1. Muscular System: Histology and Physiology

2. Muscular System Functions
Body movement (Locomotion)
Maintenance of posture
Respiration
Diaphragm and intercostal contractions
Communication (Verbal and Facial)
Constriction of organs and vessels
Peristalsis of intestinal tract
Vasoconstriction of b.v. and other structures
Heart beat
Production of body heat

3. Properties of Muscle
Excitability: capacity of muscle to respond to a stimulus
Contractility: ability of a muscle to shorten and generate pulling force
Extensibility: muscle can be stretched back to its original length
Elasticity: ability of muscle to recoil to original resting length after stretched

4. Types of Muscle Skeletal Smooth Cardiac Attached to bones
Makes up 40% of body weight
Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement
Voluntary in action; controlled by somatic motor neurons
Smooth
In the walls of hollow organs, blood vessels, eye, glands, uterus, skin
Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow,
In some locations, autorhythmic
Controlled involuntarily by endocrine and autonomic nervous systems
Cardiac
Heart: major source of movement of blood
Autorhythmic

5. Connective Tissue Sheaths
Connective Tissue of a Muscle
Epimysium. Dense regular c.t. surrounding entire muscle
Separates muscle from surrounding tissues and organs
Connected to the deep fascia
Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle
Contains b.v and nerves
Endomysium. Loose connective tissue that surrounds individual muscle fibers
Also contains b.v., nerves
Collagen fibers of all 3 layers come together at each end of muscle to form a tendon.

7. Nerve and Blood Vessel Supply
Motor neurons
stimulate muscle fibers to contract
Neuron axons branch so that each muscle fiber (muscle cell) is innervated
Form a neuromuscular junction (= myoneural junction)
Capillary beds surround muscle fibers
Muscles require large amts of energy
Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

8. Basic Features of a Skeletal Muscle
Muscle attachments
Most skeletal muscles run from one bone to another
One bone will move – other bone remains fixed
Origin – less movable attach- ment
Insertion – more movable attach- ment

9. Basic Features of a Skeletal Muscle
Muscle attachments (continued)
Muscles attach to origins and insertions by connective tissue
Fleshy attachments – connective tissue fibers are short
Indirect attachments – connective tissue forms a tendon.

10. Skeletal Muscle Structure
Composed of muscle cells (fibers), connective tissue, blood vessels, nerves
Fibers are long, cylindrical, and multinucleated
Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length
Develop from myoblasts
Striated appearance
Nuclei are peripherally located

11. Muscle Fiber Anatomy Sarcolemma - cell membrane
Surrounds the sarcoplasm (cytoplasm of fiber)
Contains many of the same organelles seen in other cells
An abundance of the oxygen-binding protein myoglobin
Punctuated by openings called the transverse tubules (T-tubules)
Narrow tubes that extend into the sarcoplasm at right angles to the surface
Filled with extracellular fluid
Myofibrils -cylindrical structures within muscle fiber
Are bundles of protein filaments (=myofilaments)
Two types of myofilaments
Actin filaments (thin filaments)
Myosin filaments (thick filaments)
At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma
When myofibril shortens, muscle shortens (contracts)

12. Sarcoplasmic Reticulum (SR)
SR is an elaborate, smooth endoplasmic reticulum
runs longitudinally and surrounds each myofibril
Form chambers called terminal cisternae on either side of the T-tubules
A single T-tubule and the 2 terminal cisternae form a triad
SR stores Ca++ when muscle not contracting
When stimulated, calcium released into sarcoplasm
SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction

13. Sarcoplasmic Reticulum (SR)
Figure 9.5

14. Parts of a Muscle

15. Sarcomeres: Z Disk to Z Disk
Sarcomere - repeating functional units of a myofibril
About 10,000 sarcomeres per myofibril, end to end
Each is about 2 µm long
Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.
A bands: a dark band; full length of thick (myosin) filament
M line - protein to which myosins attach
H zone - thick but NO thin filaments
I bands: a light band; from Z disks to ends of thick filaments
Thin but NO thick filaments
Extends from A band of one sarcomere to A band of the next sarcomere
Z disk: filamentous network of protein. Serves as attachment for actin myofilaments
Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment
Sarcomeres: Z Disk to Z Disk

16. Structure of Actin and Myosin

17. Myosin (Thick) Myofilament
Many elongated myosin molecules shaped like golf clubs.
Single filament contains roughly 300 myosin molecules
Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.
Myosin heads
Can bind to active sites on the actin molecules to form cross-bridges. (Actin binding site)
Attached to the rod portion by a hinge region that can bend and straighten during contraction.
Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction
Myosin (Thick) Myofilament

18. Actin (Thin) Myofilaments
Thin Filament: composed of 3 major proteins
F (fibrous) actin
Tropomyosin
Troponin
Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.
Composed of G actin monomers each of which has a myosin-binding site (see yellow dot)
Actin site can bind myosin during muscle contraction.
Tropomyosin: an elongated protein winds along the groove of the F actin double helix.
Troponin is composed of three subunits:
Tn-A : binds to actin
Tn-T :binds to tropomyosin,
Tn-C :binds to calcium ions.
Actin (Thin) Myofilaments

19. Sliding Filament Model of Contraction
Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree
In the relaxed state, thin and thick filaments overlap only slightly
Upon stimulation, myosin heads bind to actin and sliding begins

20. Sliding Filament Model of Contraction
Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere
As this event occurs throughout the sarcomeres, the muscle shortens
PLAY
InterActive Physiology®: Muscular System: Sliding Filament Theory

21. Neuromuscular Junction
Region where the motor neuron stimulates the muscle fiber
The neuromuscular junction is formed by :
1. End of motor neuron axon (axon terminal)
Terminals have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)
2. The motor end plate of a muscle
A specific part of the sarcolemma that contains ACh receptors
Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

22. Neuromuscular Junction
Figure 9.7 (a-c)

23. Motor Unit: The Nerve-Muscle Functional Unit
A motor unit is a motor neuron and all the muscle fibers it supplies
The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds ( )
Muscles that control fine movements (fingers, eyes) have small motor units
Large weight-bearing muscles (thighs, hips) have large motor units

24. Motor Unit: The Nerve-Muscle Functional Unit
Figure 9.12 (a)

25. Motor Unit: The Nerve-Muscle Functional Unit
Muscle fibers from a motor unit are spread throughout the muscle
Not confined to one fascicle
Therefore, contraction of a single motor unit causes weak contraction of the entire muscle
Stronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)

26. Smooth Muscle Cells are not striated
Fibers smaller than those in skeletal muscle
Spindle-shaped; single, central nucleus
More actin than myosin
No sarcomeres
Not arranged as symmetrically as in skeletal muscle, thus NO striations.
Caveolae: indentations in sarcolemma;
May act like T tubules
Dense bodies instead of Z disks
Have noncontractile intermediate filaments
Smooth Muscle

27. Smooth Muscle Grouped into sheets in walls of hollow organs
Longitudinal layer – muscle fibers run parallel to organ’s long axis
Circular layer – muscle fibers run around circumference of the organ
Both layers participate in peristalsis
Figure 9.24

28. Smooth Muscle Is innervated by autonomic nervous system (ANS)
Visceral or unitary smooth muscle
Only a few muscle fibers innervated in each group
Impulse spreads through gap junctions
Who sheet contracts as a unit
Often autorhythmic
Multiunit:
Cells or groups of cells act as independent units
Arrector pili of skin and iris of eye

29. Cardiac Muscle
Found only in heart where it forms a thick layer called the myocardium
Striated fibers that branch
Each cell usually has one centrally-located nucleus
Fibers joined by intercalated disks
IDs are composites of desmosomes and gap junctions
Allow excitation in one fiber to spread quickly to adjoining fibers
Under control of the ANS (involuntary) and endocrine system (hormones)
Some cells are autorhythmic
Fibers spontaneously contract (aka Pacemaker cells)

30. Cardiac Muscle Tissue
Figure 10.10a

31. Disorders of Muscle Tissue
Muscle tissues experience few disorders
Heart muscle is the exception
Skeletal muscle – remarkably resistant to infection
Smooth muscle – problems stem from external irritants

32. Disorders of Muscle Tissue
Muscular dystrophy – a group of inherited muscle destroying disease
Affected muscles enlarge with fat and connective tissue
Muscles degenerate
Types of muscular dystrophy
Duchenne muscular dystrophy
Myotonic dystrophy

33. Disorders of Muscle Tissue
Myofascial pain syndrome – pain is caused by tightened bands of muscle fibers
Fibromyalgia – a mysterious chronic-pain syndrome
Affects mostly women
Symptoms – fatigue, sleep abnormalities, severe musculoskeletal pain, and headache

34. Developmental Aspects: Regeneration
Cardiac and skeletal muscle become amitotic, but can lengthen and thicken
Myoblast-like satellite cells show very limited regenerative ability
Cardiac cells lack satellite cells
Smooth muscle has good regenerative ability
There is a biological basis for greater strength in men than in women
Women’s skeletal muscle makes up 36% of their body mass
Men’s skeletal muscle makes up 42% of their body mass

35. Developmental Aspects: Male and Female
These differences are due primarily to the male sex hormone testosterone
With more muscle mass, men are generally stronger than women
Body strength per unit muscle mass, however, is the same in both sexes

36. Developmental Aspects: Age Related
With age, connective tissue increases and muscle fibers decrease
Muscles become stringier and more sinewy
By age 80, 50% of muscle mass is lost (sarcopenia)
Decreased density of capillaries in muscle
Reduced stamina
Increased recovery time
Regular exercise reverses sarcopenia