Special senses

Describe the general function of the outer, middle and inner ear.

• Outer: funneling of sound

• Middle: Relaying tympanic vibrations to inner ear

• Inner: Transduction of vibrations of neuronal signals. Balance organs

Describe the general anatomy of the outer, middle and inner ear.

• Outer: the outer bit (aka auricle) and the auditory canal

• Middle: the tympanic membrane and tympanic cavity (containing the 3 small bones: malleus, incus and stapes which connect eardrum to inner ear)

• Inner: inside temporal bone (locked in position and protected) full of fluids.

1. cochlea (hearing)

2. vestibule and semi-circular canals (balance)

Roles of the bones in the middle ear

Malleus, incus, Stapes

• transduction of vibration

• amplify force of vibration

Explain the function of the tympanum (ear drum).

Focuses sound waves: Sound waves in air directed to tympanic mb (ear drum) and transduced into vibration of tympanum

Tympanum innervation

• Trigeminal nerve (sensory motor)

• Vagus nerve (sensory)

Describe the function of the muscles found in the missile ear.

• Tensor tympani – tensioning of tympanum

• Stapedius – Dampening movement of incus (when activated)

Acoustic reflex triggers tension - Reduction of vibrations/ movements during load noises = protective

Describe the function of the bones found in the missile ear.

• Transduction of vibration.

• amplification force of vibration

The bones are: Malleus, Incus, Stapes

Describe the function of the windows found in the missile ear.

• Oval – transfer of energy to inner ear acts as second amplifier (stapes acts as actuator) inner ear fluid filled

• Round – Relieves pressure waves that develop in the fluid filled cochlea. Membrane flexes ‘in and out’ to relive pressure

Briefly describe the function of the outer ear.

Funnelling of sound

Cochlea

Cochlea: a fluid filled spiral tube

Role: transduce sound waves to nerve impulses

Key structures of cochlea: tectorial membrane, hair cells, basilar membrane, organ or Corti

Explain how the tectorial membrane, hair cells, basilar membrane, organ or Corti and are involved in transduction of sound energy to nerve impulses

• Organ of Corti – supports hair cells

• Hair cells – physical transduction takes place (the sound waves change the relative position of membrane-activating hair cells) 

• Tectorial membrane – gel-like sheet.

• Hair cell tips connect to membrane

• Basilar membrane – supports organ of Corti

• Cochlea nerve – carries electrical signals to the brain

Cochlea

Fluid filled spiral tube that transduces sound waves to nerve impulses

Explain the role of the cochlear nerve and the spiral ganglion.

Cochlear nerve: carries electrical signals to the brain

Spiral Ganglion: contains sensory neuron cell bodies that project axon to join cochlea nerve

Describe how an electrical signal is generated from hearing a sound wave

1. Sound waves come into inner ear and displace 2 membranes (basilar membrane and tectorial membrane) which displaces the hairs

2. hair cells allow K+ to enter cell = depolarisation

3. calcium enters cell = releases neurotransmitter (glutamate) at synapse with neuron

4. Action potentials flow to spiral ganglion

Describe the role of the spiral ganglion

Relays the information through to the cochlear nerve which carries info to brain stem and then auditory processing centres of brain

8.Explain how hair cells of the cochlea are activated and how they can release glutamate to activate auditory nerve fibres.

Hair cells:

A. Deflection of stereocilia – K+ enters cell

B. Voltage-gated Ca++ channels open

Glutamate released at synapse with neuron

Action potentials flow to spiral ganglion

Spiral ganglion:

Contain sensory neuron cell bodies

Projects axon to join cochlea nerve

Cochlea nerve enters brain stem

Target – auditory processing centres of brain

Briefly describe the location of the vestibular apparatus.

Inner ear

Embedded in the temporal bone so that when the head moves, the vestibular apparatus move with it

Outline the general role of the vestibular apparatus.

Balance! = Sensing acceleration/ movement

Explain the function of the semicircular canals and otolith organs.

• Semi-circular canals: angular acceleration

• Otolith organs:

1. utricle: horizontal acceleration

2. saccule: vertical acceleration

Explain that Scarpa’s ganglion and the auditory nerve relay vestibular signals to the brain.

Scarpa’s ganglion contains neuronal cell bodies

bipolar arrangement: carries info from vestibular organs, through vestibular nerve, back to brainstem

Explain how otolith organs function to sense vertical and horizontal acceleration.

• Stereocilia (shortest) move away from kinocilium (tallest) – Hyperpolarisation of hair cell

• Stereocilia (shortest) move towards kinocilium (tallest) – depolarisation of hair cell

• Signals change in acceleration

The gel (gelatinous mas) wants to stay still but the microvilli are embedded inside it. so when you bend your head and the gel remains stationary, the hairs bend.

Describe how the cilia of the semicircular canals can encode angular acceleration.

Angular acceleration = vectorial measurement (direction and magnitude)

At the base of each canal = swelling called ampulla containing a cupula = gelatinous

Flow of endolymph (fluid) deflects cilia

Hair cells signal to brain stem via scarpa’s ganglion

Describe pathway of vestibular information from ear to brain

1. Vestibular organs

2. vestibular nerve

3. arrives in brain stem at the vestibular nuclei

Describe the four main outputs of the vestibular ganglia found in the brain stem.

1.Contribution to control of eye muscle

2.Contribution to control of lower motor neuron limb extensors

3.Contribution to cervical spinal cord - control of head movement

4.‘Balance’ information sent to cerebellum

Explain some effects of ageing on balance.

1.Reduction in hair cells – 40% by age 75

2.Deterioration of  brainstem and cerebellum

3.Decline in oculomotor control – visual instability when moving head

4.‘Balance’ information sent to cerebellum

5.Can cause dizziness, disequilibrium, and an increased risk of falls

Identify and list the layers of the eye.

1. Fibrous: cornea and sclera (shape + structure)

2. Vascular: ciliary body, choroid, iris

3. Inner: retina (photoreceptors of neural layer and pigmented layer, macula, fovea, optic disc)

Explain the three-layer structure of the eye wall

1. Fibrous tunic

Sclera – heavy fibrous white of eye

Cornea – transparent / continuous with sclera

2. Vascular tunic

Choroid – pigmented layer

Ciliary body – ciliary muscle

Iris- colour pigment, aperture of pupil

3. Neural tunic

Retina –  3 layers

Optic Nerve – carries signals to brain

Describe and explain the three layers of the retina.

1. Photoreceptor layer: rods and cones

2. Bipolar cell layer

3. ganglion layer

Rods

Function: specialized for low-light

Sensitivity: extremely sensitive to light

Do not detect colour

Location: Mostly in the peripheral retina.

Role: provide night vision

Cones

Function: specialized for bright-light

Sensitivity: Less sensitive to light than rods

Allow for high acuity

Location: concentrated in centre of the retina(fovea)

Role: enable colour vision

Role of the bipolar cell layer

• process signals from rods and cones

• transmits signals to ganglion layer

Role of ganglion layer

• converts graded potentials from retinal cells into Action Potentials

Optic nerves

visual signals from the retina to chiasm

Understand the role of the optic chiasm

Combines visual signals

Left hemisphere of both eyes combine in left optic tract

Right hemisphere of both eyes combine in right optic tract

Important for binocular vision and depth perception

Describe the visual pathways of the brain.

Carry combined visual information to visual cortex.

Define what proprioception is.

The body’s ability to sense its position, movement and the amount of force it is using in a space

Explain that the visual system provides proprioceptive information.

You can use your visual system for proprioception! to find out where your limbs are.

The proprioceptive system, visual system and vestibular system all play important roles in the nervous coordination

Gustation

the sense of taste/ activity of tasting

• chemical sense

Structure and function of taste buds

Taste receptor

Modified epithelial cells

Replaced 10 – 14 days

Microvilli on apical surface – taste pore

Synapse with nerve cells

Support cells

Physical / metabolic support

Basal cell

Produce new taste receptors

Papillae

The small natural bumps on your tongue which present taste buds:

3 types:

• circumvallate (largest, dark-reddish colour)

• foliate (smaller, paler)

• fungiform (smallest with rich blood supply = red colour. Group of taste buds on the apex)

Different cell types found in taste buds and their functions

Taste receptors = modified epithelial cells (not nerve cells)

Taste buds

Components:

• Taste receptors (the multicoloured cell on the image):

• Support cells

• Basal cells

Taste Receptors

• modified epithelial cells

• replaced 10-14 days

• microvilli on apical surface = taste pore

• synapse with nerve cell

Support cells

physical/ metabolic support

act like glial cells (support, protect, provide nutrients to neurons)

Basal cells

produce new taste receptors every 10-14 days

Describe which cell type is associated with perception of which basic taste

Type 1 cells

• Support cells – glial like

• possibly saltiness

Type 2 cells

• Sweet

• Bitter

• Umami

Type 3 cells

• Sour

• possibly saltiness

Saltiness and tastebuds

• not sure which cells trigger saltiness taste (possibly type 1 or type 3 cells)

• salt = NaCl

• Na+ passes through sodium channels on receptor cells so Na+ influx amplifies signals triggered by other taste molecules

Genetic variants of different taste buds

propylthiouracil tastes different to people depending on genetic variants (some taste it as bitter, others dont) - dont need to remember this!

Taste perception pathway

Cranial nerves carry info from tongue into

1. medulla oblongata (autonomic response like vomit reflex) and hypothalamus

2. amygdala and insula (emotional integration, linked to memory which makes flavour a perception thing)

Flavour

• perception

• Olefaction and gustation = Highly integrated to provide perception of flavour

• integrated in frontal cortex

Olfaction

The activity of smelling (or the ability to smell)

• one of the CHEMICAL senses where the sensed chemicals are odorants

Olfactory bulbs

• base of frontal lobe

• olfactory epithelium that sits high up in nasal cavity, part of brain. pokes through bone and dangles into nasal cavity.

provides direct neuronal connection to olfactory epithelium

Primary olfactory neurons

• project through ethmoid bone

• into olfactory epithelium

Olfactory epithelium

• Receptor cells: olfactory neurones (replaced 30-60 days)

• Support cells: (like glials) physical/ metabolic support, detoxify odorants

• basal cells: create new support cells

• Olfactory cilia: embedded in layer of mucus, odorants dissolve in mucus, then detected by cilia

• odorant binding proteins: sit in mucus, detect odorants and collect them which leads to depolarisations of nerve cell

Olfactory processing

1. olfactory bulb: early processing/ filtering

2. lateral olfactory tract

3. olfactory cortex

4. together they form CNS olfactory system

5. sent to other areas of the brain:

• hippocampus (formation of memories linked to smell)

• limbic system (emotional responses, directs someone behaviour to smell)

• amygdala 9associate learning between smell, emotion, situations, behaviour

• signals bypasses thalamus

Explain the locations of the three different forms of lingual papillae

Explain why food tasted bland if olfaction is compromised

Gustation and olfaction highly integrated to provide perception of flavour – no smell, not much flavour

Key area for integration in frontal cortex

Understand that cranial nerves carry gustatory signals into the brain

Explain the main brain areas involved in gustation and taste perception

Explain the general role and location of the olfactory bulbs

general role: provide direct neuronal connection to olfactory epithelium

location: base of the frontal lobe

Describe the olfactory epithelium and explain the role of each cell type found within it

Receptor cells – olfactory neurons (replaced – 30 to 60 days)

Support cells -

Physical / metabolic support

Detoxify odorants

Basal cells - Create new support cells

Olfactory cilia –

Embedded in mucus

Odorants dissolve into mucus

Odorant binding proteins – ‘collect’ odorant molecules

Receptors bind odorant molecules. depolarisation and AP to brain

Describe the olfactory tract and how it’s main parts function to facilitate olfactory signal processing

Explain the role of the hippocampus in olfactory processing and responses

Formation of memories linked to smell

Explain the role of the limbic system in olfactory processing and responses

Emotional responses to smell

Directing some behavioural responses to smell

Explain the role of the amygdala in olfactory processing and responses

Associative learning between smell, emotion, situations and behaviour

Explain the role of the Thalamus in olfactory processing and responses

Signals /information bypasses thalamus

Label the layers of the retina

1. Sensory layer (pigment epithelium layer): absorbs light detected by rods and cones

2. bipolar layer: transparent - light passes through most of retinal thickness

3. ganglion layer: transparent: light projected from lens

Note: Choroid layer - provides nutrients and removes waste

Rod cells

• Light sensitive discs contain rhodopsin (~1000)

• Rhodopsin responses - same for all wavelengths of light

• So – CAN’T distinguish between colours

• Highly light sensitive

• Blunt rod-like shape

Cone cells

• Light sensitive discs contain iodopsin

• Iodopsin responses – encode  different wavelengths of light

• So – CAN distinguish between colours

• Less light sensitive ~100x

• Pointed tip

• But shorter than rods

• No disks – folds instead

• Allows for rapid replacement of pigment layers

Define the three steps of sensation, collection, transduction, processing

1. Light entering eye is refracted

2. Image reversed and inverted

3. Neural processing corrects this

4. Lens changes shape – focusing light

5. Ciliary muscle control lens

6. Rounder lens – visual field closer

7. Flatter lens – visual field distant

Explain the dorsal and ventral visual streams

Explain the two main retinal specialisation, the fovea and the optic disk

Fovea: 

Most light projected onto fovea

Retinal layers pushed aside

Direct access to photoreceptors

Mainly cones

High acuity and colour sensitive

Optic Disk:

Axons from ganglion cells

Converge on optic disk to create optic nerve which carries signal to brain

Briefly explain the role of the pupil

Controls ‘amount’ of light

Dark – pupil dilates

But this allows ‘stray light’ to degrade focus / depth of field

Describe how accommodation works

The focussing of light- by the lens changing shape

Ciliary muscle control lens

Rounder lens – visual field closer

Flatter lens – visual field distant

Explain why the retinal image is inverted because of refraction and that neural processing corrects for this

Pupil constriction

• controls the amount of light that enters eye

• problematic: scatterable light = not quite as accurate in dark

Retinal specialisations

• optic disc

• fovea

Fovea

• most light is focussed to

• 2 outer layers of retina (ganglion and bipolar layer) are pushed aside to provide better access to photoreceptors

• high acuity and colour sensitive

Lateral geniculate nucleus

• relay center

• there is some facsilitation to focus and concentrate on specific areas of visual field

Outputs from visual cortex

Visual info and proprioceptive info integrated in parietal cortex

1. dorsal visual stream (the “where/’how” pathway): occipital lobe to sensory associated areas. allows us to have spacial awareness in movements and relationships

2. ventral visual stream (the “why” pathway): insula and temporal lobe memory areas (what is this thing that I am looking at, what does it mean to me?), object and facial recognition

3 steps to the broad understanding of our senses

1. collection

2. transduction

3. processing