Learning & Test Objectives
- The structural and functional organization of the lower CNS.
- Cerebellum external and internal structure. Its neuronal connections.
- Brain stem components and their external and internal structure.
- Rhomboid fossa. Topography of the nuclei.
- Spinal cord divisions. External and internal structure.
- Pathways (tracts) within spinal cord, brain stem, cerebellum (excluding diencephalon and neocortical projections) and their function.
- Blood supply of the CNS.
Description of the test
The test is held by the general rules of written tests (see Continuous Testing – Organisation).
The test of the central nervous system is in written format and follows the general rules for written tests (see Continuous testing – organisation). The test will be focused on the spinal cord, brain stem, cerebellum, reticular formation, pathways (tracts) of the spinal cord and brainstem, meninges, infratentorial subarachnoid cisterns, the circle of Willis and the dural venous sinuses. For the spinal cord students should know its external appearance, division, internal structure (organisation of grey and white matter), blood supply and main functions. Other topics included in the test are the general structure of the central nervous system, basic histology and embryology of the brain, clinical and other interesting correlations and radiological images. At the end of the test the student is required to draw and described one of the required schemes.
In the CNS, oligodendrocytes produce myelin. In the PNS, Schwann cells have this function. Oligodendrocytes and Schwann cells differ in their ability to regenerate damaged neuronal projections. Oligodendrocytes are limited in this function; injury of the CNS is almost always permanent. In contrast, Schwann cells are endowed with a better ability to regenerate damaged nerve tissue. Thus, severed peripheral nerves can be treated by neurosurgical microsuturing, which helps the Schwann cells repair the damaged axons.
The pyramidal system consists of two bineuronal tracts: the corticospinal and corticobulbar tracts.
The first order neurones are located in the cerebral cortex and are referred to as upper motor neurones (UMN). The second order neurones are located in the spinal cord (corticospinal tract) and in the brainstem (corticobulbar tract) and are referred to as lower motor neurones (LMN). The clinical manifestations of upper and lower motor neurone diseases are very different.
Conus medullaris syndrome is caused by damage to the conus medullaris (S3–S5). It manifests as a decrease or complete loss of sensation in a saddle-shaped distribution in the perianal, perigenital and inner femoral regions, faecal and urinary incontinence and erectile dysfunction. The short flexors of the toes are affected, but this does not cause a noticeable loss of function.
Epiconus syndrome is caused by damage to the epiconus (L5 to S2). It manifests as palsy and atrophy of the extensors of the foot and muscles of the leg. Extension and flexion of the foot becomes difficult or even impossible. Sensation is impaired in dermatomes L5 to S2. Bowel and bladder dysfunction can also sometimes occur.
Persistent vegetative state (formerly known as apallic syndrome and coma vigile) is a condition in which a patient is not able to react to external stimuli or make voluntary movements. Reflexes are usually intact, as is the sleep-wake cycle. The patient is in a state of non-awareness, but is able to breathe without support. The vegetative state results from extensive damage to the cortex and its connections to subcortical nuclei or from bilateral damage of the thalami.
Elevated intracranial pressure (intracranial hypertension) can lead to herniation of the cerebellar tonsils through the foramen magnum. This can quickly lead to death by compression of the cardiorespiratory centres in the reticular formation of the brainstem.
Disorders of the vestibulocerebellum and spinocerebellum manifest as gait disturbances, vertigo, nystagmus, titubation (rhythmic tremor of the head or trunk) and falls. Neocerebellar damage is characterised by ataxia, dysmetria, intention tremor (a tremor that increases in intensity during movement and is not present at rest), dysdiadochokinesis (impairment of rapid repetitive movements), saccadic movements (disruption to the fluidity of movements), dysarthria, megaphonia and macrographia. Movements are fragmented and speech is saccadic (jerky and irregular).
Arteria haemorrhagica of Charcot is a clinical term for the anterolateral central arteries and their lateral striate branches (clinically termed the lenticulostriate arteries). These arteries are susceptible to damage from hypertension. Rupture of these arteries causes bleeding and both ischaemic and mechanical damage to the internal capsule and can lead to paresis of the contralateral side of the body.
Other interesting notes
The CNS contains more than 100 billion neurones in childhood. During development many of these neurones perish in the process of programmed cell death. Other neurones become specialised. They sprout projections and connect to various neuronal circuits and pathways. The final result is the creation of a complex network known as the connectome of the human brain.
The ascent of the spinal cord (ascensus medullae spinalis) is a term used for non-proportional growth of the vertebral column and spinal cord. The vertebral column grows faster so that in the adult the caudal end of the spinal cord lies between the first and second vertebrae. To reduce the risk of damaging the spinal cord, lumbar puncture is performed beneath this, typically between the third and fourth lumbar vertebrae. At this level, the needle penetrates into the extension of the subarachnoid space (lumbar cistern) where only the cauda equina passes.
The decussation of the medial lemniscus (decussatio lemnisci medialis), also known as the sensory decussation, is the crossing of axons of the spino-bulbo-thalamo-cortical tract after they synapse in the gracile and cuneate nuclei.
The inferior olivary complex (complexus olivaris inferior) contains the inferior, posterior and accessory olivary nuclei.
The pars compacta of the substantia nigra and the red nucleus are easily visible on magnetic resonance imaging due to their high iron content.
The substantia nigra has a dark colour due to the production of the pigment neuromelanin.
The interpeduncular nucleus integrates the olfactory and limbic systems with the nuclei of the autonomic nervous system in the brainstem and spinal cord. It plays a role sleep-wake cycle by regulating the rapid eye movement phase of sleep.
The ventral tegmental area of Tsai (area ventralis tegmentalis) delivers dopamine to the cerebral cortex via the mesocortical tract and to the nucleus accumbens via the mesolimbic tract. It is known as the centre of love and addiction because an elevated concentration of dopamine in the cortex and nucleus accumbens evokes happiness, joy, desire and craving (besides other feelings).
Motor memory is involved in creating and imprinting permanent motor circuits. Structures responsible for motor memory include the premotor area (area 6), association areas, basal ganglia, hippocampus and cholinergic nuclei. The cerebellum plays an important role in verbal working memory, primarily through connections between Broca's motor speech area and the right cerebellar hemisphere.
Injury to the cerebellum can thus lead to language disorders.
The carotid syphon (sipho caroticus) is a term for the sinuous course of the intracranial part of the internal carotid artery. The cavernous part has a dorsocranially oriented convexity; the cerebral part has a ventrally oriented convexity.
In clinical practice the vessels of the brain are often referred to by the following abbreviations:
ICA – internal carotid artery
ACA – anterior cerebral artery
MCA – middle cerebral artery
PCA – posterior cerebral artery
PICA – posterior inferior cerebellar artery
AICA – anterior inferior cerebellar artery
SCA – superior cerebellar artery
Pcom – posterior communicating artery
Acom – anterior communicating artery
Required schemes for drawing
- Cross section through of the spinal cord – tracts, nuclei and Rexed laminae
- Topography of the vertebral canal
- Rhomboid fossa illustrating the positions of the cranial nerves nuclei topographically.
- Cross section of the medulla oblongata (rostral section)
- Cross section of the pons (caudal section)
- Cross section of the midbrain (rostral section)
- The circle of Willis