Physiology 1, Fall 2008, LPC

Chapter 12 - Muscles

I - Skeletal Muscle – attached to bones by tendons, voluntary body movement; antagonistic pairs -> flexion & extension

A. Myofibrils

Long, cylindrical cell with many nuclei
Many myofibrils bundled together to form a muscle
Anatomy

Sarcolemma = cell membrane
Sarcoplasm = cytoplasm-contains many glycogen granules & mitochondria
Sarcomere = unit of skeletal muscle between Z disks, contractile unit of a myofibril

Sarcoplasmic reticulum = modified ER, wraps around myofibrils

Longitudinal tubules release Ca++
Terminal cisternae concentrate & sequester Ca++, connected by t-tubules

Myosin-thick filaments, many protein chains with protruding heads, position stabilized by M line (protein)
Actin-thin filaments, globular proteins in long chains, attached to Z discs
Titin –elastic molecule stretches from Z disk to neighboring M line, stabilizes myosin & actin, returns stretched muscles to resting length
Tropomyosin – long thin protein that wraps around actin
filament & partially blocks the myosin-binding sites
Troponin – a Ca++ binding protein that controls position of tropomyosin

B. Contraction

Sliding filament theory – thick & thin filaments slide past each other, moving the Z disks of the sarcomere closer together
Movement of flexible myosin crossbridges push actin filaments toward the center of the sarcomere
Contractile cycle

1. myosin heads bind to actin, no ATP or ADP occupies 2nd binding site on myosin head
2. ATP binds to myosin head, head releases from actin molecule
3. ATP releases Pi, ADP & Pi remain bound to myosin
4. myosin head straightens (to 90o) and attaches to new actin molecule
5.  Pi released  and myosin head springs back toward M line, sliding actin toward M line
6. ADP released from myosin, myosin still bound to actin

Control of contraction

Resting - Tropomyosin blocks myosin head contact with & binding to actin

Initiation of contraction

Increase of cytosolic Ca++ -> Ca++ binds to troponin
Troponin-Ca++ complex pulls tropomyosin away from actin binding site
Myosin binds to actin & completes power stroke

Relaxation

Sarcoplasmic reticulum pumps Ca++ back into its lumen
Ca++ releases from troponin
Tropomyosin slides back to block the myosin-binding site
Fiber relaxes
Titan and elastic connective tissue cause filaments to slide back to resting position

Acetylcholine starts Excitation-Contraction coupling

1. axon terminal releases acetylcholine (Ach)
2. Ach initiates action potential (opens Na+ & K+ channels, -> end-plate potential) in muscle fiber
3. muscle action potential triggers Ca++ release from sarcoplasmic reticulum
4. Ca++ combines with troponin, -> contraction

ATP supply needed for:

1. Crossbridge movement & release during contraction
2. Pump Ca++ back into sarcoplasmic reticulum during relaxation
3. Restore Na+ & K+ levels after excitation-contraction coupling

Slight amount of ATP in muscle
Few minutes’ worth made from phosphocreatine
More can be made via anaerobic respiration
Biggest supply from aerobic respiration

C. Fatigue

Can be from CNS or peripheral
Peripheral fatigue could be anything from neurotransmitter to receptors to Na+, K+, or Ca++ concentration problems or lack of energy

D. Tension developed by individual muscle fibers is a function of fiber length

If too much overlap between actin & myosin, not much space available for contraction
If not much overlap between actin & myosin, not many places for myosin to contact actin and cause sliding for contraction

E. Summation

More tension can be developed in a muscle by action potentials causing contraction before muscle fully relaxed
If action potentials too close together, -> tetanus = no time to relax between stimulations -> maximum tension (until fatigue)

F. Motor Unit

Group of muscle fibers that function together & the somatic motor neuron that controls them.
All muscle fibers in motor unit contract when somatic motor neuron fires
Fine motor control if few fibers innervated by each nerve

Force of contraction controlled by how many motor units are recruited. Motor units have different threshold levels in neurons.

II- Body Movement

Bones form levers
Flexible joints form fulcrums
Muscle attached to bones create force by contracting
Isometric contraction (load stays in same place) – muscle does not shorten. Sarcomeres shorten, generating force, but elastic elements stretch, allowing muscle length to remain the same.
Isotonic contraction (load moves) – Sarcomeres shorten, elastic elements already stretched, entire muscle shortens.

III-Smooth Muscle

Found mostly in walls of hollow organs & tubes
Often generates force to move material through the lumen of the organ
Contraction & relaxation slower than skeletal or cardiac muscle
Uses less energy to generate a given amount of force
Can maintain its force for long periods

A. Cellular Structure

Small, spindle-shaped cells
Single nucleus
Longer actin & myosin filaments than skeletal muscle
Actin & myosin arranged in long bundles that extend diagonally around the cell periphery -> cell becomes globular when contracted
Actin filaments attach to dense bodies, rather than Z disk
Entire surface of thick filament on actin filament, -> actin can slide along myosin for long distatnces

B. Contraction

Controlled by ANS neuron varicosity neurotransmitter release
Some smooth muscle has gap junctions to spread action potential from cell to cell

Increased intracellular Ca++ binds to calmodulin (rather than troponin in skeletal muscle)
Ca++-calmodulin -> cascade which phosphorylates myosin head & increases myosin ATPase activity
Active myosin crossbridges slide along actin & create muscle tension

C. Relaxation

Ca++ pumped out of cell
Ca++ comes off calmodulin
Phosphate removed from myosin
Myosin ATPase activity decreases, -> decrease in muscle tension

D. Ca++

Contraction force is graded according to strength of Ca++ signal
Ca++ entry through membrane channels opened by depolarization, membrane stretch or chemical signals
Some membranes spontaneously open & close ion channels, -> slow wave potential or pacemaker potentials

E. Regulation

Innervation – sympathetic, parasympathetic, or both via neurotransmitters
Hormones
Paracrines
Table 12 3 – very good

IV-Cardiac muscle

Striated, sarcomere structure
May be branched
Single nucleus/cell
Electrically linked via gap junctions in junctions called intercalated disks
Pacemaker potentials
Sympathetic, parasympathetic & hormonal control