ms Lecture 4
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Where are motoneurons located?
Spinal cord and cranial nerve nuclei
Motoneurons are small and have small dendritic trees. True or False?
False : LARGE neurons with extensive dendritic trees.
Inputs converge into motoneurones from...?
- 1. Sensory pathways
- 2. Interneurons
- 3. Descending pathways
Groups of MNs supplying individual muscles (eg. biceps MN pool)
Outgoing motor fibres to the muscle run through...?
Ventral root of spinal cord
How are motoneurones organized in the spinal cord? (VERY GENERALLY)
- Motoneurone pools are organized into columns going through different segments/levels of the spinal cord.
- *TBC in Q1
Limb muscle motoneurone pools are generally positioned :
a) ________ in the ventral root of the spinal cord.
b) Where are Axial (shoulder/trunk) muscle MN pools found?
- a) Dorsolaterally
- b) Axial muscle motoneurone pools are usually in the ventro-medial region of the spinal cord ventral root.
Proximal - Distal Rule
- Medial MN pools =Supply> axial/ proximal muscles. eg. axial, shoulder
- Lateral MN pools => distal muscles eg. wrist, digits
- (Some are somewhere in between eg. elbow muscle motoneurones)
Final common pathways
- = Motoneurone...
- Only route from the CNS to muscle is via motoneurones.
- ... it being called this
Types of motor neurones
- α (Alpha) + γ (gamma) } 2 main ones (ie. in spinal cord or nuclei of cranial nerve)
- & β (beta) - more important in amphibians than in ♁ ♂ ♀
- ⋅ Supply extrafusal muscle fibres(big, powerful skeletal muscle fibres producing muscle force allowing you to move your limbs).
- -> large soma (50μm diameter)
- -> large (12-20μm), fast (CV=70-120ms-1) myelinated Aα axons.
- ⋅ Supply intrafusal (muscle spindle) fibres
- -> Smaller soma (30μm diameter)
- -> Smaller, slower, myelinated Aγ axon (3-6μm) (15-30ms-1)
- ⋅ Control muscle spindle responsivenes)
Recruitment order correlated with size of the motoneurone's cell body ( and conduction velocity of MN's axon). Small MNs recruited first during contraction.
Why recruitment of MNs follows the size principle?
- ⋅Smaller surface are
- ↳ High density of synaptic inputs
- ↳ High electrical input resistance
- [V=IR (Ohm's Law) -> EPSP = synaptic current × input resistance. Small MNs have large EPSPs ∴ more likely to exceed threshold for firing APs]
Advantages of the size principle
- 1. Not indefatigable muscle fibres are used first while the indefatigable ones are used sparingly.
- 2. Increase in contractile force (by recruitment of new MNs) is roughly proportional to current force.
Antagonist pairs of muscle can produce faster movements than muscles acting alone. TRUE or FALSE?
Tri-phasic contraction/ burst
- 1. Flexor burst (large initial force)
- 2. Extensor burst (act as brakes)
- 3. Flexor maintains force (better xontrol using both muscles) (speeds movement)
- *if just flexor, force rise is fairly slow
What would you like to do?
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