Nervous System

Structures and Functions of Neuron Structures

Neurons are the primary cells of the nervous system, specialized for communication. Each part of a neuron has a specific function:

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Dendrites: Branch-like extensions that receive signals from other neurons.

• Axon: Long projection that transmits electrical impulses away from the cell body.

• Axon Terminals: Endings where neurotransmitters are released to communicate with other cells.

• Myelin Sheath: Insulating layer that increases the speed of impulse transmission.

• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal conduction.

Three Main Functions of the Nervous System

• Sensory Input: Detects changes inside and outside the body.

• Integration: Processes and interprets sensory input, deciding what action is needed.

• Motor Output: Activates effector organs (muscles and glands) to cause a response.

Classification of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord; processes information.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS; connects CNS to limbs and organs.

• Divisions of PNS:

  • Somatic Nervous System: Controls voluntary movements.

  • Autonomic Nervous System (ANS): Controls involuntary functions; subdivided into sympathetic and parasympathetic divisions.

Synapses and Related Terms

• Synapse: Junction between two neurons or a neuron and an effector cell.

• Synaptic Cleft: Small gap between pre- and post-synaptic neurons.

• Pre-synaptic Neuron: Sends the signal.

• Post-synaptic Neuron: Receives the signal.

• Axosomatic Synapse: Synapse between axon and soma.

• Axodendritic Synapse: Synapse between axon and dendrite.

Characteristics and Classification of Neurons

• Characteristics: Excitability, conductivity, longevity, amitotic (do not divide), high metabolic rate.

• Structural Classification:

  • Multipolar: Many dendrites, one axon (most common).

  • Bipolar: One dendrite, one axon (rare, e.g., retina).

  • Unipolar: Single process (sensory neurons).

• Functional Classification:

  • Sensory (Afferent): Transmit impulses toward CNS.

  • Motor (Efferent): Transmit impulses away from CNS.

  • Interneurons: Connect sensory and motor neurons within CNS.

Supporting Cells (Neuroglia) of CNS and PNS

• CNS:

  • Astrocytes: Support neurons, form blood-brain barrier.

  • Oligodendrocytes: Form myelin sheath in CNS.

  • Microglia: Immune defense.

  • Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

• PNS:

  • Schwann Cells: Form myelin sheath in PNS.

  • Satellite Cells: Support neuron cell bodies in ganglia.

Blood-Brain Barrier (BBB)

• Definition: Selective barrier formed by endothelial cells and astrocytes.

• Function: Protects brain from toxins and pathogens.

• Permitted Molecules: Water, glucose, oxygen, CO2, lipid-soluble substances.

Regeneration of Nerve Fibers

• CNS: Limited regeneration due to inhibitory environment and lack of growth factors.

• PNS: Schwann cells promote regeneration by forming regeneration tubes.

Autonomic Innervation and Functional Classification

• Autonomic Nervous System: Controls involuntary functions (heart rate, digestion, etc.).

• Sympathetic Division: "Fight or flight" responses.

• Parasympathetic Division: "Rest and digest" responses.

Special Senses: Eye and Ear

Extrinsic Muscles of the Eye

• Six muscles control eye movement: superior, inferior, medial, and lateral rectus; superior and inferior oblique.

• Allow for precise movement and tracking.

Lacrimal Apparatus and Tear Pathway

• Lacrimal Gland: Produces tears.

• Tear Pathway: Lacrimal gland → lacrimal ducts → surface of eye → lacrimal puncta → lacrimal canaliculi → lacrimal sac → nasolacrimal duct → nasal cavity.

Anatomy of the Eye and Its Tunics

• Fibrous Tunic: Sclera (white, protective) and cornea (transparent, refracts light).

• Vascular Tunic (Uvea): Choroid (blood supply), ciliary body (lens shape), iris (controls pupil size).

• Sensory Tunic (Retina): Contains photoreceptors (rods and cones).

Muscles of the Iris and Nervous System Response

• Sphincter Pupillae: Constricts pupil (parasympathetic).

• Dilator Pupillae: Dilates pupil (sympathetic).

Eye Cavities and Fluids

• Anterior Cavity: Contains aqueous humor.

• Posterior Cavity: Contains vitreous humor.

Accommodation and Lens Shape

• Accommodation: Adjustment of lens shape to focus on near or distant objects.

• Ciliary Muscles: Contract to thicken lens for near vision; relax to flatten lens for distance.

• Suspensory Ligaments: Connect ciliary muscles to lens.

Key Eye Structures

• Optic Nerve: Transmits visual information to brain.

• Optic Disc: Blind spot; no photoreceptors.

• Fovea Centralis: Area of sharpest vision; high concentration of cones.

Common Eye Conditions

• Myopia (Nearsightedness): Eye too long; image focuses before retina.

• Hyperopia (Farsightedness): Eye too short; image focuses behind retina.

• Astigmatism: Irregular curvature of cornea or lens.

Photoreceptors: Rods vs. Cones

• Rods: Sensitive to low light; no color vision; more numerous in peripheral retina.

• Cones: Detect color; high acuity; concentrated in fovea centralis.

• Visual Pigment: Rhodopsin in rods; photopsins in cones.

Pathway of Sensory Signal from Retina

• Photoreceptors → Bipolar Cells → Ganglion Cells → Optic Nerve → Brain

Path of Light Through the Eye

• Cornea → Aqueous Humor → Lens → Vitreous Humor → Retina (photoreceptors)

Distribution of Rods and Cones

• Highest Concentration of Cones: Fovea centralis.

• Highest Concentration of Rods: Peripheral retina.

• Lowest Concentration: Optic disc (no rods or cones).

Lens Accommodation and Focal Distance

• Lens changes shape to focus light on fovea.

• Myopia: Retina is farther from lens; corrected with concave lenses.

• Hyperopia: Retina is closer to lens; corrected with convex lenses.

Anatomy of the Ear

• External Ear: Auricle, external auditory canal.

• Middle Ear: Tympanic membrane, ossicles (malleus, incus, stapes).

• Inner Ear: Cochlea (hearing), vestibule and semicircular canals (balance).

• Nerves: Cochlear nerve (hearing), vestibular nerve (balance).

Internal Ear Structures, Membranes, and Fluids

• Membranous Labyrinth: Contains endolymph.

• Bony Labyrinth: Contains perilymph.

• Cochlear Duct: Houses organ of Corti (hearing receptor).

Path of Sound Through the Auditory System

• Sound waves → Tympanic membrane → Ossicles → Oval window → Cochlea → Hair cells → Cochlear nerve → Brain

Heart

Pacemaker Cells, Contractile Cells, and Intercalated Discs

• Pacemaker Cells: Generate spontaneous action potentials (e.g., SA node).

• Contractile Cells: Produce force for heart contraction.

• Intercalated Discs: Specialized junctions for electrical and mechanical connection between cardiac cells.

Functions of Cardiac Tissue

• Contracts rhythmically to pump blood.

• Conducts electrical impulses for coordinated contraction.

Electrical Conduction System of the Heart

• SA node → AV node → Bundle of His → Bundle branches → Purkinje fibers

Autonomic Influence on Heart Activity

• Sympathetic Stimulation: Increases heart rate and contractility.

• Parasympathetic Stimulation: Decreases heart rate.

• Both systems influence SA node, AV node, and myocardium.

Pacemaker Potentials and Autonomic Effects

• Sympathetic: Increases slope of pacemaker potential (faster depolarization).

• Parasympathetic: Decreases slope (slower depolarization).

Diastole and Systole

• Diastole: Relaxation phase; chambers fill with blood.

• Systole: Contraction phase; blood is ejected.

Ventricular Contraction, Relaxation, and One-Way Blood Flow

• Contraction increases pressure, closing AV valves and opening semilunar valves.

• Relaxation decreases pressure, closing semilunar valves and opening AV valves.

Heart Sounds and Valves

• First Sound ("lub"): Closure of AV valves at start of systole.

• Second Sound ("dub"): Closure of semilunar valves at start of diastole.

• Valves ensure one-way flow of blood.

Oxygenated vs. Deoxygenated Blood

• Left Side: Carries oxygenated blood from lungs to body.

• Right Side: Carries deoxygenated blood from body to lungs.

Blood Vessels

Pathway of Blood in Systemic and Pulmonary Circuits

• Systemic Circuit: Left ventricle → aorta → body → right atrium

• Pulmonary Circuit: Right ventricle → pulmonary arteries → lungs → left atrium

Tissue Layers of Blood Vessels

• Tunica Intima: Endothelial lining.

• Tunica Media: Smooth muscle and elastic fibers.

• Tunica Externa (Adventitia): Connective tissue.

Differences Between Arteries and Veins

Capillary Structure and Function

• Thin walls (single endothelial layer) facilitate exchange of gases, nutrients, and wastes.

• Small diameter allows close contact with tissues.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.