1. Basic topography of CNS Flashcards
what does the CNS consist of?
- Cerebral hemispheres
- Brainstem and cerebellum
- Spinal cord
what is the function of the cerebral hemispheres?
Higher functions, motor and sensory (conscious), emotion, memory
what is the function of the brainstem and cerebellum?
o Communication via cranial nerves including functions such as eye movement, swallowing and cardiorespiratory homeostasis
o Cerebellum involved with motor sequencing and co-ordination
what is the function of the spinal cord?
o Ascending (sensory) and descending (motor) pathways
o Spinal reflex arcs
o Control of upper and lower limbs at level of cervical and lumbosacral enlargements
what does the PNS consist of?
- Dorsal and ventral roots
- Spinal nerves
- Peripheral nerves
What is grey matter composed of?
Cell bodies and dendrites
• There are axons in grey matter, but volume is predominantly composed of cell bodies and
dendrites
what does the vascularity of the grey matter reflect about its function?
Highly vascular which reflects its computational role (higher functions) - huge amount of metabolic activity
What is white matter composed of?
(myelinated and non-myelinated) axons (with their
supporting cells) and no cell bodies
Why is white matter white?
Due to presence of fatty myelin (aqueous fat - mayonnaise)
What are the PNS equivalents of white and grey matter?
White - peripheral nerves
Grey - ganglions
why does the grey matter contain axons?
to communicate with white matter
• White matter pathways connect areas of grey matter, like cables between components of a computer
What is a nucleus of the grey matter?
A collection of functionally related `cell bodies
What is the cortex of grey matter?
A folded sheet of cell bodies found on the
surface of a brain structure. Typically 1-5mm thick
Define fibre (white matter).
Relating to an axon in association with its supporting cells (e.g. oligodendrocytes)
What are the different types of white matter fibres?
Association, Commissural and projection
What are association fibres?
Connect cortical regions within the same hemisphere
What are commissural fibres?
Connect left and right hemispheres or cord halves(e.g. corpus callosum or the ventral white commissure of the cord)
What are projection fibres?
Connect the cerebral hemispheres with the cord/brainstem and vice versa
How many spinal cord segments are there?
31
each supplying a given dermatome and myotome on each side
What is the arrangement of grey and white matter in the brain and spinal cord?
Spinal cord: inner grey and outer white
Brain: same but has third outer grey matter layer
what are each segments of the spinal cord connected to?
Each segment connects with a spinal (mixed) nerve through dorsal (sensory) and ventral (motor) roots
what forms roots?
Roots are actually formed from the convergence of multiple rootlets, which plug directly into the cord
What allows localisation of lesions to a given cord segment(s)
Knowledge of dermatomal and myotomal supply
what does A sensory deficit in a dermatomal pattern suggest?
Lesion is at the level of dorsal roots or spinal nerves
what does A sensory deficit across multiple segments suggest?
a cord lesion
what does A sensory deficit in a homuncular pattern suggest?
a lesion above the thalamus
what does a motor deficit without sensory loss suggest
lesion in ventral root
what does a sensory and motor deficit suggest?
lesion in spinal nerve
what does the ventral and dorsal roots join to form?
mixed spinal nerve
why does the cauda equina form?
vertebral column grows faster than spinal cord so one to one correspondence at cervical level but as you go down the spinal cord, the spinal nerves get drawn down by the growing vertebral column resulting in an area of spinal nerves without spinal cord known as cauda equina.
where does spinal cord end?
L1 L2 vertebral levels
What are funiculi?
a segment of white matter containing multiple distinct tracts. Impulses travel in multiple directions - both ascend and descend
What are the different funiculi of the spinal cord?
Each half of the spinal cord has a ventral, lateral and dorsal funiculi.
What are tracts?
An anatomically and functionally defined white matter pathway connecting two distinct regions of grey matter. Impulses travel in one direction
Give examples of tracts.
spinothalamic tract (connecting spinal cord dorsal horn to thalamus), corticospinal tract (connecting cerebral cortex to spinal cord ventral horn)
What are fasciculi?
A subdivision of a tract supplying a distinct region of the body (singular: fasciculus)
What are the 2 fasciculi of the dorsal funiculi?
Fasciculus gracilis (subdivision of dorsal column tract supplying lower half of body) and fasciculus cuneatus (subdivision of dorsal column tract supplying upper half of body, excluding the head)
What are Rexed’s laminae?
Organisation of grey matter within the spinal cord into columns (I-X)
if a particular level like L1 was damaged then why might the muscle it supplies still be able to function?
most muscles have innervation from more than one spinal level
What are the general functions of the midbrain?
Eye movements and reflex responses to sound and vision
what does the Cerebral peduncles (white matter) in midbrain contain?
descending corticospinal fibres from the ipsilateral hemisphere)
what does the Substantia nigra (grey matter) in midbrain contain?
dopaminergic neurones that project to the striatum (nigrostriatal fibres)
what is the red nucleus (grey matter) in midbrain?
a well distinct region that gives rise to axons that travel to the cord in the vestigial rubrospinal tract. It also has some other less important motor functions
what does the Oculomotor nucleus (grey matter) in midbrain contain?
lower motor neurone cell bodies that project through the oculomotor nerve to all bar two of the extraocular muscles
what does the Edinger-Westphal nucleus (grey matter) in midbrain contain?
contains parasympathetic preganglionic neurones that project to the ciliary ganglion in the orbit to cause pupillary constriction
what is the Periaqueductal grey matter in the midbrain?
an area surrounding the cerebral aqueduct that has roles in pain transmission and micturition
what is the function of the Cerebral aqueduct in midbrain?
connects the third ventricle (found between the halves of the thalamus) and the further ventricle (found beneath the cerebellum)
what do the Superior/inferior colliculus (grey matter) in midbrain regulate?
reflex responses to visual
and auditory stimuli respectively
What are the general functions of the pons?
Feeding and Sleep
What are the general functions of the medulla?
- Cardiovascular and respiratory centres
* Contains a major motor pathway (medullary pyramids)
What are some of the key sulci and gyri of the brain? (6)
- Central sulcus: Sitting in the coronal plane.
- Precentral gyrus: Contains primary motor cortex (anterior)
- Postcentral gyrus: Contains primary sensory cortex (posterior)
- Lateral/Sylvian fissure: Separates temporal from frontal/parietal lobes
- Parieto-occipital sulcus: Separates parietal from occipital lobe
- Calcarine sulcus: Primary visual cortex surrounds this
what are key features of the Pons?
Trigeminal nerve exits from its lateral aspect
Corticospinal fibres travel ventrally (hence susceptible to damage by basilar artery occlusion causing locked in syndrome)
Sits beneath the fourth ventricle so can get compressed if this ventricle expands
Contains reticular formation (grey matter) regions important for sleep
what are key features of the medulla?
Pyramids (white matter, ventral swellings on each side containing corticospinal fibres from ipsilateral hemisphere. These decussate in the caudal medulla at the decussation of the pyramids)
Gracile and cuneate nuclei (grey matter, relays onto second order neurones in the dorsal column-medial lemniscus pathway)
Important nuclei (grey matter) for cardiorespiratory homeostasis such as solitary nucleus and dorsal motor nucleus of the vagus
define gyrus, sulcus and fissure?
Gyrus (pl. gyri): a raised fold of cerebral cortex
Sulcus (pl. sulci): the valley between adjacent gyri
Fissure: a large groove between adjacent hemispheres/lobes
What is the calcarine sulcus associated with and where is it located?
Primary visual cortex
- medial surface of the occipital lobe
Visual cortex above the calcarine sulcus supplies the (contralateral) inferior field, and visual cortex below the calcarine sulcus supplies the (contralateral) superior visual field
what are key feature on the inferior aspect of the brain?
- Optic chiasm
- Uncus
- Medullary pyramids
- Parahippocampal gyrus
what is the optic chiasm?
A site where fibres in the visual system cross over (decusate)
what is the uncus?
the most medial part of the temporal lobe, containing olfactory cortex. This structure can also herniate below the tentorium cerebelli, compressing adjacent midbrain
what is the medullary pyramids?
Location of descending motor
fibres (each has around 1 million axons!)
What is the parahippocampal gyrus associated with, where is it located?
Key cortical region for memory encoding (temporal lobe, lateral to the uncus)
what are key features of the midline of the brain?
- Corpus callosum
- hemispheres
- Thalamus
- Cingulate gyrus
- Hypothalamus
- Fornix
- Tectum
- Cerebellar tonsil:
what is the corpus callosum?
Fibres connecting the two cerebral hemispheres
What is the function of the thalamus?
Sensory relay station
It contains the cell bodies of third order neurones and projects to the primary sensory cortex in a homuncular pattern
WHat is the function of the cingulate gyrus?
Cortical area important for emotion and memory - participates in an important circuit involving the hippocampus and thalamus
what is the function and location of hypothalamus?
found in the walls of the third ventricle (just like the thalamus is) and has important neuroendocrine and homeostatic roles
WHat is the fornix, what type of fibres does it contain and where is it located?
Major output pathway from the hippocampus. Located inferior to the septum that separates the lateral ventricles
- commuissural fibres
- and is part of the Papez circuit
What is the tectum?
Dorsal part of the midbrain involved in involuntary responses to auditory and visual stimuli - contains the superior and inferior colliculi (four in total, forming the so-called corpora quadrigemina)
What is the cerebellar tonsil?
Part of the cerebellum that can herniate (through foramen magnum) in cases of raised intracranial
pressure, often leading to death due to cardiorespiratory compromise through compression of the medulla
What is responsible for production of CSF, how much is produced per day?
choroid plexus, which is highly vascular and makes a total of 600-700ml of CSF per day
where are choroid plexuses found?
in ventricles
What are the different ventricles of the brain?
Lateral ventricles, third ventricle and fourth ventricle
What connects the lateral and third ventricles?
interventricular foramen
What separates the the lateral ventricles?
septum pellucidum
which are the largest ventricles?
lateral
function of CSF?
CSF has both metabolic and mechanical functions
o Contains glucose and maybe even hormones
o Shock absorbs the brain and renders it effectively weightless
where does CSF circulate and where is it reabsorbed?
CSF circulates through the ventricular system and subarachnoid space before being reabsorbed at the arachnoid granulations (and some other sites)
which ventricles make CSF?
- All ventricles have choroid plexus, hence all make CSF
* However, most CSF is made in the large lateral ventricles
describe the flow of CSF from lateral ventricles
• From the lateral ventricles CSF circulates through the
interventricular foramen into the third ventricle
• then drains into the fourth ventricle via the cerebral aqueduct (in the midbrain)
• The CSF can drain from fourth ventricle via 3 apertures
o These apertures are direct holes in the brain, permitting CSF to drain from the ventricular system into the subarachnoid space.
• Once in the subarachnoid space, CSF percolates around the superficial surfaces of the brain and spinal cord before being reabsorbed in the arachnoid granulations into dural venous sinuses
where is the third ventricle?
squashed flat in the midline by the thalamus on each side
where is the fourth ventricle?
pons beneath the cerebellum
what are the 3 apertures that drain CSF from fourth ventricle into subarachnoid space?
2 lateral (of Luschka) and 1 median (of Magendie) apertures
does CSF drain from fourth ventricle to subarachnoid space through spinal cord central canal
no only negligible amounts
What connects the third and fourth ventricles?
cerebral aqueduct
What are the 4 openings of the fourth ventricle?
- foramen of magendie (median aperture)
- foramen of luschka (lateral apertures, bilateral)
- central canal
what happens if there is a blockage of ventricles?
Blockage of a part of the ventricular system will lead to upstream dilatation and potential damage to structures surrounding the dilated ventricles
where is the most common blockage in the ventricular system and what does it lead to?
• The cerebral aqueduct
o Blockage of the aqueduct would cause dilatation of the lateral and third ventricles but with a normal fourth ventricle (downstream)
