Sensory Pathways

Specific regions of the CNS coordinate different somatic processes using sensory inputs and motor outputs of peripheral nerves. A simple case is a reflex caused by a synapse between a dorsal sensory neuron axon and a motor neuron in the ventral horn. More complex arrangements are possible to integrate peripheral sensory information with higher processes. The important regions of the CNS that play a role in somatic processes can be separated into the spinal cord brain stem, diencephalon, cerebral cortex, and subcortical structures.

A sensory pathway that carries peripheral sensations to the brain is referred to as an ascending pathway, or ascending tract. Each of the various sensory modalities follows a specific pathway through the CNS. Tactile and other somatosensory stimuli activate receptors in the skin, muscles, tendons, and joints throughout the entire body. However, the somatosensory pathways are divided into two separate systems on the basis of the location of the receptor neurons.

Somatosensory stimuli from below the neck pass along the sensory pathways of the spinal cord, whereas somatosensory stimuli from the head and neck travel through the cranial nerves—specifically, the trigeminal system. The dorsal column system (sometimes referred to as the dorsal column–medial lemniscus) and the spinothalamic tract are two major pathways that bring sensory information to the brain (Figure 1). The sensory pathways in each of these systems are composed of three successive neurons.  First order neurons are sensory neurons that detect the stimulus.  Sensory neurons that pass through a spinal nerve have their cell bodies in the dorsal root ganglion.  Second order neurons are interneurons whose cell bodies are in the spinal cord or brainstem that relay the signal along an ascending path through the central nervous system.  Third order neurons have their cell body in the thalamus, and carry the signal to the appropriate area of the cerebrum.

Figure 1. Ascending Sensory Pathways of the Spinal Cord. The dorsal column system and spinothalamic tract are the major ascending pathways that connect the periphery with the brain.

Dorsal Column System

The dorsal column system begins with a first order sensory neuron sensitive to fine touch, vibration, pressure, and proprioception.  The cell body of that neuron resides in the dorsal root ganglion, and its axon passes through the dorsal root and into the dorsal column of white matter in the spinal cord. As axons of this pathway enter the dorsal column, they take on a positional arrangement so that axons from lower levels of the body position themselves medially, whereas axons from upper levels of the body position themselves laterally.

The dorsal column is separated into two component tracts, the fasciculus gracilis that contains axons from the legs and lower body, and the fasciculus cuneatus that contains axons from the upper body and arms. The axons in the dorsal column terminate in the nuclei of the medulla, where each synapses with the second order neuron in their respective pathway. The nucleus gracilis is the target of fibers in the fasciculus gracilis, whereas the nucleus cuneatus is the target of fibers in the fasciculus cuneatus.

The second order neuron in the system projects from one of the two nuclei and then decussates, or crosses the midline of the medulla. These axons then continue to ascend the brain stem as a bundle through a tract called the medial lemniscus. These axons terminate in the thalamus, where each synapses with the third neuron in their respective pathway.

The third neuron in the system projects its axons to the postcentral gyrus of the cerebral cortex, where somatosensory stimuli are initially processed and the conscious perception of the stimulus occurs.

Spinothalamic Tract

The spinothalamic tract begins with sensory neurons for crude touch, temperature and pain whose cell bodies are in a dorsal root ganglion. These neurons extend their axons to the dorsal horn, where they synapse with the second neuron in their respective pathway. The name “spinothalamic” comes from this second order neuron, which has its cell body in the spinal cord gray matter and connects to the thalamus. Second order neurons decussate within the spinal cord and ascend through one of two spinothalamic tracts.  Those carrying signals for crude touch sensations have axons in the ventral spinothalamic tract, while those transmitting signals for temperature and pain pass through the lateral spinothalamic tract (Figure 1). The second order neurons of both spinothalamic tracts ascend to the brain and enter the thalamus, where each synapses with the third neuron in its respective pathway.  The third order neurons in the thalamus then project their axons to the postcentral gyrus of the cerebral cortex.

Comparing Dorsal Column and Spinothalamic Pathways

The dorsal column system is primarily responsible for touch sensations and proprioception, whereas the spinothalamic tract pathway is primarily responsible for pain and temperature sensations. Another similarity is that the second neurons in both of these pathways are contralateral, because they project across the midline to the other side of the brain or spinal cord. In the dorsal column system, this decussation takes place in the brain stem; in the spinothalamic pathway, it takes place in the spinal cord at the same spinal cord level at which the information entered. The third neurons in the two pathways are essentially the same. In both, the second neuron synapses in the thalamus, and the thalamic neuron projects to the somatosensory cortex.

Somatosensory and Special Sense Information Pathways for Cranial Nerves (Excluding Vision and Sound)

The trigeminal pathway carries somatosensory information from the face, head, mouth, and nasal cavity. As with the previously discussed nerve tracts, the sensory pathways of the trigeminal pathway each involve three successive neurons.

First, axons from the trigeminal ganglion enter the brain stem at the level of the pons. These axons project to one of three locations. The spinal trigeminal nucleus of the medulla receives information similar to that carried by spinothalamic tract, such as pain and temperature sensations. Other axons go to either the chief sensory nucleus in the pons or the mesencephalic nuclei in the midbrain. These nuclei receive information like that carried by the dorsal column system, such as touch, pressure, vibration, and proprioception.

Axons from the second neuron decussate and ascend to the thalamus along the trigeminothalamic tract. In the thalamus, each axon synapses with the third neuron in its respective pathway.

Axons from the third order neuron then project from the thalamus to the primary somatosensory cortex of the cerebrum.

The sensory pathway for gustation travels along the facial and glossopharyngeal cranial nerves, which synapse with neurons of the solitary nucleus in the brain stem.  Axons from the solitary nucleus then project to the ventral posterior nucleus of the thalamus.

Finally, axons from the ventral posterior nucleus project to the gustatory cortex of the cerebral cortex, where taste is processed and consciously perceived.