Sensory Systems and Physiology of Pain

Question 1

Give examples of sensory receptors

Answer 1

Mechanoreceptors
Chemoreceptors
Thermoreceptors
Nociceptors
Proprioceptors

Question 2

What is the name for the area to which a specific sensory receptor responds?

Answer 2

Receptive field

Question 3

What does the term adequate stimulus refer to in relation to sensory receptors?

Answer 3

The type of stimulation to which they respond e.g. mechanoreceptors - touch

Question 4

What initials happens to all adequate stimulus in a sensory receptor?

Answer 4

It is transduced into a graded potential called the receptor/generator potential

Question 5

What information is carried in the APs from sensory receptors?

Answer 5

Information on the modality, intensity and location of the stimulus

Question 6

Describe the relationship between the receptive field and sensitivity and acuity

Answer 6

The larger the receptive field, the less acuity that sensory receptor will be capable of, but it is no less sensitive

Question 7

What is carried in A Beta fibres? Are they myelinated?

Answer 7

Touch, pressure, vibration

Large unmyelinated

Question 8

What is carried in A Delta fibres? Are they myelinated?

Answer 8

Cold, fast pain

Myelinated

Question 9

What is carried by C fibres? Are they myelinated?

Answer 9

Warmth, slow pain

Unmyelinated

Question 10

What two types of afferent fibres mediate proprioception?

Answer 10

A Alpha

A Beta

Question 11

What tract is used by the A alpha and A beta fibres of touch, pressure and vibration?

Answer 11

Dorsal column
Synapse in the cuneate & gracile nuclei
Decussate in the brainstem

Question 12

What tract is used by the A delta and C fibres? What does it carry?

Answer 12

Spinothalamic tract - temperature, all pain

Decussate in dorsal horn

Question 13

What is the result of damage to the dorsal column?

Answer 13

Loss of touch, vibration, proprioception below the lesion on ipsilateral side

Question 14

What is the result of damage to the anterolateral quadrant?

Answer 14

Loss of nociceptive and temperature sensation below the lesion on the contralateral side

Question 15

Where does sensory information ultimately terminate?

Answer 15

The somatosensory cortex (S1) of the post central gyrus

Question 16

What does the arrangement of sensory information arriving in the somatosensory cortex produce?

Answer 16

The sensory homunculus

Question 17

Describe the process of adaptation in sensory pathways

Answer 17

This is a process whereby sensory receptors are able to mediate their response to constant stimulation. If stimulus intensity is sustained over lengthened periods of time, the sensory receptors can adapt, preventing the continuous firing of unnecessary APs. Receptors can be either fast reacting or slow reacting e.g. the hat response

Question 18

Describe the process of convergence in sensory pathways

Answer 18

This refers to the process whereby signals form multiple sensory receptors can converge at a synapse and continue towards the brain as a combined signal input

Question 19

What are some benefits and downsides to convergence in sensory pathways?

Answer 19

The advantages are that it saves on neurons, and makes the transmission of sensory information more efficient in terms of the number of occupied neurons. However, the downsides are that it results in a reduction in acuity, and is the underlying principle of referred sensation

Question 20

What are the two forms of convergent pathway in the sensory channels? Describe each

Answer 20

Specific ascending pathways - here receptor neurons synapse together with other neurons of the same adequate stimulus, and continue from the synapse in the spinal cord with information form multiple inputs but that are from the same type of stimulus. These pathways remain distinct through the second synapse in the midbrain and reach the cortex still as separate pathways

Nonspecific ascending pathways - here receptor neurons of different adequate stimuli synapse together, creating a post-synaptic pathway which contains information of more than one kind of sensory input e.g. touch and temperature merging together

Question 21

Describe the process of lateral inhibition in sensory pathways

Answer 21

This refers to the principle that when one sensory fibre is activated, it causes synaptic inhibition of its neighbours, which are activated to a lesser degree by the original stimulus, in order to give better definition of borders, improving acuity and localisation of the input, and cleaning up sensory information

Question 22

Give some of the different ways in which pain can be described

Answer 22

Sharp
Stabbing
Diffuse
Throbbing
Crushing

Question 23

What are the two major forms of pain?

Answer 23

Acute

Chronic

Question 24

Describe the analgesic action of NSAID drugs

Answer 24

NSAIDs inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins. Prostaglandins acting on prostanoid receptors on the surface of nociceptors increase the nociceptor sensitivity to braykinin acting on Beta-bradykinin receptors. This increased sensitivity causes stimulation of ion channels which allow entry to the nociceptor to the toxins that generate the depolarisation and subsequent APs - “pain” signals. Inhibition of the cyclo-oxygenase and prostaglandins therefore reduces the pain signals = analgesia

Question 25

What is the (‘analgesic’) action of local anaesthetics?

Answer 25

They block sodium action potentials and therefore all axonal transmission - so are not strictly analgesic

Question 26

Describe the principle of segmental controls with regard to analgesia

Answer 26

This is the principle that stimulation of other sensory fibres such as A alpha or beta fibres activates inhibitory interneurons which act on and inhibit, via an opiate-like mechanism, the A delta / C fibres carrying the ‘pain’ signals

Question 27

Outline the theory of gate control and pain

Answer 27

The gate control theory of pain asserts that non-painful input closes the “gates” to painful input, which prevents pain sensation from traveling to the central nervous system. Therefore, stimulation by non-noxious input is able to suppress pain. The key contributor is the Substantia Gelatinosa, which essentials exerts an inhibitory effect on the nociceptive inputs to the ‘gate’. Mechanoreceptors stimulate the SG to inhibit = segmental control, descending inhibitory pathways inhibit directly as well as stimulating the SG to inhibit

Question 28

Describe the descending pathways involved in gate control of pain

Answer 28

Descending pathways from the peri-aqueductal grey matter (PAG) and nucleus raphe magnus use inhibitory neurons to directly inhibit the nociceptive neurons as well as stimulating the SG to further inhibit - closing the gate

Question 29

Describe the analgesic actions of opiates e.g. morphine

Answer 29

They reduce the sensitivity of the nociceptors, blocking transmission in the dorsal horn (hence epidural administration) and activating the descending inhibitory pathways = closing the gate