Motor units consist of a single motor neuron and the muscle fibers it innervates. Muscle tension can be increased by recruiting more motor units through increasing the rate and strength of stimulation. Muscles are made up of motor units of varying sizes, with small motor units having few fibers used for precision movements and large motor units having many fibers used for gross movements.

1. Objective 7: Define the term and discuss how
motor units are varying sizes are recruited to
produce movements of varying strength and
precision.
1

2. Objective 7 Motor Unit
Motor Unit: a motor unit consists of a single motor neuron and all
the muscle fibers it innervates.
When the motor neuron is stimulated all the fibers in the motor
unit will contract 2

3. Motor Unit Recruitment
Muscle tension can be
increased by increasing
the number of active
motor units
3

4. Motor Units:
The average number of muscle fibers per motor unit is 150
*Muscles used for precision movements have small motor units with a few
muscle fibers
*Muscles used for gross (large) movements have large motor units with
large numbers of muscle fibers
Not all motor units are activated asynchoronously to prevent fatigue. The
strength of contraction increases when increasing numbers of motor units
are activated
MUSCLE NUMBER OF MOTOR
UNITS
FIBERS/MOTOR UNIT
BICEPS BRACHII 774 750
FIRST LUMBRICAL 119 110
GASTROCNEMIUS 580 1720
LATERAL RECTUS 98 10
4

5. The Motor Unit
Which of these motor units do you think would have
a stronger contraction? 5

6. Objective 8: Describe the following types of muscle
responses to stimulation: twitch, graded muscle
contraction (wave summation, incomplete tetanus,
complete tetanus, multiple motor unit summation), treppe,
isotonic contraction and isometric contraction.
Muscle tone: refers to the fact that all muscles are in a slightly
contracted state, even if they appear to be relaxed;
this partial contraction is maintained by the nervous
system and keeps muscles from atrophy; it also
helps to maintain stability and posture
6

7. Contraction: the process of generating force in a muscle as a
result of cross bridge activity; contraction may or may not lead
to shortening
Tension (effort): the force exerted by a muscle on a load
Load (resistance): any force that opposes the tension (effort)
generated by muscle contraction
Muscle twitch/contraction: the response of a skeletal muscle to a single
brief stimulus
7

8. Latent (lag) period: short period of time after stimulation when
excitation/coupling is occurring; or from AP in nerve to
Ca++
binding to troponin
Contraction period: cross bridge activity leads to tension development;
the amount of tension depends on the number of
motor units recruited (activated)
Relaxation period: cross bridge activity stops as Ca2+
is pumped back
into the sarcoplasmic reticulum; muscle tension falls
Myogram: the graph of a muscles mechanical contractile activity
(tension or shortening) vs. time (t vs. T)
8

9. Types of Contraction: Isotonic vs. Isometric
Isotonic contraction: muscle changes length and moves a load
Tension generated by the muscle exceeds resistance (load)
and the muscle shortens, or T > L
Concentric Action:
muscle shortens
Eccentric action:
muscle lengthens
9

10. Isometric Contraction: muscle generates tension but
does not shorten or lengthen (isometric means same
length)
The resistance (load) is greater than the tension
developed by the muscle or L > T
10

11. Objective 9: Describe the role of the following in
supplying energy for muscle contraction: ATP,
creatine phosphate, anaerobic respiration and
aerobic respiration. Define the following related
terms: aerobic endurance, anaerobic threshold,
muscle fatigue, contractures and oxygen debt.
11

12. Objective 9 Metabolism
ATP is the preferred fuel for muscle contraction; it is needed for muscle
contraction and for muscle relaxation:
1. ATP fuels the power stroke: moves the cross bridges from low energy
to high energy positions
2. ATP is needed to detach cross bridges after the power stroke; new
ATP binds to cross bridges, allowing them to break free from the actin
filaments
3. ATP is needed for sarcomere relaxation since it is used to pump
calcium back into the sarcoplasmic reticulum
12

13. Where does a muscle cell get all of this ATP?
1) Muscle fibers store very little ATP; when activity starts, stored
ATP is hydrolyzed (1st
8 seconds of contraction)
Adenosine P-P-P
ATPase
Adenosine P-P + Pi + energy
Three additional sources of ATP for contracting
muscle fibers:
2) Creatine phosphate (8-10 seconds of contraction)
creatine
creatine P + ADP
kinase
creatine + ATP
13

14. 3. Anaerobic glycolysis: quick but low yield (up to 3 minutes of
contraction)
glucose lactic acid + 2 ATP
- takes place in the sarcoplasm
- little ATP is generated from each glucose
- relatively quick pathway
- used primarily for short term, vigorous activity
(sprinting, diving, tennis, soccer, etc)
4. Aerobic respiration: slow but high yield (endurance activities)
glucose + O2 CO2 + H2O + 36 ATP
- takes place in mitochondria
- slower pathway
- used for prolonged activities, such as jogging
14

15. Definitions:
Aerobic endurance: the length of time that a muscle can continue to
contract using aerobic pathways
Anaerobic threshold: the point at which muscle metabolism switches
from aerobic to anaerobic
Muscle fatigue: a state in which a muscle is physiologically unable
to contract; in this state, the rate of ATP
production is less than the rate of ATP consumption
Oxygen debt: the amount of oxygen needed to restore muscle to
its resting metabolic state
In the muscle cell:
lactic acid + O2 pyruvic acid ATP + CO2 + H2O
-the ATP generated will restore resting levels of ATP and CP
In the liver:
lactic acid glucose glycogen
15

16. 16

17. Objective 8/10 Strength/Duration/Velocity
Strength (tension) of Contraction
The amount of strength (tension) generated by a
muscle depends on the following factors:
1. The rate (frequency) of stimulation; increased rate of
stimulation to a single fiber allows more force to develop (from
wave summation, to tetnus)
2. The number of motor units recruited (motor unit recruitment);
the greater the number of motor units stimulating a whole
muscle, the greater the force of contraction
3. The starting length of the sarcomere (degree of muscle
stretch); there is an optimum sarcomere length which allows for
maximum contraction; the resting length of skeletal muscle
sarcomeres is the optimum length
17

18. Graded Muscle Contractions: strength (tension) of a
contraction can range from weak to strong (pat vs
slap)
Rapid stimuli, strong stimuli
Many activated motor units Strong contraction
High actin/myosin overlap
Low frequency stimuli, weak stimuli
Few activated motor units Weak contraction
Low actin/myosin overlap
18

19. 1. Graded contractions produced by changing the rate (frequency)
of stimulation:
Wave (temporal) summation: a second stimulus is applied to a
muscle before it finishes relaxing
Incomplete tetanus: like wave summation, but the stimuli are
delivered more frequently
Complete tetanus: prolonged, smooth contraction that results
from very rapid stimulation 19

20. 2. Graded contractions produced by changing stimulus
strength:
Multiple motor unit summation: increasing the voltage (strength) of
stimulation to the motor units increases the number of motor units
recruited; as a result, stronger muscle contractions are produced
Note: small motor units are recruited first then larger and larger
ones
Other terms:
Treppe: also called the staircase
effect; the strength of muscle
contraction increases with
repeated stimulation; the first few
contractions cause the muscle to
“warm up” , temperature rises,
blood flow increases, etc.
20

21. So, to increase the strength (tension) of a
contraction:
1. Increase the rate
(frequency) of
stimulation
– Go from a twitch
contraction to
complete tetanus
2. Increase the strength of the
stimulus
– Recruit more motor units and
larger motor units (Treppe)
21

22. 3. Length-Tension Relationship
A maximum tetanic contraction is produced at an optimal overlap of thick
and thin filaments
22

23. By decreasing
filament overlap
no tension can
develop
By increasing fiber
(actin/mysosin) overlap,
Less tension can develop
Optimal overlap,
maximal tension can
develop
http://labyrinth.mvm.ed.ac.uk/mnode.asp?id=644
23

24. Velocity/Duration:
Velocity: the rate of contraction, speed over unit time (how fast)
Duration: how long the contraction lasts before fatigue sets in
Velocity and duration depend on characteristics of the load:
As load increases velocity decreasesAs load increases duration decreases
The Bottom Line
As load increases, velocity and duration decrease, or, the
heavier the weight the slower the lift and shorter the time
you can hold the weight 24

25. Figure 9.21 Factors that increase the force of skeletal muscle contraction.
Large
number of
muscle
fibers
recruited
Large
muscle
fibers
High
frequency of
stimulation
(wave
summation
and tetanus)
Muscle and
sarcomere
stretched to
slightly over 100%
of resting length
Contractile force (more cross bridges attached)
25

26. Figure 9.23 Factors influencing velocity and duration of skeletal muscle contraction.
Predominance
of fast glycolytic
(fatigable) Type III
fibers
Contractile
velocity
Small load Predominance
of slow oxidative
(fatigue-resistant)
Type I fibers
Contractile
duration
26

27. Velocity and duration also depends on the
characteristics of the muscle fiber
27

28. Fiber type (Type I) Slow Oxidative (Type II) Fast Oxidative (Type III) Fast Glycolytic
Structure slow myosin ATPase fast myosin ATPase fast mysosin ATPase
Aerobic adapted Aerobic adapted Anaerobic adapted
- myoglobin (red fibers) - some myoglobin (pinks) - no myoglobin (white fibers)
- well vascularized - well vascularized -poorly vascularized
-many mitochodnria - many mitochondria -few mitochondria
-small diameter fibers - intermediate diameter -large diameter
Functional -slow speed of contraction fast speed of contraction fast speed of contraction
Properties -fatigue resistant fatigue resistant fatigable
-used for endurance and used for endurance used for intense, short
prolonged contraction and rapid contraction term movements
28
-Repeated low-level
contractions e.g. walking
or low intensity cycling for
30 mins.
-Postural muscles, e.g. of
the neck and spine, & leg
muscles (which have Type
I & Type IIa fibres).
-Activities involving
speed, strength and
power, e.g. moderately
weight training and fast
running e.g. 400 metres.
-Leg muscles have large
quantities of both Type I
and Type IIa fibers.
Short, fast, bursts of
power (but rapid
fatigue) e.g. heavy
weight training,
power lifting, and
100 metre sprints.
-Arm muscles. N.B.
Type IIb fibres can
be converted into
IIa fibres by
resistance training.

29. Contraction: the process of generating force in a muscle as a
result of cross bridge activity; contraction may or may not lead
to shortening
Tension (effort): the force exerted by a muscle on a load
Load (resistance): any force that opposes the tension (effort)
generated by muscle contraction
Fulcrum (joint):elbow, shoulder…etc
Objective 12: Lever Systems
29

30. • Basic principle of levers
Mechanical Advantage: Mechanical Disadvantage:
Effort farther/load closer to fulcrum Effort closer/load farther from
fulcrum
• Three classes of levers
– Depends on relative position of effort, fulcrum, load
Effort
Load
Fulcrum
Effort
Load
Fulcrum
30

31. Figure 10.4a Lever systems. (1 of 2)
First-class lever
Arrangement:
load-fulcrum-effort
Load
L
Effort
Fulcrum
Load
L
Effort
Fulcrum
Load
Fulcrum
Effort
In the body: A first-class lever system
raises your head off your chest. The
posterior neck muscles provide the effort,
the atlanto-occipital joint is the fulcrum,
and the weight to be lifted is the facial
skeleton.
Example: scissors, seesaw
First-class lever
31

32. Figure 10.4b Lever systems. (1 of 2)
Second-class lever
Arrangement:
fulcrum-load-effort
Load
L
EffortFulcrum
Load
L
Effort
Fulcrum
Load
Fulcrum
Effort
In the body: Second-class leverage is
exerted when you stand on tip-toe. The
effort is exerted by the calf muscles
pulling upward on the heel; the joints of
the ball of the foot are the fulcrum; and
the weight of the body is the load.
Example: wheelbarrow, standing on toes
Second-class lever
32

33. Figure 10.4c Lever systems. (1 of 2)
Third-class lever
Arrangement:
load-effort-fulcrum
Load
L
Effort
Fulcrum
Load
L
Effort
Fulcrum
Load
Fulcrum
Effort
In the body: Flexing the forearm by the
biceps brachii muscle exemplifies
third-class leverage. The effort is exerted
on the proximal radius of the forearm, the
fulcrum is the elbow joint, and the load is
the hand and distal end of the forearm.
Example: tweezers or forceps
Third-class lever
33

34. Objective 13 Terms For Muscle Contraction
Prime Mover (agonist): principle muscle performing a movement
Antagonist: muscle the reverse the action of a prime mover
Synergist: muscle that assists the prime mover by performing the
same action or by preventing an opposite action
Fixator: muscle that stabilizes the bone of origin of the muscle
Example: Arm abduction
Prime Mover: deltoid
Antagonist: latissimus dorsi
Synergist: supraspinatus
Fixator: pectoralis major
34

35. Objective 14 Disuse Atrophy
Disorder Cause(s) Characteristics
Disuse Atrophy prolonged inactivity due to such pathologic
reduction in the factors as bed rest, casting or normal size of a
muscle or local nerve damage muscle fibers
35

36. Disorder Cause(s) Characteristics
Muscle Cramps circulatory impairment, heat sustained involuntary
disorders which lead to contractions of a
electrolyte disorders skeletal muscle
diuretic intake, unknown
causes
Muscular dystrophy
X linked recessive disorder see a reduction in the
in ½ of all cases; defective number of muscle
gene for dystrophin fibers and necrosis
causing dystrophin to be and replacement
absent or abnormal with endomysial
connective tissue and
fat; delayed sitting and
walking and
standing with
progressive weakness
in the shoulder and
pelvic girdles
36

37. Disorder Cause(s) Characteristics
Myasthenia Gravis defective transmission at fatigue; chronic
the neuromuscular junction respiratory infections
due to a reduction in the muscle weakness
number of Ach receptors
37

38. Objective 11 Smooth Muscle
A. Structure
• Small spindle shaped fibers, with a single centrally located nucleus
• No striations
• Poorly developed sarcoplasmic reticulum, no T tubules
• Sarcolemma has caveoli which sequester calcium (like SR in skeletal
muscle)
Calveoli 38

39. • Troponin is absent; tropomyosin is present but not at active
site
• No sarcomeres -thick and thin myofilaments are arranged
diagonally so that when the cell contracts, it twists like a
corkscrew
• No Z discs - non-contractile intermediate filaments are
present and are attached to dense bodies; dense bodies are
attached to the sarcolemma and to the thin myofilaments
No sarcomeres Dense bodies 39

40. -Individual muscle fibers are covered by endomysium
-Fibers may or may not be organized into fascicles
-In most organs, smooth muscle fibers are arranged into cellular
sheets with circular or longitudinal orientation
40

41. Smooth Muscle Contraction – mechanism
• Ca2+
concentrations increase in the sarcoplasm
• Ca2+
activates calmodulin (a troponin like molecule)
• Calmodulin activates myosin light chain kinase
• myosin light chain kinase uses ATP to phosphorylate myosin
cross bridges
• phosphorylated cross bridges bind to actin filaments
• cross bridge cycle produces tension and contraction/shortening
41

42. Contraction regulation
Neural regulation- stimulated by the ANS
- neurotransmitters include acetylcholine, norepinephrine
Local factors - some smooth muscle is stimulated by pacemaker cells,
hormones, anoxia, histidime, pH, CO2
Special Properties
Stress relaxation response- can stretch beyond resting length and
still contract (think bladder)
Hyperplasia– cells divide to increase cell numbers
42

43. Single – Unit (Visceral) Multi-unit
•Gap junctions
•Circular and longitudinal layers
•Contracts as a unit
•Walls of hollow organs bladder,
stomach, GI tract, uterus
•Few gap junctions
•Many nerve endings
•Walls of large airways, large
arteries, ciliary muscle (eye),
arrector pili
Types of Smooth Muscle
Intestine Artery and Vein 43