cessation of brain activity: no more electrical activity in brain (brain dead)
Clinical Criterion for Death
towards the forehead
towards the spinal cord
-men: 1600g (3.5lb)
-women: 1450g (3.2lb)
Major Components of Brain
deep groove that separates cerebral hemispheres
thick folds of the brain
shallow grooves of the brain
thick nerve bundle at bottom of longitudinal fissure that connects hemispheres
What are the components of the brain stem from highest to lowest
-has neuron cell bodies, dendrites and synapses
-forms surface layer (cortex) over cerebrum and cerebellum
-forms nuclei deep within brain
*nuclei: cluster of cells like the ganglion
Gray Matter of Brain
-bundles of axons
-lies deep to cortical gray matter (opposite in spinal cord)
-composed of tracts, or bundles of axons, that connect one part of the brain to another, and to the spinal cord
White Matter of Brain
Which totipotent layer of the zygote creates the nervous system?
dorsal streak appears that appears along the length of the embryo in the third week
-What is it?
-When does it happen?
-formed when the neuroectoderm thickens
-destined to give rise to most neurons and all glial cells except microglia (which comes from the mesoderm)
-happens in the third week
-How is it formed?
-What does it create
-When does it happen?
-as thickening progresses in neural plate, the neural groove in formed with raised neural folds on each side that will eventually fuse along the midline
*beginning in the cervical region and progressing rostrally and caudally
-happens in the third week
-How is it formed
-What does it look like
-Where does it begin
-When does it happen
-a hollow channel that that is created in the fourth week of the embryo
-separates from the overlying ectoderm
-as it sinks deeper, it grows lateral processes that later will form motor fibers
-lumen of neural tube becomes fluid filled space in the central canal of spinal cord and ventricles of the brain
-What is it
-When does it happen
-What does it separate
-What does it grow
-What is contained in the tube
-formed from the ectodermal cells that lay along the margins of the groove and separate from the rest forming a longitudinal column on each side
-happens in the fourth week
-gives rise to the:
1.arachnoid mater and pia mater
3.skeletal, integumentary and endocrine systems
-what is it formed from
-what does it does it look like
-when does it happen
-what does it give rise to
-forebrain (prosencephalon): most advanced; forms neocortex
What anterior primary vesicles will the neural tube exhibit in the fourth week?
1.telencephalon: cerebral hemispheres
2.diencephalon: has optic vesciles that become retina of the eye
-midbrain: remains undivided
What does each vesicle subdivide into by the fifth week?
outer: dura mater
middle: arachnoid mater
inner: pia mater
Meninges of the Brain
1.outer periosteal: equivalent to periosteum of cranial bones. folds inward to extends btwn parts of the brain. no epidural space since it is pressed closely against the cranial bone but NOT attached to bone
2.inner meningeal: continues into vertebral canal where it forms the dural sac around the spinal cord
What are the layers of the dura mater?
-around the foramen magnum
-sutures of the skull
Where does the periosteal layer of the dura mater attach to the bone?
-where the two layers of the dura are separated
-these spaces collect blood that has circulates through the brain
What are dural sinuses
transparent membrane over brain surface
space btwn arachnoid mater and pia mater
space betwen dura mater and arachnoid mater
-only in some places
-very thin membrane that follows contours of brain, even dipping in sulci
-not usually visible w/o a microscope
-inflammation of the meninges
-serious disease in infancy and childhood
-caused by bacterial, viral and fungal invasion of the CNS usually by way of nose and throat
-pia and arachnoid mater are most often affected
what is is?
who does it seriously affect?
what is is caused by?
how does it get through?
which meninges does it usually affect?
-stiff neck (neucorigidity)
-may progress to coma then death within hours of onset
Signs of Meningitis
-examine the CSF for bacteria, or increase in lympocytes if viral
-uses a lumbar puncture (spinal tap) to draw fluid from subarachnoid space btwn L4-L5
How is meningitis examined and diagnosed?
-four internal chambers within brain
What are ventricles?
-two lateral: one in each cerebral hemisphere
*interventricular foramen: a tiny pore that connects to third ventricle
-third ventricle: single narrow medial space beneath corpus callosum
*cerebral aqueduct: runs through midbrain and connects third to fourth ventricle
-fourth ventricle: small triangular chamber btwn pons and cerebellum
*connects to central canal, runs down through spinal cord
What are the ventricles of the brain and where does it connect?
spongy mass of blood capillaries on the floor of each ventricle
-neuroglia that lines the ventricles and covers choroid plexus
-produces spinal fluid
-clear, colorless liquid that fills the ventricles and canals of CNS
How much CSF is produced and absorbed each day?
How much CSF is normally present at one time?
-40% in subarachnoid space external to brain
-30% by the general ependymal lining of the brain ventricles
-30% by choroid plexus
Where and what percentage of CSF is produced from each area?
ependymal cells modify the filtrate so CSF has more Na+ and Cl- than plasma, but less K+, Ca++, glucose and very little protein
What modifies the CSF filtrate?
Why do they modify the CSF filratate
continuously flows around the CNS: driven by its own pressure, beating of ependymal cilla and pulsations of the brain produced by each heart beat
What drives CSF flow?
1.CSF is secreted by choroid plexus in each lateral ventricle
2.CSF flows through interventricular foramin into third ventricle
3.choroid plexuses in third ventricle adds more CSF
4.CSF flows down cerebral aqueduct to fourth ventricle
5.choroid plexuses in fourth ventricle adds more CSF
6.CSF flows out two lateral apertures and one median aperture in fourth ventricle
7.CSF fills subarachoid space and bathes external surfaces of brain and spinal cord
8.at arachnoid granulations, CSF is reabsorbed into venous blood of dural venous sinuses
What are the steps of CSF flow/filtration?
-cauliflower-shaped extension of the arachnoid meninx
-protrudes through dura mater into the superior sagittal sinus
-CSF penetrates the walls of the villi and mixes w/ the blood of the sinus
1. buoyancy: allows brain to attain considerable size w/o being impaired by its own weight. if rested heavily on floor of cranium, the pressure would kill the nervous tissue
2.protection: protects brain from striking the cranium when the head is jolted. shaken child syndrome and concussions do occur from severe jolting
3.chemical stability: flow of CSF rinses away metabolic wastes from nervous tissue and homeostatically regulates its chemical environment
*mostly done by blood, CSF only aids
Functions of CSF
What percentage of weight does the brain have on the overall adult body?
What percentage of blood does the brain receive from the overall body?
What percentage of oxygen and glucose does the brain consume?
-Neurons have a high demand for ATP and therefore oxygen and glucose, so a constant supply of blood is critical to the nervous system
Why does the brain need so much blood?
loss of conciousness
What may happen if there is a 10 second interruption of blood flow to the brain?
significant impairment of neural function
What may happen if there is a 1-2 minute interruption of blood flow to the brain?
irreversible brain damage
What may happen if there is a 4 minute interruption of blood flow to the brain?
antibodies, macrophages, bacterial toxin and other harmful agents that can damage the brain
In addition to the beneficial stuff blood carries, what else does it carry?
-strictly regulates what substances can get from the bloodstream and into the tissues of the brain
*when inflamed, it is more susceptible to damage
What is the brain barrier system?
-blood capillaries throughout the brain tissue
-capillaries of the choroid plexus
What are the points of entry of the brain barrier system that must be guarded?
-protects the blood capillaries throughout the brain tissue
-consists of tight junctions btwn endothelial cells that form the capillary walls
-astrocytes reach out and contact capilliaries w/ their perivascular feet
-induce the endothelial cells to form tight junctions that completely seal off gaps btwn them
-anything leaving the blood must pass through the cells, and not between them
-endothelial cells can exclude harmful substances from passing to the brain tissue while allowing necessary ones to pass
-protects the brain at the choroid plexus
-forms tight junctions btwn the ependymal cells
*tight junctions are absent from ependymal cells elsewhere to allow exchange btwn brain tissue and CSF
What is the blood barrier system highly permeable to?
-wastes products from urea and creatinine
What is the blood barrier system slightly permeable to?
obstacle for delivering medications such as antibiotics and cancer drugs
What is an obstacle of the blood barrier system?
can allow pathogens to enter brain tissue
Trauma and inflammation of the BBS
-places in the third and fourth ventricles where the barrier is absent and the blood has direct assess to the brain neurons
Where are the Circumventricular Organs?
-enables the brain to monitor and respond to fluctuations in blood glucose, pH, osmolarity, and other variables
What are the functions of the circumventricular organs?
allows a route for HIV to enter
What virus is the circumventricular organs susceptible to?