BMA End of Semester Test

Topics 5-8, Pracs 3-5

Spinal Cord

Communication highway between the brain and nerves

• linking peripheral receptors to the brain

• then to the effector muscles and glands

External Anatomy of the Spinal Cord

Long, thin and delicate tubular structure

• Protected by the vertebral column, passes through the vertebral foramen

• ends at L1-2

Regions of the Spinal Cord

1. Cervical (C1-8)

2. Thoracic (T1-12)

3. Lumbar (L1-5)

4. Sacral (S1-5)

5. Coccygeal (1)

→ Spinal cord ends at L1-2, therefore, L1-5, S1-5 and 1 coccygeal lie between T12-L2 of the VERTEBRAL COLUMN

• not located in their corresponding regions, but will extend out to them

Spinal Nerves

Pairs of spinal nerves → axons

• Dorsal (back) = sensory

  • sensory neurons = UNIPOLAR

• Ventral (front) = motor

  • autonomic and somatic motor neurons

→ Mixed nerves (sensory + motor)

Protection of Spinal Cord

Mechanical forces, protected by:

• Vertebral column

• CSF

• Spinal Meninges

Parts of the Spinal Cord

Caude Equina: Collection of nerve Roots (L1-5, S1-5, 1 coccygeal)

• That extend beyond the conus medullaris (End structure of spinal cord)

• emerge out of corresponding vertebral column

Filum Terminale: extends from the conus medullaris → coccyx vertebral column

• Anchors the spinal cord in place

  • so that spinal cord isn’t affected by mechanical forces

Internal Anatomy of Spinal Cord

Neural tissue divided into left and right: separated by 2 grooves

• Anterior (front) median fissure

  • *THINK → front = deeper into body, = deeper groove

• Posterior (back) median sulcus

Grey matter surrounded by White matter

Grey Commissure

Axons cross from one side to the other

• Explains why right brain = left movement, vice versa

Central Canal = contains CSF, nourishing neural tissue

Subdivided into gray horns: → Processing

• Anterior

  • Somatic motor function

  • cell bodies of lower motor neurons

• Lateral → only thoracic and lumbar

  • Autonomic motor function

  • cell bodies of preganglionic neurons

• Posterior = sensory input

  • cell bodies of spinal interneurons

    

White Commissure

“Conduction highway”

→ Myelinated and unmyelinated axons

→ Anterior white commissure = axons cross from one side to the other

White columns, subdivided into:

• Anterior

• Posterior

• Lateral

→ Ascending and descending spinal cord tracts (axons)

Ascending Spinal Cord Tracts

• Sensory input → BRAIN

Posterior White Columns:

• Fasciculus Gracillus

• Fasciculus Cuneatus

  • Fine touch, light pressure, vibration, conscious proprioception

Lateral White Columns:

• Lateral Spinothalamic

  • Pain and temperature

• Anterior + Posterior Spinocerebellar

  • Unconscious proprioception

Anterior White Columns:

• Anterior Spinothalamic

  • crude touch, deeep pressure

    • *THINK → more INSIDE = deep

Fine Touch vs Crude Touch

• Fine touch = discriminative -> sense and identify location of touch stimulus on the skin

  • Details like texture, size and shape

• Crude touch = non-discriminative -> cannot precisely locate the stimulus

  • No specific details, e.g. when someone bumps into you

Descending Spinal Cord Tracts

Lateral White Columns

• Lateral Corticospinal

  • Somatic motor output - skeletal muscles of LIMBS

Anterior White Columns

• Anterior Corticospinal

  • Somatic motor output - skeletal muscles of AXIAL

Somatosensory Neural Pathways

Connect receptors to brain

• Conduct general sensory input

Relay of neurons:

• First-order - sensory input from receptors → posterior gray horns

  • synapses with second-order, within sensory nucleus

• Second-order - sends sensory input to designated area of brain

  • ascends through spinal cord tracts

  • synapses with third-order, within region of brain

• Third-order

  • sensory input to designated cortex of the area of the brain

3 Main Somatosensory Pathways

1. Spinothalamic (crude touch, deep pressure - anterior)

   (pain and temperature - lateral)

   • First order - sensory input from tactile, nociceptors and thermoreceptors into posterior gray horn

   • Second order - sensory input to thalamus

     • through anterior/lateral

   • Third order - sensory input from thalamus to PSC

2. Spinocerebellar (unconscious proprioception)

   Maintains posture, balance, standing sitting and walking, ensure smooth coordinated movements

   • First order - sensory input from proprioceptors to posterior gray horn

   • Second order - sensory input to cerebellum

     NO THIRD ORDER

3. Fasciculus G/C (fine touch, light pressure, vibration and proprioception)

   • First order - sensory input from proprioceptors and tactile into posterior gray horn

     • Synapses w second in medulla oblongata

   • Second order - sensory input to thalamus

   • Third order - sensory input to PSC

Somatic Motor Pathways

Conducting somatic motor output from PMC to skeletal muscles

Relay of 2 neurons:

1. lower motor

2. upper motor

Main Somatic Motor Pathways

1. Lateral corticospinal

   • Upper motor neurons: somatic motor output from PMC to anterior gray horn

     • Descending spinal cord in tract

   • Lower motor neurons: Somatic motor output to skeletal muscles of limbs

2. Anterior corticospinal

   • Upper motor neurons: somatic motor output from PMC to anterior gray horn

     • Descending spinal cord in tract

   • Lower motor neurons: Somatic motor output to axial muscles

Spinal Reflex Arcs

• Neural pathways that produce a rapid, predictable, involuntary somatic/autonomic reflex response to stimulus

• 5 components

  • Receptors

  • Sensory neurons

  • Integration centre

  • Lower motor neurons

  • Skeletal Muscles

Somatic Reflexes

• Mediated by reflex integration centres (in spinal cord)

• Stimulates skeletal muscle contractions

• Lower motor neurons (LMN)

• Motor output to skeletal muscles → can be modified by the PMC

  • Can be modified by the PMC (upper motor neurons)

• Can diagnose disorders of the Nervous System

Patellar Reflex - Knee Jerk

• Normal response = extension of the knee

• Damage to spinal segments L2-4 OR Lower Motor Neurons → loss of reflex

• Damage to Primary Motor Cortex or Corticospinal Tracts → hyperreflexia

Plantar Reflex

• Normal response = flexion of toes

• Damage to spinal segments L4-S2 or LMN = loss of reflex

• Damage to PMC or corticospinal tracts = Bakinski sign

Spinal Cord Injuries

• Sensory and/or motor losses → at or below level of injury

Damage to posterior gray horns (ascending) = loss of sensation

Damage to anterior gray horns (descending) = spastic or flaccid paralysis

Spastic Paralysis

Upper Motor neurons are damaged

• Lateral White Columns affected

• Loss of voluntary movement

• Reflex activity intact → abnormal (hyper reflexia)

Flaccid Paralysis

Lower Motor neurons are damaged

• Loss of voluntary movement and reflex activity

Complete Transaction of Spinal Cord

• Loss of all sensations and voluntary movement

• Both sides of body are affected

• Transection in cervical region = quadriplegia (electric wheelchair)

• Transection in thoracic or lumbar regions = paraplegia (leg muscles)

• Reflex activity below level of Transection = present but abnormal

Spinal shock: transient period of complete sensory and motor loss

• loss of all reflexes below level of lesion

  • reflex activity will slowly restore

Autonomic Nervous System

Dual innervation

• Most effector organs are innervated by both sympathetic and parasympathetic divisions

• ANS divisions can work independently or together

  • Opposing effects, counterbalance

Sympathetic Nervous System

Preparing the body for heightened activity

• Exercises, excitement, emergency, embarrassment (fight or flight)

Parasympathetic Nervous System

Stimulating abdominal visceral activity

• Conserving energy, rest and digest

• Blood vessels, sweat glands, adrenal and spleen do not have parasympathetic function

Somatic vs Autonomic NS: General

Similarities

• Both are efferent divisions

• CNS to effectors

Differences

• Neural pathways

• Effectors - target organs

• Organ responses

• Neurotransmitters

Somatic vs Autonomic NS: Pathways

Somatic

• Corticospinal pathway

• Upper motor neurons cell bodies are in PMC

• Travel down the spinal cord to LMN in anterior grey horn

• Ventral root of spinal nerve to skeletal muscles

Autonomic

• Peripheral pathways

• Visceral motor nuclei in the hypothalamus (cerebral cortex ~ limbic system)

• Preganglionic neurons → synapse with postganglionic neurons and innervate target organs

  • Smooth muscles, glands, cardiac muscles

Peripheral Motor Neurons

Somatic

• Targets skeletal muscle fibres

• Lower motor neuron cell body

  • spinal cord motor nucleus (anterior gray horn)

  • axon exists through ventral root

  • Brain stem nucleus (Cranial Nerve)

Autonomic

• Targets smooth muscles, glands, cardiac muscles

• 2 different peripheral motor neurons

  • Preganglionic

    • cell bodies in brain stem nucleus, axon exits through CNs

      OR motor nucleus → cell body in lateral gray horns, exits the ventral root

  • Postganglionic

    • found in peripheral ganglion

Neurotransmitters

Transmits nervous signal across a synapse OR between neuron and target

Somatic NS

• NT is ALWAYS ACH → acetylcholine

  • binding receptors on targets, causing greater potential = action potential

  • Excitatory NTs, increasing target activity

Autonomic NS

• Synapsing between pre- and postganglionic neurons, NTs is always ACH

• Cross the synapse, binding receptors of the cell body of postganglionic

• Depolarises to (- to +) to neuron, causing graded potential and leading to an action potential

  • Signals to be transmitted across synapse + reach target

• The synapse between postganglionic and target organs

  • Acetylcholine

  • Noradrenalin released by postganglionic neuron

    • Depending on type of receptor that target has, NA:

      • Excitatory → increasing target activity

      • Inhibitory → reducing target activity

Sympathetic vs Parasympathetic NS

• Similarities:

  • Serving many of the same organs

  • Pre- and postganglionic neurons

  • Preganglionic release Acetylcholine

  • Presence of ganglia (cell bodies in PNS)

• Differences:

  • Both divisions don’t innervate ALL organs

  • Location of preganglionic cell bodies + axons exit points

  • Length of axons, location of ganglia

Parasympathetic NS - Craniosacral Division

• Preganglionic neurons arise from opposite ends of the CNS

• Preganglionic: Long Axons → postganglionic

• Postganglionic: Short axons → target organs

• Brain stem (cranial nerves)

  • Cranial pathways

  • CN III: Oculomotor nerve

  • CN VII: Facial nerve

  • CN IX: Glossopharyngeal nerve

  • CN X: Vagus nerve (organs)

 

• Sacral region (S2-S4), (spinal nerves) -> lateral grey matter

  • Pelvic organs

  • Distal portion of large intestine

Sympathetic NS - Thoracolumbar Division

• Preganglionic fibres arise from T1-L2

  • Cell bodies in motor nuclei of lateral grey horns, axons exit through spinal nerves

• Ganglia is close to spinal cord

• Pre- axon: short

• Post- axon: long

• Chain of 23 connected by fibres,

  to form sympathetic trunk

• Located on both sides of vertebral column

• Extend from neck to pelvis

 

• Segments T1-T6 innervate the head and thorax

• Segments T5-L2 innervate abdominopelvic organs

Adrenal Medulla

• Adrenal glands are located on top of the kidneys

Medulla - inner region

• Release Adrenaline and Noradrenaline into blood

• Axons innervates adrenal medulla

  • release ACH when an action potential reaches axon terminal

• Stimulates medullary cells to release adrenaline and noradrenaline

  • Affecting any cell bearing adrenergic receptors

  • Enhance and prolong sympathetic response → widespread

  • Single preganglionic neuron can synapse with many postganglionic, to active multiple effectors

CHOLINERGIC RECEPTORS (NTs)

Acetylcholine (ACh)

• Cholinergic receptors

  • Nicotinic : EXCITATORY

    • increasing target activity

    • ALL ANS postganglionic neurons

    • Skeletal muscles

    • Sympathetic Division

  • Muscarinic : EXCITATORY OR INHIBITORY

    • excitation of most targets (Increasing activity)

    • Inhibition of cardiac muscles (decreasing activity)

ADRENERGIC RECEPTORS (NTs)

Alpha (1-2)

• A1: blood vessels (except heart, liver, skeletal muscles) - all sympathetic targets

  • Excitatory and increasing activity

    • constricting blood vessels and visceral organs, dilating pupils

Beta (1-3)

• B1: Heart

  • Excitatory, increases heart rate and force of contraction

• B2: Airways, blood vessels (heart, liver, skeletal muscles), digestive tract, urinary organs

  • Inhibitory, decreasing activity → relaxing smooth muscles

  • Dilates airways, blood vessels, relaxes muscles in digestive and urinary organs

Receptors → Symp + Parasym

• Preganglionic releases ACh = cholinergic receptors

• ACh binds to cell bodies of postganglionic neurons = cholinergic nicotinic receptors

• Postganglionic

  • Sympathetic = Noradrenaline

    • Adrenergic receptors, Alpha or Beta

  • Parasympathetic = ACh

    • Cholinergic receptors, Muscarinic

Effects of NTs on Targets

• Synaptic Transmission

  1. Action potential arrives at axon terminal

  2. Voltage-gated Calcium 2+ channels open, Ca2+ ions enters

  3. Ca+ entering, triggering the vesicles (filled with NT) to fuse with the plasma membrane of the terminal, releasing NTs into synaptic cleft

  4. NT diffuses across the cell to bind receptors (chemically gated ion channels) on the postsynaptic membrane

  5. Binding of the NT open the ion channels, graded potential, threshold = action potential

  6. NT effects end when diffuses away, through enzymatic degradation or taken back up by axon terminal to be stored and reused

Postsynaptic Potential

AKA graded potential

EPSP - Excitatory Postsynaptic Potential

1. Neurotransmitters bind receptors on the postsynaptic membrane

2. Opens gated ion channels

3. Graded depolarisation (Na+ ions entering the cell, - to +)

4. Postsynaptic cells are EXCITED!!, membrane potential closer to threshold

5. Action potential is more likely to occur

6. = Increased target activity

IPSP - Inhibitory Postsynaptic Potential

1. Neurotransmitters bind receptors on the postsynaptic membrane

2. Open gated ion channels

3. Graded hyperpolarisation (K+ exit the cell, - to --)

4. Postsynaptic cell are INHIBITED, membrane potential further from threshold

5. Action potential is less likely to occur

6. = Decreased target activity

 

Drugs with Target Neurotransmitter Receptors

• Agonists - mimicking the action of the NT

• Antagonists - bind to receptor and block the action of NT

  • Affect the sympathetic targets

• Adrenergic

  • B1 receptors -> Cardiac muscle

    • Normal response: increasing cardiac activity = increasing BP

    • Typical agonist drug (increasing) = dobutamine

    • Type antagonist drug (blocking) = beta blockers -> propranolol or timolol

  • B2 receptors -> smooth muscles in airways, blood vessels

    • Normal response: dilation of airways, blood vessels to increase airflow + blood flow

    • Typical agonist drug = Albuterol (Asthma puffers)

    • Typical antagonist drug = Beta blockers -> propranolol

Central NTs and Major Effects

(in the CNS)

• Induce excitatory or inhibitory responses in the postsynaptic membranes -> 50 neurotransmitters identified

• Neuropsychological disorders are traditionally thought to be caused by imbalances in neurotransmitters, but now questionable

Acetylcholine (ACh)

• Motor control outside CNS

• Released by:

  • Somatic lower motor neurons

  • Parasympathetic postganglionic neurons

Within the CNS

• cerebral cortex - cognition

• hippocampus - memory

• brain stem - consciousness

  Low levels of ACh found in Alzheimer's patients

Biogenic Amines

• broadly distrbuted in the brain, emotional behaviours

• Regulate our biological clocks

• Imbalances associated with mental illness (no consistent evidence)

  • However, drugs that enhance the release of these neurotransmitters or block the uptake are associated with relief of symptoms -> but we don’t know how they work :(

 

Noradrenaline

• (feel good)

  • Stimulates brain reward and pleasure centres

  • Involved in reducing stress, enhancing attention

    • Amphetamines: stimulant for CNS

      • Can treat ADHD, help to improve cognitive control

Dopamine

• Stimulating brain reward, pleasure centres

• High levels noted with schizophrenia

• Deficient in Parkinson's disease

Serotonin

• Mood regulation, sleep, appetite, nausea, migraines/headaches

GABA

• Main inhibitory neurotransmitter in brain

• Regulate sleep and wakefulness

• Involve in motor control -> regulate skeletal muscle tone, by dampening activity

• GABA blockers: convulsions, rapid repeated muscle contractions

• Enhancing GABA (alcohol intake) = increase in dampening effect, impaired motor coordination

Glutamate

• Excitatory neurotransmitter

• Involved in learning and memory

• Excessive release = excitotoxicity, overexcited and kill surrounding neurons

  • CVA, Alzheimer's disease

Substance P

• Produced by damaged tissue

• Stimulates peripheral nociceptors (pain)

• Mediates pain transmission to the CNS

Endorphins

• Includes endorphins and enkephalins

• Inhibit perception of pain in CNS

• Natural opiates, chemically similar to morphine (agonist), reduces pain, induce sleepiness, wellbeing

Special Senses

Smell, taste, see, hear and maintain balance

• Specific information to specialised receptors

Stimuli detected by special senses:

• Chemical molecules → taste and smell, chemoreceptors

• Movement of fluids → hearing and balance, mechanoreceptors

• Light (vision) → photoreceptors

Physiology + Neural Pathway of Olfaction

Smell → olfactory sensory neurons

• located in the olfactory epithelium

• hav cilia, surrounded by mucus

• Odourant must be gaseous / volatile

• Must reach olfactory epithelium, dissolved in mucus

• odourant binds and opens gated channels → generating action potential

Olfactory Pathway

Olfactory sensory neurons (CN 1)

• Action potentials travel to olfactory cortex of temporal lobe

• 2 different pathways

  • Frontal lobe

  • Hypothalamus + limbic system: emotional response to odour elicited

Role of Nose and Smell

Nociceptors

Thermoreceptors

→ smells can also signal danger: smoke/gas

• Triggering fight or flight response

• Protection by triggering sneezing and choking → in response to unpleasant odourants

Physiology + Neural Pathway of Gustation

Taste

Gustatory epithelial cells in taste buds

→ microvilli (gustatory hairs)

Food chemicals dissolved in saliva (chemoreceptors)

Gustatory Pathway

Cranial Nerves:

• VII → Facial

• IX → Glossopharyngeal

• X → Vagus

→ all carry action potentials from taste buds to:

• gustatory cortex in the insula

• hypothalamus nd limbic system (appreciation of taste)

Role of Mouth and Taste

• Taste sensations

• Nociceptors

• Thermoreceptors

• Triggering reflexes involved in digestion

  • secretion of saliva, gastric juice

• Protective reactions

  • gagging, reflexive vomiting

• 5 tastes -> homeostatic value

  • Influence food choices that help satisfy our body's needs

  • Control quality of food that will be ingested

  • Sweet = energy, carbohydrates

  • Bitter = warning (vomiting)

  • Sour = unripen, or spoilt food

  • Salty = mineral and water balance

  • Umami = protein rich foods

Influence of Smell on Taste

Combined effect when interpreted in cerebral cortex → 80% of taste is smell

Anatomy of the Ear

External → hearing

Middle → hearing
Internal → hearing and balance

External Ear: Structures and Functions

Auricle / Pinna 👂

• Funnels sound waves into the external acoustic meatus

External Auditory Meatus

• Sound waves enter EAM, travel to vibrate the tympanic membrane

Tympanic Membrane (Eardrum)

• Thin, translucent membrane

• Vibrates in response to sound waves, transfers sound energy to bones of middle ear

  • Mechanical energy to auditory ossicles

Middle Ear: Structures and Functions

Tympanic Cavity

• air-filled cavity in temporal bone

• Tympanic membrane is lateral to CAVITY

• Bony wall is MEDIAL to cavity

  • i.e. cavity lies in between both

Bony wall = oval and round window

Auditory ossicles = malleus, incus, stapes

Pharyngotympanic tube: connects middle ear to nasopharynx

• Equalises air pressure in middle ear cavity

→ open to nose and throat, microorganisms can travel and cause infections

• *middle ear infections can cause tympanic membrane to be inflamed and red

• more common in children

Auditory Ossicles (mid ear)

Malleus

Incus

Stapes

→ transmit and amplify the vibratory motions of tympanic membrane to the oval window

Bony Wall: Oval and Round Window (mid ear)

Oval

• Opening in the bone, partitions middle ear to start of cochlea

• Movement causes pressure waves within perilymph

Round

• thin membrane that separates end of cochlea from middle ear

• absorbs pressure waves

Inner Ear: Structures and Functions

Temporal Bone

2 divisions of internal ear

1. Bony Labyrinth

   1. Vestibule

   2. Semicircular canals

   3. Cochlea

   • Filled with perilymph

2. Membranous Labyrinth

   1. Series of interconnecting sacs and ducts

   2. Within the bony labyrinth

   • Filled with endolymph

Cochlea (inner ear)

Spiral, bony chamber, extends from vestibule

→ contains the cochlear duct, housing the spiral organ of corti

Cochlear Nerve → Cranial Nerve VIII: Vestibulocochlear

→ contains 3 chambers:

• 2 perilymph, 1 endolymph

→ Membranous Labyrinth → cochlear duct (endolymph)

• basilar membrane = floor of duct

  • Spiral Organ: on top of basilar membrane, with cochlear duct

Spiral Organ (within Membranous Labyrinth)

Detects sound, organ of hearing

• Composed of cochlear/cilia hair cells

• Embedded in tectorial membrane

  • mechanoreceptors, auditory receptors

• Pressure waves travel through perilymph of cochlea = movement of basilar membrane

  • this causes the hair cells to move or bend against tectorial membrane

  • = initiating a depolarisation = generating action potentials

    • sent to CN VIII: Vestibulocochlear nerve for interpretation

Cochlea + Stapes

Embedded in oval window, wall of bony labyrinth

• stapes moves back and forth in response to mechanical movement

  • sound waves hit tympanic membrane

• When moving back and forth, pressure waves are created within perilymph (in vestibule)

Route of Sound Waves Through Ear

1. Sound waves vibrate the tympanic membrane (funnelled through by pinna and EAM)

2. Auditory ossicles vibrate oval window + pressure is amplified

3. Pressure waves created by stapes pushing on oval window move through perilymph

4. Pressure waves of perilymph vibrate basilar membrane

5. Receptor hair cells embedded in tectorial membrane get moved/bent

6. Creating action potentials in vestibulocochlear nerve

Auditory Pathway to Brain

Bending of hair cells in Spiral Organ

• opens sodium ion channels, sodium rushes in = generating action potentials in auditory neurons

Axons of auditory neurons for the cochlear branch of CN VIII

• Action potential travels to:

• Medulla oblongata → auditory reflex centre of midbrain

• To the auditory cortex (temporal lobe) → stimulus interpreted as sound

Balance / Equilibrium

Response felt by head movements

• Role of equilibrium: providing information about location and movement of head

Equilibrium depends on inputs from external ear

• As well as: Vision and Information from proprioceptors of muscles nd tendons

Mechanoreceptors for equilibrium are found in the inner ear, specifically:

• Vestibule

• Semicircular Canals

Static vs Dynamic Equilibrium

Static

Position of head, relative to gravity + forwards/backwards

• motionless, moving in a straight line

→ when head positions move, gravity pulls on stones, distorting jelly

→ Bending the hair cells in the vestibule

• Forward = depolarisation

• Backward = hyperpolarisation

Dynamic

Evaluating position of head while it is rotating, or moving in an angular direction

→ detected by hair cells in the semi circular canals

Vestibule

2 membranous sacs, containing equilibrium receptors

→ embedded in jelly-like substance

→ Ear stones (called otoliths) embedded into the jelly

• Saccules: continuous with the cochlear duct = static

• Utricles: continuous with the semicircular canals = dynamic

Semicircular Canals

3 fluid filled canals

• Equilibrium hair cells respond to the rotational movement of head

• Hair cells are surrounded by endolymph

• Angular movement moves the endolymph, bending and distorts hair cells

Equilibrium Pathway

1. Distortion of the hair cells stimulates depolarisation of vestibular division (CN VIII)

2. Action potentials travel to the:

   1. Reflex centres of brainstem (vestibular nuclei)

   2. Cerebellum

   3. Vestibular cortex of insula

Maintaining Balance

• Hair cells distorted in response to stimulus

• Linear movement (vestibule) and rotational movement (semicircular canal)

• Stimulate depolarisation of the vestibular division (CN VIII) + cochlea division -> sent to brain

• Action potentials travel to vestibular nuclei (brain stem)

  • Integrative centre for balance

• Sends commands to brain stem motor centres

  • Direct eye movement and reflex actions of the neck, limbs and trunk

    • Adjust body positions to maintain/regain balance

• Sends information to the cerebellum

  • Comparing where the body is now to where it should actually be

• Reaches vestibular cortex of the insula, becoming consciously aware of our balance (where position is in space)

Vision

Dominant sense

→ 70% of our sensory receptors are found in eye

→ half of the cerebral cortex is involved in some aspect of visual processing

Movement of the Eyeball

• 6 eye muscles

• Innervated by Cranial Nerves:

  • III - Oculomotor

  • IV - Trochlear

  • VI - Abducens

• Motor movement of eyeballs**

• Enables eyes to be fixed on objects

Structure of the Eyeball

Cushion of fat and walls of bony orbit

Cornea

• most exposed, transparent layer of the eye

• Forms window to let light in, no blood

• Transplanted between individuals

• Well-supplied with nerve endings → nociceptors

• Reflexive blinking when touched

Iris

• Coloured part of the eye

• Adjustable diaphragm, controls the diameter of round central opening (pupil)

• Contains muscles that constrict and dilate

• Controlled by Sympathetic/Parasympathetic

Lens

• Transparent and flexible, changing shape to focus light on the retina

• Suspensory ligaments hold in place

• Round and thickened

Pupil

• Round and central

+ Ciliary Muscles and Zonules

Ciliary Muscles - Distant and Close Vision

Suspensory ligaments (zonules) - extend from ciliary muscle to lens to hold in place

Round and thick lens = ciliary muscles contract, suspensory ligaments loosen

• Close vision, parasympathetic NS

• 6 metres>, reflect lights on divergent paths

• Requires refraction to direct onto retina

• Accommodation = lens gets fatter

• Convergence of eyeballs required

• Constriction of pupil, decreasing diameter to prevent the MOST divergent rays from entering

Flattened lens = ciliary muscles relax, suspensory ligaments tighten

• Distant vision, sympathetic NS

• 6 metres<, light rays that are parallel to each other

• Light rays will focus on retina, lens flattened

Internal Chambers and Fluids of Eyeball

2 Chambers:

Anterior (between cornea and lens)

• Aqueous humour → nutrients and oxygen to lens and cornea

Posterior (behind the lens)

• Vitreous humour: gelatinous substance, supporting posterior surface of eye → retina

Layers of the Eyeball

Fibrous (outer layer)

• Sclera

  • White, tough connective tissue

  • Continuous with dura mater of brain

• Cornea

  • Transparent region of front of eye

Vascular

• Choroid

  • Vascularised area

  • Blood vessels nourish all layers

  • Brown pigment produced by melanocytes

    • absorbs light, prevents from scattering and reflecting inside eye

• Ciliary Body

  • forms ciliary muscle rings around lens

• Iris

  • Adjustable muscle layer

Retina (inner)

• Nervous layer

• Contains photoreceptors - light detecting receptor cells

  • Rods → dim light, peripheral vision

  • Cones → bright light, colour and visual activity

• Fovea centralis → visual activity

  • Most cones, enhanced visual activity and detailed colour

• Optic disc → blind spot

  • Optic nerve exits eye

  • Lacks photoreceptors

Light on the Retina

Refraction

• Bending of light through different mediums → cornea, aqueous humour, lens, vitreous humour

• Light travels in a straight line, until hitting an object or medium

  • Light reflects off of objects

  • Light gets refracted/bent through mediums

Pathway of Light into Eye

1. Cornea

2. Aqueous Humour

3. Lens

4. Vitreous Humour

→ combined effect of all refractions = focus on the image in retina

• refraction is constant for all, EXCEPT the lens

  • Lens is highly elastic, can change shape to focus

  • More convex and fat = bigger refraction

Converting light to Action Potentials

• Focused on retina (photoreceptors)

• Rods contain Rhodopsin, which is sensitive to light

  • Light promotes breakdown of Rhodopsin = cell membrane depolarises = action potential

• Cones are similar, with a different type of Rhodopsin

Pathway of signals from Eye to Brain

Light Into the Eye

• Cornea → Aqueous Humour → Lens → Vitreous Humour → Retina

  • Photoreceptors of retina create action potential, travelling through Cranial Nerve II - Optic Nerve

Light through the Brain

• Optic nerve → Optic chiasma → Optic Tract → Thalamus

  • → Optic Radiations to Primary Visual Cortex in the Occipital Lobe

Binocular Vision and Depth Perception

Field of vision overlaps

• 2 optic nerves meet at the Optic Chiasma (overlapping point)

• Medial fibres cross to the other side

• Both hemispheres of brain receive visual images (from both eyes)

• Visual cortex fuses the 2 images, providing depth perception