Anatomy & functions of the Brainstem & Cerebellum

Provides a good description of the anatomy of the brainstem & cerebellum; their parts, structure, blood supply & a brief description of their functions.

1. Cerebellum & Brainstem Card-6 Item-7

2. Cerebellum: Introduction Description: The cerebellum, which stands for “little brain”, is a structure of the central nervous system. It has an important role in motor control of the ipsilateral side of the body . In particular, it is active in the coordination, precision and timing of movements , as well as in motor learning. Embryological formation: The hindbrain subsequently divides into the  metencephalon  (superior) and the  myelencephalon  (inferior). The cerebellum develops from the metencephalon division. Anatomical position The cerebellum is located at the back of the brain, immediately inferior to the  occipital  and  temporal   lobes ,   and within the posterior cranial fossa . It is separated from these lobes by the  tentorium cerebelli , a tough layer of dura mater. It lies at the same level of and posterior to the  pons , from which it is separated by the fourth ventricle.

3. Cerebellum: External Features

4. Cerebellum: Anatomical Divisions There are 3 anatomical lobes divided by 2 fissures – the  primary fissure  and  posterolateral fissure can be distinguished in the cerebellum. These are Anterior lobe : extends from the level of the cerebellar peduncles anteriorly and includes the anterior 2/3s of the superior vermis, along with the anterior third of each hemisphere. This lobe terminates at the primary fissure. Posterior lobe: lying posteriorly and laterally and continuing along the inferior surface to the posterolateral fissure. Flocculonodular lobe: flattened lobe that lies between the posterolateral fissure (inferiorly) and the inferior medullary velum and the cerebellar peduncles (superiorly).

5. Cerebellum: Functional Divisions The cerebellum can also be divided by function. There are 3 functional areas of the cerebellum Cerebrocerebellum / Pontocerebellum/ Neocerebellum   – the largest division, formed by the lateral hemispheres.It receives inputs from the cerebral cortex and pontine nuclei , and sends outputs to the thalamus and red nucleus. Functions : It is involved in planning movements and motor learning. Regulates coordination of muscle activation and is important in visually guided movements. Spinocerebellum/ Paleocerebellum  – comprised of the vermis and intermediate zone of the cerebellar hemispheres. Functions : It is involved in regulating body movements by allowing for error correction. Receives proprioceptive information from spinal nerves Vestibulocerebellum/ Archicerebellum   – the functional equivalent to the flocculonodular lobe. It receives inputs from the vestibular system , and sends outputs back to the vestibular nuclei. Functions : Involved in controlling balance and ocular reflexes, mainly fixation on a target.

6. Functions of the Functional Divisions

7. Cerebellum: Lobules The vermal lobules are assigned Roman Numerals I – X. They can be better appreciated when the cerebellum is divided along the midline of the vermis. The lobules can be better remembered with the aid of the following mnemonic: “ L oving  C aring  C hildren  D onate  F ood  T o  P oor  U nfed  N eedy”. L – Lingula (I) C – Central lobule (II & III) C – Culmen (IV & V) D – Declive (VI) F – Folium (VII A) T – Tuber (VII B) P – Pyramid (VIII) U – Uvula (IX) N – Nodule (X)

8. Cerebellum: Nuclei There are 2 major parts to the cerebellum, the cerebellar deep nuclei and the cerebellar cortex. Output from the cerebellum originates solely in the deep nuclei, where almost all of the neurons of the cerebellum reside. The 4 cerebellar deep nuclei include: Dentate nuclei Emboliform Globose Fastigial ( D on’t E at G reasy F ood ) 3 distinct fiber bundles carry all signals into and out of the cerebellum, these include the  superior cerebellar peduncles.   middle  cerebellar peduncles. inferior cerebellar peduncles.

9. Cerebellum: Pathways The figure demonstrates the afferent nerve (blue) and efferent nerve (red) of the cerebellum Tracts passing through superior cerebellar peduncles: C erebellothalamic tract Cerebellorubral tract Ventral Spinocerebellar tract Tracts passing through middle cerebellar peduncles: Corticopontocerebellar tract Tracts passing through inferior cerebellar peduncles: Cerebellovestibular tract C erebelloreticular tract Vestibulocerebellar tract Reticulocerebellar tract Olivocerebellar tract Dorsal Spinocerebellar tract

10. Cerebellum: Histology Molecular layer Superficially located stellate cells and basket cells are found in this layer. The stellate cells usually bear short dendrites in which make contact with small number of Purkinje cell dendrites. In comparison, basket cells have extensive dendritic processes that can make contact with much larger number of Purkinje cells. Both cells receive excitatory input from the parallel fibers and in turn exhibit inhibitory influence on the Purkinje cells Purkinje cell layer The middle layer (Purkinje cell layer) consists of a single layer of large pear-shaped  Purkinje cells. Their cell bodies are largest in the cerebellum. The dendrites of these cells reside in the molecular layer, while their axons project deep through the granular layer and synapse into the deep nuclei of cerebellum. Granular layer The inner granular layer contains many, tightly packed granule cells and Golgi type II cells .  Granule cells, , usually represent the extensions of the mossy fibers . Their axons extend into the outer molecular layer where they branch in T shape forming parallel fibers and synapse with the dendrites of Purkinje, basket and stellate cells.

11. Cerebellum: Circuitry The  cerebellum  as a whole has  afferent  connections (input) and  efferent  connections (output). Within the cerebellar cortex, ongoing circuits driven by cerebellar input define cerebellar output. Cerebellar afferents (input):   Afferent axons to the cerebellum are excitatory . They send collateral branches to   cerebellar nuclei  before terminating in  cerebellar cortex . The afferents generally excite neurons in cerebellar nuclei and, via cortical circuits, selectively excite Purkinje neurons in cerebellar cortex. Cerebellar afferents include:   mossy fibers : from vestibular nuclei & vestibular nerves multiple spinocerebellar tracts motor-related cerebral cortex, via pontine nuclei    climbing fibers: from the contralateral olivary nucleus Cerebellar cortex circuitry: Mossy fibers  excite  granule cells , whose axons enter the molecular layer and bifurcate into  parallel fibers  that run longitudinally in a folium. The parallel fibers excite bands of   Purkinje cells  ,  basket cells and stellate cells the basket cells and stellate cells inhibit (Feed forward inhibition) Purkinje cells along the edges of the excited band.  Climbing fibers  excite individual Purkinje cells to fire action potentials repetitively.          Axons of selectively excited Purkinje cells inhibit selected neurons within cerebellar nuclei (or within vestibular nuclei in the case of the flocculonodular lobe). Cerebellar efferents (output):   Cerebellar output consists of axons from neurons within   cerebellar nuclei  (or cerebellar cortex of the flocculonodular lobe). The axons synapse on neurons in brainstem motor centers or in thalamic nuclei that project to motor-related cerebral cortex.

12. Cerebellar control

13. Cerebellum: Vasculature Arterial supply The cerebellum receives its blood supply from three paired arteries: Superior cerebellar artery (SCA) Anterior inferior cerebellar artery (AICA) Posterior inferior cerebellar artery (PICA) The SCA and AICA are branches of the  basilar artery , which wraps around the anterior aspect of the pons before reaching the cerebellum. The PICA is a branch of the  vertebral artery . Venous drainage Venous drainage of the cerebellum is by the  superior cerebellar veins   inferior cerebellar veins . They drain into the superior petrosal, transverse and straight dural venous sinuses.

14. Cerebellum Dysfunction Dysfunction of the cerebellum can produce a wide range of symptoms and signs. The clinical picture is dependent on the functional area of the cerebellum that is affected. The symptoms can be remembered using the acronym ‘ 3DAPANISH ’: 3D- Dysdiadochokinesia, Dysmetria, Dizziness Dysdiadochokinesia - inability to engage in rapid alternating movements Dysmetria - inability to judge distances when moving , leading to either overshooting or undershooting the amount of movement required Dizziness - when someone experiences a distinct sense that things are moving even when they are not A- Ataxia Ataxia is the uncontrolled voluntary movement caused by errors in the rate, force, range and the direction of movement P- Pendular Knee Jerks Pendular knee jerks occur due to loss of cerebellar influences on stretch reflexes A- Absence of Rebound phenomenon Rebound phenomenon  is a reflex that occurs when one attempts to move a limb against resistance that is suddenly removed.   Normally : When the resistance is removed, the limb will usually move a short distance due to the braking motion of antagonistic muscles. Absence of the reflex: The patient with cerebellar dysfunction can’t brake the movement of the limb so it flies in the original direction. N- Nystagmus (coarse) Saccadic movement of the eye when patient tries to fixate eyes on one side of the head I- Intention tremor Tremors that occur while performing voluntary motor movements S- Scanning speech/ dysarthria Ataxia of laryngeal muscles causes speech to sound gibberish H- Hypotonia Decrease in muscle tone due to loss of cerebellar influence

15. Cerebellum Dysfunction: Signs

16. Cerebellum: Functional tests

17. Brainstem: Parts The brainstem can be divided into 3 parts: Midbrain The  midbrain  (also known as the mesencephalon) is the most superior of the 3 regions of the brainstem. It acts as a conduit between the forebrain above and the pons and cerebellum below. Pons The   pons  is the largest part of the brainstem, located above the  medulla  and below the  midbrain. Medulla Oblongota The medulla oblongata (medulla) is the most inferior of the three and is continuous above with the pons and below with the spinal cord.

18. Midbrain: External Anatomy The midbrain is the smallest of the three regions of the brainstem, measuring around 2cm in length. As it ascends, the midbrain travels through the opening in the  tentorium cerebelli. It can be divided into two main parts: Tectum  – located posterior to the cerebral aqueduct Paired cerebral peduncles  – located anteriorly and laterally. Internally, the cerebral peduncles are further separated by the substania nigra into the crus cerebri (anterior) and the tegmentum (posterior). Tectum The tectum houses four rounded prominences named  colliculi  (collectively the corpora quadrigemina) which sit directly inferior to the  pineal gland. The colliculi are separated by the cruciform sulcus; there are 2 superior colliculi: centers for visual reflexes 2 inferior colliculi: lower auditory centers Extending laterally from each colliculi are the quadrigeminal brachium: Superior quadrigeminal brachium  forms a pathway between the superior colliculus and the retina of the eye. Inferior quadrigeminal brachium  conveys fibres from the lateral lemniscus and inferior colliculus to the medial geniculate body. Inferior to the colliculi, the  Trochlear nerve  (CN IV)  emerges before sweeping across to the anterior surface .

19. Midbrain: External Anatomy Cerebral Peduncles The paired cerebral peduncles extend from the cerebral hemispheres to converge as they meet the pons. They are separated anteriorly in the midline by the  interpeduncular fossa , the floor of which is termed the posterior perforated substance (as many perforating blood vessels can be identified). The  oculomotor nerve  (CNIII) is seen exiting from between the peduncles while the  optic tract  runs around the superior border of the midbrain.

20. Midbrain: Internal Anatomy Grey matter: oculomotor nucleus- somatic motor of CN III Edinger Westphal nucleus- Parasympathetic nucleus of CN III Mesencephalic nucleus of CN V Pretectal nucleus- involved in visual reflexes Periaqueductal Grey mattter : Control centre for descending pain modulation (inhibits pain sensation). Red Nucleus : Red/ pink in colour due to the presence of iron. It is only found at the level of the superior colliculus and is involved in motor co-odination via the rubrospinal tracts. Substantia Nigra : A large motor nucleus situated between the tegmentum, and the crus cerebri. It is composed of medium-size multipolar neurons that possess neuromelanin. The substantia nigra is concerned with muscle tone and addiction. White Matter: Cerebral Peduncle : contains important descending tracts to contralateral side. Medial Leminiscus : Relays vibration and proprioreciption from contralateral side.

21. Midbrain: Internal Anatomy Grey matter nucleus of trochlear nerve, mesencephalic nucleus of trigeminal nerve. Periaqueductal Grey mattter nucleus of inferior colliculus Substantia nigra. The reticular formation White matter The anteriolateral surface of the midbrain houses the paired crus cerebri. 4 fibre tracts run within the crus: Frontopontine fibres  – located most medially. Corticospinal fibres  – motor fibres from the primary motor cortex. Corticobulbar tracts  – motor fibres from the primary motor cortex. Temporopontine fibres  – located posterolaterally. decussation of the  superior cerebellar peduncles

22. Midbrain: Vasculature & Functions The  midbrain  receives vascular supply from the  basilar artery  and its branches. The major vessels are: Posterior cerebral artery and its peduncular branch Superior cerebellar artery Posterior choroidal artery Interpeduncular branches of the basilar artery. Functions: Co-ordinate eye movements Participates in visual and auditory reflexes Consciousness and arousal

23. Pons Description: The  pons  is the largest part of the brainstem, located above the medulla and below the midbrain. It is a group of nerves that function as a connection between the cerebrum and cerebellum  ( pons is Latin for bridge ). Embryonic origins: The pons develops from the  embryonic metencephalon  (part of the hindbrain, developed from the rhombencephalon), alongside the cerebellum. Anatomical relations: The pons is a horseshoe-shaped collection of nerve fibres located in the anterior part of the posterior cranial fossa. Its anatomical relations are as follows: Posteriorly  – the cerebellum, separated by the fourth ventricle. Inferiorly  – the medulla oblongata. Superiorly  – the midbrain lies immediately above the pons.

24. Pons : External Anatomy Ventral Surface Ventral Surface of the pons is marked by a bulging formed by the  transverse pontocerebellar fibres . These fibres wrap around the otherwise vertically oriented brainstem. It measures around 2.5 cm in adults. The basilar groove demarcates the midline of the ventral surface and is where the basilar artery is located. The  pontomedullary  junction is an important anatomical landmark defined by the angle between the lower border of the pons and the superior border of the medulla. Several cranial nerves originate from the ventral surface of the pons: Cranial nerve V :  trigeminal – originates from the lateral aspect of mid pons Cranial nerve VI: abducens  – originates from the pontomedullary junction, close to the midline Cranial nerve VII: facial  – originates from the cerebellopontine angle, the more lateral aspect of the pontomedullary junction. Cranial nerve VIII:vestibulocochlear  – originates laterally to the facial nerve.

25. Pons : External Anatomy Posterior Surface The pons is intimately related to the cerebellum and is connected to it by the middle cerebellar peduncles. Removal of the cerebellum will reveal the underlying  fourth ventricle . The floor of the fourth ventricle is composed of the dorsal surface of the pons and the medulla. There are some important anatomical landmarks here: The  medial eminence  marks the midline of the floor. The  facial colliculus  is a bulging formed by the fibres of the facial nerve looping around the abducens nucleus. The  stria medullaris  of the fourth ventricle is a bundle of nerve fibres crossing transversely from the lateral aspect into the midline. They mark the posterior border between the pons and the medulla. The angle formed at the junction of the pons, medulla, and cerebellum is another anatomical landmark and is named  cerebellopontine angle . Here, the cerebellar  flocculus , the ventricular  choroid plexus  and the emerging CNs VII and VIII surround the lateral apertures of the fourth ventricle (the foramen of Luschka) .

26. Pons : Internal Anatomy The pons is comprised of 2 major components – the ventral pons and the tegmentum. Ventral pons  contains the pontine nuclei, which are responsible for coordinating movement. Fibres from the pontine nuclei cross the midline and form the middle cerebellar peduncles on their way to the cerebellum. Tegmentum  forms part of the reticular formation – a set of nuclei found throughout the brainstem that are responsible for arousal and attentiveness. Damage to this part of the pons may result in anosognosia for hemiplegia, where patients are unaware of their paralysis. The rest of the pons is made up of tracts passing through the pons including: Descending corticospinal tracts  – responsible for voluntary motor control of the body. Descending corticobulbar tracts  – responsible for voluntary motor control of face, head and neck. Ascending medial leminiscus tracts  – responsible for fine touch, vibration and proprioception. Ascending spinothalmic tracts  – responsible for pain and temperature sensation

27. Pons : Internal Anatomy

28. Pons : Cranial Nerve Nuclei Principal trigeminal sensory nucleus and the  trigeminal motor nucleus  are located in the midpons – at the level where the fibres originate from the lateral aspect of the pons. Abducens nucleus   controls the abducens nerve, which innervates the ipsilateral lateral rectus muscle. It is located in the caudal pons, on the medial aspect of its dorsal surface. Facial nucleus   is located more anteriorly and laterally at the same level of the abducens nucleus. Cochlear  and  Vestibular   nuclei sit dorsolaterally from the inferior pons to the superior medulla.

29. Pons : Vasculature & Functions Vasculature: Arterial supply The blood supply of the pons is formed by branches of the  vertebrobasilar system: Most of the pons is supplied by the  pontine arteries , branches of the basilar artery A smaller part of its blood supply comes from the  anterior inferior cerebellar artery (AICA) and the  superior cerebellar artery  (SCA). Venous drainage consists of the anterior pontomesencephalic vein , which drains superiorly into the basal vein , that in turn drains into the  cerebral veins . Inferiorly, the pons drains into the  inferior petrosal sinus , which drains into the internal jugular veins. Function of the pons : to house the pontine nuclei To facilitate  cortico-ponto-cerebellar comunication . Enables communication between the left and right hemispheres of the cerebellum.

30. Medulla oblongata Description: The medulla oblongata is the direct upward continuation of the spinal cord, extending from foramen magnum to the lower border of the pons. It forms the lowest part of the brainstem. Measurements : Medulla oblongata measures about 3 cm in length, 2 cm in breadth (at the widest part) and 1.25 cm in thickness). Importance: The medulla contains vital centres which are essential for life. These are: (a) cardiac centre, (b) vasomotor centre (c) respiratory centre. Medulla provides attachment to last 4 cranial nerves . Divisions :  It is divided into two parts based on whether or not the fossa is present at a level:  There is an upper, rostral or open part where the dorsal surface of the medulla is formed by the rhomboid fossa. There is a lower, caudal or closed part where the fourth ventricle has narrowed at the obex in the caudal medulla and surrounds part of the central canal. Embryological origins : The medulla develops from the myelencephalon, which is a secondary brain vesicle that arises from the rhombencephalon (the hindbrain). 

31. Medulla oblongata: External Anatomy Anterior Surface There are several structures visible on the anterior surface of the medulla – namely the  3 fissures/sulci , the pyramids , the olives , and 5 cranial nerves. Anterior median fissure is in the midline of the medulla and is continuous along the length of the spinal cord. However, it is interrupted temporarily by the decussation of the pyramids (see below). As we move away from the midline, two sulci are visible – the  ventrolateral sulcus  and the  posterolateral sulcus . Pyramids -  are paired swellings found between the anterior median fissure and the ventrolateral sulcus. Olives -  are another pair of swellings located laterally to the pyramids – between the ventrolateral and posterolateral sulci. 5 cranial nerves : Arising from the junction between the pons and medulla is the abducens nerve (CN VI) . Extending out of the ventrolateral sulcus is the hypoglossal nerve (CN XII). In the posteriolateral sulcus, three more cranial nerves join the medulla (CN IX, CN X, and CN XI).

32. Medulla oblongata: External Anatomy Posterior surface The posterior surface of the medulla can be divided into an upper open area and a lower closed area. Features of the closed part: 3 longitudinal elevations. From medial to lateral these are: fasciculus gracilis, fasciculus cuneatus, inferior cerebellar peduncle. The upper ends of the fasciculus gracilis and fasciculus cuneatus expand to form the gracile and cuneate tubercles respectively due to underlying nuclei of the same name. Another elevation present lateral to cuneate tubercle, the tuber cinereum is produced by the spinal nucleus of trigeminal nerve Features of the open part The open part of the medulla forms the lower part of the floor of fourth ventricle, which presents number of features like, median sulcus , hypoglossal triangles- location of the nucleus of CN XII vagal triangles - location of CN X nucleus vestibular areas, area postrema, Stria medullaris,

33. Medulla oblongata: Internal Anatomy The internal structures of the medulla must be viewed in cross section to understand the layout. 3 levels of the medulla are typically discussed (inferior – superior): Level of decussation of the pyramids Level of decussation of the medial lemnisci Level of the olives Level of the Decussation of the Pyramids This is the major decussation point of the  descending motor fibres . Roughly 75% of motor fibres housed within the  pyramids  cross diagonally and posteriorly, and continue down the spinal column as the lateral corticospinal tracts . The posterior white matter contains the  fasiculus gracilis  and the more lateral  fasiculus cuneatus . Corresponding portions of gray matter extend to these regions and are the nucleus gracilis and nucleus cuneatus respectively. Unchanged from the spinal cord, the  spinocerebellar tracts  (posterior and anterior) are located laterally, with the lateral spinothalamic tract situated between them. The large trigeminal nucleus and tracts can be found posterior to these tracts. This is a continuation of the substantia gelatinosa of the spinal cord.

34. Medulla oblongata: Internal Anatomy Level of Decussation of the Medial Lemniscus This level marks the sensory decussation occurs of the medial lemniscus. Internal arcuate fibres  run from the nucleus gracilis and nucleus cuneatus around and anterior to the central gray matter to form the medial lemniscus. Lateral to the medial lemniscus, the trigeminal nucleus and  spinal tract  can once again be seen, as can the spinocerebellar tracts and the lateral spinothalamic tract. Similarly, the posterior structures are much the same at this level. Centrally, the  hypoglossal nucleus  and medial longitudinal fasciculus are seen. Moving laterally, the nucleus ambiguous can be seen. Between this structure and the pyramids is the  inferior olivary nucleus .

35. Medulla oblongata: Internal Anatomy Level of the Olives The central canal has now expanded into the 4 th ventricle and as such makes this region the  open medulla . The large  inferior olivary nucleus  is responsible for the external expansion of the olives. The related medial and dorsal accessory olivary nuclei can be seen medial and posterior to this structure respectively. The large inferior cerebellar peduncles come into view and are surrounded by multiple nuclei. The two  vestibular nuclei  (medial and inferior) are both found towards the midline while the two cochlear nuclei are found somewhat above and below the peduncles. Now a much smaller structure, the trigeminal tract and nucleus is seen adjacent to the peduncle. The  nucleus ambiguous  remains as it was previously, while the hypoglossal nucleus has migrated with the central canal posteriorly, joined by the medial longitudinal fasciulus. An additional cranial nucleus comes into view lateral to the hypoglossal – the dorsal vagal nucleus. Moving further lateral, the nucleus of tractus solitarius comes into view. Centrally, the  medial lemniscus  hugs the midline posterior to the pyramids, as does the tectospinal tract. Between the peduncle and the olivary nuclei resides the lateral spinothalamic tract and the more lateral anterior spinocerebellar tract.

36. Medulla oblongata: Vasculature Vasculature The vessels that supply the medulla include: the anterior spinal posterior spinal posterior inferior cerebellar anterior inferior cerebellar vertebral arteries . Throughout the medulla, the anterior spinal artery supplies a region beginning at the central canal (or anterior border of the fourth ventricle), and fans out to encompass the pyramids. Below the level of the olives the posterior half of the medulla is supplied by the posterior spinal artery. No other regions are supplied by this vessel. The remaining portions are supplied by the posterior inferior cerebellar and vertebral arteries. In cross section through the olives both the posterior inferior cerebellar and vertebral arteries take on greater territories posterolaterally and anterolaterally respectively. They continue to do so as the medulla ascends. At the highest point in the medulla , the anterior inferior cerebellar artery supplies the outermost portions of the posterior region.

37. Reticular Formation The reticular formation is a vast network of neurons that are involved in maintaining consciousness and initiating arousal. This neuronal tract extends from the spinal cord to the diencephalon  and occupies different parts of the brainstem throughout. The nuclei of the reticular formation are situated deep within the brainstem along its vertical axis. On each half of the brainstem, there is a lateral group of nuclei. medial group of nuclei. median group of nuclei. Functions: Given the vast number of connections of the reticular formation to all parts of the nervous system it serves many functions. A few of the more important functions are considered here. Control of skeletal muscle. Control of facial expression muscles. Control of somatic and visceral sensations. Control of the Autonomic nervous system Control of the endocrine system. Influence on biologic clocks. Reticular activating system.