Motor Units, Muscle Spindles, Golgi Tendon Organs and Joint Receptors

Question 1

the somatic motor system is responsible for what type of skeletal muscle movement?

Answer 1

voluntary movement (and involuntary spinal reflexes)

Question 2

what two neurones make up the somatic motor system?

Answer 2

upper motor neurones (UMNs) within brain

lower motor neurones (LMNs) within brainstem and ventral horn of spinal cord

Question 3

what is the main function of UMNs in relation to LMNs?

Answer 3

UMNs modulate the activity LMNs

Question 4

LMNs receive input from UMNs and what other places?

Answer 4

proprioceptors and interneurones

Question 5

LMNs command the “final common pathway” - what is the function of this?

Answer 5

to cause muscle contraction

Question 6

what two types of muscle fibres are found in LMNs - one supplies the bulk of muscle whilst the other supplies the muscle spindle?

Answer 6

alpha (a) motor neurones (a-MNs) supply muscle bulk

gamma (y) motor neurones (y-MNs) supply spindle

Question 7

what is meant by synergistic muscles such as the biceps brachii and brachioradialis?

Answer 7

they both aim to complete the same movement (ie arm flexion)

Question 8

what name is given to muscles which oppose each others action?

Answer 8

antagonist eg tricep is an antagonist of biceps brachii

Question 9

what is the difference between axial and distal muscles?

Answer 9

axial muscles control movements of the trunk (posture)

distal muscles move the hands, feet and digits (allowing fine manipulation of objects by the hand)

Question 10

why are the ventral horns of the spinal cord larger at the cervical (C3-T1) and lumbar (L1-S3) enlargements?

Answer 10

more alpha motor neurones are found here for relay of motor info to upper and lower limbs

Question 11

what is encompassed in a motor unit?

Answer 11

α-MN and all of the skeletal muscle fibres that it innervates

Question 12

what name is given to all of the alpha motor neurones innervating one muscle?

Answer 12

motor neurone pool

Question 13

what does each AP cause in a muscle fibre?

Answer 13

a muscle twitch

summation of twitches causes a sustained contraction

Question 14

describe the somatotopic map of the ventral horn of the spinal cord?

Answer 14

axial muscles = medial
distal muscles = lateral
flexor muscles = more dorsal in ventral horn
extensor muscles = anterior of ventral horn

Question 15

what 3 neurones can input to an alpha-motor neurone?

Answer 15

dorsal root ganglion cells
upper motor neurones
spinal interneurones

Question 16

activation of muscle fibres depends on what?

Answer 16

firing rates of LMNs
number of LMNs that are all active
co-ordination of movement (agonist vs antagonist)

Question 17

force production by innervated muscles depends on what?

Answer 17

muscle fibre size (hypertrophy)
fibre phenotype (fast/slow)

Question 18

if the AP frequency is low, why does no prolonged contraction occur?

Answer 18

enough time is given for the muscle to relax

“relaxation phase” involved calcium reuptake into the SR and the reversal of toponin cross-bridges

Question 19

give an example of muscles in the body which require small motor units to carry out their function?

Answer 19

extraocular eye muscles as fine movements are required

Question 20

give an example of muscles in the body requiring large motor units to carry out their function?

Answer 20

large postural (antigravity) muscles eg leg muscles

Question 21

motor units only contain fibres of one type ie fast or slow - true or false?

Answer 21

true - slow and fast twitch fibres are interspersed throughout a full muscle but are specific in each individual motor unit

Question 22

each muscle fibre is innervated by a single motor axon at endplate (neuromuscular junction) - where along the fibre are these found?

Answer 22

midway along the fibre, usually in the centre

Question 23

why are the muscle fibres divided into slow and fast twitch types?

Answer 23

they differ in how quickly myosin ATPase splits ATP to provide energy for cross bridges

Question 24

what are the main types of slow and fast twitch muscle fibres?

Answer 24

slow oxidative (type I)

fast (IIa, IIb, IIx)

Question 25

what makes a muscle fibre slow type I?

Answer 25

get ATP oxidative phosphorylation
slow contraction and relaxation
fatigue resistant
red fibres as high myoglobin content

Question 26

what is the difference between fast twitch type II muscle fibres?

Answer 26

IIa - ATP from oxidative phosphorylation

• fast contraction and relaxation
• resistant to fatigue
• well vascularised = relatively red in colour

IIb, IIx - ATP from glycolysis

• fast contraction
• not fatigue resistant
• white in colour

Question 27

what is the henneman size principle?

Answer 27

the susceptibility of an α-MN to discharge AP is a function of its size

smaller α-NM units (part of slow motor units) have lower threshold - are recruited first

then larger ones recruited

LMNs are recruited in an order appropriate to the physical task that is being performed

Question 28

describe the recruitment of LMNs from standing to running?

Answer 28

standing - slow type I
walking - fast type IIa
run - fast type IIx (IIb doesnt exist in many mammals including humans)

Question 29

describe the myotatic reflex?

Answer 29

when a muscle is stretched, it pulls back (eg knee jerk response from quads)

Question 30

what structure is responsible for the myotatic reflex?

Answer 30

muscle spindle (sensory organ)

this is stretched with the muscle as it is parallel to the fibres

Question 31

what is found inside the muscle spindle?

Answer 31

fibrous capsule
intrafusal muscle fibres
sensory afferents (very fast conducting)
gamma motor neurone efferents

Question 32

describe how the myotonic reflex is initiated?

Answer 32

stretch of muscle spindle 
activation of Ia afferent 
excitatory synaptic transmission in spinal cord (monosynaptic)
activation of α-MN
contraction of homonymous muscle

Question 33

what mediates the excitatory synaptic transmission in spinal cord?

Answer 33

glutamate

Question 34

what manoeuvre can be attempted if a spinal reflex is deemed to be absent?

Answer 34

jendrassik manoeuvre

• patient interlocks fingers and tried to pull hands apart strongly when instructed
• reflex is usually exaggerated

Question 35

what spinal levels are tested in the biceps, supinator, triceps, knee and ankle jerks?

Answer 35

biceps C5/6
supinator C5/6
triceps C7
knee L3/4
ankle S1

Question 36

why are α and γ-MNs normally co-activated to make the intrafusal muscle fibres in the muscle spindle contract in parallel with the extrafusal fibres?

Answer 36

to prevent the intrafusal muscle spindle fibres getting slack and not firing any APs

Question 37

what different named fibres are found in muscle spindle?

Answer 37

nuclear bag fibres: bag 1 (or dynamic) and bad 2 (or static)

chain fibres

Question 38

what sensory afferents wrap around the intrafusal fibres in the muscle spindle?

Answer 38

IIa afferents - wind around the centre of all intrafusal fibres

II fibres - wrap around endings on all intrafusal fibres except the bag 1 dynamic type

Question 39

when are dynamic and static muscle fibres activated respectively?

Answer 39

dynamic y-NMs are active when muscle length changes rapidly and unpredictably

static - active when muscle length changes slowly and predictably only static y-MNs are active

Question 40

where are golgi tendon organs found and what are they involved in?

Answer 40

found at junction of muscle and tendon

involved in inverse myotatic reflex

Question 41

golgi tendon organs lie in series with muscle fibres rather than in parallel - true or false?

Answer 41

true

Question 42

what is the function of the golgi tendon organs and the inverse myotatic reflex?

Answer 42

regulate muscle tension - protect muscle from overload

Question 43

describe the neurone pathway in the reverse myotatic reflex?

Answer 43

group IIb afferent (not IIa like normal myotatic) enter spinal cord
synapse upon inhibitory interneurones
these synapse upon the alpha motor neurones of the muscle to prevent contraction

Question 44

what types of nerve endings contribute to proprioception and where are they found?

Answer 44

free nerve endings (in capsule and CT - nociceptive)
golgi type endings (found only in ligaments - protective)
paciniform endings (found in periosteum - acceleration detectors) 
ruffini endings (found mainly in joint capsule - static position and speed of movements)

Question 45

where does proprioception information arise from?

Answer 45

muscle spindles
golgi tendon organs
joint receptors

Question 46

a loss of proprioception leads to what type of movement?

Answer 46

vision guided movement ie must look at limb before moving it or must focus on an object in distance to sit/stand without falling over

Question 47

what is meant by reciprocal inhibition?

Answer 47

the antagonist of a muscle must relax for the other to perform its function eg for biceps to contract and flex the arm, the triceps must relax

Question 48

excitatory interneurones mediate the flexor reflex and the crossed extensor reflex - explain how these occur is a patient were to stand on a damaging stimulus such as drawing pin?

Answer 48

sensory afferents detect damage
neuron sent back to spinal cord which diverges into 4 pathways
1 excitatory pathway to flexor muscle and 1 inhibitory pathway to extensor muscle on affected side

1 excitatory pathway to extensor and inhibitory to flexor on opposite side (to provide stability whilst affected leg flexes upwards)