Question

1. Compare the anatomy of the dorsal column system and the anterolateral system and the way that information in each system reaches the cortex. What types of sensation does each carry? Where are the primary afferent neurons located? Where are the tracts and what are they called? Where are the synapses? In what area of cortex do the projections terminate? Are they crossed or uncrossed?

2. somatic motor column, and describe the nuclei found in that column. Specify the cranial nerves that they are connected to, the names and locations of the nuclei within the brainstem, and the function of the nuclei.

3. describe its organization the organization of the olfactory system. What are the receptors? What nerve or nerves carry information to the CNS and where does it (or they) terminate? What areas of the thalamus and cortex are involved?

Answer 1

1. Anatomy of Dorsal column system: Dorsal column system carry all sensory information of whole body through the sensory nerve fibre. The sensory nerve fibre enters in the spinal cord through dorsal roots of the spinal column. The first order neuron from the dorsal root ganglion projects into the fasciculus gracilis(medial portion of spinal cord), that carry information from the lower portion of the body and into the fasciculus cuneatus, that carry information from the upper part of the body. From this area the neurons are project directly into the caudal medulla. before medulla region this dorsal column system doesn't cross the midline of the spinal cord to the opposite side. The projection from the spinal cord synapes in the nucleus cuneatus (for the information from upper body) and in the nucleus gracilis (for the information of lower body). After synapsing in the medulla, the nerve fibre crosses the midline as internal arcuate fibres, and goes on the opposite side of the spinal cord. After crossing the midline the nerve fibre ascend the rostral medulla, pons and midbrain as medial leminiscus and terminates into the ventral posterior lateral nucleus of thalamus (VPL). From VPL the sensory fibre projects in the somatosensory cerebral cortex for the perception of the sensory signal. This total column of tract from the dorsal root of the spinal cord termed as dorsal system.

Anatomy of Anterolateral system: In this type of somatosensory pathway, the sensory fibres enters through the dorsal root as similar to dorsal column system,the first order neuron from the dorsal root ganglion directly enters in the grey ,matter of the spinal cord, but the second order neuron in the central grey matter crosses the midline and enters into the anterior commissure of the white matter and ascend laterally to the medulla. This why this pathway is named as Anterolateral system. In Dorsal column system second order neuron doesn't crosses the midbrain in the spinal region. The ascending anterolateral fibre ascends medulla, pons and midbrain anterolaterally. This ascending fibre also projects into reticular system (spinoreticular tract), mesencephalon (spinomesencephalic tract), thalamus (spinothalamic tract) and ultimately synapes into ventral posterior lateral nucleus of thalamus (VPL). and then somatosensory cortex.

Type of sensation carry: The Dorsal column system carries sensory signals of vibration, Propioception, Light touch. The Anterolateral system carries signals of pain, temperature crude touch to the somatosensory cortex.

As described the primary afferent neurons of both tract/system located in the dorsal root ganglion (DRG) of the spinal cord.

In the dorsal system Medial leminiscus is the main tract that carry sensory signal to the cortex. and in the Anterolateral system there are several tract are present like, spinoreticular tract that carry emotion and arousal signal, spinomesencephalic tract  modulates pain sensation, and spinothalamic tract carry pain and temperature signals.

The synapes of the both systems lies in the Dorsal column system are, dorsal root ganglion, fasciculus gracilis or fasciculus cuneatus, nucleus cuneatus of medulla, in pons, midbrain and in VPL.

In case of Anterolateral system the synapes are lies in the dorsal root ganglion, anterolateral white matter, medulla, pons, midbrain, Thalamus.

In the somatosensory cortex area of the cerebral cortex, these two system terminate their projection.

Crossed or Uncrossed: The both systems are crossed but at the different level. in case of Dorsal column system the tract is crossed at the caudal medulla region, but in case of Anterolateral system the tract crosses the midline of spinal cord in the spinal cord region through the anterior commissure in to the anterolateral white matter.