Motor Units and Control of Muscle Force Flashcards
What does the somatic motor system consist of?
Skeletal muscles and the elements of the nervous system that control them
What does the neural element of the somatic motor system comprise of?
Upper motor neurons within the brain and lower motor neurons within the brainstem of the ventral horn of the spinal cord
What is the function of upper motor neurons?
Supply input to LMNs to modulate their activity
What do lower motor neurons receive input from, other than UMN?
Proprioceptors and interneurons
What is the function of lower motor neurons?
Command muscle contraction and form the final common pathway
What do lower motor neurons consist of?
An alpha motor neuron that innervates the bulk of the fibres within a muscle that generates force
What do gamma motor neurons innervate?
A sensory organ within the muscle known as a muscle spindle
Where do axons of LMN exit the spinal cord?
In ventral roots or via cranial nerves
What does the joining of each ventral root to a dorsal root form?
A mixed spinal nerve = contains both sensory and motor fibres
What do motor neurons belong to?
A spinal segment (e.g C8)
What areas of the spine have a greater number of motor neurons?
Cervical enlargement = C3-T1, to supply upper limb
Lumbar enlargement = L1-S3, to supply lower limb
Where are motor neurons that innervate the distal and proximal musculature located?
The cervical and lumbo-sacral segments
Where are motor neurons that innervate axial musculature located?
Occur at all levels
What does a motor unit consist of?
An alpha motor neuron and all of the skeletal muscle fibres that it innervates
What does muscle contraction result from?
Individual and combined actions of motor units which must be co-ordinated
What is a motor neuron pool?
The collection of alpha motor neurons that innervate a single muscle
How is force of muscle contraction graded by alpha motor neurons?
Frequency of AP discharge of the alpha motor neuron
The recruitment of additional synergistic motor units
How are the cell bodies of LMNs innervating axial muscles distributed in the ventral horn?
They are medial to those innervating distal muscles
How are the cell bodies of LMNs supplying flexors arranged in the ventral horn?
Dorsal to those supplying extensors
What are the three sources of input to an alpha motor neuron that regulate its activity?
Central terminal of dorsal root ganglion cells whose axons innervate the muscle spindles
UMNs in the motor cortex and brainstem
Spinal interneurons
What does muscle strength depend on?
Activation of muscle fibres and force production by innervated muscle fibres (fibre size and phenotype)
What does the activation of muscle fibres depend?
The firing rates of the LMNs involved
The co-ordination of the movement
What does a single AP in an alpha motor neuron cause?
Makes a muscle fibre twitch
What does the summation of twitches cause?
A sustained contraction as the number of incoming APs increases
How does the size of motor units impact the function of the muscle?
Small size for fine movements
Large size for large postural muscles
Does the size of the motor unit impact the size of the alpha motor unit which innervates it?
Yes = small units innervated by small alpha motor neurons
What does force of contraction that each motor unit produces depend on?
The size of the motor unit
What are the features of alpha motor neurons innervating fast type motor units?
Tend to be larger and have faster conducting axons than those of slow units
What is each muscle fibre innervated by at the endplate?
A single motor neuron = usually at the centre of the fibre
How do skeletal muscle fibres differ?
Differ in how quickly myosin ATPase splits ATP provide energy for cross bridge cycling and express different myosin heavy chains
What are the features of slow oxidative (type 1) fibres?
ATP largely derived from oxidative phosphorylation
Slow contraction and relaxation, and fatigue resistant
Red fibres because of high myoglobin content
Used for antigravity sustained movement
What are the features of fast (type IIa) fibres?
ATP largely derived from oxidative phosphorylation
Fast contraction and relaxation, and fatigue resistant
Red and reasonably well vascularised
Used for sustained locomotion
What are the features of fast (type IIb) fibres?
ATP mainly derived from glycolysis
Fast contraction but not fatigue resistant
Pale in colour and poorly vascularised
Used for burst power
What is the Henneman Size Principle?
The susceptibility of an alpha motor neuron to discharge APs is a function of its size
How does the size of the alpha motor neuron affect its threshold?
Smaller alpha motor neurons have a lower threshold than larger ones
Are slow motor units more easily activated?
Yes = more easily activated and trained by any activity which activates the muscle
What does recruitment of alpha motor neurons by size allow?
Fine and graded development of muscle force = allows for fine control over a wide range of tensions
What does the activation of an UMN cause?
Causes the LMN it supplies to be activated
How are motor units recruited?
In order of their size = small LMNs are more easily excited than large LMNs
How are LMNs recruited?
In an order appropriate to the physical task being performed
What does recruiting motor neurons in order of slow before fast allow?
Results in increasing increments towards the maximal force the muscle exerts
What is the myotatic reflex?
When a skeletal muscle is pulled, it pulls back
What registers non-conscious proprioception?
Muscle spindle = registers change in length
What do spindles consist of?
Fibrous capsule, intrafusal muscle fibres, sensory afferents, gamma motor neurons
What supplies intrafusal fibres?
Sensory afferents and gamma motor neurons
What is the only monosynaptic reflex in humans?
The myotatic reflex = most prominent in extensor muscles
What does stimulation of intrafusal fibres by gamma motor neurons cause?
Spindles contract
What do intrafusal fibres consist of?
Contractile polar ends = receive efferent input from gamma motor neurons with cell bodies in the ventral horn
Non-contractile equatorial region innervated by Ia sensory neurons
What occurs during voluntary movement?
Alpha and gamma motor neurons normally co-activated so that intrafusal fibres contract in parallel with extrafusal fibres
What is the purpose of co-activation of gamma and alpha motor fibres during voluntary movement?
Maintains sensitivity of the spindle to stop it going slack when extrafusal fibres contract
