Pons and Cerebellum
The pons
-middle part of the brain stem.
-connecting the midbrain with the medulla.
-pons means “bridge”. It connects the spinal cord with the brain and links parts of the brain with one another by the tracts'
- relays nerve impulses related to voluntary skeletal movements from the cerebral cortex to the cerebellum.
- It contains the nuclei of V-VII cranial nerves.
- has the pneumotaxic and apneustic centers which help to control respiration.
External structure
- two surfaces, ventral and dorsal
- ventral or anterior surface is convex in both direction and is transversely striated.
- median plane it shows a vertical basilar groove, sulcus basilaris which lodges the basilar artery.
- Laterally the surface is continuous with the middle cerebellar peduncles, pedunculi cerebellares medii.
- formed by the axons of pontine nuclei
- connect the right and left sides of the pons with the cerebellum.
- rootlets of the trigeminal nerve (V pair of cranial nerve) emerge at the junction of the pons with the peduncle.
- rootlets of the abducent (VI pair of cranial nerve), facial (VII pair of cranial nerve) and vestibulocochlear (VIII pair of cranial nerve) nerves emerge at the lower border of the ventral surface.
- line that connects the roots of the trigeminal and facial nerves is called linea trigeminofacialis – this is the border line between the middle cerebellar peduncle and the pons.
- dorsal or posterior surface is hidden by the cerebellum, and forms the upper half of the floor of the fourth ventricle that is bordered with the upper cerebellar peduncles, pedunculi cerebellares superiores.
- These peduncles are the structures of isthmus rhombencephali and connect the midbrain with the cerebellum.
Internal structure
- transverse section, the pons is seen to be divisible into ventral and dorsal parts.
- Between the dorsal and ventral parts is the trapezoid body, corpus trapezoideum and its nucleus, nucl. dorsalis corporis trapezoidei – a transverse band of fibers that arise in the cochlear nuclei (VIII pair of cranial nerve) of both sides.
part of the auditory pathway.
- ventral side contact each side cerebellam and saame as medula oblongata through middle cerellar peduncles.
- basilar part of the pons has a uniform structure throughout its length.
-grey matter is presented by the pontine nuclei which are scattered among longitudinal and transverse fibers.
- pontine nuclei form an important part of the pontocerebellar pathway.
-Fibers from these nuclei go to the opposite half of the cerebellum through the middle cerebellar peduncle.
- The white matter consists also of the corticospinal and corticonuclear (pyramidal) tracts and the corticopontine fibers, ending in the pontine nuclei.
- dorsal or tegmental part is a continuation of the medulla (excluding the pyramids).
- grey matter of this part includes the nuclei of V-VIII pairs of cranial nerves.
- white matter consists of the following fibers – lemniscus medialis, fasciculus longitudinalis medialis and formatio reticularis.
The cerebellum
The cerebellum (little brain) is the largest part of the hindbrain
situated in the posterior cranial fossa behind the pons and medulla are situated
important functions for maintaining muscle tone, posture equilibrium and movements of the body
subcortical structure that coordinates voluntary movements of the body
serves to smooth and coordinate complex sequences of skeletal muscle contractions
makes possible all skilled motor activities (catching, dancing, swimming etc.). Besides coordinating skilled movements, the cerebellum is one of the autonomic (vegetative) nervous system centers; it controls the activity of internal organs, glands and the process of regeneration.
consists of two cerebellar hemispheres, hemispheria cerebelli, that are united to each other through a median vermis, vermis.
vermis has several parts: lingula, the central lobule, nodule etc.
external surface of the cerebellum is covered by the layer of grey matter; that is cerebellar cortex.
narrow ridges – folia cerebelli
inferior surface of each hemisphere there is an isolated lobule – flocculus.
flocculus is connected with a nodule of the vermis with the help of the pedunculus floccularis.
Morphological and functional divisions of the cerebellum:
1.The archicerebellum phylogenetically is the oldest part of the cerebellum.
- made of the flocculus and the lingula of the vermis.
- chiefly vestibular in its connections
- controls the axial body musculature and the bilateral movements used for locomotion and maintenance of equilibrium.
2.The paleocerebellum is the next part of the cerebellum to appear.
- connections are chiefly spinocerebellar
- controls tone, posture and crude movements of the limbs.
3.The neocerebellum is the newest part of the cerebellum
- It is made up with the hemispheres
- It is primarily concerned with the regulation of fine movements of the body.
Deep to the gray matter are white matter tracks called arbor vitae that resembles branches of a tree.
Deep within the white matter are masses of gray matter.
- the cerebellar nuclei. They are:
1) the nucleus dentatus (neocerebellar);
2) the nucleus globosus and
3) the nuceus emboliformis are paleocerebellar
4) the nucleus fastigii is archicerebellar
The nuclei give rise to nerve fibers that convey information out of the cerebellum to other parts of the brain and the spinal cord.
fibers entering or leaving the cerebellum are grouped to form the three peduncles which connect the cerebellum to the midbrain, the pons and the medulla.
1. Inferior cerebellar peduncles, pedunculi cerebellares inferiores,
connect the cerebellum with the medulla.
The constituent fibers passing through them are:
1) tractus spinocerebellaris posterior;
2) fibrae olivocerebellares;
3) tractus vestibulocerebellaris et cerebellovestibularis;
4) fibrae arcuatae externae – from the nuclei gracilis et cuneatus in the medulla oblongata.
2. Middle cerebellar peduncles, pedunculi cerebellares medii,
connect the cerebellum with the pons,
they are formed by the fibers starting from the pontine nuclei (nn. pontini).
They connect the pons with the cerebellum and form tr. pontocerebellaris.
3. Superior cerebellar peduncles, pedunculi cerebellares superiores,
connect the cerebellum with the midbrain.
They are formed due to the tractus spinocerebellaris anterior
and tractus cerebellotegmentalis (connects the cortex of the cerebellum and nucleus dentatus with the gray matter of the midbrain tegmentum (nucleus ruber, substantia nigra and formatio reticularis)).
Cerebellar lesions give rise to symptoms and signs which together constitute the cerebellar syndrome.
It is characterized by muscular hypotonia;
intention tremor (tremor only during movements) tested by finger-nose and heel-knee tests;
adiadochikinesia which is the inability to perform rapid and regular alternating movements, like pronation and supination;
nystagmus (to- and fro-oscillatory movements of the eyeballs while looking to either side); scanning speech (jerky and explosive) and ataxic gait (unsteady gait).
Isthmus rhombencephali
junction of the rhombencephalon with the mesencephalon.
includes:
1) superior cerebellar peduncles;
2) superior medullary velum, velum medullare superius – the plate situated between the superior cerebellar peduncles;
3) trigonum lemnisci, – the triangle area that is placed laterally from the superior cerebellar peduncles
- borders with the cerebral peduncle by súlcus laterális mesencéphali and the brachium of the inferior collicle of the midbrain.
- triangle contains the auditory fibers of the lateral lemniscus.
isthmus of rhombencephalon the narrow segment of the fetal brain, forming the plane of separation between the rhombencephalon and cerebrum
The fourth ventricle
ventriculus quartus is the cavity of the rhombencephalon
fourth ventricle is a tent-shaped space situated between the pons and the medulla in front, and the cerebellum behind.
floor of the fourth ventricle is formed by the posterior surface of the pons and the medulla; also called the rhomboid fossa because it is rhomboidal in shape.
roof of the fourth ventricle, tegmen is tent-shaped, projecting into the cerebellum
- upper part is formed by superior medullary velum that is stretched between the superior cerebellar peduncles.
- Inferiorly the roof is more complex but mostly a thin sheet
- there nothing(devoid) any nervous tissue
-formed by the ventricular ependyma and the pia mater of the tela choroidea inserted in the structure of the paired velum medullare inferius that lie laterally.
- The inferior medullary velum fills the angle between the flocullar peduncles.
The tela choroidea of the fourth ventricle is a double layer of pia mater tela vascular fringes form the fourth ventricle’s
choroid plexuses are networks of capillaries covered by ependymal cells that form cerebrospinal fluid from blood plasma by filtration and secretion.
Superiorly, the fourth ventrical communicates with the third ventricle by the cerebral aqueduct.
Inferiorly it is continuous with the central canal of the spinal cord.
Dorsally in lower part of the roof there is a median aperture (foramen of Magendi), apertura mediana ventriculi quarti and
two lateral apertures (foramen of Luschka), aperturae laterales ventriculi quarti.
Through this aperture the ventricle communicates with the subarachnoid space.
The rhomboid fossa
Rhomboid, fossa romboidea, is the floor of the fourth ventricle. It is formed by the posterior surface of the pons and the posterior surface of the medulla.
borders of the rhomboid fossa are:
superior cerebellar peduncles (superior border) and inferior cerebellar peduncles (inferior border).
Deep to the floor there is a layer of grey matter containing various cranial nerves. floor is lined by ependyma.
The fossa is divided by a median groove, sulcus medianus, flanking which are paired medial eminences, eminentiae mediales.
groove divides the floor into right and left halves..
superior area each eminence is expanded into facial colliculus, colliculus facialis, which is formed by the underlying motor nucleus of the VI pair of cranial nerves and rootlets of the motor nucleus of the facial nerve.
In the superior area each eminence is expanded into facial colliculus, colliculus facialis, s formed by the underlying motor nucleus of the VI pair of cranial nerves and rootlets of the motor nucleus of the facial nerve.
e inferior area each medial eminence is a hypoglossal triangle, trigonum n. hypoglossi.
Laterally from the hypoglossal triangle, there is a vagal triangle, trigonum n. vagi.
The vestibular area, area vestibularis lies in the lateral sides of the fossa.
Here are the auditory and vestibular nuclei of the VIII pair of cranial nerves.
In the rhomboid fossa, there are the nuclei of XII-V pairs of cranial nerves.
XII pair, n. hypoglossus, has one motor nucleus that is placed deep in the trigonum nervi hypoglossi.
XI pair, n. accessorius, motor, has two nuclei: nucleus n. accessorii, that elongates up till the spinal cord and nucleus ambiquus, common with the nucleus of X and IX pairs.
X pair, n. vagus, mixed, has three nuclei: motor – nucleus ambiguus, common with the nucleus of XI and IX pairs, sensitive – nucleus solitarius, common with the nucleus of IX and VII pair and vegetative – nucleus dorsalis n. vagi, placed in the vagal triangle.
IX pair, n. glossopharyngeus, mixed, has three nuclei: motor – nucleus ambiguus, common with the nucleus of XI and X pairs, sensitive – nucleus solitarius, common with the nucleus of X and VII pairs and vegetative – nucleus salivatorius inferior, placed in formatio reticularis of the medulla.
VIII pair, n. vestibulocochlearis, sensitive, consists of two parts: 1) cochlear nerve (auditory), n. cochlearis, has ventral and dorsal nuclei; 2) vestibular (static), n. vestibularis, has four nuclei (superior, inferior, medial, lateral). These nuclei are placed in the vestibular areas of the fossa.
VII pair, n. facialis, motor, has one motor nucleus that is placed in the colliculus facialis. The VII pair also adjoins nervus intermedius, that meets the facial nerve after its origin. The intermedial nerve is mixed, has two nuclei: sensitive – nucleus solitarius, common with the nucleus of X and IX pairs and vegetative – nucleus salivatorius superior, placed in formatio reticularis of the pons.
VI pair, n. abducens, motor, has one motor nucleus V pair, n. trigeminus, mixed, has three sensitive nuclei: nucleus spinalis, nucleus pontinus, nucleus mesencephalicus, and one motor - nucleus motorius, placed in the anterior portion of the rhomboid 29 fossa. The spinal nucleus is like the elongation of the pontine nucleus and reaches the upper cervical segments of the spinal cord.
