S1: topography of the nervous system Flashcards
Describe grey matter
Composed of cell bodies and dendrites
Highly vascular
Contains axons (far less prevalent than white matter) allowing it to communicate with white matter
PNS equivalent of grey matter = ganglion
Describe white matter
Composed of axons with their supporting cells
White due to presence of myelin
PNS equivalent of white matter = peripheral nerve
Define nucleus (grey matter)
Collection of functionally related cell bodies
Define cortex (grey matter)
Folded sheet of cell bodies found on the surface of a brain structure
Define fibre (white matter)
An axon in association with its supporting cells
Define the three types of fibres
Association fibres = connect cortical regions within the same hemisphere
Commissural fibres = connect left and right hemispheres
Projection fibres = connect the cerebral hemispheres with the cord/brainstem and vice versa
Describe the segments of the spinal cord
31 segments, each supplying a given dermatome and myotome on each side
Cord has a central core of grey matter and outer shell of white matter
Segment connects with spinal nerve through dorsal and ventral roots
Define funiculus
A segment of white matter containing multiple distinct tracts
Impulses travel in multiple directions
Define tract
Anatomically and functionally defined white matter pathway connecting two distinct regions of grey matter
Impulses travel in one direction
Define fasciculus
Subdivision of a tract supplying a distinct region of the body
Outline the 3 main parts of the brainstem
Midbrain (mesencephalon): eye movements and reflex responses to sound and vision
Pons: feeding and sleep
Medulla: cardiovascular and respiratory centres, contains a major motor pathway (medullary pyramids)
List the important gyri and sulci
Central sulcus: separates frontal and parietal lobes
Precentral gyrus: contains primary motor cortex
Postcentral gyrus: contains primary sensory cortex
Lateral/sylvian fissure: separates temporal from frontal/parietal lobes
Parieto-occipital sulcus: separates parietal from occipital lobe
Calcarine sulcus: primary visual cortex surrounds this
Outline key features on the inferior aspect of the brain
Optic chiasm: a site where fibres in the visual system cross over
Uncus: medial part of temporal lobe that can herniate, compressing the midbrain
Medullary pyramids: location of descending motor fibres
Parahippocampal gyrus: key cortical region for memory encoding
Outline key features of the brain in the midline
Corpus callosum: fibres connecting the two cerebral hemispheres
Thalamus: sensory relay station projecting to sensory cortex
Cingulate gyrus: cortical area important for emotion and memory
Hypothalamus: essential centre for homeostasis
Fornix: major output pathway from the hippocampus
Tectum: dorsal part of the midbrain involved in involuntary responses to auditory and visual stimuli
Cerebellar tonsil: part of the cerebellum that can herniate and compress the medulla
Describe the function of the ventricles
Cavities filled with CSF
Each contain the choroid plexus, which is highly vascular and makes a total of 600-700ml of CSF per day (most made in the lateral ventricles)
Describe the function of the CSF
Both metabolic and mechanical functions
- contains glucose (& maybe hormones)
- shock absorbs the brain and renders it effectively weightless
Describe the flow of CSF through the brain ventricles
From the lateral ventricles, the CSF circulates through the interventricular foramen into the third ventricle
CSF drains from the third ventricle to the fourth ventricle via the cerebral aqueduct
In the fourth ventricle, the CSF can drain through the lateral and median apertures (direct holes of the brain, which permits the CSF to drain into the subarachnoid space; negligible drainage via the spinal cord central canal)
CSF is reabsorbed at the arachnoid granulations
What happens if there is blockage of the ventricular system?
Leads to upstream dilatation and potential damage to structures surrounding the dilated ventricles
Cerebral aqueduct is a common place for such occlusions
-would cause dilatation of the lateral and third ventricles but with a normal fourth ventricle (downstream)
Describe the process of neurulation
Process of formation of the neural tube, induced by the notochord
Elevation of the neural folds
Fusion of the folds in the midline
When the folds fuse, the neural crest cells detach and migrate to their ultimate destinations
Neural tube zips up rostrally and caudally
-process fails in the rostral direction: anencephaly
-process fails in the caudal direction: spina bifida
Outline the three major swellings on the rostral neural tube
Prosencephalon – becomes the forebrain
Mesencephalon – becomes the midbrain
Rhombencephalon – becomes the hindbrain
What are the divisions of the prosencephalon?
Telencephalon – becomes most of the cerebral hemisphere
Diencephalon – becomes thalamus, hypothalamus and optic nerve/retina
What are the divisions of the rhombencephalon?
Metencephalon – forms the pons and cerebellum
Myelencephalon – forms the medulla
Describe the fundamental relationship between the sensory and motor systems
Motor structures tend to sit anteriorly, and sensory structures tend to sit posteriorly
This pattern exists due to the development of the basal and alar plates in the neural tube
1) Notochord induces the ventral (anterior) portion of the neural tube to become the basal plate – gives rise to motor neurones
2) Alar plate forms in the absence of influences – gives rise to inter- and sensory neurones
Describe the development of the cauda equina
The spine grows faster than the spinal cord, particularly at the lumbar levels
Lower portions of the cord are stretched, drawing out the cauda equina
Outline how neural tube defects can predispose to hydrocephalus
Tethering of the cord at the site of the defect
As the spine grows, the cord cannot move within the vertebral canal, resulting in the brainstem being pulled down through the foramen magnum and becoming occluded
Describe the developmental basis of neural tube defects
Failure of the neural tube to ‘zip up’ in the cranial or caudal directions
Failure of closure in the cranial direction involves the brain eg. anencephaly
Failure of closure in the caudal direction involves the spine/spinal cord eg. classic spina bifida
All of these disorders have failure of development of the posterior vertebral arches at one or more levels (neural tube is partly responsible for inducing the migration of the sclerotome from somites to form the posterior bony arch)
Describe craniorachischisis
Entire neural tube remains open
Failure of both brain and spinal cord to form
Incompatible with life
Describe anencephaly
Cranial neural tube fails to close
Failure of brain to form
Children may be born alive but do not live for long
Describe myelocoele
Spinal cord fails to develop
Usually associated with a CSF filled cyst
Children frequently have neurological deficits and are susceptible to meningitis due to presence of exposed neural tissue
Describe myelomeningocoele
CSF-filled cysts containing the spinal cord
Transilluminates relatively poorly (due to prescence of solid tissue in the cyst)
Children may have neurological deficits & repair is necessary
Describe meningocoele
Presence of CSF filled cyst
Cord is sited within the vertebral canal
Transilluminates brilliantly & children tend to have a good neurological prognosis
Cyst will need repair as it predisposes to infection
Describe spina bifida occulta
Only anomaly is the lack of the posterior vertebral arch
May manifest as a tuft of hair or skin marking over the defect
Not associated with significant neurological problems
Occurs in about 10% of the population
What should be taken to prevent neural tube defects?
Folic acid
400mg daily to be taken from around 3 months before conception until week 12 of pregnancy
Mechanism of action of folate in preventing neural tube defects is unknown
What is the neural crest?
A highly specialised population of cells derived from the point at which the neural folds fuse when the surface ectoderm is reconstituted
Become detached from the ectoderm and then migrate to their distant targets
Outline cells derived from the neural crest
Primary sensory neurones Autonomic postganglionic neurones Enteric neurones Schwann cells Cells of the adrenal medulla Melanocytes Leptomeninges Head mesenchyme
Outline tissues receiving a significant contribution from the neural crest
Thymus
Thyroid
Parts of the heart (eg. spiral septum)
Parts of the teeth
Describe Di George syndrome
Neural crest cell disorder Immunodeficiency (due to involvement of the thymus) Facial anomalies Heart anomalies Hypocalcaemia
Describe Hirschprung’s disease
Lack of enteric neurones in sections of the large intestine
This leads to hypomotility and constipation
