Motor units and muscle spindles Flashcards
UMNs supply input to LMNs to ________ their activity
UMNs supply input to LMNs to modulate their activity
LMNs command muscle ___________ and form the ‘_____ ______ _______’
LMNs command muscle contraction and form the ‘final common pathway’
LMNs comprise _____ _____ _____ that innervate the bulk of fibres within a muscle that generate force, _____ ______ _____ innervate a sensory organ within the muscle known as a muscle spindle
LMNs comprise alpha motor neurones that innervate the bulk of fibres within a muscle that generate force, gamma motor neurones innervate a sensory organ within the muscle known as a muscle spindle
Biceps brachii and brachialis work together as _________. Similarly triceps brachii and anconeus are _________ ______
Biceps brachii and brachialis work together as synergists. Similarly triceps brachii and anconeus are synergistic muscles
Biceps brachii and brachialis (as _____) oppose the triceps brachii and anconeus (as _______), these groups are __________ to each other
Biceps brachii and brachialis (as flexors) oppose the triceps brachii and anconeus (as extensors), these groups are antagonists to each other
____ muscles control movements of the trunk, _______ ____ muscles are found in the shoulder, elbow and ______ _______ are found in the digits
axial muscles control movements of the trunk, proximal girdle muscles are found in the shoulder, elbow and distal muscles are found in the digits
Axons of the LMNs exit the spinal cord in the _______ roots (or via ______ nerves)
Axons of the LMNs exit the spinal cord in the ventral roots (or via cranial nerves)
Each ventral (anterior) root joins with a dorsal (posterior) root to form a _____ _____ _____ containing…..
Each ventral (anterior) root joins with a dorsal (posterior) root to form a mixed spinal nerve containing;
- sensory fibres
- motor fibres
What do motoneurons belong to?
A spinal segment;
- cervical (C) 1-8
- thoracic (T) 1-12
- lumbar (L) 1-5
- sacral (S) 1-5
How are motoneurones distributed within the spinal cord?
Unevenly, more in the cervical and lumbar enlargements
What forms a motor unit?
An a-MN and all of the skeletal muscle fibres that it innervates
what is a motor neurone pool?
The collection of an a-MNs that innervate a single muscle
What are the two principle mechanisms that allow a-MN to grade muscle contraction?
- frequency of action potential discharge of the a-MN
- the recruitment of additional, synergistic motor units
Where are the cell bodies of LMN distributed in the spinal cord?
They show a somatotopic distribution in the ventral horm
LMNs innervating axial muscles are ______ to those innervating distal muscles.
LMNs innervating flexors are ______ to those supplying extensors.
LMNs innervating axial muscles are medial to those innervating distal muscles.
LMNs innervating flexors are dorsal to those supplying extensors.
What are the three sources of input to an a-MN that regulate its activity?
- central terminals of dorsal root ganglion cells whose axons innervate muscle spindles
- UMNs in the motor cortex and brainstem
- spinal interneurones
Muscle strength depends on which two things?
Activation of muscle fibres and force production by innervated muscle fibres
What components determine level of activation of muscle fibres?
- the firing rates of the LMNs involved
- the number of LMNs that are simultaneously active
- the co-ordination of the movement
What components determine force production of muscle fibres?
Fibre size (hypertrophy) Fibre phenotype (fast or slow contracting muscle)
A single action potential in an a-MN causes a muscle fibre to ______. The summation of ________ causes a _________ __________ as the number of incoming action potentials increases.
A single action potential in an a-MN causes a muscle fibre to twitch. The summation of twitches causes a sustained contraction as the number of incoming action potentials increases.
What are the discriminatory features of motor units?
- variable size
- fast and slow types
describe the variable sizes of motor units
small (a few fibres)- for fine movement- innervated by small a-MNs (diameter)
large (hundreds to thousands of fibres) in large postural muscles (antigravity muscles)
Motor units contain muscle fibres of only a single type (_______ ________ or ____ ________) a-MNs innervating fast type tend to be _____ and have faster conducting axons than slow units
Motor units contain muscle fibres of only a single type (rapidly fatuguing or slow fatiguing) a-MNs innervating fast type tend to be larger and have faster conducting axons than slow units
Each muscle fibre is innervated by a single motor neurone at _______ (___________ junction) which is usually at the centre of the fibre
Each muscle fibre is innervated by a single motor neurone at endplate (neuromuscular junction) which is usually at the centre of the fibre
What are the two major types of skeletal muscle fibres?
Slow-twitch and fast-twitch
Describe slow-oxidative (type I) fibres?
ATP largely derived from oxidative phosphorylation. Slow contraction and relaxation. Fatigue resistant..
What colour are slow-oxidative fibres?
Red fibres because of high myoglobin content
Describe fast (type II) fibres?
There are two categories-
IIa, IIb, IIx
Describe type IIa fibres?
ATP largely derived from oxidative phosphorylation. Fast contraction and relaxation. Fatigue resistant.
What colour are type IIa fibres?
Red and reasonably well vascularised
Describe type IIb,IIx fibres?
ATP mainly derived from glycolysis. Fast contraction but not fatigue resistant
What colour are type IIb and IIx fibres?
white and poorly vascularised
Which fibres do humans have?
I, IIa
Which kind of motor units are responsible for burst power?
fast fatiguing, type IIx fibres
Which kind of motor units are responsible for sustained locomotion?
Fatigue resistant type IIa fibres
Which kind of motor units are responsible for antigravity, sustained movement?
Slow type I fibres
What is the henneman size principle?
The susceptibility of an a-MN to discharge action potentials is a function of its size, smaller a-MNs have a lover threshold than larger ones. Slow motor units are more easily activated and trained by any training that activates the muscle.
Activation of an upper motor neurone causes the ____ _____ ________ that it supplies to be excited
Activation of an upper motor neurone causes the ____ _____ ________ that it supplies to be excited
Motor units are recruited in order of their ____, progressively increasing
Motor units are recruited in order of their size, progressively increasing
Recruitment in the order ____ (___) before ____ _______ _________ (___) preceding ____ __________ (___) results in increasing increments towards the maximal force the muscle exerts
Recruitment in the order slow (I) before fast fatigue resistant (IIa) preceding fast fatiguable (IIx) results in increasing increments towards the maximal force the muscle exerts
What is the myotactic reflex
when a skeletal muscle is pulled it pulls back
Describe how the myotactic reflex occurs
Change in length (and rate of change) is registered by a sensory organ within the muscle, the muscle spindle
What does a muscle spindle consist of ?
- fibrous capsule
- intrafusal muscle fibres
- sensory afferents (Ia class, myelinated and very fast conducting) that innervate intrafusal fibres
- gamma motor neurone efferents that innervate intrafusal fibres
The myotactic reflex is a __________ reflex ___
The myotactic reflex is a monosynaptic reflex arc
Stretch of muscle spindle –> activation of __ ________ –> excitatory synaptic transmission in _____ ____ (modified by release of _________) –> activation of ____ –> contraction of ___________ muscle
Stretch of muscle spindle –> activation of Ia afferent –> excitatory synaptic transmission in spinal cord (modified by release of glutamate) –> activation of a-MN –> contraction of homonymous muscle
Where is monosynaptic reflex most prominent in humans?
In extensor muscles
What can the myotactic reflex be used to assess?
The functional integrity of the spinal cord at specific levels
Which spinal level is assessed in biceps (elbow) deep tendon reflex?
C5-C6
Which spinal level is assessed in supinator (wrist) deep tendon reflex?
C5-C6
Which spinal level is assessed in triceps (elbow) deep tendon reflex?
C7
Which spinal level is assessed in quadriceps (knee) deep tendon reflex?
L3-L4
Which spinal level is assessed in gastrocnemius (ankle) deep tendon reflex?
S1
What is jendrassik manoeuvre
Interlocking fingers and pilling apart when instructed- modulate descending control
What do intrafusal fibres consist of?
- non-contractile equatorial region
- contractile polar ends
What innervates the non-contractile equatorial region of intrafusal fibres?
innervated by Ia sensory neurones
What input do contractile polar ends of intrafusal fibres receive?
Efferent input from y-MNs with cell bodies in the ventral horn of the spinal cord (driven by higher centres)
stimulation of y-MNs causes the _______ __ ________
stimulation of y-MNs causes the spindle to contract
During ________ movement _ and _ MNs are normally co-activated, so that the ________ fibres contract in parallel with the ________ fibres
During voluntary movement a and y MNs are normally co-activated, so that the intrafusal fibres contract in parallel with the extrafusal fibres
What function does co-acitvated a and y MNs have?
Maintains the sensitivity of the spindle to keep it on the air, to stop it going slack when extrafusal fibres contract
