Neuroanatomy: Key Concepts

Cerebrospinal Fluid (CSF)

• CSF is produced by the choroid plexuses of the third, lateral, and fourth ventricles, with about 30% coming from brain capillaries.
• The total volume of CSF is about 130 ml, with a pressure of approximately 130 mm of water.
• Approximately 30 ml of CSF are within the ventricular system.
• Approximately 100 ml are in the subarachnoid space (75 ml in the spinal part and 25 ml in the cranial part).
• The total production of CSF is over 500 ml per day.
• Constant circulation and resorption take place mainly through the arachnoid granulations.
• Some drainage occurs through the cribriform plate of the ethmoid bone into the tissues of the nose and cervical lymphatics.
• Changes in arterial pressure have little effect on CSF pressure.
• Increases in venous pressure quickly reflect in CSF pressure.
• Lumbar puncture reduces CSF pressure; leakage at the puncture site can keep the pressure low for days.
• CSF acts as a protective buffer for neural tissue, effectively reducing the brain's weight from 1500 g to 50 g by allowing it to float.
• CSF serves as a pathway for removing brain metabolites.
• There is no "brain/CSF barrier."
• Ependymal cells of the ventricles, covering the choroid plexuses, have selective transport mechanisms and tight junctions, forming a "blood/CSF barrier" similar to the blood/brain barrier.

Cranial Nerve Nuclei

• Cranial nerves with motor nuclei for skeletal muscle send fibers directly to those muscles.
• Nuclei for cardiac and visceral muscle and glands send fibers for peripheral relay in autonomic ganglion cells.
• Sensory nuclei of cranial nerves contain the cell bodies of the second sensory neurons.
• Cell bodies of the first neurons are outside the CNS in ganglia on the nerves themselves, similar to posterior root ganglia of spinal nerves.
• Central processes from sensory nuclei go to motor nuclei for reflexes, the cerebellum, and the opposite thalamus for relay to the sensory cortex.
• A solitary exception is the mesencephalic part of the trigeminal nucleus, which contains the cell bodies of the first neuron on the proprioceptive pathway from muscles of the orbit, face, and tongue.

Oculomotor Nerve Nuclei

• Includes two motor nuclei
• Somatic motor nucleus (oculomotor nucleus): Near midline in floor of aqueduct of midbrain, level with superior colliculi, for superior, medial, and inferior rectus, inferior oblique and levator palpebrae superioris.
• Visceral motor nucleus (Edinger-Westphal nucleus): Cranial to somatic part; goes to ciliary ganglion for sphincter pupillae and ciliary body.

Trochlear Nerve Nucleus

• Motor, somatic: Near midline of floor of aqueduct of midbrain, level with inferior colliculi, for superior oblique.

Trigeminal Nerve Nuclei

• One motor and three sensory nuclei.
• Motor (branchial): Motor nucleus of trigeminal; off-center in upper pons deep to floor of fourth ventricle, for mastication muscles, mylohyoid, and tensor palati.
• Sensory (somatic): divided in three continuous parts throughout the brainstem and extending into the upper spinal cord
• Mesencephalic nucleus: In grey matter lateral to aqueduct of midbrain; for proprioception from muscles of mastication, face, tongue, and orbit.
• Main sensory nucleus: In lateral part of upper pons, lateral to motor nucleus; for touch from trigeminal area.
• Spinal nucleus: In lower pons and throughout medulla, continuous below with gelatinous substance of spinal cord; for pain and temperature from trigeminal area. Afferent fibers from glossopharyngeal and vagus nerves may also reach these nuclei.

Abducent Nucleus

• Motor, somatic nucleus: Near midline in pons deep to facial colliculus in floor of fourth ventricle, for lateral rectus.

Facial Nerve Nuclei

• Two motor and two sensory nuclei.
• Motor (branchial): Facial nerve nucleus; off center in pons, deep and lateral to facial colliculus.
• Motor (visceral): Superior salivary nucleus adjacent to facial nucleus; secretomotor to pterygopalatine and submandibular ganglia, mainly for lacrimal and salivary secretion.
• Sensory (branchial): Nucleus of tractus solitarius, lateral to dorsal nucleus of vagus in upper medulla, for taste fibers of chorda tympani from tongue and of greater petrosal nerve from taste buds of soft palate.
• Sensory (somatic): Sensory nuclei of trigeminal nerve for skin of external acoustic meatus and tympanic membrane.

Vestibulocochlear Nerve Nuclei

• Six special sensory nuclei.
• Cochlear nuclei: Two in medulla buried ventrally and dorsally in the inferior cerebellar peduncle for hearing.
• Vestibular nuclei: Four in pons and medulla. Under vestibular area in the lateral angle of the floor of the fourth ventricle, for equilibrium.

Glossopharyngeal Nerve Nuclei

• Two motor and two sensory nuclei.
• Motor (branchial): Nucleus ambiguus deep in upper medulla, for stylopharyngeus.
• Motor (visceral): Inferior salivary nucleus adjacent to facial nucleus in lower pons, for otic ganglion and parotid secretion.
• Sensory (visceral): Nucleus of tractus solitarius, lateral to dorsal nucleus of vagus in upper medulla, for taste fibers from the posterior third of the tongue and baroreceptors of carotid sinus (blood pressure) and chemoreceptors of the carotid body (O2 and CO2 of blood).
• Sensory (somatic): Sensory nuclei of trigeminal nerve for ordinary sensation from the mucous membrane of the tongue, palate, pharynx, and tonsil.

Vagus Nerve Nuclei

• Two motor and two sensory nuclei.
• Motor (branchial): Nucleus ambiguus deep in upper medulla, for skeletal muscle of pharynx, upper oesophagus, palate, and larynx.
• Motor (visceral): Dorsal motor nucleus of vagus, below vagal trigone of upper medulla, for cardiac muscle and visceral muscle of thoracic and abdominal viscera.
• Sensory (visceral): Connects to the Nucleus of tractus solitarius, lateral to dorsal nucleus of vagus in upper medulla, for afferent fibres from heart, lungs, and abdominal viscera, for baroreceptors of the aortic arch (blood pressure) and chemoreceptors of aortic bodies (O2 and CO2 of blood), and for taste fibres from the epiglottis.
• Sensory (somatic): Connects to the Sensory nuclei of trigeminal nerve, for skin of external acoustic meatus and behind the auricle, and perhaps for mucous membrane of the pharynx and larynx (though these fibres may pass to the nucleus of tractus solitarius).

Accessory Nerve Nuclei

• Two motor nuclei
• Motor (branchial): Cranial part- nucleus ambiguus deep in upper medulla, fibres joining the vagus for skeletal muscle of palate and pharynx.
• Motor (somatic): Spinal part- anterior horn cells of the upper five or six cervical segments of spinal cord, for sternocleidomastoid and trapezius.

Hypoglossal Nucleus

• Motor, somatic nucleus: Near midline below hypoglossal trigone of upper medulla, for muscles of tongue.

brain - Death

• Brain death requires irreversible cessation of brainstem function.
• Exclude effects of drugs, hypothermia, and endocrine/metabolic disturbances before diagnosis.
• Absence of brainstem reflexes are needed to diagnose brain death.
• Criteria include:
  • Fixed pupils not reacting to light (testing midbrain function).
  • No corneal reflexes (testing pontine connections between trigeminal and facial nerve nuclei).
  • No vestibulo-ocular reflexes (testing connections between vestibular nerve and eye-muscle nerves).
  • No gag reflex or response to bronchial stimulation (testing vagal connections in the medulla).
  • No motor responses in cranial nerves on adequate nociceptive stimulation.
  • No respiratory movements when disconnected from the ventilator long enough to cause arterial CO2 to rise above the threshold for respiratory stimulation (Pco2 > 50 mm Hg or 6.7 kPA).

Cerebellum

• The cerebellum can be considered a brainstem nucleus grown backwards onto the brainstem's dorsal surface during vertebrate evolution.
• It is found in the posterior cranial fossa.
• Two hemispheres are united by the vermis, its midline.
• Three peduncles connect each hemisphere to the brainstem: the superior peduncle (to the midbrain), the middle peduncle (pons), and the inferior peduncle (medulla).
• The ventral surface of the vermis sits on the superior medullary velum and the roof of the fourth ventricle's medullary portion.
• The parts of the vermis that the ancients named have morphological and functional significance.
• Parts of the hemispheres that the ancients named are functionally meaningless and can safely be ignored.
• Terms correlated with evolutionary history:
  • Archaeocerebellum: the oldest part; concerned with balance (vestibular mechanisms).
  • Palaeocerebellum: assists in limb movement control (spinal mechanisms).
  • Neocerebellum: the most recent development; associated with the cerebral hemispheres.
• The superior surface of the cerebellum lies below the tentorium cerebelli.
• Posteroinferior surfaces occupy concavities in the occipital bone.
• The vallecula is a deep groove between them that houses the inferior vermis parts.
• The surface is marked by fine sulci and parallel cerebellar folia.
• A horizontal fissure indents the posterior border.
• A primary fissure (significant) separates the palaeocerebellum and neocerebellum.
• Hemispheres have an anterior lobe (palaeocerebellum) and a large posterior lobe (neocerebellum).
• The vermis has superior and inferior parts, separated by the neocerebellar posterior lobes.
• The lingula is the superior vermis part in contact with the superior medullary velum.
• The inferior vermis in the vallecula has three lobules: pyramid, uvula, and nodule.
• The nodule sits on the fourth ventricle's roof.
• The flocculus is a slender band of white matter projecting laterally from the nodule.
• The flocculonodular lobe (flocculi and nodule) and the lingula represent the archaeocerebellum.
• The uvula and pyramid, along with the anterior lobe, form the palaeocerebellum.
• The paraflocculus is a slender lobule projecting laterally (mainly attached to the pyramid).

Cerebellum: Morphology

• Three distinct morphological parts evolved sequentially, each with different functions.
• The archaeocerebellum has vestibular connections only (lingula, uvula, and flocculonodular lobe); lesions cause trunk ataxia.
• The palaeocerebellum has spinal connections (spinocerebellar tracts) and is concerned with postural and righting reflexes (anterior lobe, pyramid and paraflocculus, and uvula); lesions cause disturbances of postural mechanisms and increased muscle reflexes.
• The neocerebellum has cerebropontine connections and is involved in feedback with basal nuclei and the cerebral cortex (the cerebellar hemispheres behind the primary fissure); lesions lead to hypotonia, intention tremor, dysdiadochokinesia, and nystagmus.
• The essential function of the cerebellum is movement coordination.
• Cerebellar lesions cause disturbances of movement and balance such as ataxia, intention tremor, nystagmus, and dysarthria, but not paralysis.

Cerebellum: Internal Structure

• The Cerebellum contains a cortex of grey matter.
• White matter is internal.
• Subcortical nuclei of grey matter lie near the roof of the fourth ventricle.
• The dentate nucleus is the largest and most important of the subcortical nuclei.
• The neocerebellum mainly connects to the dentate nucleus.
• The dentate nucleus contains efferent fibres and goes to the contralateral red nucleus and thalamus.
• Emboliform, globose, and fastigial nuclei lie medial to the hilum of the dentate nucleus.
• The emboliform, globose are known collectively as roof nuclei.
• The fastigial nucleus belongs to the archaeocerebellum.
• The emboliform, globose nuclei belong to the palaeocerebellum.
• The cerebellar cortex has two layers- molecular and granular- that sandwich a single layer of Purkinje cells.
• Basket cells are scattered among fibres in the molecular layer, and their axons arborize around the Purkinje cell bodies.
• Round granular cells that are tightly packed together overwhelm the granular layer.
• Purkinje cells are large cells located between two layers.
• Incoming afferent fibres activate Purkinje cells.
• Climbing fibres activate the Purkinje cells directly
• Mossy fibres activate the Purkinje cells through intermediary granular and basket cells.
• Incoming fibres synapse in the dentate and other nuclei.
• Purkinje dendrites form a tree shape in the molecular layer.
• Purkinje axons go the dentate and other nuclei.
• Superior peduncle fibres run in the red nucleus, thalamus, and cerebral cortex that is on the opposite side.
• The climbing pontine and vestibular fibres weave around the Purkinje dendrites in the molecular layer.
• Mossy fibres arborize as a tuft around a granular cell.
• The axon of the granular cell bifurcates and arborizes the basket cells.
• An axon of the basket cell divides and arborizes 500 Purkinje cells around the cell bodies.
• A climbing fibre activates a Purkinje cell.
• A mossy fibre activates Purkinje cells.
• Incoming fibres pass to the cortex.
• Efferent fibres arise from subcortical nuclei.
• Cerebellovestibular fibres run straight from the cortex to the vestibular nuclei.
• Some vestibulocerebellar fibres enter the fastigial nucleus, skipping the cortex.
• Reticulocerebellar & Olivocerebellar fibres pass to subcortical nuclei.

Cerebellum Peduncles and Connections

• Superior and middle peduncles are relatively simple.
• The inferior peduncle contains a mixture of fibres.
• Superior peduncle contains efferent fibres.
• Fibres pass from the dentate nucleus to the red nucleus, thalamus, and opposite cortex side.
• Superior peduncle contains the anterior spinocerebellar tract that pass the anterior lobe (palaeocerebellum) and tectocerebellar fibres from the midbrain.
• Middle peduncle contains pontine nuclei fibres of the opposite side (afferent to the neocerebellum).
• Inferior peduncle is mainly afferent.
• Cerebellovestibular tract is the only tract of note.
• Cerebellovestibular tract goes from the roof nuclei to the vestibular nucleus on the same side.
• Inferior peduncle contains Cerebello-olivary and cerebelloreticular fibres.
• The Vestibulocerebellar tract (from the vestibular nucleus on the same side) forms the afferent fibres.
• The afferent fibres (posterior spinocerebellar and cuneocerebellar tracts) comprise the anterior lobe (palaeocerebellum).
• Cuneocerebellar tract contains of posterior external arcuate fibres from the accessory cuneate nucleus for proprioception from the upper limb.
• Olivocerebellar tract, pontine nuclei fibres, and the reticular formation form the posterior lobe (neocerebellum).

Cerebellum: Blood Supply

• Two Arteries are used to supply the large convex surface. As well as one artery which supplies the small upper surface.
• The arteries anatomize with each other on the cerebellar surface.
• The posterior inferior cerebellar artery supplies blood to one artery.
• Posterior inferior cerebellar artery arises ventrally from the vertebral artery. The spiral has backs around the medulla below the hypoglossal rootlets.
• The Anterior inferior cerebellar artery supplies blood to the basilar artery.
• The Superior cerebellar artery supplies blood to the termination of the basilar and passes.
• Venous drainage travels from the surface of the cerebellum into the nearest available venous sinus of the dura meter.

Spinal Cord

• The spinal cord's lower end tapers into a cone.
• Extends from the foramen magnum to the conus medullaris at L1-L2 vertebral level in adults.
• Possesses an anterior median fissure and a posterior median sulcus on the midline.
• The arachnoid is attached to the Posterior median septum in the subarachnoid space.
• Terminates around the S2 vertebra level as a fetus.
• At birth it is around the L3 vertebrae.
• C3 to T1 vertebrae are located in the Cervical vertebrae.
• T9 to L1 are located in the Lumbosacral
• Spinal nerves lie within the spinal theca. The spinal nerves lie between the intervertebral foramina.
• Has cervical and lumbosacral enlargements for limb plexuses (C5-T1 and L2-S3).
• Cervical and lumbosacral enlargements contain the spinal cords mass of motor cells.
• Spinal nerve roots unite within the intervertebral foramina.
• Anterior and posterior roots have rootlets attached to the spinal cord.
• Each anterior root is formed by rootlets and form irregularly.
• Each posterior root is formed by rootlets, and are attached to the surface.
• Anterior and posterior roots pass to their correct intervertebral foramina.
• Each evaginates the dura matter separately before uniting the form mixed spinal nerve.
• Posterior root ganglia lie just lateral to the intervertebral foramina, touching the vertebral artery.
• Denticulate ligament is found in all levels from C1 to LI.
• Nerve roots slope down to the spinal cord shortness.
• Below the LI vertebrae, the spinal column passes the arachnoid space (Cauda equina).
• The cauda equina consist of the nerve roots.
• The filum terminale (pia mater) starts at the tip of the conus medullaris among the nerve roots of the cauda.
• Roots of the spinal accessory nerve emerge from the lateral surface of the upper five or six segments of the cord (behind the denticulate ligament)

Spinal Cord: Internal Structure

• The spinal cord has a central mass of grey matter in the shape of a fluted column, surrounded by white matter (fibres).
• The cord is almost divided into two halves by the anterior median fissure and the posterior median septum.
• Extends as far as the grey commissure connects the grey matter halves.
• The central canal extends into the filum terminale.
• The anterior fissure does not separate the white matter.
• The grey and white matter of the right and left halves is divided into anterior, lateral and posterior columns of cells or fibres.
• From the cross sectional appearance, the grey columns are usually referred to as horns.
• Transverse sections of the cord define the positions of tracts and groups of cells.
• Most activity within the cord occurs vertically.
• The posterior white columns are largest The cervical spine.
• The anterior grey horns are largest The cervical and lumbar regions.
• An Hgirder is found in the Central grey matter
• Thoracic regions make this.
• The medial anterior horn cells lead to trunk muscles and limbs and more laterally located cells lead to limbs.
• The Ventral cells supply movement to limb muscles, and cells lead to the distal movement of limb muscles.
• Between the limb enlargements are cells.
• Sympathetic and Parasympathetic cells have groups of similar types.
• Laminates- cells groups of the grey matter.
• Rexed areas that were researched found this to be of correct function, and that it could be verified in primates.
• Lamina II is of gelatinous substance.
• lamina V main source of anterolateral fibres
• lamina VII Medially contains the thoracic nucleus.
• a and 7 motor neurons are found within the lateral (IX lamina).

Spinal Cord: White Matter

• The spinal cord's white matter contains three main kinds of fibres: ascending, descending, and intersegmental.
• It divides into three columns due to the grey matter and nerve roots.
• The posterior white column lies between the posterior median septum and the posterior grey horn.
• The Gracile and cuneate tracts are for touch.
• The two columns (lateral and anterior) have ascending and descending tracts.
• Propriospinal fibres that communicate between segments run along the central grey matter.
• The most important tracts are the lateral corticospinal tract (motor) and the anterolateral/spinothalamic tract (pain and temperature). There is (much intermingling of fibres and, except in the posterior columns).
• There are three pathways used for Afferent.
• The 3 Fibres are- Cortex, Cerebellum, and Brainstem.
• Cerebral hemispheres are controlled by the 1st neutron.
• First Neurons Are the Cell bodies which lies outside the central nervous systems
• Interneurons in central systems are responsible for modifying the conduction. The cell bodies also send into interneurons with this function.