Motor neurons and spinal reflexes Flashcards
What are myotomes?
Volunraty movements
Where are motor neurons found in the spinal cord?
In the ventral horn
What is a motor unit?
A single alpha motorneuron and the muscle fibers that it innervates
True or false: one motor neuron innervates many fibers, but one fiber is only innervated by one motor neuron.
True
What happens when a motorneuron fires an AP?
It activates all muscle fibers in the motor unit
Describe the signaling pathway of a muscle contraction.
ACh released in neuromuscular juntion –> end plate are depolarized –> AP travels deep into the muscle down T-tubules –> voltage sensitive DHP receptors are activated –> DHPRs activate ryanodin receptors –> Ca2+ release from the SR –> cross bridges are formed
How are the twitch duration tuned?
To the relative refractory period duration, to ensure instant summation with increased firing rate
What are the three types of motor units?
Slow, fest fatique resistant and fast fatigable
Describe the slow motor units.
- type I, red oxidative muscle fibers
- slow contracting
- recruited first
- most useful for posture support
Describe the fast fatigue resistant motor units.
- type IIa oxidative/glycolytic muscle fibers
Describe the fast fatigable motor units.
- type IIb, white glycolytic muscle fibers
- fast contracting
- recruited last
How are some diseases affecting the motor units?
Neuropathies, peripheral neuropathies, neuromuscular junction, myopathies, the whole motor unit
Mention some neuropathies.
Spine muscular atrophy
Mention some peripheral neuropathies.
Charcot-Marie tooth disease
Mention some neuromuscular junction diseases.
Myasthenia gravis
Mention some myopathies.
Duchennes muscular distrophy
Mention some diseases affecting the whole motor unit.
Amyotrophic latereal sclerosis
Where does info on muscle length come from?
Muscle spindles
What are the three types of muscle spindles?
Dynamic nuclear bag fiber, static nuclear bag fiber, nuclear chain fiber
What are the three types of muscle spindels sensitive to?
DNBF: velocity of stretch
SNBF: static length
NCF: static length
What are the two types of afferents innervating the muscle spindles, and how do they differ?
Ia and II
Ia: innervate all three types
II: innervate only nuclear bag and chain –> signal length only
What happens to the spindle when we contract?
co-activation of alpha and gamma motorneurons maintains spindle sensitivity during muscle contraction
Describe how the stretch reflex works.
- a sudden stretch of the muscle activates Ia afferents, which make monosynaptic excitatory connections with the same muscle and synergist muscles
- the Ia afferents also project to inhibitory Ia interneurons which inhibit motorneurons of antagonist muscles
What does the golgi tendon organs report?
Muscle force
Describe how the golgi tendon organs work.
contraction causes compression and activation of Ib afferents —> inhibition of motorneurons from the same muscle (under resting conditions)
What does the flexor reflex cause?
Rapid removal of limb from a painful stimulus
What does the crossed extensor reflex do?
Stops you from falling
What is the Hoffman (H) reflex?
The electrical analogue to the stretch reflex
