Pain is a vital function of the nervous system in providing the body with a warning of potential or actual injury. It is both a sensory and emotional experience, affected by psychological factors such as past experiences, beliefs about pain, fear or anxiety.
When you roll your ankle, or pull a muscle in your lower back, pain is usually immediate. In these cases, pain is “acute“ – meaning new or sudden. Acute pain often originates from nociception – the nervous systems way of detecting, transmitting and processing potential damage. This potential damage is detected by nociceptors – a kind of nerve ending (sensory receptor) found all over your body.
Nociceptors are the specialised sensory receptors responsible for the detection of noxious (unpleasant) stimuli, transforming the stimuli into
electrical signals, which are then conducted to the central nervous system. They are the free nerve endings of primary afferent Aδ and C fibres.
Distributed throughout the body (skin, viscera, muscles, joints, meninges) they can be stimulated by mechanical, thermal or chemical stimuli.
Inflammatory mediators (eg bradykinin, serotonin, prostaglandins, cytokines, and H+) are released from damaged tissue and can stimulate nociceptors
directly. They can also act to reduce the activation threshold of nociceptors sothat the stimulation required to cause activation is less. This process is called primary sensitisation.
Once nociceptors are activated, they send signals through peripheral nerves to the spinal cord, which then travel to the brain. The signals are processed at each stage of transmission, with the brain arguably playing the largest role in how pain is consciously experienced.
People often refer to nociceptors as “pain receptors” – but that’s not quite right. Nociceptors actually detect the same sensations as other receptors (pressure, temperature, etc). However, nociceptors have a higher threshold of activation, meaning that they require a stronger than usual stimulus before sending a signal to your brain. In a way, they are the “danger” nerves, telling you that something intense is happening – and therefore, you should pay some attention! Nociceptors are kind of like alarms.