Venous Structure	Location and Area Drained
Superior sagittal sinus	Courses along the midline at the superior border of the falx cerebri; superior cerebral veins empty into it
Straight sinus	Lies in the midline attachment of the falx cerebri and the tentorium; drains the system of internal cerebral veins (including the inferior sagittal sinus and great cerebral vein of Galen)
Transverse sinuses	Lie in the bony groove along the fixed edge of the tentorium cerebelli; drain the straight sinus and the superior sagittal sinus
Sigmoid sinuses	Lie on the mastoid process of the temporal bone and jugular process of the occipital bone; receive blood from the transverse sinuses and empty into the internal jugular veins
Inferior sagittal sinus	Lies along the free inferior border of the falx cerebri just above the corpus callosum; receives blood from the medial aspects of the hemispheres
Emissary veins	Connect the dural sinuses with veins outside the cranial cavity

(b) Internal jugular veins collect blood from the large dural venous sinuses and return blood to the heart

iv. Blood-brain barrier: Specialized permeability of the brain capillaries that limits transfer of certain substances from blood into brain tissue. Barrier formed by tight junctions between brain capillary endothelial cells, reduced transport mechanisms of these cells, and footlike projections from the astrocytes that encase the capillaries.

(a) Water, carbon dioxide, oxygen, glucose, and lipid-soluble substances cross the cerebral capillaries with ease. Uptake of other substances, such as dyes and ions (e.g., Na⁺, K⁺), is much slower.

(b) Regulates the entry or removal of various substances to maintain a homeostatic environment for the central nervous system (CNS)

(c) Clinically significant in treating and diagnosing CNS disease. Blood-brain barrier disruption and increased permeability occurs with brain injury, tumors, infections, and stroke.

d. Ventricular system and CSF (Figure 4-11)

FIGURE 4-11 Ventricular system. A, Circulation of cerebrospinal fluid. B, Ventricles of the brain. (From Applegate EJ: The anatomy and physiology learning systems textbook, ed 2, Philadelphia, 2000, Saunders, p 169.)

i. Ventricles: Four cavities containing CSF

(a) Lateral ventricles: Largest ventricles, one in each cerebral hemisphere. The anterior (frontal) horns lie in the frontal lobes; the bodies extend back through the parietal lobes to the posterior (occipital) horns, which project into the occipital lobes; the inferior (temporal) horns lie in the temporal lobes.

(b) Third ventricle: Midline between the two lateral ventricles, surrounded by the diencephalon

(c) Fourth ventricle: In the posterior fossa bordered by the pons, medulla, and superior cerebellar peduncles; continuous with the cerebral aqueduct (aqueduct of Sylvius) superiorly and the central spinal canal inferiorly

ii. CSF functions

(a) Cushions the brain and spinal cord from injury

(b) Provides support and buoyancy for the brain, decreasing its effective weight on the skull

(c) Its displacement out of the cranial cavity (and, to an extent, its increased reabsorption) compensates for increases in intracranial volume and pressure

(d) Regulates the nervous system chemical environment to preserve homeostasis

(e) Enables water-soluble metabolites to diffuse from the brain

(f) Serves as a channel for neurochemical communication within brain

iii. CSF properties: See Table 4-4

TABLE 4-4

Normal Properties of Cerebrospinal Fluid (CSF)

iv. CSF formation

(a) Rate of synthesis estimated as 500 ml/day or 22 to 25 ml/hr

(b) Choroid plexus: Tuft of capillaries covered by epithelial cells found in all ventricles; principal source of CSF; lateral ventricles produce most

v. Circulation and absorption of CSF (see Figure 4-11)

(a) CSF circulates from the lateral ventricles through the interventricular foramina (foramina of Monro) to the third ventricle and to the fourth ventricle via the aqueduct of Sylvius; CSF then circulates to the subarachnoid space via the foramina of Luschka and Magendie

(b) Most CSF is absorbed via the arachnoid villi into the dural sinuses

vi. Blood-CSF barrier: Choroid plexus epithelium imposes a barrier analogous to the blood-brain barrier; permits selective transport of substances from the blood into the CSF

e. Brain metabolism

i. Brain has high metabolic energy requirements; energy primarily used for neuronal conductive and metabolic activities

ii. At rest, the brain consumes 25% of body glucose and 20% of body oxygen; cerebral oxygen consumption averages 49 ml/min

iii. Brain utilizes glucose as its principal energy source

iv. Minimal storage of oxygen and glucose in the brain necessitates a constant supply for normal neuronal function

v. Anaerobic glucose metabolism (glycolysis) yields insufficient adenosine triphosphate (ATP) to meet cerebral energy demands. Rate of glycolysis increases markedly during hypoxia in an attempt to maintain functional neuronal activity.

vi. Within seconds to minutes of anoxia, the energy-dependent sodium-potassium pump fails; cytotoxic cerebral edema results

vii. Hypoglycemia causes neuronal dysfunction and may lead to convulsions, coma, and death

f. Cells of the nervous system

i. Neuron: Basic functional unit of the nervous system; transmits nerve impulses

(a) Components of each cell (Figure 4-12)

FIGURE 4-12 Structure of a typical neuron. (From Thibodeau GA, Patton K: Anatomy and physiology, ed 5, St Louis, 2003, Mosby, p 348.)

(1) Cell body: Carries out the metabolic functions of the cell; contains a nucleus, cytoplasm, and organelles surrounded by a lipoprotein cell membrane

(2) Dendrites: Short branching extensions of the cell body; conduct impulses toward the cell body

(3) Axon hillock: Thickened area of the cell body from which the axon originates

(4) Axon: Long extension of the cell body; conducts impulses away from the cell body; usually myelinated. Outside the brain, axons are also covered with neurilemma. Branch into several processes at the terminal end.

(5) Myelin sheath: White protein-lipid complex that surrounds some axons; laid down by oligodendrocytes in the CNS and by Schwann cells in the PNS

(6) Nodes of Ranvier: Periodic interruptions in the myelin covering along the axon. Impulses are conducted from node to node (saltatory conduction), which makes conduction more rapid and efficient.

(7) Synaptic knobs: At the terminal ends of the axon; contain vesicles that store neurotransmitter substances

(b) Functions

(1) Receive input from other neurons, primarily via the dendrites and cell body

(2) Conduct action potentials or impulses along the axon

(3) Transfer information by synaptic transmission to other neurons, muscle cells, or gland cells

ii. Neuroglial cells: Support, nourish, and protect the neurons; about 5 to 10 times as numerous as neurons. Four types:

(a) Microglia: Phagocytize tissue debris when nervous tissue is damaged

(b) Oligodendroglia: Responsible for myelin formation on axons in the CNS

(c) Astrocytes: Contribute to the structure of the blood-brain barrier. Provide nutrients for neurons. Constitute the structural and supporting framework for nerve cells and capillaries. Remove excess potassium and neurotransmitters. Contribute to scar formation in response to neuronal cell injury or death.

(d) Ependyma: Line the ventricles of the brain and the central canal of the spinal cord. Regulate the flow of substances from these cavities into the brain. Aid in CSF production.

g. Synaptic transmission of impulses: Unidirectional conduction of an impulse from a presynaptic neuron across a junction or synapse to a postsynaptic neuron

i. Resting membrane potential (RMP): Voltage difference across the cell membrane when the neuron is resting. Determined by the difference in ion concentrations on either side of the membrane. At rest, cells are positively charged outside and negatively charged inside.

ii. Depolarization: Stimulus causes sodium channels to open, which results in an intracellular influx of sodium ions (Na⁺)

(a) These ionic fluxes decrease RMP

(b) This depolarization is called the excitatory postsynaptic potential (EPSP)

iii. Action potential: If a transient voltage change that occurs with depolarization is of sufficient magnitude (threshold level), an action potential is produced and transmitted (conducted as an impulse) along the nerve fibers in an active, self-propagating process

iv. Summation: Simultaneous excitation of numerous excitatory presynaptic terminals (or rapidly successive discharges from the same presynaptic terminal) can add together to cause a progressive increase in the postsynaptic potential that may eventually reach threshold to generate an action potential

v. Neurotransmitters (Table 4-5): Chemicals secreted by presynaptic knobs or vesicles (usually located at the axon terminal) that excite, inhibit, or modify the response of a postsynaptic neuron. When an action potential reaches the synaptic knob, calcium channels are opened, allowing Ca⁺⁺ influx into the knob, which triggers neurotransmitter release.