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Central Nervous System
Brain & spinal cord
Peripheral Nervous System
Cranial & spinal nerves and the peripheral components of the autonomic nervous system
Nervous System's 2 types of Cells
- Neuron - Neuroglia
Neuron
- Primary functional unit of nervous system - Nonmitotic, not replicating or replacing themselves if irreversibly damaged - consists of cell body, axon & several dendrites
Neuroglia
- glial cells: provide support, nourishment & protection to neurons - 5 to 10 times more numerous than neurons - mitotic & can replicate - when neurons are destroyed, the tissue is replaced by proliferation of neuroglia cells
Neuroglia: Different types of glial cells = Oligodendrocytes
- specialized cells that produce myelin sheath of nerve fibers in CNS [Schwann cells myelinate nerve fibers in periphery] & are primarily found in white matter of CNS
Neuroglia: Different types of glial cells = Astrocytes
- provide structural support to neurons & their delicate processes form the blood-brain barrier w. endothelium of blood vessels, & play a role in synaptic transmission - found primary in gray matter - injury, acts as phagocytes for neuronal debris - proliferation = formation of scar tissue [gliosis] in CNS
Neuroglia: Different types of glial cells = Ependymal Cells
- line brain ventricles & aid in secretion of CSF
Neuroglia: Different types of glial cells = Microglia
- type of macrophage, relatively rare in normal CNS tissue - are phagocytes & are important in host defense
Most primary CNS tumors involve?
Neuroglia 'coz they are mitotic
Nerve Impulse
- neurons initiate, receive, & process msgs about events both w.in & outside body - initation of neuronal msg [nerve impulse] involves generation of an action potential
Action Potential
- cells @ resting state: [-] electric charge inside compared to outside of cell. - Na+ are high outside - K+ are high inside - depolarization: inside becomes + - repolarzation: inside becomes negative - metabolic process accomplished by Na+K+ pump [requires energy:ATP]
Resting Membrane Potential
- difference in electric charge across cell memebrane
Myelination
- insulating capacity - myelination of nerve axons facilitates conduction of action potential - white, lipid substance
Neurotransmitters
- chemical agent involved in transmission of an impulse across synaptic cleft - either Excitatory or Inhibitory
Excitatory Neurotransmitters
- cause an increase in Na+ permeability at postsynaptic cell membraine = action potential generated
Inhibitory Neurotransmitters
- cause an increase in permeability of K+ & Cl- ions, decreasing likelihood of generation of action potential
A presynaptic cell that releases an excitatory neurotransmitter doesn't always cause postsynaptic cell to depolarize enough to generate an action potential
However, when many presynaptic cells release excitatory neurotransmitters on a single neuron, the sum of their input is enough to generate an action potential
Spatial Summation
presynaptic input can be summed by the number of presynaptic cells firing
Temporal Summation
Frequency of firing of a single presynaptic cell
Cerebral Circulation
- blood supply of brain arises from internal carotid arteries [anterior circulation] & the vertebral arteries [posoterior circulation]
Internal Carotid Arteries
Supplies the ipsilateral hemisphere
Vertebral Arteries
- Basilar artery, formed by the junction of the 2 vertebral arteries, supplies structures w.in the posterior fossa [cerebellum & brainstem]
Circle Of Willis
- arises from basilar artery & two internal carotid arteries - acts as a safety valve when differential pressure are present in these arteries - also acts as an anastomotic pathway when occlusion of a major artery on one side of brain occurs
2 Anterior Cerebral Arteries
supply the medial & anterior portions of frontal lobes
2 Middle Cerebral Arteries
supply the outer portions of the frontal, parietal, & superior temporal lobes
2 Posterior Cerebral Arteries
supply the medial portions of the occipital & inferior temporal lobes
Where does Venous Blood drain from in the brain?
through the dural sinuses, which form channels that drain into the 2 jugular veins
Blood-Brain Barrier
- astrocytes form the BBB - a physiologic barrier btwn blood capillaries & brain tissue - allows nutrients & gases to enter - only certan drugs can enter CNS from bloodstream [NOT ALL CHEMO DRUGS!] - lipid-soluble compounds enter brain easily - water-soluble & ionized drugs enter brain & spinal cord slowly
Meninges
- protective structure - 3 layers that surround the brain & spinal cord: 1. Dura Mater = thick, outermost layer 2. Arachnoid Layer = Middle. Delicate, impermeable membrane 3. Pia Mater = Inner
Subarachnoid Space
- lies between arachnoid layer & pia mater; filled with CSF - All cerebral arteries & veins & CNs lie in this space - larger SA space is in region of 3rd & 4th lumbar vertebrae [area penetrated to obtain CSF during lumbar puncture] - anything that passes to & from brain & skull or its foramina, must pass through subarachnoid space
Spinal Cord
Ends between 1st & 2nd Lumbar Vertebrae
Tentorial Herniation
Expansion of mass lesions in cerebrum forces brain to herniate through the opening created by the brainstem [FORAMEN MAGNUM?]
Skull
- protects brain from external trauma - composed of 8 cranial bones & 14 facial bones - top & sides are smooth; bottom surface is uneven [many ridges, prominences, & foramina]
Skull
- largest hole is FORAMEN MAGNUM, which brainstem extends to spinal cord - this foramen offers the only major space for expansion of brain contents when increased ICP occurs
Vertebral Column
- protects spinal cord, supports head, & provides flexibility - made up of 33 individual vertebrae: 1. 7 cervical 2. 12 thoracic 3. 5 lumbar 4. 5 sacral [fused into 1] 5. 4 coccygeal
Vertebral Column
- each vertebra has a central opening for spinal cord - vertebrae held together by a series of ligaments - intervertebral disks occupy spaces btwn vertebrae
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