Motor Units and Muscle Spindles

Question 1

the somatic motor system consists of what?

Answer 1

skeletal muscles and the elements of the nervous system that control them

Question 2

function of upper motor neurones?

Answer 2

within the brain

supply input to lower motor neurones to modulate their activity

Question 3

function of lower motor neurones?

Answer 3

soma within the brain stem and ventral horn of spinal cord
receive input from UMNs, proprioceptors and interneurons
command muscle contraction and form the final common pathway

Question 4

LMNs consist of what?

Answer 4

alpha motor neurones that innervate the bulk of fibres within a muscle that generate force
gamma motor neurones which innervate a sensory organ within the muscle known as a muscle spindle

Question 5

describe 2 groups of synergistic muscles?

Answer 5

biceps and brachii and brachialis (flexors)
triceps brachii and anconeus (extensors)
these groups are antagonists to each other

Question 6

axons of LMNs exit the spinal cord where?

Answer 6

in ventral roots (or via cranial nerves)

Question 7

how are motor neurones distributed along the spinal cord?

Answer 7

not equally - greater number in cervical and lumbar enlargements
motor neurones that innervated distal and proximal muscles are mainly in the cervical and lumbar-sacral segments of the spinal cord
motor neurones innervating the axial musculature occur at all levels

Question 8

what forms a motor unit?

Answer 8

alpha motor neurone and all the skeletal muscle fibres that it innervates
- the smallest functional component of the motor system

Question 9

what is a motor neurone pool?

Answer 9

collection of alpha motor neurones that innervate a single muscle

Question 10

the force of muscle contraction is graded by alpha motor neurones by which 2 principle mechanisms?

Answer 10

frequency of action potential discharge of the alpha motor neurone (each AP = twitch)
the recruitment of additional, synergistic motor units

Question 11

how are cell bodies of LMNs organised in spinal cord?

Answer 11

somatotopic distribution in ventral horn

• LMNs innervating axial muscles = medial to those innervating distal muscles
• LMNs innervating flexors = dorsal to those supplying extensors

Question 12

what are the 3 sources of input to an alpha motor neurone that regulate its activity?

Answer 12

central terminals of dorsal root ganglion cells who’s axons innervate the muscle spindles
UMNs in the motor cortex and brain stem
spinal interneurones

Question 13

muscle strength depends on what 2 factors?

Answer 13

activation of muscle fibres

force production by innervated muscle fibres (fibre size and phenotype)

Question 14

what determines amount of activation of muscle fibres?

Answer 14

firing rates of LMNs
number of LMNs that are simultaneously active
co-ordination of the movement (activation of agonist/antagonist)

Question 15

a single action potential causes what in a muscle fibre and why is this important?

Answer 15

causes a twitch in a muscle fibre

summation of twitches causes a sustained contraction as the number of APs increases

Question 16

types of muscle contraction depending on rate of motor neurone activity (AP firing)?

Answer 16

5Hz = twitch
10Hz = wave summation
20Hz = unfused tetanus
40Hz = fused tetanus

Question 17

how does size of a motor unit relate to its function?

Answer 17

small = fine movement (extraocular eye muscles)
- innervated by small alpha motor neurones
large = large postural antigravity muscles (e.g legs)
- innervated by large alpha motor neurones
larger motor units = larger force of contraction

Question 18

how are motor units divided into fast and slow types?

Answer 18

motor unit can contain only fibres of one type

alpha motor neurones innervating fast type tend to be larger and have faster conducting axons than those of slow units

Question 19

what are the two major types of skeletal muscle and how do they differ?

Answer 19

slow twitch and fast twitch
differ in how quickly myosin ATPase splits ATP to provide energy for cross bridge cycling (reflected in time taken to reach peak tension)
also express different myosin heavy chains

Question 20

describe slow (type 1) fibres?

Answer 20

ATP derived from oxidative phosphorylation
slow contraction and relaxation
resistant to fatigue
red fibres due to high myoglobin content

Question 21

describe fast (type 2) fibres?

Answer 21

type 2a
- ATP derived fom oxidative phosphorylation
- fast contraction and relaxation
- fatigue resistant
- red and well vascularised
type 2b
- ATP derived from glycolysis
- fast contraction
- fatigues easily
- white fibres and poorly vascularised

Question 22

what are the 3 types of motor unit?

Answer 22

fast fatiguing
fatigue resistant
slow

Question 23

describe fast fatiguing motor units?

Answer 23

very high tension
fast fatigue
large alpha motor neurones
high threshold
type 2b fibres
used for burst power (e.g weight lifting)

Question 24

describe fatigue resistant motor units?

Answer 24

high tension
slow fatigue
intermediate alpha motor neurones
intermediate threshold
type 2a fibres
sustained locomotion (e.g distance running)

Question 25

describe slow motor units?

Answer 25

low tension
fatigue resistant
small alpha motor neurones
low threshold
type 1 fibres
antigravity, sustained movement (e.g yoga)

Question 26

describe the henneman size principle?

Answer 26

the susceptibility of an alpha motor neurone to discharge an AP is a function of its size
- smaller ones have a lower threshold than larger ones, therefore slow motor units are more easily activated and trained by any training that activates that muscle

Question 27

motor units (LMNs and the muscle they innervate) are recruited in order of their size (i.e small LMNs more easily excited than larger ones) what does this allow?

Answer 27

allows for fine control of muscle force across a wide range of tensions developed
recruitment in order slow (type 1) > fast fatigue resistant (type 2a) > fast fatigable (type 2b) results in increasing increments towards the maximal force that the muscle exerts

Question 28

what is the myotatic reflex?

Answer 28

type of monosynaptic reflex arc where when a skeletal muscle is pulled, it pulls back (e.g knee jerk)
- change in length (and rate of change) is registered by a sensory organ within the muscle (muscle spindle)

Question 29

a muscle spindle consists of what?

Answer 29

fibrous capsule
intrafusal muscle fibres
type Ia sensory afferents that innervate the intrafusal fibres
gamme motor neurone efferents that innervate the intrafusal fibres

Question 30

how does the myotatic reflex work?

Answer 30

stretch of muscle spindle > activation of Ia afferents > excitatory synaptic transmission in spinal cord (monosynaptic) > activation of alpha motor neurone > contraction of homonymous muscle

Question 31

intrafusal fibres consist of what?

Answer 31

non-contractile equatorial region innervated by the Ia sensory neurones
contractile polar ends that receive efferent input from gamma motor neurones with cell bodies in the ventral horn of the spinal cord (driven by higher centres, no Ia afferents)

Question 32

stimulation of gamma motor neurons causes what in muscle spindle?

Answer 32

contraction

Question 33

how are alpha and gamma motor neurones affected in voluntary movement?

Answer 33

they are normally co-activated by higher centres to that the intrafusal muscle fibres contract in parallel with the extrafusal fibres
this maintains the sensitivity of the spindle to stop it going slack when the extrafusal fibres contract