S1: topography of the nervous system

Question 1

Describe grey matter

Answer 1

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

Question 2

Describe white matter

Answer 2

Composed of axons with their supporting cells
White due to presence of myelin
PNS equivalent of white matter = peripheral nerve

Question 3

Define nucleus (grey matter)

Answer 3

Collection of functionally related cell bodies

Question 4

Define cortex (grey matter)

Answer 4

Folded sheet of cell bodies found on the surface of a brain structure

Question 5

Define fibre (white matter)

Answer 5

An axon in association with its supporting cells

Question 6

Define the three types of fibres

Answer 6

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

Question 7

Describe the segments of the spinal cord

Answer 7

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

Question 8

Define funiculus

Answer 8

A segment of white matter containing multiple distinct tracts
Impulses travel in multiple directions

Question 9

Define tract

Answer 9

Anatomically and functionally defined white matter pathway connecting two distinct regions of grey matter
Impulses travel in one direction

Question 10

Define fasciculus

Answer 10

Subdivision of a tract supplying a distinct region of the body

Question 11

Outline the 3 main parts of the brainstem

Answer 11

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)

Question 12

List the important gyri and sulci

Answer 12

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

Question 13

Outline key features on the inferior aspect of the brain

Answer 13

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

Question 14

Outline key features of the brain in the midline

Answer 14

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

Question 15

Describe the function of the ventricles

Answer 15

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)

Question 16

Describe the function of the CSF

Answer 16

Both metabolic and mechanical functions

• contains glucose (& maybe hormones)
• shock absorbs the brain and renders it effectively weightless

Question 17

Describe the flow of CSF through the brain ventricles

Answer 17

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

Question 18

What happens if there is blockage of the ventricular system?

Answer 18

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)

Question 19

Describe the process of neurulation

Answer 19

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

Question 20

Outline the three major swellings on the rostral neural tube

Answer 20

Prosencephalon – becomes the forebrain
Mesencephalon – becomes the midbrain
Rhombencephalon – becomes the hindbrain

Question 21

What are the divisions of the prosencephalon?

Answer 21

Telencephalon – becomes most of the cerebral hemisphere

Diencephalon – becomes thalamus, hypothalamus and optic nerve/retina

Question 22

What are the divisions of the rhombencephalon?

Answer 22

Metencephalon – forms the pons and cerebellum

Myelencephalon – forms the medulla

Question 23

Describe the fundamental relationship between the sensory and motor systems

Answer 23

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

Question 24

Describe the development of the cauda equina

Answer 24

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

Question 25

Outline how neural tube defects can predispose to hydrocephalus

Answer 25

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

Question 26

Describe the developmental basis of neural tube defects

Answer 26

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)

Question 27

Describe craniorachischisis

Answer 27

Entire neural tube remains open Failure of both brain and spinal cord to form Incompatible with life

Question 28

Describe anencephaly

Answer 28

Cranial neural tube fails to close Failure of brain to form Children may be born alive but do not live for long

Question 29

Describe myelocoele

Answer 29

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

Question 30

Describe myelomeningocoele

Answer 30

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

Question 31

Describe meningocoele

Answer 31

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

Question 32

Describe spina bifida occulta

Answer 32

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

Question 33

What should be taken to prevent neural tube defects?

Answer 33

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

Question 34

What is the neural crest?

Answer 34

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

Question 35

Outline cells derived from the neural crest

Answer 35

``` Primary sensory neurones Autonomic postganglionic neurones Enteric neurones Schwann cells Cells of the adrenal medulla Melanocytes Leptomeninges Head mesenchyme ```

Question 36

Outline tissues receiving a significant contribution from the neural crest

Answer 36

Thymus Thyroid Parts of the heart (eg. spiral septum) Parts of the teeth

Question 37

Describe Di George syndrome

Answer 37

``` Neural crest cell disorder Immunodeficiency (due to involvement of the thymus) Facial anomalies Heart anomalies Hypocalcaemia ```

Question 38

Describe Hirschprung’s disease

Answer 38

Lack of enteric neurones in sections of the large intestine | This leads to hypomotility and constipation