Biol 2401

Biol 2401 A&P I Lecture Notes Muscle Tissue Dr. Weis

Muscle Tissue Properties ::

1. Muscle cells can be excited to produce an action potential transmitted along their cell membranes

a. chemically

b. electrically

c. mechanically

2. Contractile -->

Contractile proteins that have a contractile mechanism will be activated by the action potential (AP)

3. Stretched --> extended (warm-up exercises to recruit more muscle fibers)

4. Elastic --> resume resting length

Muscle tissue is divided into three types

a. Skeletal

b. Cardiac

c. Smooth

Muscle Similarities :

1) Muscle cells ....elongated, termed fibers

2) Contractile proteins.... called myofilaments

3) terminology .... myo, mys --> muscle

sarco- --> flesh

A. Skeletal muscle characteristics

striations

contract with nervous stimulation

voluntary control

function : movement, support, protect, temperature, (heat generation),joint stability, posture

location : attatched to skeleton

B. Cardiac muscle characteristics

striations, in-voluntary

contracts rhythmically in absence of external innervation due to pacemaker cells in the myocardium that discharge spontaneously (Na+/K+ involvement) to cause an action potential

C. Smooth muscle characteristics

lacks cross striations

primarily found in hollow organs of the digestive, vessels, respiratory system, urogenital

spontaneously active (pacemaker cells)

involuntary, slow sustained contractions

I. Skeletal Muscle

40% of body mass

all muscles are made up of numerous fibers and each fiber is made up of successively smaller subunits.

Most skeletal muscle will begin (termed the origin point) and end (termed the insertion point) in tendons.

These tendons are formed from the collagen fibers of the epimysium ( the connective tissue that surrounds the entire muscle)

[The deep fascia is fibrous c.t. that is external to the

epimysium and will bind muscles into functional groups]

Several muscle fibers are divided into bundles called fasicles by a connective tissue called the perimysium.

Each muscle fiber is a single cell, multinucleated, long and cylindrical in shape and is surrounded by a c.t. called the ENDOMYSIUM.

All these c.t. coverings support muscle cells and provide routes for blood vessels and nerves.

Most fibers extend the entire length of muscle and are arranged parallel to each other so that the force of contraction of each unit is additive.

Theses fibers can create various shapes and sizes of muscles such as pennate (feather-like), convergent, circular, or parallel.
Each fiber is innervated by only one nerve ending, near the middle of the fiber at a specialized junction called the NEUROMUSCULAR JUNCTION.

The reason each skeletal muscle fiber is a long cylindrical cell with many nuclei due to the fusion of many cells during embryologic development.
This specialized cell will have different names for their structures :

Plasma membrane .... is called the sarcolemma

The sarcolemma will be modified in ways to form tube like structure called the T-tubules

Cytoplasm .....sarcoplasm

fluid + organelles

fluid --> K+, Mg++, P, proteins

Myoglobin (O2 binding protein)

organelles --> mitochondria, nucleus

also specifically modified structures such as the sarcoplasmic

reticulum (modified smooth E.R.) and the myofibrils

Each muscle fiber is made up of several 100-1000 myofibrils

each myofibril is made up of myofilaments.

These filaments are made up of contractile proteins called :

1. Myosin .... thick filament

contains 200 myosin molecules

each peptide chain has 2 regions : tail and

head (also called cross-bridge)

with ATP binding sites and enzymes to

split ATP to generate Energy for muscle

contraction

2. Actin ... thin filament

three different protein components

a. G actin (globular) linked in long strands.

The final structure being called F actin (fibrous)

Two strands of F actin coil around each other.

G actin molecules have active sites for binding with myosin

b. tropomyosin --> rod shaped regulatory protein that spiral about the

F actin to help stiffen

c. troponin --> polypeptide structure

three binding sites :

1. actin inhibitory site

2. tropomyosin position

3. Ca++ binding site

The cross striations are due to the arrangement of thick & thin filaments with in
the Sarcomere (the functional contractile unit of muscle) to give it an alternating light and dark band appearance.

These cross striations are named for filament areas andcan be used to observe the mechanism for contraction.

Bands

1. I band.... isotropic, only actin fibers, light area

2. A band.... anisotropic

area delineating the length of myosin will also contain parts of actin

3. H band.... only contains thick filaments

found in the middle of the A band, visible only in relaxed muscle

Lines

1. Z line... filamentous protein that appears as a

dark line (disc) for attachment of thin (actin) filaments

The portion of the myofibril that lies between two adjacent

Z lines is called a Sarcomere, the smallest functional unit of the muscle fiber

2. M line... link between thick filaments

will be in the center of the A band

(and therefore in the center of the H band)

When the sarcomere contracts

a. Z LINES MOVE CLOSER

b. H and I bands get smaller

c. A band stays the same width

The myofibrils are surrounded by a plasma membrane called the sarcolemma, and openings in it's structure have a tubular appearance.

These structures form the sarcotubular system, (2 parts)

1. Sarcoplasmic reticulum (SR)

2. Transverse tubular (T tubular) system

The sarcoplasmic reticulum stores Calcium ion and is involved in its release

it surrounds each myofibril and expands into sac-like channels called

terminal cisternae and located around the region of the T-tubules

The T system are transverse tubules continuous with the

sarcolemma (plasma membrane) of the muscle fibers to form a hollow elongated tube

that runs between the terminal cisternae of the SR.

These tubes will transmit the AP and conduct the impulses to the sarcomere.

The T tubules can also allow passage of O2, glucose, and ions by way of ECF entry.

The junction of the sarcoplasmic reticulum and the T system occurs at the junction of the A and I bands, and forms an area known as the TRIAD.

PHYSIOLOGY ::

each skeletal muscle fiber is innervated by only one nerve ending near the middle of the fiber
at a specialized junction called the NEUROMUSCULAR JUNCTION (NMJ).

The entire physiologic process is called the Excitation-Contraction coupling.

Initiation and execution of muscle contraction occurs in the following steps ::

1. Action Potential (AP) travels along motor nerves to its ending at the NMJ. This junction consists of

a. axonal terminal branches called synaptic knobs

b. synaptic cleft, a space between knobs and muscle

c. sarcolemma that is folded to create invaginations called junctional folds

2. At each synaptic knob (nerve ending) a chemical is synthesized, stored and released from the synaptic vesicles.
Release from these vesicles is dependent on Ca++ influx into the axonal terminals due to the transmission of the AP (nerve impulse).
This chemical is called a neurotransmitter, and the specific one for skeletal NMJ is called acetylcholine (ACH).

3. ACH when released from the vesicles diffuses across the cleft and binds to receptors on the sarcolemma.
ACH will act to open up specific ACH protein channels in the sarcolemma.

4. Opening of channels allows for Na+ to flow in.

The resting membrane potential (rmp) for skeletal muscle is between -80 -> -90 mV.

Na+ in causes a depolarization of the membrane. If the mV is sufficient enough to reach threshold, then an AP is initiated in the muscle fiber.

5. The AP in the muscle membrane travels down the T tubule system past the Triad to the terminal cisternae of the SR, thus causing release of Ca++ into the sarcoplasm.

6. Ca++ initiates attractive forces between actin and myosin in a process called the sliding mechanism of contraction.

7. ACH is destroyed by an enzyme ACHase located in the synaptic cleft and sarcolemma.
This distruction prevents continued muscle fiber contraction in the abscence of any additional nervous system stimulation.

8. Without any further stimulation, Ca++ is returned to the SR by way of active transport and stored here bound to proteins.
Uptake causes muscle fibers to relax & contraction ends.

MUSCLE CONTRACTION --> Sliding Filament Theory

explains how muscle contraction occurs due to the mechanical forces generated by the interactions of the cross-bridges (heads) of the myosin filaments with the actin filaments.

The active sites on the actin filaments of a relaxed muscle are inhibited because they are physically covered by the troponin- tropomyosin complex.

The role of Ca++ is to bind with troponin @ the Ca++ binding site causing a conformational change in the troponin that helps move the tropomyosin
deeper into the grooves between the actin strands. This uncovers the actin active sites and allows the myosin heads to bind.

---> Ratchet or walking theory

When myosin heads bind to actin it causes a new alignment between the heads and the arm. The head tilts toward the arm at the hinge and is called the POWER STROKE to move the actin filament

ADP + P are released and a new ATP molecule binds to the myosin head.
This then caused a the release of myosin from actin and readies the myosin head for further binding @ an actin active site further down on the filament.

When the cross bridges bind to a new site, the head tilts --> power stroke --> move actin --> release.

This creates a step by step "walking" to pull the ends of the actin filament toward the center of the myosin filament.

Important Physiology Terms ::

1. Depolarization... change in the membrane potential such that the interior becomes less negative.

Usually involves Na+ coming into the cell

2. Repolarization... restores membrane potential to initial resting state.

Usually involves K+ movement out of the cell

3. Refractory Period... time during which a cell is not responsive to further stimulation. This time IS dependent on

the restoration of ELECTRICAL conditions NOT IONICconditions during the repolarization phase.

4. All or None...once an action potential is initiated (after reaching threshold), it runs to completion.

Muscle cells contract or they dont (nothing half way in between).

Remember that the strengthof contraction depends on the NUMBER of cells contracting.

Types of contraction

1. Isometric....contraction occurs with out appreciable decrease in length of the whole muscle.
No movement of the load is achieved. i.e., posture, joint stability

iso = same metric = measure

2. Isotonic....contraction against a constant lead with approximation of the ends.

Movement of the load is achieved andwork is done.

iso = same tonic = tension

Each muscle is served by 1 motor nerve containing hundreds of motor neuron axons.
As the axon enters the muscle, it branches into a number of terminals that form a neuromuscular junction with a single muscle fiber.

A motor neuron and all the muscle fibers it supplies is called a MOTOR UNIT

Response of a muscle to a single brief threshold stimulus is called a MUSCLE TWITCH.
The muscle twitch involves a contraction followed by relaxation.
The duration is variable and if enough tension is added to overcome resistance, the muscle will shorten.

The events of contraction and relaxation will form a graph if recorded on a myogram.

This graph will have 3 identifiable areas :

1) latent period

appears as no response after a stimulus, short duration

event : excitation-contraction coupling

2) period of contraction

appears as a peak

shortening of muscle to overcome resistance and increase in tension

3) period of relaxation

tension drops, no contractile force, tracing returns to baseline

a) Increasing rapidity of stimulation (change speed of stimulation) to produce a WAVE SUMMATION

b) Recruiting more motor units to produce a MOTOR UNIT SUMMATION

Summation of contraction occurs because contractile mechanisms do NOT have a refractory period.

1) Repeated stimulation, before relaxation has occurred, will produce additional activation and an additive response.
If the rate of stimulation increases, the tension produced will rise to a peak, and the periods of relaxation will be very brief. (Incomplete tetanus)

2) With repeated stimulation, the individual responses fuse into one contraction with no relaxation phase and is called a complete tetanic contraction.

3) If the stimulus arrives after the completion of the relaxing the twitch, it forms a staircase like graph, and

is called TREPPE. (The basis for warmup exercises)

4) Muscles are always in a slightly contracted state. No active movements are produced but it creates muscle tone
to keep muscles firm, healthy, and ready to respond, help maintain joint stability, and maintain posture.

Energy Sources

1. ATP from

a. Anaerobic glycolysis

b. Aerobic respiration in the Mitochondria

2. Creatinine Phosphate (CP)

CP + ADP ------> creatine + ATP

The energy comes from the breakdown of glucose into CO2 and H2O.

Glucose can come from foodstuffs, or from storage sites in the muscle and liver known as GLYCOGEN

Glucose is broken down into two 3 carbon molecules called Pyruvic acid, in the process known as GLYCOLYSIS.

In the prescence of Oxygen, pyruvic acid will undergo aerobic glycolysis in the mitochondria and produce ATP, H2O and CO2

In the absence of oxygen, anaerobic glycolysis occurs and lactic acid is produced.
The lactic acid can be converted back to pyruvic acid by an enzyme (LDH) or it can go to the liver to get converted back to glucose.

When muscle exertion is very great, an oxygen debt occurs as Oxygen consumption exceeds available oxygen supply.
Lactic acid production increases due to anaerobic breakdown of glucose.

After a period of exertion is over, extra Oxygen is consumed to remove excess lactic acid and replenish ATP and creatinine phosphate stores.
Muscular activity generates heat that must be carried away.

When muscle fibers are completely depleted of ATP and creatinine phosphate, they develop a state of extreme rigidity called rigor.
When this occurs after death, the condition is called RIGOR MORTIS. The process takes several hours, as the ATP that is lost would
normally be required to cause the separation of the cross bridges between the filaments.
The muscles remain in rigor until the muscle proteins are destroyed by enzymes 13-25 hours later.

Energy output appears as

work done by the muscle

Energy bonds that are formed

heat production

Healthy muscle does not contract except in response to stimulation by its motor nerve supply.

Destruction of this nerve supply causes

1. muscle atrophy

2. flaccid paralysis

3. abnormal exciteablity

4. irregular contraction called fibrillations

Hormones important to muscle activity include

Thyroid hormone....to increase the rate of Energyconsumption by Skeletal Muscle

Adrenalin...from the adrenal gland to help increase the force of contraction

Muscle Types

Fast units, slow units, intermediate units

Fast units have short rapid specialized for fine skilled movement (such as the hands).

Referred to as WHITE muscle

Slow units have a slow response and are adapted forlong slow posture maintaining contration.

Referred to a RED muscle

Intermediate units have characteristics of both fast and slow

The difference between the two are due in part to their innervation.

Attachments of muscle will reflect isometric contractions, so that the body movement is integrated in ways that make maximal motion with a minimal muscular exertion.

Muscle tone occurs even when muscles are at rest. A certain amount of tautness usually remains as a result from nerve impulses coming from the spinal cord.

Prolonged strong contraction of a muscle leads to muscle fatigue.

The nerve impulses pass normally creating normal action potentials,
but the contraction becomes weaker because of reduction of ATP formation in the muscle fibers and the interruption of blood flow.

Muscles operate by applying tension to their points of insertion into bones.
The bones in turn, form various types of lever systems. Therefore, the lever system depends on

1. point of muscle insertion

2. distance from the fulcrum of the lever

3. length of the lever arm

4. postion of the lever.

Kinesiology is the study of different types of muscle, lever systems, and their movements.

Muscle hypertrophy is the result of the foreful muscle activity that causes the muscle SIZE to increase, by the increase in the DIAMETER of individual muscle fibers.

Muscle atrophy will result any time a muscle is not used or used for only weak contractions. It is a decrease in size, tone, and power.

Muscle Aging

With increased age

1. decreased size and power of all muscles

2. decreased diameter due to decreased number of myofibrils

3. increase in amounts of fibrous connective tissue called fibrosis

4. rapid fatigue

5. decrease in ability to repair itself

Muscle Problems ::

ACH receptors

1) autoimmune disease --> Myasthenia Gravis

Muscular Dystrophy (MD)

fibers degenerate, replaced by fat and C.T.

Chemicals

Organophosphates (OPs) --> interfer with the action of ACHase. Muscle contraction

continues because ACH is available

Curare --> binds to ACH receptors, blocks ACH therefore no muscle contraction

CARDIAC MUSCLE SUMMARY

striated, involuntary

single nucleus, centrally located

NO TRIADS

short T tubules

no terminal cisternae in SR

Ca++ in SR and ECF

Intercalated discs --> gap junctions create electrical connections

Pacemaker cells --> spontaneously discharge

ANS to change rate of pacemaker cell discharge

Longer contraction, NO tetanic contractions (would stop heart)

SMOOTH MUSCLE SUMMARY

spindle shaped cells organized into sheets

single nucleus

lack structured NMJ

ANS innervated (involuntary)

NO Sarcomeres or T tubules

Decreased SR, therefore Ca++ ions in cytoplasm interact with calmodulin
Slow speed of contraction, anaerobic

No striations, but do contain interdigitating actin and myosin

Electrical coupling by gap junction

Some smooth muscles have pacemaker cells that are self excitatory.

Types of smooth muscle ::

a. single unit (visceral smooth muscle)

most common

walls of most hollow body organs

in two planes -->

inner circular

outer longitudinal

contract as a unit

above summary list applies

b. Multiunit

fibers are independent

more specialized, so above summary list is different

e.g. pupillary m of iris

arrector pili m. of hair shaft

bronchiolar sm. m. in lungs