Comprehensive Study Guide: Nervous System, Special Senses (Vision & Audition) – Anatomy & Physiology I

Nervous System Overview

Basic Functions of the Nervous System

• Sensory Input: Detects changes in the internal and external environment via receptors.

• Integration: Processes and interprets sensory input, deciding what should be done at each moment.

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

• Homeostasis: Maintains internal balance by regulating bodily functions.

• Mental Activity: Responsible for consciousness, memory, and thought.

Structural and Functional Classification of the Nervous System

• Structural Classification:

  • Central Nervous System (CNS): Brain and spinal cord; integration and command center.

  • Peripheral Nervous System (PNS): Cranial and spinal nerves; communication lines between CNS and body.

• Functional Classification:

  • Somatic Nervous System (SNS): Voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands; subdivided into sympathetic and parasympathetic divisions.

Types of Neurons

• Afferent (Sensory) Neurons: Transmit impulses from sensory receptors to the CNS.

• Efferent (Motor) Neurons: Carry impulses from the CNS to effectors (muscles/glands).

• Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS; involved in integration.

Neuroglia (Glial Cells)

• CNS Neuroglia:

  • Astrocytes: Support neurons, maintain blood-brain barrier, regulate ion/nutrient environment.

  • Oligodendrocytes: Form myelin sheaths around CNS axons.

  • Microglia: Phagocytic cells; remove debris and pathogens.

  • Ependymal Cells: Line ventricles; produce and circulate cerebrospinal fluid (CSF).

• PNS Neuroglia:

  • Schwann Cells: Form myelin sheaths around PNS axons.

  • Satellite Cells: Support neuron cell bodies in ganglia.

Myelin Sheath

• Importance: Increases speed of nerve impulse conduction (saltatory conduction); insulates axons.

• Formation:

  • CNS: Oligodendrocytes wrap around multiple axons.

  • PNS: Schwann cells wrap around a single axon segment.

Structure of a Typical Motor Neuron

• Cell Body (Soma): Contains nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Conducts impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

White and Gray Matter

• White Matter: Myelinated axons; transmits signals between different CNS regions.

• Gray Matter: Neuron cell bodies, dendrites, and unmyelinated axons; site of synaptic integration.

• Ganglia: Clusters of neuron cell bodies in the PNS.

• Tracts: Bundles of axons in the CNS.

• Nerves: Bundles of axons in the PNS.

Neuron Structure Classification

• Multipolar: Many dendrites, one axon; most common in CNS.

• Bipolar: One dendrite, one axon; found in retina and olfactory epithelium.

• Unipolar (Pseudounipolar): Single process splits into two branches; sensory neurons in PNS.

Reflexes

• Definition: Rapid, automatic response to a stimulus.

• Functional Elements: Receptor, sensory neuron, integration center, motor neuron, effector.

Action Potential Generation

• Resting Membrane Potential: Maintained by Na+/K+ pumps; inside negative relative to outside.

• Depolarization: Na+ channels open, Na+ enters cell.

• Repolarization: K+ channels open, K+ exits cell.

• Hyperpolarization: K+ channels remain open briefly.

• All-or-None Principle: If threshold is reached, action potential occurs fully.

Equation:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Additional info: This is the Nernst equation for potassium, a key determinant of resting membrane potential.

Central Nervous System (Brain)

Embryonic Neural Tube Development

• Neural Tube: Forms the CNS; anterior end becomes the brain, posterior end forms the spinal cord.

• Primary Brain Vesicles: Prosencephalon, mesencephalon, rhombencephalon.

Major Brain Regions and Structures

• Cerebrum: Higher brain functions; divided into lobes.

• Diencephalon: Thalamus (sensory relay), hypothalamus (homeostasis), epithalamus.

• Brain Stem: Midbrain, pons, medulla oblongata; controls vital functions.

• Cerebellum: Coordinates movement and balance.

Cerebral Anatomy

• Lobes: Frontal, parietal, temporal, occipital, insula.

• Gyri: Elevated ridges.

• Sulci: Shallow grooves.

• Fissures: Deep grooves (e.g., longitudinal fissure).

Meninges and Meningeal Spaces

• Dura Mater: Outermost, tough layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Innermost, delicate layer adhering to CNS surface.

• Spaces:

  • Epidural (spinal only): Between vertebrae and dura mater.

  • Subdural: Between dura and arachnoid.

  • Subarachnoid: Between arachnoid and pia; contains CSF.

• Difference: Epidural space is present in the spinal cord but not around the brain.

Key Brain Structures and Functions

• Thalamus: Sensory relay station; different nuclei process different modalities.

• Hypothalamus: Regulates autonomic, endocrine, and homeostatic functions.

• Midbrain: Visual and auditory reflexes.

• Pons: Relays signals; regulates breathing.

• Medulla Oblongata: Controls heart rate, breathing, vasomotor tone.

• Cerebellum: Coordinates voluntary movements.

Functional Areas of the Cerebral Cortex

• Motor Areas: Primary motor cortex (precentral gyrus), premotor cortex.

• Sensory Areas: Primary somatosensory cortex (postcentral gyrus), visual, auditory, gustatory, olfactory cortices.

• Association Areas: Integrate information (e.g., visual association, Wernicke's area for language comprehension, prefrontal cortex for executive function).

Ventricles and CSF Circulation

• Lateral Ventricles: Paired, in cerebral hemispheres.

• Third Ventricle: In diencephalon.

• Cerebral Aqueduct: Connects third and fourth ventricles.

• Fourth Ventricle: Between pons/medulla and cerebellum.

• CSF Flow: Produced by choroid plexus → lateral ventricles → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space via apertures → arachnoid villi → venous circulation.

• Choroid Plexus: Produces CSF.

• Arachnoid Villi: Absorb CSF into venous blood.

Blood-Brain Barrier

• Structure: Endothelial cells with tight junctions, astrocyte end-feet.

• Function: Regulates passage of substances from blood to CNS; protects brain from toxins.

Brain Disorders and Injuries

• Hydrocephalus: Excess CSF accumulation due to impaired flow or absorption.

• Concussion: Temporary brain dysfunction from trauma.

• Contusion: Bruising of brain tissue.

• Cerebral Edema: Swelling due to fluid accumulation.

• Cerebrovascular Accident (CVA)/Stroke: Blood flow interruption causing brain damage.

• Alzheimer’s Disease: Progressive neurodegeneration; memory loss, cognitive decline.

Spinal Cord and Peripheral Nervous System

Spinal Cord Structure

• Gross Anatomy: Extends from foramen magnum to L1/L2; cervical and lumbar enlargements.

• Microscopic Anatomy: Central gray matter (H-shaped), surrounding white matter.

Ascending and Descending Pathways

• Ascending (Sensory) Tracts: Carry sensory information to the brain (e.g., spinothalamic, dorsal columns).

• Descending (Motor) Tracts: Carry motor commands from brain to effectors (e.g., corticospinal tracts).

Structure of a Nerve

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds fascicles (bundles of axons).

• Epineurium: Surrounds entire nerve.

Spinal Nerves and Plexuses

• 31 Pairs of Spinal Nerves:

  • 8 cervical (C1–C8)

  • 12 thoracic (T1–T12)

  • 5 lumbar (L1–L5)

  • 5 sacral (S1–S5)

  • 1 coccygeal (Co1)

• Plexuses:

  • Cervical: Neck, diaphragm (phrenic nerve).

  • Brachial: Upper limb.

  • Lumbar: Anterior thigh.

  • Sacral: Posterior thigh, lower leg, foot.

Cranial Nerves

• 12 Pairs: Olfactory (I), Optic (II), Oculomotor (III), Trochlear (IV), Trigeminal (V), Abducens (VI), Facial (VII), Vestibulocochlear (VIII), Glossopharyngeal (IX), Vagus (X), Accessory (XI), Hypoglossal (XII).

• Key Nerves and Testing:

  • Olfactory (I): Smell test.

  • Optic (II): Visual acuity and field tests.

  • Trigeminal (V): Facial sensation, jaw movement.

  • Facial (VII): Facial expressions, taste.

  • Vestibulocochlear (VIII): Hearing and balance tests.

  • Vagus (X): Gag reflex, voice quality.

Autonomic Nervous System (ANS)

Definition and Relationship to PNS

• ANS: Subdivision of PNS; controls involuntary effectors (smooth/cardiac muscle, glands).

Somatic vs. Autonomic Nervous Systems

Feature	Somatic	Autonomic
Effectors	Skeletal muscle	Cardiac/smooth muscle, glands
Efferent Pathways	One neuron	Two-neuron chain (preganglionic & postganglionic)
Neurotransmitters	Acetylcholine (ACh)	ACh, norepinephrine (NE)

Sympathetic vs. Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1–L2)	Craniosacral (brainstem & S2–S4)
Ganglia Location	Near spinal cord (sympathetic chain)	Near/in target organs
General Function	"Fight or flight"	"Rest and digest"

Effects on Organs

• Sympathetic: Increases heart rate, dilates bronchioles, inhibits digestion, dilates pupils, stimulates sweat glands.

• Parasympathetic: Decreases heart rate, constricts bronchioles, stimulates digestion, constricts pupils.

Special Senses: Visual System

Primary and Supporting Organs of Vision

• Primary: Eyeball (globe).

• Supporting: Eyelids, eyelashes, eyebrows, lacrimal apparatus, extrinsic eye muscles.

Key Eye Structures and Functions

• Cornea: Transparent anterior part; refracts light.

• Sclera: White, fibrous outer layer; protection.

• Choroid: Vascular layer; nourishes retina.

• Ciliary Body: Produces aqueous humor; controls lens shape.

• Iris: Colored part; regulates pupil size.

• Pupil: Opening for light entry.

• Lens: Focuses light on retina.

• Retina: Contains photoreceptors (rods and cones).

• Macula Lutea: Central region for sharp vision.

• Central Fovea: Area of highest visual acuity.

• Optic Disk (Papilla): Blind spot; exit for optic nerve.

• Anterior/Posterior Chambers: Contain aqueous humor.

• Vitreous Humor: Gel filling posterior cavity; maintains eye shape.

• Canal of Schlemm: Drains aqueous humor.

Retinal Cell Arrangement

• Light passes through ganglion cells → bipolar cells → photoreceptors (rods/cones).

Common Eye Disorders

• Myopia (Nearsightedness): Image focuses in front of retina; corrected with concave lenses.

• Hyperopia (Farsightedness): Image focuses behind retina; corrected with convex lenses.

• Glaucoma: Increased intraocular pressure damages optic nerve.

• Cataract: Clouding of the lens.

Accommodation Reflex and Pupil Constriction

• Accommodation: Lens changes shape to focus on near/far objects (ciliary muscle contraction/relaxation).

• Pupil Constriction: Reduces light entry, increases depth of focus.

Rods vs. Cones

Feature	Rods	Cones
Function	Dim light, peripheral vision	Color, sharp vision
Location	Periphery of retina	Central retina (fovea)
Light Sensitivity	High	Low

Visual Fields and Cerebral Cortex Reception

• Each visual field projects to the opposite side of the visual cortex (e.g., left visual field to right occipital lobe).

Special Senses: Auditory System

Ear Anatomy and Functions

• Auricle (Pinna): Collects sound waves.

• External Auditory Meatus: Channels sound to tympanic membrane.

• Tympanic Membrane: Vibrates with sound; transmits to ossicles.

• Ossicles: Malleus, incus, stapes; amplify and transmit vibrations to inner ear.

• Eustachian Tube: Equalizes pressure between middle ear and pharynx.

• Mastoid Air Cells: Air spaces in temporal bone.

• Tympanic Cavity: Middle ear chamber.

• Semicircular Canals/Ducts: Balance (rotational acceleration).

• Cochlea/Cochlear Duct: Hearing; contains organ of Corti.

• Oval/Round Window: Transmit vibrations to/from inner ear.

• Vestibule, Saccule, Utricle: Balance (linear acceleration).

• Vestibular/Cochlear Nerves: Transmit balance and hearing signals to brain.

Bony and Membranous Labyrinths

• Bony Labyrinth: Cavities in temporal bone; filled with perilymph.

• Membranous Labyrinth: Membranous sacs/ducts within bony labyrinth; filled with endolymph.

Specialized Sensory Structures

• Crista (in semicircular canals): Detects rotational movement; contains hair cells.

• Macula (in utricle/saccule): Detects linear acceleration; contains hair cells.

• Organ of Corti (in cochlear duct): Main hearing organ; contains hair cells on basilar membrane.

• Basilar Membrane: Supports organ of Corti; vibrates in response to sound.

Endolymph and Perilymph

• Endolymph: Fluid within membranous labyrinth; high K+ concentration.

• Perilymph: Fluid between bony and membranous labyrinths; similar to CSF.

Sequence of Auditory Signal Transmission

1. Sound waves enter auricle and external auditory meatus.

2. Tympanic membrane vibrates.

3. Ossicles transmit/amplify vibrations to oval window.

4. Vibrations create waves in perilymph of cochlea.

5. Basilar membrane vibrates; hair cells in organ of Corti bend.

6. Hair cells generate nerve impulses in cochlear nerve.

7. Signals travel to primary auditory cortex (temporal lobe).

Brain Regions for Hearing

• Primary Auditory Cortex: Temporal lobe; processes auditory information.

• Auditory Association Areas: Interpret sound meaning.