Nervous System Divisions

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Nervous System Structure and Function

• The nervous system is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS)
• CNS consists of the brain and spinal cord, enclosed within protective cranial vault (skull) and vertebral column
• PNS consists of cranial nerves and spinal nerves, functioning as an input-output system for relaying impulses to the CNS and transmitting output messages to control effectors (muscles and glands)

Divisions of the Peripheral Nervous System

• Somatic Nervous System: consists of motor and sensory pathways that regulate voluntary control of skeletal muscle
• Autonomic Nervous System: consists of motor and sensory components that regulate body's internal viscera through involuntary control of organ systems (sympathetic and parasympathetic systems)

Nervous System Cells

• Neurons: primary cells of the nervous system, respond to stimuli and conduct impulses
• Supporting cells (neuroglial cells): provide structure, support, and nutrition for neurons

Neuron Structure

• Three components: soma (cell body), dendrites (multiple branch extensions that receive nerve impulses), and axon (long, conductive projection that carries impulses away from the cell body)
• Classified into bipolar, multipolar, and unipolar neurons based on structure and function
• Functionally, neurons can be sensory (afferent), motor (efferent), or interneurons

Neuroglial Cells

• Four types: astrocytes (fill spaces between neurons and surround blood vessels, contribute to blood-brain barrier), oligodendrocytes (deposit myelin in the CNS), microglia (remove debris in CNS), and ependymal cells (line CSF-filled cavities in CNS)

Myelin Sheath

• An important component of the axon, acts as an insulating substance, and helps increase nerve conduction velocity
• Formed by Schwann cells in the PNS and oligodendrocytes in the CNS
• Disorders of the myelin sheath, such as multiple sclerosis and Guillian-Barre syndrome, can lead to demyelination and axon death

Brain and Spinal Cord

• Brain: consists of the cerebrum, cerebellum, brain stem, and limbic system
• Cerebrum: divided into two hemispheres, with gray matter (cerebral cortex and basal ganglia) and white matter (myelinated nerve fibers)
• Spinal cord: divided into gray matter (cell bodies of neurons) and white matter (myelinated nerve fibers)

Brain Stem

• Consists of the midbrain, pons, and medulla
• Regulates vital reflexes, including cardiac and respiratory functions

Forebrain

• Divided into the telencephalon (cerebrum) and diencephalon (thalamus and hypothalamus)
• Cerebrum: divided into two hemispheres, with a cerebral cortex and basal ganglia
• Thalamus: major integrating center for afferent impulses to the cerebral cortex
• Hypothalamus: regulates body temperature, endocrine function, and emotional expression

Hindbrain

• Consists of the cerebellum, pons, and medulla
• Cerebellum: responsible for maintaining balance and posture
• Pons: an important center for control of respiration
• Medulla: regulates vital reflexes, including heart rate, blood pressure, and respiration

Neurodegenerative Disorders

• Parkinson's disease: characterized by the degeneration of dopamine-producing neurons in the substantia nigra
• Alzheimer's disease: characterized by the presence of senile plaques and neurofibrillary tangles, leading to brain atrophy and cognitive decline

Protective Mechanisms

• Meninges: three layers of protective tissue that cover the CNS (pia mater, arachnoid, and dura)
• Blood-Brain Barrier: a selective barrier that permits the passage of essential substances while restricting the entry of toxins
• Autoregulation: the ability of the CNS to maintain constant blood flow despite changes in systemic arterial pressure### Neural Cells and Cerebral Blood Flow
• Neural cells function only with aerobic metabolism
• Increases in carbon dioxide or hydrogen ion (acidosis) increase cerebral blood flow
• Decreased oxygen levels increase cerebral blood flow
• Acidosis depresses neural activity, and increased blood flow helps to wash away acidic materials from brain tissue

CVA (Stroke)

• A stroke is an acute focal deficit from a vascular disorder that injures brain tissue
• Risk factors for stroke include:
  • Age (risk increases with age, 1%/yr in persons 65-75 yrs old)
  • Gender (men have a 19% greater incidence than women)
  • Ethnicity (African-Americans have a 60% greater risk of death & disability from stroke)
  • Heart disease (especially atrial fibrillation & other conditions that predispose to clot formation)
  • Polycythemia and sickle cell anemia predispose to clot formation
  • Alcohol consumption contributes to stroke through induction of cardiac arrhythmias, hypertension, and reduced cerebral blood flow
  • Cocaine use can cause both ischemic and hemorrhagic strokes

Types of Strokes

• Ischemic stroke: caused by cerebrovascular obstruction by thrombus or emboli
• Hemorrhagic stroke: caused by spontaneous hemorrhage into the brain substance

Ischemic Stroke

• Caused by thrombus or emboli in cerebrovascular vessels
• Can cause a central core of dead or dying cells, surrounded by a penumbra (area of minimally perfused cells)
• Thrombotic stroke is the most common cause of ischemic strokes, often occurring in atherosclerotic blood vessels
• Transient ischemic attack (TIA) is a temporary disturbance in cerebral blood flow, often providing a warning of impending stroke
• Embolic stroke is caused by a moving blood clot that travels from its origin to the brain

Hemorrhagic Stroke

• Most fatal type of stroke
• Causes include:
  • Spontaneous hemorrhage into the brain substance
  • Aneurysms
  • Arteriovenous malformation (AVM)
  • Vasculitis
  • Drug use

Lacunar Infarcts (Small Vessel Strokes)

• Small strokes with a 50% chance of bleeding
• Can occur with acute increases in ICP

Aneurysms

• Classification based on shape and size:
  • Saccular (berry)
  • Fusiform (giant)
  • Mycotic
  • Traumatic

Subarachnoid Hemorrhage

• Bleeding into the subarachnoid space, often due to ruptured cerebral aneurysm
• Manifestations include:
  • Atypical headache (often with a history of headache preceding onset)
  • Meningeal irritation (nuchal rigidity, photophobia)
  • Cranial nerve deficits (diplopia, blurred vision)
• Complications can include:
  • Rebleeding
  • Vasospasm
  • Hydrocephalus