Download indirect pathways

SECTION #1 Continued
Motoneurons and Local Circuits
(Group #1)
Review: General Control Theory
We’ll finish up local and
“lower” MNs today, as
well as discuss brainstem pathways:
-reflexes
-CPGs
-Models
Current Research Topics
Historically, any patterned behaviour was termed a “reflex.”
What is a reflex?
What are the defining characteristics of a reflex?
There was a time when reflexes were to be feedback circuits.
Current Research Topics
Many patterned motor actions are loosely independent of sensory
feedback. Circuits that produce a patterned output (unchanging)
and do not involve sensory integration are referred to as
feedforward.
Current Research Topics
- It remains unclear to what extent reflexes such as the H-reflex
control muscle tone during normal postural events and holistic
changes in position.
- Sign change in activation / inactivation of reflexes:
mechanism is partially understood, but upstream controller isn’t.
- There is an important distinction between tonic and phasic postural
events and systems.
Jane MacPherson in cat
Local Circuits: CPGs
Central Pattern Generators:
- Oscillatory circuits… “Push – Pull”
- Responsible for appropriate coordination of flexion and extension
during locomotion
- Each limb has own in primate, and presumably human
check out: http://www.bio.brandeis.edu/biomath/mike/net_osc.html
Flexor
Extensor
Limb Position
CPGs Continued
experimental
disruption
Sources of Modulation:
- Local Circuits
- Descending inhibition
- Afferent input (1b)
- Propriospinal pathways
coordinate upper & lower limbs
- STATE!
CPGs Continued
Clinical Implications:
- Some patients with partial spinal damage have been able to
regain limited motor control with a combination of:
- psychological training (readiness?)
- electrical repetitive stimulus directly to the lower spinal cord
- extensive physical training (Jane MacPherson: cat)
- Our understanding of autonomous behaviors has improved
- stroke patients are better guided to bi-lateral modality
- Local applications of pharmacology?
- NMDA
Now that you have an introduction to the
SIMPLEST elements of motor control:
what do you see happening here?
Now that you have an introduction to the
SIMPLEST elements of motor control:
what do you see happening here?
Now that you have an introduction to the
SIMPLEST elements of motor control:
what do you see happening here?
Appropriate Models…
How can we improve the prognosis of spinal patients?
As mentioned, quantitative evaluations of neural networks
CANNOT, or as of yet, HAVE NOT taken place in human:
Every model has some implicit limitation; it is only an
approximation of the system of interest.
- Computer models
- “Lower” Species
- primates (new and old world monkeys)
- cat
- lamprey
- crustacean
- Inference-based reasoning
- from clinical trials
Appropriate Models Continued
Why in the world work on a hermit crab (or ANY model)?
1) Clearly observable, measurable, and simple behavior
2) Robust preparation (survivability)
3) No vertebrate ethics committees
Appropriate Models Continued
Why in the world work on a hermit crab?
It’s a simple system, with
so few components, that
we can compute circuit /
model equations with
some precision.
Compared to its
phylogenetically close
relatives, the hermit crab’s
abdominal motor control
system has fewer
premotor elements
Appropriate Models Continued
Why investigate model species? Answer: If a crab is this
complicated, then imagine the
human models.
SECTION #2
Brainstem and Motor Cortex
(Group #2a)
Brainstem
Indirect Pathways: ~semi-autonomous
-to proximal muscles
-maintain an initiated motor behavior
-some amount of brainstem
“processing”
Direct Pathways: ~primarily voluntary
-fine movement
-distal muscle groups
Brainstem Continued
Consists of multiple “centers” or nuclei
It is within these loci that integration / processing takes place
We will discuss some:
- Superior Colliculus (eyes, not to inferior sp. cord)
- Vestibular Nuclear Complex (balance)
- Reticular Formation (postural coordination)
indirect pathways
- Red Nucleus (generally associated with direct pathways*)
* though direct pathways also refers to M. Cortex to spinal cord
Brainstem Continued
indirect pathways
Vestibular Nucleus:
- Posture / Body Position
- Compensatory (rapid but only post-event)
- Input: 8th C.N. = Aud. / Vestibular
- Output: medial spinal cord gray matter
- Some output to more lateral sections
controls proximal set of ‘limb muscles’
Brainstem Continued
indirect pathways
Reticular Formation:
- Body Stability During Movement
(also respiration, heart rate, and other functions)
- feedforward / RT integrative control
- neuron clusters intertwined with passing axons
- Input: cortex / other brainstem centers
- Output: primarily medial spinal gray matter
Brainstem Continued
direct pathways
Red Nucleus:
- Fine movements (direct pathway)
- Feedforward mechanism
stabilizes body during movements
- Inputs: Motor cortex
- Output: act on lateral spinal gray matter
& local circuits in spinal cord
Brainstem Continued
indirect pathways
Decussate: To cross the midline
Brainstem Continued
direct pathways
Rubrospinal: via the Red Nucleus
An example…
could be the
action of a
corticospinal
or rubrospinal
pathway
likely the
action of a
reticulospinal
pathway
An example…
DIRECT
PATHWAY
INDIRECT
PATHWAY
What occurs upon hand contact?
1) The a-MN innervates
what muscle?
2) What sensory fiber is
involved?
3) Where are these cells?
Clinical Implications
Partial Lesions
Local Sensitivity
- Medial (gross)
walking, sitting
- Lateral (fine)
piano, spoons
Section Level
“relative severity”
Sacral: OK (why?)
Cervical/Thoracic: Bad (why?)
Lumbar: 50/50 (why?)
Current Research Topics
Where is control of simple
postural commands (Cat;
MacPherson J):
Current Research Topics
Determining
the necessary
signals
(Bizzi E):
Brainstem and Motor Cortex
(Group #2b)
Primary Motor Cortex
Primary Motor Cortex
- Pre-Central Gyrus
- Identification by stimulus response and morphology
Primary Motor Cortex
- Mostly direct pathways, some indirect pathways
D: dorsolateral pathways to lateral sp. cord
via red nucleus (functions?)
I: ventromedial pathways to medial sp. cord
via reticular formation (functions?)
- Does 1° motor cortex encode whole movement?
not individual muscles
not vectors
 command neurons
Primary M. Cortex Continued
Topographical
Orientation:
Homunculus
Rough Estimate;
great diversity
Primary M. Cortex Continued
Directionality:
yellow = one cell’s firing vectors (broad)
red = “best” vector
No one (1) cell can generate an appropriate directional code!
Population Tuning
Current Research Topics
How is the
vertebrate
spinal cord
arranged?
Could it be by
function
(Bizzi E):
PreMotor Cortex
PreMotor Cortex
- Rostral Pre-Central Gyrus
- Activated by cues for well-learned events (monkey)
PreMotor Cortex Continued
Lateral PreMotor elements
- conditional tasks
- fire before action
- external cues (in monkey anyway)
- damage = inability to select appropriate response movements
Medial PreMotor elements
- initiate internally / voluntarily driven motor tasks
- internal cues
- damage = reduced spontaneous movements
Review: Pathways
Be familiar with the contribution
of each particular anatomical
structure / region:
-Cortex
2 regions discussed
-Brainstem
the 3 nuclei discussed & pathways
-Spinal Cord
spatial significance / distribution
-Local Circuitry / MNs / Muscle
those properties discussed