Neuromuscular system Flashcards
subdivisions of nervous system
CNS: integration (brain, spinal cord)
PNS: peripheral nervous system
- SNS: motor (afferent and efferent)
- ANS: involuntary (cardiac, smooth muscles) (PSNS, SNS)
Neuromuscular system
Interaction between nervous system and musculoskeletal systems
Isotonic contraction
Constant force producing (shortening/lengthening)
- concentric: muscle shortening (positive work; acceleration)
- eccentric: muscle lengthening (negative work; deceleration)
Isokinetic contraction
Constant velocity while shortening/lengthening
- positive or negative work; acceleration or deceleration
- cannot go faster than machine
Isometric contraction
Constant muscle length
- static: limb does not displace/change rotation
- zero work done
- fixed contraction
Tools to study muscle
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
Motor Unit
A motor neuron (MU) and all the muscle fibers it innervates.
- basic unit of contraction
Cross intervention
Efferent neurons of the skeletal muscles
- A1 = FT ; A2 = ST
- originate from spinal cord
- end plates (NMJ)
- innervate MF by myelinated MN
Innervation ratio
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)
Types of Motor Units
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)
Types of Motor Fibers
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
Slow Twitch Fibers
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
Intermediate Twitch Fibers
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
Fast Twitch Fibers
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
Polymorphic Muscle Fibers
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)
Muscle Fiber Type Distribution
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
Motor Unit Recruitment
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
Motor Unit during Strength Exercise
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
Force development in muscle
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
Neural factors of muscle force
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
Tetanus
- 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»_space; irregular/unfused tetanus
Length - Tension Relationship
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)
Joint Angle
“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
Force-Velocity Relationship
shortening (concentric) = positive work
lengthening (eccentric) = negative work
concentric: higher velocity = less force
eccentric: higher velocity = higher force
