2.25 Motor SC 1 Flashcards
sarcomere
contractile units of muscle
neuromuscular junction
Chemical synapse between the axon terminal of an alpha motor neuron and the sarcolemma of the muscle fiber
sarcolemma
membrane of muscle cell
What is released from presynaptic terminals at the NMJ?
ACh
EPP
excitatory endplate potential
excitatory endplate potential (EPP)
- sets off a chain of events that leads to muscle contraction
- ACh released into invaginations in sarcolemma (motor end plate)
7 steps from AP propagation of axon to AP on muscle cell
- AP travels to axon terminal
- voltage gated Ca2+ channels open, Ca2+ diffuses into terminal
- synaptic vesicles release ACh
- ACh diffuses across synaptic cleft and binds to receptors (ligand gated)
- ligand gated channels open
- Na+ enters fiber, K+ leaves » depolarization
- membrane potential reaches threshold and an AP propagates along sarcolemma
When does neurotransmission stop?
when ACh is removed from synaptic cleft
How might ACh be removed from synaptic cleft?
- diffuses away from synapse
- broken down by acetylcholinesterase to acetic acid and choline
What happens to choline after ACh is broken down?
transferred to axon terminal for resynthesis of ACh
excitation-contraction coupling
conversion of AP into a contraction
Where does the AP go once it reaches the sarcolemma?
transverse tubules conduct AP to interior of muscle fiber
t-tubules
- regularly spaced infoldings of sarcolemma
- make contact with sarcoplasmic reticulum (SR)
terminal cisternae
formed by SR, helps for triad
triad
- 1 portion of a t-tubule + 2 terminal cisternae = triad
- linked by series of proteins that control Ca2+ release
What happens to the AP once it reaches the t-tubule?
- opens voltage gated Ca2+ channel in triad
- allows Ca2+ ions to flood sarcoplasm
- rapid influx of Ca2+ triggers contraction
sarcomere
functional unit of contraction
contraction initiation
initiated when Ca2+ released from SR bind to troponin
- troponin changes shape
- tropomyosin moves out of the way and allows troponin to bind
- ATP binds to myosin head and activates
4 phases of crossbridge cycle
- cross bridge formation
- power stroke
- cross bridge detachment
- reactivation of myosin head
power stroke
myosin head pivots, sliding actin toward center of sarcomere
cross bridge detachment caused by
ATP binding to myosin head
reactivation of myosin head
returns to cocked position
When does the contraction cycle end?
when Ca2+ ions actively transported back to SR
What happens to troponin and tropomyosin once contraction cycle ends?
- troponin returns to original shape
- allows tropomyosin to cover myosin binding site on actin
When can sarcomeres disappear and reappear?
when healthy innervated muscle is immobilized
LMN cell bodies
cell bodies in ventral horn of spinal cord ALWAYS
2 types of LMN
- alpha LMN
- gamma LMN
alpha LMN innervates
extrafusal fibers (contractile portion of muscle)
gamma LMN innervates
intrafusal fibers
coactivation of alpha and gamma fibers
coactivated to maintain stretch on intrafusal fibers while extrafusal fibers contract
motor unit
a single alpha motor neuron (LMN) and the group of muscle fibers it innervates
motor unit action potentials
electrically recorded intramuscular potentials (needle electrode)
characteristics of motor unit AP
- no activity when at rest
- normally biphasic or triphasic recording when voluntary recruitment of muscle
- maximal recruitment produces “interference pattern”
- changes can be seen with various peripheral or central disease
damage to LMN cell body or axon characterized by
- hyporeflexia
- hypotonia
- paralysis or paresis
- atrophy
- denervation pattern of EMG
denervation pattern of EMG
- spontaneous EMG activity at rest
- abn MUAPs
spontaneous EMG activity at rest
- fibrillations (fibs)
- positive sharp waves (PSWs)
