2/5 Cerebellum motor system

chemically defined afferents

The primary function of these is to act as a neuromodulator. They alter the responsiveness of a cerebellar neurons.

What nuclei are in the lateral hemisphere and what happens from here

The projection of the dentate nucleus is similar to interposed nuclei (except there is no descending limb) -axons leave via the ipsilateral SCP and decussate in the rostral pons. -Red nucleus recieve this input, but there are not the ones that give rise to the rubrospinal tract. -Rather, they terminate in the inferior olive, the source of climbing fibers to the cerebellum.

What happens once spinocerebellar tract projects to the interposed nuclei (descendingly)

The spinocerebellum projects to the interposed nuclei (glovose and emboliform). The axons of these nuclear neurons leave the cerebellum via the superior cerebellar pedunce. -some axons form a descending limb that courses back to the medulla where they terminate in the reticular formation to modulate the activity of neurons giving rise to the reticulospinal tract, which modulates motor neurons in the spinal cord. -NOTE: this occurs on the contrallateral side.

Where do we find climbing fibers and what are some of their characteristics?

These axons arise from neurons located in 1 and only 1 place- the INFERIOR OLIVARY NUCLEUS in the medulla. -axons are unique in that they form a one to one relationship with purkinje cells (no convergence). -unlike mossy fibers that have a relay in the granule cell layer, climbing fibers go directly to the purkinje cell where they literally climb along the dendrite of the neurons. -they excite purkinje cells by releasing glutamate.

what happens once the spinocerebellar tract projects to the interposed nuclei (ascendingly)

These axons leave the cerebellum via the superior cerebellar peduncle, cross in the rostral pons in the decussaiton of the superior cerebellar peduncle and ascend to the midbrain and diencephalon. -here a majority terminate in the red nucleus, giving rise to the rubrospinal tract. Note these axons return to the same side of the body which proprioceptirve info carriedy by the posterior spinocerebellar tract originated. -finally, a few axons continue to the thalamus, where they terminate in the ventral lateral nucleus). This is "feeding" back the boss.

what's significant about a purkinje cell

This is the only cell with an axon that leaves the cortex. All of the other neurons are considered "interneurons" in that their axons remain within the cortex and function to modulate the activity of purkinje cells. -The axons go to the cerebellar nuclei where it is inhibitory to neruon in all of the cerebellar nuceli. -It is GABAergic.

cerebellar peduncles (which way does info go)

all of tehse are fiber tracts that bring info into or out of the cerebellum. -ICP is most cadual and primarily an input pathway for axons coming from the spinal cord and the medulla. -MCP- largest and connects the cerebellum to the pons. Information from the cerebral cortex synapses on neurons in the pons, their axons cross the midline to form the MCP. This is exclusively input. -SCP- connects the cerebellum to the midbrain and is the pimary OUTPUT pathway of the cerebellum. Axons leaving the cerebellum in the SCP cross as they enter the midbrain.

fcns of the cerebellum

also important in controlling eye movements

cortico-ponto-cerebellar input

axons from all areas of the cortex descend and synapse on neurons in the pontine nuclei located within the basilar pons. The axons of these neurons cross the midline and form the large middle cerebellar peduncle which relays cortical information to the lateral hemisphere of the cerebellum.

Which peduncle does not cross?

inferior cerebellar peduncle.

where can be find mossy fibers in the brainstem? and where do they synapse

neurons located in various nuclei in the spinal cord and brainstem (Clarke's nucleus and external cuneate nucleus in the spinal cord. vestibular nuclei and pontine nuclei in the brainstem). Axon terminals are large. Mossy fibers are excitatory to neurons in the granule cell layer. Neurons in this layer relay the excitatory info to the purkinje cell.

Anterior sipnocerebellar tract

proprioceptive info enters the cord, but instead of synapse in CLarke's nucleus, it synapses on interneurons in the spinal cord that receive info from other areas of the spinal cord as well as the cerebral cortex. -axons double cross and enter the cerebellum via the SUPERIOR CEREBRAL PEDUNCLES

now what happens to the purkinje cells in the flocculondular love

they project to the fastigial nucleus located adjacent to the Midline. The axons of the fastigial nucleus neurons leave the cerebellum via the ICP to reach the IPSILATERAL vestibular nuclei in the brainstem. some of the axons cross within the cerebellum to reach the contralateral side. -The cerebellar input to the vestibular nuclei modulates neurons that give rise to the medial and lateral vestibulospinal tracts that modulate the activity of alpha and gamma motor neurons. Remember:VS tract Coordination of head & eye movements (medial)and Postural responses Balance reactions (lateral).

functional divisions of the cerebellum

-flocculondular lobe- vestibulocerebellum. -vermis and immediately adjacent medial hemishere is designated the spinocerebellum -lateral hemispher- called the ponto or cerebro cerebellum

divisions of the cerebellum

-primary fissure splits the cerebellum into a posterior and anterior love. -A third division, called the flocculonodular lobe is tucked between the anterior and posterior lobes. -It can also be divided medial-to-lateral. We call these the hemispheres.

general considerations for basic cerebellar circuitry

1. Climbing and mossy fibers synapse on cerebellar nuclei BEFORE continuing on to the cerebellar cortex (excitatory). 2. Their is a precise sagittal organization in the projection between the cerebellar cortex and the cerebellar nuclei. 3. Vast majority of axons leave via the superior cerebellar peduncle. 4. The output of the nuclei are excitatory to their target neurons in the brainstem nuclei or in the thalamus. 5. A few are inhibitory to neurons in the inferior olive

afferent input to the cerebellum

2 anatomically distinct afferents that project to the cerebellum called mossy fibers and climbing fibers. As they enter the cerebellum, they give off branches to cerebellar nuclei before continuing on to the cortex. -Both systems are excitatory.

What happens once we sent input to the olivary neurons?

Axons of the olivary neurons cross the midline and project to the cerebellum as climing fibers. THis pathway represents a FEEDBACK LOOP between the cerebellum, red nucleus, and inferior olive that regulates climbing fiber activity.

vestibulocerebellar projection and flocculondular lobe

Flocculondular lobe receives input form 2 areas. -First order neurons in the vestibular ganglion project directly to the ipsilateral cerebellum. (only sensory system that has a direct projection to the cerebellum). -In addition, the first order vestibular neurons also project to the vestibular nuclei. These 2nd order neurons in the vestibular nuclei project bilaterally to the cerebellum.

what is the neurotransmitter of purkinje cells?

GABA

basic anatomy of the cerebellum

Grey matter on the outside and on the inside. -these nuclei are critical for normal cerebellar function. It is the neurons within the nuclei that carry the modulatory output of the cerebellum to brainstem nuclei and feedback to the cerebral cortex. -the cortex basically functions to regulate the output of the cerebellar nuclei as we'll see later in this presentation. -finally, the peduncles are fiber tracts that link the cerebellum to the underlying brainstem.

afferents form the dentate nucleus terminate primarily in the:

ventral lateral nucleus of the thalamus

spinocerebelluar afferents form upper limbs

Input from the upper limb beings with DRG above T1. The axons ascend in the dorsal funiculus and synapse in a nucleus located adjacent to the cuneate nucleus in the medulla called the EXTERNAL or LATERAL CUNEATE NUCLEUS. 2nd order axons leave this nuclues and enter the ipsilateral cerebellum via the ICP.

Example of dysmetria

Object enterir field of view and the eye follows it smoothly. If the target stops abruptly, the normal response is for the eyes to continue in anticipation of the object continuing along its path.The eyes will then rapidly fixate on the now stationary target. -in individuals with a cerebellar lesion, the eyes overshoot in the opposite direction, and then come back and so forth until they ultimately land on the object. (dysmetria)

Cerebellum histology

Purkinje cell layer are neurons that form a row between the granule and molecular layers

climbing fibers input

Remember, this is unique afferent system that forms a one to one relationship between a neuron in the inferior olivary nucleus and a purkinje cell in the cerebral cortex. -The inferior olive receives input from many areas of the CNS including the cerebral cortex, vestibular nuclei, red nucleus, as well as feedback form the cerebellum. -They also have a complex action potential consisting of special voltage gated calcium channels.

spinocerebellar afferents from lower limbs

SPinocerebellum is made up of the vermis and adjacent medial hemisphere. -1st order neurons carrying proprioceptive info from the lower limb is in a DRG. These synpase on CLarke's nucleus. These axons from the posterior Spinocerebellar tract that courses rostrally and enters the cerebellum through the ipsillateral inferior cerebral peduncle

What is the role of the cerebellum (big picture)

The cerebellum is a comparator. It is like middle management. It takes the command makes the fine adjustments needed for efficient motor function. -there is a pathway form the cortex to the cerebellum, but it is not direct -the cerebellum also needs info on where the body is in space as well as the status of the muscles at an instant in time. -Now that is has both info, it can compare the "command" given from the cortex with the current status of the muscle. -The output of the cerebellum is to the brainstem nuclei taht project ot he pinal cord to adjust the activity of the motor neurons as necessary. -Finally, the cerebellum also projects back to the cortex