Week 1 and 2: The Brain and Environs Flashcards
CNS: Forebrain
cerebral cortex, subcortical structures (basal ganglia, thalamus, hypothalamus)
CNS: Midbrain
midbrain and cerebral peduncles
CNS: Hindbrain
pons, cerebellum and medulla (brainstem)
What does “Lateral” mean?
away from the midline
What does “Medial” mean?
towards the midline
WHat are foramina/foramen?
The holes within the skull are called foramina, and these allow blood vessels, nerves, and the spinal cord to exit the skull compartments within the skull. The vertebral column also has these holes to allow nerves to pass through
Foramen MAgnum
a. The foramen magnum is the largest foramen in the skull and is where the spinal cord exits the skull cavity
b. The foramen magnum is also the point at which the medulla, which is at the base of the brainstem, ends and the spinal cord begins
What are fossa? 3 types + components?
The grooves within the skull create compartments, called fossa, with each forming a base for the lobes of the brain. Both cerebral hemispheres have these grooves in the skull
- ANterior cranial fossa (frontal lobe)
- Middle cranial fossa (temporal lobe)
- Posterior cranial fossa (cerebellum and brainstem)
3 layers of the meninges?
- Dura
- Arachnoid
- Pia
Dura
this tough, fibrous layer is comprised of two distinct layers: an outer periosteal layer that adheres to the skull and an inner meningeal layer
Dura: Exceptions where the outer layer stays attached to the skull and the inner layer folds away are:
the falx cerebri and the falx cerebelli (at the tentorium cerebelli)
where does the Falx cerebri occur?
Falx cerebri – this occurs at the longitudinal fissure that separates the two hemispheres
where does the Tentorium cerebelli occurs and what does it divide?
Tentorium cerebelli – this occurs over the cerebellum
- The tentorium cerebelli divides the posterior fossa, which contains the brainstem and cerebellum, from the rest of the cranial cavity
What is the tentorial incisura?
There is a hole in the tentorium cerebelli, called the tentorial incisura, that allows the brainstem to “pass through” into the foramen magnum
What is the Arachnoid layer?
Arachnoid – is a wispy, “spidery” layer that is attached to the inner layer of the dura. The cerebrospinal fluid (CSF) flows in this layer and blood vessels exit here as well
- subarachnoid space
What is the Pia?
- This innermost meningeal layer is formed from a very thin layer of cells. Unlike the other layers, this layer follows the gyri and sulci of the brain very closely.
- Blood vessels from the arachnoid space need penetrate through the pia to get to the brain
- The pia very closely wraps around these blood vessels at the site of penetrance
Hemorrhage
bursting of a blood vessel that causes blood to leak out
Hematoma
A collection of blood outside of a blood vessel
Aneurysm
enlargement of a blood vessel. This can lead to a leaking of blood, a hemorrhage, if the vessel breaks. If the blood pools outside of the vessel, a hematoma can develop
EPidural space
between the skull and the dura. The middle meningeal artery supplies the dura in this space. This is oxygenated blood.
Middle Menigeal Artery
supplies the dura in the epidural space. This is oxygenated blood.
Subdural space
– between the inner layer of the dura and the arachnoid. Innervation of this space by blood vessels is as follows:
Bridging veins dural venous sinuses sigmoid sinuses jugular vein
Blood vessels in Subdural space
Innervation of this space by blood vessels is as follows:
Bridging veins dural venous sinuses sigmoid sinuses jugular vein
Subarachnoid space + components
– between the arachnoid and the pia (more likely to get hemorrhage)
Contains CSF and major arteries
Oxygen’s importance
- the brain is an energy demanding organ, and lack of oxygen for about 4 minutes and longer will cause neurons to die
- Without oxygen, the brain is at a severe energy deficit. Processes that require energy (i.e. ATP) include sodium potassium pumps and transport of proteins down the axon. These very quickly cause neuronal death.
- Stroke, hemorrhages can both impede oxygen delivery to neurons
Glucose’s Importancce
– glucose is also necessary to generate ATP to use as energy. As with oxygen, you generally do not have stores of massive glucose as a reserve in the brain
- Certain glial cells might have small stores of a stored form of glucose called glycogen, but not enough for your brain to function on
- Unlike with oxygen deficits that cannot be compensated for, other areas in the body do store glycogen and can provide the brain with glucose in times of starvation
Where does CSF Flow?
Cerebrospinal fluid (CSF) is a fluid that flows in cavities of the brain, called ventricles, and in the subarachnoid space throughout the nervous system
The CSF has many functions for the nervous system
- buoyancy
- protection
- excretion of waste products
- endocrine medium
What produces CSF?
CSF is produced by the choroid plexus that lies inside the ventricles. Specialized cells within this region, called choroid epithelial cells, produce the CSF
Flow of CSF in the ventricles
a. Made in the lateral ventricles and flows through the foramen of Monro into the third ventricle
b. From the third ventricle, CSF flows through the sylvian/cerebral aqueduct into the fourth ventricle (thinnest, gets blocked up easily)
c. From the fourth ventricle, CSF flows out through the foramina of Luschka and Magendie into the subarachnoid space and then is reabsorbed into the blood stream
d. From the fourth ventricle, CSF also passes into the central canal of the spinal cord, where it runs the length of the spinal cord
cisterns
- In addition to the ventricles and subarachnoid spaces, there are areas within the brain and spinal cord where the subarachnoid space becomes wide enough to form cisterns
- These regions are important clinically as they are spaces where CSF can be safely removed for testing. The lumbar cistern, for example, is used during lumbar punctures (aka spinal taps) to remove CSF
Blood Brain Barrier
- The brain and spinal cord, as a consequence of their relative inability for neuronal repair and extreme importance, are protected from most substances by the blood brain barrier (BBB)
- In the brain and spinal cord, the capillaries do not allow free passage due to tight junctions that block the capillary openings.
- The brain needs substances, like glucose, to enter however. To facilitate the solutes being able to enter, active transport usually has to be used. This means carrier proteins that will shuttle a substance from the capillary into the nervous system
- Some substances are lipid-soluble enough (and small) that they can just diffuse through the membrane and directly into the nervous system
BLOOD-CSF Barrier
- There also exists a blood-CSF barrier such that substances are prevented from entering the areas containing CSF by the cells that make up the choroid plexus, called the choroid epithelial cells
- The separation of substances in blood and the CSF is important because CSF has a different composition than blood—concentrations of K+, Ca2+ and glucose are lower
Circumventricular Organs (CVOs)
- There exists regions in the brain where the BBB is interrupted to allow the brain to respond to chemical changes in the body. These regions are known as circumventricular organs (CVOs)
- These regions that have no BBB generally serve two functions: detecting molecules in the blood or secreting hormones into the blood.
CVO examples + functions
1) The area postrema is one such region and it triggers vomiting after sensing toxic substances in blood
2) The pineal is another region and it releases melatonin for circadian rhythms
3) The median eminence is involved in the release of pituitary hormones
