1. We can think in terms of two types of receptor cells:-
Mechano-nociceptors are responsive to touch, especially injurious
pressure. Some of these cells, called mechano-thermo-nociceptors, also
respond to high temperatures (>48°C).
Polymodal nociceptors, as their name implies, respond to a wide
range of stimuli, including pain-producing chemicals, high or low temperatures,
and strong mechanical pressure.
2. The two types of nociceptors contrast in their sensory neurons-
Mechano-nociceptors connect to the CNS via Ad
fibres which are thin (only 1-6µm) but myelinated and so are fast-conducting
(5 - 36 m.s-1) - signal sharp 'first pain' (p 159 of Bk.3.).
Polymodal nociceptors connect via C fibres which are thin (0.2 - 1µm)
and unmyelinated and so slow-conducting (0.2 - 1 m.s-1) - signal
duller 'second pain'.
See Table 5.2 and §5.2.2, 5.2.3 in Book 3.
3. As described in §5.2.4, especially Figs 5.12 and 5.13, the two specific (DC and STT) and one non-specific (SRT) pathways are:-
DC = dorsal column; axon from dorsal root ganglion (DRG) cell ascends on same side (ipsilateral) to medulla, were it synapses on cell with axon crossing to other side (contralateral) and on up to the thalamus, finally synapsing with cell going to the cortex.
STT = spinothalamic tract; the DRG cell synapses in its own segment with interneuron which crosses over and ascends contralaterally to the thalamus where it synapses with cell going on up to the cortex.
SRT = spinoreticular tract; either uncrossed - the DRG cell synapses with a neuron which ascends on the same side, or crossed - the DRG cell synapses with a neuron which crosses over and ascends contralaterally; this second neuron in the sequence goes to the reticular formation, synapses with another neuron there and this then synapses with a neuron up to non-specific thalamic nuclei where there's a final synapse with neurons connecting up to the cortex.
Thus, there are just two synapses in the DC and STT pathways, but four synapses in the SRT.
These tracts also differ in the kinds of stimuli involved (pp.162-163 of Bk. 3.).
4. Neurons in the dorsal horn ascend to the higher centres to signal pain. But some of these cells receive two inputs from DRG cells, for example one from the skin and the other from deeper organs. Once the signal is ascending its common projection to the brain it's no longer possible to discriminate between the two peripheral pathways. See Fig. 5.14 and associated text.
5. Yes. According to the Gate Theory of analgesia, signals along the slower fibres from mechanoreceptors can have an inhibitory effect in the substantia gelatinosa. See §5.7 in Bk. 3.
6. As shown in §2.4.3 of Bk. 4, the three criteria are:
8. Apart from the spelling (the extra letter 'h' in chemotrophic), one is a kind of tropism, i.e. directed growth , in which a substance acts as a target at a distance to attract the growing axon (in contrast to chemotactic guidance (contact mediated) - §2.4.5. This is chemotropism.
A chemotrophic factor acts to promote cell survival, rather than direction of growth. It acts once the axon has reached its target so that the cell survives - so its not really guidance at all, as you can see in §3.2.2; for example nerve growth factor (NGF).
9. It dies! See §3.2.3.
10. Plasticity refers to the ability of connections and synapses to change, either structurally or functionally. In the mature animal, this could be important in two main ways: (a) recovery from damage and (b) changes during learning. See §3.6 of Bk. 4.