THE MUSCULAR SYSTEM

 

I.                  Types and Functions of Muscles

 

A.     Contractile cells, called fibers (well, really skeletal muscle normally)

 

B.   Types of Muscles

 

1.   Smooth

a.   Uninucleated

b.    Non-striated

c.    Involuntary

d.    Spindle shaped cells, usually found in parallel lines forming sheets

e.   Found in walls of hollow organs (also called visceral)

f.      Slow contractions but can sustain – doesn’t fatigue easily

 

2.   Cardiac

a.   Uninucleated

b.    Striated

c.    Involuntary

(1)           Auto-rhythmic – doesn’t require nervous system stimulation to contract

d.    Cylindrical and branched cells (fibers)

(1)           Joined by intercalated disks – allow passage of electrical activity throughout muscle

e.   Relax completely between contractions, avoids fatigue

 

3.   Skeletal

a.   Multinucleated

b.    Striated

c.    Voluntary

d.    Cylindrical fibers are long – run the length of the muscle

e.   Can contract multiple times between relaxations, can fatigue (use up all the available ATP)

 

C.   Functions of Muscles

 

1.   Stabilize joints – tendons may extend across joints

 

2.   Maintains posture, which also takes strain off bones and joints

a.   “Opposes the force of gravity and keeps us upright”

b.    “Muscle contraction refinements allow us to assume different positions”

 

3.   Movement

a.   Skeletal muscle

(1)           Body movements

(2)           Eye movements

(3)           Facial expressions

(4)           Breathing

b.    Smooth muscle

(1)           Moves food through the digestive tract

(2)           Constricts blood vessels

c.    Cardiac muscle

(1)           Pumps blood

 

4.   Contraction produces heat

 

II.               Skeletal Muscle Structure and Contraction – since SkM is pretty much what we’re talking about when we say “Muscular System”

 

A.    Anatomy of a Skeletal Muscle

 

1.   CT coverings (fascia) and hierarchy

a.   Epimysium – outer covering of muscle (covers bundles of fasicles), extends beyond muscle to become tendon

b.    Perimysium – surrounds bundles of muscle fibers called fasicles

c.    Endomysium – covers individual fibers (cells)

 

2.   Muscle Fiber

a.   Contains bundles of myofibrils, the contractile elements of the cell, which run the length of the fiber

b.    Myofibrils contain actin and myosin myofilaments arranged in sarcomeres

c.    Sliding filament theory of contraction (very briefly, just myosin-actin interactions, forget troponin and tropomyosin but mention that calcium and ATP are required).

 

3.   Innervation of Muscle

a.   Motor unit – motor neruon from spinal cord branches and innervates several muscle fibers

b.    Neuromuscular junction – site of interaction between neuron and muscle fiber

c.    Sarcolemma (plasma membrane) forms tubes that dip into cell –T-tubules (transverse)

d.    T-tubules come in contact with sarcoplasmic reticulum (ER) at sites of calcium storage (calcium storage sacs)

e.   Nerve impulse travels from neuromuscular junction, along sarcolemma, down T-tubules, stimulates calcium release from sarcoplasmic reticulum

f.      Calcium allows myosin binding to actin and if ATP is present the sarcomere contracts

 

B.   Physiology of Muscle Contraction

 

1.   All-or-None Law

a.   When stimulated enough to contract a muscle fiber (cell) completely contracts

b.    A muscle (a bunch of fasicles, which is a bunch of fibers) doesn’t follow the All-or-None Law; how much the muscle contracts depends on how many fibers are stimulated to contract

 

2.   Muscle Twitch, Summation, and Tetanus

a.   Single stimulus, muscle contracts and relaxes

b.    Stimulation in succession before the muscle can relax results in summation of tension – tension gets greater with same level of stimulation because it doesn’t get back to “0” tension before stimulated to contract again

c.    Tetanus is maximal sustained contraction – muscle can’t shorten anymore despite continued stimulation

d.    Fatigue – muscle runs out of ATP, can’t maintain contraction in spite of continued stimulation

 

C.   Chemistry of Muscle Contraction

 

1.   Energy Supply

a.   ATP required for contraction, made in mitochondria by aerobic means

(1)           As long as demand is below anaerobic threshold ATP by this mechanism will fuel muscle activity

(2)           Under anaerobic conditions only about 4-6 seconds worth of ATP present

b.    Creatine phosphate quickly regenerates ATP by directly donating a phosphate (good for about 15 seconds, first ATP regeneration mechanism to kick in under anaerobic conditions)

c.    Anaerobic production of ATP by glycolysis occurs during strenuous exercise after CP stores are exhausted

(1)           Very inefficient, good for only 1-2 minutes before muscle fatigue occurs

(2)           Lactic acid builds up (pH drops, muscles fatigue, ache)

 

2.   Oxygen Debt

a.   Lactic acid is transported to the liver where it is metabolized

b.    Requires oxygen

c.    The amount of oxygen required to finish metabolism of lactic acid is the oxygen debt

d.    That’s the wind-sucking you do after going anaerobic

 

D.   Aspects of Muscle Contraction

 

1.   Functional Identification

a.   Muscles connect two bones, when they contract one bone moves and one remains stationary

b.    Origin – where the muscle attaches to the stationary bone

c.    Insertion – attachment site on the bone that moves

d.    Prime mover – muscle that does most of the work

e.   Synergists – assisting muscles

f.      Antagonist – muscle that opposes the action of the prime mover

(1)           Muscles can only move bones in one direction since they shorten when they contract –don’t cause movement by lengthening

(2)           Have to have an opposing muscle to move the bone back in the opposite direction

 

2.   Isotonic Vs. Isometric Contraction

a.   Isotonic – muscle contracts, shortens, movement occurs (tension remains constant)

b.    Isometric - muscle contracts, can’t move load, tension increases but length of muscle remains constant

 

3.   Muscle Tone – partial contraction of muscles, maintains posture

a.   Muscle spindles are sensors that send information to the CNS to allow partial contraction or tone to be maintained

 

4.   Effect of Contraction on Size of Muscle

a.   Hypertrophy – increase in size due to repeated forceful contraction (exercise)

b.    Requires 75% maximum effort to stimulate hypertrophy

c.    Number of muscle fibers may also increase (but this is due to splitting of fibers; basically ripping them into shreds and allowing them to heal, not hyperplasia)

d.    Atrophy – disuse or very low intensity use causes muscles to shrink – fibers shorten and are replaced with fat and CT

 

III.            Skeletal Muscles of the Body

 

A.   Naming Muscles

1.   Size – gluteus maximus, the largest buttock muscle

2.   Shape – deltoid, shaped like a delta

3.   Direction of Fibers – rectus abdominus – longitudinal abdominal muscle (rectus means straight)

4.   Location – frontalis overlies the frontal bone

5.   Number of Attachments – biceps, triceps

6.   Action – extensor digitorum, flexors, adductors

 

B.   Muscles of the Head

 

1.   Muscles of Facial Expression

a.   Frontalis – corn row muscle

b.    Orbicularis oculi – blinking muscle, crow’s feet

c.    Orbicularis oris – pucker muscle

d.    Buccinator – in cheek, compresses cheek

e.   Zygomaticus – cheekbone to corners of mouth, smiley muscle

 

2.   Muscles of Mastication

a.   Masseter – chewing muscle\

b.    Temporalis

 

C.  Muscles of the Neck and Trunk

 

1.   Muscles That Move the Head

a.   Sternocleidomastoids (2) – sternum to mastoid process, both: head to chest (flex neck); one: head turns

b.    Trapezius – shoulder shrug, neck extension

 

2.   Muscles of the Trunk

a.   External intercostals – ribs up and out

b.    Internal intercostals – ribs down and in

c.    External oblique – abdominal wall, lateral rotation

d.    Internal oblique – same

e.   Transversus abdominis – abdominal wall

f.      Rectus abdominis – flexes vertebral column

 

D.  Muscles of the Shoulder and Upper Limb (Arm)

 

1.   Muscles That Move the Pectoral Girdle and Arm

a.   Deltoid – abducts the arm

b.    Pectoralis major – flexes and adducts the arm (pulls across chest)

c.    Latissimus dorsi – extends and adducts the arm

d.    Serratus anterior – pulls scapula down and forward (pushing)

 

2.   Muscles That Move the Forearm

a.   Biceps brachii – flexes forearm and supinates hand

b.    Triceps brachii – extends forearm

c.    Brachialis – flexes forearem

 

3.   Muscles That Move the Hand

a.   Extensor and flexor carpi – move wrist and hand

b.    Extensor and flexor digitorum – move fingers

 

E.   Muscles of the Thigh and Lower Limb (Leg)

 

1.   Muscles That Move the Thigh

a.   Iliopsoas – flexes thigh

b.    Gluteus maximus – extends thigh

c.    Gluteus medius – abducts thigh

d.    Adductor group - adducts thigh

 

2.   Muscles That Move the Lower Limb (Leg)

a.   Quadriceps femoris group – extends lower leg

b.    Hamstring group – flexes lower leg and extends hip

c.    Sartorius – flexes, abducts, and rotates leg

 

3.   Muscle That Move the Ankle and Foot

a.   Gastrocnemius – plantar flexion and eversion of foot

b.    Tibialis anterior – dorsiflexion and inversion of foot

c.    Peroneus group – plantar flexion and eversion of foot

d.    Soleus – plantar flexes foot

e.   Flexor and extensor digitorum longus – moves toes

 

IV.            Effects of Aging

A.    Mass and Strength  - tend to decrease

B.   Endurance – tends to decrease

C.   Exercise – anti-aging effect, combats loss in mass, strength and endurance

 

V.               Medical Focus – Muscular Dystrophy

 

A.    A group of muscular disorders characterized by muscle degeneration.

 

1.   Symptoms result from progressive skeletal muscle weakness

 

2.   Due to degeneration of the fibers

 

3.   Several types, all inherited, cause unknown in most

 

B.   Duchenne muscular dystropy

 

1.   Passed from the mother and nearly always only seen in male offspring

 

2.   Muscle weakness, difficulty in walking, curvature of the spine

 

3.   Gradual wasting of muscle, replacement with fat and CT

 

4.   Confinement to a wheelchair by about 12 an death by 20 or so

 

5.   Recently discovered that the cause is lack of a protein that maintains the integrity of the sarcolemma (dystrophin)

 

6.   Gene therapy:

a.   Myoblast transfer therapy – inject with health myoblast cells that fuse with health ones, this provides the normal gene and allows the fibers to produce normal dystrophin

b.    Insertion of plasmids with the correct genes also being tried

 

C.   Myotonic muscular dystrophy

 

1.   Failure of muscles to relax after contraction

 

2.   Inherited from either parent

 

3.   Face and neck affected first

 

4.   Problems:

a.   Lifting and turning head

b.    Swallowing

c.    Speech

 

Later problems:

d.    Abnormal heart rhythms

e.   Cataracts

f.      Abdominal cramps

g.    Sometimes leads to confinement to bed or wheelchair

 

VI.            MedAlert – Benefits of Exercise

 

A.    Improves strength

 

1.   The force a muscle can exert against resistance in one maximal effort.

 

2.   The size of the muscle and the number of myofibrils and even fibers will increase as the strength increases

 

3.   Capillaries and CT will increase also, including CT of tendons and ligaments

 

4.   Strength training (resistance) benefits all adults (remember it also increases bone density)

 

B.   Improves endurance

 

1.   Ability of the muscle to contract repeatedly or to sustain a contraction.

 

C.   Improves flexibility

 

1.   Range of motion around a joint

 

D.   Any level of exercise can improve health:

 

1.   Lowers risk of heart attack

a.   Increases HDL levels

b.    Lowers resting heart rate, lowers blood pressure

 

2.   Reduces pain and swelling in arthritis patients

a.   Also reduces fatigue and depression associated with the disease

b.    May be beneficial in a variety of other long-term diseases that include fatigue and depression as part of their secondary symptoms

 

3.   Stimulates osteoblast activity, helps prevent osteoporosis

 

4.   Cancer prevention:

a.   Linked to decreases in liklihood of developing colon, breast, cervical, uterine, and ovarian cancers

b.    Increases intestinal motility and fat mobilization (and utilization)