Central Motor Mechanisms

Purves et al., Chapters 17-20, Chap 10, Appendix
Sylvius is also very useful here.
Note that the Washington University Medical School's Neuroscience Tutorial has good coverage on these topics:
Basic Motor Pathway
Basal Ganglia and Cerebellum

Pep-talk:

There are many deficits in motor function and coordination. After you have learned how much of the brain is dedicated to motor function and coordination, you will appreciate what a gift it is not to be spastic.

Spinal tracts have names like cortico-spinal tract (from -> to).
Above and beyond spinal reflexes, these tracts mediate descending influences on spinal motor neurons.

Anatomical review:

Ventral view of sheep brain shows cerebral peduncle and pyramids
Horizontal section of sheep brain shows caudate and lentiform n. (putamen and globus pallidus), as well as internal capsule (and cerebellum)

Even before considering basal ganglia and cerebellum, paths are complicated and numerous

Fig. 17.9
Pyramidal system with corticospinal tract
Corticospinal tract Pyramidal motor system (75-90% crosses) 10 to the 6th axons
Named because it goes through pyramids on ventral medulla
(though it might have been named from pyramidal shaped neurons in layer V incl. Betz cells)
Lateral and ventromedial pathways
Summary of corticospinal:
precentral gyrus -> internal capsule -> cerebral peduncles -> pyramids ->
decussate in brain stem -> lateral cortical spinal tract (see below)
(uncrossed in medial cortical spinal tract) (see below)
(only primates among mammals have monosynaptic pathway to motor neurons)
Initiation of voluntary motor movements

Refer back to the lecture on the somatosensory system:
Fig. 10.4
A half spinal cord injury would cause contralateral loss of spinothalamic below injury and ipailateral loss of lemniscal.
Brown-Sequard syndrome include motor (ipsilateral impairment)

The pathway is neatly organized topographically at the levels of internal capsule, Cerebral peduncle in midbrain, Pyramid in medulla

Fig. 17.9
Ventral corticospinal tract

Table A2 (Appendix)
Corticobulbar
Output for face and upper body via facial nerve (and trigeminal, vagus, accessory, hypoglossal).

Fig. Box 17B
Interesting in that upper face has bilateral innervation, lower face is only contralateral in its control
The famous Sunday night TV anchorman Ed Sullivan ("We have a really big shew for you tonight") had the lopsided mouth described in Box B for unilateral damage (or stroke).

Fig. not in 4th edition
Red nucleus adds control to arm muscles
Rubrospinal tract from red nucleus replaced by corticospinal in evolution

Fig. 17.2B
corticoreticulospinal tract
Reticular formation controls axial muscles and proximal limbs
pontine reticulospinal- help to maintain posture
medullary reticulospinal - liberates antigravity from reflex

Fig. 17.6

Fig. 17.2C
Superior colliculus (tectospinal) goes down to control head movements

Fig. 17.2A
Vestibular control for posture and catching balance

Fig. 17.10
topographic map of motor cortex- compare with corresponding sensory homumculus
work by neurosurgeon Penfield, note relative "magnifications"
Precentral gyrus = Brodmann area 4 = M1

Fig. 17.7
premotor area (area 6)

Basal ganglia and cerebellum

Basal Ganglia get input from Cerebral Cortex and feed back to motor Cortex
Cerebellum gets input from Cerebral Cortex and feed back to motor Cortex
In early coverage of functional neuroanatomy, this is where the statement that the thalamus is a motor relay proves true.

Fig. 18.4
Not just motor cortex, but huge parts of cortex feed to basal ganglia.

Fig. 19.4
Not just motor cortex, but huge parts of cortex feed to cerebellum.

Basal ganglia and cerebellum take a lot into account to integrate motor control.

Basal ganglia

Extrapyramidal (because it lies outside the pyramids)
caudate + putamen = striatum (striated because strands of internal capsule make it look striated)
putamen + globus pallidus = lentiform nucleus [lens shaped] (see sheep brain horizontal section)

Fig. 18.2
inputs to basal ganglia
cortex and substantia nigra and pars compacta

Fig. 18.5 A
outputs from basal ganglia
The globus pallidus is a relay nucleus for the caudate and putamen and so is the subthalamus.
To VA/VL complex of thalamus to motor cortex
also to substantia nigra pars reticulata to superior colliculus

Fig. 18.3
connections are to spiny and aspiny neurons in caudate and putamen

Box 18A
Parkinson's
(see the neurotransmitter lecture)

Fig. 18.11A
Lowered excitatory input from substantia nigra via D1 dopamine receptors leads (through globus pallidus and thalamus) to decreased excitation at motor cortex, explaining the hypokinesia of motor cortex.
Also there is another interaction via D2 receptors to subthalamic nucleus

Box 18B
Huntington's (1872) disease (chorea) choreoathetosis
Dominant late onset - many interesting genetic counseling issues. The Folk singer Woodie Guthrie died of Huntingtons. There is a big family tree derived from Venezuela near lake Maracaibo
On post-mortem, degeneration of putamen and caudate is observed.
It is on short arm of chromosome 4
1983 and since: cloning -CAG repeat (polyglutamine repeat), 15-34 (normal) -> 42-66 (Huntington's)
Other trinucleotide repeat diseases: fragile X syndrome, myotonic dystrophy, and others
sometimes they get worse from generation to generation (anticipation)

Fig. 18.11B
The diagram is more complicated, but in some ways, Huntingtons is the opposite of Parkinsons in that circuit has thalamus increasing excitation to cortex.

Cerebellum

Dysmetria (cannot approach target), ataxia, intentional tremor if cerebellar damage.
Cerebellum highly developed in electric fish.
Cerebellum is involved in rhythmic activity and plasticity.
An additional decussation makes it so that cerebellum controls the ipsilateral side of the body.

Fig. 19.3
input to cerebellum
especially from cerebrum, vestibular apparatus and spinal cord

Fig. 19.7
output from cerebellum
via deep cerebellar nucleus via superior cerebellar peduncle to VL complex of thalamus to motor and premotor cortex

Fig. 19.10
cerebellum is a fairly "simple circuit"

Fig. 19.11
excitatory and inhibitory interactions are known
Mossy fibers input to 10-100 billion granule cells to parallel fibers, and many connect to each spectacular Purkinje cell.
Also inpput from climbing fiber makes more 1:1 connection to Purkinje fiber.
Also local circuits from basket cells, Golgi cells, and stellate cells

Fig Box 19 B
There are very interesting mouse mutants, reeler, weaver, leaner, lurcher, nervous, Purkinje cell degeneration (those last two interestingly cause blindness too) and staggerer.
reeler is cloned, had a defect in protein like extracellular matrix proteins and has defect in migration of cells during development.
weaver is a K+ channel.

Eye movements make an interesting example

Fig. Box 20 A
if image is stabilized on the retina the image disappears

Fig. 20.3
a reminder of muscles and wiring
Abducens (VI) to lateral rectus
Trochlear (IV) to (contralateral) superior oblique
Occulomotor (III) to the rest (and eyelid control and the pupil)

Types of eye movements: saccades (also in REM sleep), smooth pursuit, vergence, drift, and vestibular control

Fig. 20.8
horizontal saccades are controlled by paramedian pontine reticular formation PPRF (gaze center)

Fig. 20.11
Superior colliculus involved (and frontal eye field)

Fig. 20.0
Stimulate superior colliculus and bring fixation to receptive field of area stimulated

Exam questions from 2005 - 2007 related to this outline

What is the main motor nerve to control facial expression?

facial (7)

What portion of the cerebral cortex projects to the basal ganglia?

most

Why is Huntington's referred to as a "triplet repeat disease?"

CAG nucleotide triplet increases coding for polyglutamine

Why would paralysis be a more likely consequence of stroke in the lower portion of the face than in the upper portion?

upper is bilaterally innervated, lower is not

Which nerve innervates more than one extraocular muscle?

occulomotor (III)

Increased motor excitation from thalamus to motor cortex is the hallmark of what genetic disease?

Huntington's

"The left side of the brain is for the right side of the body" is the conventional wisdom that most people learn early in their science education. Where is the cross-over for the direct
output from the primary motor cortex to the spinal motor neurons?

caudal medulla

Here is a partial list of descending motor tracts to spinal cord: colliculospinal, rubrospinal, reticulospinal and vestibulospinal. What major descending tract is missing from that list?

corticospinal

What is the extrapyramidal system?

motor system outside pyramids involving basal ganglia

What part of the brain is especially altered in the ataxic weaver mouse mutant ?

cerebellum

When specialized optics are used to stabilize the image on the retina despite eye movements, what does the subject perceive?

everything disappears

What conspicuous midbrain structure is missing from the following list of areas involved in eye movement: PPRF (gaze center), vestibular system, abducens, trochlear, occulomotor?

superior colliculus

What cell, a well-known cell type, is the output to the deep cerebellar nuclei?

Purkinje

"The thalamus is a major motor relay station." Justify this statement when the pyramidal motor system does not synapse in the thalamus.

extrapyramidal system feeds back to cortex via thalamus

The rectus muscles move the eyes up and down and side to side. What are the muscles that rotate the eyes?

oblique

"Diminished excitation and increased inhibition to the globus pallidus inner segment contribute to chorea in Huntington's disease." Name one of the brain areas that input to the globus pallidus.

caudate, putamen

What cranial nerve is missing from the following list for motor output to the face: trigeminal, vagus, accessory, hypoglossal.

facial

A unilateral lesion in the cranial nerve would cause unilateral weakness in the top and bottom of the face. Why might only the lower part of the face be afflicted if the lesion were in the brain?

because there is ipsilateral and contralateral control

Where is the first synapse of a cell of the precentral gyrus whose axon is in the pyramidal motor system?

on the spinal motor neuron

Where would there be a motor defect below a hemisection of the spinal cord?

ipsilateral to the lesion

In somatotopic organization, there is a homunculus of the body, with face and hands over-represented, on the postcentral gyrus. Where is the corresponding motor map, also with exaggerated face and hands, located?

precentral gyrus

Basal ganglia feed via the globus pallidus to the cerebral cortex. What relay area is between the two?

thalamus

While discussing extrapyramidal control of motor movements, there was a figure showing the entire cerebral cortex shaded (except primary auditory and visual cortex areas). What does this shaded area represent?

the part of the cerebral cortex with input to the basal ganglia

"Polyglutamine repeat." Translate.

glutamine is an amino acid. polyglutamine repeat is a string of them, with a low number in normals and a high number in Huntington's

Granule cells and climbing fibers both connect to Purkinje cells, but in different numbers. Specify.

1:1::CF:PC, many:1::GC:PC

In the diagrams, middle and inferior cerebellar peduncles are inputs to the cerebellar cortex. The superior cerebellar peduncle is output from deep cerebellar nuclei to (where)?

thalamus

The trochlear is the contralateral output. Which two nerves, subserving the same overall function, have ipsilateral output?

occulomotor and abducens

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