Neuromuscular and Spinal Cord Flashcards
Membrane potential value
-70mV
2 directional alterations to membrane potential of post-synaptic neurones
It can be made less negative – i.e. brought closer to threshold for firing; this is an excitatory post synaptic potential (EPSP)
Or it can be made more negative – i.e. brought further away from threshold for firing; this is an inhibitory post-synaptic potentials (IPSP)
Sequence at the NMJ
When an action potential arrives at the MNJ, Ca2+ influx causes ACh release
ACh binds to receptors on motor end plate
ACh diffuses across the synapse, activate ACh receptors and propagate AP
Ion channel opens – Na+ influx causes action potential in muscle fibre
mEPP
At rest, individual vesicles release ACh at a very low rate causing miniature end-plate potentials (mEPP). These potentials tend to be graded.
Alpha motor neurone
Lower motor neurone (α): final neurone going from the CNS to the muscle
They innervate the extrafusal muscle fibres of the skeletal muscles
(standard skeletal muscles that cause contraction)
Intrafusal muscle fibres
Contain specialised sensory organs that tell the CNS information
Motor Neurone pool
All of the neurones going to a single muscle
Arrangement of alpha motor neurones
They are found in the anterior/ventral horn of grey matter
Motor Unit
Made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron’s axonal terminals.
How many motor units can a muscle fibre be innervated by
Only 1
Types of motor unit
Slow (S)
Fast (FR and FF)
Characteristics of slow motor units
Smallest diameter cell bodies
Small dendritic tress
Thinnest axons
Slowest conduction velocity
Characteristics of Fast fatigue resistant
Resistant to fatigue
Large diameter cell bodies
Larger dendritic trees
Thickets axons
Faster conduction velocity
Characteristics of Fast fatiguable
Fatigue easily
Larger diameter cell bodies
Larger dendritic trees
Thickets axons
Faster conduction velocity
Recruitment
Changing the number of motor units active at any one time
Sequence of recruitment
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.
Define Rate coding
Change he frequency with which action potentials travel down to the muscle
Process of 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.
Neurotrophic Factors
Growth factors- produced in the nerve and transported throughout nerve to maintain nerve integrity and function
Allows growth after injury
Plasticity of motor unit/muscle fibres- which muscle fibres change
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 possible in cases of severe deconditioning or spinal cord injury
Microgravity during spaceflight results in shift from slow to fast muscle fibre types
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
Motor tracts in the spinal cord
Corticospinal /pyramidal Tract = voluntary movement pathway (it goes from the motor cortex to the spinal cord)
There are many extrapyramidal tracts that are concerned with automatic movements in response to stimuli.
* RUBROSPINAL TRACT: involved in automatic movements of arms in response to posture/balance changes
- RETICULOSPINAL TRACT: coordinated movements of locomotion/posture resulting from painful stimuli
- VESTIBULOSPINAL TRACT: regulates posture to maintain balance – allow us to maintain head/neck position
Pathway of upper and lower neurone
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. Pyramidal tracts are the major voluntary pathways (tracts outside of this are the extrapyramidal tracts).
The lower motor neurone then projects out of the spinal cord and joins with a sensory nerve coming in to form a peripheral nerve.
Define Reflex
An automatic and often inborn response to a stimulus
Components of a reflex arc
- Sensory receptor
- Sensory neurone
- Integrating centre- the point at which the neurons that compose the grey matter of the spinal cord or brainstem synapse
- Motor neurone
- Effector
Using reflex tests to determine where the damage is
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
How many synapses in a reflex arc
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).
The monosynaptic (stretch) reflex
When you hit the patellar ligament with a tendon hammer, not only do you excite the quadriceps muscle, you also INHIBIT the hamstrings
- This sends an afferent signal that excites the efferents to the quadriceps and inhibits the efferents to the hamstrings - the leg kicks up
The Hoffman Reflex
The stimulus can be identical every time the reflex is tested
APs are sent down the motor neurone to the muscle (quick). 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). There are 2 contractions.
M wave- from motor neurone
H(Hoffman) wave- from sensory
This allows us to see what part of the system is injured
Supraspinal control of 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
How higher 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
Higher centres and pathways involved in reflexes
o Cortex – corticospinal (fine control of limb movements, body adjustments)
o Red nucleus – rubrospinal (automatic movements of arm in response to posture/balance changes)
o Vestibular nuclei – vestibulospinal (altering posture to maintain balance)
o Tectum – tectospinal (head movements in response to visual information).
Gamma motor neurone
Supply sensory organs of the muscle
These neurones change the sensitivity of the sensory organ in order to sense the extent of contraction
Stroke causes what lesions- and what this then results in
Causes upper motor neurone lesions
Strokes lead to a loss of descending inhibition of reflexes so you get hyperreflexia
- Clonus
- Babinski’s sign
Clonus
Muscular spasm involving repeated, often rhythmic, contractions
Babinski’s Sign
If you stroke the bottom of the foot you see plantar extension (the toes fan out
Causes of hypo-reflexia
Mostly associated with Lower motor neurone lesions
