Download Lecture 26 revised 03/10 Upper Motor Control Last lecture we

Lecture 26
revised 03/10
Upper Motor Control
Last lecture we concentrated on the motor neurons and spinal circuitry that modulates
them… sometimes to result in complex movements. Thus, today… Descending control
of spinal cord circuitry- How is movement controlled by the brain? Must explain how
alpha motor neurons are controlled since they control the muscles.
Recall- medial alpha motor neurons innervate axial and proximal limb muscles; lateral motor
neurons innervate distal limb muscles.
in turn, these alpha motor neurons are innervated by interneurons that are located either
medially or laterally; respectively
interestingly, medial and lateral interneurons of the spinal cord differ in their
projection patternslateral interneurons project to a restricted number of segments where they synapse w/
lateral motor neurons (recall, these control distal muscles); medial interneurons can
project to many or even all segments and often cross the midline connecting w/ medial
motor neurons; fig. 17.1
descending projections typically have similar differences in how focused their terminal
fields are; those projecting to medial part of cord are more diffuse, often both sides of
midline; to lateral part of cord more focused
why does this make sense? medial interneurons involved in postural control; lateral
interneurons in control of independent/fractionated movements of limbs and digits
Brainstem projections to spinal cord; see also Fig. 17.2
most significant:
Widespread effects on body position
vestibular nuclei----- postural adjustments to maintain balance- recall input from n. VIIIotolith organs and semicircular canals signalling position of head in space
reticular formation---- nuclei estending from rostral midbrain to caudal medulla ventral
of cerebral aqueduct (e.g. raphe, locus coeruleus, others) functions in cardiovascular and
respiratory control, regulation of sleep and wakefulness, aspects of motor control;
primary projections are on medial spinal cord gray matter
Functions focused on position of specific parts of body
superior colliculus-- projects to medial cell groups in cervical cord- orienting
movements of the head- recall inputs from retinas
red nucleus-- projections to cervical spinal cord- lateral cell groups- control of arm
muscles
brainstem signals and posture- feedforward controle.g. leg muscles contract in anticipation of lifting w/ arm, prior to contraction of arm
muscles (fig. 17.4)
reticular formation is heavily involved in feedforward adjustments; receives input from
motor ctx; can demonstrate in an anesthetized cat where have trained to strike object; can
stimulate motor ctx and get paw movement and accompanying stance adjustment (to
ensure the cat doesn't fall over); if inhibit reticular formation don’t get stance adjustment
but cat still tries to strike object w/ paw
feedback info from vestibular system, visual system, and somatic sensory system (muscle
spindles, golgi tendon, etc)
thus, thru feedforward and feedback, reticular formation and vestibular nuclei especially
important in postural control (Fig 17.5)
2 Pathways for flow of information from cortex to spinal cord- Direct and Indirect
Direct route, corticospinal tract, fig. 17.9 at caudal end of medulla, about 3/4 of axons
of pyramidal tract decussate (cross) and enter lateral columns of spinal cord, forming
lateral corticospinal tract; largely from ctx representing limbs
remaining 1/4 of axons pass directly into spinal cord w/out crossing, forming the ventral
corticospinal tract- these are from ctx representing neck, shoulder, trunk; terminate on
neurons in medial ventral horn and intermediate zone; many also have branches that cross
the midline
Indirect route
projections to red nucleus- from ctx innervating lateral ventral horn
projections to reticular formation- from ctx innervating medial spinal cord gray matter
What is the functional relevance of the direct and indirect routes?
Hint- transection of pyramidal tracts in monkeys interferes w/ detailed independant
movements of extremeties; but they can still stand, walk, run
Thus, brainstem projections are sufficient for much of the motor behavior of the animal!
But the direct connections between motor ctx and neurons of ventral horn appears key in
maximal speed, agility of fractionated movements.
Notably, the direct projections are a relatively recent addition in evolution; not extensive
in rodents (raccoon an exception).
the direct pathway is not present in humans at birth; hasn’t yet innervated neurons of
ventral horn
Descending projections from ctx
From layer V of cortex, axons descend
travel in internal capsule of forebrain, cerebral peduncle of midbrain
intermingle w/ cell bodies and crossing fibers at base of pons
coalesce again on ventral surface of medulla to form the pyramids (pyramidal tracts); fig.
17.9
motor ctx is about 1/2 of pyramidal tract fibers; the rest are axons from premotor area and
somatosensory ctx
motor and premotor axons terminate on ventral horn and intermediate zone of spinal cord
somatosensory ctx axons terminate in dorsal horn; thought to regulate influx of
somatosensory info
There is somatotopic organization of descending projections in internal capsule, cerebral
peduncle, pyramids; this can be useful in diagnosing site of damage to pyramidal tracts;
internal capsule, etc. w/ a stroke or injury.
Motor ctx (area 4)- precentral gyrus- has a somatotopic representation of body Fig.
17.10
note how much the disproportions relative to body size look like those in the
somatosensory map- corresponds to need for finer motor control of some structures
(ears?)
Box 17D
A difference from how somatotopy works in the somatosensory system, where a point on
the surface of the body is processed in a specific region of cortex- The cortical map
seems to be organized such that individual neurons control the activation and inhibition
of multiple muscles related to a movement- not a single muscle- i.e. a single upper motor
neuron influences multiple pools- can obtain evidence by spike-triggered averaging;
recording simultaneously from motor ctx and muscle fibers
Premotor ctx (area 6)- just anterior of motor ctx and supplementary motor ctx (more
medially than primary motor ctx)
more difficult to elicit movement by stimulation of these areas; requires more current
movements stimulated tend to be more complex than those elicited by stim of primary
motor ctx; often involve both sides of the body
Seems to be important in selecting appropriate movements based upon activity in other
cortical areas
General model- premotor area operates to plan/program movements; primary motor
area governs their actual execution
Upper Motor Neuron Syndromes
The pyramidal tracts are rarely selectively damaged, but damage to internal capsule or motor
ctx typically causes flaccid paralysis first on side contralateral to lesion- initial period of
spinal shock- reduced activity in spinal circuits due to loss of descending input.
The trunk is usually unaffected by such injuries; brainstem projections and/or crossing of
corticospinal axons still controlling these motor neurons.
after several days:
Babinski sign- stroke foot from heel to toes- usually get flexion in adult; extension in
infants (recall corticospinal pathway not mature)
Spasticity- increased muscle tone, hyperactive stretch reflexes and clonus (oscillating
contraction/relaxation of muscles in response to stretching)
severe lesions often have rigidity of extensors of leg and flexors of arm (decerebrate
rigidity)
hyporeflexia of superficial reflexes- decreased vigor of corneal reflex, superficial
abdominal reflex (tensing of abs in response to stroking of overlying skin), thought to
result from loss of inhibitory effects of cortical input on the descending pathways from
vestibular nuclei and reticular formation- which have net excitatory effect on reflex
pathways
can show in experimental animals that increased tone can be eliminated by severing
dorsal roots; presumably interruption of Ia afferent reflex eliminates problem with
excessive gain in stretch reflexes