Sensory Pathways for Transmitting Somatic Signals into the Central Nervous System
Almost all sensory information from the somatic segments of the body enters the spinal cord through the dorsal roots of the spinal nerves. However, from the entry point into the cord and then to the brain, the sensory signals are carried through one of two alter-native sensory pathways: (1) the dorsal column–medial lemniscal system or (2) the anterolateral system. These two systems come back together partially at the level of the thalamus.
The dorsal column–medial lemniscal system, as its name implies, carries signals upward to the medulla of the brain mainly in the dorsal columns of the cord. Then, after the signals synapse and cross to the oppo-site side in the medulla, they continue upward through the brain stem to the thalamus by way of the mediallemniscus.
Conversely, signals in the anterolateral system, immediately after entering the spinal cord from the dorsal spinal nerve roots, synapse in the dorsal horns of the spinal gray matter, then cross to the oppo-site side of the cord and ascend through the anterior and lateral white columns of the cord. They terminate at all levels of the lower brain stem and in the thalamus.
The dorsal column–medial lemniscal system is com-posed of large, myelinated nerve fibers that transmit signals to the brain at velocities of 30 to 110 m/sec, whereas the anterolateral system is composed of smaller myelinated fibers that transmit signals at velocities ranging from a few meters per second up to 40 m/sec.
Another difference between the two systems is that the dorsal column–medial lemniscal system has a high degree of spatial orientation of the nerve fibers with respect to their origin, while the anterolateral system has much less spatial orientation. These differences immediately characterize the types of sensory information that can be transmitted by the two systems. That is, sensory information that must be transmitted rapidly and with temporal and spatial fidelity is transmitted mainly in the dorsal column–medial lemniscal system; that which does not need to be transmitted rapidly or with great spatial fidelity is transmitted mainly in the anterolateral system.
The anterolateral system has a special capability that the dorsal system does not have: the ability to transmit a broad spectrum of sensory modalities— pain, warmth, cold, and crude tactile sensations;. The dorsal system is limited to discrete types of mechanoreceptive sensations.
With this differentiation in mind, we can now list the types of sensations transmitted in the two systems.
1. Touch sensations requiring a high degree of localization of the stimulus
2. Touch sensations requiring transmission of fine gradations of intensity
3. Phasic sensations, such as vibratory sensations
4. Sensations that signal movement against the skin
5. Position sensations from the joints
6.Pressure sensations having to do with fine degrees of judgment of pressure intensity
2. Thermal sensations, including both warmth and cold sensations
3. Crude touch and pressure sensations capable only of crude localizing ability on the surface of the body
4. Tickle and itch sensations
5. Sexual sensations