Motor Systems Flashcards
What are the 3 main categories of motor systems?
Reflexive - involuntary in response to peripheral stimuli
Rhythmic - occilating controlled by brain stem and spinal cord
Voluntary - goal directed, higher centres involved
How is the motor system hierarchally organised?
spinal cord -> brain stem -> cortex
How does the brainstem feedback to other motor systems?
- Feedback to cortex and spinal cord to regulate planning and execution
(affected in Parkinsons, Huntington’s and Cerebral ataxia)
What are the two areas of output of the spinal cord?
Dorsal -> sensory
Ventral -> motor
What are the two different categories of motor neurons?
Lateral - limb buds
Medial - axial muscles
What are local interneurons?
Confined to the same or adjascent segments, local networks and rythmic neurons
What are propriospinal neurons?
Axons that project to distant segments
What are projection neurons?
Axons that ascend to higher brain areas
What are the medial brainstem pathways?
- Control basic posture control
- Vestibulospinal, reticulospinal and tectospinal tracts
- Terminate on ventromedial interneurons and some medial interneurons
Describe the lateral brainstem pathways?
- Goal directed
- Lateral motorneurons
- Dosolateral interneurons
What does the cortex control?
- Coordination of individual joint and whole limb actions
- Complex goal directed and precise movements
- Code parameters such as direction and force
What does the ventral corticospinal tract control?
Posture, ventral medial motor neurons
What does the lateral corticospinal tract control?
Activates lateral neurons (limbs)
Loss of these affects fine digit control
What do the axons of the primary motor cortex ennervate?
- Single neuron synapses with motor neurons innervating a number of muscles
- Different groups activate different combinations of spinal interneuron networks
What are premotor areas influential in?
Complex movements, motor planning and learning
What are mirror neurons?
- Discharge when motor act is performed and done by another individual
- Activated also by sounds of task
Describe the role of the basal ganglia in motor systems
Gets input from cortex which it relays through the thalamus
striotum - major recipient of inputs
globus pallidus - major output projections of the basal ganglia
What is the direct pathway?
Facilitates desired movement
- inhibits GPi input (thalamus) and allows more activity in the cortex
- Absent in Parkinsons
What is the indirect pathway?
- Inhibits GP less
- Inhibits movement
Define locomotion
Rythmic and alterating movements of the body which generate propulsion involving functional agonists
In limbed animals what are the two modes of locomotion?
Power stroke - stance (extensor)
Return stroke - swing (flexor)
What were the earliest studies of locomotion?
Marey and Muybridge used high speed photography to show that horses had all legs off the ground at point during a gallop
What changes during the different gaits in the locomotion of quadripeds?
Swing phase changes a little however stance phase becomes much shorter with speed
What did Charles Sherrington propose?
That simple reflexes are the basic units of movement
What did T. Graham Brown find?
- Isolated spinal chords can still produce rythmic motor activity
- Central pattern generators -> half centre hypothesis
What has been the more recent addition to the half-life hypothesis?
- Reflexes integrated with cpg to produce adaptive movements
e. g sstimulation of high threshold, small diameter afferents in cat spinal chords treated with L-DOPA led to rythmic activity
What is the function of the mesencephalic locomotor region?
- Exites interneurons in the medial reticular formation
- Axons descend to the spinal locomotor system via the ventrolateral funiculus
- Increasing stimulation strength changes gait and speed of circuit
What are the descending pathways of locomotion in the brain?
- MLR activates locomotion
- Processing of afferent signals involving the cerebellum refines the motor pattern
- Limb movements guided by visual input
What are central pattern generators?
Rhythm generating networks which may be conveyed to pattern forming circuits for appropriate spatial and temporal muscle pattern activity
What is the output of central pattern generators determined by?
- Intrinsic properties e.g ion channels, neurotransmitters
- Synaptic connectivity between neurons
- Synaptic properties
- Neuromodulation
Where is rostro-caudal delay found?
In invertebrates
What can the rhythmicity of a CPG reflect?
- Occilating neural networks
- Endogenous bursters (pacemaker cells)
What is the half-centre hypothesis?
Idea of 2 neurons that receive tonic exitation also exite inhibitory neurons connecting the two resulting in occilating activity (reciprocal inhibition) where the inhibitory neuron fatigues
What is the hyperpolarisation-activated inward current (Ih)?
- Mixed cationic (N+/K+) current activated by hyperpolarisation
- Activates and deactivates slowly causing slow depolarisation
- Can give rise to a post-inhibitory rebound potential
What are low voltage activated calcium currents(ILVA)?
- Transient (inactivating) Ca2+ currents activated at low v
- Can also mediate post-inhibitory rebound potentils
- Can boost membrane depolarisation threshold
What are transient K+ channels (IA)?
- Transient outward current
- Require hyperpolarisation to remove inactivation
- Can delay spike and burst initiation
What are persistent Na+ currents (INAP)?
- Non-inactivating persistent
- Low activation threshold
- Can contribute to plateau potentials
- Help initiate depolarisation and are then important for maintenance
What are high voltage activated calcium currents (IHVA)?
- Activated by depolarisation from resting potential
- Various subtypes with slow/little activation
- Mediate persistent inwards currents which contribute to plateau potentials
- Help initiate and main depolarisation and maintain it
What are slow non-selective cationic current (ICAN) ?
- Calcium activated current
- Non-inactivating and persistent with no voltage sensitivity
- Maintains depolarisation
What are two channels which contribute to the termination of action potential firing?
- Inactivation of fast inactivating sodium currents (INA) which inactivate when depolarised
- KCa channels which produce long hyperpolarising pauses in activity
What is the Calcium activated K current (IKCA)?
- Activated by Ca2+ entry via voltage activated channels
- Underlies post action potential hyperpolarisation (AHP)
- Can control spike timing leading to spike frequency adaptation
How are isolated rodent spinal cords used to study locomotion?
- Early spinal tissue kept alive
- Motor activity induced by stimulating brainstem or applying NMDA and serotonin or dopamine (older)
- Attach electrodes to ventral roots of left and right extensor (L2 )and extensor (L5)
- See alternating burst of activity
Where is the vertebrate CPG believed to be located?
- In the ventral horn of the spinal cord
- Distributed along the lumbar cord however has a rostrocaudal exitability gradient
How are interneurons studied in the vertebrate spinal cord?
- Identification of transciption factors involved in the differentiation of specific types of spinal neurons
- Labeling of specific interneurons with fluorescent markers
What are genetic knock-outs?
Knock-outs of specific genes using homologous recombination to replace gene of interest with an inoperable version
How does the cre-lox system work?
- Cre mediates recombination between two loxP sites
- Pair gene of interest with Cre and flank gene we want to cut out with loxP
- Can be used for conditional deletion and expression at the level of specific tissues
What are the criteria for rhythm generating neurons in vertebrates?
Must be ipsilaterally projecting glutaminergic excitatory neurons
Why might HB9 expression neurons be rhythm generating?
- Glutaminergic with intrinsic bursting (revealed by TTX)
- Rhythmically active in time with fictive locomotion
- Coupled by gap junctions (allows coordination)
- Ipsilaterally projecting neurons
What are the characteristics of Dbx-1 (V0) expressing interneurons and what might they be involved in?
- Have monosynaptic connections with motor neurons
- knockout of these leads to increased number of cobursts between left and right
- Could be involved in left-right coordination
What are V1 motor neurons and what do they appear to control?
- Express Pax6 and En1
- Inhibitory and ipsilareally projecting
- When deleted locomotor burst duration increases and rhythm slows - could therefore control speed
What is the advatage of inducible silencing techniques?
Avoids compensation
What do deletions of activity during fictive locomotion suggest?
A layered organisation of the CPG with a central rythm generator (clock) and a bottom pattern forming layer which distribute this rhythm
