MUSCLES HUMAN ANATOMY AND PHYSIOLOGY SUMMER, 2002

I          GENERAL CHARACTERISTICS - Constitutes 40-50% body wt.; MYOLOGY - study of muscles

A) EXCITABILITY - receive/respond to stimuli

STIMULI - change in internal/external environment strong enough to initiate a nerve potential (ACTION POTENTIAL)

B) CONTRACTILITY - ability to lengthen/shorten when receive sufficient stimuli

C) EXTENSIBILITY - ability to be stretched; muscles set in opposing pairs: one contract, one
relaxes

D) ELASTICITY - return to original shape after B & C

II FUNCTIONS - Motion, maintain posture, heat production (85% of body heat generated by muscle
contraction

III TYPES - SKELETAL, CARDIAC, SMOOTH

A) SKELETAL - attached to bones and moves skeletal parts; Striated & Voluntary 1) CONNECTIVE TISSUE COMPONENTS:

FASCIA. - sheet of fibrous C.T. beneath skin or around muscle or other body organs; Avascular - where sutures are anchored

a SUPERFICIAL FASCIA (SUBCUTANEOUS LAYER) - deep to skin; has adipose and loose C.T.: storehouse for water, fat ( >50% total weight); insulation; mechanical protection: pathway for nerves and vessels.

b. DEEP FASCLA - dense C.T.: lines body wall; extremities and holds muscles together

(separating them into functional groups); allows free muscle movement; carries nerves and „ vessels: fife spaces between muscles

EPIMYSIUM - fibrous C.T. that wraps around entire muscle

PERIMYSIUM - invaginations of perimysium; divide muscle into bundles of fibers

(FASCICULI)

3.      ENDOMYSIUM - invaginations of perimysium; penetrate into interior of each fasciculi

•     TENDON - where all 3 extend beyond muscles; continuous with C.T.; cord of

C.T. attaches muscle to periosteum

• APONEUROSIS- when CT. elements extend as broad flat layer

• TENDON SHEATH - enclosure of fibrous CT: inner (Visceral), outer (Parietal); cavity containing synovial film between inner & outer layer

2) NERVE/BLOOD SUPPLY - ample: 1 artery and 1-2 veins accompany each nerve that penetrate skeletal muscle; capillaries in endomysium: each fiber makes contact with portion of nerve cell (SYNAPTIC END BULB)

3) HISTOLOGY-

a.    MYOFIBERS (muscle fiber) - numerous elongated cylindrical cells which make up muscle; parallel to each other; range from 10 - 100um dia.; up to 12"; multinucleated lying close to sarcolemma; composed of MYOFIBRILS; special high energy molecules & enzymes

1. Sarcolemma - plasma membrane enveloping each fiber

2. Sarcoplasm - cytoplasm enveloped by sarcolemma

3. Sarcoplasmic Reticulum (SR) - network of membrane enclosed
tubule.

Terminal Cistererns( TC) - dilated end sacs of Transverse Tubles

4. Transverse Tubules (T tubules) - tubes running transverse through muscle
perpendicular to SR and is continuous with sarcolemma (external openings)

5. Triad - T tubules + TC on either side

b. MYOFIBRILS - subunits of myofibers; l-2um dia.; 100 - 1000's; run longitudinally

through fiber; composed of myofilaments

c. MYOFILAMENTS:

1.   Contractile Elements:

•  Thick Myofilaments (16nm dia.): contain Myosin (golf club shaped arranged

parallel to each forming filament shaft, head projects outward); overlap free ends of thin; form cross bridges; contain actin binding site and ATP binding sites; anchored to Z Line with Thin

•  Thin Myofilaments(6nm dia.): composed of protein ACTIN (arranged in 2

single strand entwined helix; each actin has MYOSIN binding site (for cross bridges); forms

•  Titin Myofilaments(1nm dia): contains protein elastin: anchors and stabilizes

THICK filaments to Z disc

2. Regulator. Elements:

• Tropomyosin - loosely attached to actin - covers myosin binding sites on actin

• Troponin - regular intervals on tropomyosin surface; contains Ca binding sites

d.    Additional Landmarks

• SARCOMERES - myofilament compartments: don't run entire fiber length

• Z LINE/DISC - dense mat'l separating sarcomeres

• A BAND (ANISTROPIC) - dark dense area; shows length of THICK myofilaments;
edges darkened by thick/thin overlapping

• I BAND (ISOTROPHIC) - light, less dense area; shows THIN myofilaments only

• H ZONE - THICK myofilaments only

• M LINE - center of H zone; series of fine threads connecting middle part of adjacent
thick myofilaments

• MUSCLE STRIATIONS alternation of A&I band

4)      CONTRACTION (SLIDING FILAMENT THEORY)

a. THIN myofilaments slide inward toward H zone; sarcomere shortens
(length of THICK/THIN don't change)

b. Myosin (Thick) cross bridges connect with portions of actin (Thin). Crossbridges
move like oars on surface of thin resulting in thick & thin slide past each other

c. H zone narrows/disappears as thin meet & may overlap at sarcomere center. As
thin move inward, Z lines drawn towards each other and sarcomere shortens

d. Sliding of myofilament with shortening of sarcomere - Contraction

5)     NEUROMUSCULAR JUNCTION (MOTOR END PLATE) - need stimuli for muscle contraction

a. MOTOR NEURON - neuron stimulate muscle tissue

Axon branches inside muscle into axon terminal (TELODENDRIA) - in sarcolemma

b. MOTOR END PLATE - telodendria + adjacent part of sarcolemma

c. SYNAPTIC END BULBS - bulb like structures at distal ends of
telodendria

d. SYNAPTIC VESICLES - membrane enclosed sacs containing
neurotransmitters

e. SYNAPTIC GUTTER - invaginated area of sarcolemma under telodendria

f. SYNAPTIC CLEFT - space between axon terminal & sarcolemma

g. SUBNEURAL CLEFT - numerous folds of sarcolemma along synaptic gutter

6)      MOTOR UNIT - motor neuron together with all muscle fibers it stimulates

up to 150 muscle fibers; for precise control (e.g. eye - only 10 fibers), for gross motion (e.g. gastrocnemius - up to 500 fibers)

•    RECRUITMENT - process of increasing # of motor units; fire asynchronously ; prevents fatigue, maintains partial contraction (muscle tone)

7)     CONTRACTION PHYSIOLOGY a. Relaxed State: Maintained by

• Low Ca in sarcoplasm (Ca stored in SR)

• High ATP concentration (attached to ATP binding site on Myosin).

• Tropomyosin-Troponin Complex attached to Actin (covering Myosin binding

sites on Actin).

•  Muscle stays relaxed as long as these conditions are maintained

b.       Activation

1. Nerve impulse reaches axon terminal, synaptic vesicles release Ach. initiates impulse in fiber sarcolemma which changes it's permeability - Muscle Action Potential. Impulse travels to t tubules and SR.

2. Transmitted impulse releases Ca that combines with troponin causing structural

changes. This causes a pull on tropomyosin units, thus exposing myosin binding sites on actin.

3. Energy released from ATP breakdown causes myosin X bridges to attach to actin
and their movement results in the sliding of thin myofilaments towards the H
zone (Power Stroke).

c.       Relaxation

1. Ach rapidly destroyed by AchE (acetylcholinesterase) which is stored on
subneural cleft surface. This terminates the conduction of a nerve impulse.

2. Ca is actively transported back into SR (Carriers: Calsequestrin,
Calcium-ATPase)

3. Tropomyosin/troponin complex re-attaches to actin strand covering myosin

binding sites on actin & forcing separation of myosin X bridges from actin (use ATP here)

4. Thin myofilaments slip back to relaxed position

ADP + P = ATP and attaches to ATP binding sites of myosin X bridges

5. Sarcomeres return to resting length, muscle fibers relax

8)     ENERGY FOR CONTRACTION - muscle synthesize ATP; vary with inactivity and great activity

a. Phospagen System - immediate energy (< 120 seconds)

1. CREATTNE PHOSPHATE - ADP = ATP (ENERGY) - CREATINE

Phosphocreatine - 6 times greater than ATP in skeltal muscle; Good for

Creatne Kinase — gets phosphate from Creatine Phosphate

2. Myokinase - enzyme system controlling phosphate transfer

b. SS and grycogen utilization - good for 10-20 minutes maximum contraction

Glycogen - Stored in liver and skeletal muscles

c. Lipid Utilization - predominant form of fuel for aerobic activities lasting longer that
15 minutes

9)      CONTRACTION FORCE - myosin cross bridges of thick myofilaments connect with portions of thin myofilaments within sarcomere

• GREATEST TENSION- maximum overlap between thick and thin myofilaments

• OPTIMUM LENGTH (OL) - maximum # of myosin X bridges contacting
thin myofilaments

• STRETCH MUSCLE - fewer myosin X bridges contacts; stretch 175%beyond
optimum length - no myosin X bridge contacts and so no muscle contraction

a. ALL OR NONE PRINCIPLE - once stimulated, individual fiber of motor unit will
contract to fullest extent; (Muscle = many motor units)

• Threshold Stimulus (TS) - weakest neuronal stimuli to initiate a contraction

• Sub-Threshold Stimulus - stimuli less than TS.

b. MYOGRAM- visual record of muscle contraction
1.     PHASES:

•  Latent Period (.01 sec) - time between stimuli application and contraction

initiation (Ca released from reservoirs and myosin X bridge formation)

• Contraction Period (.04 sec)

• Relaxation Period (.05 sec)

• Refractor. Period - temporary loss of irritability (Cardiac- .3 sec. skeletal .005

sec)

c. TYPES OF CONTRACTIONS

1. TETANUS (Wave Summation) - when 2 equal stimuli are applied after refractory
period but before muscle is totally relaxed - 2nd contraction is stronger than first.
Mech: Addition of Ca released from SR by 2nd stimuli   has additive effect of ions
left from 1st: results in rapid succession of separate twitches

• INCOMPLETE (UNFUSED) TETANUS - muscle partially relaxes
between contractions (20 - 30 stimuli/sec)

• COMPLETE (FUSED) TETANUS - muscle maintains sustained
contraction and lacks even partial relaxation (>35 stimuli/sec)

2. Isotonic- minimal shortening of muscle but increase in tension and no movement

3. Isometric - contraction over range of muscle and produces movement

d. MUSCLE TONE - sustained partial contraction of muscle in response to stretch receptor activation; asynchronous firing of various motor units

• Muscle Spindle - receptors in muscle which monitor degree of tension

• Flaccid - less than normal tone

• Atrophy - wasting away; decrease in cell size and loss of myofibrils; replaced by

fibrous tissue: once complete is irreversible

•  Hypertrophy - increase in fiber dia. due to production of myofibrils, etc.; results

from forceful or repetitive activity

10)    FIBER TYPES - (based on MYOGLOBIN content ) By structure:

a. RED MUSCLE FIBERS - high myoglobin content; small dia.: abundant capillaries and
mitochondria

b. WHITE MUSCLE FIBERS - low myoglobin content: large dia.; extensive SR

By function:

a. Type 1- Slow Twitch; Oxidative; Red - high ATP output through aerobic
processes; splits ATP at slow rate, contraction velocity slow; resistant to fatigue

b. Type II - Fast Twitch: giycolytic; white - large glycogen concentration; generates
ATP through anaerobic processes; splits ATP at a fast rate: contraction velocity is
fast; muscle fatigue easily

• Endurance exercise transforms b into a; weight training increases b and increases
myo filament production.

• All muscle fibers of any 1 motor unit are only one type; activation of units
controlled by CNS

11)   Aging and the Muscular System

a. Fiber diameter decreasing of myofibrils

b. Becomes less elastic (fibrosis) increased amounts of fibrous connective tissue
incorporated into muscle

c. Reduced exercise tolerance - rapid fatigue and thermoregulation considerations

d. Delayed recovery from injury- increased fibrosis and scar tissue formation

e. Fast twitch exhibit earliest and most severe atrophy (Use vs. Disuse)

B) CARDIAC - forms bulk of heart wall

1) COMPOSITION: Striated and Involuntary

a. Quadrangular/cylinderical cells with single central nucleus commonly branched

b. Authorhythmic - cells are capable of self excitation

c. More sarcoplasm and mitochondria; less SR

d. Larger T tubules located at Z line rather than A-I band junction

e. Myofilaments - not arranged into discrete myofibrils: fibers branch freely

f. Intercalated Disc - thickenings of sarcolemma containing desmosomes and gap
junctions. These facilitate muscle action potential conduction

g. Non regenerative

2) PHYSIOLOGY

a. Contract and relax rapidly, continuously & rhythmically

b. Numeorus mitochondria needing fuel (glycogen, fat)

c. Can contract without extrinsic stimuli (e.g. SA node) and remain contracted longer
(prolonged deliver}' of Ca into sarcoplasm)

d. Long refractory period (.5 sec) - prevents tetanus and allows chambers to fill

C) SMOOTH - involved in processes maintaining internal environment

1)      COMPOSITION: Non-striated & Involuntary

a. Spindle shaped cells with a singular oval nucleus

b. Thick and thin myoffilamnets but not orderly arranged; more numerous
myofilaments/myofibrils

c. Intermediate Fibers - attach to Dense Bodies which act as Z line: dispersed thruout
cytoplasm or attached to sarcolemma)

2) Types

a. Visceral (Single Unit) - fibers bound as a continuous network that contract
together; have many Gap Junctions between adjacent cells; most common type

•    Location - walls of stomach, intestines, urinary bladder, hollow viscera and
small blood vessels)

b. Multiunit - individual fibers have own motor nerve ending so stimulation of 1
multiunit results in the contraction of a singular fiber

•    Location - large blood vessels, eye muscles (iris and ciliary body), arrector pili)

3) Physiology

a. Contraction starts slowly duration is longer (5 - 500X) - no T tubules

b. Respond to nerve impulses, hormones and local factors

c. Can shorten and stretch considerably without developing tension (Stress Relaxation)

- great size changes without losing contraction ability

d. Regenerative more than other muscle types