Central Pattern Generators & Locomotion Flashcards
Motorneuron pools in the spinal cord
grey matter
Medial & ventral: proximal limb
Dorsal & lateral: distal limb
medial - flxs and lat - extensors
Rexed laminae
I: Marginal zone
II /III: Substantia gelatinosa (post. horn)
IV-VI: Nucleus proprius (IV); spinothalamic tract
VII: intermediate grey - critical area for inducement of mvmt
VIII: propriospinal
IX: motorneuron pools (ant. horn)
Spinal interneurons:
Neurotransmitter
• Inhibitory (GABA / glycine)
• Excitatory (glutamatergic)
Projections:
• intrasegmental - stay within segment of spinal cord in which located
• intersegmental (incl. propriospinal) - connect area of SC over long distances
• commissural - close to midline connects L > R
Fact:
Spinal interneurons make up the majority of grey mater cells
central pattern generator: CPGs
A flexible network of interneurons that can produce purposeful movement
- Connections within the spinal cord contribute to:
- reflexes
- coordination of movement
=> CPG
Organisation of the locomotor system in mammalian vertebrates
Forebrain: motor cortex, basal ganglia,
Midbrain: MLR
Hindbrain: cerebellum, pons
SC:locomotor CPG ntw <> proprioreceptive fdbk 2 muscles
The framework of locomotor control
medial= flexors -cortico and rubospinal tracts
lateral - extensors - reituclo and vestibulospinal tracts
CPG: reciprocal inhibition
During gait, the amount of reciprocal inhibition is modulated by upper motor neurons
- once contract mn one side > inhibit mn other side
alternative activation via CPG
Note: stimulation can be achieved via incubation with e.g. serotonin / N-methyl-DL-aspartic acid
What is driving the CPG?
walking can be induced by stimulating
locomotor areas in the brain stem
and so
Muscle activity is NOT dependent on sensory input into the spinal cord
Importance of sensory feedback in motor function
Sensory feedback is needed to change from ‘swing’ to ‘stance’ and back
CPG rhythm generation
Model 1: Pacemaker concept
Pacemaker cell triggers rhythmic activity of non-pacemaker cells
Model 2: Emergency network concept
Rhythmic activity results from reciprocally coupled non-pacemaker cells
different interneurons do different jobs
VO: L>R ALTERNATIONS V1: LOCOMOTOR RYTHMIC REGULATION V2a: LR ALTERNATIONS AND RYTHM ROBUSTNESS V3:RYTHM ROBUSTNESS HB9: RYTHYM GENERATORS
FUNCTION ORGANISATION OF CPG DURING WALKING
INITITATION OF LOCOMOTION: prominent emergent ntw patterns (all non-pc mkrs)
ONGOING/SLOW WALKING: hybrid ntw properties (half pm and non pm)
RUNNING: prominent pacemakers properties (all pm)
FUNCTION ORGANISATION OF CPG DURING WALKING
* key points
• Locomotor CPG is activated, modulated (adaptive control of goal-directed locomotion) and silenced by supraspinal structures
• The mesencephalic locomotor region (brainstem) controls the intensity of locomotion:
progressively increasing stimultion intensity => faster locomotor rythm
• Mesencephalic (mesencephalon: midbrain!) locomotor region influences the CPG mainly via the reticular formaTon and the reticulospinal tract:
=> MAIN EXCITATORY DRIVE!!
• When locomotion is initiated, Ca2+ outside decreases while K+ increases;
this provides positive feedback for I’NaP’ and gap juntiTons => pacemaker
activities are induced (Fig. 7, middle)
• Faster locomotion, more cells start to act like pacemakers and increase burtiTng frequency (Fig. 7 , right)
FUNCTION ORGANISATION OF CPG DURING WALKING
* key points 2
- Note that inhibitory connections responsible for alterations are NOT shown!!
- Current theory is that Hb9 interneurons can funcTon as pacemaker cells, because: - they can synchronise their firing (electric coupling)
- they can switch from spiking to bursTng
- they project to motorneurons
- Firing paLerns change based upon ionic composiTon of extracellular fluid (CSF)
THUS: LOCOMOTOR NETWORK IS A HYBRID PACEMAKER WHERE PACEMAKER NEURONS ARE THE SEEDS FOR RYTHM GENERATION
