Answer: The anterolateral system is an ascending somatosensation tract that carries pain and temperature information from the skin to the brain.
The anterolateral system refers to a set of neurons that send signals towards the brain. These neurons carry information regarding temperature and pain, but also crude touch. Activity at this pathway often has a salient motivational component, since painful stimuli or temperature information encourages an action - get away from the painful stimuli or the hot object. Additionally, the brain needs to learn to avoid these potentially damaging stimuli.
The first order sensory neurons have their receptors in the skin, and the cell bodies in the dorsal root ganglion. These cells project into the spinal cord, where they can ascend or descend one or two segments via the posterolateral tract (also called Lissauer’s tract). Then, these neurons synapse onto the second order neurons in the substantia gelatinosa or the nucleus proprius. These cells send their axons across the midline to the other half of the spinal cord, which is the process of decussation. Then, these second order neurons ascend towards the brain, where they synapse onto the third order neurons in the rostral ventromedial medulla. This incoming pain and temperature information then gets passed to the thalamus before eventual processing in the cortex.
At the level of the skin, the first order neurons are sensitive to crude touch, temperature, and pain.
Crude touch information is usually not able to be identified very well. It is produced when an object is in contact with the skin. The two point discrimination for crude touch is very large. Compare this with fine touch, which can identify textures and dots very close together (as in Braille).
The sensation of pain is detected by free nerve endings. A painful stimulus activates specialized receptor cells called nociceptors. Painful stimuli signal to the brain that tissue damage may occur if the muscles are not moved away from the offending stimulus.
Temperature information is also detected by free nerve endings. The receptors that respond to temperature are called thermoreceptors. Cold information is sent to the brain via two types of nerve fibers, the slower C-fibers as well as the faster A delta fibers. A delta fibers transmit information faster because they are myelinated.
The anterolateral system is also called the ventrolateral system or the spinothalamic tract.
Compare this somatosensation pathway with the dorsal column-medial lemniscus pathway, which sends fine touch, vibration, and proprioception information into the brain. Whereas the dorsal column-medial lemniscus pathway decussates at the level of the brain, the anterolateral system decussates at the level of the spinal cord. This means that an injury to one lateral half of the spinal cord may result in a loss of fine touch or vibration information from the same side of the injury (ipsilateral) with a loss of pain or temperature sensation from the side opposite the injury (contralateral). This is referred to as Brown-Sequard syndrome.