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.
Where are golgi tendon organs located? What do they act like?
They sit in the junction of a muscle and its tendon. They act like a strain gauge.
Do the nerve endings of muscle spindle fibres run in parallel to the muscle fibres or across the fibres?
In parallel. Hence they send information about the muscle length
Do the nerve endings of golgi tendon organ fibres run in parallel to the muscle fibres, or across it?
They run across the muscle fibres. Hence they send information about tension within the muscle
Where does the information from the golgi tendon organs synapse and with what?
In the ventral horn, and it synapses with the interneurons (unlike the muscle spindles which synapse with the alpha motor neurons). Some of these interneurons form inhibitory connections with the alpha motor neurons to fibres of the same muscle.
Where do alpha motor neuron received the majority of their input from? (3)
Interneurons that relay primary sensory input
- Golgi tendon organs
- Muscle spindle fibres
Descending axons from the cerebrum
Collaterals of other LMN's
Hows do inhibitory interneurons play a role in normal motor function?
They use reciprocal inhibition to avoid an antagonistic myotatic reflex (say the muscle spindle of the patellar tendon fires and causes the quads to contract, if there were no inhibitory interneurons to reciprocally inhibit the hamstrings muscle spindles, the sudden contraction of the quads would cause the hamstrong muscle spindle fibres to fire...causing the hamstrings to also contract).
Is there such a thing as excitatory interneurons?
Describe the flexor reflex. Are both inhibitory and excitatory interneurons required for this to work?
A polysynaptic reflex to activate the flexors at multiple cord levels to withdraw a limb from a noxious stimuli.
Nociceptive fibres bring information into the cord. Collaterals communicate with interneurons over several levels. Interneurons excite the alpha motor neurons to the flexors of the affected limb. The limb is drawn away from the stimulus.
Excitatory interneurons excite the alpha motor neurons.
Inhibitory interneurons inhibit the extensor muscles from resisting the sudden change in length and tension of the flexor muscles.
Which is faster, the flexor reflex or the myotatic reflex?
What is the crossed extensor reflex and why do we get it?
It is a follow on from the flexor reflex, where the extensors on the other side of the body are activated while the flexors are inhibited.
This happens as a compensation for the shift in body mass which occurs with the flexor reflex.
Information: spinal motor programs for walking:
The crossed extensor reflex appears to be the building block or starting point for locomotion. Walking is simply rhythmic repetition of alternating the withdrawal and activation of crossed extensor reflexes bilaterally. The only thing missing is the mechanism for the regulation of timing.
One theory is that we have descending control from the cerebrum and also, though this could be possible, we know there must also be some kind of spinal regulation.
Circuitry that produces rhythmic motor patterns is referred to in a generic sense as a central pattern generator.
We are yet to identify the pathways that form these central pattern generators.
What are the 2 types of motor units?
Fast and slow
What kind of muscle fibres do fast motor units contain? Describe their fatiguability
They contain white muscle fibres which fatigue easily
Why do fast motor fibres appear white?
Because of their RELATIVE lack of mitochondria
What are the subtypes of fast motor units?
IIa and IIb
What kind of energy do IIa fast motor units use?
They use oxidative metabolism and glycolysis
What kind of energy do IIb fast motor units use?
They use almost soley glycolysis
What kind of muscle fibres do slow motor units contain? Describe these fibres in terms of their fatiguability
They contain red muscle fibres which are slower to fatigue than fast motor uni muscle fibres
In terms of axon size, what is the difference between fast and slow motor units and how is this reflected in their function?
Fast motor units will have larger diameter axons for faster conduction and are able to generate periodic bursts of high frequency firing.
Slow motor units will have smaller diameter axons which hinder conduction velocity and thus makes it impossible to generate high frequency impulses
What happens if we remove a fast neuron from a fast fibre and attach a slow neuron instead?
The function of the muscle fibre will change from a fast fibre to a slow fibre.
What is Brodmanns area 4? Where is it located on the brain?
The primary motor cortex (M1)
Located on the precentral gyrus of the frontal lobe
What is Brodmanns area 6? Where is it located on the brain?
The pre and supplementary motor cortices
Located just anterior to M1 on the frontal lobe. The SMA is located just medial to the PMA.
What is normal goal related movement often referred to as? Why?
This is due to the massive input from the sensory cortex
List what is needed in terms of sensory input in order to be able to execute a normal goal related movement.
We need to know where in space and time we are
- proprioceptive, visual and vestibular input
We need to use prior experience to decide on the best way to execute the movement
- prefrontal input
Once the appropriate motor plan has been decided upon, we need to execute the plan
- prefrontal, thalamic and basal ganglia input
What happens when an area of M1 is electrically stimulated? How does this differ from what happens when an are of the premotor cortex is electrically stimulated? What does this mean in terms of the PMA neural code?
When an area of the M1 is stimulated, a located group of muscles of the contralateral side of the body twitch
When the PMA is stimulated we see a more complex movement pattern elicited in the proximal limb muscles bilaterally.
This means that the PMA neurons code for not only the muscles to excited but also the direction of movement of the limbs. i.e. a specific PMA neuron would increase its activity if the limb moved to the right, but not to the left. We can assume that another PMA neuron would have the encoding for movement to the left.
What is the difference in function between the premotor cortex and the supplementary motor cortex?
Their function appears to be similar but for the distal limb muscles instead of the proximal limb muscles (done by the PMA)
When are the most pronounced deficits noted with lesions to the SMA/PMA?
They occur during activities that require coordination of both hands
What is apraxia?
The inability to perform previously acquired complex motor skills
Motor planning in the SMA info:
Its function appears to be quite similar to PMA but for the distal limb muscles
This is supported by electrical stimulatory tests
Again somatotopic arrangement was noted in these tests
More recent imaging on behaving animals show that the activity of SMA neurons about a second prior to voluntary movement of the hands
Supporting the theory that it acts as a motor planning centre
This increase in activity occurs with movement of either hand, indicating an intense interconnection via the corpus callosum.
The most pronounced deficits noted with lesions to either SMA occur during activities that require coordination of both hands
This inability to perform previously acquired complex motor skills is known as apraxia.
These imaging studies appear to support theories on a “motor hierarchy’ which emphasises the function of these motor cortices in calculating tactics to achieve goals
According to this hierarchy, accurate motor planning relies on sensory information about our position in space from cortical association areas
The decision making prefrontal cortex helps us decide how we might get from our current position to our desired position
The basal ganglia also play a role in this highest level of the hierarchy by determining which of any number of strategies is most feasible
The ‘feed forward’ or predictive functions of the cerebellum are also implicated in this function
What is the posterior parietal cortex?
An advanced sensory integration area which allows us to form an accurate mental body image of our relationship to our environment.
What is Brodmanns area 5? What is its function?
It is the superior parietal lobule
Receives details and partically pre-integrated information from the primary somatosensory cortex (S1)
The superior lobule is essential for the completion of self image and lesions of this region may lead to disorders in the recognition or visualisation of the contralateral side of the body.
What is Brodmanns area 7? What is its function?
It is the inferior parietal lobule
Receives detailed and partially pre-integrated information from higher order visual cortex
The inferior lobule integrates diverse sensory information for speech and perception and is more concerned with creating a complete perception of our environment.
What is the 'motor hierachy'? What areas of the brain supply the highest input into this and how? What parts of the brain are implicated at which levels of this hierachy?
This hierachy states that accurate motor planning relies on sensory information about our position in space from cortical association areas.
The areas of that brain that contribute:
Prefrontal cortex: help us decide how we might get from our current position to our desired position
Basal ganglia: helps to determine which of any number of strategies identified by the prefrontal cortex is the most feasible. (also some input from the cerebellum in terms of 'feed forward' or predictive functions)
The posterior parietal cortex: allows us to form an accurate mental body image of our relationship to the environment.
The neurological connection and communication between the PPC and the prefrontal association cortex is imperative to the anticipation of the consequences of the actions that are to be performed and therefore it is the intercommunication between these 2 areas of the brain that help to determine what the potential action should be.
Therefore the PPC and the prefrontal cortex form the cortical portion of the highest level of the motor hierachy, and the basal ganglia forms the subcortical portion of this highest level of the motor hierachy.