Neuromuscular and Spinal Cord Movement of Control Flashcards
Is the contact ratio of synapses higher in the CNS or the muscles?
CNS
If there are more neurones attaching to another neurone, what does that mean?
There is more potential for the post-synaptic neurone to be altered
What are the two types of inputs altering the post synaptic neurone?
Excitatory post synaptic potential (EPSP)
Inhibitory post synaptic potential (IPSP)
What can graded potentials result in and how?
summation leading to an AP
What is the neuromuscular junction?
A specialised synapse between the motor neuron and the motor end plate
What is the motor end plate?
The muscle fibre cell membrane that is highly folded
What happens at rest - vesicles?
Individual vesicles release ACh at a very low rate causing miniature end-plate potentials (mEPP) - graded
Describe the process of an AP arriving at the NMJ leading to contraction
- When an action potential arrives at the NMJ, Ca2+ influx causes ACh release
- ACh binds to receptors on motor end plate
- ACh diffuses across the synapse, activates ACh receptors and propagates AP
- Actin and myosin -> muscle contraction
- Ion channel opens – Na+ influx causes action potential in muscle fibre
What is an alpha motor neurone?
Lower motor neurons of the brainstem and the spinal cord (final neurone going from the CNS to the muscle)
They innervate the extrafusal muscle fibres of the skeletal muscles
What are intra and extrafusal muscle fibres?
Extrafusal muscle fibres: standard skeletal muscles that cause contraction
Intrafusal muscle fibres: contain specialised sensory organs that tell the CNS information
What is a motor neurone pool?
All of the neurones going to a single muscle - contains all alpha motor neurones innervating a muscle
How are alpha motor neurones arranged in the spinal cord?
In the anterior/ventral horn of grey matter
Alpha motor neurones are also called ventral/anterior horn cells
What is extension and flexion?
Flexion - movement that decreases the angle between two body parts. Flexion at the elbow, arm moves towards you. Knee flexion the leg bends back.
Extension - movement that increases the angle between two body parts. Extension at the elbow moves arm back and at knee straightens leg.
What is a motor unit?
Describes a nerve and all of the muscle fibres that it innervates
It is the smallest functional unit to produce a contraction
Stimulation of that one motor unit causes contraction of all the fibres in it
Where are cell bodies of the alpha neurones found?
ventral horn
Can a muscle fibre be innervated by more that one motor unit?
NO
Unless there is a pathology
How can motor units be categorized?
slow (S) and fast (FF, FR)
Describe the relationship between the nerve and the muscle it innervates
Muscles take on characteristics depending on which nerve supplies them.
Describe the features of slow fibres
- smallest cell body diameter
- small dendritic trees
- thinnest axons
- slowest conduction velocity
Describe the features of FR (fast fatigue resistant) fibres
- larger diameter of cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
- resistant to fatigue
Describe the features of FF (fast fatiguable) fibres
- larger diameter of cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
How are the 3 types of motor units classified?
According to:
- The amount of tension generated
- Speed of contraction
- Fatigability of the motor unit
When nerved are stimulated they shorten and then relax, hence force can be measured as well as speed. How do the different fibre types vary in this?
Slow units don’t generate much force, and take a while to do it. However, they can continue to generate this force indefinitely – these muscles don’t tire out.
Fast units generate more force and do it quickly, but this cannot be sustained for very long periods of time.
All muscles have more of one type of muscle fibre over the other type.
How does the brain regulate the forces that a single muscle can produce? (2 mechanisms)
Recruitment – Changing the number of motor units active at any one time
Rate coding - Change the frequency with which action potentials travel down the muscle
What is recruitment?
Motor units are not randomly recruited. 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), under which low force levels are required. We go from slow, to fatigue resistant, to fast fatigable. This happens in reverse when coming down from a contraction.
What is rate coding?
A 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.
Coding changes at the same time recruitment changes. As the motor unit coding increases, frequency increases. At low levels of force, you expect the slow motor units to be recruited, but within an individual set of motor units, impulses are changing in frequency at the same time.
What are neurotrophic factors?
A type of growth factor
Compare CNS and PNS nerve regeneration
- Peripheral axon damage is repairable
- Something in the peripheral environment that allows for axonal regeneration
- They prevent neuronal death and promote growth of neurons after injury
- CNS neurones don’t regenerate after injury unlike peripheral nerves
What happens if a fast and slow twitch fibre are cross innervated?
Motor unit and fibre characteristics are dependent on the nerve that innervates them
If a fast twitch muscle and a slow muscle are cross-innervated, the slow becomes fast and the fast becomes slow. The motor neurone has some effect on the properties of the muscle fibres which it.
Hence something must be occurring in the nerve, maybe something else is delivered to the muscle.
Describe the plasticity of motor units/muscle fibres
- Fibre types can change properties under many different conditions
- The change from type IIB (fast fatigable) to IIA (fast fatigue resistant) is most common following training
- There is normally no way of changing fast to slow (or vice versa)
- The change from type I to II is possible in cases of severe deconditioning or spinal cord injury
- Microgravity during spaceflight results in shift from slow to fast muscle fibre types
What effect does ageing have on motor units/muscle fibres?
- Ageing is associated with loss of type I and II fibres but also preferential loss of type II fibres
- This results in a larger proportion of type I fibres in aged muscle (evidence: slower contraction times)
- This loss of muscle is called sarcopenia
What is one of the tracts controlling voluntary movement?
Corticospinal /pyramidal Tract
What are extrapyramidal tracts?
many are concerned with automatic movements in response to stimuli
Describe the arrangement of upper and lower motor neurones
The upper motor neurone crosses over at the decussation of the pyramids in the medulla and synapses with a lower motor neurone in the ventral horn of grey matter. The lower motor neurone then projects out of the spinal cord and joins with a sensory nerve coming in to form a peripheral nerve.
What does the rubrospinal tract do?
Involved in automatic movements of arms in response to posture/balance changes
What does the reticulospinal tract do?
Coordinated movements of locomotion/posture resulting from painful stimuli
What does the vestibulospinal tract do?
Regulates posture to maintain balance – allow us to maintain head/neck position
What happens during a reflex?
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.
Why is a reflex test done?
If there is indication that there might be damage to the CNS/PNS. For a reflex you need an afferent signal, some kind of relay neurone (not always) and a motor neurone.
What does it mean if you can voluntarily contract a muscle but there is no tendon reflex?
Since you can voluntarily contract it there is nothing wrong with the motor neurones and indicates a sensory loss.
How many synpases are there in the reflex arc?
There can be more than one synapse in the reflex from beginning to end.
Muscles work in pairs – flexors and extensors. An afferent signal will travel to the spinal cord. The afferent stimulates certain motor neurones (e.g. those supplying the flexor), and inhibits others (e.g. those supplying the extensor) to allow for movement If the afferent fibres from the muscle are stimulated you will get a monosynaptic connection with the efferent to get contraction (excitatory). It can also synapse with an interneuron, which inhibits the motor neurone supplying another muscle (inhibitory)..
What happens when the patellar ligament is hit?
You excite the quadriceps muscle and inhibit the hamstrings. Stretching of the quadriceps sends a signal to excite the quadriceps and inhibit the hamstrings (efferent)
What is the Hoffman reflex?
APs are sent down the motor neurone to the muscle (quick - its the M wave). Simultaneously, it sends sensory signals to the spinal cord, which synapse onto the motor neurone, and come back to the same motor nerve -> contract the muscle again (slower, longer - H wave). So there are 2 contractions
Why is Hoffman reflex better than the standard knee jerk test?
The reflex depends on which part of the tendon is hit and how hard it is hit.
With Hoffmans the stimulus can be identical every time the reflex is tested (ensures stimulus has the same duration and amplitude – any change in reflex size is not due to the input
What is Jendrassik’s manoeuvre?
- We think of reflexes as being automatic (knee jerk) and stereotyped behaviours (sneeze, cough) in response to stimulation of peripheral receptors but we can control reflexes to a certain degree
- Try clenching teeth or making a fist when having patellar tendon tapped
- This is reflected in disease processes as well
How do higher centres influence reflexes and compare normal and abnormal?
- Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex
- Inhibitory control dominates in normal conditions
- Decerebration reveals the excitatory control from supraspinal areas
- Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex
What happens that causes spasticity in someone who had a stroke?
- If you remove the descending inhibitory control, you get very brisk reflexes and spasticity in the muscles
- In upper motor neurone lesions you get an upregulation of the reflex control of these muscles such that tone is generated when you don’t want tone to be generated and reflexes will be much larger
How can higher brain centres influence reflexes?
- Activating alpha motor neurons (-> contraction)
- Activating inhibitory interneurons (inhibit)
- Activating propriospinal neurons (posture)
- Activating gamma motor neurons
- Activating terminals of afferent fibres
What are some of the higher centres and pathways involved in the reflex modulation?
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).
What is the gamma reflex loop?
- Gamma motor neurones supply sensory organs of the muscle
- These neurones change the sensitivity of the sensory organ in order to sense the extent of contraction
What kind of lesions are strokes?
Why does hypereflexia occur?
upper motor neurone lesion
- strokes lead to a loss of descending inhibition of reflexes
What is a clonus?
muscular spasm involving repeated, often rhythmic, contractions
What is Babinski’s sign?
- If you stroke the bottom of the foot you see plantar extension (the toes fan out)
- If you do a babinski test on a child (<18 months) it will be positive
- This is because the child’s corticospinal tract is not fully developed
What is hyporeflexia?
- Below normal or absent reflexes
- Mostly associated with lower motor neurone lesions
