Week 4 - Special Senses

Question 1

Describe the anatomy of the ear:

Diagram 14

Answer 1

1. External ear
   a) Involves the collection of sound and direction of sound
   b) Involves the pinna and meatus
   c) The pinna helps with sound collection - a larger pinna increases its efficiency
   d) Ridges in pinna reflect different frequencies differently depending on elevation
   e) Differences in loudness between left and right ears help find horizontal direction for high frequencies but not good at finding direction of low frequencies
2. Middle ear (tympanic cavity)
   a) Consists of ossicles and ear drum (tympanic membrane)
   b) Involves amplification of sound and protection
3. Inner ear
   a) Consists of the oval window, auditory-vestibular nerve, and cochlea
   b) Involved in computation and transduction

Question 2

Describe the middle ear:

Answer 2

1. Consists of the tympanic membrane and ossicles
   a) Ossicles include malleus, incus, and stapes
2. Limitation: sound has to go from air into fluid-filled cochlea
   a) It is very difficult for this to happen - 99.9% of sound energy is reflected when travelling from air to water
3. Middle ear makes up for this loss of sound via area difference and lever action - diagram 15
   a) Area difference: Ear drum is larger than stapes - difference in area produces an amplification of pressure
   b) Lever action: Length of malleus is longer than length of pivot point on other sides - difference in length on malleus side amplifies the sound
4. Second function of middle ear is protection to inner ear from very loud sounds
   a) Muscles stapedius and tensor tympani attach to small inner ear bones and when they tense up they prevent large vibrations from being passed through
   b) These muscles are reflex activated - so there is a 10 ms delay
   c) Delay means that no protection will be provided for very quick sounds such as gun shot and these muscles can fatigue and become less effective
   d) Muscles are also ineffective at frequencies >2 kHz

Question 3

Describe the inner ear:

Answer 3

1. The basilar membrane is narrow at the stapedal end and wide at the apical end
   a) Stapedal end responds to high frequencies whilst apical end responds to low frequencies
2. The basilar membrane separates and sorts out different frequencies along the basilar membrane
   a) Fourier transformation - mechanical computation
3. Inner hair cells within organ of Corti perform a transduction
   a) Hair cells are arranged into tectorial membrane
   b) Mechanical deformation of inner hair cell cilia depolarises hair cell membrane to produce an action potential further downstream

Question 4

Describe the events involved in sound processing:

Answer 4

1. Outer ear takes in sound through the pinna
2. Middle ear pressurises the vibrations
   a) Vibrations are quicker at higher frequencies
3. Gets to basilar membrane - responds to different frequencies
4. Causes transduction - mechanical deformation of inner hair cell cilia depolarises hair cell membrane causing action potential along the fibres of the 8th cranial nerve at the cochlea
   a) Each fibre carries information on a particular frequency
5. Signal travels along auditory nerve fibre and crosses over at the medulla and then into the auditory cortex
6. The primary auditory cortex is tonotopically mapped
   a) Means different frequencies cause different stimulations along the fibres

Question 5

Describe the vestibular system:

Answer 5

1. The system that provides sense of balance and information about body position
2. There are 2 main types of vestibular sensors:
   a) Semi-circular canals
   b) Otolith organs

Question 6

Describe the anatomy and role of the otoliths:

Answer 6

1. Anatomy: Diagram 16 + 17
   a) Two subtypes of otoliths; Utricles and saccules
2. Role:
   a) If head tilts then otoliths pull back on gelatinous cap which puts a bending force on cilia (hair cells) - this causes depolarisation and if threshold is reached it leads to action potential along vestibular nerve axons
   b) If head accelerates forward, the otoliths get left behind because of the inertia which also causes a bending force on cilia and lead to depolarisation
   c) Utricles (horizontal movement) and saccules (vertical movement) are oriented at different planes and so are stimulated by different directions = hence, how we can tell the difference between acceleration and tilting

Question 7

Describe the anatomy and role of the semi-circular canals:

Answer 7

1. Anatomy: Diagram 17
   a) Three canals oriented about 90 degrees to each other (orthogonal)
2. Role:
   a) When head is rotated side to side the endolymph (fluid) gets left behind due to inertia which causes the cilia to bend

Question 8

Describe the role of the vestibular-ocular reflex:

Answer 8

1. Signal from semi-circular canal to the extraocular muscles of the eye allow for steady gaze

Question 9

Describe the anatomy of the eye: Diagram 18

Answer 9

1. Accessory structures of the eye
2. Cornea
3. Ciliary muscles
4. Retina
5. Optic nerve

Question 10

What are some control systems in the eye for protection, intensity, focus, and direction?

Answer 10

1. Protection:
   a) Blink reflex
   b) Lacrimation (tears)
2. Intensity
   a) Pupillary control - controls light
3. Focus:
   a) Accommodation control - muscles which change shape of eye lenses to accommodate focus on objects whether they are far or close
4. Direction:
   a) Vestibulo-ocular reflex
   b) Saccadic movements
   c) Pursuit movements
   d) Convergence/divergence movements
   e) Fixation maintenance
   f) Optokinetic movements

Question 11

Describe the organisation of the retina:

Answer 11

1. Retina is layered - diagram 18
   a) Light propagates from the front whereas nerve signals propagate from the back
2. Anatomy:
   a) Photoreceptors - rods and cones
   b) Bipolar cells
   c) Ganglion cells
   d) Sclera and choroid

Question 12

Describe the events occurring during phototransduction leading to the activation of the optic nerve:

Answer 12

1. Pigments found in rods and cones absorb photons
2. Absorption of light causes receptor cell to be hyperpolarised
   a) Ligand-gated channel on outer receptor cell need cyclic guanosine monophosphate (cGMP) to remain open and allow potassium and sodium in
   b) In inner segment of cell, channels allow potassium to exit cell
   c) Light reduces levels of cGMP by converting it to GMP which causes the ligand-gated channel to close so potassium and sodium cannot pass through, however, the channels on the inner segment are still releasing potassium making the cell more negative (hyperpolarised)

Question 13

Describe the role of the visual pathways leading to the stimulation of the primary and associative visual cortical areas and hypothalamus: Diagram 19

Answer 13

1. When light leaves the retina, it goes into optic chiasm via the optic nerve
   a) Information crosses contralaterally at the optic chiasm
2. The information then travels through the lateral geniculate body of the thalamus
3. This information is then distributed along the visual cortex retinotopically

Question 14

What is the difference between rods and cones?

Answer 14

1. Rods are not colour sensitive and are great in dim light
2. Cones are colour sensitive and are broken down into three types:
   a) Blue cones
   b) Red cones
   c) Green cones
3. Cones are very densely distributed at the site of the fovea
4. Rods however are distributed evenly across the retina
   a) There is a spike of rods between fovea and optic disk

Question 15

List the 5 main taste categories:

Answer 15

1. Salt
2. Sour
3. Sweet
4. Umami
5. Bitter

Question 16

Outline the mechanisms for detection of tastant molecules:

Answer 16

1. Taste buds are specialised structures on papillae of tongue, palate, epiglottis, and oesophagus diagram 20
   a) Three types of papillae; circumvallate, foliate, fungiform
   b) Each taste bud has a taste pore and many taste cells
2. Mechanisms:
   a) Taste cells are polarised
   b) Tasted ions (salt, sour) are positive and enter the cell leading to depolarisation
   c) Tasted molecules (sweet, bitter, umami) are not charged molecules but lead to 2nd messenger mechanisms lead to depolarisation and calcium to be released into cytoplasm (progressing depolarisation)
   d) Depolarisation of cell causes release of neurotransmitter serotonin into synaptic cleft

Question 17

Identify the main neural pathways for taste information:

Answer 17

1. Tongue is innervated by cranial nerves VII, IX, and X
2. Those signals go from tongue to nucleus of solitary tract (gustatory nucleus)
3. Signal is then sent to ventral posterior medial nucleus of thalamus
   a) Signal also has separate pathway to hypothalamus and amygdala
4. Signal then sent to the insular cortex and frontal taste cortex
5. Signal then goes to Amygdala