When the activity of the transmission cells reaches a critical threshold level we experi -
ence pain with greater pain intensity associated with greater activity. Once the pain
centres in the brain have been activated, we are able to respond quickly to remove
ourselves from danger. It is worth noting that the brain produces its own pain-relieving
chemicals in the form of endorphins (i.e. a chemical similar to opiates), which inhibits
the pain fibres from releasing substance P, which subsequently reduces the experience
of pain. This is why endorphins are often described as being associated with a ‘jogger’s
high’.
Moreover, pain sensations from the injury site are transmitted to the gating mech -
anism by pain fibres (or nerves) known as nociceptors. This is known as an afferent
pathway because it indicates information is travelling towards the CNS. As outlined
above, in addition to A-beta fibres two other key fibre types have been identified:
- A-delta fibres (types I and II):
- transmit information about sharp, brief pain;
- wrapped in layers of ‘fatty’ cell membranes (i.e. myelinated) which increases
the speed of action.
- C-fibres:
- transmit information about dull, throbbing pain;
- not wrapped in ‘fatty’ cell membranes, therefore they have a slower speed of
action.
Each of the three types of nociceptors is important as stated by Melzack and Wall
(1965: 972) ‘The degree to which the gate increases or decreases sensory transmission
is determined by the relative activity in large diameter (A-beta) and small diameter
(A-delta and C) fibres and by descending influences from the brain.’ The other group
of nerves that transmit information to the gating mechanism are the other peripheral
fibres. In particular, A-beta fibres carry information about harmless stimulation or mild
irritation, such as gentle touch or stroking or lightly scratching the skin, to the spinal
cord. When A-beta fibres are stimulated the gate is likely to close and pain perception
is inhibited, explaining why people experience a reduction in pain during a gentle
massage or when heat is applied to aching limbs.
The final factor that influences the opening and closing of the gate is the impact
of messages descending from the brain. Neurons in different parts of the brain send
impulses, via what are known as efferent pathways (i.e. indicating they lead away from
the CNS or are descending from the brain), to the spinal cord. Various brain processes
such as anxiety, distraction, hypnosis and excitement have the capacity to influence
this neural activity by releasing chemicals such as endorphins and therefore the open -
ing and closing of the gate. From a biopsychosocial point of view, this is the most
important component of gate-control theory as it provides a clear route through which
cognitive, emotional and social factors can influence pain perception. Focus 2.1 pro -
vides an overview of various conditions that individuals may experience that may open
and close the gate. Since the introduction of this theory, researchers have been inspired
to investigate the efficacy of psychological and behavioural approaches to pain
management. We will consider these approaches in Chapter 10.
BIOPSYCHOSOCIAL PATHWAYS 29