Muscle Histology and Physiology

Muscle tissue

Near half body’s mass; transforms chemical energy (ATP) to directed mechanical energy —> exerts force

Skeletal, cardiac, smooth

What are the three types of muscle

Myo, mys, and sarco

What are the prefixes for muscle

Skeletal muscles

Organs attached to bones and skin; elongated cells called muscle fibers, striated, voluntary, contract rapidly, tire easily, powerful; require nervous system stimulation

Cardiac muscle

Only in heart, bulk of heart walls; striated, involuntary, can contract without nervous system stimulation

Smooth muscle

In walls of hallow organs (stomach, bladder, airways); not striated, involuntary, can contract without nervous system

Body movement, maintenance of posture, protection and support, regulating elimination of materials, heat production

What are the functions of skeletal muscle

Excitability

Ability to respond to a stimulus by changing electrical membrane potential

Conductivity

Involves sending an electrical change down the length of the cell membrane

Contractility

Exhibited when filaments slide past each other; enables muscle to cause movement

Extensibility

Ability to be stretched

A whole muscle contains many fascicles; a fascicle contains many muscle fibers/cells

What is the hierarchy if structures in a muscle

Epimysium

Dense irregular connective tissue wrapping the whole muscle

Perimysium

Dense irregular connective tissue wrapping fascicle; houses many blood vessels and nerves

Endomysium

Areolar connective tissue wrapping individual fibers; delicate layer or electrical insulation, capillary support, binding of neighboring cells

Tendons or aponeurosis

Tendons-cordlike structure of dense regular CT

Aponeurosis- thin, flattened sheet of dense irregular CT

Connective muscle components that attach muscle to bones or to skin or another muscle

Deep fascia

Sheet of dense irregular CT located external to epimysium; separates different ,uncles while binding them together; contains nerves, blood vessels, and lymph nodes

Superficial fascia

Areolar and adipose tissue located superficial to deep fascia; separates muscles from skin

Sarcoplasm

Has typical organelles plus contractile proteins and other specializations

Sarcolemma

Has T-tubules that extend deep into the cell; this and its T-tubules have voltage gated ion channels that allow for conduction of electrical signals

Myofibrils

Hundreds of thousands per cell; bundles of myofilaments enclosed in sarcoplasmic reticulum; makes up most of the cells volume

Sarcoplasmic reticulum

Internal membrane complex similar to smooth endoplasmic reticulum

Terminal cisternae

Blind sacs of sarcoplasmic reticulum; serves as reservoirs for calcium ions

Thick filaments

Consist of bundles of many myosin protein molecules; each myosin molecule has 2 heads and 2 intertwined tails; heads have binding site for actin of thin filament and ATPase site; heads point towards ends of the filament

Thin filaments

Consist mostly of two twisted strands of fibrous actin (F-actin); each strand is a necklace of hundreds of actin globules (G-actin); each G-actin has a myosin binding site to which myosin heads attach during contraction

Troponin and tropomyosin

When Ca2+ binds to troponin, it pulls tropomyosin off actin allowing contraction

What are the 2 regulatory proteins in thin filaments and what happens when Ca2+ binds to troponin

Tropomyosin

Twisted string like protein covering actin in a non contracting muscle

Troponin

Globular protein attracted to tropomyosin

Sarcomeres

Myofilamens arranged in repeating units; composed of overlapping thick and thin filaments; delineated at both ends by Z discs; the position of thin and thick filaments give rise to altering I-bands and A-bands

I-bands

Light appearing regions that contain only thin filaments; bisected by Z disc; get smaller when muscle contracts (can disappear wit maximal contraction)

A band

Dark appearing region that contains thick filaments and overlapping thin filaments; contains H zone and M line; makes up central region of sarcomere

H zone

Central portion of A band; only thick filaments present, no thin filament overlap; disappears with maximal muscle contraction

M line

Protein meshwork structure; attachment site for thick filaments

Mitochondria

Abundant for aerobic ATP production

Myoglobin

Within cells allows storage of oxygen used for aerobic ATP production

Glycogen

Stored for when fuel is needed quickly

Creatinine phosphate

Can quickly give up its phosphate group to help replenish ATP supply

Motor unit

A motor neuron and all the muscle fibers it controls; axons of motor neurons from spinal cord innervate numerous muscle fibers; the number of muscle fibers a neuron innervates varies; fibers of this are dispersed throughout the muscle, not just in one clustered compartment

Small- less than 5 muscle fibers. Large- thousands of muscle fibers

How many muscle fibers do small motor units have? Large motor units?

Neuromuscular junction

Location where motor neuron innervates muscle; usually mid-region of muscle fibers; has synaptic knobs, synaptic cleft, and motor end plate

Motor end plate

Specialized region of sarcolemma with numerous folds; has many ACh receptors

Synaptic cleft

Narrow fluid-filled space; separates synaptic knob from the motor end plate; acetylcholineterase resides here

Resting membrane potential (RMP

Exhibited by muscle fibers; fluid inside cell is negative compared to fluid outside cell; is about -90mV in muscle cell; set by leak channels and Na+/K+ pumps (voltage-gated channels are closed)

Neuron excites muscle fibers

What is the first step in the process of contraction

Calcium enters synaptic knob

Synaptic knob releases ACh

ACh binds to its receptors at motor end plate

Steps of neuron exciting muscle fibers Calcium enters during the process of contraction

End plate potential (EPP)

ACh receptors are chemically gated channels that open when ACh binds to them; Na+ diffuses into the cell through the channels (while little K+ diffuses out; cell membrane briefly becomes less negative at the end plate region; EPP is local but it does lead to the opening of voltage gated ion channels in the adjacent region of the sarcolemma

The sliding of filaments, coupling included EPP, muscle action potential, and release of Ca2+ from the sarcoplasmic reticulum

Stipulation of the fiber is coupled with what And what does the coupling include?

Release of Ca2+ from the sarcoplasmic reticulum

Action potential opens voltage gated Ca2+ channels of sarcoplasmic reticulum; Ca2+ diffuses out of cisternae ito sarcoplasm; Ca2+ interacts with myofilaments triggering contraction

At low intracellular Ca2+ concentration

tropomyosin blocks active sites on action, myosin heads cannot attach to actin, muscle fiber is related

At higher intracellular Ca2+ concentration

Ca2+ binds to troponin, troponin changes shape and moves tropomyosin away from myosin binding sites; myosin heads bind to actin, causing sarcomere shortening and muscle contraction

When nerves stimulation ceases, Ca2+ pumped back into SR contraction ends

1. Crossbridge formation

2. Power stroke

3. Release of myosin head

4. Reset myosin head

What are the 4 repeating steps of cross bridge cycling

Crossbridge cycling

Cycling continues as long as Ca2+ and ATp are present; results in sarcomere shortening as Z discs move closer together; narrowing or disappearance of H zone and I bands; thick and thin filaments remain the same length but slide past each other

Events at muscle relaxation

Termination of nerve signal and ACh release from motor neuron; hydrolysis of ACh by acetylcholinesterase; closure of ACh receptor causes cessation of end plate potential; no father action potential generation; closure of calcium channels in sarcoplasmic reticulum by pumps; return of troponin to original shape; return of tropomyosin blockade of actin’s myosin binding sites; return of muscle to original position due to its elasticity

Isometric contraction

Although tension is increased, it is insufficient to overcome resistance; muscle length stays the same (ex. Holding a weight while arm doesn’t move)

Isotonic contraction

Muscle tension overcomes resistance resulting in movement; tone stays constant, but length changes

Concentric contraction and eccentric contraction

Two types of isotonic contraction

Concentric contraction

Muscle shortens as it contracts (ex. In the bicep brachii when lifting a load)

Eccentric contraction

Muscle lengthens as it contracts (ex. In the bicep brachii when lowering a load)

Motor unit

Motor neuron and all (four to several hundred) muscle fibers it supplies; smaller number = fine control; muscle fibers spread throughout muscle so single motor unit causes weak contraction of entire muscle; usually contract asynchronously which helps prevent fatigue

Muscle tension

Forced generated when a muscle is stimulated to contract; lab experiments measure this in myogram and graph it

Twitch

Latent periods, contraction period, relaxation periods

Brief contraction to a single stimulus

What are its 3 periods

Threshold

The minimum voltage that triggers a twitch

Latent period

Time after stimulus but before contraction begins; no change in tension

Contraction period

Time when tension is increasing; begins as power strokes pull thin filaments

Relaxation period

Time when tension is decreasing to baseline; begins with release of cross bridges; generally lasts a little longer than contraction period

Recruitment

Also called multiple motor unit summation; explains how muscles exhibit varying degrees of force; ordere based on side of motor units (small first, large last)

Wave summation temporal summation)

If stimulus frequency set at about 20 per second, relaxation is not completed between twitches, contractile forces add up to produce higher tensions; incomplete tetany and tetany

Incomplete tetany

If frequency is increased further, myogram exhibits this; tension increases and twitches partially fuse

Tetany

If frequency is increased further still (for example 40 to 50 per second) myogram exhibits this; tension trace is a smooth line without relaxation

Fatigue

High frequency stimuli lead to this (decreased tension production)

Size principle of graded muscle responses

Motor units with smallest muscle fibers recruited first; motor units with larger and larger fibers recruited as stimulus intensity increases; largest motor units activated only for most powerful contractions

Muscle tone

Constant, slightly contracted state of all muscles; due to spinal reflexes; keeps muscles firm, healthy, and ready to respond

Number of muscle fibers stimulated (recruitment); relative size of fibers- hypertrophy of cells increases strength; degree of muscle strength

Force contraction depends on number of Crossbridges attached, which is affected by?

Slow or fast fibers and metabolic pathways for ATP synthesis

muscle fibers type classified according to what

Oxidative fibers; glycolytic fibers

Use aerobic pathways; use anaerobic glycolysis

Slow oxidative fibers, fast oxidative fibers, fast glycolytic fibers

Three types of muscle fibers

Aerobic (endurance) exercise

Leads to increased muscle capillaries, number of mitochondria, and myoglobin synthesis; results in greater endurance, strength, and resistance to fatigue; may convert some fast glycolytic fibers into fast oxidative fibers

Resistance exercise (typically anaerobic)

Muscle hypertrophy; increased mitochondria, myofilaments, glycogen stores, and connective tissue; results in increased muscle strength and size

Overload principle

Forcing muscle to work hard promotes increased muscle strength and endurance; muscle adapt to increased demands; muscles must be overloaded to produce further gains; overuse injuries may result from lack of rest

Disuse atrophy

Result of immobilization

Without neural stimulation muscles could atrophy to ¼ initial size; fibrous CT replaces muscle tissue, rehabilitation impossible

Female- 36% Male- 42%

Female skeletal muscle makes up how much % of body mass? Male skeletal muscle?

CT increases, muscle fibers decrease

What happens to connective tissue and muscle fibers with age

Sarcopenia

Loss of muscle mass begins usually by age 30 is called what/ regular exercise reverses this

Cardiac muscle cells

Short, branching fibers; one or two nuclei; striated; many mitochondria; intercalated discs join ends of neighboring fibers; contraction started by hearts autorythmic pacemaker cells; heart rate and contraction force influenced by autonomic nervous system

In blood vessels of cardiovascular system, in bronchioles of respiratory system, in intestines of digestive system, in ureters of unitary system, in uterus of female reproductive system

Where is smooth muscle found (variety of organ systems with variety of roles)