Flashcards in Week 7 - voluntary movement - finished Deck (63)
How many axons from the CNS is a muscle fibre innervate by?
Supplied by a single axon from the CNS
Muscles can be classified by what features and list some examples of these categories
They can be classified by location and/or function.
What is the axon muscles are innervated by and where do these axons originate?
They are innervated by LMN's which originate in the ventral horn of the spinal cord
Generally, what is the organisation of flexors/extensors and proximal and distal neurons in the ventral horn of the spinal cord? What image can we use to help describe this?
Neurons for flexors are more dorsal, extensors are more ventral
Neurons for promixal muscles are more medial, and distal are more lateral.
We the image of a bicep curling arm.
Why do we have cervical and lumbar enlargements of the ventral horn?
Because the distribution of muscles supplied by LMN's is not even throughout the body, so we have variations in ventral horn size in areas of the spine that correlate to the areas that supply the upper and lower limbs, i.e. the lumbar and cervical enlargements
What neurons are soley responsible for the generation of force by a muscle?
Alpha motor neurons
Is a muscle fibre supplied by more than one alpha LMN? Can one alpha LMN supply more that one muscle fibre?
One muscle fibre is supplied by ONE alpha LMN only, however an alpha LMN can supply more than one muscle fibre.
What is the name for one alpha LMN and the muscle fibres it supplies?
A motor unit
What is the name for the alpha motor neurons required to supply an entire muscle?
A motor neuron pool
Where and how to alpha motor neurons communicate with skeletal muscle fibres?
They communicate with them at a neuromuscular junction via the release of Ach
What happens when an action potential is generated at an alpha motor neuron?
It produces an EPSP (excitatory post synaptic potential) which produces a twitch in the muscle fibres.
What is a muscle twitch?
A rapid contraction followed by a rapid relaxation
In terms of action potential being received by an alpha motor neuron and producing a twitch in a muscle fibre, how is a sustained contraction achieved?
By a continuous barrage of action potentials
What does high frequency activity at the presynaptic terminal result in?
Temporal summation at the post synaptic terminal
What does twitch summation cause in a muscle contraction?
It causes increased tension in the muscle fibre and smoothes the muscle fibre contraction.
What is a way to grade muscle contraction in relation to action potential?
Looking at the frequency of neuron firing. This is because twitch summation causes increased tension in the muscle fibre and the increased rate or twitch leads to a smoother muscle contraction.
What is the principle behind motor recruitment patterns?
Smaller motor units require a smaller alpha motor neuron which must be easier to excited, thus smaller motor units are recruited first.
What does innervation ratio dictate?
What dictates dexterity in movement?
How do muscle spindles provide proprioceptive feedback?
They relay information about a muscles length back to the DRG via alpha motor neurons
Where do alpha motor neurons receive their input from? Which of these gives the most input?
- UMN from the cerebrum
- interneurons of the spinal cord which give both inhibitory and excitatory signals.
The interneurons give the most input.
What is a muscle spindle? How is it shaped?
A group of modified muscle fibres (intrafused fibres) within a fibrous capsule.
This complex is shaped like a spindle with a type A axon wrapped around it.
What are intrafused fibres? What are extrafused fibres? Where do each of these receive their input from?
Intrafused fibres are a group of modified muscle fibres within a fibrous capsule (also called a muscle spindle).
They receive their input from gamma motor neurons, NOT alpha motor neurons.
Extrafused fibres are fibres not within this capsule. These receive their input from alpha motor neurons.
What causes a muscle spindle neurons to fire?
When the modified muscle fibres wrapped with a type A axon are stretched.
Where is the signal from a fired neuron in a muscle spindle sent? What then happens to this information?
To the DRG and cord. This information the synapses at spinal interneurons and alpha motor neurons.
What makes muscle spindle input so powerful?
One muscle spindle may communicate with an alpha motor neurons for every muscle fibre within the muscle that the signal came from. I.e. if a single muscle spindle in a bicep muscle is activated, the muscle spindle can communicate that information to all alpha motor neurons that suuply every muscle fibre in the biceps muscle.
What is the myotatic reflex? Give an example.
A theoretical monosynaptic response between the muscle spindle input and the alpha motor neuron output.
e.g. knee jerk
You pull on a muscle and it pulls back
What is the key controller of posture?
Antigravity muscles are monitored and maintained by the myotatic response as the the myotatic response maintains muscle tone.
Describe the UMN activation of gamma and alpha motor neurons.
UMN activate both alpha and gamma motor neurons, in order to make sure that the gamma motor neurons can always report muscle length.
Gamma motor neurons act ONLY on intrafused fibres (the modified fibres inside a muscle spindle) and therefore have control over the contraction of these fibres. Alpha fibres only act on the extrafusal fibres, which essentially are all muscle fibres outside of the muscle spindle fibres.
If the UMN contracted only the fibres supplied by the alpha motor neurons then the muscle length would shorten but the muscle spindle fibres would be the same length and would essentiall go "offline", as due to their length being longer than the whole muscle length, they cant relay muscle length information. So to counter this, the UMN always contracts both the alpha and gamma fibres, so that the muscle length always shortens in relation to the muscle spindle length and visa versa, meaning the muscle spindle can always send muscle length information to the DRG.