9. Motor Pathways: Neuromuscular and Spinal Cord control of movement Flashcards
1
Q
What does the potential of the post-synpaptic neurone depend on?
A
- The membrane potential of the post synaptic neurone can be altered in two directions by inputs
- It can be made be made less negative (brought closer to the threshold for firing) - this is an excitatory post-synaptic potential (EPSP)
- Or it can be made more negative (hyperpolarised) - this is an inhibitory post-synaptic potential (IPSP)
- You get GRADED effects - whether the post-synaptic neurone fires or not is dependent on the summation of the various inputs
- The degree of summation will determine how readily a neuron can reach threshold to produce an action potential
2
Q
Describe the activation of the neuromuscular junction.
A
- A NMJ is a specialised synapse between the motor neurone and the motor end plate on the muscle fibre cell membrane
- When an action potential arrives at the MNJ, Ca2+ influx causes ACh release. ACh binds to receptors on motor end plate.
- Ion channel opens – Na+ influx causes action potential in muscle fibre
- Acetylcholine gets released from the presynaptic cell when the SNARE proteins interact with the membrane (they are involved in the amalgamation of the vesicle membrane with the presynaptic membrane)
- Calcium influx triggers the acetylcholine release
- If you record the membrane potential across the muscle fibre, you can see that at any one point there are small changes in membrane potential
- These are NOT action potentials but rather just small changes in membrane potential that happens as vesicles are constantly dumping their contents into the synaptic cleft
- These are called miniature end plate potentials (mEPP)
3
Q
What do alpha neurons innervate?
A
- These are also called ventral horn cells, anterior horn cells or lower motor neurones
- They innervate the extrafusal (contracting muscle) muscle fibres of the skeletal muscle
- Activation of alpha motor neurones causes skeletal muscle contraction
- There are coiled, spring like sensory receptors in the muscle called spindles that, when stretched, feedback to the CNS and allows an excitatory reflex to be generated which is what you want when your patella ligament gets hit by a tendon hammer
4
Q
Define Intrafusal muscle fibres
A
skeletal muscle fibres that serve as specialised sensory organs (proprioceptors) that detect the amount and rate of change in length of a muscle
5
Q
Define Extrafusal muscle fibres.
A
- standard skeletal muscle fibres that are innervated by alpha motor neurones and generate tension by contracting, thereby allowing for skeletal muscle movement
6
Q
Define Motor Neurone Pool
A
Motor Neurone Pool = collection of lower motor neurones that innervate a single muscle
7
Q
Describe the arrangement of alpha motor neurons.
A
- They are found in the anterior/ventral horn of grey matter
- Flexors = flex the muscles and allow you to curl up into a ball
- Extensors = allow you to be as tall and long as possible
- They have some kind of arrangement within the ventral horn
8
Q
Describe the motor unit and innervation of motor fibres.
A
- Motor Unit Definition: a single motor neurone together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force.
- IMPORTANT: one alpha motor neurone can innervate SEVERAL muscle fibres
- But it is also important to note that every muscle fibre is only innervated by ONE ALPHA NEURONE
- However under pathological conditions, e.g. when a nerve has been cut, the axon can sprout and being to innervate muscle fibres that are already innervated by other motor neurones
- The number of muscle fibres innervated by a single alpha motor neurone varies and is reflected by the function of the muscle
- Muscles in the EYE have a low innervation ratio (number of fibres innervated by a single motor neurone) because this needs to be finely controlled
- If loads of muscle fibres are innervated by a single motor neurone, then when that motor neurone fires, ALL of the muscle fibres will contract
- The quadriceps do not need a low innervation ratio because you want POWER from this muscle rather than delicate control
- Humans have around 420,000 motor neurones and 250,000,000 muscle fibres
9
Q
Describe the types of motor units
A
- We have SLOW and FAST muscles
- Slow muscles don’t produce much force but they can work for a long time
- Postural muscles, muscle in the shin are mainly slow muscles (e.g. soleus)
- Fast muscles are further subdivided into:
- Fast Fatigue Resistant (Type 2a)
- Fast Fatiguable (Type 2b)
- The alpha motor neurones that innervate these different types of muscle have specific characteristics (listed above)
- Thicker axon = faster conduction velocity
- In terms of distribution, specific types of muscle fibre aren’t grouped into specific areas, they are fairly spread out
- If you stimulate a slow muscle fibre you will find that it will generate its peak force much more slowly than the fast fibres
- Fatigue resistant muscles produces more force than the slow fibres and the force is produced more quickly
- Fatiguable - this produces a LOT of force and does this very quickly but it also gets fatigued very easily
10
Q
How are muscle forces regulated?
A
- There are TWO mechanisms by which the brain regulates the force that a single muscle can produce
- Recruitment - recruiting more motor units (smaller units (generally slow twitch) are recruited first)
- Rate Coding - changing the frequency with which you send action potentials down the nerves
11
Q
Describe the order of recruitment.
A
- Slow
- Fast Fatigue Resistant
- Fast Fatiguable
12
Q
Explain what neurotrophic factors.
A
- There are a whole host of factors that are produced within the nerve and are transported throughout the nerve to maintain the nerves integrity and function
- These are neurotrophic factors
- They are a type of growth factor and they prevent neuronal death
- They promote the growth of neurons after injury
- CNS neurones don’t regenerate after injury unlike peripheral nerve - the explanation is that in the CNS you have millions of axons as opposed to a few thousand so the consequences of rewiring incorrectly is not worth it
13
Q
Describe how plasticity of motor neurons affect how neurotrophic factors work.
A
- It is not easy to switch from one type of motor unit to another
- Type 2B (fast fatiguable) to Type 2A (fast fatigue resistant) is the most common following training
- But there is normally NO WAY of changing fast to slow or vice versa
- Type I to type 2 is only possible in cases of severe deconditioning or spinal cord injury
- Microgravity during space flight results in a shift from slow to fast muscle fibre types
- Ageing is associated with a loss of Type 1 and Type 2 muscle fibres but with preferential loss of Type 2 fibres
- This results in a large proportion of type 1 fibres in aged muscle
- This is why aged muscle generally has slower contraction times
- This loss of muscle is called sarcopenia
14
Q
Define a reflex.
A
- A reflex is an automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness
- The magnitude and timing of the coordinated muscle contraction and relaxation is determined by the intensity and onset of the stimulus
- E.g. if the biceps were tapped, the reflex occurs quickly and is related in size to how hard the biceps were hit
- Reflexes differ from voluntary movements in that once they are released, they can’t be stopped
- Reflexes are important for maintaining upright posture and for reducing damage to parts of the body
15
Q
How would you test the reflex arc??
A
- If there is any indication that there might be some damage to the central or peripheral nervous system, you will do a reflex test
- For a reflex you need an afferent signal, some kind of relay neurone (not always) and a motor neurone
- Reflexes need afferents:
- A muscle is stretched and the amount of force the muscle produces in the reflex action is recorded
- The amount of force the muscle produces increases as a result of the reflex but this doesn’t happen if you do NOT have the dorsal roots
- So you need a sensory input for a reflex to take place
- This is why reflexes can be lost when you’ve damaged motor nerves OR sensory nerves
- Reflex testing can help determine whether there has been a sensory loss or a motor loss
- If you can voluntarily contract the muscle then there is probably nothing wrong with the motor neurones
- If you then hit the tendon and nothing happens, since you can voluntarily contract it, is indicates a sensory loss
16
Q
How would you test to see how many synapses are present?
A
- We measure the volley (action potential) as a stimulus is set up in one of the two nerves
- This is a set up where a sensory nerve innervating a flexor or a sensory nerve innervating an extensor has been stimulated
- The recording device to the top left is recording the direction of the afferent signal coming past the recording device
- The recording device on the right is measuring a change in membrane potential
- When you stimulate one of the nerves going through the dorsal root, you will record a volley (action potential) going past the recorder
- If the afferent fibres from the extensor are stimulated you will get a monosynaptic connection with the efferent then you get contraction of the extensor
- This is equivalent to tapping the patellar tendon and getting a reflex contraction
- Bottom left graph - shows the afferent volley - a recording of the set of action potentials going past the recording device on the top left
- The graph directly above shows a huge excitatory potential recorded in the motor neurone
- This occurs just about 0.7 ms after the afferent volley has gone past the recording equipment
- This is indicative of a SINGLE SYNAPSE between the afferent and efferent
- When you hit the patellar ligament with a tendon hammer, not only do you excite the quadriceps muscle, you also INHIBIT the hamstrings
- So there is an excitatory signal to the quadriceps and an inhibitory signal to the hamstrings
- In general terms: there is an inhibitory signal to the antagonist at the same time as the excitatory signal to the agonist
- If you stimulate the flexor nerve, e.g. nerve to the hamstrings, and record the volley as it goes past - the quadriceps will be inhibited
- Not only will the membrane potential be going in the OPPOSITE DIRECTION (as it is inhibitory), but it will also take TWICE AS LONG from the start of the volley to the change in membrane potential of the efferent
- This is indicative of there being more than one synapse between the afferent and efferent
- Excitatory tends to be monosynaptic and inhibitory tends to be polysynaptic the time between the response and action potential is indicative of how many synapses there are.
- Generally it is about 0.7 ms for each synapse in this set up
17
Q
Describe the Hoffman (H-) Reflex.
A
- You can’t rely on the knee-jerk reflex on its own with a tendon hammer because the reflex depends very much on which part of the tendon is hit and how hard it is hit – human error
- Hoffman came up with a way in which the stimulus can be identical every time the reflex is tested - it makes sure that the stimulus has the same duration and amplitude so you know that any change in reflex size is NOT due to the input (this can’t be guaranteed with a tendon hammer)
- Hoffman reasoned that he could bypass the physical stretch of the muscle
- If he had a nerve containing sensory and motor fibres - if he delivered an electrical stimulus to this nerve then it would carry the impulse along the sensory fibre to the spinal cord and via a reflex arc back to the muscle
- This is the Hoffman Reflex and it is commonly used to test the integrity of reflex pathways
- If you stimulate the nerve at the back of the knee you will see two twitches:
- Direct motor response - going from the motor neurone that has been stimulated, directly to the muscle causing contraction
- This is the M wave (motor wave)
- A short time later you will see another response in the EMG and there will be another twitch
- This is caused by the action potential in the sensory neurone going back to the spinal cord and exciting the motor neurone - H wave
- Direct motor response - going from the motor neurone that has been stimulated, directly to the muscle causing contraction
- Sensory nerves are more amenable to electrical stimuli because they are larger so you can get a response from a sensory nerve (H wave) at lower stimulus intensity than the M wave
- Stimulation of the sensory neurone also means that you can feel the stimulus before you get the twitch
20
Q
What are the higher centres and pathways involved in supraspinal control of reflexes?
A
- Cortex – corticospinal (fine control of limb movements, body adjustments)
- Red nucleus – rubrospinal (automatic movements of arm in response to posture/balance changes)
- Vestibular nuclei – vestibulospinal (altering posture to maintain balance)
- Tectum – tectospinal (head movements in response to visual information).
Facilitation from higher centres acts:
- On the motor neurone, increasing its sensitivity to afferent input or…
- Indirectly via gamma motor neurones and the muscle spindle, increasing afferent input to the alpha motor neurons
