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1. Gross Anatomy Overview

Major Divisions

  • Forebrain

    • Cerebrum (most obvious part of forebrain; responsible for higher-level functions like thought, voluntary movement, language, reason, and perception)

    • Diencephalon (hidden beneath cerebrum; contains thalamus, hypothalamus, epithalamus; primarily involved in relaying sensory information, autonomic control, and hormone regulation)

  • Brainstem

    • Midbrain

    • Pons

    • Medulla oblongata

    (Collectively controls vital body functions like breathing, heart rate, and consciousness, and serves as a conduit for nerve pathways)

  • Cerebellum (posterior to brainstem; primarily controls balance, coordination, motor learning, and accurate timing of movements)

2. Cerebrum

Surface Anatomy

  • Gyri = ridges (e.g., precentral gyrus for primary motor cortex, postcentral gyrus for primary somatosensory cortex)

  • Sulci = shallow grooves (e.g., central sulcus separates frontal and parietal lobes)

  • Fissures = deep grooves

    • Longitudinal fissure → separates left and right cerebral hemispheres

    • Transverse fissure → separates cerebrum & cerebellum

    • Lateral sulcus (Sylvian fissure) → separates temporal lobe from frontal and parietal lobes

    • Central sulcus (Rolandic fissure) → between frontal & parietal lobes; separates primary motor from primary somatosensory cortex

Lobes

  • Frontal (anterior; executive functions, planning, voluntary motor control, speech production via Broca's area, personality)

  • Parietal (superior-posterior; processes sensory information like touch, temperature, pain, pressure; spatial awareness, navigation)

  • Temporal (lateral; processing auditory information, memory formation, language comprehension via Wernicke's area, emotion)

  • Occipital (posterior; visual processing and interpretation)

  • Insula (deep to lateral sulcus; involved in taste, visceral sensation, emotion, empathy, and self-awareness)

Gray vs White Matter

  • Gray matter: Neuron cell bodies, dendrites, unmyelinated axons, neuroglia, synapses → forms the cerebral cortex (outer layer) and deep basal nuclei

  • White matter: Myelinated axons → organized into tracts that transmit signals between different brain regions and to/from the spinal cord

White Matter Tracts

  • Projection tracts

    • Extend vertically, connecting higher–lower brain centers and the spinal cord

    • Example: internal capsule (a compact bundle of afferent and efferent fibers) → fans out into corona radiata (radiating to different cortical areas)

  • Commissural tracts

    • Connect corresponding areas of the two cerebral hemispheres, enabling communication between them

    • Corpus callosum (the largest and most significant one, containing millions of axons)

    • Also anterior/posterior commissures (smaller tracts)

  • Association tracts

    • Connect regions within the same cerebral hemisphere, integrating diverse information

    • Short = connect adjacent gyri; long = connect different lobes within the same hemisphere

Basal Nuclei (also known as basal ganglia)

  • Deep gray matter structures (e.g., caudate nucleus, putamen, globus pallidus, substantia nigra, subthalamic nucleus) involved in:

    • Motor planning and control (initiating and stopping movements, regulating intensity of movements; damage can lead to movement disorders like Parkinson's or Huntington's disease)

    • Memory

    • Learning

    • Emotional processing (limbic system connections)

3. Diencephalon

Epithalamus

  • Smallest, most dorsal part of the diencephalon, forming the roof of the 3rd ventricle

  • Pineal gland (epiphysis cerebri) → secretes melatonin, an amine hormone that helps regulate sleep-wake cycles (circadian rhythms) and reproductive functions

  • Habenula → involved in emotional/motivational behaviors, particularly those related to reward and aversion, by connecting the limbic system to the midbrain

Thalamus

  • The gateway to cerebral cortex, acting as a major relay and processing station for most sensory and motor input

  • Relays:

    • Ascending sensory input (except olfaction) from the body and internal organs to appropriate cortical areas for conscious perception

    • Descending motor signals from the cerebral cortex to lower motor centers

    • Also involved in emotion, memory, and motor control through its diverse nuclei

  • Connected L–R via intermediate mass (interthalamic adhesion) in ~70% of people

  • Located around the 3rd ventricle

Hypothalamus

Major homeostasis center located inferior to the thalamus, forming the floor of the 3rd ventricle. It influences virtually all organs via the autonomic nervous system and pituitary gland:

  • Autonomic nervous system (ANS) control → regulates heart rate, blood pressure, gastrointestinal motility, respiration via brainstem centers

  • Thermoregulation → coordinates sweating/shivering, vasodilation/vasoconstriction to maintain body temperature

  • Hunger/thirst → monitors blood glucose and osmolarity, stimulating feeding/drinking behaviors

  • Sleep/circadian rhythms → contains the suprachiasmatic nucleus (SCN), the body's main biological clock

  • Emotional responses (links limbic system to autonomic systems; involved in pleasure, fear, rage, sex drive)

  • Endocrine control:

    • Produces ADH (antidiuretic hormone, for water balance) and oxytocin (for uterine contractions and milk ejection), which are stored and released by the posterior pituitary

    • Controls anterior pituitary with releasing and inhibiting hormones, regulating growth, metabolism, stress response, and reproduction

4. Brainstem

The brainstem contains the reticular formation throughout all levels (midbrain → pons → medulla), a diffuse network of neurons involved in sleep, consciousness, pain modulation, and visceral functions.

Midbrain (Mesencephalon)

Located between diencephalon and pons. Structures:

  • Cerebral aqueduct → channel for CSF flow from 3rd to 4th ventricle

  • Corpora quadrigemina (four colliculi on the dorsal surface)

    • Superior colliculi → visual reflex centers (tracking moving objects, pupillary reflexes to light)

    • Inferior colliculi → auditory reflex centers (startle reflex to loud noises, relay for auditory pathways)

  • Cerebral peduncles → paired ventral pillars that 'hold up' the cerebrum, containing corticospinal and corticobulbar tracts

  • Cerebral crus → contains the descending corticospinal tracts (motor pathways from cortex)

  • Substantia nigra → darkly pigmented nucleus containing dopamine-producing neurons; crucial for motor control and reward; degeneration associated with Parkinson’s disease

  • Periaqueductal gray → surrounds the cerebral aqueduct; involved in pain modulation (suppression of pain) and autonomic control

Functions:

  • Motor control (via substantia nigra, red nucleus)

  • Reflexive visual and auditory responses

  • Consciousness & filtering of sensory input (reticular formation activity helps maintain alertness and attention)

Pons

  • Appears as a prominent “pregnant bulge” anterior to the cerebellum; primarily involved in relaying information and regulating respiration

  • Cerebellar peduncles connect pons ↔ cerebellum, providing pathways for motor and sensory information to and from the cerebellum

  • Contains reticular formation nuclei involved in sleep, respiration (apneustic/pneumotaxic centers), and posture

  • Contains ascending sensory and descending motor tracts that pass between higher brain centers and the spinal cord

Medulla Oblongata (Myelencephalon)

Lowest part of the brainstem, blending into the spinal cord; critical for vital reflex control.

Functions (contains vital centers, part of the reticular formation):

  • Cardiac center → regulates heart rate and force of contraction

  • Vasomotor center → regulates blood pressure by controlling blood vessel diameter

  • Respiratory center → sets the basic rhythm of breathing, interacting with pons centers

  • Reflexes: mediates numerous non-vital reflexes like swallow, cough, sneeze, gag, vomit, sweat, salivate

Structures:

  • Pyramids → two anterior ridges containing the large corticospinal tracts (voluntary motor pathways)

    • Decussation of the pyramids → occurs at the inferior medulla, where most corticospinal fibers cross to the opposite side, explaining contralateral motor control

  • Olives → prominent lateral bulges, containing inferior olivary nuclei, which relay sensory information to the cerebellum, particularly important for motor learning and coordination

  • First-order sensory neurons from the gracile and cuneate fasiculi synapse here to form the medial lemniscus (ascending sensory pathway for touch, pressure, vibration, proprioception)

Processes:

  • General senses: touch, pressure, pain, temperature

  • Special senses: taste, hearing, equilibrium (relays from cranial nerves)

5. Cerebellum

Located posterior to the brainstem; critical for motor control and coordination.

Anatomy

  • Folia = slender, transverse ridges on the surface, increasing surface area for gray matter (cerebellar cortex)

  • Sulci = grooves separating the folia

  • Arbor vitae = distinctive branching pattern of white matter within the cerebellum, resembling a tree

  • Cerebellar Peduncles = thick bundles of fibers that connect the cerebellum to the brainstem, facilitating communication between different parts of the brain.

Chapter 15

The ANS as the Visceral Motor Branch

  • PNS = all nerves outside CNS; has sensory (afferent) and motor (efferent) divisions.

  • Somatic = body wall; sensory from skin & muscles; motor effectors = skeletal muscle.

  • Visceral = internal organs; visceral motor = ANS (involuntary).

  • You are not consciously aware of ANS activity.

  • ANS effectors = cardiac muscle, smooth muscle, glands.

  • ANS to body wall includes: arrector pili, sweat glands, blood vessels.

  • Heart & intestines have pacemakers → ANS modulates, does not initiate.

  • If ANS nerves cut → activity becomes independent, not paralyzed.

  • ANS maintains homeostasis.

Visceral Reflex Arc (vs Somatic)

  • Visceral reflexes are slower and use 2 motor neurons.

  • Five parts: receptor → afferent neuron → integration center → efferent neurons → effector.

  • Stimuli sensed: stretch, chemicals, pH, oxygen, CO₂, blood pressure.

  • Integration: brainstem, spinal cord.

  • Baroreceptors monitor blood pressure → carried on glossopharyngeal nerve (IX) → integrated in medulla.

  • If BP high → vagus nerve slows heart → negative feedback.

  • Standing up drops BP → sympathetic ↑ heart rate → restores BP → negative feedback.

  • Visceral motor neurons use varicosities and diffuse release.

Two Divisions of the ANS

Sympathetic & Parasympathetic

  • Autonomic tone = baseline firing level.

  • Somatic LMN cell bodies: anterior horn of spinal cord; NT = ACh, opens Na⁺ channels → depolarizes → Ca²⁺ release → contraction.

ANS motor

  • Neurons synapse in ganglia (outside CNS).

  • CNS → ganglion = preganglionic, myelinated, white matter.

  • NT at all ganglia = ACh on nicotinic receptors.

  • Ganglion → effector = postganglionic.

  • ANS uses two neurons.

Parasympathetic Division (Rest & Digest)

  • Parasympathetic tone keeps GI tract moving.

  • Without parasympathetic input, heart rate = 100 bpm.

  • Normal HR ~75 bpm due to vagal tone.

  • Preganglionic somas in lateral horn of sacral cord & brainstem nuclei.

  • Preganglionic = long, myelinated, white.

  • Ganglia are in/near organs = terminal ganglia.

  • Postganglionic are short.

  • Parasympathetic to effectors releases ACh.

Sympathetic Division (Fight or Flight)

  • Increases: HR, BP, airflow, glucose.

  • Decreases: GI activity & skin blood flow.

  • Sympathetic tone maintains baseline vasoconstriction → normal BP.

  • Preganglionic somas in lateral horn T1–L2.

  • Preganglionic = short, myelinated, white.

  • Ganglia: chain (paravertebral), collateral, adrenal medulla.

  • Postganglionic = long.

  • NT to effectors = norepinephrine (NE).

Thoracolumbar vs Craniosacral; Ganglia

Sympathetic = Thoracolumbar

  • Preganglionic exit T1–L2.

  • Go to sympathetic chain ganglia.

  • Typical # per side: 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, 1 coccygeal.

  • White ramus = preganglionic (myelinated).

  • Gray ramus = postganglionic (unmyelinated).

  • Splanchnic nerves pass through chain but synapse in collateral ganglia.

  • Carotid plexus = sympathetic fibers regulating head (pupil dilation, sweat, salivary inhibition).

Parasympathetic = Craniosacral

  • Carried by III, VII, IX, X; most important = vagus (X).

  • Ganglia = terminal/intramural, embedded in organ wall.

  • Effects = localized (not widespread).

Enteric Nervous System

  • Independent network controlling GI motility & secretion.

Vagus Nerve & Adrenal Medulla

Vagus (parasympathetic superstar)

  • Goes to: cardiac, pulmonary, esophageal, abdominal (celiac) plexuses.

  • Vagal trunks: left/right branches that can trigger vasovagal drop → HR ↓ → BP ↓.

Adrenal Medulla

  • Cortex = outer, Medulla = inner.

  • Preganglionic sympathetic goes through chain → splanchnic nerves → celiac plexus → adrenal medulla.

  • Releases epinephrine & norepinephrine into blood.

  • Widespread response (hormones reach all cells with adrenergic receptors).

Dual vs Single Innervation

Dual Innervation

Most organs receive both sym & parasym.

Antagonistic examples:

  • Sym ↑ HR; Parasym ↓ HR.

  • Sym ↑ respiration; Parasym ↓ respiration.

  • Sym ↓ digestion; Parasym ↑ digestion.

  • Sym dilates pupils; Parasym constricts.

  • Sym dries nasal mucosa; Parasym moistens.

Cooperative examples:

  • Salivation: Sym = thick mucus; Parasym = watery saliva.

Single Innervation (Sym Only)

  • Blood vessels, adrenal medulla, sweat glands, piloerector muscles.

Vasomotor tone: baseline sympathetic firing

  • ↑ firing → vasoconstriction

  • ↓ firing → vasodilation

ANS Neurotransmitters

  • Effect depends on receptors, not NT.

  • ACh released by all preganglionics, and all parasympathetic postganglionics.

  • Sympathetic postganglionics mostly release NE (except sweat glands & some vessels which use ACh).

  • Cholinergic fibers release ACh.

  • Adrenergic fibers release NE/Epi.

ANS Agonists & Antagonists

  • Agonist = activates a receptor.

  • Mimetic = acts like that division.

  • Antagonist = blocks a receptor.

  • Lytic = breaks/blocks function.

Sympathomimetics (act like NE):

  • Phenylephrine → vasoconstriction.

Sympatholytics (oppose NE):

  • Beta blockers → block β-receptors.

Parasympathomimetics:

  • Pilocarpine → stimulates muscarinic ACh receptors.

Parasympatholytics:

  • Atropine → blocks muscarinic ACh receptors.

SSRIs (like Prozac): block serotonin reuptake → ↑ serotonin.
MAOIs: block monoamine breakdown → ↑ NE, serotonin, dopamine.
Cocaine: blocks reuptake → huge ↑ NE, dopamine.

Adenosine slows ACh release → tired.
Caffeine blocks adenosine receptors, so you don’t feel tired yet.

Higher-Level Control

  • Cerebral cortex influences ANS via limbic system.

  • Main control center = hypothalamus (HR, BP, digestion, thermoregulation).

  • Hypothalamus controls both sympathetic & parasympathetic.

  • Sends commands to brainstem nuclei → cranial nerves → spinal cord.

  • Brainstem regulates HR, BP, respiration.

  • Spinal cord controls reflexes for defecation & urination.

  • Potty training = gaining voluntary cortical control.

Chapter 16

Sensory Receptors & General Senses (Condensed Exam Version)

1. Sensory receptors, transduction, sensation

  • Sensory receptor: any structure that detects a stimulus

  • Simplest = free nerve ending

  • Complex receptors use connective tissue (muscle spindles, taste buds, photoreceptors)

  • Transduction: converting stimulus → nerve signal

  • Sensation: conscious awareness of stimulus

  • 4 types of sensory info: modality, location, intensity, duration

Origin of stimulus

  • Exteroceptors: outside body (touch, vision, taste, hearing)

  • Interoceptors: internal organs (stretch, pH, pain)

  • Proprioceptors: position/movement (muscles, tendons, joints)

2. General vs Special Senses

  • General senses = widely distributed

  • Found in skin, muscles, joints, viscera

  • Include touch, pressure, temp, pain, stretch, vibration, etc.

  • Special senses = head only, in cranial nerves

  • Special senses: vision, hearing, equilibrium, taste, smell

3. Encapsulated vs Unencapsulated

  • Unencapsulated: bare dendrites

    • free nerve endings, tactile discs, hair receptors

  • Encapsulated: wrapped in glial/connective tissue

    • tactile corpuscles, end bulbs, lamellar corpuscles, bulbous corpuscles, muscle spindles, tendon organs

4. Modality / Stimulus

  • Modality determined by type of receptor

  • Brain interprets modality by where the signal ends up (labeled line)

5. Receptor potentials

  • Stimulus → local electrical change

  • Receptor potentials are local

  • If receptor is neuron: transduction occurs at dendrite, AP begins at axon hillock

  • If receptor cell: releases neurotransmitter; afferent fires AP if strong enough

6. How receptors convey information

Modality

  • Determined by where pathway leads

  • Receptor types:

    • Thermo: temperature

    • Photo: light

    • Noci: pain

    • Chemo: chemicals

    • Mechano: pressure/stretch/vibration

Location

  • Determined by projection pathway

  • Receptive field: area monitored by one neuron

  • Large field → poor discrimination

  • Small field → high discrimination

  • Projection pathway explains referred pain

Intensity

  • Encoded by:

    1. Threshold

    2. Recruitment (more neurons fire)

    3. Firing frequency

  • Strong stimulus = more neurons + faster firing

Duration

  • Phasic receptors: burst → adapt (smell, hair movement, pressure, temperature)

  • Tonic receptors: slow adapting (pain, proprioception)

7. Projection pathway (general senses)

  • Head: receptors → 1st order → CN V (mostly) → pons/medulla → 2nd order → thalamus → 3rd order → cortex

  • Body: receptors → spinal nerve → posterior horn → 2nd order crosses → thalamus → cortex

  • Proprioception → spinocerebellar tract

  • Visceral sensory → vagus nerve

  • Visceral pain → spinothalamic tract

8. Pain receptors & chemicals

  • Pain protects from injury

  • Pain = activation of nociceptors (free nerve endings)

  • Chemicals: bradykinin, prostaglandins, serotonin, histamine

  • Somatic pain: skin, muscles, joints

  • Visceral pain: organs

  • Referred pain: shared pathways

  • Fast pain: myelinated, sharp

  • Slow pain: unmyelinated, dull

  • Pain from head: CN V, VII, IX, X

  • Pain from body: spinothalamic + spinoreticular + gracile/cuneate

9. Spinothalamic Pathway & Modulation

  • Nociceptor → 1st order → posterior horn

  • Releases substance P to excite 2nd order

  • 2nd order crosses → thalamus → 3rd order → cortex

  • Limbic activation = emotional response

  • Endogenous opioids: endorphins, enkephalins, dynorphins

  • Opioids block pain in midbrain, spinal cord

  • Spinal gating: descending fibers → release serotonin → interneurons release enkephalins → block pain

  • Massage activates mechanoreceptors → interneurons release enkephalins → reduced pain

Special Senses

10. Taste bud structure

  • Chemicals dissolve in saliva

  • Cell types:

    • Gustatory cells (receptors)

    • Supporting cells

    • Basal cells (replace others)

  • Papillae:

    • Filiform (texture)

    • Foliate (side ridges, degenerate young)

    • Fungiform (few taste buds, tip/sides)

    • Vallate (most taste buds, back of tongue)

  • Other locations: palate, pharynx, epiglottis

11. Taste transduction

  • Tastants bind taste hairs

  • Salty: Na+ enters → depolarize

  • Sour: H+ enters → depolarize

  • Sweet, bitter, umami: GPCR → 2nd messenger → NT release

  • NT excites cranial nerves

12. Taste projection pathway

  • Anterior 2/3 → CN VII

  • Posterior 1/3 → CN IX

  • Epiglottis/pharynx → CN X

  • 1st order → medulla

  • 2nd order → hypothalamus + amygdala

  • 2nd order → thalamus

  • 3rd order → insula (primary gustatory cortex)

  • Flavor integration → orbitofrontal cortex

13. Olfactory epithelium & ethmoid

  • Airborne odorants dissolve in mucus → bind bipolar neurons

  • Olfactory epithelium at roof of nasal cavity

  • Contains: supporting cells, basal cells

  • Olfactory hairs = dendrites

  • Bipolar axons → form fascicles → pass through cribriform plate → olfactory bulb

14. Olfactory transduction

  • Odorant binds receptor proteins

  • GPCR → cAMP → opens Na+/Ca2+ channels → depolarize

  • If threshold reached → AP fires

  • Irritants stimulate nociceptors via CN V

15. Olfactory projection

  • Bipolar neurons synapse in glomeruli

  • Mitral & tufted cells → olfactory tract → primary olfactory cortex (inferior temporal lobe)

  • Signals also → amygdala, hippocampus (emotion + memory)

  • Thalamus → orbitofrontal cortex (smell identification)

Auditory (Hearing) and Equilibrium

16. Gross anatomy of outer, middle, and inner ear

A. Hearing:

  • Sound waves compress air, move down ear canal, vibrate tympanic membrane.

  • Middle ear bones (malleus, incus, stapes) amplify motion, press on oval window of inner ear.

  • Inner ear fluid (perilymph) moves, causing basilar membrane to push against hair cells.

  • Hair bending releases neurotransmitter to cochlear nerve.

  • Pitch processed along basilar membrane location; loudness comes from amplitude of membrane motion.

B. Equilibrium:

  • As body moves, endolymph in inner ear moves, specifically in semicircular canals and vestibule.

  • Fluid pushes against hair receptors; hair bending signals brain along vestibular nerve.

C. Outer ear: Flap = auricle/pinna, directs sound to external auditory canal.

D. Passage from outside to eardrum: external acoustic meatus.

E. Wax trap: cerumen, hair; moves naturally, don’t Q-tip.

F. Middle ear begins at: tympanic membrane.

G. Middle ear bones (ossicles): malleus, incus, stapes.

H. Tympanic cavity also receives: auditory tube from throat.

I. Stapes pushes on: oval window, moves inner ear fluid.

J. Protective muscles: tensor tympani (malleus), stapedius (stapes).

K. Other window: round window.

L. Otitis externa: outer ear, fungi/bacteria.

M. Otitis media: middle ear, more common in children.

N. Inner ear: labyrinth of canals in temporal bone. Perilymph fills bony labyrinth, endolymph in membranous labyrinth.

O. Sound enters: cochlea (snail shell).

P. Linear equilibrium: saccule & utricle, contain maculae.

Q. Rotational equilibrium: 3 semicircular canals.

R. Hearing nerve: cochlear nerve; equilibrium: vestibular nerve; merge as vestibulocochlear nerve, cranial nerve VIII.

S. Obstruction/fusion: conductive deafness; bone vibration bypasses.

17. Bony and membranous labyrinth

A. Bony canals: bony labyrinth.
B. Perilymph: fills space between bony and membranous labyrinth.
C. Membranous labyrinth: contains endolymph.
D. Endolymph ion: high K+.

18. Sound wave collection and amplification

A. Collected by: auricle, sent into external auditory canal.
B. Vibrates: tympanic membrane.
C. Amplified by ossicles: malleus, incus, stapes.
D. Stapes pushes on: oval window.
E. Motion crosses: basilar membrane in cochlea.

19. Cochlea chambers and cochlear duct

A. Cochlea: bony labyrinth = perilymph, membranous labyrinth = endolymph.
B. Vestibular membrane = superior membrane; cavity above = scala vestibuli.
C. Basilar membrane: inferior part of membranous labyrinth.
D. Perilymph below basilar membrane: scala tympani.
E. Cochlear duct: between basilar & vestibular membrane, contains endolymph.
F. Organ of Corti: functional hearing unit, also called spiral organ.
G. Hair cells detect stimuli, supporting cells hold them.
H. Hairs = stereocilia; 3 outer, 1 inner hair cell.
I. Tectorial membrane overlays stereocilia; loud noises can cause sensorineural deafness.

20. Transduction in Organ of Corti

A. Fluid moves = perilymph in cochlea.
**B. Wave reaches correct basilar membrane area.
**C. Hair cells push against tectorial membrane.
**D. Tip links open mechanically-gated K+ channels.
**E. K+ enters hair cell → depolarization.
**F. Hair cells release glutamate to cochlear nerve.
**G. Cochlea wraps 2.5 times around modiolus.
H. High frequency: near oval window.
I. Low frequency: near apex.
**J. Scala vestibuli fluid passes to scala tympani, pushes round window.

21. Projection pathway for hearing

A. Cochlea has bipolar sensory neurons. Peripheral process gets neurotransmitter from hair cells.
B. Cell bodies in spiral ganglion.
**C. Central process = cochlear nerve.
**D. Joins vestibular nerve = cranial nerve VIII.
**E. Fibers to both sides of medulla, cochlear nuclei; 1st order synapses to 2nd order.
**F. 2nd order crosses as trapezoid body.
G. Cochlear tuning adjusts outer hair cells; inhibits background noise via outer hair cells.
H. Middle ear muscles: tensor tympani & stapedius.
I. Binaural hearing: compare sound arrival to locate direction.
**J. Cochlear nuclei → inferior colliculus (startle, speech processing).
**K. 3rd order → medial geniculate nucleus (thalamus).
**L. 4th order → primary auditory cortex.

22. Role of saccule, utricle, semicircular ducts

A. Inputs: vestibular apparatus, proprioception, vision.
B. Vestibular apparatus: 3 semicircular canals + vestibule (saccule & utricle).
C. Head in space: static equilibrium.
D. Motion/acceleration: dynamic equilibrium.
E. Linear acceleration: saccule & utricle; rotational acceleration: semicircular canals.
F. Maculae detect linear acceleration & head tilt.
**G. Semicircular canals detect angular acceleration.
**H. Macula contains hair cells + supporting cells.
**I. Gel = otolithic membrane; crystals = otoliths.
J. Head tilt: stereocilia bend; otoliths provide weight & inertia.
K. Saccule orientation: vertical → detects vertical acceleration (elevator).
L. Utricle orientation: horizontal → detects horizontal acceleration (car).
**M. Semicircular canals = X, Y, Z planes.
**N. Filled with endolymph.
**O. Enlarged end = ampulla; sensory organ = crista ampullaris.
**P. Crista: tuft of hair cells + cupula (gel blob).
**Q. Supporting cells hold hair cells.
**R. Base = vestibular nerve.

23. Transduction in equilibrium

Cristae ampullaris:

  • Endolymph lags behind motion, bends hair cells.

  • Hair cells release glutamate → vestibular nerve.

Otolith organs:

  • Otolithic membrane moves; bends hair cells.

  • Hair cells release glutamate → vestibular nerve.

24. Projection pathway for equilibrium

A. Vestibular nerve merges with cochlear → CN VIII.
B. Fibers → vestibular nuclei in pons & medulla.
**C. Info → 5 places: cerebellum, brainstem, spinal cord, thalamus, eye muscles.
D. Cerebellum: adjust muscles.
E. Reticular formation: adjust posture & eye movements.
F. Spinal tracts: skeletal muscles for balance.
G. Thalamus: sends to inferior postcentral gyrus, precentral cortex.

Vision

24. Accessory structures of the eye

A. Conjunctiva: wet mucous membrane lining interior eyelids & anterior eye except cornea.

B. Lacrimal apparatus: tear glands (lacrimal glands) → ducts → sweep tears across eye → puncta → canaliculi → sac → nasolacrimal duct.

C. Extrinsic eye muscles: move eyeball in socket.

D–G. Rectus muscles:

  • Superior rectus → up

  • Medial rectus → medially (adduction)

  • Inferior rectus → down

  • Lateral rectus → laterally (abduction, CN VI)

H–J. Oblique muscles:

  • Superior oblique → moves eye down & laterally, CN IV

  • Inferior oblique → moves eye up & laterally, CN III

25. Three tunics of the eye

A. Fibrous tunic:

  • Sclera → white, protects, attaches extrinsic muscles

  • Cornea → clear, anterior eye, refracts light, irregular shape → astigmatism

B. Uvea (vascular tunic):

  • Choroid → dark, absorbs stray light, vascularized to feed retina

  • Ciliary body → forms muscle ring, suspensory ligaments attach lens

  • Iris → colored, surrounds pupil, constrict/dilate pupil

C. Neural tunic (retina): photoreceptors + neurons; attached at optic disc & ora serrata.

26. Lens

A. Lens fibers = transparent cells; allow light passage
B. Protein clumps → cataracts
C. Surgery → artificial lens (IOL)
D. Suspensory ligaments attach lens to ciliary body
E–G. Lens shape & focusing:

  • Tight ligaments → flat lens → far vision

  • Relaxed ligaments → round lens → near vision

  • Aging → less elasticity → presbyopia

H. Emmetropia: normal vision, light focused on retina, ciliary muscle relaxed, lens flat

I–M. Near response:

  1. Convergence → medial rectus

  2. Pupil constriction → block stray light

  3. Lens rounds → accommodate

N–T. Refractive errors:

  • Hyperopia → eyeball too short → behind retina → convex lens corrects

  • Myopia → eyeball too long → in front of retina → concave lens corrects

27. Eye fluids

A. Vitreous humor: fills posterior cavity, holds retina against choroid, maintains intraocular pressure
B. Canal of Schlemm: drains aqueous humor
C. Aqueous humor: constantly produced by ciliary body, fills anterior & posterior chambers
D. Flow: posterior → anterior → pupil → drains into scleral venous sinus
E. Glaucoma: high IOP → compromises retinal blood supply

28. Rods vs Cones

A. Two photoreceptor types: rods & cones
B. Fovea centralis: highest cone density, sharp color vision
C. Peripheral retina: mostly rods, low acuity, gray vision
D. Blind spot: optic disc, no photoreceptors
E. Rods: outer segment = stacked discs with rhodopsin, scotopic (night), monochromatic, high convergence → fuzzy image
F. Cones: outer segment = opsin pigments, photopic (day), trichromatic → sharp image

29. Phototransduction

A. Retinal pigment molecule: retinal cis (bent) attached to opsin in dark
B. Light → retinal straightens (trans), separates from opsin → bleaching of photopigment
C. Trans-retinal → cis-retinal (ATP) → rejoins opsin → regenerates photopigment

D. Dark: rod secretes glutamate, inhibits bipolar cell
E. Light: glutamate inhibition stops → bipolar excites ganglion → ganglion fires → optic nerve

30. Projection pathway for vision

A. Ganglion cell axons → optic nerve, exits at optic disc
B. Optic nerves meet at optic chiasm, medial fibers cross, lateral fibers stay ipsilateral
**C. Optic tract → lateral geniculate nucleus (thalamus)
**D. Thalamus → primary visual cortex (occipital lobe)
**E. Some fibers → midbrain (superior colliculus) → visual reflexes
**F. Right hemisphere sees left visual field, left hemisphere sees right visual field
G. Colorblindness: lack of functional opsin genes resulting in difficulty distinguishing certain colors, most commonly reds and greens.