The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017

Afferent Tracts

Nerves and tracts travel in two basic directions within the central nervous system (CNS), toward—which is called afferent—and away from—which is called efferent. This makes up the two general types of pathways within the CNS, which are essential for proper functioning throughout the body. Afferent and efferent tracts can be remembered by A for “afferent connections arrive” and E for “efferent connections exit.” For afferent tracts, the general function is to bring sensory information from the body to the brain.

Anatomy and Function

Afferent tracts carry sensory information to the brain, where the sensory information is then interpreted. This sensory information tells an animal important information about its surroundings, such as the temperature or if a predator is nearby. It also gives the animal information about its internal organs that are necessary for survival, such as whether its stomach is empty and it needs to eat. The traditional sensory or afferent information includes sight, smell, sound, taste, and touch. However, there are other sensory modalities, which include acceleration and balance, kinesthetic sense, pain, proprioception, temperature, time, and visceral senses (interoception).

The afferent pathways from the sensory neurons to the brain generally include three neurons: a first-order neuron, a second-order neuron, and a third-order neuron. The first-order neuron receives a stimulus and sends the signal to the spinal cord. Within the spinal cord the neuron passes the information on to a second-order neuron, which will carry the information up the spinal cord to the thalamus. In the thalamus the signal will be passed to a third-order neuron that will take the signal to a specific region of the brain that will process the information.

Specific Afferent Tracts

Within the spinal cord, afferent tracts are also called ascending tracts. These tracts are divided into groups that carry similar sensory information. Some of these tracts are the gracile fasciculus, cuneate fasciculus, and spinothalamic. These tracts are named for where the neuron’s cell body is located and where it will synapse on to the next neuron. These pathways can vary on which side of the spinal cord the signal travels after being received in the body. Some of the pathways enter the spinal cord and travel on the contralateral side to the brain. Other pathways enter the spinal cord and travel on the ipsilateral side and then cross in the brain.

Pain, however, is considered to be one of the most important sensory systems; this is because an animal must know if its body is hurt and how to self-preserve. Thus, pain afferent tracts are brought to both sides of the spinal cord. When pain occurs, the afferent tracts on the ipsilateral side of the pain will carry the information toward the brain as well as send the information to the contralateral side of the spinal cord, so it too can send the pain information to the brain. This allows pain to have a “backup” system to ensure that the pain information reaches the brain for interpretation.

The gracile fasciculus (Latin meaning “thin bundle”) receives signals from the mid-thoracic and lower parts of the body. Below the sixth thoracic vertebra, the gracile fasciculus makes up the posterior column of the spinal cord. This pathway carries vibration, visceral pain, deep touch, discriminative touch, and proprioception signals for the lower limbs and lower trunk of the body. After the sixth vertebra the posterior column of the spine becomes split between the gracile fasciculus in the medial portion and the cuneate fasciculus in the lateral portion of the column. The cuneate fasciculus (Latin meaning “wedge-shaped bundle”) is responsible for signals from the upper limbs and chest. The cuneate fasciculus is larger than the gracile fasciculus because in mammals (and particularly in humans) the forelimbs or arms carry much more sensory information than the hind limbs or legs. This is because the hands are used to grasp small items and have more sensory receptors than most body regions. The types of signals that the cuneate fasciculus pathway carries are the same as those of the gracile fasciculus. From the sixth thoracic vertebra up, the posterior column is divided unequally between the cuneate fasciculus and the gracile fasciculus. These two pathways lead to the ipsilateral side of the medulla oblongata of the brainstem. From the medulla oblongata the gracile and cuneate pathways cross and form the medial lemniscus, which leads to the contralateral thalamus. From here, third-order neural fibers lead to the cerebral cortex of the brain where the sensory information will be processed.

The spinothalamic tract is another major tract of the afferent nervous system. This tract along with some smaller tracts form the anterolateral system in the spinal cord. This tract is responsible for carrying signals for pain, temperature, pressure, tickle, itch, and light or crude touch. Within the anterolateral system, the first-order neurons end in the posterior horn of the spinal cord where the second-order neurons carry the signals to the contralateral side of the spinal cord and up the cord to the thalamus. The signals are then passed on to third-order neurons that take the information to the cerebral cortex to be processed.

Stephen Mazurkivich and Jennifer L. Hellier

See also: Discriminative Touch; Nociception; Sensory Receptors; Somatosensory Cortex; Somatosensory System; Taste System; Touch; Vestibular System

Further Reading

Spilman, Bernard. (2003). Ascending spinal tracts. In Neurokinesiology: A new path to human health. Retrieved from http://neurokinesiology.nuxit.net/Neurological_Background/ascending_spinal_tracts.html