Flashcards in Motor units and muscle spindles Deck (74):
Where can upper motor neurones be found?
Where can lower motor neurones be found?
Soma of brain stem and ventral horn of spinal cord
What is the relationship between upper and lower motor neurones?
UMN supply input to LMN to modulate their activity
From which sources do LMNs receive input?
What are the different types of LMNs?
Alpha motor neurones that innervate the bulk of fibres within a muscle that generate force
Gamma motor neurones that innervate a sensory organ within the muscle known as the muscle spindle
Where are the spinal cord enlargements?
What is a motor unit?
An alpha motor unit and all of the skeletal muscle it innervates
What is a motor neurone pool?
The collection of alpha motor neurones that innervate a single named muscle
What determines the force of muscle contraction?
Frequency of action potential discharge of the alpha motor neurone
Recruitment of additional, synergistic motor units
Where can the cell bodies of LMNs be found?
Where can LMNs that innervate axial muscles be found?
Medial to those innervating distal muscles
Where can MNs supplying flexors be found?
Dorsal to those supplying extensors
What are the sources of input to an alpha motor neurone?
Central terminals of dorsal root ganglion cells whose axons innervate the muscle spindles
UMNs in motor cortex and brain stem
What does muscle strength depend on?
Activation of muscle fibres
Force production by innervated muscle fibres
What does activation of muscle fibres depend on?
Firing rate of LMNs (force of motor unit increases to a max as a function of LMN firing frequency)
Number of LMNs that are simultaneously active
Co-ordination of movement
What does the force production of innervated fibres depend on?
Fibre size - hypertrophy
Fibre phenotype - fast or slow contracting muscle
What does a single action potential in an alpha motor neurone result in?
Twitch in muscle fibre
What does a sustained twitching of muscles result in?
Describe the variable sizes of motor units
Small - extraocular eye muscles
Large - postural antigravity muscles
Describe the relationship between the size of a cell body of an alpha motor neurone and excitability
Smaller cell bodies are more excitable
What is each muscle fibre innervated by?
Single motor axon at endplate (neuromuscular junction) which is usually at the centre of the fibre
What is the difference between fast and slow muscle fibres?
Alpha motor neurones innervating fast type tend to be larger and have faster conductive axons than those of slow units
What determines a slow and fast twitch muscle?
Differs in how quickly myosin ATPase splits ATP to provide energy for the cross bridge formation
Expression of different myosin heavy chains
Describe a slow oxidative type 1 muscle fibre
ATP derived from oxidative phosphorylation
Slow contraction and relaxation
Red fibres due to high myoglobin content
Describe a fast type 2a muscle fibre
ATP derived from oxidative phosphorylation
Fast contraction and relaxation
Red and well vascularised
Describe a fast type 2b muscle fibre
ATP derived from glycolysis
Fast contraction but not fatigue resistant
Pale in colour and poorly vascularised
What are the 3 types of motor units?
Describe a fast fatiguing motor unit
Large alpha motor neurone
Type 2b fibres
Utilised in burst power
Describe a fatigue resistant motor unit
Intermediate alpha motor neurone
Type 2a fibres
Utilised in sustained locomotion
Describe a slow motor unit
Small alpha motor neurone
Type 1 fibres
Utilised in antigravity and sustained movement
What is the henneman size principle?
Smaller alpha motor neurones (slow motor units) have a lower threshold than larger ones
Slow motor units are more easily activated and trained by any training that activates the muscle
Describe what occurs with the activation of an UMN
Describe muscle fibre recruitment
Motor units (LMN and muscle fibres) are recruited in order of size i.e. progressively increasing - small LMNs are more easily excited than large ones
What does this activation in terms of increasing size of LMNs allow for?
Fine control of muscle force across a wide range of tensions developed
Which will be recruited first, type 1 or type 2 fibres?
Slow type 1 before fast fatigue resistant type 2a which comes before type 2b
This results in increasing increments towards the maximal force the muscle exerts
What detects changes in length and rate of change of a muscle?
What is contained within the sensory organ the muscle spindle?
Intrafusal muscle fibres
Sensory afferents; 1a class
Gamma motor neurone efferents which innervate intrafusal fibres
Describe a 1a sensory afferent
Very fast conduction
What do extrafusal fibres generate?
Which neurotransmitter mediated the myotatic reflex?
Which muscles is the monosynaptic myotatic reflex most prominent in?
Which spinal levels mediates the biceps reflex?
Which spinal level mediates the supinator reflex?
Which spinal level mediates the triceps reflex?
Which spinal levels mediate the quads (knee) reflex?
Which spinal levels mediate the gastrocnemius (ankle) reflex?
What are intrafusal fibres?
Non contractile equatorial regions innervated by 1a sensory afferents
Contractile polar ends that receive efferents from gamma motor neurones
Where are gamma motor neurones cell bodies?
Ventral horn of spinal cord
What will cause the intrafusal fibres of the muscle spindles to contract?
Stimulation of gamma motor neurones
What are the different types of nuclear bag fibres?
Bag 1/dynamic: very sensitive to the rate of change of muscle length
Bag 2 / static: very sensitive to the absolute length of muscles
What innervates bag 1 nuclear fibres?
Dynamic gamma motor neurones
What innervates bag 2 nuclear fibres?
Static gamma motor neurones
What are chain fibres?
Sensitive to absolute length
What innervates chain fibres?
Static gamma motor neurones
What are the 2 types of afferent fibres that innervate the intrafusal fibres?
Which nerve ending will 1a afferents form?
Annulospiral nerve ending that winds around the centre of all intrafusal fibres
Which nerve ending will 2 fibres form?
Flowerspray endings on all intrafusal fibres EXCEPT bag 1 dynamic type
What will 1a fibres respond to?
Rate of change of muscle length (dynamic) and absolute length (steady state)
What will stimulation of the static gamma fibre result in?
Steady state response
What will stimulation of the dynamic amma fibre result in?
Dynamic response to stretch
When will static gamma motor neurones be active?
In activities in which muscle length changes slowly and predictably
When will dynamic gamma motor neurones be active?
In activities in which muscle length changes rapidly and unpredictably
Where are golgi tendon organs found?
Junction of muscle and tendon
What will golgi tendon organs monitor?
Changes in muscle tension
Are golgi tendon organs in series or parallel to extrafusal fibres?
(Muscle spindles are parallel)
What innervates golgi tendon organs?
Group 1b sensory afferents
What will golgi tendon organs protect the muscles from?
Overload e.g. weight lifting
Regulates muscle tension to an optimal range
Where will group 1b afferents enter the spinal cord and synapse?
Upon inhibitory interneurons which will synapse on alpha motor neurones of homonymous muscle forming the reverse myotatic reflex
Which reflex does the golgi tendon organ mediate?
Inverse myotatic reflex; tells a contracting muscle to relax
Where can proprioceptive axons be found?
Connective tissue of joints (joint capsules and ligaments)
What will proprioceptive axons respond to?
Changes in angle, direction and velocity of movement of a joint
Prevents excessive flexion and extension
Where will proprioceptive information arise from?
Golgi tendon organs
Describe the golgi tendon pathway
Activation of 1b afferents from tendon organ
Excitation of inhibitory spinal interneuron
Inhibition of alpha motor neurone supplying homonymous muscle
Relaxation of muscle