Final answer:
Peripheral nerves contain thermoreceptors and nociceptors that transmit painful stimuli to the brain. The spinothalamic tract in the spinal cord is primarily responsible for carrying pain and temperature sensations and crosses the midline at the level it enters the spinal cord.
Step-by-step explanation:
The two types of peripheral nerve fibers that conduct painful stimuli are thermoreceptors and nociceptors. Thermoreceptors are sensitive to temperature changes and can be stimulated when local temperatures differ from the body temperature. Nociceptors, on the other hand, are responsible for the sensation of potentially damaging stimuli, such as mechanical, chemical, or thermal stimuli beyond a set threshold. These receptor proteins in nociceptors are activated by chemicals released from stressed or damaged tissues and are crucial for the sensation of pain.
In the spinal cord, pain and temperature sensations are primarily carried by the spinothalamic tract, which crosses the midline at the level at which it enters the spinal cord. This tract, in conjunction with the trigeminal nerve which adds similar sensation information from the head, is responsible for transmitting pain signals to the brain. Notably, the spinothalamic tract is different from the dorsal column system, which carries information about touch and proprioception and decussates in the medulla.