Descending Motor Flashcards
Ascending and descending pathways
Consist of three general types:
Long, ascending fibers going to thalamus, cerebellum or various brainstem nuclei
Long, descending fibers going from cerebral cortex or various brainstem nuclei to spinal cord gray matter
Short, propriospinal fibers interconnecting different spinal cord levels
These fibers help coordinate flexor reflexes
Pars caudalis
pain processed here.
Lower Motor Neuron
Innervates striated muscle, directly signals muscle to contract, only way movement can be initiated
Last neuron in chain of neurons
LMN includes
Alpha, Gamma motor neuron
Alpha motor neuron
extrafusal muscle fibers, directly contract
Gamma motor neuron
intrafusal muscle fibers, anterior horn as well. Work with cerebellum to make sure force is precise.
LMN lesion
Atonia - loss of muscle tone
Areflexia - loss of myotatic (knee jerk) reflex
Flaccid paralysis
Fasciculations - spontaneous muscle contractions
Atrophy - loss of muscle tissue
Atonia
loss of muscle tone
Areflexia
loss of myotatic (knee jerk) reflex
Fasciculations
spontaneous muscle contractions
Upper Motor Neurons
Axons descend from cortex, end on or near LMN
UMN lesions
Spastic paralysis (paresis) Hyperreflexia
Hypertonia UMN Lesion
(increased resting tension)
Arm flexors, leg extensors
Pathologic refluxes UMN Lesion
Pathologic reflexes, e.g. negative plantar reflex or Babinski sign
Big toe dorsoflexion with fanning of other toes when side of heal is stroked
Fasiculations are
spontaneous contractions of small groups of muscle fibers that can be visible at the skin surface
Fibrillations
contractions of individual muscle fibers that can not be seen visually but are detected using electrical monitoring.
Clonus is a rapid
series of alternating muscle contractions that occur in response to the sudden stretch of a muscle
Lower motor neurons & motor units
Cell bodies in anterior horn
Axons in ventral root LMN+motor units
divide into terminal branches widely distributed in target muscle
Each branch ends at one neuromuscular junction
Systematic arrangement of motor neurons - Neurons controlling axial muscles are
medial to those controlling distal muscles
Neurons controlling flexors are -Systematic arrangement of motor neurons
located posterior to the extensor groups
Motor units
1 motor neuron + all myofibers it innervates = motor unit
Extraocular muscle 10 myofibers/ motor unit
Large antigravity muscle like gastrocneumius 100s up to 1000myofibers/ motor unit
MU size
Vary in size, related to control we have over the muscle
Darker
more mito
Three types of muscle fibers
Standing, running, jumping
Standing
contract weakly for long periods
Running
contract strongly for short/long periods
Jump
contract very strongly for very short periods
Each muscle fiber type populates
one motor unit, no mixing
Type 1 muscle fiber - Action
Sustained force
weight bearing
Type 1 muscle fiber- Lipids
Abundant
Type 1 muscle fiber - Glycogen
Scant
Type 1 muscle fiber - Ultrastructure
Many mito
Type 1 muscle fiber- physio
Slow twitch
Type 1 muscle fiber- prototype
Turkey leg/ duck breast muscle
Type 2 muscle fiber - action
Sudden movements
purposeful motion
Type 2 muscle fiber - lipids
scant
Type 2 muscle fiber- glycogen
Abundant
Type 2 muscle fiber - ultrastructure
few mito
Type 2 muscle fiber - physio
Fast twitch
Type 2 muscle fiber- prototype
Turkey breast muscle
S
slow-twitch; small amounts of force over long time
FR
fast twitch, fatigue resistant
FF
fast twitch, fast fatigue, large amount of force, for brief period
Motor control
Basal ganglia & cerebellum
Basal ganglia & cerebellum influence
cerebral cortical output to cord and brainstem
Basal ganglia, cerebellum and association cortex are
vital in design, choice and monitoring of movement, but have no direct effect on LMN
Cortex initiates
movement, BG and cerebellum refine it.
Damage to these areas does not cause weakness may have: (basal ganglia, cerebellum)
involuntary movements,
incoordination,
difficulty initiating movement
Hierarchical - motor control
in that cortex “decides” what movement should occur
Premotor cortex plans and tells motor cortex and then the LMN what to do
Parallel - motor control
arrangement as premotor cortex can directly “talk to” LMN
Basal ganglia and cerebellum involved in planning and monitoring movements, have
no (few) outputs to spinal cord most go to motor & premotor cortices
Descending Motor Pathways - mostly terminate or synapse on
Interneurons in spinal cord, but some directly synapse with primary motor neuron (hand and CST)
Descending Motor Pathways - mostly terminate or synapse on
Interneurons in spinal cord, but some directly synapse with primary motor neuron (hand and CST)
Corticospinal tract (pyramidal tract)
Cortex to spinal cord (classic upper motor neuron)
Corticobulbar (corticonuclear) tract
Cortex to brainstem
Corticopontine tract
Cortex to basilar pons
Corticospinal Tract - motor
Primary motor area; precentral gyrus (Area 4) ~ 40% of fibers
CST Somatic sensory area
postcentral gyrus (Areas 3, 1, & 2) ~ 25%
CST Premotor area (Area 6)
lateral surface of cerebrum ~ 20%
CST Supplementary motor area (Area 6)
medial surface of cerebrum ~ 10%
Primary Motor Area function
Execution of contralateral voluntary movements
Control of fine digital movements
Primary motor area projections
Projects to brainstem & spinal cord – some monosynaptic terminations on spinal cord motor neurons (hand)
Synapse to interneurons, then to lower motor neurons (hence fine digital movements).
Primary motor area lesion
results in paralysis of contralateral muscles
Premotor Area
area 6
Premotor Area function
Plans movements in response to external cues (e.g., instructions)
Control of proximal and axial musculature (trunk, shoulder, hip)
May assemble empathetic facial movements
Premotor area Projects to
primary motor area and reticular formation
Some fibers project to all spinal cord levels
Premotor area lesions
Moderate weakness of contralateral proximal muscles
Loss of ability to associate learned hand movements to verbal or visual cues
Supplementary Motor Area (SMA) Function
Plans movements while thinking (internally paced)
Assembles (learns) new sequence (playing new music)
Assembles previously learned sequence (music scale)
“Imagines” movements
SMA projection
Projects to premotor and primary motor areas
Parietal Lobe
Somatic sensory area and superior parietal lobule
Parietal lobe projection
Project to primary motor area
Direct motor patterns in response to sensory input
Project to sensory areas of brainstem and spinal cord
Modulate sensory signals
CST COLLATERALS
project to: Basal ganglia, thalamus, reticular formation, various sensory nuclei (dorsal column nuclei), posterior & intermediate horns of spinal cord
CST
many functions
3Not all movements are dependent on CST
If cut in monkeys, after a period of flaccid paralysis, they move again, fine finger movement lost permanently
Corticospinal tract (CST) originates in
cerebral cortex, precentral gyrus and nearby areas
Corticospinaltract Decussation
Lateral CST- 80% of fibers cross in decussation in medulla descend in lateral funiculus
Uncrossed Lateral CST- 10%
Anterior CST- 10% of fibers, uncrossed descend in anterior funiculus
Axial muscle activity, some actually cross in anterior commissure prior to synapsing
Somatotopically organized
cerebral peduncle descending fibers
Corticospinal, corticobulbar, and corticopontine fibers descend in the cerebral peduncle
The corticospinal fibers descend in the middle third of the cerebral peduncle.
In the pons the corticospinal tract is
broken up into multiple small bundles on each side.
In the pons the corticospinal tract is
broken up into multiple small bundles on each side.
CST rostral medulla
The corticospinal tract goes through the pyramid and for this reason some refer to it as the pyramidal tract
Rubrospinal tract
Control of shoulder and proximal arm musculature
Reticulospinal tract
Control of axial musculature - walking
Vestibulospinal tract
Control of axial musculature - balance
Tectospinal tract
believed to be important in head turning reflexes in response to visual stimuli, unclear function in humans
Vestibulospinal Tract(s)
Origin: vestibular nuclei in pons
Receives input from:
Vestibular system and cerebellum (balance)
Lateral vestibulospinal tract
Projects via lateral funiculus to:
Ipsilateral spinal cord, facilitates antigravity muscles
Medial vestibulospinal tract
Projects via anterior funiculus to:
Spinal cord cervical levels bilaterally, controls head movements in response to gravity
Function:
Mediates postural adjustments & head movements
Antigravity reflexes
Righting reflex (cats); righting reflex to head inversion
Rubrospinal Tract
Origin: red nucleus (“rubro”)
Receives input from:
Primary and premotor areas – shoulder and arm control
Cerebellum
Course:
Ventral tegmental decussation
Lateral funiculus
Projects to contralateral spinal cord
Function: like that of vestibulospinal tract;
Facilitates upper extremity flexor muscle tone
Believed to be small in humans, some question significance
RF and movement control
Two reticulospinal tracts:
Medial: pons; ipsilateral, descends near MLF & in anterior funiculus
Lateral: medulla, descend bilaterally, in lateral funiculus
RF and movement control function
Function:
Supports rhythmic motor actions, i.e. walking
May support recovery of motor function via projections to motor neurons controlling arm and hand
Corticobulbar pathway
Some fibers end directly on motor neurons (XII), but most end on interneurons in the reticular formation
III, IV, VI receive no direct input
Figure shows V, VII, XII, nucleus ambiguus (part of X) & XI
V, VII, XII, nucleus ambiguus & XI CBP
receive bilateral input
Originate in face/mouth portion of motor cortex and other nearby areas
Descend with CST to level of target nucleus, then splits off so no corticobulbar decussation exists
Facial motor nucleus
Exception to typical CBP pattern
Motor neurons to lower facial muscles mainly innervated by
contralateral cortex, but upper facial muscles innervated bilaterally
Unilateral damage to CBP (e.g. in cerebral peduncle) result
Inability to smile or show teeth symmetrically; but ability to wrinkle forehead is unaffected
Atrophic hands
both upper and lower neuron involvement issues.
Fasciculations
classic lower neuron. Sensation is preserved, so dorsal column is probably fine. Anterolateral pathway and PNS are fine. Purely motor.
CST runs through damaged area. Losing upper motor neurons and lower motor neurons. This is ALS. Lou Gherigs.
