CNS anatomy

• Cerebrum external structure:

  ·       Highly convoluted appearance consisting of sulci (grooves or depressions) and gyri (ridges or elevations)

  ·       Largest part of the brain, located superiorly and anteriorly in relation to the brainstem

  ·       Two cerebral hemispheres (left and right)

  ·       It is divided into two anatomically symmetrical hemispheres by longitudinal fissure (major sulcus)

  ·       The falx cerebri (fold of dura mater) descends vertically to fill this fissure

  ·       Two cerebral hemispheres are connected by the corpus callosum
• Main sulcus:

  • Central sulcus – separates 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
• Main Gyri:

  • 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
• Internal structure of the cerebrum:

  • Grey matter - surface of each cerebral hemisphere - processing and cognition
  • White matter - forms bulk of the deep brain - glial cells and myelinated axons - internal capsule carries the ascending and descending tracts
• Frontal lobe:

  ·       Most anterior region

  ·       Separated from the parietal lobe posteriorly by the central sulcus

  ·       Separated from the temporal lobe inferoposteriorly by the lateral sulcus

  ·       Planning and commanding voluntary movements

  ·       Higher intellect, personality, mood, social conduct and language (if dominant side)
• Parietal lobe:

  ·       Between the frontal lobe anteriorly and the occipital lobe posteriorly

  ·       Separated from the frontal lobe by the central sulcus

  ·       Separated from occipital lobe by the parieto-occipital sulcus

  ·       Separated from the temporal lobe by the lateral sulcus

  ·       Dominant side – control of language and calculation

  ·       Non – dominant side – visuospatial functions (e.g. 2 point discrimination)
• Temporal lobe:

  ·       Separated by the lateral sulcus

  ·       Receives and processes auditory information

  ·       Location of primary auditory cortex
• Occipital lobe:

  ·       Most posterior part of the cerebrum

  ·       Separated by the parieto-occipital sulcus

  ·       Location of the primary visual cortex
• Functional motor areas of the cerebrum:

  ·       Primary motor cortex – anterior to central sulcus – provides the most important signal for the production of voluntary skilled movements. Electrical stimulation of this area results in focal movements of muscle groups on the opposite side of the body

  ·       Premotor cortex – movements of groups of muscles with common function (less precise). Includes Broca’s area which is responsible for the motor element of speech

  ·       Prefrontal cortex – complex human attributes such as solving complex multitask problems
• Functional sensory areas:

  • Primary sensory cortex – posterior to central sulcus- all forms of somatic sensation.
  • Somatosensory association cortex – integration of somatic sensory information from the primary sensory cortex with other sensory information. Includes visual and auditory association area
  • Primary visual cortex – is a primary sensory cortex located in the occipital lobe
  • Primary auditory cortex – located in the temporal lobe
  • Wernicke's area - sensory aspect of language in the dominant hemisphere
• The basal ganglia:

  ·       Consists of a number of subcortical nuclei

  ·       Grouping of these nuclei is related to function rather than anatomy

  ·       Part of a basic feedback circuit, receiving information from several sources including the cerebral cortex

  ·       The basal ganglia feeds this information back to the cortex via the thalamus

  ·       Acts to modulate and refine cortical activity – controlling descending motor pathways through either inhibitory or excitatory synapses

  ·       Prevents excessive and exaggerated movements

  ·       Modulates cognitive and emotional responses
• ·       Arterial supply to the basal ganglia comes mainly from the middle cerebral artery – a continuation of the internal carotid artery
• ·       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 . Most common affected artery in stroke.
• ·       Posterior cerebral arteries – branches of the basilar arteries supplying both the medial and lateral sides of the cerebrum posteriorly
• Internal carotids and vertebral arteries are paired arteries that arise in the neck

  2 vertebral arteries converge to form the basilar artery

  Within the cranial vault, the terminal branches of the internal carotids and basilar artery form the circle of willis
• The circle of willis is within the sub-arachnoid space. Provides collateral blood flow between the anterior and posterior circulation. Protects against ischaemia in the event of vessel disease or damage in one or more areas
• Dural venous sinuses:

  ·       Multiple venous channels within the cranial cavity which are sandwiched between the two layers of the dura mater

  ·       The venous system represents the main pathway of returning venous blood from the brain into the circulation via the internal jugular vein

  ·       The dural venous sinuses receive blood from the veins associated with the cerebrum, cerebellum and brainstem

  ·       Venous blood then flows through the dural system and eventually empties into the internal jugular veins, where deoxygenated blood is returned to the systemic circulation
• Cerebellum anatomy:

  ·       Located at the back of the brain

  ·       Immediately inferior to the occipital and temporal lobes – separated by the tentorium cerebelli (tough layer of dura mater)

  ·       Lies at the same level of and posterior to the pons – separated by the forth ventricle

  The cerebellum consists of two hemispheres which are connected by the vermis

  ·       Consists of grey and white matter

  ·       The cerebellum can be divided in 3 ways: anatomical lobes, zones and functional divisions
• Cerebellar function:

  • Important role in motor control
  • Co-ordination, precision and timing of movement
  • Motor learning
  • Receives information regarding balance from vestibular nuclei
  • Controls ipsilateral side of the body
• Causes of cerebellar dysfunction- posterior stroke, MS, tumour, trauma and alcohol excess. Signs of dysfunction can be remembered with DANISH mnemonic
• Cerebellar dysfunction:

  ·       D= DYSDIADOCHOKINESIA

  ·       A = ATAXIA (coordination, BALANCE, and speech)- ataxic gait, heel to shin test

  ·       N = NYSTAGMUS (rhythmical involuntary movements of eyes)

  ·       I = INTENTION TREMOR

  ·       S = SCANNING speech (broken up into syllables)

  ·       H = HYPOTONIA
• The cerebellum is supplied by 3 paired arteries

  • The superior cerebellar artery and anterior inferior cerebellar artery are branches of the basilar artery
  • Posterior inferior cerebellar artery is a branch of the vertebral artery
• The brain stem = midbrain, pons and medulla oblongata
• The midbrain:

  ·       Most superior of the three regions of the brainstem

  ·       Acts as a conduit between the forebrain above and the pons and cerebellum below

  ·       Receives vascular supply from the basilar artery and its branches
• Pons:

  ·       Largest part of the brainstem located above the medulla and below the midbrain

  ·       Functions as a connection between the cerebrum and cerebellum

  ·       Cranial nerve nuclei are located in the dorsal pons
• Medulla oblongata:

  ·       Houses essential ascending and descending nerve tracts as well as brainstem nuclei

  ·       Medulla exits the skull through the foramen magnum

  ·       Inferior margin is marked by the origin of the first pair of cervical spinal nerves

  ·       Anterior surface gives rise to cranial nerves
• Function of the ventricles:

  • Production, transport and removal of CSF
  • Protection - limits neural damage in cranial injuries
  • Buoyancy
  • Chemical stability - maintains low extracellular potassium for synaptic transmission
• The ventricles of the brain are lined with ependymal cells, which form a structure called the choroid plexus - where CSF is produced
• There are 4 ventricles in the brain - right and left lateral ventricles, third and fourth ventricles

  The fourth ventricle is the last in the system, and receives CSF from the third ventricle via the cerebral aqueduct

  From the fourth ventricle CSF drains either into the central spinal canal or into the subarachnoid space where it is reabsorbed back into the circulation
• Drainage of the CSF occurs in the subarachnoid space
• The meninges are the membranous coverings of the brain and spinal cord. Consists of three layers - dura mater, arachnoid mater and pia mater. The major functions of the meninges are to provide a supported network and to allow blood supply to the brain
• The dura mater:

  • outermost layer of the meninges
  • two layered sheets of connective tissue
  • Dural venous sinuses are located within the 2 layers of the dura mater
  • Receives its own vascular supply mostly from the middle meningeal artery
• Arachnoid mater:

  • Middle layer of the meninges
  • Avascular and does not receive any innervation
  • sub-arachnoid space contains CSF
  • Lumbar puncture is via the sub-arachnoid space in the spine
• Pia mater:

  • Very thin and tightly adhered to the surface of the brain and spinal cord
  • Highly vascularised
• The pineal gland is a small endocrine gland located between the two cerebral hemispheres

  Responsible for the secretion of melatonin - regulation of the circadian rhythm
• The hypothalamus lies deep within the centre of the brain - responsible for the regulation of pituitary hormones, body temperature and appetite
• The hypothalamus is attached to the pituitary gland by the pituitary stalk
• The pituitary gland:

  • Anterior lobe is composed of glandular epithelium
  • Posterior lobe is composed of nervous tissue
• The pituitary gland sits within a small depression in the sphenoid bone known as the sell turcica

  It is close to the optic chiasm - a pituitary adenoma can cause bitemporal hemianopia