Week 9 Content Flashcards
Peripheral Nervous System (PNS)
Connects the CNS to the rest of the body
Includes nerves and ganglia
Transmit sensory input and motor output
Functional Divisions of the PNS
Sensory (input) and motor (output)
Somatic (body surface, muscles) and visceral (internal organs)
General (widespread) and special (localized senses)
Autonomic Nervous System (ANS)
General visceral motor division of the PNS
Regulates involuntary functions (ie; heart rate, digestion)
Main Divisions
1. Parasympathetic
- Rest and digest
- Sympathetic
- Fight or flight
Cranial Nerves
Originate from the brain and pass through specific foramina of the skull
Numbered from I-XII
- Crainial Nerves I and II: Attach to the forebrain
- Cranial Nerves III-XII: Attach to the brainstem
Serve head and neck structures
Only vagus nerve (X) extends into the abdomen
Cranial Nerves Mnemonic
Open One Or Two Textbooks And Find Very Generic Vague Study Habits
- Olfactory
- Optic
- Oculomotor
- Trochlear
- Trigeminal
- Abducens
- Facial
- Vestibulocochlear
- Glossopharyngeal
- Vagus
- Spinal Accessory
- Hypoglossal
I - Olfactory Nerves
Special visceral sensory
- Smell
Origin: Olfactory receptor cells located in the olfactory epithelium of the nasal cavity
Pathway: Pass through cribriform foramina of the ethmoid bone
II - Optic Nerves
Special somatic sensory
- Vision
Origin: Retina of the eye
Pathway: Pass through the optic canals of the sphenoid bone
III - Oculomotor Nerves
Somatic Motor (Innervates extrinsic eye muscles)
1. Superior rectus
2. Medial rectus
3. inferior rectus
4. Inferior oblique
Visceral Motor (Constricts pupil, controls lens shape)
Origin: Oculomotor nucleus of midbrain
Pathway: Pass through the superior orbital fissure
IV - Trochlear Nerves
Somatic Motor
- Innervate superior oblique muscle
Origin: Trochlear nucleus of midbrain
Pathway: Pass ventrally and laterally around midbrain
- Exit through superior orbital fissure
V - Trigeminal Nerves
Largest cranial nerve, sensory and motor functions
- Ophthalmic Division (V1): Sensory, upper face
- Maxillary Division (V2): Sensory, midface
- Mandibular Division (V3): Sensory and motor, lower face
Origin: Sensory cell bodies in the trigeminal ganglion and motor nucleus of trigeminal nerve
V - Trigeminal Nerve Pathways
Ophthalmic (V1)
- Passes through the superior orbital fissure
Maxillary (V2)
- Passes through the foramen rotundum
Mandibular (V3)
- Passes through the foramen ovale, enters mandible through mandibular foramen
VI - Abducens Nerves
Somatic Motor
- Innervates the lateral rectus muscles
Origin: Abducens nucleus in the pons
Pathway: Travels through the superior orbital fissure to the eye
VII - Facial Nerves
Special visceral sensory
- Taste (anterior two-thirds of tongue)
Somatic Motor
- Innervates five branches of facial muscles
1. Temporal
2. Zygomatic
3. Buccal
4. Mandibular
5. Cervical
Visceral Motor
- Innervates lacrimal glands, submandibular and sublingual salivary glands
Origin: Facial nucleus of pons in the brain stem
Pathway: Enters temporal bone through the internal acoustic meatus
- Travels through the facial canal to target glands
VIII - Vestibulocochlear Nerves
Sensory Nerve
- Hearing and balance
Vestibular Branch: Equilibrium
Cochlear Branch: Hearing
Origin: Vestibular apparatus and cochlea
Pathway: Passes through the internal acoustic meatus to the brainstem
IX - Glossopharyngeal Nerves
Posterior third of the tongue
Special Visceral Sensory
- Taste
General Visceral Sensory
- Pharyngeal mucosa
- Chemoreceptors in the carotid body
- Baroreceptors in the carotid sinus
Somatic Motor
- Elevate pharynx during swallowing
Visceral Motor
- Innervate the parotid salivary gland
Origin: Medulla oblongata
Pathway: Fibers exit through the jugular foramen
X - Vagus Nerves
Sensory Functions
- General Visceral Sensory: From thoracic and abdominal viscera
- Special Visceral Sensory: Taste from taste buds on the epiglottis
Somatic Motor Functions
- Innervates skeletal muscles of the pharynx and larynx
Visceral Motor (Parasympathetic Innervation)
- Heart, lungs, abdominal viscera
Origin: Medulla oblongata
Pathway: Fibers exit the skull through the jugular foramen
XI - Accessory Nerves
Somatic Motor
- Innervates the trapezius and sternocleidomastoid muscles
- Formed from ventral rootlets of the spinal cord (C1-C5)
Pathway: Enters the skull through the foramen magnum, exits the skull through the jugular foramen
XII - Hypoglossal Nerves
Somatic Motor
- Innervates the tongue muscles
- Formed from ventral rootlets of the medulla oblongata
Pathway: Exits skull through the hypoglossal canal
Cranial Nerve Function Mnemonic
Some Say Marry Money, But My Brother Says Big Brains Matter Most
S: Sensory
M: Motor
B: Both
- S (Olfactory)
- S (Optic)
- M (Oculomotor)
- M (Trochlear)
- B (Trigeminal)
- M (Abducens)
- B (Facial)
- S (Vestibulocochlear)
- B (Glossopharyngeal)
- B (Vagus)
- M (Accessory)
- M (Hypoglossal)
Spinal Nerves
31 pairs connect to the spinal cord
- Cervical (C1-C8): 8 pairs
- Thoracic (T1-T12): 12 pairs
- Lumbar (L1-L5): 5 pairs
- Sacral (S1-S5): 5 pairs
- Coccygeal (Co1): 1 pair
Spinal Nerve Connections
Dorsal Root: Sensory fibers, cell bodies in dorsal root ganglion
Ventral Root: Motor fibers from the anterior gray column
Spinal Nerve Branches
Dorsal and Ventral Rami: Both carry sensory and motor fibers
Rami Communicantes: Connect ventral ramus to sympathetic chain ganglia
Innervation of the Back
Dorsal rami supplies back muscles and skin in segmented strips
Follow emergence points of the vertebral column
Thoracic and Abdominal Wall Innervation
Ventral Rami: Simple, segmented pattern
Intercostal Nerves: Supply intercostal muscles, skin, and abdominal wall
Branches: Lateral and anterior cutaneous
Introduction to Nerve Plexuses
Networks of ventral rami (except T2-T12)
Found in cervical, brachial, lumbar, and sacral regions
Serve limbs; fibers crisscross for redundancy
The Cervical Plexus
C1-C4
Deep to the sternocleidomastoid
Mostly cutaneous nerves; some serve anterior neck muscles
Cervical Plexus Sensory Branches
Lesser Occipital Nerve
Great Auricular Nerve
Transverse Cervical Nerve
Supraclavicular Nerves
Mnemonic: Let’s Go To Sleep
Cervical Plexus Motor Branches
Muscular Branches
Ansa Cervicalis
Phrenic Nerve
Acronym: MAP
Phrenic Nerve
Key nerve of the cervical plexus
Formed by C3, C4, and C5 fibers
Controls the diaphragm
Brachial Plexus
Located in the neck and axilla
Formed by C5-C8 ventral rami
Cords give rise to main upper limb nerves
Upper Limb Innervation
Components (Medial to Lateral)
1. Ventral rami
2. Trunks
3. Divisions
4. Cords
Brachial Plexus Structure
Ventral Rami: Form the roots of the brachial plexus
Trunks: 3 trunks formed from merging rami
Divisions: Each trunk splits into anterior and posterior divisions
Cords: 6 divisions converge to form 3 cords
Terminal Branches from Lateral and Medial Cords
- Musculocutaneous
- From lateral cord, innervates biceps brachii and brachialis - Median
- From lateral and medial cords, innervates anterior forearm muscles and lateral palm
- Muscular and digital branches - Ulnar
- From medial cord, innervates intrinsic hand muscles and medial hand skin
- Dorsal, superficial, and digital branches
Terminal Branches from the Posterior Cord
Axillary: Innervates deltoid and teres minor
- Deep and superficial branch
Radial: Continuation of posterior cord, largest branch, innervates posterior upper limb muscles
- Deep and superficial branch, posterior cutaneous nerve
Mnemonic: Most Alcoholics Must Really Urinate
Mnemonic for the Brachial Plexus
3 Musketeers Assassinated 5 Rats, 4 Mice, and 2 Unicorns
C5, C6, C7 Fingers: Musculocutaneous nerve
C5 and C6: Axillary nerve
C5 to T1: Radial nerves
C6 to T1: Median nerves
C8 and T1: Ulnar nerve
Muscular Innervation of the Upper Limb
Musculocutaneous Nerve: Coracobrachialis, Biceps Brachii, Brachialis
Median Nerve: Forearm flexors, 3 Thenar muscles, Lumbricals (digit 2 and 3)
Ulnar Nerve: Flexor Carpi Ulnaris, Flexor Digitorum Profundus, 3 Hypothenar Muscles, Lumbricals (digit 4 and 5)
Cutaneous Innervation of the Upper Limb
Medial Cutaneous Nerve
- Sensory input to Musculocutaneous, Ulnar, and Median Nerves
Muscular Innervation of the Upper Limb
Radial Nerve: Triceps, Brachioradialis, Extensors (Wrist, Digits), Supinator, Abductor Pollicis Longus, Aconeus
Axillary: Teres Minor, Deltoid
Cutaneous Innervation of the Upper Limb
Axillary Nerve: Provides sensory input to the shoulder
Radial Nerve: Provides sensory input to the posterior arm, forearm, and hand
Lumbar Plexus
L1 to L4
Smaller Branches: Innervate posterior abdominal wall and psoas muscle
Femoral Nerve: Innervates anterior thigh muscles
Obturator Nerve: Innervates adductor muscles
Ventral Rami and Major Branches
Iliohypogastric: L1
Ilioinguinal: L1
Genitofemoral: L2
Lateral Femoral Cutaneous: L2-L3
Obturator: L2-L4
Femoral: L2-L4
Lumbosacral Trunk: L4-L5
Mnemonic: I Twice Got Laid On Friday, Luckily
Muscular Innervation of the Lower Limb
Femoral Nerve
- Anterior thigh
- Innervates: Iliacus, Sartorius, Pectineus, Rectus Femoris, Vastus Lateralis, Vastus Intermedius, Vastus Medialis
Obturator Nerve
- Medial thigh
- Innervates: Pectineus, Obturator Externus, Adductor Brevis, Adductor Longus, Adductor Magnus, Gracilis
Cutaneous Innervation of the Thigh
Lateral Femoral Cutaneous Nerve: Lateral sensory innervation
Obturator Nerve: Medial sensory innervation, upper thigh
Femoral Nerve: Anterior thigh, medial thigh, knee
Sacral Plexus
Arises from spinal nerves L4-S4
Located caudal to the lumbar plexus
Often considered together with the lumbar plexus
- Lumbosacral plexus
Ventral Rami and Major Branches
Superior Gluteal Nerve: L4-S1
Inferior Gluteal Nerve: L5-S2
Posterior Femoral Cutaneous Nerve: S1-S3
Sciatic Nerve: L4-S3
Pudendal Nerve: S2-S4
Nerve to Quadratus Femoris: L4-S1
Nerve to Obturator Internus: L5-S2
Mnemonic: Some Irish Sailors Pester Polly Quite Often
Innervation of the Pelvis
Superior and Inferior Gluteal Nerves
- Innervate gluteal muscles
Superior: Gluteus medius, minimus, and tensory fasciae latae
Inferior: Gluteus maximus
Pudendal Nerve: Innervates perineum
Innervation of the Lower Limb
Sciatic Nerve
- Largest nerve of the sacral plexus
2 Nerves in One Sheath
1. Tibial Nerve
- Innervates posterior lower limb
2. Common Fibular (Peroneal) Nerve
- Innervates anterolateral leg
Tibial Nerve
Passes through popliteal fossa, innervates posterior leg and foot muscles and skin
Divides into…
- Medial Plantar Nerve
- Lateral Plantar Nerve
Common Fibular Nerve
Innervates anterolateral leg
Divides into…
- Superficial Fibular Nerve: Fibularis Longus and Fibularis Brevis
- Deep Fibular Nerve: Tibialis Anterior, Extensor Hallucis Longus, Extensor Digitorum Longus, and Fibularis Tertius
Cutaneous Innervation of the Lower Leg
Common Fibular Nerves: Dorsum of foot and anterolateral leg
Tibial Nerve: Posterior leg and sole of foot
Innervation of the Skin
Dermatome: Area of skin innervated by cutaneous branches of a single spinal nerve
Pain along dermatome indicates nerve root damage
General pattern similar, precise area innervated unique like fingerprints
Autonomic Nervous System (ANS)
System of motor neurons that innervates…
- Smooth muscle
- Cardiac muscle
- Glands
General visceral motor division of the peripheral nervous system (PNS)
Functions of the ANS
Regulates visceral functions…
- Heart rate
- Blood pressure
- Digestion
- Urination
Autonomic vs Somatic Motor Systems
Somatic
- 1 motor neuron from CNS to skeletal muscle
- Axons well myelinated; fast conduction
Autonomic
- 2 motor neurons: preganglionic and postganglionic neuron
- Axons thinly myelinated or unmyelinated; slower conduction
Divisions of the Autonomic Nervous System
Sympathetic Division
- Mobilizes the body during stressful situations
- Activates ‘fight or flight’ response
Parasympathetic Division
- Controls routine, restful functions
- Activates ‘rest and digest’ response
Innervate same structures; produce opposite effects
Sympathetic Division
Activated during extreme situations
Increased heart rate and breathing
Redirected blood flow
Dilated pupils and bronchioles
Suppressed digestion and urination
Parasympathetic Division
Active during rest and recovery
Digestion
Elimination
Maintains heart rate, blood pressure, and respiration at low/normal levels
Sympathetic/Parasympathetic Key Differences
Origin
- S: Thoracolumbar (T1-L2)
- PS: Craniosacral (cranial nerves + S2-S4)
Postganglionic Fibers
- S: Long, reach distant targets
- PS: Short, near, or within target organs
Fiber Branching
- S: Highly branched, systemic response
- PS: Minimal branching, localized effect
Neurotransmitter Release
- S: Norepinephrine (adrenergic)
- PS: Acetylcholine (cholinergic)
Parasympathetic Division
Cranial Outflow
- Origin: Brain
- Targets: Organs of the head, neck, thorax, and abdomen
Sacral Outflow
- Origin: S2-S4 spinal segment
- Targets: Distal digestive tract and pelvic organs
Cranial Outflow (Parasympathetic)
Preganglionic axons run via…
1. Oculomotor Nerve (CN III)
2. Facial Nerve (CN VII)
3. Glossopharyngeal Nerve (CN IX)
4. Vagus Nerve (CN X)
Cell bodies in gray matter of brain stem
Outflow via Oculomotor Nerve (CN III)
Targets
- Iris Sphincter: Constricts pupil
- Ciliary Muscle: Adjusts lens
Outflow via Facial Nerve (CN VII)
Targets
- Lacrimal Gland: Tear secretion
- Submandibular and Sublingual Gland: Saliva secretion
Outflow via Glossopharyngeal Nerve (CN IX)
Target
- Parotid Gland: Saliva secretion
Outflow via Vagus Nerve (CN X)
Postganglionic neurons in walls of target organs
- Thoracic and abdominal visceral organs
- Increased digestion, reduced heart rate and blood pressure
Autonomic Nerve Plexuses
Cardiac Plexus: Regulates heart function
Pulmonary Plexus: Controls bronchial and lung function
Esophageal Plexus: Controls motility in esophagus
Celiac Plexus: Innervates abdominal organs
Superior Mesenteric Plexus: Supplies small intestine and part of large intestine
Sacral Outflow
S2 to S4
Preganglionic cell bodies in spinal gray matter
Axons travel via ventral roots and rami
Form pelvic splanchnic nerves
Run through the inferior hypogastric plexus
Target
- Pelvic organs and lower abdomen
Sympathetic Division
Spans T1 to L2
Preganglionic fibers form the lateral horn of gray matter
Innervates visceral organs, internal cavities, and superficial body regions
More ganglia than parasympathetic division
Sympathetic Trunk Ganglia
On both sides of the vertebral column
Connected by short nerves into sympathetic trunks
Joined by ventral rami by white and gray rami communicants
Fusion of ganglia; fewer than spinal nerves
Pathways to the Body Periphery
Preganglionic Fibers
- Synapse in sympathetic trunk ganglia
Postganglionic Fibers
- Exit via gray rami communicans to peripheral structures
White Rami Communicans
- Carry myelinated preganglionic fibers to the trunk
Sympathetic Targets in Body Periphery
- Sweat glands
- Arrector pili muscles
- Peripheral blood vessels
Pathways of Sympathetic Innervation
- Synapse at the same level
- Synapse at higher or lower level
- Pass through trunk to collateral ganglion
Sympathetic Division - Adrenal Medulla
Major organ in the sympathetic system
- Larger sympathetic ganglia
Secretes norepinephrine and epinephrine (Adrenaline) when stimulated by preganglionic fibers
Visceral Sensory Neurons
Monitor sensations in visceral organs
- Stretch
- Temperature
- Chemical changes
- Irritation
Cell bodies located in dorsal root ganglion
Visceral Reflexes
Reflex arc
Visceral sensory and autonomic neurons
- Defecation reflex
- Micturition reflex
Some involve CNS (spinal reflexes)
Others strictly peripheral reflexes
Central Control of the ANS
Operates involuntarily
Regulated by…
- Brain stem
- Spinal cord
- Hypothalamus
- Amygdaloid body
- Cerebral cortex
Peripheral Neuropathy
Sensory Symptoms (tingling, pain, burning, loss of sensation)
Motor Symptoms (Muscle weakness, paralysis)
Caused by diabetes, trauma, repetitive use, alcohol abuse, or virus (polio, HIV)
Nerve Compression or Irritation
Carpal Tunnel: Compression of the medial nerve at the wrist
Sciatica: Pain along the sciatic nerve due to compression from a herniated disk or bone spur
Radiculopathy: Compression or irritation of a spinal nerve root
Viral Infection
Shingles (Herpes Zoster): Reactivation of chicken pox virus in dorsal root ganglion
Affects sensory nerves
Causes pain and a rash
Inflammatory Conditions
Bell’s Palsy: Inflammation of the facial nerve, temporary paralysis on one side of the face
Guillain-Barré Syndrome: Autoimmune disorder, attacks peripheral nerves
Aging and the PNS
Slower nerve conduction
Decreased sensory function
Muscle weakness due to nerve degeneration
Increased susceptibility to injury
Slower and less coordinated reflexes
Special Senses
Taste, smell, sight, hearing, and balance
Localized to the head region
Confined to one organ with distinct sensory receptors
General Senses: Touch
Classification of Specialized Sensory Receptors
- Chemoreceptors
- Detect chemical stimuli (ie; taste buds, olfactory epithelium) - Photoreceptors
- Detect light stimuli (ie; rods and cones in the retina) - Mechanoreceptors
- Detect mechanical forces (ie; vibration, pressure, and movement) (ie; hair cells of cochlea, vestibular apparatus)
How Special Senses Work
- Detect Stimuli
- Receptors convert stimuli into receptor potentials, initiating an action potential - Pathway to the Brain
- Afferent Pathways: Sensory neurons carry action potentials to the CNS for processing
Receptor Structure and Function
Chemical Senses: Taste and Smell
Taste (Gustation)
- Chemoreceptors detect food molecules in saliva
Smell (Olfaction)
- Chemoreceptors respond to airborne chemicals dissolved in mucous
Taste
Taste receptors are found within taste buds of the oral cavity
- 10,000 taste buds
Most are on the surface of the tongue
- Housed in tongue papillae
Types of Papillae
3 Types with Taste Buds
1. Fungiform Papillae
- Scattered across tongue surface
2. Vallate Papillae
- V-shaped, on posterior tongue
3. Foliate Papillae
- Along lateral tongue
5 Flavours
- Sweet: Sugars and carbohydrates
- Salty: Na+
- Sour: Acidity (H+)
- Bitter: Alkaloids and toxins
- Umami: Savory flavours from glutamate (ie; meat, cheese)
Taste Buds
Collection of 50-100 epithelial cells
2 Major Cell Types
- Gustatory Epithelial Cells: Taste receptor cells, detect tastants
- Basal Epithelial Cells: Stem cells
How Gustation Works
- Stimulus Detection
- Chemicals in saliva activate gustatory receptors - Signal Transduction
- Receptors generate signals via neurotransmitters - Neural Pathway
- Signals travel via cranial nerves to the brainstem
Cranial Nerves in Taste Perception
Vagus Nerve (CN X)
- Pharynx and epiglottis
Glossopharyngeal Nerve (CN IX)
- Posterior one-third of tongue
Facial Nerve (CN VII)
- Anterior two-thirds of tongue
Gustatory Pathway
- Pathway
- Signals travel to the medulla oblongata
- Relayed to the thalamus
- Processed in the gustatory cortex of the insula - Integration
- Gustation combines with smell, texture, and temperature for flavour perception
Gustatory Disorders
Ageusia: Complete loss of taste
- Cause: Nerve damage or severe oral infections
Hypogeusia: Reduced ability to taste certain flavours
- Causes: Nutritional deficiencies, medications, or radiation therapy
Dysgeusia: Distorted or unpleasant taste perception
- Causes: Chemotherapy, zinc deficiency, or infections
Olfactory Epithelium
Specialized tissue responsible for detecting smell
- House olfactory receptors
- Lines roof of the nasal cavity
Role of Mucous in Olfaction
Traps and dissolves odour molecules
Facilitates contact between odour molecules and cilia
Activates sensory neurons and initiates sense of smell
How Olfaction Works
- Odour molecules dissolve in mucous and interact with olfactory receptors on sensory neurons
- Signals transmitted via the olfactory nerve to the olfactory bulb
- Triggers action potentials that travel to the brain via the olfactory tract
Olfactory Tract and Brain Regions
Olfactory Tract: Axons that carry signals to the brain
Limbic System: Emotional and memory-related response
Primary Olfactory Cortex: Perception and identification
Olfactory Disorders
Anosmia: Complete loss of smell
- Causes: Injury, colds, allergies, neurodegenerative disease, or zinc deficiency
Hyposmia: Reduced ability to smell
- Causes: Aging, polyps, sinusitis
Dysosmia: Distorted sense of smell
- Parosmia: Perceiving unpleasant odours
- Phantosmia: Olfactory hallucinations
Uncinate Fits: Psychomotor seizure beginning with olfactory area
- Causes: Epilepsy, head trauma, tumour, infection
Age-Related Changes in Taste and Smell
Ability begins to decline in 40s
Fewer taste buds and olfactory receptors
- Reduced sensitivity
May affect appetite and food enjoyment
Sense of Sight
Gather, focus, and process light into precise images
Dominant sense in humans
70% of sensory receptors are in the eyes
40% of the cerebral cortex is dedicated to processing visual information
Anterior one-sixth of the eye is externally visible
Accessory Structures of the Eye
Eyebrows: Coarse hairs on the superciliary arches
- Shade eyes, redirect sweat
Eyelashes: Hairs along edges of eyelids
- Protect the eye, trap dust and debris
Eyelids
Palpebrae: Separated by the palpebral fissure
Meet at the medial and lateral angles (Canthi)
Lacrimal Caruncle: Red bump at medial angle
Conjunctiva
Transparent mucous membrane
- Palpebral Conjunctive: Lines the inner surface of the eyelids
- Bulbar Conjunctiva: Covers sclera
- Conjunctival Sac: Space between palpebral conjunctiva and bulbar conjunctiva where tears collect
Lacrimal Apparatus
Eye moisture and protection
Lacrimal Gland: Produces lacrimal fluid (tears)
Lacrimal Sac: Collects fluid and drains into the nasolacrimal duct
- Empties into the nasal cavity
6 Extrinsic Eye Muscles
Rectus Muscles
1. Lateral Rectus
2. Medial Rectus
3. Superior Rectus
4. Inferior Rectus
Oblique Muscles
1. Superior Oblique
2. Inferior Oblique
Eye Muscles, Actions, and Nerves
Components of the Eye
External Walls: Three layers or tunics
1. Fibrous layer
2. Vascular layer
3. Nervous layer
Internal Cavity: Contains fluid-filled humors, maintain shape and provide nutrients
Fibrous Layer
Outermost layer, made of connective tissue
Sclera: White, tough, and opaque
- Shapes eye and anchors muscles
Cornea: Clear, dome-shaped, focuses light
Scleral Venous Sinus: Drains aqueous humor
Vascular Layer
Uvea: Intermediate layer of the eye
1. Choroid
2. Ciliary body
3. Iris
Posterior Segment
Lens and ciliary zonules divide the eye
Posterior segment filled with vitreous humor
Transmits light
Supports the lens
Maintains intraocular pressure
Anterior Segment
Filled with aqueous humor
Supplies nutrients to the lens and cornea
Iris
Coloured part of the eye
Smooth muscle, attached to ciliary body
Pupil: Central opening that controls light entry
- Sphincter Pupillae: Contricts pupil
- Dilator Pupillae: Enlarges pupil
Pupillary Light Reflex: Pupil constricts in response to bright light
Retina
- Photoreceptors: Detect light (rods and cones)
- Bipolar Cells: Relay signals
- Ganglion Cells: Transmit signals to the brain
Regional Specializations of the Retina
Macula Lutea: Contains mostly cones
Fovea Centralis: Center of macula, only cones
- Region of highest visual acuity
Optic Disc: Blind spot where the optic nerve exits
Blood Supply of the Retina
Outer Third: Supplied by capillaries in the choroid
Inner Two-Thirds: Supplied by the central artery and vein of the retina
Eye Diagram
Accommodation
Structures in the eye bend light to focus on the retina
- Bent by cornea
- Adjustable lens curvature
Enables focus on nearby objects
Visual Pathway
Most visual information travels to the cerebral cortex
- Responsible for conscious vision
Simplified Pathway
Retina - Optic Nerve - Optic Chiasm - Lateral Geniculate Nucleus (LGN) - Optic Radiations - Visual Cortex
Disorders of the Eye and Vision
Myopia (Nearsightedness): Eye too long or cornea too curved; blurred vision for distant objects
Hyperopia (Farsightedness): Eye too short or cornea too flat, blurred vision for near objects
Astigmatism: Irregularly shaped cornea or lens, blurred or distorted vision
Vision and Aging
Reduced lens elasticity
Difficulty focusing on nearby objects (presbyopia)
Lens clouding
Decreased pupil size
Retinal Changes: Reduced sensitivity and slower adaptation to light
Disorders of the Eye and Vision
Glaucoma: Increased intraocular pressure, damages optic nerve, tunnel vision
Cataracts: Clouded lens, causes blurred vision, sensitivity to light, difficulty seeing at night
Macular Degeneration: Deteriorating macula, darkened central vision, difficulty reading or recognizing faces
Colour Blindness
Defective or absent cones
Genetic, non-progressive
The Ear
Receptor organ for hearing and equilibrium
- External Ear: Hearing
- Middle Ear: Hearing
- Internal Ear: Hearing and equilibrium
External Ear Components
Auricle (Pinna): Directs sound
External Acoustic Meatus: Contains hairs, sebaceous (sebum), and ceruminous glands (wax)
Tympanic Membrane: Boundary between external and middle ear
Features of the Auricle
Helix: Outer rim
Antihelix: Curved ridge inside helix
Concha: Hollow part outside ear canal
Tragus: Flap covering ear canal
Antitragus: Protrusion opposite tragus
Lobule: Fleshy lower part
Middle Ear
Tympanic Cavity: Air-filled space in temporal bone
Medial Wall
- Oval Window: Transmits vibrations
- Round Window: Pressure release valve
Pharyngotympanic (Auditory) Tube: Connects middle ear to the pharynx
Ossicles
Smallest bones in the body
- Malleus: Attached to the eardrum
- Incus: Connects malleus to stapes
- Stapes: Vibrates on oval window
Stabilized by tensor tympani and stapedius
Internal Ear (Labyrinth)
Cavity located in the petrous part of the temporal bone
- Filled with perilymph
Bony labyrinth contains…
1. Semicircular canals
2. Vestibule
3. Cochlea
Membranous Labyrinth
Series of membrane sacs and ducts within the bony labyrinth
- Filled with endolymph
Main Parts
1. Semicircular ducts
2. Utricle and saccule
3. Cochlear duct
Cochlea
Convert sound vibrations into action potentials interpreted as sound
- Spiral chamber in the bony labyrinth
Coils around the modiolus (bone pillar)
Osseous Spiral Lamina: Spiral bone in the modiolus
Cochlear nerve runs through the modiolus
Cochlear Duct (Scala Media)
Contains hearing receptors
Located between two chambers
1. Scala vestibuli (superior)
2. Scala tympani (inferior)
Separated by two membranes
1. Vestibular membrane (roof)
2. Basilar membrane (floor)
Spiral Organ
Receptor epithelium for hearing
Components
- Supporting cells
- Hair cells: Covered in tectorial membrane, connected to the spiral ganglion
- Inner hair cells: Transmit vibrations
- Outer hair cells: Tune and amplify the signal
Hearing Diagram
Vestibule
Central part of the bony labyrinth, medial to the middle ear
Utricle: Detects horizontal movement
Saccule: Detects vertical movement
Both suspended in perilymph
Both houses maculae, sensory epithelium for balance
Macula
Contains hair cells (Receptor cells) for detecting head position
Hair cells synapse with the vestibular nerve
Hair cell tips are embedded in the otolith membrane, which contains otoliths
Monitors head position when still
Semicircular Canals
Detect rotational movements of head (rotational acceleration)
Posterior and lateral to the vestibule
Anterior and Posterior Canals: Vertical, at right angles
Lateral Canal: Horizontal plane
Membranous Ampulla
Found within the bony ampulla
Contains the sense organ; crista ampullaris
Cristae have supporting cells and hair cells (receptors)
Filled with endolymph; movement bends hair cells and action potential sent to the brain
Movement and Endolymph Diagram
Disorders of Equilibrium
Motion Sickness: Nausea and vomiting caused by macular fluctuations during travel
Ménière’s Syndrome: Blockage in the cochlear duct, excess endolymph and ducts balloon
Vertigo: Nausea and vomiting caused by false sense of motion; neuritis or nerve damage
Hearing Disorders
Conduction Deafness: Sound vibrations fail to reach the inner ear
- Ruptured eardrum, otitis media, otosclerosis, or earwax buildup
Sensorineural Deafness: Damage to the auditory pathway
- Hair cells, cochlear nerve, or brain regions
Hearing and Aging
Gradual loss of high-pitched sound perception (presbycusis)
Hair cell damage
Stiffening of ear structures
Increased difficulty understanding speech and localizing sounds
