Physiology: Somatic Motor System Flashcards
what does the somatic motor system consist of?
skeletal muscles and the nervous system that controls them
where are UMNs located?
within the brain
where are LMNs located?
within the brainstem and anterior horn
what do LMNs receive input from?
UMNs
proprioceptors
interneurons
what do LMNs consist of?
alpha MNs that innervate muscle fibres (extrafusal fibres)
what do gamma-MNs innervate?
intrafusal fibres within muscle spindle
what do alpha-MNs innervate?
muscle fibres (extrafusal fibres)
where do the LMNs supplying muscles exit?
anterior horn
those supplying axial muscles are more medial than those innervating distal muscles
flexors are dorsal to extensors
what is the Henneman Size Principle?
susceptibility of an alpha-MN to discharge an AP is a function of its size
small alpha-MNs have lower thresholds than larger ones
what activates the myotatic reflex?
stretch of muscle spindle
what does the stretch of muscle spindle cause?
excites alpha MNs causing contraction
why are gamma MNs activated alongside alpha MNs?
prevents muscle spindle from slackening
two categories of intrafusal fibres
- nuclear bag fibres
2. chain fibres
two types of nuclear bag fibres
bag 1/ dynamic= very sensitive to change in muscle length
bag 2/static= sensitive to absolute length
what innervates intrafusal fibres?
A-alpha (Ia) and A-beta (II)
