Tuesday, April 19, 2011

Post #8 - The Eye- Structures, Components, and Mechanisms of Disease- FINAL BLOG!!!!

         I guess the final topic that I would like to talk about is the eye. Arguably, one of the most complex parts of the human body. Each of these sections will highlight a different structure of the eye, and also a pathology that is associated with that part of the eye. 

The Conjunctiva

        The conjunctiva is the thin, transparent mucous membrane that covers the posterior surface of the lids (the palpebral conjunctiva) and the anterior surface of the sclera (the bulbar conjunctiva). It is continuous with the skin at the lid margin (a mucocutaneous junction) and with the corneal epithelium at the limbus.

        The palpebral conjunctiva lines the posterior surface of the lids and is firmly adherent to the tarsus. At the superior and inferior margins of the tarsus, the conjunctiva is reflected posteriorly (at the superior and inferior fornices) and covers the episcleral tissue to become the bulbar conjunctiva.

        The bulbar conjunctiva is loosely attached to the orbital septum in the fornices and is folded many times. This allows the eye to move and enlarges the secretory conjunctival surface. (The ducts of the lacrimal gland open into the superior temporal fornix.) Except at the limbus (where Tenon's capsule and the conjunctiva are fused for about 3 mm), the bulbar conjunctiva is loosely attached to Tenon's capsule and the underlying sclera.

        Disease of the conjunctiva is farily common, but there are a host of different disorders that can present very differently. One example is Congenital Conjunctival Lymphedema
This is a rare entity, unilateral or bilateral, and characterized by pinkish, fleshy edema of the bulbar conjunctiva. Usually observed as an isolated entity at birth, the condition is thought to be due to a congenital defect in the lymphatic drainage of the conjunctiva. It has been observed in chronic hereditary lymphedema of the lower extremities (Milroy's disease) and is thought to be an ocular manifestation of this disease rather than an associated anomaly.

The Sclera & Episclera
          The sclera is the fibrous outer protective coating of the eye, consisting almost entirely of collagen. It is dense and white and continuous with the cornea anteriorly and the dural sheath of the optic nerve posteriorly. Across the posterior scleral foramen are bands of collagen and elastic tissue, forming the lamina cribrosa, between which pass the axon bundles of the optic nerve. The outer surface of the anterior sclera is covered by a thin layer of fine elastic tissue, the episclera, which contains numerous blood vessels that nourish the sclera. The brown pigment layer on the inner surface of the sclera is the lamina fusca, which forms the outer layer of the suprachoroidal space.

          One particular pathology is the discoloration of the sclera. The normal sclera is white and opaque, so that the underlying uveal structures are not visible. Structural changes of the scleral collagen fibers and thinning of the sclera may allow the underlying uveal pigment to be seen, giving the sclera a bluish discoloration. Blue scleras also occur in several disorders that lead to disturbances in the connective tissues, such as osteogenesis imperfecta, Ehlers-Danlos syndrome, pseudoxanthoma elasticum, and Marfan's syndrome. Blue scleras are sometimes noted in normal newborn infants and in patients with keratoconus or keratoglobus.
The Cornea

          The cornea is a transparent tissue comparable in size and structure to the crystal of a small wristwatch . It is inserted into the sclera at the limbus, the circumferential depression at this junction being known as the scleral sulcus. The average adult cornea is 550 m thick in the center, although there are racial variations, and about 11.75 mm in diameter horizontally and 10.6 mm vertically. From anterior to posterior, it has five distinct layers : the epithelium (which is continuous with the epithelium of the bulbar conjunctiva), Bowman's layer, the stroma, Descemet's membrane, and the endothelium. The epithelium has five or six layers of cells.

          An associated pathology with the cornea are corneal ulcers. Central ulcers usually are infectious ulcers secondary to corneal epithelial damage. The lesion is situated centrally, away from the vascularized limbus. It is often accompanied by hypopyon, a collection of inflammatory cells seen as a pale layer in the inferior anterior chamber that also occurs in severe anterior uveitis. Although hypopyon is sterile in bacterial corneal ulcers unless there has been a rupture of Descemet's membrane, in fungal ulcers it may contain fungal elements.


Garcia-Ferrer Francisco J, Schwab Ivan R, Shetlar Debra J, "Chapter 5. Conjunctiva" (Chapter). Riordan-Eva P, Whitcher JP: Vaughan & Asbury's General Ophthalmology, 17e: http://www.accessmedicine.com/content.aspx?aID=3090544.

Riordan-Eva Paul, "Chapter 7. Disorders of the Eyes & Lids" (Chapter). McPhee SJ, Papadakis MA: CURRENT Medical Diagnosis & Treatment 2011: http://www.accessmedicine.com/content.aspx?aID=2002.


Thursday, April 14, 2011

Neuroanatomy- Post #7- Cranial Nerves

     Well group, this is the next to last blog I have to do! This is actually going to be the last one on neuroanatomy. I promise. This blog is going to strictly be on the cranial nerves. This is probably one of the most interesting topics in neuroanatomy. Or at least I think that it is. They should really let me teach a class on all this neuroanatomy stuff. Ha. I guess the best place to start would to be at the first.

CN I: Olfactory Nerve
      The olfactory nerves are short connections that project from the olfactory mucosa within the nose and the olfactory bulb within the cranial cavity. There are 9 to 15 of these nerves on each side of the brain. The olfactory bulb lies just superior to the cribriform plate and below the frontal lobe. Axons from the olfactory bulb run within the olfactory stalk, synapse in the anterior olfactory nucleus, and terminate in the primary olfactory cortex (pyriform cortex) as well as the entorhinal cortex and amygdala.

CN II: Optic Nerve
       The optic nerve contains myelinated axons that come from the ganglion cells in the retina. It passes through the optic papilla to the orbit, where it is contained within the meningeal sheaths. The nerve changes its name to optic tract when the fibers have passed through the optic chiasm. Optic tract axons project to the superior colliculus and to the lateral geniculate nucleus within the thalamus, which relays visual information to the cortex.

CN III: Oculomotor Nerve

       The oculomotor nerve contains axons that arise in the oculomotor nucleus (which innervates all of the oculomotor muscles except the superior oblique and lateral rectus) . The oculomotor nerve leaves the brain on the medial side of the cerebral peduncle, behind the posterior cerebral artery and in front of the superior cerebellar artery. It then passes anteriorly, parallel to the internal carotid artery in the lateral wall of the cavernous sinus, leaving the cranial cavity by way of the superior orbital fissure.
The somatic efferent portion of the nerve innervates the levator palpebrae superioris muscle; the superior, medial, and inferior rectus muscles; and the inferior oblique muscle. The visceral efferent portion innervates two smooth intraocular muscles: the ciliary and the constrictor pupillae.

CN IV: Trochlear Nerve
         The trochlear nerve is the only crossed cranial nerve. It originates from the trochlear nucleus, which is a group of specialized motor neurons located just caudal to the oculomotor nucleus within the lower midbrain. Trochlear nerve axons arise from these neurons, cross within the midbrain, and then emerge contralaterally on the dorsal surface of the brain stem. The trochlear nerve then curves ventrally between the posterior cerebral and superior cerebellar arteries. It continues anteriorly in the lateral wall of the cavernous sinus and enters the orbit via the superior orbital fissure. It innervates the superior oblique muscle.

CN V: Trigeminal Nerve

        The trigeminal nerve contains a large sensory root, which carries sensation from the skin and mucosa of most of the head and face, and a smaller motor root, which innervates most of the chewing muscles (masseter, temporalis, pterygoids, mylohyoid), and the tensor tympani muscle of the middle ear.

CN VI: Abducens Nerve

           The abducens nerve arises from neurons of the abducens nucleus located within the dorsomedial tegmentum within the caudal pons. These axons project through the body of the pons and leave it as the abducens nerve. This nerve emerges from the pontomedullary fissure, passes through the cavernous sinus close to the internal carotid, and exits from the cranial cavity via the superior orbital fissure. Its long intracranial course makes it vulnerable to pathologic processes in the posterior and middle cranial fossae. The nerve innervates the lateral rectus muscle.

CN VII: Facial Nerve

        Both parts of the facial nerve pass through the internal auditory meatus, where the geniculate ganglion for the taste component lies. The facial nerve contains axons that arise in the facial nucleus. The nerve exits through the stylomastoid foramen; it innervates the muscles of facial expression, the platysma muscle, and the stapedius muscle in the inner ear.

CN VIII: Vestibulocochlear Nerve
       Cranial nerve VIII  passes into the cranial cavity via the internal acoustic meatus and enters the brain stem behind the posterior edge of the middle cerebellar peduncle in the pontocerebellar angle. The cochlear nerve is concerned with hearing; the vestibular nerve is part of the system of equilibrium.
CN IX: Glossopharyngeal Nerve

Cranial nerve IX contains several types of fibers. Branchial efferent fibers from the ambiguus nucleus pass to the stylopharyngeal muscle.

CN X: Vagus Nerve

       Efferent fibers from the ambiguus nucleus contribute rootlets to the vagus nerve and the cranial component of the accessory nerve (XI). Those of the vagus nerve pass to the muscles of the soft palate and pharynx. Those of the accessory nerve join the vagus outside the skull and pass, via the recurrent laryngeal nerve, to the intrinsic muscles of the larynx.

CN XI: Accessory Nerve

        The accessory nerve consists of two separate components: the cranial component and the spinal component. In the cranial component, branchial efferent fibers (from the ambiguus nucleus to the intrinsic muscles of the larynx) join the accessory nerve inside the skull but are part of the vagus outside the skull. In the spinal component, the branchial efferent fibers from the lateral part of the anterior horns of the first five or six cervical cord segments ascend as the spinal root of the accessory nerve through the foramen magnum and leave the cranial cavity through the jugular foramen. These fibers supply the sternocleidomastoid muscle and partly supply the trapezius muscle. 

CN XII: Hypoglossal Nerve

            Somatic efferent fibers from the hypoglossal nucleus in the ventromedian portion of the gray matter of the medulla emerge between the pyramid and the olive to form the hypoglossal nerve . The nerve leaves the skull through the hypoglossal canal and passes to the muscles of the tongue. A few proprioceptive fibers from the tongue course in the hypoglossal nerve and end in the trigeminal nuclei of the brain stem. The hypoglossal nerve distributes motor branches to the geniohyoid and infrahyoid muscles with fibers derived from communicating branches of the first cervical nerve. A sensory recurrent meningeal branch of nerve XII innervates the dura of the posterior fossa of the skull.

The table below gives a great summary of the CN's. 

Table 8–1 Overview of Cranial Nerves.
Location of Cell bodies
Functional Type*
Motor Innervation
Sensory Function
Parasympathetic Function
Within Sensory Organ or Ganglia
Within Brain Stem
Major Connections
Special Sensory:
I Olfactory
Sense of smell
Olfactory mucosa
Mucosa projects to olfactory bulb
II Optic
Visual input from eye
Ganglion cells in retina
Projects to lateral geniculate; superior colliculus
VIII Vestibulocochlear
Auditory and vestibular input from inner ear
Cochlear ganglion
Projects to cochlear nuclei, then inferior colliculi, medial geniculate
Vestibular ganglion
Projects to vestibular nuclei
Motor for Ocular System:
III Oculomotor
Medial rectus, superior rectus, inferior rectus, inferior oblique
Oculomotor nucleus
Receives input from lateral gaze center (paramedial pontine recticular formation; PPRF) via median longitudinal fasciculus
Constriction of pupil
Edinger–Westphal nucleus
Projects to ciliary ganglia, then to pupil
IV Trochlear
Superior oblique
Trochlear nucleus
VI Abducens
Lateral rectus
Abducens nucleus
Receives input from PPRF
Other Pure Motor:
XI Accessory
Sternocleido-mastoid, trapezius
Ventral horns at C2–5
XII Hypoglossal
Muscles of tongue, hyoid bone
Hypoglossal nucleus
V Trigeminal
Sensation from face, cornea, teeth, gum, palate. General sensation from anterior 2/3 of tongue
Semilunar (= gasserian or trigeminal) ganglia
Projects to sensory nuclei and spinal tract of V, then to thalamus (VPM)
Chewing muscles
Motor nucleus of V
VII Facial
Muscles of facial expression, platysma, stapedius
Facial nucleus
Taste, anterior 2/3 of tongue (via chorda tympani)
Geniculate ganglion
Projects to solitary tract and nucleus, then to thalamus (VPM)
Submandibular, sublingual, lacrimal glands (via nervus intermedius)
Superior salivatory nucleus
IX Glossopharyngeal
Parotid gland
Inferior salivatory nucleus
General sensation from posterior 1/3 of tongue, soft palate, auditory tube. Sensory input from carotid bodies and sinus. Taste from posterior 1/3 of tongue
Inferior (petrosal) and superior glossopharyngeal ganglia
Projects to solitary tract and nucleus
Stylopharyngeus muscle
Ambiguus nucleus
X Vagus
Soft palate and pharynx
Ambiguus nucleus
Autonomic control of thoracic and abdominal viscera
Dorsal motor nucleus
External auditory meatus
Superior (jugular) ganglion
Projects to thalamus (VPM)
Sensation from abdominal and thoracic viscera
Inferior vagal (nodose) and superior ganglia
Projects to solitary tract and nucleus

*Efferent (motor): SE—somatic; general SE; BE—branchial; special VE; VE—visceral; general VE. Afferent (sensory): VA—visceral; general VA, special VA; SA—somatic; general SA; SS—sensory.
*Most nerves with SE components have a few SA fibers for proprioception

In this blog I'm not going to list any pathologies because essentially everyone did the extra credit assigment that told you what to look for and how damage presents itself. 

Neuroanatomy- Post #6- The Skull, the Ventricles, and the Spaces

     As you probably already know there are many components of the body that serve as protective mechanisms. Within the realm of neuroanatomy this is especially true. This blog is going to be covering the Skull, the ventricles, and the spaces of the brain. All of these to some extent contribute to protection of the brain. At this point, I'm not going to delve into the specifics of the skull itself, but rather what passes into it.
    The skull (cranium), surrounds the brain and meninges completely and forms a strong mechanical protection.Essential structures (eg, cranial nerves, blood vessels) travel to and from the brain through various openings (fissures, canals, foramens) in the skull and are especially subject to compression as they traverse these small passageways.

Table 11–3 Structures Passing through Openings in the Cranial Floor.

Cribriform plate of ethmoid
Olfactory nerves
Optic foramen
Optic nerve, ophthalmic artery, meninges
Superior orbital fissure
Oculomotor, trochlear, and abducens nerves; ophthalmic division of trigeminal nerve; superior ophthalmic vein
Foramen rotundum
Maxillary division of trigeminal nerve, small artery and vein
Foramen ovale
Mandibular division of trigeminal nerve, vein
Foramen lacerum
Internal carotid artery, sympathetic plexus
Foramen spinosum
Middle meningeal artery and vein
Internal acoustic meatus
Facial and vestibulocochlear nerves, internal auditory artery
Jugular foramen
Glossopharyngeal, vagus, and spinal accessory nerves; sigmoid sinus
Hypoglossal canal
Hypoglossal nerve
Foramen magnum
Medulla and meninges, spinal accessory nerve, vertebral arteries, anterior and posterior spinal arteries
     One of the more obvious problems or pathologies associated with the skull would be fractures of the skull. This can lead to many problems such as brain damage or morphological changes in the skulls shape. Leaving the person more suseptible to future damage. These fractures can occur in many places, but the most common places are referred to as Le Forts I, II, and III. See the image below.

Red = I ; Blue = II, & Green = III

The next topic I would like to focus on are the ventricles of the brain. Within the brain there is a communicating system of cavities that are lined with ependyma and filled with cerebrospinal fluid (CSF): There are two lateral ventricles, the third ventricle (between the halves of the diencephalon), the cerebral aqueduct, and the fourth ventricle within the brain stem.
         The lateral ventricles are the largest of the ventricles. They each include two central portions (body and atrium) and three extensions (horns).
          The third ventricle is a narrow vertical cleft between the two halves of the diencephalon. And the fourth ventricle is a pyramid-shaped cavity bounded ventrally by the pons and medulla oblongata.
          The last structure to point out is the cerebral aqueduct. The cerebral aqueduct is a narrow, curved channel running from the posterior third ventricle into the fourth. It contains no choroid plexus.
See the image below for details.
Lastly , I would like to talk about the spaces and meningies in the brain.
Three membranes, or meninges, envelop the brain: the dura, the arachnoid, and the pia. The dura, the outer membrane, is separated from the thin arachnoid by a potential compartment, the subdural space, which normally contains only a few drops of CSF. An extensive subarachnoid space containing CSF and the major arteries separates the arachnoid from the pia, which completely invests the brain.  The pia, together with a narrow extension of the subarachnoid space, accompanies the vessels deep into the brain tissue; this space is called the perivascular space.
         The dura, is a tough, fibrous structure with an inner (meningeal) and an outer (periosteal) layer. The dural layers over the brain are generally fused, except where they separate to provide space for the venous sinuses and where the inner layer forms septa between brain portions. The outer layer is firmly attached to the inner surface of the cranial bones and sends vascular and fibrous extensions into the bone itself; the inner layer is continuous with the spinal dura.
        The arachnoid, a delicate avascular membrane, covers the subarachnoid space, which is filled with CSF. The inner surface of the arachnoid is connected to the pia by fine arachnoid trabeculae.
     The pia is a thin connective tissue membrane that covers the brain surface and extends into sulci and fissures and around blood vessels throughout the brain.

      Damage to any of the meningeal coverings can be fatal. That is why most of the coverings are very strong and durable. They are able to withstand massive wear and tear, and are very rarely ruptured, unless intracranial pressure is increased dramatically.

Neuroanatomy- Post #5- Vasculature of the Brain

       Ok readers. After that last post I'm going to try to simply how I'm doing this. Inserting tables apparently just doesn't work well with this program. Oh well. Anyway, the next topic I want to write about is the arterial supply of the brain. Something we should all know like the back of our hand by now, right.....? Well in case you still don't know it, maybe this blog will serve as a refresher.
     The circle of Willis  is a confluence of vessels that gives rise to all of the major cerebral arteries. It is supplied by the paired internal carotid arteries and the basilar artery. When the circle is complete, it contains a posterior communicating artery on each side and an anterior communicating artery. The circle of Willis shows many variations among individuals. The posterior communicating arteries may be large on one or both sides (embryonic type); the posterior cerebral artery may be thin in its first stretch (embryonic type); and the anterior communicating artery may be absent, double, or thin. Despite these variations, usually the arteries are fairly consistent.
        The course of the large arteries  is largely ventral to the brain in a relatively small region. The arteries course in the subarachnoid space, before entering the brain itself. Each major artery supplies a certain territory, separated by border zones from other territories; sudden blockage in a vessel affects its area immediately, sometimes irreversibly.

          The arterial blood for the brain enters the cranial cavity by way of two pairs of large vessels: the internal carotid arteries, which branch off the common carotids, and the vertebral arteries, which arise from the subclavian arteries. The vertebral arterial system supplies the brain stem, cerebellum, occipital lobe, and parts of the thalamus, and the carotids normally supply the remainder of the forebrain. The carotids are interconnected via the anterior cerebral arteries and the anterior communicating artery; the carotids are also connected to the posterior cerebral arteries of the vertebral system by way of two posterior communicating arteries, part of the circle of Willis.
         After passing through the foramen magnum in the base of the skull, the two vertebral arteries form a single midline vessel, the basilar artery. This vessel terminates and bifurcates as the left and right posterior cerebral arteries. These may be thin, large, or asymmetric depending on  the embryonic pattern.  
        These branches make up a very complex and unique system that is worht knowing about. Naturally, the next topics will be on problems or pathologies that occur within the circle. There can be many disorders that can occur because of either anatomical differences between people or sheer trauma to a certain area. Examples follow. Occlusive cerebrovascular disorder can result from arterial or venous thrombosis, or embolism, and can lead to infarction of well-defined parts of the brain. Because each artery irrigates a specific part of the brain, it is often possible, on the basis of the neurologic deficit, to identify the vessel that is occluded.
Another disorder that can occur is Transient Cerebral Ischemia: Transient ischemia, if brief enough, can occur without infarction. Episodes of this type are termed transient ischemic attacks (TIAs). As with occlusive cerebrovascular disease, the neurologic abnormalities often permit the clinician to predict the vessel that is involved.
Finally another occurence is a Hemorrhage. Which is the rupture of a blood vessel  often associated with hypertension or vascular malformations or with trauma.
Hopefully this review was helpful.


Waxman SG, "Chapter 12. Vascular Supply of the Brain" (Chapter). Waxman SG: Clinical Neuroanatomy, 26e: http://www.accessmedicine.com/content.aspx?aID=5273762.

Barrett KE, Barman SM, Boitano S, Brooks H, "Chapter 34. Circulation through Special Regions" (Chapter). Barrett KE, Barman SM, Boitano S, Brooks H: Ganong's Review of Medical Physiology, 23e: http://www.accessmedicine.com/content.aspx?aID=5245769.

Neuroanatomy- Post #4- The Hypothalamus

            The hypothalamus, which serves a number of autonomic, appetitive, and regulatory functions, lies below and in front of the thalamus; it forms the floor and lower walls of the third ventricle. External landmarks of the hypothalamus are the optic chiasm; the tuber cinereum, with its infundibulum extending to the posterior lobe of the hypophysis; and the mamillary bodies lying between the cerebral peduncles.

The hypothalamus can be divided into an anterior portion, the chiasmatic region, including the lamina terminalis; the central hypothalamus, including the tuber cinereum and the infundibulum (the stalk connecting the pituitary to the hypothalamus); and the posterior portion, the mamillary area.

The right and left sides of the hypothalamus each have a medial hypothalamic area that contains many nuclei and a lateral hypothalamic area that contains fiber systems (eg, the medial forebrain bundle) and diffuse lateral nuclei.
            It is through these pathways that the hypothalamus relays afferent and efferent signals. The hypothalamus receives (afferent) inputs from limbic structures, thalamus and cortex, visceral and somatic afferents, and sensors such as osmoreceptors, which permit it to monitor the circulation.
Efferent tracts from the hypothalamus include the hypothalamohypophyseal tract, which runs from the supraoptic and paraventricular nuclei to the neurohypophysis, the mamillotegmental tract going to the tegmentum; and the mamillothalamic tract, from the mamillary nuclei to the anterior thalamic nuclei. There are also the periventricular system, including the dorsal fasciculus to the lower brain levels; the tuberohypophyseal tract, which goes from the tuberal portion of the hypothalamus to the posterior pituitary; and fibers from the septal region, by way of the fornix, to the hippocampus. The table below gives a summary of the major tracts and their roles.
********* TABLE WOULD NOT POST !!!!!!!!!! ***********
            The next topic to talk about would be the functions of the hypothalamus. These include: eating, autonomic functions, body temperature regulation, water balance, Circadian Rhythm regulation, anterior pituitary functions, some control of emotions. Naturally problems can occur within each of these categories. So that’s what I’m going to talk about next.
            For example, damage to the feeding center leads to anorexia and severe loss of body weight. This damage can occur either through physical trauma or through lesions within this area. Generally, the first employment route for treatment would be appetite increasing medications. This is used to avoid drastic health problems. Of course, this doesn’t solve the problem. So the next route is surgery. Of course, if lesions are the problem then they are removed before any medication is given, but if the cause is from blunt trauma, then usually medications are used.
Another problem can occur if there is misregulation of water balance. Hypothalamic influence on vasopressin secretion within the posterior pituitary is activated by osmoreceptors within the hypothalamus, particularly in neurons within a "thirst center" located near the supraoptic nucleus. Lack of secretion of vasopressin caused by hypothalamic or pituitary lesions can result in diabetes insipidus, which is characterized by polyuria (increased urine excretion) and polydipsia (increased thirst). Generally this is treated with a combination of surgery and diet management.
Finally, a third pathology can be associated with Circadian Rhythm malfunction. Within the hypothalamus, a specific cell group, the suprachiasmatic nucleus, functions as an intrinsic clock. Cells within this nucleus show circadian rhythms in metabolic and electrical activity, and in neurotransmitter synthesis, and appear to keep the rest of the brain on a day–night cycle. Obviously lesions within this area can lead to a malfunction of all circadian rhythms throughout the body, resulting in many problems.

1.)     Molina PE, "Chapter 2. The Hypothalamus & Posterior Pituitary Gland" (Chapter). Molina PE: Endocrine Physiology, 3e: http://www.accessmedicine.com/content.aspx?aID=6169265.
2.)     Hopper AH, Samuels MA, "Chapter 27. The Hypothalamus and Neuroendocrine Disorders" (Chapter). Ropper AH, Samuels MA: Adams and Victor's Principles of Neurology, 9e: http://www.accessmedicine.com/content.aspx?aID=3634491.

Wednesday, April 13, 2011

Neuroanatomy- Post #3- Development, the Spinal Cord, etc.

         This blog is going to be divided into three section: more divisions of the brain, spinal cord overview, and Cerebrospinal fluid. This is a semi-continuation of the last blog, but this one is going to be shorter than the last one. I got kinda carried away with that. ha. Hope you enjoy!
        The first topic that I would like to mention is how the brain develops, and it's embryonic origins. This is important to understand because each of the brain structures rise from some common origins. There are three main divisions that are associated with development: the Hindbrain, the Midbrain, and the Forebrain. Each of these is comprised of embryonic originations that give rise to individual brain structures. The image below gives a very simplistic view, but it is the most straighforward of any I've been able to find.

The Hindbrain (Rhombencephalon)

   Myelencephalon (Medulla oblongata): The base of the brain, which is formed by the enlarged top of the spinal cord. This part of the brain directly controls breathing, blood flow, and other essential functions.

   Metencephalon (Pons & Cerebellum): The metencephalon develops from the hindbrain, and is differentiated from the myelencephalon in the embryo by approximately 5 weeks of age. By the third month, the metencephalon differentiates into its two main structures, the pons and the cerebellum.

The Midbrain (Mesencephalon)

   Mesencephalon (Tectum, Tegmentum (Substantia Nigra) comprises the tectum and tegmentum. The mesencephalon is considered part of the brain stem. The substantia nigra is closely associated with motor system pathways of the basal ganglia (as mentioned in the last blog).

Forebrain (Prosencephalon)

   Diencephalon (Thalamus, Hypothalamus): The diencephalon (“interbrain”) is the region of the brain that includes the thalamus, hypothalamus. It combines with the telencephalon. The diencephalon is located near the midline of the brain.
   Telencephalon (Cerebral Cortex): The cerebrum or telencephalon, together with the diencephalon, constitute the forebrain. It is the most anterior or, especially in humans, most superior region of the vertebrate central nervous system.

     All of these areas combine to form the brain. This can be important because it allow us to pinpoint when certain structures will develop, and if something goes wrong, it can be determined when this might have happened. If you refer to the table below, you can see when certain milestones are for neural development.

Table 28-2 Timetable of Growth and Nervous System Development in the Normal Embryo and Fetus

Age, Days
Size (Crown–Rump Length), mm
Nervous System Development
Neural groove and tube
Optic vesicles
Closure of anterior neuropore
Closure of posterior neuropore; ventral horn cells appear
Anterior and posterior roots
Five cerebral vesicles
Primordium of cerebellum
Differentiation of cerebral cortex and meninges
Primary cerebral fissures appear
Secondary cerebral sulci and first myelination appear in brain
8–9 months
Further myelination and growth of brain (see text)

      As you can see there are many stages in which something can go wrong in this development. One such example is Anencephaly. Anencephaly is a neural tube defect that occurs when the cephalic (head) end of the neural tube fails to close, usually between the 23rd and 26th days of pregnancy, resulting in the absence of a major portion of the brain, skull, and scalp. Infants with this disorder are born without a forebrain. The remaining brain tissue is often exposed - not covered by bone or skin. Unfortunately it is not known what causes this disorder, but it is suggested that women who are pregnant should take folic acid supplements. Recent studies have shown that the addition of folic acid to the diet will significantly reduce the incidence of neural tube defects.

         The next topic that I would like to discuss is the spinal cord, in a general overview sense. You probably already know what is to follow, but it may be a good review for you anyway.  The spinal cord is the the major column of nerve tissue that is connected to the brain and lies within the vertebral canal and from which the spinal nerves emerge. Thirty-one pairs of spinal nerves originate in the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. The spinal cord and the brain constitute the central nervous system (CNS). The spinal cord consists of nerve fibers that transmit impulses to and from the brain. Like the brain, the spinal cord is covered by three connective-tissue envelopes called the meninges. The space between the outer and middle envelopes is filled with cerebrospinal fluid (CSF), a clear fluid that protects the spinal cord from blunt impact.

         Of course there are many pathologies that can be associated with the spinal cord, but one of the most obvious would be damage to a spinal nerve. There are cases in which the nerve can either be "pinched" or completely severed. Depending on the extent of the nerve involvement, the damage can have widespread effects. Usually the injuries are work related, and can leave a person permanently injured. Usually treatment will focus on the pain management aspect of medicine.


1.)      Moore Keith L., Dalley Arthur F., Agur Anne M.R., "Chapter 6: Upper Limb". Lippincott Williams & Wilkins. Clinically Oriented Anatomy, 6e. 2010
2.)      Larson, J.L., Hurston, M.T., & Felli, G.J., "Chapter 3: Cerebrum". McGraw-Hill Inc. Neuroanatomy Through Clinical Cases. 2011.
3.)      Waxman SG, "Chapter 4. The Relationship Between Neuroanatomy and Neurology" (Chapter). Waxman SG: Clinical Neuroanatomy, 26e: http://www.accessmedicine.com/content.aspx?aID=5271560. 4.)      Lomen-Hoerth Catherine, Messing Robert O, "Chapter 7. Nervous System Disorders" (Chapter). McPhee SJ, Hammer GD: Pathophysiology of Disease, 6e: http://www.accessmedicine.com/content.aspx?aID=5368376.
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