Descending Pathways Flashcards
lower motor neurons
- innervates striated m
- directly signals m to contract (only way movement can be initiated)
- last neuron in chain of neurons
types of lower motor neurons
- alpha: extrafusal m. fibers (actively contract m)
- gamma: intrafusal m. fiber (regulatory)
lesions associated with lower motor neurons
- atonia: loss of muscle tone
- areflexia: loss of myotonic (knee jerk) reflex
- flaccid paralysis
- fasciculations: spont m contractions
- atrophy: loss of m tone
where are cell bodies of motor units located?
anterior horm
upper motor neurons
- axons descend from cortex
- end on or near LMN
lesions associated with upper motor neurons
spastic paralysis
- hypertonia (increased resting tensor): flexors, leg extensors
- hyperflexia
- pathoreflexes (neg plantar reflex or Babinski sign)
- big toe dorsoflexion, other toes fan when heel stroked
- atrophy not as severe
motor unit
1 motor neuron and all myofibers it innervates
-vary in size, related to control we have over the muscle
extraoccular mm: 10 myofibers/motor unit
gastrocnemius mm: 1000 myofibers/motor unit
type I fibers
- slow twitch
- for sustained force, weight bearing
- abundant lipids, less glycogen
- many mitochondria
- *turkey leg/duck breast m
type II fibers
- fast twitch
- for sudden movement and purposeful motion
- few lipids, abundant glycogen
- few mitochondria
- turkey breast m
Basal ganglia/cerebellum motor control
- influence cerebral cortical output to SC and BS
- vital in design, choice of monitoring of movement, no direct effect on LMN
lesions in basal ganglia/cerebellum and effect on motor control
- does not mean weakness
- involuntary, incoordination, difficulty initiating movement
higher centers motor control
hierarchical bc cortex “decides” what movement should occur
-parallel arrangement as premotor cortex can directly “talk to” LMN
where do descending motor pathways mostly terminate/synapse?
interneurons in SC
*but some directly synapse with primary motor neuron (hand and CST)
location of primary motor area of corticospinal tract
precentral gyrus (area 4)
location of somatic sensory area of CST
post central gyrus (areas 1-3)
location of premotor area of CST
lateral surface of cerebrum (area 6)
location of supplementary motor area of CST
medial surface of cerebrum (area 6)
location of superior parietal lobe of CST
areas 5 and 7
primary motor area of CST
- fncnt: execution of contralat vol movements and control of fine digital movements
- projects to BS and SC
- some monosynaptic terminations on SC MN (hand)
- usually synapse on interneurons
- lesions = contralat m paralysis
fnctn of premotor area of CST
- plans movmements in response to external cues (instructions
- control of proximal and axial mm (trunk, shoulders, hip)
- empathetic facial expression
projections and lesions of premotor area of CST
- to primary motor area and reticular formation
- some fibers project to all spinal levels
- lesions cause moderate weakness of contralat prox mm
- loss of ability to link learned hand movements to verbal/visual cues
supplementary motor area of CST
- fnctn: plans movements while thinking
- learns new sequences, assemble prev learned sequences, imagines movements
-projects to premotor and primary motor areas
parietal lobe and CST
- somatic sensory area and sup parietal lobule
- project to primary motor area (direct motor patterns in response to sensory input)
- project to sensory areas of BS and SC (moderate sensory signals)
characteristics of CST
- complex: multiple origins and destinations
- collaterals project to basal ganglia, thalamus, RF, sensory nuclei (dorsal column nuclie), post and intermed horns of SC
- not all movements depend on CST
