Guyton Chapter 48: Somatic Sensations

Question 1

What 3 stimuli can elicit pain?

Answer 1

Mechanical
Thermal
Chemical

Question 2

Name some chemicals that excite the chemical type of pain.

Answer 2

Bradykinin, serotonin, histamine, potassium ions, acids, acetylcholine and proteolytic enzymes.

Prostaglandins and Substance P enhance the sensitivity of pain endings but do not directly excite them.

Question 3

What are the difference in peripheral pain fibres: fast and slow fibres

Answer 3

Fast sharp pain signals are elicited by either mechanical or thermal pain stimuli. Transmitter in the peripheral nerves to the spinal cord by small type A fibres at velocities 6-30 m/sec.

Slow chronic type of pain is elicited mostly by chemical types of pain stimuli. Transmission to the spinal cord by type C fibres at velocities 0,5-2 m/second.

Question 4

What 2 different pathways do the pain signals take to the brain?

Answer 4

1. Neospinithalamic tract
2. Paleospinothalamic tract

Question 5

What happens in the neospinothalmic tract for fast pain?

Answer 5

The fast A-fibres terminate mainly in lamina 1 (lamina marginalis) of the dorsal horns and there excite second order neurons of the neospinothalmic tracts. These give rise to long fibres that cross immediately to the opposite side of the cord through the anterior commisure and then turn upward passing to the brain in the anterolateral columns.

Question 6

Where does the neospinothalmic tract terminate in the brain stem and thalamus?

Answer 6

A few fibres of the neospinothalmic tract terminate in the reticular areas of the brain stem but most pass all the way to the thalamus without interruption terminating in the ventrobasal complex along with the dorsal column medial lemniscal tract for tactile sensations. A few fibres also terminate in the posterior nuclear group of the thalamus. From these thalamic areas, the signals are transmitted to other basal areas of the brain as well as to the Somatosensory cortex.

Question 7

What neurotransmitter substance is secreted in the spinal cord at the type A pain nerve fibre endings?

Answer 7

Glutamate

This is one of the most widely used excitatory transmitters in the central nervous system usually having a duration of action lasting for only a few milliseconds.

Question 8

How does the paleospinothalamic pathway for transmitting slow chronic pain?

Answer 8

Transmits pain mainly from the peripheral C-fibres. The peripheral fibres terminate in the spinal cord almost entirely in laminae II and III of the dorsal horns (together called the substantia gelatinosa) —> then entering lamina V also in the dorsal horn. The last neurons in the series give rise to long axons that mostly join the fibres from the fast pain pathway passing first through the anterior commisure to the opposite side of the cord then then upward to the brain in the anterolateral pathway.

Question 9

What is the probable slow chronic neurotransmitter of type C endings?

Answer 9

Substance P

Glutamate transmitter acts instantaneously and lasts only for a few milliseconds. Substance P is released much more slowly.

Question 10

Projection of the paleospinothalamic pathway into the brain. Only one tenth to one fourth of the fibres pass all the way to the thalamus. Instead most terminate in 1/3 areas - which are they?

Answer 10

1. The reticular nuclei of the medulla, pons and mesencephalon
2. The tectal area of the mesencephalon deep to the superior and inferior colliculli
3. The periaqueductal gray region surrounding the aqueduct of Sylvius.

Question 11

Which 3 major components does the analgesia system consist of?

Answer 11

1. The periaqueductal gray and periventricular areas of mesencephalon and upper pons surround the aqueduct of Sylvius and portions of the third and fourth ventricle. Neurons from these areas send signals to
2. The raphe magnus nucleus located in the lower pons and upper medulla and the nucleus reticularis paragigantocellularis located laterally in the medulla. From these nuclei, second order signals are transmitted down the dorsolateral columns in the spinal cord to
3. A pain inhibitory complex located in the dorsal horns of the spinal cord

Question 12

What areas in the brain can suppress pain?

Answer 12

Stimulation of areas of the brain that excite the periaqueductal gray area can also suppress pain. Some of these areas are:
1. Periventricular nuclei in the hypothalamus
2. The medial forebrain bundle

Question 13

Which transmitted substances are especially involved in the analgesia system?

Answer 13

Enkephalin and serotonin

Question 14

What Is enkephalin believed to do in the analgesia system?

Answer 14

Causes both presynaptic and postsynaptic inhibition of incoming type C and type A pain fibres where they synapse in the dorsal horns.

Question 15

What 3 protein molecules result in breakdown products that are natural opioid substances?

Answer 15

Pro-opiomelanocortin
Proenkephalin
Prodynorphin

Question 16

Name 4 of the natural opiate like substances and where they are found:

Answer 16

B-endorphin: hypothalamus and pituitary gland
Met-enkephalin: Brain system and spinal cord
Leu-enkephalin: brain system and spinal cord
Dynorphin: mainly the same areas as the enkephalins but in lower quantities

Question 17

How are thermal signals transmitted?

Answer 17

When entering the spinal cord downward in the tract of Lissauer and then terminate mainly in laminae I, II and III of the dorsal horns - the same as for pain. The signals then enter long ascending thermal fibres that cross to the opposite anterolateral sensory tract and terminate in both the
1. Reticular areas of the brain stem
2. The ventrobasal complex of the Thalamus
A few thermal signals are also related to the cerebral somatic sensory cortex from the ventrobasal complex