Motor Nervous System Flashcards
Upper motor neurons are also known as what cells?
Betz cells
How are muscle action potentials generated at the neuromuscular junction?
1) AP arrives at motor nerve terminal
2) AP triggers opening of voltage gated calcium channels and lets in Ca2+
3) Ca2+ releases acetylcholine (ACh) by exocytosis
4) ACh travels across synaptic cleft to bind to its receptor which is a ligand gated ion channel
5) the ion channel opens and allows a large amount of sodium in and a small amount of potassium out of the muscle cell
6) depolarisation occurs
7) depol causes voltage gated Na+ channels to open in the adjacent membrane
8) Na+ entering causes the membrane potential to rise allowing the generation of an AP
9) ACh is destroyed by acetylcholinesterase in the synaptic cleft
Total failure of neuromuscular synapses results in what? What are some examples of diseases where this occurs?
leads to death as respiratory muscles need innervation from lower motor neurons to work
i.e. motor neuron disease & spinal muscular atrophy (SMA) disease
Partial failure of neuromuscular synapses leas to what? What are some examples of diseases that result in this?
muscle weakness due to a failure to recruit successively large motor units for strong muscle contractions
i.e. Lambert-Eaton syndrome (defects in motor nerve terminal) or Myasthenia Gravis (defects in muscles post synaptic region)
Muscle weakness can also be caused by diseases of skeletal muscle i.e. Duchenne’s Muscular dystrophies. What causes this disease?
due to a mutation in the dystrophin gene, during eccentric contraction muscle damage occurs because the muscle membrane is sheared as dystrophin is not present to hold it together
over time more and more muscle cells die
What is rate modulation in terms of muscle action potentials?
the frequency at which action potentials are delivered in order to increase muscle tension (i.e. the closer together AP’s are delivered the more likely to cause twitch summation or even tetanus)
Henneman’s size principle states what?
smallest motor units are activated first followed by larger motor units
slow twitch fibres are activated before fast twitch fibres
What is the latent period (in terms of AP and muscle force generation)?
the time between an muscle AP and that muscle fibre actually producing force
What is fibre type grouping?
chronic denervation re-innervation events lead to type 1 fibre predominance
basically type 2 fibres start acting like type 1 fibres due to re-innervation by type 1 neurons
occurs in motor neuron disease and also as we age as fast twitch neurons may be more vulnerable with age
True or false… neuron activity is the main driver for muscle fibre type (i.e. whether a muscle fibre is fast or slow twitch)
true
What are the 3 levels of input control for motor neuron output?
1) afferent information i.e. reflexes
2) primary motor cortex i.e. pyramidal
3) multi-neuronal motor system i.e. extra pyramidal
Where does afferent (sensory) input come from?
muscle spindles which gain information from muscle length and velocity of contraction
golgi tendon organs which gain information from muscle tension
free nerve endings in joints/joint receptors which gain information from joint position
What are the two types of muscle spindles?
1) muscle spindle chain which is sensitive to muscle length
2) muscle spindle bag which is sensitive to velocity of muscle length
True or false… muscle spindles have both an afferent and efferent nerve. Why?
true
muscle spindles must have motor innervation to maintain length the same as the muscle to give accurate readings of length and velocity of length
in other words the gamma motor neuron is responsible for maintaining sensitivity of the muscle spindles
True or false… golgi tendon organs have both afferent and efferent neurons
false, only afferent
