Download Commentary on slides Lecture 16

Commentary on slides Lecture 16
1. Central pattern generators can, in turn, be accessed by higher levels of motor control.
These higher levels include the cerebellum, a large structure at the junction of the pons
and medulla, the basal ganglia in the forebrain and the motor cortex. Let’s begin by
considering the functions of the cerebellum.
2. The cerebellum, or little brain, is a large collection of neurons that hangs off the pons
just behind the cortex. The cerebellum is connected with the vestibular system and spinal
cord and with motor cortex.
3. These connections are carried via 3 large fiber bundles, the superior, middle and
inferior cerebellar peduncles.
In cross section (here mid-saggital) the cerebellum can be see to consist of a number of
leaflets or folia.
4. The midline portion of the cerebellum is called the vermis and is flanked by lateral
cerebellar lobes. The part of the cerebellum that is most intimately connected to the
vestibular system is the posteriorly-located floccular-nodular lobe. The output of
cerebellar processing consists of inhibitory discharges from the Purkinje cells. These are
directed towards the deep cerebellar nuclei: the globose, embolliform (these two make
up the interpositus nucleus), fastigial and dentate. These nuclei also receive excitatory
input from the collaterals of afferent fibers; Purkinje cell outputs thus modify discharges
of the deep cerebellar nuclei.
The neocerebelllum is basically a self-contained processing unit. It communicates with
the rest of the CNS via the depp cerebellar nucei: the dentate, the emboliform, the
globose and the interpositus. The neocerebellum has a complete motor and sensory map
of the body surface. Incoming fibers excite the cells of the deep nuclei; their activity is
modified by cerebellar output and they then affect motor systems. The entire output of
the cerebellum arises from Purkinje cells which are inhibitory to the deep nuclei.
5. Each folium of the cerebellum contains a well defined set of cells in orderly positions.
The largest cells are the Purkinje cells, a cell type with a highly characteristic dendritic
arbor (“like a pear tree espaliered against a wall). The dendritic tree is oriented at right
angles to a group of fibers called parallel fibers that run across the folium. Parallel fibers
are the axons of a second cell type called granule cells.
6. The two major inputs to Purkinje cells arise from climbing fibers and mossy fibers.
The cells of origin of climbing fibers are in the inferior olivary nucleus of the pons.
These fibers ensheathe the cell bodies of the Purkinje cells and form very powerful
excitatory synapses. The mossy fiber input arises from the pontine relay nuclei. These
synapse on granule cells which innervate the dendritic tree of Purkinje cells.
7. The basic circuit diagram for these cell types is shown here together with the
contribution of two local inhibitory neurons, the Golgi and basket cells. These latter cell
types perform a function that is similar to lateral inhibition in the somatosensory system.
8. For example, the inhibitory basket cells sharpen up the patter of excitation produced
by parallel fibers.
9. We know that the cerebellum is important in general for movement because of deficits
produced by damage. What precisely, however, does it do? One way to understand
cerebellar function is to consider the role of the flocular-nodular lobe (the
vestibulocerebllum) in controlling gain in the oculomotor system. The position of each
eye in the obit is determined by contraction of 3 pairs of extraocular muscles: the medial
and lateral recti, the superior and inferior recti and the superior and inferior obliques).
Study question: Name the cranial nerve nuclei that contain the motor neurons that
innervate each muscle.
The actions of these pairs are opposing. For example, the lateral rectus adducts the eye
while the medial rectus abducts the eye (moves the eye towards the nose; hence the
abducens nucleus).
10. One of the functions of the extraocular muscles is to maintain gaze. As you walk, for
example, your head moves. If you are trying to keep a particular object in view
(“foveate” the object) while you move, the extraocular system can control eye position
using sensory information delivered by the vestibular system. The vestibular system is
located in the inner ear and consists of two components: one is driven by movement of
fluid within the semicircular canals. These are paired structures with hoops containing
fluid and are located at right angles to each other. Movement of the fluid is sensed by the
sterocilia of hair cells within the canals. The hair cells in this system are particularly
sensitive to linear acceleration. The inner ear also contains small “rocks”: the otoconia.
These roll around on the surface of the saccule and utricle and deform the sterocilia of
hair cells on those surfaces; each has a characteristic pattern of hair cell orientations.
This arrangement permits translation of hair cell depolarization into head position.
11. The movements of the eye and controlled by motor neurons located in three
oculomotor nuclei: the oculomotor (III), the trochlear (IV) and the abducens (VI). The
abducens motor neurons, for example, innervate the lateral rectus muscle and pull the eye
towards the temple (abduct the eye). The trochlear nucleus controls the superior oblique
muscles. All of the other muscle, notably the medial rectus (which we will be
discussing), are controlled by motor neurons in the oculomotor nucleus.
12. One of the functions of the cerebellum is to correct the gain of motor commands on
the basis of sensory feedback. We can consider how this works by imagining a situation
in which the lateral rectus muscle of the left eye has been damaged. When a motor
command to move this eye to the left is generated, the muscle cannot comply and the
visual image on the retina deviates from what would be expected if the muscle were
intact. If visual feedback is provided to the system, the commands sent to the abducens
nucleus can be boosted to achieve the desired result. If, however, visual feedback is
denied (for example, if a patch is placed over the eye), the motor program will not be able
to compensate. In fact, the patch placed on the intact eye (the right eye) will cause a
maladapative change in the motor program driving its lateral rectus muscle.
13. How does the cerebellum participate in changing the gain of motor programs driving
the extraocular system? This is a schematic diagram based on the work of Matsuo Ito
and his colleagues. In his work, Ito examined the inputs from the vestibular system to the
floccular-nodular lobe of the cerebellum. Afferent fibers from the vestibular organs enter
the cerebellum as part of the mossy fiber input to granule cells. Vestibular input also
enters the cerebellum via the vestibular nuclei (dorsal, medial lateral and superior
divisions are shown here). The vestibular nuclei are the deep cerebellar nuclei of the
floccular nodular lobe. They provide afferent input to the motor neurons in the abducens
nucleus (VI) that innervate the lateral rectus muscle and to the motor neurons in the
oculomotor nucleus that innervate the medial rectus muscle. Let’s say your head turns to
the left. To maintain the position of your gaze in sppace you need to move your eye ball
to the right. This movements involved contraction (excitation) of the medial rectus
muscle of the left eye and relaxation (inhibition via decreased motor neuron activity) of
the lateral rectus muscle of the right eye. Now suppose that one of these muscles (say the
medial rectus) is weak (due to damage, fatigue or disease). The failure of the motor
program to produce foveation will generate an error signal on the retina. This signal will
be sent to a midbrain nucleus (PA) and then on to a pontine nucleus (NRTP) to generate a
mossy fiber input. It will also generate a climbing fiber input via the inferior olive. This
climbing fiber input instructs the Purkinje cell to fire, producing enhanced inhibition of
its output to the vestibular nuclei (remember that these are analagous to the deep
cerebellar nuclei). This out put disinhibits the input to the medial rectus muscle (making
it more effective) and inhibits the input to the lateral rectus muscle.
One of the consequences of cerebellar circuitry is that this structure plays a very
important role in motor learning (riding a bicycle for example, or attaining proficiency
with a particular piano piece). All forms of sensory input have access to the cerebellum
and all areas of the motor cortex project onto the cerebellum. Putting feedback together
with the intended motor program generates motor learning.