Motor units and neurons

Question 1

What is the somatic motor system?

Answer 1

Skeletal muscle and their motor neurons (MN)

Question 2

How many MN are found in the chain?

Answer 2

2: UMN and LMN

Question 3

From where do LMN receive their input?

Answer 3

UMN and interneurons from somatic sensory (reflexes)

Question 4

What NT do UMN release onto LMN?

Answer 4

Glutamate

Question 5

Where are the somas of LMN found?

Answer 5

BS and ventral horn of SC

Question 6

What two kinds of LMN are there?

Answer 6

alpha MN

gamma MN

Question 7

What do aMN do?

Answer 7

Innervate muscle fibres

Question 8

What do yMN do?

Answer 8

Innervate muscle spindles (sensory organs in muscles)

Question 9

What do synergistic muscles do?

Answer 9

Help each other

Question 10

What do antagonistic muscles do?

Answer 10

Oppose each other

Question 11

What do axial muscles do?

Answer 11

Maintain posture

Question 12

What do distal muscles do?

Answer 12

Move hand and feet

Question 13

What do proximal muscles do?

Answer 13

Move limbs

Question 14

Describe the path of a LMN

Answer 14

Exit SC in ventral root -> Spinal Nerve -> Anterior or posterior rami

Question 15

Where do you find lots of LMN?

Answer 15

Cervical (C3-T1) and lumbar (L1-S3) enlargements.

Question 16

What makes up a motor unit?

Answer 16

LMN and all the skeletal muscle it innervates

Question 17

What is a MN pool?

Answer 17

All the aMN that innervate one muscle

Question 18

How do AP generate force?

Answer 18

Each AP causes twitch which surmise to become contraction.

Question 19

In relation to each other where are the somas of LMN innervating axial and distal muscles?

Answer 19

Axial more medial to distal

Question 20

In relation to each other where are the somas of LMN innervating flexors and extensors?

Answer 20

Flexors more dorsal to extensors

Question 21

What five factors determine muscle strength?

Answer 21

Firing rates of LMN
Number of LMN/Motor units recruited
Coordination (synergism or antagonism)
Fibre size (hypertrophy)
Fast or slow fibres

Question 22

What happens at maximum AP firing on muscles?

Answer 22

Tetanus

Question 23

Why do you want large (many fibres) or small (few fibres) motor units?

Answer 23

Large of strength

Small for fine movement

Question 24

How does MN size relate to motor unit size?

Answer 24

Small motor unit = small diameter MN

Question 25

Can motor units contain fast and slow fibres?

Answer 25

No

Question 26

How do the MN innervating fast and slow fibres differ?

Answer 26

Fast- Large diameter and fast conduction. | Slow- Opposite

Question 27

How many MN innervate one muscle fibre?

Answer 27

Only 1- synapse in the middle normally

Question 28

What distinguishes if a fibre is fast or slow?

Answer 28

How quickly myosin ATPase splits ATP

Question 29

Compare slow and fast fibres

Answer 29

Slow (Type I): Red, slow and fatigue resistant. Oxidative phos generates ATP. Antigravity. Fast (Type IIa): Red, fast and fatigue resistant. Oxidative phos generates ATP. Sustained locomotion. Fast (Type IIb): White, fast and fatigues easily. Glycolysis generates ATP. Not in humans. Fast (Type IIx): Very high tension and fast fatiguing. Burst power

Question 30

What does the Henneman Size Principle say?

Answer 30

The susceptibility of an α-MN to discharge action potentials is a function of its size. Smaller α-MNs (part of slow motor units) have a lower threshold than larger ones (part of fatigue resistant, or fast fatiguing, motor units).

Question 31

What is gradation of muscles?

Answer 31

Stepwise increase in force with number of motor units recruited.

Question 32

What does a fine gradation of muscles mean?

Answer 32

Small motor units therefore small increase in muscle power.

Question 33

What does coarse gradation of muscles mean?

Answer 33

Large motor units therefore large increase in muscle power

Question 34

What two kind of muscle gradation are there?

Answer 34

Fine and coarse.

Question 35

What does Henneman's SIze Principle mean for muscle recruitment?

Answer 35

Smaller motor units are recruited first before larger ones thus allowing fine control over generation of force.

Question 36

Give the order in which LMN are recruited

Answer 36

Type I -> Type IIa -> Type IIx

Question 37

What is the myotatic reflex?

Answer 37

When a muscle is pulled it automatically pulls back (allows muscle reflex tests)

Question 38

How is change in muscle length detected?

Answer 38

Sensory organs in muscle spindle

Question 39

What is a muscle spindle?

Answer 39

Specialised muscle cell

Question 40

What 4 things make up a muscle spindle?

Answer 40

Fibrous capsule Intrafusal muscle fibres Sensory afferents yMN

Question 41

How do intrafusal and extrafusal muscle fibres differ?

Answer 41

Extrafusal generate force

Question 42

Describe the myotatic reflex?

Answer 42

Stretch of muscle spindle Activation of afferents Activation of aMN Contraction of muscle

Question 43

How does stretch affect AP generation of sensory afferents?

Answer 43

Increase AP rate

Question 44

How does increased AP generation from sensory afferents affect aMN firing?

Answer 44

Increase AP rate

Question 45

What can deep tendon reflexes (myotatic reflex) be used to test?

Answer 45

Spinal nerves

Question 46

Which deep tendon reflexes test which spinal nerves?

Answer 46

``` Biceps (elbow)- C5/6 Supinator (wrist)- C5/6 Triceps (elbow)- C7 Quadriceps (knee)- L3/4 Gastrocnemius (ankle)- S1 ```

Question 47

What two things make up intrafusal fibres?

Answer 47

Non-contractile equatorial region innervated by Ia afferents | Contractile ends with yMN input (soma in ventral horn)

Question 48

What role do yMN play?

Answer 48

During voluntary movement they fire along with aMN to cause contraction of intrafusal fibres in parallel to prevent them going slack thus keeping signal being transduced.

Question 49

What are the two broad categories of intrafusal fibres?

Answer 49

Nuclear bag fibres | Chain fibres

Question 50

What do chin fibres do and what innervates them?

Answer 50

Sense absolute length | Static yMN

Question 51

What two categories can nuclear bag fibres be split into?

Answer 51

Dynamic nuclear bag fibres | Static nuclear bag fibres

Question 52

What do dynamic nuclear bag fibres do and what efferents innervates them?

Answer 52

Sense rate of change | Dynamic yMN

Question 53

What do static nuclear bag fibres do and what efferents innervates them?

Answer 53

Sense absolute length | Static yMN

Question 54

What two afferent fibres innervate intrafusal fibres?

Answer 54

Aa and Ab

Question 55

Which one intrafusal fibre does Ab not innervate?

Answer 55

Dynamic nuclear bag

Question 56

What information do Aa fibres transmit?

Answer 56

Rate of change of stretch

Question 57

What info do Ab fibres transmit?

Answer 57

Absolute length

Question 58

When are static yMN recruited?

Answer 58

Most of the time but especially when muscle length change is slow and predictable

Question 59

When are dynamic yMN recruited?

Answer 59

When muscle lengths change rapidly and unpredictably.

Question 60

What are golgi tendon organs and where are they located?

Answer 60

Junction of muscles and tendons where they monitor changes in muscle length of extrafusal fibres.

Question 61

What afferents innervate golgi tendon organs?

Answer 61

Ab

Question 62

What do golgi tendon organs do?

Answer 62

Regulate muscle tension to protect from muscle overloading and keep muscle tension in optimal range.

Question 63

Where do the Ab fibres of golgi tendon organs synapse?

Answer 63

Onto inhibitory interneurons in the SC which synapse onto the aMN of their muscle.

Question 64

What reflex do the golgi tendon organs regulate?

Answer 64

Reverse myotatic reflex

Question 65

How does the reverse myotatic reflex work?

Answer 65

Golgi tendon organs fire causing activation of inhibitory interneurons which inhibit aMN to prevent muscle contraction.

Question 66

Why is the reverse myotatic reflex important?

Answer 66

In fine motor control to prevent over gripping