Muscular SystemPhysiology

General Functions

• Movement – skeletal muscle (s.m.) contractions move the body in whole or in part

• Heat Production – muscle cells produce heat via catabolism; S.M. contractions are one of the most important parts of maintaining temperature homeostasis

• Posture – continued partial contraction of s.m. makes it possible to sit, stand, and maintain a stable position while walking, running, etc.

Function of S.M. Tissue

• Excitability / Irritability – ability to be stimulated

• Contractility – ability to contract or shorten to pull on bones and allow movement

• Extensibility – ability to extend or stretch to allow muscles to return to resting length

Function of S.M. Tissue

• Sarcolemma: plasma membrane of a muscle fiber• Sarcoplasm: muscle cell cytoplasm

• Sarcoplasmic Reticulum – network of tubules and sacs; membrane pumps calcium ions from sarcoplasm to store in its sacs

• Myofibrils – bundles of fibers; almost fill the sarcoplasm– Myofilaments ( protein molecules)

» thin filaments-actin: beads of two strands twisted-tropomyosin: covers active site on actin-troponin: holds tropomyosin in place

» thick filaments-myosin: shaped like golf clubs with “myosin heads”; attracted to actin and forms cross bridge

Sarcomere: basic contractile unit of muscle cell; segment of myofibril between Z lines; many sarcomeres make up a myofibril

A bands – wide, dark stripesI bands – narrow, light stripesZ line – begins/ends each sarcomere and anchors thin filamentsM line –anchors thick filamentsH zone – space between actin filamentsT tubule (transverse) – extend transversely across sarcoplasm; allows impulses traveling along sarcolemma to move deeper into muscle cell

- Triad: T tubule sandwiched between two sacs; allows electrical impulse traveling along T tubule to stimulate membranes of adjacent sacs.

Excitation1. Muscle fiber at rest

2. Nerve impulse travels to motor endplate via a motor neuron, which forms a junction with the sarcolemma (neuromuscular junction – type of connection called a synapse characterized by a synaptic cleft / small gap across which neurotransmitters send signals)

3. Synaptic vesicles release the neurotransmitter acetylcholine into the synaptic cleft

4. Acetylcholine receptors on the sarcolemma of adjacent muscle fibers receive the signal

5. Electrical impulse is stimulated in sarcolemma

Contraction1. Impulse conducted over sarcolemma and along T tubules.

2. Calcium ions release from SR sacs

3. Calcium ions combine with troponin molecules

4. Causing tropomyosin to shift and expose active sites on actin

5. Myosin heads bind to actin molecules

6. Actin filaments are pulled past the myosin head, head unattaches and reattaches to a new site….repeat the process

7. Sliding Filament Theory – Sliding of thin filaments toward center of each sarcomere shortens the myofibril and muscle fiber

• Action Potential– http://highered.mcgraw-hill.com/sites/0072495855/

student_view0/chapter10/animation__action_potentials_and_muscle_contraction.html

• Myofilament Contraction– http://highered.mcgraw-hill.com/sites/0072495855/

student_view0/chapter10/animation__myofilament_contraction.html

• Sarcomere Contraction– http://highered.mcgraw-hill.com/sites/

0072495855/student_view0/chapter10/animation__sarcomere_contraction.html

Relaxation

1. Sarcoplasmic reticulum starts actively pumping the calcium ions back into its sacs so the calcium ions are stripped off the troponin molecules

2. Contraction is stopped as the troponin without calcium once again blocks actin’s active sites.

3. Myosin heads have nothing to hook on to and thin filaments are no longer held or pulled by thick

4. This process takes a fraction of a second

Energy SourcesATP

• Hydrolysis of ATP (splitting into

ADP) yields the energy required

for muscular contraction

• ATP binds to myosin head and

transfers its energy to the head

to pull the actin during contraction

• Muscle fibers continually

resynthesize ATP from breaking

down creatine phosphate (high

energy compound in muscle cells).

Catabolism by muscle fibers

requires glucose and oxygen to

synthesize ATP and CP

(aerobic respiration).

Creatine Phosphate

• Creatine is made of three amino acids: arginine, glycine, and methionine.

• Creatine is produced naturally in our kidneys, liver, and pancreas and flows via the blood stream to our muscle fibers, where it is converted to creatine phosphate.

• Creatine phosphate is stored in muscle fibers and can be broken down to provide quick ATP for muscle cell contraction.

• Red meat, herring, and salmon are all excellent food sources of creatine.

Three Main Sources of ATP for Muscle Fibers

1. Creatine phosphate - quick energy for 8-10 seconds

2. Glycogen – a little slower by will provide approx. 90 seconds of energy. Oxygen not needed but lactic acid is a by-product

3. Aerobic respiration- takes longest, but lasts longest as well. Usually starts after about 2 minutes into exercise.

Energy SourcesC6H12O6 & O2

• Oxygen delivered to muscle cells via hemoglobin• Muscle fibers store glucose in the form of glycogen• Oxygen is also stored in muscle cells• At rest:

– Excess O2 in the sarcoplasm is bound to myoglobin

– Myoglobin: large protein molecule with reddish pigment, contains Fe that attracts O2 molecules temporarily

– Exercise→ O2 conc. decreases in muscle cells→myoglobin resupplies the muscle with O2

Energy SourcesAerobic vs. Anaerobic Respiration

• Aerobic Respiration

– Requires O2

– Produces max. amt. of energy available from each C6H12O6 molecule

• Anaerobic Respiration– Occurs when there is NO oxygen– Forms lactic acid that can accumulate in muscle tissue

• During exercise• Muscles “burn”

– “Oxygen debt”: Heavy breathing during (or after) exercise will convert the lactic acid (eventually diffused into and carried by the bloodstream to the liver), back into glucose.

Energy Sources

• Muscle Fatigue– Muscle loses ability to contract– Lactic acid accumulates– Cramp: muscle contracts spasmodically, but

does not relax completely

Energy SourcesHeat Production

• Skeletal Muscle contraction (including shivering) produces waste heat that can be used to help maintain the set point body temperature (homeostasis)

Functions of S.M. Organs

• Motor Unit– One somatic motor neuron & muscle fibers

*Smaller # of fibers, more precise movement

*Larger # of fibers, more powerful contraction

Studying Contractions of S.M. Organs

• Electromyography

– “muscle graphing”

– Procedure where the force or tension from the contraction of an isolated muscle is recorded as a line that rises and falls as the muscle contracts and relaxes

– Electrical impulse produces a “twitch contraction”

Functions of S.M. Organs

• Twitch contraction– Quick jerk of a muscle that is produced as a result of

a single, brief threshold stimulus/nerve firing (if were to isolate one motor neuron to one muscle fiber)

– Threshold stimulus: electrical stimulus of sufficient intensity to cause twitch

• One entire twitch last 1/10 sec.– Twitch contractions rarely happen (our nervous

system “smoothes out” movements to prevent injury and make them more useful)

Twitch Contraction

– Three phases• Latent period-nerve impulse travels to the SR to

trigger release of calcium• Contraction phase-calcium binds to troponin to pull

it and tropomyosin off actin and sliding of filaments occurs

• Relaxation phase-sliding of filament ceases as myosin heads unlock from actin

Functions of S.M. Organs

• Treppe: A series of “twitches” leads to this “Staircase Phenomenon”– Gradual, step-like increase in contraction strength that can be

observed in a series of twitch contractions that occur about 1 sec apart

– The series of twitch contractions is a response to a frequency of stimulation that results in only a partial relaxation between contractions

– A muscle contracts more forcefully after it has contracted a few times then when it first contracts: Ca+ builds up in sarcoplasm of warmer muscle fibers to provide longer and stronger contractions over time

– Athletes warm-up by this principle

Functions of S.M. Organs

• Factors of treppe in warm muscles*Calcium diffuses to sarcoplasm more efficiently so more actin-myosin reactions occur

*Calcium accumulates in sarcoplasm of muscles that have not relaxed which would get calcium into the SR

1. Cool fiber starts to respond after a few successive stimuli

2. Relaxation phase becomes shorter and disappears (muscle stays partially contracted

3. Muscle reaches max contraction

4. After time the fibers respond with less and less strong contractions due to muscle fatigue

– Contracture: abnormal state of prolonged contraction

5. Repeated stimulation of muscle in time lessens it excitabilibility

Functions of S.M. Organs

Treppe leads to: Tetanus: common, normal, smooth, sustained contractions resulting from the coordinated contractions of different motor units within the same muscle organ– Multiple wave summation: multiple twitch waves

added together to sustain muscle tension for a longer time (no time for relaxation)

• Incomplete tetanus – very short periods of relaxation that occur between peaks of tension

• Complete tetanus – twitch waves fuse into a single, sustained peak (no relaxation period)

Functions of S.M. Organs

• Muscle Tone – Tonic contraction: continual, partial

contraction of a muscle (posture)– At any one time, a small number of muscle

fibers within a muscle contract, producing a tightness or muscle tone

– Flaccid: muscles with less tone than normal– Spastic: muscles with more tone than normal

Functions of S.M. Organs

• Graded strength principle – s.m. contract with varying degrees of strength at different times– Factors

• Metabolic condition of individual

fibers (ability to maintain a high level of ATP)• # of muscle fibers contracting

simultaneously– Greater # fibers = stronger contraction

• # motor units recruited• Intensity and frequency of

stimulation

Functions of S.M. Organs

• Length-tension relationship– Max strength that a muscle can develop bears a direct

relationship to the initial length of its fibers– A shortened muscle’s sarcomeres are compressed, so the

muscle cannot develop much tension– An overstretched muscle cannot develop much tension b/c thick

filaments are too far from thin filaments– Strongest max contraction is possible only when the s.m. has

been stretched to its optimal length

Functions of S.M. OrgansIsotonic & Isometric Contractions

• Isotonic – “same tension”– Tone/tension within muscle stays the same,

while the muscle length changes (constant pull at same tension)

– Concentric: muscle shortens as it contracts

Ex. picking book up– Eccentric: muscle lengthens as it contracts

Ex. Slowly putting book down

Functions of S.M. OrgansIsotonic & Isometric Contractions

• Isometric - “same length”– Muscle length stays the same & tension

increases– Myosin cross bridges hold their own against the

load, but can not slide the thin filaments

Ex. Pulling up on a stationery railing (feel increase in tension but no movement)

** Most body movements occur as a result of both types of contractions

Functions of Cardiac Muscle

• Only in heart• Striated, involuntary• Contracts rhythmically and continuously• Cardiac muscle fiber does not taper and has

strong intercalated disks between fibers to form a continuous, electrically coupled mass (syncytium)

• Does not run low on ATP (synovial node is built in pacemaker)

• Does not experience fatigue• Self-stimulating

Cardiac Muscle

• Cardiac muscle fibers contain sarcomeres with myofibrils that branch to interlock with adjacent cell fibers.

• Autonomic nervous system regulates speed of contractions.

• W/O nerve stimulation, the heart will still contact

Functions of Smooth Muscle

• No T tubules• Small, tapered cells with single nuclei• Calcium comes from outside the cell• Contraction triggered by calcium binding to

calmodulin (not troponin) directed by hormone signals controlled by automonic nervous system

• Myofilaments not arranged in sarcomeres, so no thin or thick striated bands

Functions of Smooth Muscle

Two Types of Smooth Muscle

A. Visceral (single-unit): • Gap junctions join fibers into large, continuous

sheets (walls of hollow organs)• Autorhythmicity: rhythmic self-excitation

spreading across the entire tissue (peristalsis that pushes contents of organ)

Functions of Smooth Muscle

B. Multiunit: • Composed of many independent, single-cell

units.• Responds only to nerve impulses• Most often found in bundles (arrector pili of skin)

or as single fibers (surrounding small blood vessels)

Myopathies• Muscle Injury

– Strain: caused by overexertion or trauma to muscle usually in area of joint also possibly damaging a tendon

– Myalgia: muscle pain due to overstretching or tearing muscle fibers– Myositis: any muscle inflammation– Fibromyositis: when tendon inflammation occurs with myositis (charley

horse)– Cramps: painful muscle spasms; due to mild myositis or fibromyositis,

irritation, overuse, ion (protein, calcium, potassium) and water imbalance, lactic acid

– Contusion: muscle bruise, local internal bleeding and inflammation• Crush injury – severe trauma to s.m., greatly damages affected tissue, can

cause release of muscle fibers into bloodstream, myoglobin pigment can accumulate in blood and cause life threatening kidney failure

Myopathies

• Muscle Infections– Infected by bacteria, viruses, and parasites

(trichinosis, influenza, etc.)– Often producing local or widespread myositis

(muscle inflammation)– Poliomyelitis:

• Viral infection of the nerves that control s.m. movement

• Causes paralysis that could lead to death• Vaccine essentially eliminated this in US

Myopathies

• Muscular Dystrophy– Group of genetic diseases characterized by atrophy

(wasting away) of s.m. tissues– Some forms are fatal (cardiac or respiratory muscle

weakness)– Duchenne muscular dystrophy (DMD):

• most common form of MD• Called pseudohypertrophy – “false growth” where the

atrophy of muscle is masked by excessive replacement of muscle by fat and fibrous tissue.

• Involves lack of protein dystrophin that forms strands in muscle fiber that holds their cytoskeleton (for support) to the sarcolemma, thus helping to keep the muscle from breaking during contraction

Myopathies

• Myasthenia Gravis– Chronic disease (persistent and long lasting)– Autoimmune disease: immune system attacks muscle

cells at neuromuscular junctions and motor neurons are therefore not able to stimulate associated muscles

– Muscle weakness, especially in face and throat– Can become a “myasthenia crisis” and affect all 4

limbs• Could die of respiratory failure

Myopathies

• Hernias– “protrusion” of abdominal organ (usually small intestine) due to

weakness of abdominal muscles when lifting something too heavy– Reducible: protruding organ manipulated back into abdominal cavity– Strangulated: not reducible and blood flow to affected organ is stopped;

obstruction, gangrene, pain, vomiting, emergency surgery required– Different types

• Inguinal – Hernia extends down inguinal canal into scrotum or labia– More males than females experience this

• Femoral– Women– Below groin – bulge in upper part of thigh– Due to changes during pregnancy