Cranial Nerves and Nervous System Overview

Cranial Nerves and Spheres

• Cranial nerves are part of the peripheral nervous system.
• Olfactory (smell), Optic (vision), Oculomotor (eye movement/pupils), Trochlear (eye movement), Trigeminal (facial sensation/chewing), Abducens (eye movement), Facial (facial expressions/taste), Vestibulocochlear (hearing/balance), Glossopharyngeal (taste/swallowing), Vagus (visceral organs), Accessory (neck muscles), Hypoglossal (tongue movements)

Central Nervous System (CNS) and Peripheral Nervous System (PNS)

• CNS: Brain and spinal cord; integration center.
• PNS: Nerves extending outside the brain and spinal cord; sensory (afferent) and motor (efferent) functions.
• CNS Location: Central (brain and spinal cord)
• CNS Function: Integration center
• CNS Support Cells: Oligodendrocytes – form myelin sheath
• PNS Location: Nerves extending outside the brain and spinal cord
• PNS Function: Sensory (afferent) and motor (efferent)
• PNS Support Cells: Schwann cells – form myelin sheath

Neuroglia (Supporting Cells of the CNS)

• Neuroglia support, insulate, and protect neurons.
• They cannot transmit nerve impulses and retain their ability to divide.
• Astrocytes: Star-shaped, abundant, form barriers between capillaries and neurons.
• Microglia: Spider-like phagocytes; dispose of debris (wastes, dead cells, bacteria).
• Ependymal Cells: Line central cavities of brain and spinal cord, produce and circulate cerebrospinal fluid.
• Oligodendrocytes: Found in the CNS; wrap around nerve fibers to form myelin sheaths.

Peripheral Nervous System (PNS) Glia

• Schwann cells: Form myelin sheath in the peripheral nervous system.

Action Potential Mechanism

• Resting State: Membrane polarized, fewer positive ions inside than outside.
• Depolarization: Stimulus depolarizes membrane, allowing Na+ ions to enter, initiating action potential. All-or-none response.
• Action Potential Propagation: Conducted along entire axon; faster with myelin sheaths.
• Synaptic Transmission: Axon terminal releases neurotransmitters, stimulating dendrites of the next neuron.
• Repolarization: Return to resting state by outward movement of K+ ions.
• Resting Neuron: Plasma membrane is polarized; fewer positive ions inside the cell than outside, inactive.
• Depolarization (step-by-step): Stimulus depolarizes neuron's membrane, allowing Na+ to enter the membrane. Charge upset inside the cell starts action potential.
• Action Potential (step-by-step): Conducted through the axon; impulses travel faster with myelin sheaths. Signals at synapses—axon terminal releases neurotransmitters, activating the dendrite. Action potential starts from dendrite.
• Repolarization (step-by-step): Return to resting state via outward movement of K+ ions.

Reflexes and Reflex Arcs

• Reflexes: Rapid, involuntary responses to stimuli.
• Reflex arc: Direct pathway from receptor (sensory neuron) to interneuron to motor neuron (effector).

Neuron Anatomy

• Cell Body: Nucleus and metabolic center (most organelles except centrioles).
• Dendrites: Conduct impulses towards the cell body.
• Axons: Conduct impulses away from the cell body.
• Axon Terminals: Release neurotransmitters (e.g., acetylcholine, serotonin, epinephrine, histamine) into the synaptic cleft.
• Myelin Sheath: Fatty insulation around axons, increasing conduction speed.
• Synaptic Cleft: Gap separating axon terminals and the next neuron.

Brain Anatomy

• Cerebral Hemispheres (Cerebrum): Two halves (left & right), connected by corpus callosum.
• Lobes: Frontal (motor, speech), Parietal (sensory), Occipital (vision), Temporal (sound, smell).
• Specialized Areas: Primary somatic sensory area (parietal), Primary motor area (frontal), Broca's area (speech).
• Diencephalon: Thalamus (sensory relay), Hypothalamus (autonomic center, body temperature, homeostasis, limbic system), Epithalamus (pineal gland, choroid plexus).
• Brain Stem: Midbrain (visual/auditory reflexes), Pons (breathing), Medulla Oblongata (vital centers, heart rate, blood pressure, breathing, swallowing, vomiting).
• Reticular Formation: Diffuse mass, motor control of visceral organs, reticular activating system (sleep/consciousness).
• Cerebellum: Motor functions, maintaining balance and posture, coordination of voluntary movements, motor learning.

CNS Protection

• Skull: Hard outer barrier.
• Meninges: Connective tissue membranes.
• Cerebrospinal Fluid: Watery cushion, removes waste.
• Blood-Brain Barrier: Prevents many harmful substances from entering the brain.

Head Trauma and Neurological Disorders

• Traumatic Brain Injuries (TBIs): Concussion (minor injury), Contusion (bruise, intracranial hemorrhage).
• Cardiovascular Accidents (CVAs): Stroke (blood vessel rupture/clot), brain tissue death due to oxygen/nutrient deprivation.
• Alzheimer's Disease: Progressive deterioration, abnormal protein deposits, ACh shortage, memory loss.

Somatic and Autonomic Nervous Systems

• Somatic NS: Voluntary motor control.
• Autonomic NS: Involuntary motor control, two divisions:
  • Sympathetic (fight-or-flight)
  • Parasympathetic (housekeeping)

Sympathetic & Parasympathetic Divisions

• Sympathetic: "fight or flight," response to stress.
• Parasympathetic: "rest and digest," conserves energy; maintains essential bodily functions (digestion, defecation, urination).

Neurotransmitters and Drugs

• Neurotransmitters: Chemical messengers.
• Neurotransmitters activate or inhibit postsynaptic neurons.
• Acetylcholine (ACh): Muscle contraction (excitatory).
• Norepinephrine: Associated with arousal (excitatory).
• Dopamine: Well-being (can be excitatory or inhibitory).
• Serotonin: Sleepiness (inhibitory).
• GABA: Sedative effects (inhibitory).
• Adenosine: Regulates sleep cycle (inhibitory).
• Drugs: Affect neurotransmitter action (release, blocking receptors).
• Stimulants: Increase neurotransmitter release, depleting it causing fatigue, circulatory problems, hallucinations, or depression.
• Depressants: Inhibit neurotransmitter activity (e.g., barbiturates, Xanax - GABA agonists).

Cranial Nerves

• Olfactory: Smell
• Optic: Vision
• Oculomotor: Eye movement, pupillary reflex
• Trochlear: Eye movement
• Trigeminal: Facial sensation, chewing
• Abducens: Eye movement
• Facial: Facial expression, anterior taste buds
• Vestibulocochlear: Hearing
• Glossopharyngeal: Posterior taste buds, swallowing
• Vagus: Motor and sensory for abdominal organs
• Accessory: Motor for neck muscles
• Hypoglossal: Motor and sensory for tongue movement