15 - Motor Units And Excitation-contraction Coupling

Question 1

Pathway from stimulus to response

Answer 1

Stimulus -> sensory neurons -> interneurons interneurons -> motor neurons -> effectors -> response

Question 2

T/F — stimulus can be internal/external and response can be detected internally/externally

Answer 2

True

Question 3

T/F - spinal cord arranged same as cortex w/ grey matter outside and white matter on inside

Answer 3

FALSE — arranged inside out to cortex

Question 4

What is white matter of spinal cord?

Answer 4

Myelinated axons

Question 5

Descending axons head…

Answer 5

Away from brain (ascending head towards)

Question 6

Does ventral or dorsal root carry sensory info into spinal cord?

Answer 6

Dorsal (ventral carries motor info out)

Question 7

Lower motor neurons

Answer 7

• involved in all movements
• directly innervate muscle
• cell bodies in spinal cord

Question 8

Mixed spinal nerve

Answer 8

Combines axons of motor and sensory fibres and synapses into single muscle fibre

Question 9

What neurotransmitter is involved in muscle contraction?

Answer 9

Acetylcholine (ACh)

Question 10

How many muscle fibres does individual motor neuron innervate?

Answer 10

One or more

Question 11

Motor unit

Answer 11

1 alpha motor neuron + muscle fibres it innervates

Question 12

Small motor units involve….

Answer 12

<10 muscle fibres

Question 13

Large motor units involve…

Answer 13

> 1000 muscle fibres

Question 14

Motor neuron pool

Answer 14

Collection of alpha motor neurons that innervates a single muscle

Question 15

Following each successive AP, there’s an increase in…

Answer 15

Force within muscle fibres

Question 16

As freq of APs increases ________ occurs

Answer 16

Force summation

Question 17

Unfused tetanus

Answer 17

Oscillations in summed force generation

Question 18

Fused tetanus

Answer 18

Smooth force summation

Question 19

Small motor unit

Answer 19

Motor neuron w/ small cell body that innervates few muscle fibres

Question 20

Large motor unit

Answer 20

Motor neuron with large cell body that innervates many muscle fibres

Question 21

To generate graded forces…

Answer 21

Small motor units (small forces) recruited 1st, followed by progressively larger motor units (larger forces)

Question 22

What takes care of grading motor recruitment automatically?

Answer 22

Cell body size

Question 23

Small motor neurons have a ___________ membrane resistance and reach threshold _______ easily

Answer 23

Higher, more easily

Question 24

An incoming spike causes a ________ change in mempot in a smaller neuron

Answer 24

Larger (because V = IR, so when R⬆️, V⬆️)

Question 25

T/F larger motor neurons reach spiking threshold more easily

Answer 25

False - smaller

Question 26

Inverse relationship between no. Motor units in a muscle and ...

Answer 26

Their force generating capacity

Question 27

Many small motor units needed to generate small amount of force whereas

Answer 27

Fewer larger motor units needed to generate large amount of force

Question 28

Types of muscle

Answer 28

Cardiac, skeletal and smooth

Question 29

Skeletal muscle

Answer 29

- striated and under voluntary control - each cell has multiple peripheral nuclei - orderly structure -> contractions in one direction

Question 30

Cardiac muscle

Answer 30

- striated and under involuntary control - single central nucleus per cell - branching and unordered striations -> contraction in many directions

Question 31

Smooth muscle

Answer 31

- no striations and under involuntary control | - single nucleus per cell

Question 32

Functional structure of skeletal muscle (hierarchy)

Answer 32

Whole muscle -> muscle fibre (single muscle cell) -> myofibril -> myofilaments

Question 33

Excitation

Answer 33

- AP depolarises axon terminal of motor neuron - Ca2+ influx in pre-synaptic neuron through Cav channels - ACh release, which binds to post-synaptic ACh receptors - Na+ entry through ion channels post-synaptically - Nav channels open - AP propagates along post-synaptic plasma membrane

Question 34

NMJ has probability of vesicle release of...

Answer 34

1, so AP leads to guaranteed neurotransmitter release

Question 35

AP propagates along muscle fibre plasma membrane and down...

Answer 35

Transverse tubules

Question 36

T-tubules are...

Answer 36

Electrically continuous w/ surface of plasma membrane

Question 37

Dihydropyridine receptor

Answer 37

Voltage-gated Ca2+ channel that opens in response to AP

Question 38

Both dihydropyridine and ryanodine receptors are

Answer 38

Physically coupled ca2+ channels

Question 39

AP ________cytosolic Ca2+ levels because...

Answer 39

Increases, because Ca2+ released from SR

Question 40

Contraction — cross-bridge cycling steps

Answer 40

1. Myosin head initially unbound but energised 2. Actin binding site blocked by tropomyosin 3. Ca2+ binds to troponin, moving tropomyosin 4. Myosin cross-bridges bind to actin 5. Power stroke -> myosin head rotates (+ release of ADP and Pi) 6. ATP binding to myosin breaks actin-myosin linkage 7. Bound ATP split, re-energising myosin head

Question 41

Hydrolysis of ATP by myosin energises cross-bridges by providing energy for

Answer 41

Force generation

Question 42

ATP binding to myosin dissociates...

Answer 42

Cross-bridges bound to actin, allowing repeated cycling

Question 43

What’s the role of the Ca2+-ATPase?

Answer 43

Hydrolyses ATP and actively transports Ca2+ back into SR

Question 44

Relaxation

Answer 44

1. Ca2+-ATPase actively sequesters Ca2+ back into SR | 2. Ca2+ removal leads to covering of myosin binding sites by troponin, ceasing cross-bridge cycle