S3: Introduction to Sensory Physiology and Perception

Question 1

Describe the basic pathway of transmission of a stimulus

Answer 1

• First there is an external event - usually external to the body but can be within the body e.g. interoceptors.
• This stimulates a sensory receptor cell e.g. light stimulating retina or pressure on skin.
• The sensory receptor needs to both detect the stimulus and convert it into electrical potential (transduce it). This is the stimulation being transduced into a change in the membrane potential called a receptor potential.
• If the depolarisation succeeds in getting the afferent up to threshold, an action potential will occur and information will propagate up to the CNS via a primary afferent.

Question 2

What are the three main things the CNS does/uses with information from a sensory stimulus?

Answer 2

1. Subconscious.
2. Sensation and Perception.
3. Arousal.

Question 3

Describe subconscious processes the CNS initiates

Answer 3

• Control of movement like proprioceptor and vestibular input to the motor pathways.
• Autonomic responses, for example, olefactory input stimulating salivation and increased gastric motility.
• Behavioural responses for example sight or smell of food promotes feeding behaviour (feeling of hunger).

Question 4

Describe arousal processes (sleep, wakefulness and attention) the CNS initiates

Answer 4

Modulatory pathways control the sleep/wake cycle and these pathways choose which of the stimuli we are aware of and attend to. They do not just control sensory pathways but are also affected by them:

• Sleep and wakefulness where sensory input can wake a person from sleep, sensory deprivation can induce sleep.
• Focusing attention where you concentration on one sensory thing which suppresses awareness of others.
• Switching attention where a salient (more important) stimulus can recapture awareness away from original one.

Question 5

What is sensory and perception?

Answer 5

Sensory is the awareness that an event as occurred. Perception is the ability to process and understand the information i.e. what it is, where it is, what does it mean, what should I do?

Question 6

What does somatosensory mean?

Answer 6

Somatosensory refers to sensation that can occur anywhere in the body rather than just at a single sense organ e.g. sight, taste.

Question 7

List and describe receptors that detect the stimulus

Answer 7

• Photoreceptors can detect light (electromagnetic radiation) giving vision.
• Chemoreceptors can detect chemicals in the air giving smell, chemicals in saliva giving taste and inflammatory chemicals during injury.
• Thermoreceptors detect temperature like heat, cold and injury.
• Mechanoreceptors detect pressure in skin with touch and injury, detect pressure/tension/stretch in musculoskeletal system allowing proprioception, can detect head movement and gravity to give us our sense of equilibrium and can detect sound through pressure waves in the air giving us our sense of hearing.

Question 8

List somatosensory stimulations

Answer 8

• Inflammatory chemicals from injury.
• Temperature.
• Pressure on the skin e.g. touch receptors and injury receptors.
• Pressure/tension/stretch in musculoskeletal system for proprioception.

Question 9

What are somatosensory receptors the same as?

Answer 9

Somatosensory receptors are also primary afferents (somatosensory afferent).

Question 10

Describe structure of primary afferent in touch sensory system (combined receptor and afferent)

Answer 10

• The primary afferent consists of pressure sensitive endings and afferent axon. The somatosensory receptor is a continuation of the axon so they are the same and this is also seen in the olfactory system. Normally other sensory systems have seperate receptor and afferent.
• At the end of the afferent, there is fine aborisation (branching structure at end of nerve fibre) in the skin and this is where the sensitive membrane is found. The sensitive membrane is part of the distal aborisation of an axon that runs into the dorsal root and up into the brainstem. The axon therefore runs from end of finger to brain stem.

Question 11

Describe how somatosensory receptors are resilient to injury

Answer 11

• The afferents are going to our skin and every time we cut our finger we cut them.
• However, the lose nerve endings can regrow and it should grow back in the same place restoring sensation.
• Axons can also grow into transplanted tissue and grow nerve endings.

Question 12

Give examples of systems with receptors as seperate cells to the afferents

Answer 12

• Visual systems
• Auditory and vestibular systems
• Gustatory system

Question 13

Why do most sensory systems have receptors seperate to afferents?

Answer 13

With combined receptor and afferent, the type of sensation they are detecting is quite primitive, achieved simply by a few proteins in the membrane.
For most our sensory system, more complex receptors are required such as visual, auditory, taste and equilibrium where the receptors are separate specialised cells. They can’t just grow an axon as they are too specialised. These cells communicate with afferents via excitatory synapses .

Question 14

Describe structure of sensory receptors as seperate cells

Answer 14

• Receptors are separated from axon by excitatory synapse.
• The separate cells will produce receptor potentials in response to stimulus. This causes a release of glutamate that will bind to the post synaptic afferent and cause an excitatory post synaptic potential that may also produce an action potential.

Question 15

What is action potential threshold?

Answer 15

Membrane potential at which action potentials are triggered. This is a function of the vgNa+ channel and can be treated as a constant.

Question 16

What is ‘activation’ threshold?

Answer 16

Minimum stimulus strength that will depolarise a receptor enough to generate action potentials. Sensory systems use receptors with a range of different activation thresholds.

Question 17

What is ‘perceptual’ threshold?

Answer 17

Minimum stimulus strength that will generate enough action potentials to be detected. Perceptual threshold is the but we can mostly test in a clinical environment. Whether someone detects stimulus depends on both activation of stimulus and focusing/attention of individual (modulatory systems may dampen or amplify the signal).

Question 18

What is the problem with separate receptors compared to combined receptor and afferent?

Answer 18

Unlike the touch example where we grew back the axon, separate receptors are delicate and irreplaceable. So if the receptors are damaged or die, then they will not be replaced.

Question 19

Describe how the separate receptors can be damaged in the visual and auditory system

Answer 19

• In the visual system, photoreceptors can be damaged/destroyed by light damage. genetic mutations, metabolic disease and malnourishment e.g. vitamin A deficiency.
• In the auditory system, auditory hair cells can be destroyed by noise truama, genetic mutations and ototoxicity (property of being toxic to ear e.g. chemotherapy agents, aminoglycoside antibiotics).

Question 20

How can a damaged seperate receptors be fixed?

Answer 20

A potential positive of this separation of receptor and afferent is the fact that if the receptors die the afferents may still be intact and healthy. This means if we are artificially able to gather information from the environment and stimulate the intact afferent with a pattern of stimulation like the original receptor would’ve done then you can restore sensation.
This is because the brain will reconstruct the incoming afferent APs as if they had come from a receptor. Any activity in the afferents will be interpreted as sensation so the rest of the system is intact and some function can be restored.
- e.g. in auditory system we have chochlear implants

Question 21

Is receptor signal/afferent signal in touch receptors a perfect replica of stimulus?

Answer 21

No

Question 22

Describe how spatial resolution is limited in touch receptors

Answer 22

If the skin is poked at the same time with two different sticks but one is a bit further up, the depolarisation of the two can summate to give one big depolarisation. This means that the brain can’t distinguish them as for any one afferent there will be a region of skin (called receptive field) over which that axon will relay a single stream of APs.
- Any fine detail happening on the skin will just be summed together and averaged out so you won’t be aware of it.

Question 23

What is two point discrimination and where is it poor on the body?

Answer 23

Two point discrimination (ability to tell two different stimuli on the skin) is poor on the upper arm and back.

Question 24

Describe how receptive field sizes vary across the body and use braille as example

Answer 24

• A person would be unable to read braille with the skin of their forearms because the receptive fields are larger here. The lumps of the braille pad may depolarise the different receptor endings but ultimately the signals will be brought together to one axon which will summate all the information and send it to the brain. Therefore all fine detail is lost only only the general information in the region is kept.
• In comparison, the nerves of the fingertips have much smaller receptive fields which allow signals to reach the brain giving the location of each braile lump. This is because each axon is carrying the individual information to tell the brain the pattern of lumps on a braille pad.

Question 25

What is the downside to having smaller receptive fields?

Answer 25

The downside of having small receptive fields means you need a huge number of afferents to innervate that single section of tissue whereas in other areas it would just be a single afferent.

Question 26

Relationship between receptive fields and innervation density

Answer 26

Smaller receptive fields = higher innervation density.

Question 27

How does the primary somatosensory cortex (runs along the postcentral gyrus) show the consequences of small receptive fields?

Answer 27

The cells along this strip are interesting because each receives sensory input from a different part of the body so there is a map of the body laid out along the somatosensory cortex.

• The regions which have really good spatial resolution (small receptive fields), which require high innervation to give them that resolution those regions require a larger amount of the cortex to interpret and process the info.
• The reason most of our body doesn’t have that high of a resolution is because otherwise our head would be massive.

Question 28

What is temporal resolution and describe how it is also limited?

Answer 28

Temporal resolution is precision of measurement with respect to time.
If we poked the skin twice (or more) in rapid succession, the two signals produced by the receptor ‘ride on top of each other’ so that it may feel like a single tap. The afferent may not be able to follow frequency of stimulation.
This depends on receptor type.

Question 29

How do our sensory systems ‘adapt’ to constant stimuli?

Answer 29

By dampening down responses to unchanged stimuli, adaptation highlights moments when stimulus strength changes. Different receptor types adapt at different rates.

Question 30

Describe lateral inhibition

Answer 30

• There is a poke against the skin and there are three afferents that project up to the brain stem where they activate secondary afferents.
• Each afferents are receiving the same pressure and at the same rate so the signal going to the brain says the stimulus is around the region but it is broad and sloppy as all three afferents are sending information.
• What happens to solve this occurs in all sensory systems, is that cells taking info from each part of the skins sensory map try to inhibit their surrounding neighbours.
• If the pressure against the skin is exactly the same across the region and all of the receptors and primary afferents are being activated to the same extent, then the secondary afferents and lateral inhibition will dampen down the firing rate of ALL the afferents and produce a low level of response. This is because all three have the same activation so will produce equal inhibition of their neighbours via the interneurons. I.e. homogeneous spatial stimuli are suppressed by lateral inhibition.
• If the stimulation is stronger in one area of the skin, the afferent innervating that area will produce more inhibition than its neighbours which will dampen the other afferents to the extent where they cannot dampen the other afferent. The stronger afferent then gives a stronger signal and produces a nice clean response and becomes the only afferent responding well as it is the one that received the most stimulation.
• This means that spatial discrimination is enhanced by lateral inhibition.
• Lateral inhibition damps down responses to homogeneous spatial information. It highlights “salient” locations i.e. where stimulus strength changes!
  It also allows receptors to encode stimulus contrast (difference between stimulus between x and y) over a huge range without saturation!

Question 31

What does ensuring sensory systems respond to changing through adaptation and lateral inhibition (not stimulus strength) allow?

Answer 31

• Highlight salient features
• Allow greater sensitivity while avoiding saturation
• Save the cost of unnecessary action potentials

Question 32

Describe how dynamic range is increased

Answer 32

• Adaptation and lateral inhibition damp down responses to homogenous temporal and spatial stimulation.
• Receptors encode changes rather than absolute levels of stimulus.
• They can therefore respond strongly to small changes over a very large stimulus range.
• This hugely increases their ‘dynamic range’ and reduces the problem of ‘saturation’

Question 33

How does damage and disease usually affect the sensory receptor?

Answer 33

• Can differentially affect different types of receptor.
• Damage and disease will generally increase perceptual threshold (making things easier to sense) and this occurs when receptors are lost.
• However, there are also times where damage and disease can result in lower perceptual threshold and cause hypersensitivity.

Question 34

Why are receptive fields important to learn about?

Answer 34

Important to know about these because they determine the resolution of sensory systems, if you use the wrong stimulus (e.g. of wrong size) you may think your receptors aren’t working when they actually do.

Question 35

Why is perceptual threshold important to learn about?

Answer 35

It is measured clinically during sensory testing (can you feel this cotton?).

Question 36

Why is adaptation of sensory receptor important to learn about?

Answer 36

Adaptation affects which clinical test to use in order to reveal deficits in different types of receptor, this is because different receptors adapt at different rates e.g. do you use vibration testing? (for slow adapting stimuli)

Question 37

Why is lateral inhibition important to learn about?

Answer 37

It is an essential feature of sensory processing, loss of this can lead to hypersensitivity.

Question 38

Describe how the receptor signal is distributed to multiple regions of the cerebral cortex

Answer 38

• If we look at where these different receptors axons go, we see they relay via the thalamus up to the somatosensory cortex.
• Once there the axons do not pile into the same bit of cortex they segregate themselves into columns of tissue!
• The columns may run from the surface down to the border with the white matter. One of the columns may respond/process gentle touch, another skin stretch, vibration etc.
• Each column responds to a specific quality of touch because it is receiving axons from that receptor type.
• It is the activity here in these columns that is interpreted by the brain giving rise to sensation.
• Stimulating the cells down a column would send sensory signals e,g. vibrations to the skin it innervates.

Question 39

What you experience in every sensory system is your brains best reconstruction of what may be out there in the enviroment (based on the pattern of AP firing in the cerebral cortex)! Give examples of when this goes wrong

Answer 39

Phantom limbs – A person may feel their limb or pain even once it has been amputated. This makes sense because the stump of the arm still contains afferents which will go up to the specific region of the cortex of the arm so the brain will interpret stimulation from those areas as if the arm was still there.
As far as the brain is concerned it is still there as it is still receiving information from it.

Synaesthesia – This occurs where there is miswiring of the sensory system, so for example auditory input activates the visual colour processing area (in the visual cortex) so a person “sees” sounds as coloured.

Epileptic activity is activity that spreads through the cortex, if it spreads through the somatosensory system it can generate phantom sensations running across the body surface. (person feels sensation over their body).

Pain without any injury or pathology to explain it. The pain sensitive neurone in the cerebral cortex are activated for us to feel pain but sometimes it is difficult to find the cause of this activation.