Transmission of pain on nerves before the spinal cord

Transmission of pain on nerves before the spinal cord

Pain sensations from the uterus and lower birth canal are carried to the spinal cord along nerves. Just about every structure in the body below the shoulders has nerves which travel between it and the spinal cord. Above the shoulders nerves usually travel directly to the brain. Information about sensations we feel like pain, temperature and touch travel from each structure along nerves, specific for that information, to the spinal cord and brain. Information is carried in the form of electrical impulses. Nerves also carry impulses in the other direction, from the spinal cord to the structure. These convey commands from the brain that control movement. In the case of pain, more nerves transmitting impulses and the faster these impulses are transmitted to the brain, the stronger the pain.

The nerve supply to the lower birth canal is quite distinct from that of the uterus. This explains the different location and nature of the pain in the 1st (contraction phase) and second (delivery phase) stages of labour.

Information travelling to and from the spinal cord along nerves can be temporarily blocked if the nerve is bathed in local anaesthetic drug. Every nerve in the body supplies its own set of structures or region. When a nerve is blocked the effect is to "numb" that nerve’s region. If local anaesthetic is injected into the skin a patch of numbness will result. This patch represents all of the individual regions of small nerves in the skin near the injected anaesthetic. This is how the skin near the vagina is made numb for an episiotomy.

Large nerves can be blocked high up before they branch into many small nerves. By doing this, a much larger area of numbness can be achieved from a single injection. Nerve blocks given by dentists are examples of these. Blockade of the pudendal nerve is sometimes performed by obstetricians during delivery as a means of anaesthetising the lower birth canal for an instrumental delivery.

The nerves that carry sensation from the lower half of the body including the uterus, lower birth canal and legs, can be blocked by injecting local anaesthetic near them as they leave the spinal cord together. This happens in an epidural. Epidurals work by depositing local anaesthetic drugs in the path of large nerves as they enter and leave the spinal cord. This occurs in an area just outside of the spinal cord called the epidural space.

Nerves vary in thickness. Pain and temperature nerves are thin walled and small. They are affected by local anaesthetic drugs in low concentrations. On the other hand, movement or ‘motor’ nerves are larger and require higher concentrations of local anaesthetic drugs to be blocked. Nerves responsible for touch sensation lie somewhere in the middle. This explains why, when high concentration local anaesthetic is given in epidurals, the legs and vagina are completely numb and you may not be able to push. It also explains how low concentration epidurals can provide good pain relief without necessarily making your legs numb and still allowing you to push during delivery.