Muscle II Flashcards
What is a neuromuscular junction?
Point of synaptic contact between somatic motor neuron (lower motor neuron) to a individual fibre
What is excitation-contraction coupling
An action potential initiated in the skeletal muscle fibre results in an increase in intracellular (Sarcoplasmic) Ca2+
What is the process of initiation of skeletal muscle contraction
Events in CNS -> events in neuromuscular junction ->excitation-contraction coupling ->Ca2+ signal -> contraction-relaxation cycle -> muscle twitch or sliding filament theory
What is brain region involved in voluntary movement
Primary motor cortex
What are parts of primary motor cortex?
Corticospinal tract, upper motor neuron, alpha (lower) motor neuron
What is the corticospinal tract
Descending tract (ventral and interior lateral white matter
- upper motor neuron goes down
What is upper motor neuron
Brain to brainstem or spinal cord (descending)
- from primary motor cortex down to spinal cord where it synapses with alpha motor neuron
what is alpha (lower) motor neuron
Spinal cord (or brainstem) to muscle
- exits spinal cord through ventral root, goes through nerve to muscle
What is a motor unit
Single motor neuron and all the muscle fibers it innervates
What happens to alpha motor neuron when approaching a muscle?
Axon splits into multiple collaterals
- each collateral will make contact with one muscle fiber
- axon with collaterals and muscle fibers is motor unit
Properties of alpha motor neuron
- Large, myelinated axon, 15-120 m/sec
- each axons branches and innervates several muscle fibers
- all muscle fibers respond simultaneously
How many motor units for delicate precise work or for powerful less precise contractions?
<10
>100
What is amyotrophic lateral sclerosis (ALS)?
Neurodegenerative motor neuron disease
- upper and lower motor neurons degenerate leading to muscle atrophy and weakness from disuse
How many cases of ALS are genetic?
10%
2.5% due to gene mutation producing superoxide dismutase
What are the three components of a neuromuscular junction?
- Presynaptic motor neuron (axon terminal) filled with synaptic vesicles
- Synaptic cleft (space between)
- Postsynaptic membrane of skeletal muscle fiber
What is a motor end plate
Region of sarcolemma at the neuromuscular junction (underneath axon terminal)
Where are junctional folds
On sarcolemma and increase surface area
- squeeze as many receptors as possible
What do motor neuron vesicles contain
Acetylcholine
What does the muscle sarcolemma contain
Nicotinic acetylcholine receptors
What is in the synaptic cleft
Extracellular matrix
- mesh work of proteins and preoteoglycans
What happens to an AP when it arrives at the axon terminal of a skeletal muscle?
Causes voltage gated Ca2+ channels to open
Ca entry causes synaptic vesicles to fuse with presynaptic membrane and release ACh by calcium mediated exocytosis into synaptic cleft for nicotinic receptors
What are nicotinic acetylcholine receptors
Member of cys-loop receptor family of ligand gated ion-channels
- monovalent cation channel (permeable to Na+ and K+)
How many acetylcholine molecules does the ACh receptor require to open
Two
What occurs when the nicotinic ACh receptor opens?
Massive influx of Na+ because large concentration and electrical gradient
Na+ influx depolarizes muscle fiber
Little K+ eflux
What does entry of Na+ through nACh generate?
Excitatory end-plate potential (EPP) (wave of depolarization)
- Spreads to adjacent voltage gated Na+channels on sarcolemma and initiates AP
-
How are APs stopped in muscle cells?
Have to stop firing in alpha motor neuron
- no inhibitory input
What happens to ACh when alpha motor neurons stop firing APs?
-ACh in synaptic cleft will diffuse away
- or broken down to acetate and choline by enzyme acetylcholinesterase
What happens to choline when ACh is broken down?
Choline transported back into motor neuron and combined with acetylcholine CoA (produced by mitochondria) by enzyme choline acetyltransferase to make acetylcholine at terminal
What is myasthenia gravis?
- severe weakness of muscle
- autoimmune disorder of neuromuscular transmission
How do action potentials propagate from sarcolemma into muscle fiber?
Along transverse tubule network
- t-tubules wrap around each myofibril
What are the specialized Ca2+ storage organelles and where are they located?
- Sarcoplasmic reticulum
- strategically organized with the t-tubules
What intracellular signal triggers contraction in all muscle types?
- rise in intracellular calcium
What are ways Calcium can enter the sarcoplasm?
- from Extracellular space via voltage gated Ca channels
- released from intracellular Ca storage reservoir of the Sarcoplasmic reticulum
- or combination of both
What is the excitation-contraction coupling process?
Process by which electrical excitation of the surface membrane triggers an increase of calcium in muscle
Where do t-tubules surround after penetrating muscle fibre?
Surround myofibrils at two points in each sarcomere, at the A and I band junctions
What is a triad
Tubule associated with two terminal cisternae (regions of SR)
What does AP in t-tubule do?
Alters conformation of DHP receptor and DHP receptor opens RyR Ca2+ release channels in SR and Ca enters cytoplasm
What is a DHP receptor
L-type Ca2+ channel
Voltage sensitive
On membrane of t-tubule
What is a ryanodine receptor (RyR)
Ca2+ release channel on SR
What is not vital in skeletal muscle?
Ca2+ induced Ca2+ release and Ca2+ entry from ECF
