Neuromuscular Control Flashcards
What is an alpha motor neurone?
- lower motor neurons of the brainstem and the spinal cord
- innervate the extrafusal muscle fibres of the skeletal muscles (contractile elements)
- activation causes muscle contraction
- motor neuron pool contains all alpha motor neurons innervating a single muscle
What are intrafusal fibers?
modified muscle fibers within the muscle spindle containing sensory organs (stretch).
What is a motor unit?
a single motor neuron together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force.
On average each motor neurone supplies about 600 muscle fibres.
At the time of birth there’s overlap but as you grow, they specialise
What are the types of motor unit?
• Slow (S, type I) eg postural control
- smallest diameter cell bodies
- small dendritic trees
- thinnest axons
- slowest conduction velocity
• Fast, fatigue resistant (FR, type IIA)
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
• Fast, fatiguable (FF, type IIB)
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
How are the different motor unit types classified?
- tension generated
- speed of contraction
- fatiguability
Describe regulation of muscle force
Two mechanisms by which the brain regulates the force that a single muscle can produce
- recruitment - number of motor units involved
- rate coding - how fast the units contract- standard firing rate is 8 units per second
Both these processes happen simultaneously
Order for both is Slow → FR → FF
What are the implications of innervation ratio?
- number of muscle fibres innervated per motor neurone
- lower ratio means greater control, fine movement
- higher ratio means greater force
Describe how recruitment works
- motor units are not randomly recruited
- governed by the “size principle”
> smaller units are recruited first (these are generally the slow twitch units)
> as more force is required, more units are recruited - this allows fine control (e.g. when writing), where low force levels are required
Describe how rate coding works-
motor unit can fire at a range of frequencies
- slow units fire at a lower frequency
- as the firing rate increases, the force produced by the unit increases
- summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials
What are neurotrophic factors? What do they do?
type of growth factor
- Prevent neuronal death
- Promote growth of neurons after injury
Describe factors which can affect muscle fibre characteristics
- motor unit and fibre characteristics are dependent on the nerve which innervates them
- if a fast twitch muscle and a slow muscle are cross innervated, the slow becomes fast and the FDL becomes slow.
- motor neuron has some effect on the properties of the muscle fibres it innervates
Describe the plasticity of motor units
Fibre types can change properties under many different conditions
- Can change IIB to IIA with training
- Type I to II possible in cases of severe deconditioning or spinal cord injury
> Microgravity during spaceflight results in shift from slow to fast muscle fibre types
- Ageing associated with loss of type I and II fibres but also preferential loss of type II fibres → hence vast majority is slow muscle
What is a reflex?
It is an automatic, stereotyped response to a peripheral stimulus resulting in involuntary coordinated pattern of muscle contraction and relaxation without reaching the level of consciousness
Reflexes when released cannot be stopped.
How can you influence a reflex
Jendrassik manoeuvre
- clenching the teeth, making a fist, or pulling against locked fingers when having patellar tendon tapped
- reflex becomes larger; less inhibition from UMN
What is the role of the CNS in reflexes?
- Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex
> Inhibitory control dominates in normal conditions (N)
> Decerebration reveals the excitatory control from supraspinal areas (D)
- Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex
Which pathways make up descending control of reflexes?
What is the function of gamma neurones?
- keep muscle spindles taut, thereby allowing the continued firing of alpha neurons, leading to muscle contraction
- adjust the sensitivity of muscle spindles
- responsible for altering sensitivity of sensory organs
- when stretched generate reflex contraction
- function is to sensitise organ to stretch when muscle is at different lengths
