The Cerebellum. Physiology #13 #CNS1

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Sydney Martin
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1 Physiology #13 #CNS1 The cerebellum consists of cortex and deep nuclei, it is hugely condensed with gray mater (condensed with neurons (1/3 of the neurons of the brain)). Cerebellum contains 30 million functional unit and each one involves one perkinje cell and one deep nuclear cell (all in all there is 60 million neuronal cell). Thus the cerebellum is very compact and very sophisticated. N.B: basket and stellate cells cause lateral inhibition ( when they get totally\ excessively excited all surrounding cells will be inhibited) The input to cerebellum is arranged into: 1.Climbing fibers: corrective fibers, if you lean on one side and reach a specific limit the cerebellum will receive a signal that if you lean more you are going to fall (instantaneous signaling). Type of action potential is complex spikes (very huge type of spikes and very complicated one. Thus one climbing fiber can excite perkinje cell. Climbing fibers always excite perkinje cells, why? Because they carry corrective signals which are very dangerous and if they are damaged the patient can't keep balance. 2.Mossy fibers: Both of them first cause excitation to deep cerebellar nuclei then they inhibit these nuclei through perkinje cells. Because it is moment to moment correction for body posture, so the cerebellum does not have time to think. Cerebellum is a silent area, why? Because stimulation does not elicit sensory response and rarely elicit a motor response. Functions of the cerebellum: Page 1

2 In past the scientist thought that the cerebellum was responsible only for the maintenance of balance and equilibrium., now they believe that it has got more functions. Definitely the cerebellum is part of the motor system. When we talk about the function of the cerebellum we divide it into vermis, intermediate zone and lateral hemisphere. The vermis represent the axial and midline part of the body. A lesion by neuroblastoma or chronic alcoholism gives characteristic features that differ from those of the lateral part lesions. The intermediate part represents the distal muscles, hands, feet, face area and coordination. The cerebellum evaluates every order coming from the cerebral cortex to the periphery (through the spinocerebellum), detects any discrepancy between the order and the execution and tries to correct the errors. Patients are unable to detect the error or to correct it leading to clumsy movements. Other function of the cerebellum is the regulation of posture and balance (throwing or catching a ball, dancing, speaking) Non motor functions of the cerebellum: Cerebellar lesion patients don't have mental retardation (cognition is a function of the cerebral cortex). In autism (social and behavioral disease) the cerebellum is involved. Autism patients start their life normally (and here the problem resides) at the age of 18 months or 2 years he start losing the milestones of development (ex. the language)and the interest in social interactions as if they are surrounding themselves with shields and start getting self involved. It is very difficult to care about an autistic child. Page 2

3 EEG for these patients shows abnormality as do the functional MRI, because the cerebellum is abnormal (there is motor and sensory involvement of the cerebellum) Does the cerebellum have reverberating circuits? No, the cerebellum cannot afford this because we need moment to moment regulation of equilibrium and balance. N.B: reverberating circuits mean that we have circulating information. For example: we have 10 neurons and we excited one of them, it will excite the others, then the information will return to the original neuron. In other words, information keeps coming and revolving. While in the cerebellum there is excitation followed by inhibition then clearing then it starts again excitation and so on. Thus we have million of action potentials going to the cerebellum and getting out from it. Also non motor functions of the cerebellum are non reverberating, the lateral hemisphere is strongly associated with the limbic system and with the spatial orientation of the parietal cortex, i.e has connections to others more than reverberating circuits. Expression of emotions in a motor activity still has a big question mark. Also they talk about verbal memory disturbances in the cerebellum but still under investigation. Cerebellar cells are constantly active; you need them even when you are sleeping to maintain your balance (in order not to fall from the bed). How the cerebellum operates when you pick an apple from the tree? First of all, the vermis works to maintain the axial muscle and truncal equilibrium (coordination of the axis which is needed in order to do anything peripherally). Then the intermediate zone helps you when you are moving your hand and trying to hold the apple and to pick it. Finally, the lateral hemisphere is the part that tells you the apple is too far, you have to stop trying, because if you lean more you are going to Page 3

4 fall. In other words, it is responsible for the coordination between the center and the periphery. Cerebellar lesions: Mostly in elderly they are caused by stroke, while in middle aged and young people they are mainly caused by multiple sclerosis (MS). Patients with cerebellar MS present with ataxia, nystagmus and vertigo. In children cerebellar lesion could be hereditary (ex. Friedreich's ataxia) or it could be caused by a tumor (ex. Medulloblastoma where unfortunately they are visible at the time of the diagnosis). A clinical case: Bipolar disorder causes swinging of mood, we all experience good day and bad day but when it gets huge reaching mania (because of hyperexcitability of the brain) and very deep down causing depression or might reach suicide this is dangerous. Here we give them mood stabilizers in which the prime example is the lithium. We give them antipsychotics (choosing the most effective with the lowest side effects) but on the long run they must take a mood stabilizer. But, how does the lithium work in such cases? Mania is excessive excitability of the brain, so we need to reduce the action potential. Lithium is a small particle that competes with the sodium thus reducing the occurrence of the action potential and so the excitability of the brain. BUT keep in mind that lithium at high doses causes irreversible cerebellar damge. Remember that: Cerebellar lesion patients are dependent (they can't walk by themselves) or on wheel chairs, meaning that they are already miserable, and giving Page 4

5 them lithium is not fair and not accepted. Thus, it is used in maintenance (prolonged treatment not an acute one). Also, psychiatric disorders need time to resolve. It takes 2-4 weeks for the effects of the drug to appear. One of the most diagnostic criteria in cerebellar lesion is ipsilateral hypotonia. Remember that: The cerebellum signals the cortex contralaterally and the cortex controls the contralateral side of the body. All in all, the cerebellum controls the ipsilateral side. Hyporeflexia also seen in cerebellar lesion. Pendular jerk occurs but rarely, most patients either have reduced or normal reflexes. Thus we don't depend on reflexes to test the cerebellum but we do in testing the spinal cord or the cerebral cortex. Vermal lesions: Anterior lesions are seen in chronic alcoholics and toxicity patints. Posterior lesions are seen in medulloblastoma patients which hits children in 9,10 years of life until 14 years of life, and they have gait disturbance and walk on wide based gait. Hemisphere lesions: Seen mostly in stroke patients and in MS patients. Thus not all of them are old and you might see young people. Manifested by disequilibrium and lack of coordination in the upper and lower limbs. For upper limb lesion we use the finger nose test or the finger to finger test. Page 5

6 For the lower limb lesion we use the shin heel test. How to tell the patient about performing the shin heel test? Put your finger close to the patients leg and ask him to approach your finger by his big toe. MS patient is usually educated and you can easily do the test with her. Vertigo and nystagmus: You have to differentiate between cerebellar, brain stem or peripheral lesion. N.B: nystagmus usually is horizontal, vertical nystagmus is very rare. Nystagmus: jerky eye movement when fixing the gaze on an object. We call it dysmetria of saccades. Also the fast component is ipsilateral to the lesion. REMEMBER: nystagmus is a sign. You diagnose it and the patient does not feel anything. His vision is normal. N.B: MS patients have optic neuritis (cerebral lesion), in which half of her eye does not see (partial blindness). These constitute 70% of the MS patients. Others may have cerebellar MS which causes nystagmus, dysmetria of saccades and unilateral looking to the site of the lesion. OPTIC NEURITIS AND CEREBELLAR MS ARE NOT RELATED AND RARELY OCCURE TOGETHER. Dysmetria : also seen in cerebellar lesions. What is the difference between the dysphasia and dysarthria? Dysphasia is a cerebral cortex lesion. Also it is manifested by difficulty in the formation of the idea and the word. While in dysarthria the formation of ideas and the formation of words are normal but the execution is abnormal so it is called scanning Page 6

7 dysarthria (increase in tone at a place and decrease in it on the other, as if he is scanning every letter. Rebound phenomenon: if there is a lesion the condition (that happens while testing) persists times. And that s it I tried my best, sorry for any mistake. Good luck Your colleague: Bayan Sarsour Page 7

Unit VIII Problem 5 Physiology: Cerebellum

Unit VIII Problem 5 Physiology: Cerebellum - The word cerebellum means: the small brain. Note that the cerebellum is not completely separated into 2 hemispheres (they are not clearly demarcated) the vermis