Somatosensory

Question 1

What are primary sensory neurones?

Answer 1

Or primary afferents.
The cell body is in the dorsal root ganglia.
The axon splits in two, one goes to the periphery, one goes to the dorsal horn in the CNS.
It synapses with the second order neurone, which takes the signal to the thalamus, then rest of the brain.

Question 2

What is the receptive field?

Answer 2

Each nerve axon innervates a specific receptive field within its dermatome (area of skin).
Sizes vary, determining precision of localisation of where stimulus is coming from.

Question 3

What are the classifications of axons in the skin?

Answer 3

Largest diameter and speed: A alpha, proprioceptors of skeletal muscle.
A Beta, mechanoreceptors of skin
A delta, pain and temperature
Smallest diameter and speed: C, temperature, pain, itch
From muscle, Group I, Group II, III and IV.

Question 4

How does the myelination affect conductance speed?

Answer 4

Fatter myelination and axon causes a faster conduction speed of action potential.

Question 5

What is 2 point discrimination?

Answer 5

If 2 stimulus activates 2 different receptive fields, will recognise it as 2 separate stimulus.
If it is in the same receptive field, then only feel one stimulus.
The fingers have many receptive fields innervated by many nerves, the back has few, so feel less discrimination.

Question 6

What is signal transduction?

Answer 6

A signal applied to the body will be transduced to an action potential.

Question 7

What is an adequate stimulus?

Answer 7

Or preferred.
The stimulus which generates an action potential.
Depends on the nerve ending.

Question 8

What is receptor potential?

Answer 8

A level of depolarisation directly linked to stimulus amount.
If it is below threshold, there is no action potential.

Question 9

How does the number of action potentials vary?

Answer 9

The number of action potentials relates to how much the signal goes above threshold, and varies by the type of neurones.

Question 10

What are slow adapting receptors?

Answer 10

Or non-adapting:
Important when maintaining information about a stimulus is valuble.
e.g. amount of stretch or pain.
Keeps firing action potentials while stimulus is still applied.
Detects strength of stimulus.

Question 11

What are fast adapting receptors?

Answer 11

Constantly changing stimulus required to generate action potential.
Useful where it is important to signal a change in stimulus.
e.g. touch receptors.
Detects how fast the stimulus changes.

Question 12

What are cutaneous sensory receptors?

Answer 12

Mechanoreceptors - touch, pressure, vibration.
Thermoreceptors - hot, cold
Nociceptors - noxious stimulation - pain.

Question 13

What are mechanoreceptors?

Answer 13

Tactile receptors at the end of A beta fibres.
Fast conduction - myelinated and thick.
Associated with specialised sensory apparatus.
Structure indicates function, which indicates location.
Gives information about surface texture, pressure and vibration.

Question 14

What are the types of mechanoreceptors?

Answer 14

Superficial layers of skin:
Merkel’s receptor
Meissner’s corpuscle
Deeper layers of skin:
Ruffini’s corpuscle
Pacinian corpuscle.

Question 15

What is Merkel’s receptor?

Answer 15

High density in epidermis of digits and mouth.
Slow adapting
Respond to sustained light touch.
Perception of form and texture.

Question 16

What are Meissner’s corpuscle?

Answer 16

In the papillary dermis.
Rapidly adapting
Respond to light touch and vibration.
e.g. detect putting on clothes but not wearing them.

Question 17

What is Ruffini’s corpuscle?

Answer 17

Respond to lateral movement or stretching of skin.
Deep touch.
Involved in monitoring grasped object slippage - causes stretch in skin, detected by Ruffini’s corpuscle.
Number of receptors varies by person and age.

Question 18

What are Pacinian corpuscles?

Answer 18

Found in deeper layers of dermis.
Rapidly adapting
Respond to deep touch, poking.
High frequency vibration.
Onion structure deforms to take up distortion due to mechanical stimulus.

Question 19

What are hair follicle receptors?

Answer 19

Light touch, activation in dermis.
Rapidly adapting.
Detects hair follicle deflection.

Question 20

What are thermoreceptors?

Answer 20

Bare nerve endings.
Slowly adapting.
Two types
Poor indicators of temperature but respond to changes in temperature, through comparison of signals from warm and cold receptors.

Question 21

What are thermoreceptor channels?

Answer 21

Transient Receptor Potential family.
non-specific cation channels.
TRPM8 - cold channels, open 10-38 degrees.
TRPV3/4 - warm - open 29-45 degrees.

Question 22

What are nociceptors?

Answer 22

Non-adapting
High threshold.
Adequate stimulus must be capable of damaging tissue.

Question 23

What are the types of nociceptors?

Answer 23

-High threshold mechanoreceptors
A delta fibres
Well localised pricking pain, felt first.
-Polymodal nociceptor - sensitive to mechanical stimulus and heat above 46 degrees.
C fibre.

Question 24

What is TRPV1?

Answer 24

Transducer channel opened by capsaicin (chillies) in the mouth.

Question 25

What are the sensory organs?

Answer 25

For proprioception, detects mechanical status of musculo-skeletal system. Muscle spindles Golgi tendon organs. Information about joint position, muscle length and movement, tension.

Question 26

What are muscle spindles?

Answer 26

Detect length, tone and acceleration. Specialised muscle fibres in a fibrous capsule. Group 1a afferents wrap around central portion. Stretch is picked up by fibres, causing firing, and tone.

Question 27

What are golgi tendon organs?

Answer 27

At junction of muscle and tendon. Group 1b afferents. Sensitive to tension from contraction. Parallel to muscle.

Question 28

What is the Dorsal column pathway?

Answer 28

Or Lemniscal pathway A Beta fibres Touch, vibration, 2 point discrimination, proprioception Travels to the brain quickly.

Question 29

What is the Spinothalamic pathway?

Answer 29

A delta fibres and C fibres. Pain and temperature. Slower because of thinner fibres and synapses - goes to spinal cord then brain.

Question 30

Why are smell and taste important?

Answer 30

Using chemoreceptors: Finding food Finding a mate Avoiding dangerous substances Homeostasis - internal environment - monitoring levels e.g. pH

Question 31

How is taste and smell used for environmental chemical detection?

Answer 31

Detecting poisons - usually through bitterness. Detecting food - through combined action of taste and smell, and texture, using the brain. Detecting the environment - if it's dangerous.

Question 32

What are the submodalities of taste?

Answer 32

Salt, sour, sweet, bitter, umami.

Question 33

Where are the taste cells?

Answer 33

Tongue Palate Pharynx GI tract

Question 34

What is the organisation of taste cells?

Answer 34

Papilla - bumps on tongue. Taste cells are independent of sensory neurones. Have a huge surface area. Closely associated with other neurones and basal cells.

Question 35

What are the properties of taste receptor cells?

Answer 35

They are not sensory neurones. Are constantly replaced - every 2 weeks - through basal cells dividing.

Question 36

How do taste cells detect taste?

Answer 36

Chemical binds Transduction to Receptor potential Depolarisation - VGCaC open and Ca2+ enters. Neurotransmitter released, excites sensory neurone - causes action potential.

Question 37

How is salt taste detected?

Answer 37

Sodium entry through non-gated channels through concentration gradient. Causes depolarisation.

Question 38

How is sour taste detected?

Answer 38

Acidity - H+ H+ enters through K+ channel block (can't leave) - maintains depolarisation. H+ entry via TRP channel - causes depolarisation (as H+ is positive).

Question 39

How is sweet, umami and bitter taste detected?

Answer 39

Bind to G protein coupled receptors. T1R and TR2 family. Different subunits of TR combine to form different G proteins which can detect different taste.

Question 40

How does taste go from the tongue to the brain?

Answer 40

Goes to primary neurone in the Gustatory nucleus of the brain. Then relays to secondary neurone, which carries to another synapse in the VPM of the thalamus. Relays to cortex through tertiary neurone with synapse in thalamus.

Question 41

How are more than 5 things tasted?

Answer 41

Through comparison of all inputs.

Question 42

What is ageusia?

Answer 42

Loss of taste because of damage to the brain - through a tumour or central lesion.

Question 43

What are the secondary pathways of taste?

Answer 43

Medulla - swallowing, salivation. Hypothalamus - satiety (measure of fullness), palatability (tastiness).

Question 44

Where are the smell receptors?

Answer 44

Olfactory epithelium in the nose. Receptor molecules diffuse in the lining of the nose. Axons of cells can penetrate through holes in the skull, and congregate in olfactory bulb.

Question 45

What are the properties of olfactory cells?

Answer 45

Olfactory cells are neurones. Replaced every 4-8 weeks. Each olfactory cell expresses one type of chemoreceptor molecule. Each molecule can bind a range of odorants.

Question 46

What is population coding?

Answer 46

Co-ordinated firing of multiple neurones. Odorant is identified by the brain.

Question 47

How are smells detected?

Answer 47

Odorant binds to receptor. G-protein mediated events Intracellular cascade Ca2+ or Na+ channels open. Depolarisation - action potential.

Question 48

How does smell go from the nose to the brain?

Answer 48

Smell cells with the same chemoreceptor molecules come together in a Glomerulus. Secondary neurone travels from synapse in Glomerulus up the olfactory tract to the olfactory cortex. Information either goes to the thalamus then back to the cortex, or to limbic areas - recognises smell.