Section 2 Flashcards
From where does the forebrain receive information?
from our sensors and sends controlling signals out to motor and visceral systems.
How does the forebrain perform functions?
By carrying out the complex computations we call higher cognitive processes.
Within the forebrain, what is the principal structure involved in the higher cognitive processes?
the cerebral cortex, but subcortical structures also play a role.
Cognitive deficits signal what?
forebrain, and usually, cortical pathology.
Three subcortical nuclei, are usually referred together as?
the Basal Ganglia which are major deep structures of the forebrain.
What is the striatum?
caudate, putamen, together commonly referred to as the striatum
Basal ganglia is composed of
1) caudate 2) putamen, (together commonly referred to as the striatum) 3) globus pallidus - note: besides these three subcortical structures, substantia nigra and subthalamic nucleus are part of the Basal Ganglia
What happens to the nuclei of the basal ganglia?
These nuclei are involved in motor control and diseases of the basal ganglia resulting in a variety of motor symptoms
What are some of the motor symptoms involved with damage to the basal ganglia?
difficulty in initiating movement (akinesia), abnormalities of muscle tone (rigidity), or the development of various involuntary motor movements (tremor, chorea, ballism, etc.) which are characteristic of various basal ganglia diseases.
In addition to motor control, what does the basal ganglia have a role in?
cognitive functions.
Amygdala is involved in
Another subcortical nucleus, the amygdala, is involved in controlling emotional behavior
Amygdala is an important component of what?
the limbic system
The limbic lobe comprises what?
only part of the limbic system.
The limbic system includes what?
1) amygdala 2) anterior portion of the cingulate gyrus 3) orbital and medial prefrontal gyri of the cerebral cortex 4) ventral parts of the basal ganglia 5) the hippocampus (though probably not directly involved in emotional behavior control) 6) parts of the thalamus that projects to the cortical regions
What do portions of the limbic system interact with?
the hypothalamus and autonomic areas of the brainstem
The anterior horn of the lateral ventricle lies in what?
frontal lobe
the body of the lateral ventricle does what?
extends across the frontal and parietal lobe,
The posterior horn of the lateral ventricle
there is a spur that extends into the occipital lobe.
The inferior horn lies in the
temporal lobe.
The third ventricle lies where?
At the midline where you identified the thalamus on the half-brain specimen;
What is the thalamus punctured by?
a hole where the thalamic adhesion (massa intermedia) had been.
How does the lateral ventricles communicate with the third ventricle?
by way of the interventricular foramen, sometimes called the Foramen of Monro.
How does the third ventricle communicate with the fourth ventricle?
through the cerebral aqueduct.
What is the path of the cerebral aqueduct?
This is a narrow passage (normally smaller than that represented in the model) that passes through the midbrain under the superior and inferior colliculi.
the dorsal surface of the lateral ventricles
This is where the fibers of the corpus callosum form the ventricular roof.
What is the cavity in the ventral lateral surface of the anterior horn of the lateral ventricle created by?
the head of the caudate nucleus that lies on the floor of the anterior horn.
a hollow on the ventral medial surface of the inferior horn is produced by what?
the hippocampus
The hole piercing the third ventricle is made by
the massa intermedia (or thalamic adhesion; this connection between the thalami on each side does not occur in some brains and is not a commissure).
The third ventricle extends ventrally
to a point like the beak of a bird. This point extends to the bottom of the brain where it is surrounded by the hypothalamus.
The medial surface of the posterior horn normally abuts what?
the calcarine sulcus where this sulcus extends inward from the medial surface of the occipital lobe
What is the tectum?
(superior and inferior colliculi)
Where is the tectum?
(superior and inferior colliculi) lies dorsal to the aqueduct, and the tegmentum lies below it.
Where is the cerebellum relative to the ventricles?
It lies above the fourth ventricle, forming its roof,
What forms the floor of the fourth ventricle?
the continuation of the tegmentum into the pons region forms the floor of this ventricle.
What does the blood brain barrier (BBB) do?
maintains a stable environment for neurons to function effectively, protecting the CNS from severe fluctuations in ionic concentrations, excludes many toxic compounds, and protects central neurons from circulating hormones and neurotransmitters released in other parts of the body
What is the actual locus of the BBB?
it is the capillary endothelium
How is the capillary endothelium specialized?
by having: 1) tight junctions between capillary endothelial cells, 2) few endocytotic vesicles for intracellular transport, 3) a high number of mitochondria indicative of high levels of oxidative metabolism.
This barrier is selective, how is entry into the brain is achieved?
1) by diffusion of lipid-soluble substances 2) by facilitative and energy-dependent transport of specific water soluble substances (e.g. glucose and amino acids) 3) by ion channels.
Give an example for diffusion of lipid soluble substances across the blood brain barrier?
(e.g. lipid soluble gases O2 and CO2)
Diffusion of lipid soluble substances across the blood brain barrier is related to what (give an example)?
generally to their oil/water partition coefficient, (e.g. Penicillin -hydrophilic- does not transport well, while diazepam –very hydrophobic- does)
How do substances get out of the brain?
Besides carrying substances to the brain, there is also a reverse pump
Where are the reverse pumps for the blood brain barrier?
intracellular transporters within astroglia that move lipophilic molecules of the brain into the blood
Example of the BBB reverse pumps.
glycine, glutamate which may limit the accumulation of these neurotransmitters within the brain
Why is the reverse pump within the astroglia?
This is probably related to the processes of astroglial cells that encase the capillaries.
Can whole cells cross the BBB?
Some whole cells can migrate across a healthy BBB, especially lymphocytes and macrophages,
What is the mechanism by which whole cells cross the BBB?
probably by a mechanism that opens the tight junctions. Unfortunately, this mechanism also permits the entry of the myelin-reactive lymphocytes of multiple sclerosis, and HIV-infected macrophages.
Several regions of the CNS do not have a BBB, all of which need to be able to do what?
sense fluctuations in the blood, because they are part of the circuitry for regulating these fluxes (e.g. the area postrema of the 4th ventricle may detect poisons in the blood and initiate the vomiting reflex).
What happens to the BBB in many brain tumors?
The BBB breaks down in many brain tumors, with bacterial invasion, and during ischemia.
What is the most common cause of BBB breakdown?
Ischemia
The pathology of a stroke
stems not only from the loss of tissue oxygenation, but also from the introduction of toxins to the brain, and from ionic fluxes.
Ischemia directly damages what?
the cells’ Na and K pumps.
In ischemia, what causes the cells to fill with water?
Na and K pumps together with the ionic influx causes the cells to fill with water.
What does the swelling result in?
The resulting brain swelling (edema) with its attendant increase in intracranial pressure will, if unchecked, eventually lead to coma and death.
Cerebrospinal fluid (CSF) is in equilibrium with
brain extracellular fluid and maintains a constant external environment for cells of the CNS, preserving homeostasis.
In addition to preserving homeostasis what else does the CSF do?
It also provides buoyancy for the brain, decreasing the weight of the brain on the skull, and serves as a mechanical cushion, protecting it from impact with the bones of the skull.
CSF drains what?
unwanted substances from the brain, serving as the brain’s lymphatic system.
The total volume of the CSF is estimated at
~140 ml,
What is the rate of formation of CSF?
the rate of formation of CSF is about 500 ml per day,
So the entire volume of CSF is turned over how many times?
three or four times a day.
CSF is secreted primarily by the
choroid plexus
What does the choroid plexus consist of?
a specialized capillary network surrounded by a cuboidal or columnar epithelium.
What maintains the chemical stability of the CSF?
Choroid plexus
The capillaries of the choroid plexus are freely permeable to what?
plasma solutes
What are the epithelial cells of the choroids plexus responsible for?
Its a barrier that is responsible for carrier-mediated active transport that is bi-directional.
What does the choroid plexus account for?
For the continuous production of CSF and active transport of metabolites out of the CNS and into the blood.
CSF and the extracellular fluids of the brain are in
a steady state normally and differ from blood plasma
How does the CSF compare to the blood plasma?
concentrations of K+, Ca2+, bicarbonate, glucose and protein in CSF are lower than in the blood plasma, and CSF is also more acidic
CSF is normally
clear and does not contain red blood cells and few, if any, white blood cells.
The choroid plexus is found within
the floor of the inferior horn and body of the lateral ventricle, the roof of the third ventricle, and the inferior part of the roof of the fourth ventricle
CSF is made in the
lateral ventricles flows into the third ventricle and this CSF, along with that added by the choroid plexus in the roof of the third ventricle, flows through the cerebral aqueduct to reach the fourth ventricle.
Does most of the CSF continue down the central canal of the spinal cord?
Most of the flow does not continue down the central canal of the spinal cord, which is vestigial in adults, but exits the fourth ventricle through three foramina; 1) the Foramen of Magendie 2) Foramina of Luschka (two of them)
Foramen of Magendie
a midline opening at the caudal end of the fourth ventricle,
Two Foramina of Luschka
at the lateral edge of the ventricle at its widest extent where cranial nerve VIII lies, adjacent to the flocculus.
What happens to the CSF after it exits the foramen?
it continues to flow over the whole brain and spinal cord beneath the arachnoid mater in the subarachnoid space
Describe the CSF flow in the subarachnoid space.
It flows slowly over the convexities of the cerebral hemispheres until it reaches the arachnoid villi in the walls of the dural sinuses, most notably the superior sagittal sinus.
Where does absorption of the majority of the CSF into the venous system occur?
through the arachnoid granulations or villi (in the walls of the dural sinuses, most notably the superior sagittal sinus)
The flow of CSF can be obstructed, especially at what point?
the interventricular foramen or at the cerebral aqueduct.
What is obstruction of CSF flow called?
This produces an obstructive or non-communicating hydrocephalus.
Obstruction of CSF flow such as a tumor, occurs most frequently where?
Wherever the ventricular system narrows (interventricular foramen, cerebral aqueduct, or at the outlet of the 4th ventricle)
Communicating hydrocephalus
occurs when there is no blockage of ventricular flow, but the arachnoid villi are diseased and absorption fails.
In either type of obstruction, the continued buildup of CSF (~20 ml/hr) causes what?
increased ventricular pressure, expanding the ventricles and putting pressure on brain tissue.
Clinically when can increased intracranial pressure be seen?
when inspecting the fundus of the eye with an ophthalmoscope.
What is seen in the eye under conditions of increased ICP?
The retinal vessels of the optic nerve become engorged and the optic nerve head becomes dilated. This effect is called papilledema.
What are the symptoms resulting from increased intracranial pressure?
headache, nausea, vomiting, cognitive impairment, a decreased level of consciousness, and may include impaired vision and sixth nerve palsies.
How might the frontal lobe function be affected with increased ICP?
it is often compromised including the descending white matter pathways from the frontal lobe
ICP compromising the descending white grey matter pathways from the frontal lobe may lead to what?
a characteristic unsteady gait where the feet barely leave the floor (magnetic gait) as well as incontinence.
The composition of the CSF
has clinical significance since it may be altered in different disease states.
An increased white cell count in the CSF
(greater than 4/mm3) indicates pathology and may be a thousand-fold greater in acute bacterial meningitis
Why might the white cell count in the CSF be a thousand fold greater in acute bacterial meningitis?
BBB breaks down in this case allowing entry of antibiotics to address the infection
How might protein content be increased in the CSF?
by many pathological processes due to changes in vascular permeability or CSF dynamics.