Intro to sensory physiology and perception

Question 1

Give 4 types of receptors and their stimulus

Answer 1

• Photoreceptors - light
• chemoreceptors - chemicals in air for smell and saliva for taste, also inflammatory chemicals for injury
• thermoreceptors - temperature
• mechanoreceptors - pressure on skin, pressure/tension/stretch in MSK (proprioception), head movement and sound for hearing

Question 2

In some systems, receptors are also primary afferents

Answer 2

• touch receptor - peripheral axon stretches out to tissue, has sensory receptor on end. Depolarises in response to pressure on skin
• resilient to injury, receptor can regrow as long as the injury doesnt destroy the cell body
• e.g. successful face transplants where nerves have regrown in face from someone elses tissue

Question 3

In others, receptors are separate

Answer 3

• separate receptors are delicate and irreplacable
• photoreceptors - light damage, genetic mutations, metabolic disease
• auditory hair cells - noise trauma, mutations, ototoxic drugs (chemo, aminoglycosides)
• any activity in afferent is detected as sensation, so if rest of system is intact, some function can be restored

Question 4

What are the differences in thresholds?

Answer 4

• AP threshold - membrane epotential at which APs are triggered
• Activation threshold - minimum stimulus strength that will depolarise a receptor enough to generate APs - different receptors have different activation thresholds (nociceptors = high, low and high-threshold mechanoreceptors)
• Perceptual threshold - minimum stimulus strength that will generate enough APs to be detected - high perceptual threhold will make it hard for person to feel it (changes with attention)

Question 5

Spatial resolution - limited

Answer 5

• two things tapped at once in same receptive field gives bigger peaks and 2 APs
• cant tell if it is from one very hard or two smaller simultaneously

Question 6

Receptive field sizes vary

Answer 6

• the smaller the receptive field, the finer the detail you can feel
• the smaller the receptive field, the higher the innervation density
• fingers have tiny receptive fields - allows you to read braille
• forearms for example will have a much larger receptive field

Question 7

Spatial resolution - variable

Answer 7

• the higher the innervation, the more cortex is needed to deal with the input
• e.g. hand and lips have much larger representation in sensory homunculus that other parts of body

Question 8

Temporal resolution is limited

Answer 8

• potentials are graded and can sum together if the receptor cannot repolarise fast enough to represent individual signals
• any APs that arise will just appear to be one tap rather than two separate ones
• we have a limitation that means we lose a lot og higher frequency signals

Question 9

Receptors adapt

Answer 9

• damps down homogeneous temporal signals
• highlights changes in signal - e.g. stronger stimulus
• allows receptors to encode changes in stimulus strength over a huge range without saturation

Question 10

Lateral inhibition

Answer 10

• damps down responses to homogeneous spatial information
• highlights salient locations - where stimulus strength changes
• allows receptors to encode stimulus contrast over a huge range without saturation
• e.g. pressing your finger on a very smooth surface will cause little stimulation, however one little bump will punch right through and send a signal

Question 11

Damage and disease

Answer 11

• can differentially affect different types of receptor
• will increase perceptual threshold where receptors are lost
• BUT neural damage can lower perceptual threshold and cause hypersensitivity

Question 12

There isnt a single representation of a stimulus

Answer 12

• Different receptors in the sklin detect different qualities of touch
• gentle touch, light, varying pressure, skin stretch, strong, vibration, movement of hair, damaging pressure
• information from different receptors are widely distributed between different cortical areas
• different qualities are represented in columns in primary cortical areas

Question 13

When does the brain get it wrong?

Answer 13

• phantom limbs
• epilepsy - sensations running across body surface due to somatosensory system
• synaesthesia - person sees sounds as colours as auditory input activates visual colour area