9. Neuromuscular and spinal cord Flashcards
What is the range of contact ratio of synapses in the body?
1:1 to 10^3:1
What is the resting potential difference over the membrane?
-70mV (inside is 70mv more negative than outside)
How can the membrane potential of the post-synaptic neurone be altered (2 directions)?
• Can be made less negative
- brought closer to the threshold for firing
- excitatory post-synaptic potential (EPSP)
• Can be made more negative
- brought further away from threshold for firing
- inhibitory post-synaptic potential (IPSP)
What is summation?
- Process that determines whether or not an action potential will be generated by the combined effects of excitatory and inhibitory signals
- Both from multiple simultaneous inputs (spatial summation), and from repeated inputs (temporal summation)
- Summation may or may not reach the threshold voltage to trigger an action potential
Outline how a motor neurone can induce an action potential in a muscle fibre
- Ca2+ influx causes ACh release
- ACh diffuses across the synapse and binds to receptors on motor end plate
- Ion channel opens - Na+ influx causes action potential in muscle fibre
What are extrafusal and intrafusal muscle fibres?
- Extrafusal - standard skeletal muscles that cause contraction
- Intrafusal - contain specialised sensory organs that give the CNS information
What are alpha motor neurones?
- Lower motor neurones of the brainstem and the spinal cord
- Alpha motor neurones are the final neurones going from the CNS to the muscle
- Innervate the extrafusal muscle fibres of the skeletal muscles
- Activation causes muscle contraction
What is it called when all of the neurones go to a single muscle?
- Motor neurone pool
* This occurs with many alpha motor neurones innervating a single muscle
Where are alpha motor neurones found in the CNS?
- Anterior/ventral horn of the grey matter
* these neurones are aka ventral/anterior horn cells
What do flexors and extensors allow?
- Flexors - flex the muscles and allow you to curl up into a ball
- Extensors - allow you to be as tall and long as possible
What does a motor unit describe?
- A nerve and all of the muscle fibres that it innervates
* The smallest functional unit to produce a contraction
How many motor units can innervate a single muscle fibre?
Only 1 - no muscle fibre is innervated by more than one motor unit
How many muscle fibres does each motor neurone supply?
Each motor neurone supplies about 600 muscle fibres
420,000 motor neurones and 250million skeletal muscle fibres in a human
What are the 3 different types of motor units?
- Slow (S)
- Fast (FR) - fatigue resistant
- Fast (FF) - fatiguable
What are the 3 different motor unit types classified by?
- Amount of tension generated
- Speed of contraction
- Fatiguability of the motor unit
Describe slow motor units
- Smallest diameter cell bodies
- Small dendritic trees
- Thinnest axons
- Slowest conduction velocity
- Don’t generate much force
- Don’t tire out - continue indefinitely
Describe fast (FR and FF) motor units?
- Larger diameter
- Larger dendritic trees
- Thicker axons
- Faster conduction velocity
What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?
- Recruitment - changing the number of motor units active at any one time
- Rate coding - changing the frequency of action potentials to the muscle
• Both happen together
How are motor units recruited?
- Size principle
- Smaller units are recruited first (slow twitch units)
- As more force is required, more units are recruited
- This allows fine control, under which low force levels are required
- Slow => fatigue resistant (FR) => fatiguable (FF)
- Happens in reverse when coming down from a contraction
What happens when units fire at a frequency too fast and why (as part of ‘rate coding’)?
- Summation
* Allows the muscle to relax between arriving action potentials
Where is my will to live?
not here.
What are neutrotrophic factors?
- Type of growth factor
- Prevent (peripheral) neuronal death and promote growth of neurones after injury
- CNS neurones don’t regenerate after injury unlike peripheral neurones
What is the general explanation for the inability of CNS neurones to repair?
- Millions of axons in the CNS as opposed to a few thousand
* Consequences of rewiring incorrectly is not worth it
What happens if a fast twitch and slow muscle are cross-innervated, and what does this mean?
- Slow muscle becomes fast
- Fast muscle becomes slow
- Therefore, action potentials can’t be the only thing being delivered to the muscle - something else governs the way the muscle behaves
How can fibre types change from type IIA (FR) to type IIB (FF) and vice versa?
• FR to FF - normally no way - possible in cases of sever deconditioning or spinal cord injury • FF to FR - most common following training
What affect does microgravity during spaceflights have on the fibre types?
Slow to fast muscle fibres
What affect does ageing have on muscle fibre types?
- Loss of type I (S) and II fibres
- Preferential loss of type II
- Larger proportion of type I - evidence: slower contraction times
What is a loss of muscle mass, quality and strength due to ageing called?
Sarcopenia
Which tracts control voluntary movements and automatic movements?
- Voluntary - corticospinal/pyramidal tract
* Automatic - extrapyramidal tracts
Where do the upper motor neurones cross over and meet the lower motor neurone?
- Cross at decussation of the pyramids
* Synapses in the ventral horn of grey matter
What does a lower motor neurone join with to form a peripheral nerve?
Sensory nerve
What is the rubrospinal tract involved in?
Automatic movements of arms in response to posture/balance changes
What is the reticulospinal tract involved in?
Coordinated movements of locomotion/posture resulting from painful stimuli
What is the vestibulospinal tract involved in?
Regulates posture to maintain balance - allow us to maintain head/neck position
What is a reflex?
- Automatic and often inborn response to a stimulus
* Elicited by peripheral stimuli
What is the integrating centre?
- One or more regions within the CNS that relay impulses from sensory to motor neurones
- Part of reflex arc
- Includes the interneuron
What does it mean if you can voluntary contract a muscle but there is no reflex?
Damage of the sensory neurone
How do flexors and extensors work together?
- Afferent signal travels to the spinal cord
- Afferent signal stimulates certain motor neurones (e.g. those supplying the flexor) and inhibits others (e.g. those supplying the extensor) to allow for movement
- Allows for a monosynaptic connection with the efferent
- An inhibitory signal from an interneuron allows this
When you strike the patellar ligament with a tendon, which muscles do you excite and inhibit?
- Excite the quadriceps muscle
* Inhibit the hamstrings
What is the Hoffman reflex used for?
- Used clinically to asses which set of nerves may be affected by a disorder
- The stimulus is identical every time the reflex is tested so it is more useful in identifying problems due to a change in reflex
- This is not the case when using the knee-jerk reflex, which is less reliable
Why do you see 2 twitches if you stimulate the nerve at the back of the knee?
1) M (motor) wave - direct motor response
• From motor neurone that has been stimulated, directly to the muscle causing contraction
2) H (Hoffman) wave - reflex
• AP in sensory neurone going to the spinal cord and exciting the motor neurone
• Second twitch - longer path
Are you more likely to get a response from a sensory or motor nerve stimulation?
- Sensory (H wave) rather than the M wave
* Sensory nerves are more amenable to electrical stimuli as they are larger
Explain why the H-waves peaks earlier then gets smaller as the M wave peaks?
- H wave peaks as sensory stimulus increases
- As the M wave increases, more antidromic (wrong way) action potentials are propagated, as the stimulus is in the middle of the motor neurone
- At a certain level, the sensory signal will be cancelled out by the antidromic motor impulse
- M wave continues to get bigger as stimulus increases, until no more neurones can be activated
What is it called when you have a reflex it move your leg away to a painful stimulus?
Flexion withdrawal
What is the reflex that occurs to prevent you from falling over called?
Crossed extensor
What is the Jendrassik manoeuvre?
- Clenching teeth and interlocking hands when having patellar tendon tapped
- This action can prevent conscious inhibition of the patellar reflex
How does disease affect the influence of the CNS higher centres on stretch reflexes?
- 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
- Explanation for brisk reflexes and spasticity in someone who’s had a stroke
How do higher centres of the CNS influence reflexes?
- Activating alpha motor neurones (contraction)
- Activating inhibitory interneurons
- Activating propriospinal neurones (posture)
- Activating gamma motor neurones
- Activating terminals of afferent fibres
Which higher centres and pathways are involved in the influence of reflexes?
- Cortex - corticospinal
- Red nucleus - rubrospinal
- Vestibular nuclei - vestibulospinal
- Tectum - tectospinal (head movements in response to visual information)
What are gamma neurones involved in?
- Supply the sensory parts of the muscle
- Change the sensitivity of the sensory organ
- Senses the extent of contraction or stretch
- Affects the alpha motor neurone creating a gamma reflex loop
What is clonus?
Muscular spasm involving repeated, often rhythmic, contractions
What is Babinski’s sign?
Plantar extension when stroking the bottom of the foot (with condition or a child <18 months due to undeveloped corticospinal tract)
What is hypo-reflexia mostly associated with?
Lower motor neurone lesions
