin heads or cross bridges that link to thin filaments
Thin filaments = actin filaments, contractile protein, anchored to Z disc
H zone lacks actin  looks lighter, “bare zone,” disappears during contractions
Sarcoplasmic reticulum (SR) = smooth endoplasmic reticulum, surrounds each myofibril, stores calcium to release “on demand”

17. Skeletal Muscle Activity
Irritability = ability to receive and respond to a stimulus
Contractility = ability to shorten (forcibly) when an adequate stimulus is received
Skeletal muscles must be stimulated by nerve impulses to contract
One motor neuron may stimulate a few muscle cells or hundreds
Motor unit = one neuron and all the skeletal muscle cells it stimulates

18. Skeletal Muscle Activity
Nerve fiber (axon) reaches the muscle and branches out into several axonal terminals, each of which forms a junction with the sarcolemma of a different muscle cell  neuromuscular junctions
The nerve endings and muscle cell membranes are very close, but they never touch
Synaptic cleft = space between nerve ending and muscle cell membrane, filled with interstitial fluid
Neurotransmitter = chemical released when a nerve impulse reaches the axonal terminals, for muscle cells this is acetylcholine (ACh)

19. Skeletal Muscle Activity
ACh diffuses across the synaptic cleft and attaches to receptors in sarcolemma
If enough Ach is released, the sarcolemma becomes temporarily permeable to sodium ions (Na+), which rush into the muscle cell
The interior of the cell now has an excess of positive ions, which generate an unstoppable electrical current called an action potential that travels over the entire surface of the sarcolemma
The result is the contraction of the muscle cell

20. Sliding Filament Theory
Cross bridges attach to myosin binding sites on the thin filaments
Energy from ATP allows cross bridges to attach and detach several times during each contraction, pulling the thin filaments toward the center of the sarcomere
Occurs simultaneously in sarcomeres throughout the cell, causing cell to shorten
Requires calcium ions stored in SR
Ach is broken down  a single nerve impulse = one contraction, muscle relaxes unless more signals are sent

21. Contraction of a Skeletal Muscle as a Whole
The “all-or-nothing” nature of contraction only applies to the individual muscle cell, not to the muscle as a whole
Graded response = different degrees of contraction
Graded responses can be produced in two ways:
1. Changing the frequency of muscle stimulation
2. Changing the number of muscle cells being stimulated

22. Contraction of a Skeletal Muscle as a Whole
Muscle twitches = single, brief, jerky contractions, sometimes result from problems in the nervous system
Usually, nervous impulses are delivered so quickly that the cells do not get a chance to relax completely between stimuli.
Contractions get “added” together and become stronger and smoother  muscle is in fused or complete tetanus (tetanic contraction)
Not to be confused with pathological condition
Until muscle reaches this state, it is exhibiting unfused or incomplete tetanus

23. Contraction of a Skeletal Muscle as a Whole
Primary role of tetanus is to produce smooth and prolonged muscle contractions.
How forcefully a muscle contracts depends largely on how many muscle cells are stimulated.
Only a few cells stimulated  contraction will be slight
Strongest contractions = all muscle cells are stimulated

24. Providing Energy for Muscle Contraction
Muscles only store about 4-6 seconds worth of ATP  needs to be regenerated for contraction to continue  3 main pathways
1. Direct phosphorylation of ADP by creatine phosphate (CP)  CP transfers a high-energy phosphate group to ADP, making it ATP. This supply is exhausted in about 20 seconds.
2. Aerobic respiration  During rest and light exercise, 95% of ATP comes from aerobic respiration, occurs in mitochondria, glucose is broken down into CO2 and water while the released energy is captured by ATP, fairly slow and requires constant oxygen
3. Anaerobic glycolysis and lactic acid formation  No oxygen is used, glucose is broken down into pyruvic acid while small amounts of energy are captured by ATP. If previous two pathways cannot keep up with needs of muscles, pyruvic acid becomes lactic acid. This provides enough energy for seconds of strenuous muscle activity.

25. Muscle Fatigue and Oxygen Debt
Muscle fatigue = muscle is unable to contract even though it is still being stimulated
If a muscle does not have rest, it begins to tire and contract more weakly until it finally stops reacting and contracting
Oxygen debt = occurs when a person is unable to take in enough oxygen to supply the energy needed for muscle contractions
Muscle activity is dependent on the blood supply and delivery of oxygen

26. Types of Muscle Contractions
Isotonic contractions = “same tone,” myofilaments are successful in their sliding movements, movement occurs
Isometric contractions = “same measurement,” myofilaments are not successful in their sliding motions, movement is “blocked,” tension in muscle keeps increasing

27. Muscle Tone
Sometimes, even when a muscle is voluntarily relaxed, some of its fibers are contracting
Contractions are not visible, but result in firm, healthy muscle
Muscle tone = state of continuous partial contractions

28. Effect of Exercise on Muscles
Muscle inactivity, which can be due to any number of things, such as loss of nerve supply, immobilization, etc., results in muscle weakness and wasting.
Regular exercise increases muscle size, strength, and endurance.
Aerobic or endurance exercises = result in stronger, more flexible muscles with greater resistance to fatigue, increased blood supply to muscle, more mitochondria per muscle cell, increases metabolism and coordination
Example: jogging, biking

29. Effect of Exercise on Muscle
Resistance exercises = based on isometric contractions, muscles are pitted against an immovable (or difficult to move) object, results in increased muscle size due to enlargement of individual muscle cells
Example: lifting weights

30. 5 Golden Rules of Skeletal Muscle Activity
1. All muscles cross at least one joint.
2. Typically, the bulk of the muscle lies proximal to the joint crossed.
3. All muscles have at least two attachments; the origin (attached to immovable or less movable bone) and the insertion (attached to moveable bone).
4. Muscles can only pull; they never push.
5. During contraction, the muscle insertion moves toward the origin.

31. Types of Body Movements
Flexion = movement that decreases the angle of the joint and brings two bones closer together
Extension = movement that increases angle of the joint and moves two bones further apart
Hyperextension = extension greater than 180 degrees
Rotation = movement of a bone around its longitudinal axis

32. Types of Body Movements
Abduction = moving a limb away from the midline of the body, also applies to the fanning of fingers and toes
Adduction = moving a limb toward the midline of the body
Circumduction = combination of flexion, extension, abduction, and adduction, commonly seen in ball-and- socket joints. The proximal end of the limb is stationary while the distal end moves in a circle

33. Types of Body Movements
Dorsiflexion and plantar flexion – up and down movements of the foot at the ankle
Inversion – turning the sole of the foot medially
Eversion – turning the sole of the foot laterally

34. Types of Body Movements
Supination – “turning backward,” forearm rotates laterally so that the palm faces anteriorly, radius and ulna are parallel
Pronation – “turning forward,” forearm rotates medially so that palm faces posteriorly, radius comes across ulna, forming an X
Opposition – thumb moves to touch tips of the other fingers in the same hand

35. Types of Muscles
Prime mover – muscle that has the major responsibility for causing a particular movement
Antagonists – muscles that oppose or reverse a movement
Synergists – help prime movers by producing the same movement or by reducing undesirable movements
Fixators – specialized synergists, hold a bone still or stabilize the origin of a prime mover so all the tension goes to moving the insertion bone

36. Naming Skeletal Muscles
Direction of the muscle fibers – reference to imaginary line, such as midline of body or long axis of a limb bone. Rectus = parallel to the line, oblique = slanted to the line
Relative size of muscle – maximus = largest, minimus = smallest, longus = long
Location of muscle – some muscles are named for the bone with which they are associated
Number of origins – biceps = two origins, triceps = three origins, quadriceps = four origins

37. Naming Skeletal Muscles
Location of the muscle’s origin and insertion – muscles are named for their attachment sites
Shape of the muscle – some muscles have a distinctive shape that helps identify them, deltoid = triangular
Action of the muscle – terms like flexor, extensor, and adductor are added to the name

38. Gross anatomy of skeletal muscles

39. Head and Neck Muscles
Frontalis – covers the frontal bone as it runs from the cranial aponeruosis to the skin of the eyebrows, where it inserts. Allows you to raise your eyebrows and wrinkle your forehead. At the posterior end of the cranial aponeurosis is the small occipitalis muscle, which covers the posterior aspect of the skull and pulls the scalp posteriorly
Orbicularis oculi – fibers that run in circles around the eyes. Allows you to close your eyes, squint, blink, and wink.

40. Head and Neck Muscles
Orbicularis oris – fibers that run in circles around the lips, the “kissing” muscle, closes mouth, protrudes lips
Buccinator – fleshy muscle that runs horizontally across the cheek inserts into the orbicular oris. It flattens the cheek (as in blowing a trumpet) and is used to chew food.
Zygomaticus – extends from corner of mouth to the cheek, the “smiling” muscle

41. Chewing Muscles
Masseter – covers the angle of the lower jaw, runs from the zygomatic process of the temporal bone to the mandible, closes the jaw by raising the mandible
Temporalis – fan-shaped muscle overlying the temporal bone, inserts into the mandible, acts as a synergist to the masseter in closing the jaw

42. Neck Muscles
Platysma – single, sheetlike muscle that covers anterolateral neck, originates from connective tissue covering of chest muscles and inserts into the area around the mouth, pulls down corners of the mouth
Sternocleidomastoid – paired muscles, one on each side of neck, two-headed, one head arises from the sternum, one from the clavicle. The heads fuse before inserting into the mastoid process of the temporal bone. Called the “prayer” muscle, helps flex the neck and bow the head.

43. Trunk Muscles – Anterior Muscles
Pectoralis major – large, fan-shaped muscle covering the upper part of the chest, originates from the shoulder girdle and first six ribs, inserts into proximal end of humerus, forms the anterior wall of the axilla and adducts and flexes the arm

44. Trunk Muscles – Anterior Muscles
Intercostal muscles – deep muscles found between the ribs, external intercostals are important for breathing because they raise the ribcage (inhaling), internal intercostals depress the rib cage (exhaling)

45. Trunk Muscles – Anterior Muscles
Rectus abdominus – paired straplike muscles, most superficial muscles of abdomen, run from pubis to rib cage, flex the vertebral column, compress the abdominal contents during defecation and childbirth

46. Trunk Muscles – Anterior Muscles
External oblique – paired superficial muscles that make up the lateral walls of the abdomen, fibers run downward and medially from the last eight ribs and insert into the ilium, flex vertebral column, rotate the trunk and bend it laterally

47. Trunk Muscles – Anterior Muscles
Internal oblique – paired muscles deep to the external obliques, fibers run at right angles to those of external obliques, originate from iliac crest and insert into the last three ribs, functions the same as external obliques

48. Trunk Muscles – Anterior Muscles
Transverse abdominus – deepest muscle of the abdominal wall, fibers run horizontally across the abdomen, arises from lower ribs and iliac crest and inserts into the pubis, compresses the abdominal contents

49. Trunk Muscles – Posterior Muscles
Trapezius – most superficial muscles of the posterior neck and upper trunk, form a diamond or kite-shaped muscle, broad origin, runs from occipital bone of skull to the end of the thoracic vertebrae, flares laterally to scapular spine and clavicle, extend the head, elevate, depress, adduct, and stabilize the scapula

50. Trunk Muscles – Posterior Muscles
Latissimus dorsi – large, flat muscle that covers the lower back, originates on the lower spine and ilium and then sweeps superiorly to insert into the proximal end of the humerus, extends and adducts the humerus

51. Trunk Muscles – Posterior Muscles
Erector spinae – prime mover of back extension, composite muscle consisting of three muscle colums (longissimus, iliocostalis, and spinalis), provides resistance that helps control the action of bending at the waist, will spasm after back injuries, common source of back pain

52. Trunk Muscles – Posterior Muscles
Deltoid – fleshy, triangle-shaped muscles that form the rounded shape of shoulders, bulkiness makes them a good injection site, origin at shoulder girdle from spine of scapula to clavicle, inserts into proximal humerus, prime movers of arm abduction

53. Muscles of the Upper Limb
Three groups:
Muscles that arise from shoulder girdle and cross shoulder joint to insert into humerus (pectoralis major, latissimus dorsi, deltoid)
Muscles that cause movement at the elbow joint, enclose humerus and insert into forearm bones
Muscles of forearm that insert into hand bones

54. Muscles of the Humerus That Act on the Forearm
Biceps brachii – bulges when elbow is flexed, originates from two heads on the shoulder girdle, inserts into radial tuberosity, prime mover for flexion of the forearm and helps supinate the forearm
Brachialis – lies deep to biceps, helps flex elbow

55. Muscles of the Humerus That Act on the Forearm
Brachioradialis – fairly weak muscle, originates from humerus, inserts into distal forearm
Triceps brachii – only muscle fleshing out posterior humerus, three heads, originates from shoulder girdle and proximal humerus, inserts into olecranon process of ulna, antagonist of biceps brachii, the “boxer’s” muscle because it allows the arm to punch

56. Muscles of the Lower Limb
Cause movement at hip, knee, and foot joints
Largest, strongest muscles in body
Many span two joints and can cause movement in both, therefore, the terms origin and insertion can be interchangeable

57. Muscles Causing Movement at the Hip Joint
Gluteus maximus – superficial muscle, forms flesh of buttock, powerful hip extensor, helps bring thigh in a straight line with pelvis, originates from sacrum and iliac bones and inserts on the gluteal tuberosity of the femur
Gluteus medius – lies under gluteus maximus, runs from ilium to femur, adducts hip, very fleshy, good injection site

58. Muscles Causing Movement at the Hip Joint
Iliopsoas – fused muscle composed of two muslces: the iliacus and the psoas major, runs from iliac bone and lower vertebrae deep inside the pelvis to the lesser trochanter of femur, prime mover of hip flexion, keeps upper body from falling backward
Adductor muscles – form muscle mass on medial side of thigh, press thighs together, become flabby very easily since gravity does most of their work for them, run from pelvis to proximal aspect of femur

59. Muscles Causing Movement at the Knee Joint
Hamstring group – form muscle mass of the posterior thigh, formed from three muscles: biceps femoris, semimembranosus, and semitendinosus, originate on ischial tuberosity, insert on both sides of proximal tibia, name comes from butchers using the tendons to hang hams

60. Muscles Causing Movement at the Knee Joint
Sartorius – thin, straplike, most superficial muscle of thigh, runs obliquely across thigh from anterior iliac crest to medial side of tibia, weak thigh flexor, the “tailor’s” muscle, synergist to cause cross-legged position
Quadriceps group – four muscles: rectus femorus and three vastus muscles, flesh out anterior thigh, originates from femur and pelvis, insert into tibial tuberosity, extends knee and flexes hip

61. Muscles Causing Movement at the Ankle and Foot
Tibialis anterior – superficial muscle on anterior leg, runs from upper tibia to tarsal bones, acts to dorsiflex and invert foot
Extensor digitorum longus – lateral to tibialis anterior, runs from lateral tibial condyle to phalanges, prime mover of toe extension and dorsiflexion of foot
Fibularis muscles – three muscles: longus, brevis, and tertius, found in lateral part of leg, run from fibula to metatarsals, plantar flex and evert foot

62. Muscles Causing Movement at the Ankle and Foot
Gastrocnemius – forms curved calf of posterior leg, two- headed, each head originating from each side of distal femur, inserts into heel of foot through Achilles tendon, prime mover for plantar flexion, “toe dancer’s” muscle
Soleus – deep to gastrocnemius, originates on tibia, inserts in heel, strong plantar flexor of foot

63. Muscular System Disorders
Muscular dystrophy = muscle destroying diseases, muscles enlarge due to fat and connective tissue deposits, muscle fibers degenerate and atrophy
Myasthenia gravis = disease characterized by drooping of upper eyelids, difficulty swallowing and talking, and general weakness and fatigue due to a shortage of ACh receptors at neuromuscular junctions, death usually due to respiratory failure