Psychology Notes – April. 3
- The significance of pain: Although not enjoyable, the experience of pain serves important functions –
1. Produces withdrawal that prevents further injury. 2. Serves as a basis of learning to avoid injurious
objects/situations. 3. Sets limits on activity and enforces inactivity and/or rest.
- Pain perception: When contact with injurious stimuli occurs, signals of tissue damage follow a
particular route. Nociceptors of the afferent (sensory) neurons of the PNS respond to the stimuli and
they generate impulses that travel to the CNS. Nociceptors are the afferent nerve endings that
respond to pain stimuli. Nociceptors exist as free nerve endings, which means they are not
encapsulated by any other tissue and they are freely exposed to the environment.
- Nociceptors or fibre types: Aδ – Has thinly myelinated axons that carry thermal or mechanical
nociception and is associated with sharp or pricking pain. They transmit impulses quickly (5 to 30
m/s). These fibres send messages through the thalamus and to the cerebral cortex. C – Is a polymodal
nociceptor that can be activated by high intensity mechanical, chemical, and heat (>45°C). It is slow
conducting because it is not myelinated (0.5 to 2 m/s) and the pain is dull or aching. These fibres
send pain messages to the brain stem. Aα and Aβ – Are fibres normally involved in proprioception.
Fibres that are encapsulated are not free to the environment, but pressure can be put on them and
this is responsible for proprioception. When tissue is damaged, there is release of a lot of chemicals.
- Characteristics of sensory receptors in the skin: Merkel’s disk (receptor) is stimulated by steady
indentation to produce a pressure sensation, and its adaptation is slow. Meissner’s corpuscle
(receptor) is stimulated by low frequency vibration to produce a gentle fluttering sensation, and its
adaptation is rapid. Ruffini’s corpuscle is stimulated by rapid indentation to produce a stretch
sensation, and its adaptation is slow. Pacinian corpuscle is stimulated by vibration to produce a
vibration sensation, and its adaptation is rapid. Hair receptor is stimulated by hair deflection to
produce a brushing sensation, and its adaptation is rapid or slow.
- Theories of pain: Gate control theory – States that a neural mechanism in the dorsal horns of the
spinal cord acts like a gate that can increase or decrease the flow of nerve impulses from peripheral
fibres to the CNS, thereby influencing the sensation of pain. This theory states that this gate-like
mechanism controls pain stimulation to the brain, it is affected by nerve fibres’ activity, and it is
opened/closed through central control trigger activity in the spinal cord and brain. The dorsal horn
receives sensory information and the ventral horn sends out motor information. A projection neuron
carries pain to the brain. C fibres activate it by inhibiting an inhibitory interneuron, which is
responsible for inhibiting the projection neuron. This means that the projection neuron is activated,
which opens the gate and produces pain. Aα and Aβ fibres activate the inhibitory interneuron, which
means there is less activity of the projection neuron, which means the gate is closed and there is less
pain. The gate theory says that pain sensations can be modified en route to the brain and they can be
influenced by psychological factors. The brain can control the amount of pain an individual
experiences by transmitting messages down the spinal cord to block the transmission of pain signals.
The experience of pain is influenced by past experience, attention, and other cognitive activities
through the central control trigger. Involvement in other activities, relaxation, or positive
experiences can lessen the pain experience by causing the gate to close. Neuromatrix theory – This is
an extension of the gate control theory, with a greater emphasis on the brain’s role in pain
perception. There is a neuromatrix or network of neurons that generates patterns called
neurosignature patterns. The origins of these patterns may be innate (phantom limb pain) or sensory
and the patterns are responsible for producing a multidimensional experience of pain. Pain
perception is affected by sensory input, activity of the nervous system, and past experiences and