Reflexes Flashcards
What it means to say that “the motor system is hierarchical” why is it important?
Our complex movement control is built on primitive movement control systems (reflexes) that guarantee our survival leaving the brain free to do higher/more complex motor functions.
How are MNs organized in the SC?
Medial MNs project to proximal part of limb and trunk (posture and balance, involuntary), whereas lateral MNs project to distal part of limb (fingers, voluntary). MNs on the inside project to flexor muscles, whereas MNs on the outside project to extensor muscles.
What is a motor unit?
A single MN and the muscle fibres that it contacts (the
basic unit of motor organisation)
How do small and large motor units differ regarding the number of muscle fibres they contact and the input resistance?
In small motor units, a single MN may contact 10-20 muscle fibres and the MN will be small, with high input
R. In large motor units, a single MN may contact >1000 muscle fibres and the MN will be large, with low input R.
How are motor units recruited?
Motor units are recruited according to the size principle to generate increasing amounts of tension: low levels of activation to a MN pool will activate the small, high R (thus easier to fire, remember V=IR) MNs of small motor units, whereas higher levels of activation will also activate the larger, lower R (thus harder to fire) MNs of larger motor units.
What does contraction mean?
Activation of muscles makes them contract, that is, generate force, become stiff. It doesn’t mean muscle gets shorter, that will depend on load.
Why are muscle and joint receptors important in motor control?
They provide feedback in muscles and tendons key to fine movement control.
Where do alpha and gamma MNs project to (efferent)? And type Ia, Ib and II fibers (afferents)?
Alpha MN => extrafusal fibers (force generators)
Gamma MN => intrafusal fibers (proprioception-lenght)
Ia fiber => annulospiral endings (intrafusal fibers)
Ib fiber => Golgi tendon organs (proprioception-tension)
II fiber => flower spray endings (intrafusal fibers)
What is the difference between intra and extrafusal fibers?
Intrafusal fibers are specialized muscle fibers within the muscle spindle (hence intrafusal, cause they are inside this fusiform proprioceptive organ) and they measure length due to mechanoreceptors . On the other hand, extrafusal fibers are outside the muscle spindle and are the force generators.
Why is proprioceptive input from muscle spindles processed by SC?
To increase response speed because they need to be very fast to allow fine motor control and rapid reflexes.
What are the 3 pathways of the stretch reflex?
1 - monosynaptic: alpha MN (efferent) goes to same muscle as muscle spindle (receptor) connected to Ia fibers (afferent) => excitatory synapse means if you pull muscle it automatically pulls back (controls intended muscle length during contractions against loads)
2 - disynaptic (excitatory): 2 MNs for agonists activated together (greater force)
3 - disynaptic (reciprocal inhibition): activation/contraction of agonist inhibits/relaxes antagonist (through inhibitory interneuron)
What are the main differences between passive stretch and active contraction?
In passive stretch both intra and extrafusal fibers are stretched and thus spindles are activated. Reflex via Ia fibers and alpha MNs causes secondary contraction. In active contraction, central excitation of alpha MNs only causes contraction of extrafusal fibers with consequent relaxation of intrafusal fibers and thus spindles are NOT activated. Tension is low, does not adjust to increased resistance. Golgi tendon organ activated causing relaxation.
What alpha-gamma co-activation and why is it important?
In active contraction with gamma co-activation, intra and extrafusal fibers contract; spindles activated reinforcing contraction stimulus via Ia fibers in according with resistance. Tendon organ activated, causing relaxation if load is too heavy. In a nutshell, alpha-gamma co-activation maintains spindle sensitivity.
In H-reflex, what do you observe in your EMG recordings as you increase the strength of the electrical stimulation of the motor nerve?
First you activate the largest diameter (Ia) fibres which initiate a monosynaptic reflex - H-wave. Increasing stimulus strength begins to activate motor fibres, which
activates muscle EMG directly, and at shorter latency - the M-wave. As stimulus strength increases, the
M-wave predominates, as antidromic effects cancel the reflex orthodromic drive through the motor fibres.
What is the purpose of the inverse myotatic reflex and which anatomical structures play a key role?
The inverse myotatic reflex or Tendon Organ reflex is only activated at high forces/loads, strong enough to activate the Golgi tendon organ. It’s a disynaptic reflex with an inhibitory Ib interneuron that makes you let go heavy loads to prevent injury. Apart from disynaptic agonist inhibition, there is also di- or tirsynaptic antagonist excitatation. These actions oppose the spindle stretch reflex allowing fine control over muscle tensions
