2.21 Motor System 2 Flashcards
LMN are only neurons that convey signals to: (specific)
extrafusal and intrafusal skeletal muscle fibers
How many types of LMN?
2
Cell bodies of LMN are located here
ventral horn of spinal cord
two types of motor neurons
- alpha
- gamma
alpha motor neurons: cell bodies and axons
- large cell bodies
- large, myelinated axons
alpha motor neurons: axons projecting
- project to extrafusal skeletal muscle
- branch into numerous terminals as they approach muscle
Normally, an alpha motor neurons releases enough _______ that all the muscle fibers it innervates contracts
ACh
gamma motor neurons: axons
- medium sized
- myelinated
- project to intrafusal fibers in the muscle spindle
How do most sensory stimuli act?
in ensemble fashion
How do sensory stimuli act in an ensemble fashion?
neurons and CNS interaction
- involved neurons can vary
- appropriate levels of CNS interact to produce context-dependent movement
What is one way to modify movement response to tendon tap?
changing arousal level
- relaxed: tendon elicits small movement
- anxious: greater movement with same force
What are H reflexes?
monosynaptic reflexes elicited by electrically stimulating a nerve
purpose of H reflexes
to quantify the level of alpha motor neuron facilitation or inhibition
compare H reflexes to tendon reflex
- faster
- doesn’t require activation of spindle receptors
What motor signals do UMN provide?
all motor signals from:
- brain to spinal
- from cerebrum to cranial nerve LMNs in the brainstem
What UMN activity usually occurs automatically, without conscious effort?
activity controlling posture and gross movements
What activity can occur before a person is consciously aware of a stimulus?
medial UMN activity
example of medial UMN activity
- a loud noise occurs behind a person
- eyes and face turn toward sound before person is consciously aware of auditory stimulus
What are the medial UMN tracts?
- reticulospinal tract
- medial vestibulospinal tracts
- lateral vestibulospinal tract
- medial corticospinal tract
reticulospinal tract (overall)
facilitates bilateral LMN innervating postural and gross limb movement of muscles throughout the body
medial vestibulospinal tracts (overall)
receives info about head movement and position from vestibular apparatus
lateral vestibulospinal tract responds to:
gravity information from vestibular apparatus
medial corticospinal tract has direct connection from:
cerebral cortex to spinal cord
fractionation
ability to activate individual muscles independently of other muscles
What is fractionation essential for? What would happen without it?
- essential for normal movement of the hands
- without it, the fingers and thumb would act as a single unit, as they do when picking up a water bottle
What is the lateral UMN tract called?
lateral corticospinal tract
major function of LMN
fractionation
How does the lateral corticospinal tract fractionate?
by activating inhibitory neurons to prevent unwanted muscles from contracting
most important pathway controlling voluntary movements
Lateral corticospinal tract
Where do lateral corticospinal fibers arise?
primary motor, premotor, and supplementary motor cortex
Where is the primary motor cortex located?
anterior to central sulcus in the precentral gyrus
