2.25 Motor SC 1

Question 1

sarcomere

Answer 1

contractile units of muscle

Question 2

neuromuscular junction

Answer 2

Chemical synapse between the axon terminal of an alpha motor neuron and the sarcolemma of the muscle fiber

Question 3

sarcolemma

Answer 3

membrane of muscle cell

Question 4

What is released from presynaptic terminals at the NMJ?

Answer 4

ACh

Question 5

EPP

Answer 5

excitatory endplate potential

Question 6

excitatory endplate potential (EPP)

Answer 6

• sets off a chain of events that leads to muscle contraction
• ACh released into invaginations in sarcolemma (motor end plate)

Question 7

7 steps from AP propagation of axon to AP on muscle cell

Answer 7

1. AP travels to axon terminal
2. voltage gated Ca2+ channels open, Ca2+ diffuses into terminal
3. synaptic vesicles release ACh
4. ACh diffuses across synaptic cleft and binds to receptors (ligand gated)
5. ligand gated channels open
6. Na+ enters fiber, K+ leaves » depolarization
7. membrane potential reaches threshold and an AP propagates along sarcolemma

Question 8

When does neurotransmission stop?

Answer 8

when ACh is removed from synaptic cleft

Question 9

How might ACh be removed from synaptic cleft?

Answer 9

• diffuses away from synapse

- broken down by acetylcholinesterase to acetic acid and choline

Question 10

What happens to choline after ACh is broken down?

Answer 10

transferred to axon terminal for resynthesis of ACh

Question 11

excitation-contraction coupling

Answer 11

conversion of AP into a contraction

Question 12

Where does the AP go once it reaches the sarcolemma?

Answer 12

transverse tubules conduct AP to interior of muscle fiber

Question 13

t-tubules

Answer 13

• regularly spaced infoldings of sarcolemma

- make contact with sarcoplasmic reticulum (SR)

Question 14

terminal cisternae

Answer 14

formed by SR, helps for triad

Question 15

triad

Answer 15

• 1 portion of a t-tubule + 2 terminal cisternae = triad

- linked by series of proteins that control Ca2+ release

Question 16

What happens to the AP once it reaches the t-tubule?

Answer 16

• opens voltage gated Ca2+ channel in triad
• allows Ca2+ ions to flood sarcoplasm
• rapid influx of Ca2+ triggers contraction

Question 17

sarcomere

Answer 17

functional unit of contraction

Question 18

contraction initiation

Answer 18

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

Question 19

4 phases of crossbridge cycle

Answer 19

• cross bridge formation
• power stroke
• cross bridge detachment
• reactivation of myosin head

Question 20

power stroke

Answer 20

myosin head pivots, sliding actin toward center of sarcomere

Question 21

cross bridge detachment caused by

Answer 21

ATP binding to myosin head

Question 22

reactivation of myosin head

Answer 22

returns to cocked position

Question 23

When does the contraction cycle end?

Answer 23

when Ca2+ ions actively transported back to SR

Question 24

What happens to troponin and tropomyosin once contraction cycle ends?

Answer 24

• troponin returns to original shape

- allows tropomyosin to cover myosin binding site on actin

Question 25

When can sarcomeres disappear and reappear?

Answer 25

when healthy innervated muscle is immobilized

Question 26

LMN cell bodies

Answer 26

cell bodies in ventral horn of spinal cord ALWAYS

Question 27

2 types of LMN

Answer 27

• alpha LMN

- gamma LMN

Question 28

alpha LMN innervates

Answer 28

extrafusal fibers (contractile portion of muscle)

Question 29

gamma LMN innervates

Answer 29

intrafusal fibers

Question 30

coactivation of alpha and gamma fibers

Answer 30

coactivated to maintain stretch on intrafusal fibers while extrafusal fibers contract

Question 31

motor unit

Answer 31

a single alpha motor neuron (LMN) and the group of muscle fibers it innervates

Question 32

motor unit action potentials

Answer 32

electrically recorded intramuscular potentials (needle electrode)

Question 33

characteristics of motor unit AP

Answer 33

• 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

Question 34

damage to LMN cell body or axon characterized by

Answer 34

• hyporeflexia
• hypotonia
• paralysis or paresis
• atrophy
• denervation pattern of EMG

Question 35

denervation pattern of EMG

Answer 35

• spontaneous EMG activity at rest

- abn MUAPs

Question 36

spontaneous EMG activity at rest

Answer 36

• fibrillations (fibs)

- positive sharp waves (PSWs)