Motor Units, Muscle Spindles, Golgi Tendon Organs and Joint Receptors Flashcards
the somatic motor system is responsible for what type of skeletal muscle movement?
voluntary movement (and involuntary spinal reflexes)
what two neurones make up the somatic motor system?
upper motor neurones (UMNs) within brain
lower motor neurones (LMNs) within brainstem and ventral horn of spinal cord
what is the main function of UMNs in relation to LMNs?
UMNs modulate the activity LMNs
LMNs receive input from UMNs and what other places?
proprioceptors and interneurones
LMNs command the “final common pathway” - what is the function of this?
to cause muscle contraction
what two types of muscle fibres are found in LMNs - one supplies the bulk of muscle whilst the other supplies the muscle spindle?
alpha (a) motor neurones (a-MNs) supply muscle bulk
gamma (y) motor neurones (y-MNs) supply spindle
what is meant by synergistic muscles such as the biceps brachii and brachioradialis?
they both aim to complete the same movement (ie arm flexion)
what name is given to muscles which oppose each others action?
antagonist eg tricep is an antagonist of biceps brachii
what is the difference between axial and distal muscles?
axial muscles control movements of the trunk (posture)
distal muscles move the hands, feet and digits (allowing fine manipulation of objects by the hand)
why are the ventral horns of the spinal cord larger at the cervical (C3-T1) and lumbar (L1-S3) enlargements?
more alpha motor neurones are found here for relay of motor info to upper and lower limbs
what is encompassed in a motor unit?
α-MN and all of the skeletal muscle fibres that it innervates
what name is given to all of the alpha motor neurones innervating one muscle?
motor neurone pool
what does each AP cause in a muscle fibre?
a muscle twitch
summation of twitches causes a sustained contraction
describe the somatotopic map of the ventral horn of the spinal cord?
axial muscles = medial
distal muscles = lateral
flexor muscles = more dorsal in ventral horn
extensor muscles = anterior of ventral horn
what 3 neurones can input to an alpha-motor neurone?
dorsal root ganglion cells
upper motor neurones
spinal interneurones
activation of muscle fibres depends on what?
firing rates of LMNs
number of LMNs that are all active
co-ordination of movement (agonist vs antagonist)
force production by innervated muscles depends on what?
muscle fibre size (hypertrophy) fibre phenotype (fast/slow)
if the AP frequency is low, why does no prolonged contraction occur?
enough time is given for the muscle to relax
“relaxation phase” involved calcium reuptake into the SR and the reversal of toponin cross-bridges
give an example of muscles in the body which require small motor units to carry out their function?
extraocular eye muscles as fine movements are required
give an example of muscles in the body requiring large motor units to carry out their function?
large postural (antigravity) muscles eg leg muscles
motor units only contain fibres of one type ie fast or slow - true or false?
true - slow and fast twitch fibres are interspersed throughout a full muscle but are specific in each individual motor unit
each muscle fibre is innervated by a single motor axon at endplate (neuromuscular junction) - where along the fibre are these found?
midway along the fibre, usually in the centre
why are the muscle fibres divided into slow and fast twitch types?
they differ in how quickly myosin ATPase splits ATP to provide energy for cross bridges
what are the main types of slow and fast twitch muscle fibres?
slow oxidative (type I)
fast (IIa, IIb, IIx)
what makes a muscle fibre slow type I?
get ATP oxidative phosphorylation
slow contraction and relaxation
fatigue resistant
red fibres as high myoglobin content
what is the difference between fast twitch type II muscle fibres?
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
what is the henneman size principle?
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
describe the recruitment of LMNs from standing to running?
standing - slow type I
walking - fast type IIa
run - fast type IIx (IIb doesnt exist in many mammals including humans)
describe the myotatic reflex?
when a muscle is stretched, it pulls back (eg knee jerk response from quads)
what structure is responsible for the myotatic reflex?
muscle spindle (sensory organ)
this is stretched with the muscle as it is parallel to the fibres
what is found inside the muscle spindle?
fibrous capsule
intrafusal muscle fibres
sensory afferents (very fast conducting)
gamma motor neurone efferents
describe how the myotonic reflex is initiated?
stretch of muscle spindle activation of Ia afferent excitatory synaptic transmission in spinal cord (monosynaptic) activation of α-MN contraction of homonymous muscle
what mediates the excitatory synaptic transmission in spinal cord?
glutamate
what manoeuvre can be attempted if a spinal reflex is deemed to be absent?
jendrassik manoeuvre
- patient interlocks fingers and tried to pull hands apart strongly when instructed
- reflex is usually exaggerated
what spinal levels are tested in the biceps, supinator, triceps, knee and ankle jerks?
biceps C5/6 supinator C5/6 triceps C7 knee L3/4 ankle S1
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?
to prevent the intrafusal muscle spindle fibres getting slack and not firing any APs
what different named fibres are found in muscle spindle?
nuclear bag fibres: bag 1 (or dynamic) and bad 2 (or static)
chain fibres
what sensory afferents wrap around the intrafusal fibres in the muscle spindle?
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
when are dynamic and static muscle fibres activated respectively?
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
where are golgi tendon organs found and what are they involved in?
found at junction of muscle and tendon
involved in inverse myotatic reflex
golgi tendon organs lie in series with muscle fibres rather than in parallel - true or false?
true
what is the function of the golgi tendon organs and the inverse myotatic reflex?
regulate muscle tension - protect muscle from overload
describe the neurone pathway in the reverse myotatic reflex?
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
what types of nerve endings contribute to proprioception and where are they found?
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)
where does proprioception information arise from?
muscle spindles
golgi tendon organs
joint receptors
a loss of proprioception leads to what type of movement?
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
what is meant by reciprocal inhibition?
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
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?
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)
