Makindo Medical Notes.com

About

• Thin grey layer of layers of neuronal cell bodies and connections that overlies the cortex
• Motor cortex thickest at 4.5 mm
• Visual cortex thinnest at 1.5 mm
• Layers II and IV receive input, V, VI generate output

Anatomy

Layers

• Layer I: Molecular layer
• Layer II: External granular layer
• Layer III: External pyramidal layer: output to projection and commissural fibres
• Layer IV: internal granular layer: receives thalamocortical fibres from the ventral tier and input from the lateral geniculate body
• Layer V: internal pyramidal layer: output to corticobulbar, corticospinal, corticostriatal fibres, contains large Betz cells in the motor cortex.
• Layer VI: multiform layer: receives corticothalamic fibres. Outputs to projection, commissural and association fibres

Cerebral Architecture

• Histologically the cerebral cortex is made up of a half centimetres of grey cells above a dense thick inner layer of white matter. The overall surface area is increased by the folds of sulci and gyri. The cortex can be seen to be composed of six layers of cells that vary in size and structure.
• Back in 1909 Brodmann produced a map of the cerebral cortex based on the micro-architecture of the cells and staining for Nissl substance which separates neurons from glial cells. This was before we had an understanding of many of the functions of these areas and the system has stood the test of time. Other improvements have been developed by others. It is quite clear that anatomy is related to function.
• Cells in the cortex are arranged in a columnar structure and can this can be seen with microelectrode stimulation. Areas with similar functions lie close together. For example, Broca's area lies within the inferior region of the precentral motor cortex near to those areas that control voluntary movement of the lips, tongue, larynx and pharynx. Broca's produces the content of speech content and damage here leads to expressive dysphasia. A lesion in the inferior area of the precentral motor cortex leads to dysarthria and facial weakness.
• Broca's discovery of dysphasia in a patient whose subsequent post mortem revealed a lesion due to syphilis in the area that became known as Broca's area was one of the key findings to suggest localization of function within the brain. The fibres from the cortex on their way to the brainstem pass down in a fan-like bundle called the corona radiata to enter the internal capsule. Note that neuroanatomical diagrams are 'upside down' compared with the normal appearance of imaging

Lobar Anatomy

• The Cerebral cortex is the most developed part of the human brain and often quoted as the most complicated system and poorly understood system known. The surface is the processing centre comprised of the outer layer of grey matter formed by neuronal cell bodies and their many connections. These are connected with neighbouring areas by axonal extensions which are myelinated and pass to other cortical areas or down and medially as long tracts to the brainstem and spinal cord whereas others form circuits within the deep grey matter of the basal ganglia and other structure grey matter structures such as the thalamus or pass to other areas on the cortex. These fibres contain ascending (mainly sensory) and descending signals (mainly motor). The hemispheres surround the fluid-filled ventricles.
• The cerebral hemispheres can be divided by certain sulci on the brain surface into lobes. The surface shows multiple folds - gyri and sulci which increase the amount of grey matter and cortical neurons (processing potential) making up the surface of the brain. It was Broca who showed that different parts of the brain had distinct functions in a patient with a syphilitic lesion in the precentral gyrus and dysphasia. Later others such as Brodmann mapped these areas on their cellular appearance. Let us look at the histology and then the lobes and their function.
• Histologically the cerebral cortex is made up of a 5mm layer of grey cells above a dense thick inner layer of white matter. The overall surface area is increased by the folds of sulci and gyri. The cortex can be seen to be composed of six layers of cells that vary in size and structure. Back in 1909 Brodmann produced a map of the cerebral cortex based on the micro-architecture of the cells and staining for Nissl substance which separates neurons from glial cells. This was before we had an understanding of many of the functions of these areas and the system has stood the test of time. Other improvements have been developed by others. It is quite clear that anatomy is related to function.
• Cells in the cortex are arranged in a columnar structure and can this can be seen with microelectrode stimulation. Areas with similar function lie close together. For example, Broca's area lies within the inferior region of the precentral motor cortex near to those areas that control voluntary movement of the lips, tongue, larynx and pharynx. Broca's produces the content of speech content and damage here leads to expressive dysphasia. A lesion in the inferior area of the precentral motor cortex leads to dysarthria and facial weakness. Broca's discovery of dysphasia in a patient whose subsequent post mortem revealed a lesion due to syphilis in the area that became known as Broca's area was one of the key findings to suggest localization of function within the brain. The fibres from the cortex on their way to the brainstem pass down in a fan-like bundle called the corona radiata to enter the internal capsule.

Important surface sulci to identify

• Central sulcus: separates frontal and parietal lobes where the primary motor cortex and primary sensory cortex like closely together
• Lateral sulcus (fissure) forms the upper limit of the temporal lobe
• Parieto-occipital sulcus
• Cingulate sulcus
• Preoccipital notch
• For the stroke physician you need to appreciate these anatomical markers on imaging so that you can name the borders of the lobes of the brain on axial, coronal and sagittal planes.

Lobes

• Frontal lobe
  • The frontal lobe starts at the central sulcus and forwards. Inferiorly limited by the lateral (Sylvian) sulcus. The primary motor cortex (MC) lies on the prefrontal gyrus. Input is received from the thalamus and cerebellum and is the main source of all conscious movement. Representation on the lateral surface is hand superiorly and then below that face and tongue and larynx and pharynx.
  • The trunk and leg are represented on the medial inner surface. The Premotor cortex (PMC) lies slightly anterior to MC and is also concerned with voluntary movement. Frontal eye fields lies in front of the PMC and are involved in eye movement. Broca's area (dominant hemisphere) lies inferiorly close to the primary motor areas for the mouth, oral cavity and pharynx and larynx. Broca's is involved in generating the content of speech. Damage here causes expressive dysphasia. The Frontal association cortex gets input from the thalamus, limbic system, other cortical areas and is involved in intellectual ability. The paracentral lobule found on the medial surface involved with urinary continence.
• Parietal lobe
  • The parietal lobe lies behind the central sulcus as far back as the limit of the Sylvian (lateral) sulcus and the parietoccipital sulcus. It contains the Primary sensory cortex with the representation of the hand and face laterally and leg on the inner superior medial section.
  • Areas of high sensory input such as face and hand are given proportionally large representations. All conscious sensation is perceived here. The parietal association cortex mostly involved with the integration of sensory input and processing. Input mainly from the thalamus. Deep fibres carry the optic radiations from lateral geniculate ganglion from the superior retina
• Temporal lobe
  • Temporal lobe lies below the Sylvian (lateral) fissure as far back as a line dropped from the end of the Sylvian fissure. Contains the Auditory cortex lies in the superior temporal gyrus on the lateral surface and is concerned with input from the medial geniculate body.
  • Temporal association cortex integrates auditory stimulus and language comprehension (dominant hemisphere) Damage here can cause receptive dysphasia. Memory processing and recall. Deep fibres carry the optic radiations from the lateral geniculate body from the lower retina
• Occipital lobe
  • Occipital lobe lies behind the parietal and temporal lobes. The calcarine cortex primarily deals with vision and visual processing. The optic radiations pass back from the lateral geniculate body of the ipsilateral thalamus.
  • Sight from the right side of one's world goes to the left occipital cortex. Occipital association areas concerned with image processing. The high density of neurons.
  • Main centre for processing is within the calcarine sulcus. Upper fibres from the upper retina pass to the upper calcarine sulcus called the cuneus. Lower fibres pass to the lower part of the sulcus, the lingual gyrus from the lower retina. The visual area is called Brodmann 17 area.
• Insular cortex
  • This area of grey matter is seen if one parts the lips of the lateral sulcus and is known as the insula. It is closely associated with the middle cerebral artery which passes on the surface.
  • Its functions are poorly understood and possibly involve belching, salivation, gastric movements and vomiting. It may play a role in addiction. The insula certainly is discussed in early stroke imaging 'loss of the insular ribbon sign.