Cerebellar histology

CEREBELLAR HISTOLOGY

Cerebellar cortex shows 5 types of neuronal cells. They are similar to cerebral cortex in this region.

  • Stellate
  • Basket
  • Golgi
  • Granule
  • Purkinje cells

They are arranged in 3 layers 

  • Outermost layer:  Molecular layer. Made up of 2 types of cells. Outer stellate cells where axons confines to this layer. Basket cells whose dendrite remain in this layer and axons reaches Purkinje layer and make synaptic contact with cell bodies of Purkinje cells.  Axons of the stellate cells synapse with primary and secondary dendrites of Purkinje.  It has axons of granule cell (parallel fibers), neuronal dendrites of deep layers, few inhibitory neurons, stellate, and basket cells. 
  • Middle layer:  Purkinje cell layer. It is composed of simple layer of ovoid/rounded cell bodies of Purkinje neurons.  Uniqueness of those cells are extensive branching of their dendrites (multi polar neurons) which are oriented at right angles to parallel fibers.  These cells are the output neurons, uses GABA as their neurotransmitter hence they are inhibitory in nature.  Axons of Purkinje cells travel outside the cerebellar cortex to reach the cerebellar nuclei.  Purkinje cell comprises of primary, secondary, tertiary, and quaternary dendritic branches.  The primary and secondary are smooth but tertiary and quaternary have numerous synaptic spines and are rough surfaced.  They synapse with the neurons of the granular layer beneath this. 
  • Inner layer:  Granule cell layer. It has cells which have darkly stained nucleus and little cytoplasm giving this layer granular appearance.  It has many small granule cells with few Golgi neurons.  The Golgi neurons are located at the junction between granular and Purkinje layers.  Dendrites of granular cell limit within this layer and receive synaptic influence from sensory fibers to the cerebellar cortex (mossy fibers) and axons of Golgi neurons.  Axons of granular cell project to the molecular layer through the tertiary and quaternary Purkinje branches and synapse their spines.  Among the cells of the cerebellar cortex, only the granule cells have excitatory neuron.

Cerebellum consists of 3 functionally distinct parts in birds and mammals

Vestibulocerebellum/Archicerebellum

  • The classification is based on the embryological basis. The archicerebellum is an initial evolutionary structure. It deals with control of tonic or postural reflexes of the spinal cord. Important for maintenance of balance and coordination with eye movement. It mediates coordination of vestibular reflexes.
  • It is present in the flocculonodular lobe.
  • It receives sensory information from vestibular system. It sends back orders to vestibular nuclei via cerebellar nuclei (fastigial nuclei), to coordinate the axial and proximal muscles controlling balance. These orders are mediated by vestibulospinal tracts which in turn help to coordinate head and eye movements. 

Cerebrocerebellum/Neocerebellum

  • It involves in planning and initiating voluntary activity.
  • It provides input to motor cortex concerned with graceful, intricate, appropriately timed voluntary movements. It occupies most of the cerebellum representing in the lateral cerebellar hemisphere.
  • It receives inputs predominantly from supplementary and premotor cortex via the corticopontine cerebellarly system. They have no clue of the sensory information from the peripheral receptors. Their output goes back to motor cortex from a communication loop for planning and preparation of movement ahead of actual execution and enhances appropriate timed transitions of a movement sequence. 

Spinocerebellum/paleocerebellum

  • This portion represents the medial portion of the cerebellum.
  • It is concerned with maintenance of muscle tone, coordinates skilled voluntary movements. It controls the tonic or postural motor activity of the spinal cord. They are concerned with accurate timing of muscular contractions. It maintains coordination between muscles of different parts to complete an action, by receiving information from peripheral somatosensory system that constantly inform higher centres about the body movement.
  • The cerebellum receives information from higher centres about the orders sent and at the same time they receive information from muscle spindle, vestibular, visual, and other sensory receptors about the movement the body is performing.
  • It predicts the position of the body in next minute and to make adjustments needed. This mediates the intentions of the motor cortex to the execution of muscular movements happening. It is important to mediate the corrections on the intended movement. The general pathway is that it receives inputs from muscle and cutaneous receptors, spinal reflex circuits through the spinal cord. The information from corticospinal, descending brain stem pathway, primary motor cortex, and somatosensory cortex reaches this area. The information from high centres encode the movement to be executed by the skeletal muscle and sensory inputs inform what exactly executed by the muscles.
  • Output from these areas are mediated through deep cerebellar nuclei, reticular nuclei of brainstem that controls antigravity muscles, red nuclei of the brainstem which controls musculature of distal limbs, to adjust the timing and coordination of the movement and muscle tone. Inhibition of this area, activity of medullary lateral vestibular nuclei is enhanced by way of fastigial nucleus. This nucleus is one of the cerebellar nuclei that lie over the roof of the fourth ventricle. Enhanced activity of the medullary nucleus excites α-motor neurons of antigravity muscles of the limb and results in rigidity of the antigravity muscles.

The degree of tonic inhibition exerted on the fastigal neurons and medullary neurons is depended on orders from cerebral cortex (cortico-ponto cerebellar pathway), basal gangilia and from cutaneus and propioceptive systems of the body. Spinocerebellum exerts flexible controls over the skeletal muscle tone on the posture.

 

 
Last modified: Thursday, 9 June 2011, 5:16 AM