Muscle Structure Flashcards
What is the term given to skeletal muscle due to the presence of multiple nuclei within a single cytoplasm?
syncytia
Thick filaments of skeletal muscle are called ____
myosin
Thin filaments of skeletal muscle are called ____
actin
Actin filaments are aligned and connected to the Z-line by ____
Nebulin (guidewire for actin)
Myosin filaments are connected to the Z-line by ____
Titin (tightens myosin)
What are extrafusal fibers responsible for?
movement (contraction)
What are intrafusal fibers responsible for?
stretch reception (ie., proprioception) by muscle spindles
Extrafusal fibers contain (sensory/motor/both) neurons
motor neurons
Intrafusal fibers contain (sensory/motor/both) neurons
motor AND sensory neurons
The fascial layer covering the entire skeletal muscle is called ____
epimysium
A skeletal muscle is composed of multiple ____
fascicles
The fascial layer surrounding each fascicle is called ____
perimysium
A fascicle is composed of multiple ____
muscle fibers (muscle cells)
The fascial layer surrounding each muscle fiber is called ____
endomysium
A muscle fiber is composed of multiple ____
myofibrils
A myofibril is a bundle of ____
myofilaments (myosin & actin)
What part of skeletal muscle is involved in compartment syndrome?
epimysium
(treatment = fasciotomy)
When at rest, the myosin binding site on thin filaments is physically blocked by the ____
troponin-tropomyosin complex
What event unblocks the binding sites on actin?
Ca2+ binds to troponin
What is the function of the sarcoplasmic reticulum?
Ca2+ storage (relaxation) & release (contraction)
What travels through the sarcoplasmic reticulum to trigger calcium release?
Na+
What is the functional unit of a muscle fiber?
sarcomere
What defines the length of a sarcomere?
Z-line to Z-line
What is the Z-line/band?
electron dense band that anchors thin actin filaments
(ends of sarcomere)
What is the I-band?
actin filaments extend from Z-band to A-band
What is the A-band?
composed of entire length of myosin filaments + overlapping actin filaments
____ filaments form a hexagonal array around each ____ filament
thin; thick
What is the H-zone?
pale region in midportion of A band where actin filaments end (ONLY contains myosin)
What is the M-line?
zone of intermolecular bridging & thickening of myosin at the midline of the A band, which forms a thin, slightly darker electron-dense band
Each thin filament is equidistant from how many thick filaments?
3
What happens to Ca2+ with cessation of neural activity?
SR actively transports Ca2+ back into terminal cisternae
What happens to Ca2+ with depolarization of a muscle cell?
SR passively releases Ca2+ into muscle cell –> binds to troponin
A muscle fiber is innervated by a single ____
LMN ending
What is a motor unit?
a single motor neuron and all the muscle fibers it innervates
What determines muscle fiber type?
LMN conduction speed
T/F:
muscle fibers in a motor unit are a combination of type I and type II fibers
FALSE
all fibers in a MU are same type
Type I fibers are ____ twitch
slow
Type II fibers are ____ twitch
fast
T/F:
In humans, all muscles consist of a combination of type I and type II fibers
TRUE
i.e., postural muscles (eg. soleus) still have fast twitch;
fast twitch muscles (eg. gastrocs, EDL) still have slow twitch
What muscle fiber type are red/dark?
type I
What muscle fiber type are white?
type II
Postural muscles contain mostly type ____ fibers
I
Describe the properties of motor neurons innervating type I fibers.
Alpha 2 motor neuron
- low threshold
- slower conduction velocity
Describe the properties of motor neurons innervating type II fibers.
Alpha 1 motor neuron
- high threshold
- faster conduction velocity
Describe the metabolic properties of type I fibers
Aerobic
- ^myoglobin
- ^mitochondria
Describe the metabolic properties of type II fibers
Anaerobic
- less myoglobin
- less mitochondria
What is the endurance level of type I and II fibers?
type I = high
type II = low
What would happen to muscle fibers if innervation were crossed between soleus (type I) and EDL (type II)?
Fiber types would switch over time
(innervation determines fiber type)
What happens to type I fibers with training?
- no hypertrophy due to endurance training
- ^mitochondria
What happens to type II fibers with training?
hypertrophy with strength training
