Neuromuscular system

Question 1

subdivisions of nervous system

Answer 1

CNS: integration (brain, spinal cord)
PNS: peripheral nervous system
- SNS: motor (afferent and efferent)
- ANS: involuntary (cardiac, smooth muscles) (PSNS, SNS)

Question 2

Neuromuscular system

Answer 2

Interaction between nervous system and musculoskeletal systems

Question 3

Isotonic contraction

Answer 3

Constant force producing (shortening/lengthening)
- concentric: muscle shortening (positive work; acceleration)
- eccentric: muscle lengthening (negative work; deceleration)

Question 4

Isokinetic contraction

Answer 4

Constant velocity while shortening/lengthening
- positive or negative work; acceleration or deceleration
- cannot go faster than machine

Question 5

Isometric contraction

Answer 5

Constant muscle length
- static: limb does not displace/change rotation
- zero work done
- fixed contraction

Question 6

Tools to study muscle

Answer 6

Electromyography: EMG voltage cause forced contraction, understand MU recruitment

Muscle biopsy: direct measure of muscle properties, frozen in nitrogen for analysis

Imaging techniques: CT, MRI, NIRS

Isokinetic dynamometry and force transducers: muscle can only contract as fast as machine allows for

Question 7

Motor Unit

Answer 7

A motor neuron (MU) and all the muscle fibers it innervates.
- basic unit of contraction

Question 8

Cross intervention

Answer 8

Efferent neurons of the skeletal muscles
- A1 = FT ; A2 = ST
- originate from spinal cord
- end plates (NMJ)
- innervate MF by myelinated MN

Question 9

Innervation ratio

Answer 9

Number of MF innervated by one MN
- fine control (fingers) have smaller ratio (less MF per 1 MN)
- gross control (thigh) has larger ratio (more MF per 1 MN)

Question 10

Types of Motor Units

Answer 10

In skeletal muscle: types 1, 2a, 2x
- depends on structure and function characteristics of MN and involved MF
- twitch properties: ST or FT
- contractile: slow (S), fatigue resistant (FR), fast fatigable (FF)
- metabolic: slow oxidative (SO), fast oxidative glycolytic (FOG), fast glycolytic (FG)

Question 11

Types of Motor Fibers

Answer 11

MF type depends on type of MU and myosin heavy chain (MHC)
- S (SO) MU have type 1 MFs
- FR (FOG) MU have type 2a MFs
- FF (FG) MU have type 2x MFs

• all MF in a MU have the same properties
• in general MU can vary

Question 12

Slow Twitch Fibers

Answer 12

contractile: ST metabolic: SO type 1
- small muscle diameter
- high mitochondria, capillary density, myoglobin
- slow twitch and relaxation time
- low force production and fatigability
- low PCr, glycogen, mATPase, glycolytic enzymes
- high triglyceride storage and oxidative enzymes

Question 13

Intermediate Twitch Fibers

Answer 13

contractile: FTa metabolic: FOG type 2a
- intermediate MF diameter, mitochondria, capillary, Mb
- fast twitch and relaxation time
- intermediate force production and fatigability
- high PCr
- intermediate glycogen, mATPase, glycolytic enzymes, triglycerides, oxidative enzymes

Question 14

Fast Twitch Fibers

Answer 14

contractile: FTx metabolic: FG type 2x
- large MF diameter
- low mitochondria, capillaries, Mb
- fast twitch and relaxation
- high force production and fatigability
- high PCr, glycogen, mATPase, glycolytic enzymes
- low triglycerides, oxidative enzymes

Question 15

Polymorphic Muscle Fibers

Answer 15

MF are classified mostly by their Myosin heavy chain content (MHC)(1, 2a, 2x)
- different genes code for MHC isoforms
type 1 = type 2a = type 2ax = type 2x
- more of spectrum, especially in the fast (2a and 2x)

Question 16

Muscle Fiber Type Distribution

Answer 16

MF type varies depending on
- the person (athlete in certain sports, lifestyle, genetics) (most have 45-55% ST)
- muscles (soleus type 1; gastroc type 1 and 2)
- with in muscles
- according to muscle function (postural is type 1, explosive type 2)
- possible gender differences, too

Question 17

Motor Unit Recruitment

Answer 17

First SO is recruited, then FOG and finally FG
- the more MU recruited = stronger contraction
- time to peak tension (TPT) faster in F
- F has more myofilaments and are larger

Question 18

Motor Unit during Strength Exercise

Answer 18

Need to recruit AND fatigue MU in order to train and improve them
- ST are low fatigable and need LSD to improve, which will not recruit FT or INT
- strength training in high loads for FT

Question 19

Force development in muscle

Answer 19

Force produced by a muscle depends on neural and mechanical factors
- neural: rate-coding, number-coding
- mechanical: length-tension relationship, joint angle, force-velocity relationship, power-velocity, elastic-force, architectural design

Question 20

Neural factors of muscle force

Answer 20

rate coding: as frequency of stimulation increase so does the force that the muscle produces

number coding: more MU that are recruited allow for greater force contraction

Question 21

Tetanus

Answer 21

• second stimulation occur before the fiber has relaxed = temporal summation, slightly greater contraction than before
• enough stimulation to prohibit relaxation they form to make irregular tetanus then smooth tetanus (great contractions)
• regular/fused tetanus&raquo_space; irregular/unfused tetanus

Question 22

Length - Tension Relationship

Answer 22

Optimal overlap of thin and thick filaments allow for best sarcomere length (100-120% of rest length) and thus the greatest force
- elongated: little overlap, hard for cross over bridges to form
- shortened: too much overlap, cannot cross over bridge
“active tension” (impact muscle force mechanically)

Question 23

Joint Angle

Answer 23

“strength curves” (impact muscle force mechanically)
Peak tension depends on joint angle
- lever arm mechanics, individual anatomical differences, angle of pull
- ascending curve: increase torque as joint angle increase
- descending curve: decrease torque as joint angle increase
- ascending/descending curve: force increase then decrease

Question 24

Force-Velocity Relationship

Answer 24

shortening (concentric) = positive work
lengthening (eccentric) = negative work

concentric: higher velocity = less force
eccentric: higher velocity = higher force

Question 25

Power-Velocity Relationship

Answer 25

More FT = more power!
- power is positively related to velocity
- power is greatest w intermediate shortening velocities because P = (f x d) / t

Question 26

More FT leads to more force and power

Answer 26

regardless of velocity, FT leads to more force and power
- larger MF (CSA more actin and myosin, in males)
- faster MN conduction velocities
- greater and faster Ca release
- faster mATPase activity
- greater reliance on ATP-PCr and glycolytic systems