1. Skeletal Muscle 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 (pupils)
Heart beat
Production of body heat (Thermogenesis)

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
Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle
Endomysium. Loose connective tissue that surrounds individual muscle fibers
Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

6. Nerve and Blood Vessel Supply
Motor neurons
stimulate muscle fibers to contract
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

7. Muscle Tissue Types

8. Skeletal Muscle Long cylindrical cells Many nuclei per cell Striated
Voluntary
Rapid contractions
Skeletal muscle attaches to our skeleton. *The muscle cells a long and cylindrical. *Each muscle cell has many nuclei. *Skeletal muscle tissue is striated. It has tiny bands that run across the muscle cells. *Skeletal muscle is voluntary. We can move them when we want to. *Skeletal muscle is capable of rapid contractions. It is the most rapid of the muscle types.

9. 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

10. 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 or aponeurosis
Bone markings present where tendons meet bones
Tubercles, trochanters, and crests

11. 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; numbers remain constant
Striated appearance
Nuclei are peripherally located

12. Muscle Attachments

13. Antagonistic Muscles

14. Microanatomy of Skeletal Muscle
In this unit we will primarily study skeletal muscle. Each muscle cell is called a muscle fiber. Within each muscle fiber are many myofibrils.

15. Parts of a Muscle

16. 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

17. Motor Unit: The Nerve-Muscle Functional Unit
Muscle fibers from a motor unit are spread throughout the muscle
Not confined to one fascicle (bundle of skeletal muscle fibers surrounded by perimysium)
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)

18. Motor Unit All the muscle cells controlled by one nerve cell
A motor unit is all the muscle cells controlled by one nerve cell. This diagram represents two motor units. Motor unit one illustrates two muscle cells controlled by one nerve cell. When the nerve sends a message it will cause both muscle cells to contract. Motor unit two has three muscle cells innervated by one nerve cell.

19. Power Output: The Most Physiologically Relevant
Marker of Performance
Power = work / time
= force x distance / time
= force x velocity
Peak power obtained at intermediate loads and intermediate
velocities.
Figure from Berne and Levy, Physiology
Mosby—Year Book, Inc., 1993.

20. Three Potential Actions During Muscle Contraction:
Biceps muscle shortens
during contraction
shortening
Isometric-muscle does not change its length
isometric
Most likely to cause
muscle injury
lengthening
Biceps muscle lengthens
during contraction

21. Recall The Motor Unit:
motor neuron and the muscle fibers it innervates
Spinal
cord
The smallest amount of
muscle that can be activated
voluntarily.
Gradation of force in skeletal
muscle is coordinated largely
by the nervous system.
Recruitment of motor units
is the most important means
of controlling muscle tension.
Since all fibers in the motor
unit contract simultaneously,
pressures for gene expression
(e.g. frequency of stimulation,
load) are identical in all fibers
of a motor unit.
To increase force:
Recruit more M.U.s
Increase freq.
(force –frequency)

22. Increased use: strength training
Early gains in strength appear to be predominantly due to neural factors…optimizing recruitment patterns.
Long term gains almost solely the result of hypertrophy i.e. increased size.

23. Performance Declines with Aging
--despite maintenance of physical activity
100 80
Performance (% of peak) 60 40
Shotput/Discus
Marathon 20
Basketball (rebounds/game) 10 20 30 40 50 60
Age (years)
D.H. Moore (1975) Nature 253:
NBA Register, Edition

24. Number of motor units declines during aging
AGE-ASSOCIATED
ATROPHY DUE TO BOTH…
Individual fiber atrophy
(which may be at least
partially preventable and
reversible through exercise).
Loss of fibers
(which as yet appears
irreversible).
Campbell et al., (1973) J Neurol Neurosurg Psych 36:

25. Motor unit remodeling with aging
Central
nervous
system
Muscle
Motor
neuron
loss
AGING
Fewer motor units
More fibers/motor unit

26. Mean Motor Unit Forces:
FF motor units get smaller in old age and decrease in number
S motor units get bigger with no change in number
Decreased rate of force generation and POWER!!
225
200
175
Adult
Old
150
125
Maximum Isometric Force (mN)
100 75 50 25 FF FI FR S
Motor Unit Classification
Kadhiresan et al., (1996)
J Physiol 493:

27. Refresher Course in Muscle Physiology
4/11/2003
Muscle injury may play a role in the development of
atrophy with aging.
Muscles in old animals are more susceptible to contraction-
induced injury than those in young or adult animals.
Muscles in old animals show delayed and impaired recovery
following contraction-induced injury.
By way of introduction, I’ll briefly reiterate some points that John made in the previous talk…
Increased susceptibility
Decreased ability to recover and prolonged deficits
Because muscles are injured repeatedly throughout life, these two observations, along with others provide circumstantial evidence that muscle injury plays a role in the development of atrophy and weakness with aging.
Muscles of animals of all ages, except perhaps the oldest-old, can continue to adapt to the habitual level of activity.
Following severe injury, muscles in old animals display
prolonged, possibly irreversible, structural and functional
deficits.
EB2003--Susan Brooks

28. 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

29. 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

30. 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

31. Aerobic Respiration Needs oxygen for respiration
Glucose + Oxygen  Carbon Dioxide + Water + Energy
Energy=ATP

32. ATP
ATP or adenosine triphosphate is the form of energy that muscles and all cells of the body use. *The chemical bond between the last two phosphates has just the right amount of energy to unhook myosin heads and energize them for another contraction. Pulling of the end phosphate from ATP will release the energy. ADP and a single phosphate will be left over. New ATP can be regenerated by reconnecting the phosphate with the ADP with energy from our food.

35. Anaerobic Respiration
Without oxygen for respiration
Glucose  Lactic Acid + Energy
Incomplete breakdown of glucose
5% of energy released by aerobic
respiration
Lactic Acid-produces an oxygen
debt because oxygen is needed to
oxidize lactic acid (liver)
REST

38. Muscle Fatigue Lack of oxygen causes ATP deficit
Lactic acid builds up from anaerobic respiration
Muscle fatigue is often due to a lack of oxygen that causes ATP deficit. Lactic acid builds up from anaerobic respiration in the absence of oxygen. Lactic acid fatigues the muscle.

39. Muscle Fatigue

40. Muscle Atrophy Weakening and shrinking of a muscle May be caused
Immobilization
Loss of neural stimulation
Muscle atrophy is a weakening and shrinking of a muscle. It can be caused by immobilization or loss of neural stimulation.

41. Muscle Hypertrophy Enlargement of a muscle More capillaries
More mitochondria
Caused by
Strenuous exercise
Steroid hormones
Hypertrophy is the enlargement of a muscle. Hypertrophied muscles have more capillaries and more mitochondria to help them generate more energy. Strenuous exercise and steroid hormones can induce muscle hypertrophy. Since men produce more steroid hormones than women, they usually have more hypertrophied muscles.

42. Steroid Hormones Stimulate muscle growth and hypertrophy
Steroid hormones such as testosterone stimulate muscle growth and hypertrophy.

43. Muscle Tonus Tightness of a muscle Some fibers always contracted
Muscle tonus or muscle tone refers to the tightness of a muscle. In a muscle some fibers are always contracted to add tension or tone to the muscle.

44. Tetany Sustained contraction of a muscle
Result of a rapid succession of nerve impulses
Tetany is a sustained contraction of a muscle. It results from a rapid succession of nerve impulses delivered to the muscle.

45. Tetanus
This slide illustrates how a muscle can go into a sustained contraction by rapid neural stimulation. In number four the muscle is in a complete sustained contraction or tetanus.

46. Refractory Period
Brief period of time in which muscle cells will not respond to a stimulus
The refractory period is a brief time in which muscle cells will not respond to stimulus.

47. Refractory
The area to the left of the red line is the refractory period for the muscle contraction. If the muscle is stimulated at any time to the left of the line, it will not respond. However, stimulating the muscle to the right of the red line will produce a second contraction on top of the first contraction. Repeated stimulations can result in tetany.

48. Refractory Periods Skeletal Muscle Cardiac Muscle
Cardiac muscle tissue has a longer refractory period than skeletal muscle. This prevents the heart from going into tetany.
Cardiac Muscle

49. Isometric Contraction
Produces no movement
Used in
Standing
Sitting
Posture
Isometric contractions produce no movement. They are used in standing, sitting and maintaining our posture. For example, when you are standing muscles in your back and abdomen pull against each other to keep you upright. They do not produce movement, but enable you to stand.

50. Isotonic Contraction Produces movement Used in Walking
Moving any part of the body
Isotonic contractions are the types that produce movement. Isotonic contractions are used in walking and moving any part of the body.

51. Muscle Spindle

52. Muscle Spindle Responses

53. Alpha / Gamma Coactivation

54. Golgi Tendon Organs

55. 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

56. 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

57. 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