Session 1: Structure and Development of the Nervous System Flashcards
What does the nervous system consist of? And describe the anatomical terminology and plane sectioning of the brain
The nervous system is composed of the brain, spinal cord, nerves (31 spinal, 12 cranial pairs), cavities, connective tissue coverings (membranes encasing soft tissues), a specialised blood supply and bones (cranium/skull, vertebral column).
Describe how the Nervous System is organised into the CNS and PNS
It is organised into the CNS and the PNS. The PNS is further divided into an afferent (input) and an efferent (output) division, which again sub-divides into the somatic nervous system, controlling skeletal muscle and the autonomic nervous systems regulating visceral functions.
The CNS is characterised by protection of the cranium and vertebral column.
The anatomical border between CNS & PNS is defined by the Pia Mater.
Describe clinical importance of distinguishing between the CNS and PNS
Tumours of CNS: malignant tumours are a feature of glia; tumours of neurones are not malignant (but as they are space-occupying lesions, they have to be removed).
Tumours of PNS: all tumours are benign.
What are the two broad classes of cell types in the CNS?
2 broad classes
- Neurones (10%)
- Neuroglia (90%)
There are multiple differences between these cell-types
- General functions
- Ion channel expression
- Diseases (1) that target them
- Diseases (2) they give rise to
- Diversity of connections (neurones have a large number, glia cells may have no connections)
What are the different kinds of neurones in the CNS
Afferent neurones which arise from a sense organ and whose axons diverge in the CNS to come into contact with many other neurones
Efferent neurones, with a cell body located within the CNS, upon which many other nerve cells converge
Interneurones – about 99% of all neurones – located entirely within the CNS (some exceptions in the ANS), which integrate input with output.
Although all neurones have a cell body and an axon, not all neurones possess dendrites e.g. primary sense neurones are unipolar.
It is estimated that there are between 100 billion to 1 trillion neurones (we are born with even more neurones). It is the sheer number of neurones that makes the brain so complex.
Give some examples of glial cells, explain why the CNS is very delicate and what its function depends on
In addition to neurones, the CNS contains large numbers of glial cells – astrocytes, oligodendrocytes, ependymal cells and microglial cells, which together make up about 90% of all cells and which support the neurones structurally and metabolically.
The CNS is very delicate – it is suspended within the cerebrospinal fluid, isolated from potentially harmful metabolites in the blood by the blood-brain barrier, surrounded by three meningeal layers and the whole thing is protected within the skull and vertebral column. It is highly metabolically active and will be irreversibly damaged if its blood supply is interrupted for more than 3 or 4 minutes.
The proper function of the nervous system depends upon anatomical and synaptic links between neurones, which are determined both genetically and by sensory experience. Malfunction occurs if either of these links is disrupted. In the PNS, sensory input comes from clearly demarcated regions of the body (dermatomes) and motor output affects distinct muscle groups (myotomes). In the CNS, specific function can be localised within the brain
Neurones are characterised by connections to others. These connects are known as neural circuits. Most circuits are vast, complex, have no starting or end-point therefore do not terminate.
What are emergent properties of the brain?
Neurones connect with one another through synapses in many different ways
Inter-connections between neurones give rise to neural circuits
Some neuronal circuits form neuronal networks
Most neuronal networks behave in complex manners not seen in the individual members of the network. These are known collectively as “emergent” properties of the neuronal ensemble.
They include:
- Consciousness
- Sensory awareness
- Thought processes
- Sensory attention
- Emergent properties of the brain that make it different from all other organs of nature.
Describe the circuitry of the spinal cord
Most connections between neurones of the spinal cord are said to be hard-wired – they are called reflex arcs.
A reflex arc is a neural pathway that controls an action reflex. Most sensory neurons do not pass directly into the brain but synapse in the spinal cord, allowing reflex actions to occur relatively quickly by activating spinal motor neurones without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex is carried out.
The integrity of a reflex arc is easily tested. Changes in reflex function may indicate onset of diseases or even damage to the neurones of the circuit.
Describe briefly the functional anatomy of the brain
It functions as a single organ
Anatomically, it is a bilateral structure
The two sides are morphologically symmetrical but functionally, there is asymmetry of specialisation. There is thus, the left brain and the right brain.
The Left & Right Brain: there is lateralisation of function with respect to some modalities
Some modalities are represented bilaterally with one side more dominant e.g. speech, handedness, attention etc.
The attention system of the brain is commanded by the right brain.
Implications: e.g. outcomes of stroke damage in parietal cortex depend on whether it is the right or left brain that is injured.
Right brain losses tend to be permanent and more severe.
What are Brodmann Areas?
Localisation of Cortical Function: Brodmann Areas
Originally the division of the cerebral cortex into numbered regions was based upon histological differences. However, some Brodmann areas are synonymous with a specific function
Area (also called); function
3 (S1); Somatosensory cortex
6 (PMA); Premotor cortex (area)
8 (SMA); Motor association areas
41 (A1); Auditory cortex
4 (M1); Motor cortex
7; Sensory association cortex
17 (V1); Visual cortex
45; Speech (Broca’s area)
Describe the brain at the gross level
at the gross level it consists of cerebral hemispheres, thalamic masses, brainstem (midbrain, pons and medulla), cerebellum and cavities (-ventricles)
The Cerebral Cortex has many lobes, which make up Cerebral Hemispheres. There are 4 major lobes (from Front to Back):
- Frontal Lobe
- Parietal Lobe
- Temporal Lobe
- Occipital Lobe
How is the anatomical orientation in the forebrain different compared to the brainstem?
Describe the Frontal and the Parietal Lobe
Frontal lobe, which lies anterior to the central sulcus and extends inferiorly to the lateral sulcus; medially, the frontal lobe also extends to the corpus callosum. The most prominent structure of the frontal lobe is the precentral gyrus (bounded by central and precentral sulci), which has an important role in motor function. Contains Broca’s area, important for speech.
Parietal lobe, which houses the functions that perceive and process somatosensory events, extends posteriorly from the central sulcus to the parieto-occipital sulcus. The parietal lobe contains the postcentral gyrus, bordered by the central sulcus and postcentral sulcus, which acts as the primary receiving area of somatosensory information from the periphery. The remainder of the parietal lobe can be divided into two sections, by the interparietal sulcus, into supramarginal gyrus and the angular gyrus. Wernicke’s area is found in the ventral aspect of these gyri and is vital for comprehension of spoken language.
Describe the Temporal Lobe and the Occipital Lobes
Temporal lobe which is separated via the transverse lateral sulcus and is vital in the perception of auditory signals. It consists of superior, middle, and inferior temporal gyri.
Occipital lobe which is separated from the parietal and temporal lobes by the parieto-occipital sulcus
Describe the thalamus, hypothalamus and cerebellum
The thalamus forms the central core of the brain. It is responsible for relaying and integrating information to different regions of the cerebral cortex from a variety of structures associated with sensory, motor, autonomic, and emotional processes.
The hypothalamus lies ventral and anterior to the thalamus and regulates visceral functions (temperature, endocrine functions, feeding, drinking, emotional states, and sexual behaviour) and links to the pituitary gland at the base of the brain.
The cerebellum plays a vital role in integration, regulation, and co-ordination of motor processes. It contains two symmetrical hemispheres that are continuous by a midline structure (called the vermis), and the hemispheres are divided into anterior, posterior, and flocculonodular lobe, all of which vary in the inputs they receive.
What is the brainstem? Which cranial nerves originate from the brainstem?
The brainstem is formed from the midbrain, medulla oblongata, and the pons:
The midbrain is involved in relaying information for vision and hearing. It is found caudal to the pons and rostral to the diencephalon (thalamus, hypothalamus etc.); it is composed of the tectum (which contains the superior and inferior colliculi) and the cerebral peduncle (which contains the substantia nigra)
The pons lies caudal to the medulla, rostral to the midbrain, and ventral to the cerebellum. It contains tracts passing through it as well as numerous nuclei for functioning in sleep, respiration, bladder control, and many others.
The medulla oblongata controls autonomic function (such as respiration, cardiac centre and baroreceptors, and vomiting, coughing, sneezing, and swallowing centres) and connects the higher levels of the brain to the spinal cord. It is found rostral to the pons.
The pyramids of the descending fibres can be seen in the medulla, on the anterior surface; the medulla contains all the ascending and descending tracts of the CNS. The inferior olivary nucleus is found on the rostral half of the medulla and is important in relaying information from the spinal cord and other regions of the brainstem to the cerebellum.
Many of the cranial nerves originate from the brainstem:
- CN III and IV form at the level of the midbrain
- CN V, VI, and VII form at the level of the pons
- CN VIII, IX, X, and XII form at the level of the medulla
Basal Ganglia play an important role in the regulation and integration of motor functions,
Describe the spinal cord
Aka the spinal medullaris
It is a continuation of the medulla therefore it is a neuronal mass of tissue.
Cylindric (approx.) in shape
~42-45 cm long
Somewhat flattened from front to back
Its gross shape changes from rostral to caudal
Shows two enlargements at the cervical and lumbar levels
Ends in a taper – the Conus Medullaris – at about L2 level.
- Filum Terminale and Denticulate Ligaments
It has a central cavity throughout its length, the central canal
Any segment of spinal tissue is surrounded by the vertebral column and is referred to according to the bony segment of spinal vertebrae it develops with embryonically.
Spinal vertebrae are discrete and named according to their morphology (i.e. cervical, lumbar etc) and numbered according to the numerical level in the vertebral sequence (e.g. C6, T7 etc). These vertebrae constitute vertebral levels. Neuronal segments of the spinal cord are named using the same nomenclature as the vertebrae they develop with embryonically. They are referred to as neural levels.
Where is the spinal cord enlarged?
The cervical enlargement extends from C4 to T1 segments, with most of the anterior rami at this region form the brachial plexus
The sacral enlargement extends from T11 to S1 segments of the spinal cord (i.e. inferior to the conus medullaris), with most of the anterior rami at this region forming the lumbar and sacral plexuses of nerves.
What is the cauda equina and filum terminale?
In embryos, the spinal cord occupies the full length of the vertebral canal, so the spinal nerves pass out laterally to exit the corresponding IV foramina. Yet during the foetal period, the vertebral column grows faster than the spinal cord so appears to ascend; at birth, the tip of the conus medullaris is at L4-L5 level and gradually ascends until it lies at L2 level in adults. The lumbar and sacral nerve roots are therefore the longest, extending beyond the termination of the cord at L2 level, in order to reach the remaining lumbar, sacral, and coccygeal IV foramina. This group of nerve roots running in the lumbar cistern is known as the cauda equina.
The filum terminlae is a remnant of the caudal part of the spinal cord of the embryo and descends amongst the cauda equina. It attaches to the dorsum of the coccyx, acting to anchor the inferior end of the spinal cord and spinal meninges.
Describe the fibrous joints and cranial base of the skull
Immoveable joints of the skull are called sutures – they are fibrous joints.
The coronal and saggital sutures meet at bregma
The lambdoid (parietal-occipital) and saggital sutures meet at lambda
The cranial base is irregular in appearance but divisible into 3 parts (descriptively)
- Anterior: alveolar arches of maxilla to posterior edge of hard palate
- Middle: posterior edge of hard palate to anterior edge of foramen magnum
- Posterior: lies behind the middle part
Describe the dorso-lateral appearance of the skull. What are Sulci, Fissures and Gyri?
Dorso-lateral aspect
- Shows mostly the cerebral cortex
- Highly convoluted
- Convolutions consist of:
Grooves or depressions known as Sulci (Sulcus sing.)
Major Sulci are known as Fissures
Ridges or elevations known as Gyri (Gyrus sing.)
Large Sulci are invariable between individuals and are used as important landmarks in brain mapping for surgery
The longitudinal fissure and falx cereberi divides the cortex in right and left-hemispheres, which are bilaterally symmetrical.
Hemispheres are normally interconnected by the corpus callosum and commissures (anterior and posterior). Split brain syndromes occurs when these connections are missing (some people are born like this).
Describe the medial aspect of the brain
More marked and complex presentation
Shows segmentation – segments are visible from embryonic stages
They are called vesicles. Initially there are 3 primary vesicles, 2 additional others appear later to give 5 secondary vesicles.
What are the meninges?
They support and mechanically stabilise contents of the cranium
They act as “seatbelts” of the brain within the cranium
They organise (or divide) the cranial cavity into anatomical compartments
They are membranous envelopes that completely cover the brain and consist of 3 connective tissue layers as follows: inner layer of cranium => extradural space=> dura mater (2 layers: periosteal, meningeal) => subdural space => arachnoid mater =>subarachnoid space => pia mater => brain tissue.
Recap gastrulation and the notochord
Nervous system is the most complex body system. It is one of the first systems to begin development and one of the last to complete development.
Because of the length of time taken to develop, NS is the system most susceptible to insult during pre-natal development.
The Notochord
- It is a solid rod of cells formed by prenotochordal cells migrating through the primitive pit.
- The definitive notochord serves as the basis for the midline, the axial skeleton and the neural tube.
Describe formation of the neural tube? When do the neuropores form?
First there is induction of the neural plate (thickening of ectoderm => neuroectoderm) by the notochord (day 18)
Day 19: Continued thickening and start of elevation of lateral edges of the neural plate leading to the formation of the neural groove (depressed midregion).
Neural folds gradually approach each other in the midline and fuse, producing the neural tube of neuroectoderm (folds fusing occurs day 21-23). The NT formed has anterior and posterior neuropores.
Fusion of the neural folds beings in the future cervical region.
It proceeds in both cranial and cranial and caudal directions. Defects in closure of neuropores underlie serious birth defects of the nervous system.
- The cranial neuropore closes approximately on day 25
- The caudal neuropore closes approximately on day 28
By day 28-32 post-conception, the neural tube is completely closed. The whole process takes place in 10 days yet the embryo has not yet folded.
What is Anencephaly?
Anencephaly: failure of NT closure cranially (failure of the cranial neuropore to close properly) leading to absence of cranial structures including the brain and is therefore incompatible with brain.
Rachischisis is the most severe: failure of neural fold elevation.
Describe the development of the spinal cord
Most of the length of the neural tube gives rise to the spinal cord
Up until the 3rd month, the spinal cord is the same length as the vertebral column (the 2 structures develop in parallel)
Thereafter, the vertebral column grows faster
Consequently, the spinal roots themselves must elongate to exit their original intervertebral foramina.
Forms the cauda equine (bundle of spinal nerves that originate in the conus medullaris).
Describe the development of the brain. what are the secondary brain vesicles?
During neural fold formation, three primary brain regions can be distinguished:
Forebrain: prosencephalon
Midbrain: mesencephalon
Hindbrain: rhombencephalon
After neural tube closure in the 4th week, these dilations at the cranial end become the 3 primary brain vesicles.
At 5 weeks of development, further reorganisation leads to five secondary brain vesicles forming.
What are flexures?
The cranial end of the neural tube undergoes such rapid enlargement that it rapidly exceeds the available space, so consequently begins to fold up.
Cervical flexure is at spinal cord – hindbrain junction
Cephalic flexure is at midbrain region
Thus the neuraxis does not remain straight.
Describe the development of the ventricular system
- Tubular structure of the developing CNS persists as development proceeds (the lumen remains patent)
- In the adult, it is comprised of interconnected reservoirs filled with CSF produced by cells of ventricular lining.
- Role: to cushion brain and spinal cord within their bony cases
- The lumen does change shape throughout its development
- So the lateral ventricle and third ventricle are derived from the forebrain, the cerebral aqueduct is derived from the midbrain and the fourth ventricle is derived from the hindbrain.
- Hydrocephalus is mostly common in newborns suffering from spina bifida. It can result if there is blockage of the ventricular system or impaired absorption of CSF fluid
Describe the patterning and organisation of the neural tube
From inside out:
- The neuroepithelial layer contributes to the lining of the ventricular system + secretion of CSF
- The intermediate (mantle) layer contains cell bodies of primitive neurones (neuroblasts)
- The marginal layer contains the processes of the neuroblasts
Roof and floor plates regulate dorsal and ventral patterning of the neural leading to different functions:
- Dorsal – alar – plate: sensory
- Ventral – basal – plate: motor (hence why motoneurones are present in ventral horn).
Describe the development of the neural crest and defects arising from neural crest migration
As the neural tube begins forming, cells of the lateral border of the neuroectoderm tube become displaced (before fusion of NT folds) and enter the mesoderm. They undergo epithelial to mesenchymal transition.
Defects of neural crest migration:
- Neural crest cells migrate extensively and contribute to a wide range of structures such as adrenal medulla, Schwanna cells and C cells of the thyroid gland.
- They have a complex migratory pattern thus are extremely vulnerable to environmental insult (especially alcohol) but defects can also be genetic.
- Defects can affect a single component (one structure) but can also affect multiple, resulting in recognisable syndromes.
Differentiate between grey matter and white matter. What is a fasiculus?
Nervous tissue consists of neurones and supporting glial cells. Neurones have a cell body and axonal processes or nerve fibres. In the brain and spinal cord, areas consisting mainly of cell bodies are referred to as areas of grey matter and areas mainly of nerve fibres as white matter (because of myelination).
In the brain, grey matter may exist as an outer covering (in the case of the cortex) or as discrete areas inside the brain. In sub-cortical structures of the brain, grey matter constitutes nuclei.
Within the white matter, nerves are collected into bundles (tracts), which form pathways connecting different regions of the CNS together. Each tract can be described anatomically i.e. in terms of its connections and its course or pathway through the system. They can also be described functionally e.g. as sensory or motor tracts. Ascending tracts are sensory, bringing information from the environment into the CNS where it may be consciously perceived, others convey information about muscle length etc which is not consciously perceived. Descending tracts are motor, controlling movement, which may be voluntary (conceived) or involuntary (unconscious).
In the spinal cord, a fasiculus is a bundle of axons and a funiculus is a bundle of fasiculi.
