Emergence of Pain
Transmission of pain signals
The pain stimulus is transmitted as an electrical signal by peripheral nerves which enter into the back part of the spinal cord, the so called dorsal horn. Here, the impulse switches from the peripheral to the central nervous system and is sent on to the brain. Two reactions take place, when a pain signal reaches the dorsal horn:
- The signal is deflected to motor neurons in the frontal part of the spinal cord. This triggers muscle contractions that leads to a reflex reaction takes place often before we even realize that we have hurt ourselves.
- The pain signal is transmitted to different areas in the central nervous system: spinal cord and brain. After it reaches a specific region in the brain, the cortex, we recognize the pain and where it comes from.
Nerve cells and their function
The pain signal is transmitted via nerve cells. These consist of the cell body, axon and dendrites. The so-called dendrite terminals or sensitive terminals function as the “ears” of the nerve cell. They receive information from other nerve cells or receptors. The synaptic terminals which pass the information on to other nerve cells could accordingly be called the “speakers” of the neuron. Axons transmit the information from the “ears” to the “speakers”. The Myelin sheath isolates the axon and thus increases its transmission speed. The area where the communication between neurons takes place is the loose junction of dendrites and synaptic terminals, called synapses.
How do nerve cells communicate?
Nerve cells transport information as electrical impulses. When impulses reach the synapses (a conjunction between two nerve fibres) the electrical transmission is interrupted and changed to a chemical transmission from the presynaptic (C) to postsynaptic (D) side. These chemical transmitters are called neurotransmitters (B).
After being secreted from the vesicles (A) (small bubbles) into the synaptic cleft (gap between the two nerve fibres) the transmitters reach the postsynaptic membrane. Here a specific receptor is occupied by the transmitters. After the transmitter (key) has engaged the receptor (lock), a specific reaction is caused in the receptor-carrying cell.
Different neurotransmitters have different effects on the processing of messages in the subsequent nerve cells. They can have an inhibitory or enhancing effect on the transmission of a stimulus.
The first synapse in the pain transmission is to be found at the dorsal horn – here the pain signal that comes from the periphery is switched to the central nervous system via the above explained mechanism.
Pain ‘transporting’ transmitters
Incoming pain impulses lead to a release of pain initiating transmitters from the presynaptic membrane, such as substance P and glutamate. By docking at the receptors on the post-synaptic membrane a new electrical impulse is generated which further transmits the pain signal to the brain.
Which parts of the brain are affected?
The pain stimulus reaches various structures in the brain involved in pain processing via afferent fibres. There is no defined pain centre but rather several important areas for pain sensation:
Reflexes in the brain increase the heart and respiratory rate and induce the release of the stress hormones.
The reticular formation affects the consciousness (mild pain increases concentration,severe pain causes unconsciousness).
The Medulla oblongata (the extended part of the spinal cord) stimulates the respiratory and cardio-vascular center.
The Thalamus functions as a relay station. It distributes the pain signal to various areas of the brain including the transmission to the cerebral cortex for realizing the signal as pain.
Via the hypothalamus and the pituitary gland, the hormonal reactions are aimed at the occurrence of pain: the sympathetic nervous system is activated, adrenaline is secreted, the heart rate increases, the blood vessels constrict and blood pressure rises.
The limbic system provides the connection to the emotional reactions. On the one hand emotions are influenced by pain experience; on the other hand pain perception is also influenced by emotions. Depending on mood, experience and expectation, the pain threshold may vary considerably in individual cases.
In the cerebral cortex the pain impulse becomes conscious. Here the information on the exact location and intensity of the pain is processed and specific reactions of the individual are initiated.
Descending pain inhibition on spinal level
The brain processes the pain stimulus and the relevant inhibitory mechanisms are activated (via descending pathways) leading to pain modulation on the spinal level. Descending inhibition is the body’s own defense mechanism against pain.
Two classes of inhibitory transmitters are important:
Pain modulating substances: noradrenaline and serotonin
Within certain limits the brain can decide how many pain signals it wants to receive from the dorsal horn of the spinal cord. Specialized brain structures can trigger inhibitory feedback mechanisms that influence pain signal transmission from the peripheral nerve to the spinal cord neurons in the dorsal horn. It activates neurons whose axons run downward to the dorsal horn and form synapses which prevent the generation of ascending electrical pain signals. The most important transmitters for this pain modulation are noradrenaline and serotonin.
In order to limit the duration of a signal each transmitter release automatically triggers so called reuptake mechanisms which remove the transmitter from the synaptic cleft and store them again in the presynaptic nerve cell. (See the red arrows in the diagram)
Pain modulating substances: endorphins
An additional pain inhibiting effect is caused by our body’s own opioids – the endorphins. When experiencing a very painful accident these substances are released and bind to specific (opioid) receptors. Within the spinal cord endorphins are used by local so called Interneurons to damp pre- and postsynaptic pain signals.