TBL28 - Brain

Question 1

What do three dilations in the cephalic end of the neural tube create? In the embryo, what two parts make up the forebrain and what do these two parts form, respectively?

Answer 1

1) Three dilations in the cephalic end of the neural tube create the forebrain, midbrain, and hindbrain
2) The forebrain consists of the telencephalon, which forms the cerebral hemispheres, and the diencephalon that forms the optic vesicles (to be studied later), the hypothalamus, infundibular stalk, and pars nervosa

Question 2

What forms the midbrain? What does the hindbrain consist of? What does the metencephalon form and what does the myelencephalon form? What part of the brainstem does the midbrain form?

Answer 2

1) The mesencephalon forms the midbrain
2) The hindbrain consists of the metencephalon and myelencephalon
3) The metencephalon forms the pons and cerebellum; and the myelencephalon forms the medulla oblongata of the brainstem
4) The midbrain forms the cranial part of the brainstem

Question 3

How do closure defects associated with exencephaly and craniorachischisis differ? How can these defects be prevented?

Answer 3

1) Exencephaly is characterized by failure of the cephalic part of the neural tube to close
2) In some cases, the closure defect of the neural tube extends caudally into the spinal cord, and the abnormality is called craniorachischisis
3) Women can take 400 micrograms of folic acid per day before and during pregnancy

Question 4

After closure of the cranial and caudal neuropores, what does the neural tube form? What forms the central canal of the spinal cord?

Answer 4

1) After closure of the cranial and caudal neuropores, the neural tube forms the spinal cord and brain
2) The neural tube lumen forms the central canal of the spinal cord

Question 5

What does the choroid plexus project into and how much CSF does it produce per day? Where does CSF flow from and to?

Answer 5

1) The choroid plexus, which projects into the lumen of the ventricles, produces 500 ml CSF/day
2) CSF flows inferiorly from the lateral ventricles into the 3rd ventricle between the right and left halves of the diencephalon

Question 6

Where does CSF flow inferiorly from the 3rd ventricle into and what does this enable CSF to flow directly into? Where else does CSF flow from the 4th ventricle into?

Answer 6

1) CSF flows inferiorly from the 3rd ventricle via the cerebral aqueduct into the 4th ventricle, which enables CSF to flow directly into the central canal of the spinal cord
2) From the 4th ventricle, CSF also flows into the subarachnoid space of the cerebral hemispheres, cerebellum, and spinal cord

Question 7

At certain areas along the base of the brain, what creates subarachnoid cisterns? What do the subarachnoid cisterns prevent compression of?

Answer 7

1) At certain areas along the base of the brain, enlargement of the subarachnoid space creates subarachnoid cisterns (ignore names)
2) The subarachnoid cisterns prevent compression of cranial nerve roots and the cerebral arterial circle (assessed below) against the cranial base

Question 8

What lines the central canal of the spinal cord and ventricular system of the brain? What contributes to flow of CSF within the ventricular system?

Answer 8

1) The central canal of the spinal cord and ventricular system of the brain are lined by ciliated, simple cuboidal epithelium designated the ependyma
2) Ciliary action of the ependyma contributes to flow of CSF within the ventricular system

Question 9

What do protrusions of the arachnoid through the meningeal dura form and where are these structures formed? What covers these structures? From the subarachnoid space, where is CSF absorbed across and into?

Answer 9

1) Protrusions of the arachnoid through the meningeal dura form arachnoid granulations mainly in the lumen of the superior sagittal sinus
2) Endothelium of the sinus covers the granulations
3) From the subarachnoid space, CSF is absorbed across the arachnoid and endothelium into the venous blood

Question 10

Where does obstruction usually occur during obstructive hydrocephalus and why do the calvaria expand?

Answer 10

1) Overproduction of CSF, obstruction of CSF flow, or interference with CSF absorption results in excess fluid in the cerebral ventricles and enlargement of the head, a condition called obstructive hydrocephalus
2) The excess CSF dilates the ventricles, thins the cerebral cortex, and separates the bones of the calvaria in infants
3) Although an obstruction can occur any place, the blockage usually occurs in the cerebral aqueduct or an interventricular foramen

Question 11

Where do cranial fractures occur that result in otorrhea or rhinorrhea? Why do these conditions increase the risk of meningitis?

Answer 11

1) Fractures in the floor of the middle cranial fossa may result in CSF leakage from the external acoustic meatus (CSF otorrhea) if the meninges superior to the middle ear are torn and the tympanic membrane is ruptured
2) Fractures in the floor of the anterior cranial fossa may involve the cribriform plate of the ethmoid, resulting in CSF leakage through the nose (CSF rhinorrhea)
3) CSF otorrhea and rhinorrhea may be the primary indications of a cranial base fracture and increased risk of meningitis because an infection could spread to the meninges from the ear or nose

Question 12

How do the frequency and severity of bacterial and viral meningitis differ?

Answer 12

1) Bacterial meningitis is less common than the viral form

2) However, bacterial meningitis is more severe than viral meningitis

Question 13

What are the four types of glial cells and by how much do they outnumber neurons? What function do oligodendrocytes serve in the CNS? Define the role of microglial cells.

Answer 13

1) Glial cells, which include the ependyma, microglial cells, astrocytes, and oligodendrocytes, outnumber neurons by 10:1
2) Oligodendrocytes support and myelinate axons in the CNS
3) Microglia, as their name implies, are the smallest glial cell. They act as phagocytes and remove CNS debris, protect the brain from invading microorganisms, and constitute the brain’s immune system

Question 14

Why do glial cells form most intracranial tumors, i.e., gliomas?

Answer 14

Unlike neurons, glia retain a postnatal ability to divide and are the source of most intracranial tumors, known as gliomas

Question 15

How do astrocytes respond to CNS injury?

Answer 15

In response to CNS injury, astrocytes undergo mitosis and are the main source of gliotic scar tissue (gliosis), which may impede neural regeneration

Question 16

What does the blood-brain barrier include that covers most of the basement membrane associated with capillary endothelium? What links the contiguous endothelial cells?

Answer 16

1) The blood-brain barrier includes foot processes of the astrocytes that cover most of the basement membrane associated with the capillary endothelium
2) Tight junctions link the contiguous endothelial cells

Question 17

What is the role of the blood-brain barrier?

Answer 17

1) The BBB restricts passage of large molecules from the capillary lumen to the surrounding tissue, but it allows free passage of gases and selected molecules such as glucose
2) The barrier protects neurons in the CNS from toxins, drugs, and other potentially harmful substances that may be in the bloodstream

Question 18

What arteries do vertebral arteries arise from and where do they ascend? Near the foramen magnum, what path do the vertebral arteries take?

Answer 18

1) Vertebral arteries arise from the subclavian arteries and ascend in the transverse foramina of the cervical vertebrae
2) Near the foramen magnum, the vertebral arteries perforate the dura and arachnoid to enter the subarachnoid space of the spinal cord and continue through the foramen magnum into the subarachnoid space of the medulla oblongata

Question 19

Where do the vertebral arteries unite and what artery do they form? What does this artery divide into and where?

Answer 19

1) The vertebral arteries unite at the inferior border of the pons to form the basilar artery
2) The basilar artery divides into left and right posterior cerebral arteries at the superior border of the pons

Question 20

What does the internal carotid artery traverse, where, and what does it enter?

Answer 20

The internal carotid artery traverses the carotid canal on the external surface of the cranial base into the middle cranial fossa

Question 21

Once it enters the middle cranial fossa, where does the internal carotid artery course? Under the anterior clinoid process, where does the artery travel, what does it pierce, and what does it enter?

Answer 21

1) In the middle cranial fossa, the internal carotid courses anteriorly within the carotid groove toward the anterior clinoid process
2) Under the anterior clinoid process, the artery makes a 180⁰ turn that redirects it posteriorly where it pierces the dura and arachnoid to enter the subarachnoid space as the cerebral part of the internal carotid

Question 22

What is the terminal end of the internal carotid joined to and by what other artery? What do the anterior and middle cerebral arteries arise by? What are the left and right anterior cerebral arteries united by?

Answer 22

1) The terminal end of the internal carotid is joined to the posterior cerebral artery by the posterior communicating artery
2) The anterior and middle cerebral arteries arise by terminal bifurcation of the internal carotid
3) The left and right anterior cerebral arteries are united by the anterior communicating artery

Question 23

What does the resulting cerebral arterial circle (of Willis) on the inferior surface of the midbrain create?

Answer 23

The resulting cerebral arterial circle (of Willis) on the inferior surface of the midbrain creates anastomoses between branches of the basilar artery and the left and right internal carotid arteries

Question 24

Why are normal anastomoses between cerebral arteries not capable of compensating for arterial obstruction by a cerebral embolism?

Answer 24

Branches of the three cerebral arteries anastomose with each other on the surface of the brain; however, if a cerebral artery is obstructed by a cerebral embolism (e.g., a blood clot), these microscopic anastomoses are not capable of providing enough blood for the area of cerebral cortex concerned

Question 25

What is the typical cause of an ischemic stroke and what is its cardinal symptom? What is a berry aneurysm and where does it commonly occur? How does hemorrhagic stroke differ from ischemic stroke?

Answer 25

1) An ischemic stroke is generally caused by an embolism in a major cerebral artery
2) The cardinal feature of a stroke is the sudden onset of neurological symptoms
3) The most common type of saccular aneurysm is a berry aneurysm, occurring in the vessels of or near the cerebral arterial circle and the medium arteries at the base of the brain
4) An ischemic stroke denotes the sudden development of focal neurological deficits that are usually related to impaired cerebral blood flow. Hemorrhagic stroke follows the rupture of an artery or a saccular aneurysm, a sac-like dilation on a weak part of the arterial wall

Question 26

How do the affects of interrupted blood flow to the brain differ after 30 sec, 1-2 minutes, and 5 minutes?

Answer 26

1) An interruption of blood supply for 30 sec alters a person’s brain metabolism
2) After 1–2 min, neural function may be lost
3) After 5 min, lack of oxygen (anoxia) can result in cerebral infarction

Question 27

What are the symptoms of TIAs (transient ischemic attack) and why does their occurrence increase the risk of other vascular disorders?

Answer 27

1) The symptoms of TIA may be ambiguous: staggering, dizziness, light-headedness, fainting, and paresthesias
2) Persons with TIAs are at increased risk for myocardial infarction and ischemic stroke