Structure and Function of the CNS

• The cerebrum is the largest part of the brain, located superiorly and anteriorly in relation to the brainstem.

  It consists of two cerebral hemispheres (left and right), separated by the falx cerebri of the dura mater. Embryologically, the cerebrum is derived from the prosencephalon.
• The cerebrum is located within the bony cranium. It extends from the frontal bone anteriorly to the occipital bone posteriorly.

  Within the skull, the cerebrum fills the anterior and middle cranial fossae, and is located above the tentorium cerebelli inferoposteriorly.
• The cerebrum is comprised of two different types of tissue – grey matter and white matter:

  • Grey matter forms the surface of each cerebral hemisphere (known as the cerebral cortex), and is associated with processing and cognition.
  • White matter forms the bulk of the deeper parts of the brain. It consists of glial cells and myelinated axons that connect the various grey matter areas.
• Externally, the cerebrum has a highly convoluted appearance, consisting of sulci (grooves or depressions) and gyri (ridges or elevations). It is divided into two anatomically symmetrical hemispheres by the longitudinal fissure – a major sulcus that runs in the median sagittal plane. The falx cerebri (a fold of dura mater) descends vertically to fill this fissure. The two cerebral hemispheres are connected by a white matter structure, called the corpus callosum.
• The main sulci are:

  • Central sulcus – groove separating the frontal and parietal lobes.
  • Lateral sulcus – groove separating the frontal and parietal lobes from the temporal lobe.
  • Lunate sulcus – groove located in the occipital cortex.
• The main gyri are:

  • Precentral gyrus – ridge directly anterior to central sulcus, location of primary motor cortex.
  • Postcentral gyrus – ridge directly posterior to central sulcus, location of primary somatosensory cortex.
  • Superior temporal gyrus – ridge located inferior to lateral sulcus, responsible for the reception and processing of sound.
• The frontal lobe is located beneath the frontal bone of the calvaria and is the most anterior region of the cerebrum. It is separated from the parietal lobe posteriorly by the central sulcus and from the temporal lobe inferoposteriorly by the lateral sulcus.

  The association areas of the frontal lobe are responsible for: higher intellect, personality, mood, social conduct and language (dominant hemisphere side only).
• The parietal lobe is found below the parietal bone of the calvaria, between the frontal lobe anteriorly and the occipital lobe posteriorly, from which it is separated by the central sulcus and parieto-occipital sulcus, respectively. It sits superiorly in relation to the temporal lobe, being separated by the lateral sulcus.

  Its cortical association areas contribute to the control of: language and calculation on the dominant hemisphere side, and visuospatial functions (e.g. 2-point discrimination) on the non-dominant hemisphere side.
• The temporal lobe sits beneath the temporal bone of the calvaria, inferior to the frontal and parietal lobes, from which it is separated by the lateral sulcus.

  The cortical association areas of the temporal lobe are accountable for memory and language – this includes hearing as it is the location of the primary auditory cortex.
• The occipital lobe is the most posterior part of the cerebrum situated below the occipital bone of the calvaria. Its inferior aspect rests upon the tentorium cerebelli, which segregates the cerebrum from the cerebellum. The parieto-occipital sulcus separates the occipital lobe from the parietal and temporal lobes anteriorly.

  The primary visual cortex (V1) is located within the occipital lobe and hence its cortical association area is responsible for vision.
• The blood supply to the cerebrum can be simply classified into 3 distinct paired arterial branches:

  • Anterior Cerebral Arteries – branches of internal carotid arteries, supplying the anteromedial aspect of the cerebrum.
  • Middle Cerebral Arteries – continuation of internal carotid arteries, supplying most of the lateral portions of the cerebrum.
  • Posterior Cerebral Arteries – branches of the basilar arteries, supplying both the medial and lateral sides of the cerebrum posteriorly.
• Venous drainage of the cerebrum is via a network of small cerebral veins. These vessels empty into the dural venous sinuses – endothelial lined spaces between the outer and inner layers of dura mater.
• A cerebrovascular accident (also known as a stroke) is defined clinically as “an abrupt loss of focal brain function lasting more than 24 hours due to either spontaneous haemorrhage into brain substance or inadequate blood supply to part of the brain i.e. ischaemia (thrombosis, embolism)“.

  Damage to the cerebrum in this matter can give rise to a range of clinical signs. The exact nature of the functional deficit that arises depends on the specific lobe that has been affected.
• • Frontal lobe stroke– a diverse range of presentations, often personality and behavioural changes occur and an inability to solve problems develops.
• • Parietal lobe lesion  – typically presents with attention deficits e.g. contralateral hemispatial neglect syndrome: where the patient does not pay attention to the side of the body opposite to the lesion.
• Temporal lobe lesion – presents with recognition deficits (agnosias) e.g. auditory agnosia: patient cannot recognise basic sounds, prosopagnosia: failure to recognise faces
• • Occipital lobe lesion– visual field defects: contralateral hemianopia or quadrantanopia with macular sparing.
• Global lesions – severe cognitive deficits (dementia), patients cannot answer simple questions such as their name, today’s date, where they are etc.
• Neural Tube Flexures

  Two ventrally directed foldings:

  • Mesencephalic (Cephalic) Flexure – centred at the midbrain region.
  • Cervical Flexure – located near the junction between the Myelencephalon and the Spinal Cord.

  One dorsally directed folding:

  • Pontine Flexure – located at the region of the pons.
• The cerebral hemisphere is the largest part of the forebrain. It is divided into four lobes:

  • Frontal
  • Parietal
  • Temporal
  • Occipital
• The nerve cell bodies which form the grey matter are located on the outer layer of the cerebral cortex and brainstem. The nerve fibres that form the white matter are located in the inner layer. Aggregates of nerve cell bodies called nuclei are also contained within the inner

  layer of the cerebral cortex and brainstem.
• The cerebellum has an important role in motor control, with cerebellar dysfunction often presenting with motor signs. It is active in the coordination, precision and timing of movements, as well as in motor learning.
• During embryonic development, the anterior portion of the neural tube forms three parts that give rise to the brain and associated structures:

  • Forebrain (prosencephalon)
  • Midbrain (mesencephalon)
  • Hindbrain (rhombencephalon)

  The hindbrain subsequently divides into the metencephalon (superior) and the myelencephalon (inferior). The cerebellum develops from the metencephalon division.
• 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.
• The cerebellum consists of two hemispheres which are connected by the vermis, a narrow midline area. Like other structures in the central nervous system, the cerebellum consists of grey matter and white matter:

  • Grey matter – located on the surface of the cerebellum. It is tightly folded, forming the cerebellar cortex.
  • White matter – located underneath the cerebellar cortex. Embedded in the white matter are the four cerebellar nuclei (the dentate, emboliform, globose, and fastigi nuclei).
• There are three anatomical lobes that can be distinguished in the cerebellum; the anterior lobe, the posterior lobe and the flocculonodular lobe. These lobes are divided by two fissures – the primary fissure and posterolateral fissure.
• There are three cerebellar zones. In the midline of the cerebellum is the vermis. Either side of the vermis is the intermediate zone. Lateral to the intermediate zone are the lateral hemispheres. There is no difference in gross structure between the lateral hemispheres and intermediate zones
• Three functional areas of cerebellum:

  • Cerebrocerebellum – largest division, lateral hemispheres. Planning movements and motor learning. Inputs: cerebral cortex, pontine nuclei. Outputs: thalamus, red nucleus. Regulates coordination of muscle activation, visually guided movements.
  • Spinocerebellum – vermis and intermediate zone. Regulates body movements by allowing for error correction, receives proprioceptive information.
  • Vestibulocerebellum – functional equivalent flocculonodular lobe. Controls balance and ocular reflexes, target fixation. Input: vestibular system. Output: vestibular nuclei.
• 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 of the cerebellum is by the superior and inferior cerebellar veins. They drain into the superior petrosal, transverse and straight dural venous sinuses.
• Dysfunction of the cerebellum can produce a wide range of symptoms and signs. The aetiology is varied; causes include stroke, physical trauma, tumours and chronic alcohol excess.

  The clinical picture is dependent on the functional area of the cerebellum that is affected. Damage to the cerebrocerebellum and spinocerebellum presents with problems in carrying out skilled and planned movements and in motor learning.
• Symptoms of cerebellar dysfunction

  • Dysdiadochokinesia (difficulty in carrying out rapid, alternating movements)
  • Ataxia
  • Nystagmus (coarse)
  • Intention tremor
  • Scanning speech
  • Hypotonia
• Damage to the vestibulocerebellum can manifest with loss of balance, abnormal gait with a wide stance.
• A collection of nerve fibre bundle that serve a particular function is called nerve tract. Nerve tracts run within the white matter.
• The vast majority of the axons that pass between the cerebral cortex and subcortical structures form a condensed broad sheet of white matter called the internal capsule.
• The two cerebral hemispheres are incompletely separated by a deep cleft called the great longitudinal fissure.

  In the depth of the great longitudinal fissure, the two hemispheres are united by nerve fibres called the Corpus Callosum.
• Between the internal capsule and the cerebral cortex are radiating nerve fibres in a fan-like arrangement called the Corona Radiata. Buried within the white matter are a number of nuclear masses.

  Example: Basal Ganglia or Corpus Striatum:

  • Caudate nucleus
  • Putamen
  • Globus pallidus
• The cerebral cortex is involved in conscious awareness, thought, memory and intellect.

  All sensory modalities relay to the cerebral cortex mostly via the thalamus for conscious perception and experiential interpretation.

  The cerebral cortex serves as the highest level for the representation of the motor system for the conception and initiation of actions.
• The posterior part of the cerebral cortex which include Parietal lobe for

  somatosensory, Occipital lobe for vision, and Temporal lobe for hearing, are involved in receiving sensory information from the outside environment.

  The adjacent association cortices elaborate the information in a multimodal and spatial modality-specific perception - touch, sight and hearing.

  The anterior part (frontal lobe) is involved with the organisation of movements (primary motor, pre-motor and supplementary motor areas), and strategic guidance of complex motor behaviours over time (pre-frontal area).
• Frontal lobe syndrome:

  • Difficulty initiating behaviour.
  • Inability to stop a behavioural pattern.
  • Difficulties in planning and problem solving.
  • Incapable of creative thinking.
• Apraxia:

  • Difficulty with planning and performing motor activities.