topography of CNS + Embryology Flashcards
Basic components of the central nervous system
Cerebral hemispheres • Brainstem and cerebellum • Spinal cord
Grey matter versus white matter
Grey matter is composed of cell bodies and
dendrites and is highly vascular White matter is composed of axons (with their
supporting cells) - white due to the presence of
fatty myelin
• Grey matter contains axons, allowing it to
communicate with white matter
The spinal cord as an illustration of the ‘basic plan’ of the CNS •
The cord is composed of around 31 segments, each
supplying a given dermatome and myotome on each
side
central core of grey matter and an outer
shell of white matter
Each segment connects with a spinal (mixed) nerve
through dorsal (sensory) and ventral (motor) roots
Revision of basic features of the brain and introduction to some new
ones •
Key features of the brain – gyri and sulci
Central sulcus: Sitting in the coronal plane. Key landmark separating 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
Key features of the brain – inferior aspect
Optic chiasm: A site where fibres in the visual system cross over
Uncus: Part of the temporal lobe that can herniate, compressing the midbrain. Important olfactory role Medullary pyramids: Location of descending motor fibres (each has around 1 million axons!)
Parahippocampal gyrus: Key cortical region for memory encoding
features of the brain - 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
Production and flow of cerebrospinal fluid
• Having developed from a hollow tube, the brain is
itself hollow •
These cavities are known as the brain ventricles •
The ventricles each contain choroid plexus, which
makes a total of 600-700ml of CSF per day •
CSF circulates through the ventricular system and
subarachnoid space before being reabsorbed at the
arachnoid granulations (and some other sites)
white matter
• Funiculus - a segment
of white matter containing
multiple distinct tracts. Impulses
travel in multiple directions
• Tract: white matter pathway connecting two distinct regions of grey matter. Impulses travel in one direction. Eg sensory and motor tract.
• Fasciculus - a subdivision of a tract supplying a distinct region of the body
Grey matter
grey matter in the cord is organised into cell columns – Rexed’s laminae
• The motor neurones supplying a given muscle arise from
multiple segments and form a distinct population of
neurones in the CNS – a nucleus
Brainstem
Midbrain (mesencephalon)
• Eye movements and reflex responses to sound and vision
• Pons
• Feeding
• Sleep
• Medulla
• Cardiovascular and respiratory centres
• Contains a major motor pathway -medullary pyramids
To outline the steps involved in the development of the neural tube •
Gastrulation - outer layer of the bilayer disc invaginates at the primitive streak to produce a three layered disc.
Formation of notochord from mesoderm primitive node.
Inducts the overlying ectoderm to invaginate and form the neural tube
Elevation of neural folds and fold in the midline
Neural creat cells detach and migrate
Closes first cranial then caudal.
To understand the formation of the three germ layers and how it contributes to the development of the nervous system •
Neural crest cells
Derived from the point at which the neural folds fuse when the surface ectoderm is reconstituted. Detach from ectoderm.
Form:
• Primary sensory neurones
• Autonomic postganglionic neurones
• Enteric neurones
Schwann cells
Cells of the adrenal medulla (these are after all specialised
sympathetic postganglionic neurones)
Melanocytes
The leptomeninges (i.e. the arachnoid and pia)
Head mesenchyme, which contributes to many tissues such as pharyngeal arches
o Tissues receiving a significant contribution from neural crest
Thymus
Thyroid
Parts of the heart (e.g. spiral septum)
Parts of the teeth
Dorsal part:
• The dorsal root ganglia
• The sympathetic ganglia (sympathetic postganglionic
neurones)
• The preaortic ganglia (sympathetic postganglionic
neurones that receive input from splanchnic nerves)
• The adrenal medulla (chromaffin cells, which are
homologous to sympathetic postganglionic neurones)
• The enteric nervous system)
To understand the formation of the brain and the CSF filled ventricles from the neural tube •
Outline the neural tube derivative of the white and grey matter of the spinal cord •
White = alar plate
Grey = basal plate
Outline how morphogens/ligands are involved in determining the patterning or axis of the neural tube •
Notochord produces SHH and then causes development of basal plate at ventral portion. Motor neurones.
Then alar plate has absence of influence from notochord and release of Wnts/BMP from ectoderm so develops alar plate. Sensory neurones.
Precursor to both is neuroblasts.
To describe the pathology that occurs due to neural tube formation defects
• All of these disorders have failure of development of the
posterior vertebral arches at one or more levels.
most severe to least:
Craniorachischisis - The entire neural tube remains open
• Hence failure of both brain and spinal cord to form
• Incompatible with life
Anencephaly - The cranial neural tube fails to close
• Hence failure of the brain to form
Myelocoele (coele- relating to
a fluid filled cyst)
• The spinal cord fails to develop, usually
associated with a CSF filled cyst
• These children frequently have neurological deficits and
are susceptible to meningitis due to the presence of
exposed neural tissue
Failure of development of the posterior vertebral arch (neural tube is partly responsible for inducing the migration of the sclerotome from somites to form the posterior bony arch):
• Myelomeningocoele
o A CSF-filled cyst containing the spinal cord
Transilluminates relatively poorly (due to presence of solid tissue in the cyst)
These children may have neurological
deficits
Repair is necessary
• Meningocoele
o Presence of a CSF filled cyst
o The cord is sited within the vertebral canal
o Transilluminates brilliantly
o These children tend to have a good neurological
prognosis
o However, the cyst will need repair as it does
predispose to infection
• Spina bifida occulta
o The only anomaly is the lack of the posterior vertebral arch o May manifest a sign such as a tuft of hair or a large naevus over the defect
o Not associated with significant neurological
problems
o Occurs in about 10% of the population!
Neural crest disorders
Di George syndrome
• Immunodeficiency (due to involvement of the thymus) • Facial anomalies (due to contribution of neural crest to
facial development) • Heart anomalies • Hypocalcaemia (involvement of parathyroids) •
Hirschprung’s disease
Lack of enteric neurones in sections of the large intestine • This leads to hypomotility and constipation
