General And Special Somatic Afferent Pathways Notes

Somatic Afferent Pathways

General somatic afferent pathways

Somatic afferent pathways are the fibre tracts and nuclei that convey general sensory information from a wide range of receptors to the CNS. It excludes special somatic afferent pathways from the eye and inner ear, and pathways from visceral receptors. To distinguish this, they may be referred to as general somatic afferent (GSA) pathways.

The general somatic afferent system conveys sensory information such as …

• Pain

• Temperature

• Touch/pressure

• Kinaesthesia (conscious proprioception)

The primary neurons of all these senses are located within the dorsal root ganglia of the spinal nerves (and corresponding ganglion of the trigeminal nerve for structures of the head). Their axons enter the CNS by the dorsal roots of spinal nerves (or afferent root of the trigeminal nerve).

Upon entering the CNS, the axons branch. Some branches end on interneurons in the grey matter of the segment they entered, or of an adjacent segment. The interneurons project onto the ventral horn of the same or an adjacent segment. This forms the short neuron chain that allows local reflex responses. The ventral horn neuron whose axon ends directly on the effector is called the lower motor neuron.

Other branches of the primary axons connect directly, or through interneurons, to higher centres of the CNS. Ascending tract systems such as these include …

• The spinothalamic tract – transmits some pain and temperature information.

• Gracile tract – transmits touch and kinaesthesia information from hindlimb.

• Cuneate tract – transmits touch and kinaesthesia information from forelimb.

• Trigeminothalamic tract – equivalent to all above tracts, but serves the head region.

1. The lemniscal system (gracile and cuneate tracts)

The gracile and cuneate tracts are part of the lemniscal system. They consist of a three neurone contralateral (crosses midline of the body) relay.

The primary axons (running from receptors in the skin or elsewhere) enter the spinal cord and dorsal funiculus without synapsing. Those that enter through the sacral nerves are located medially, while those that enter at more cranial levels are located laterally.

A glial septum appears within the dorsal funiculus at mid-thoracic level, which divides it into two parts – the gracile fasciculus medially, which contains nerve fibres from the hindlimb and caudal trunk, and the cuneate fasciculus, which contains nerve fibres from the forelimb, neck and cranial part of the trunk.

The heavily myelinated fibres of the primary neurons run in either the gracile or cuneate tract to the dorsal medulla oblongata, where they terminate in the gracile or cuneate nuclei respectively.

The axons of the second stage neurons leave these nuclei and decussate to the contralateral medial lemniscus fibre tract, which ascends to the thalamus.

The axons of the third stage neurons leave the ventral group of thalamic nuclei and relay via the internal capsule to the somatosensory area of the cerebral cortex.

2. The extralemniscal system

The extralemniscal system conveys information along somatic afferent fibres, but propagation is slower and the localisation of the original stimuli is less precise.

1. The spinothalamic tract

The spinothalamic tract begins with a primary neuron, which conveys information from receptors in the skin or elsewhere. The peripheral nerve fibres of the primary neuron runs within the appropriate spinal nerve to the dorsal root ganglion, and the axon of the neurone enters the dorsal horn through the dorsal root of the spinal nerve, where it terminates.

The cell body of the second neuron is located in the grey matter of the dorsal horn. The axon decussates to the contralateral spinothalamic tract. The nerve fibres ascend to the brainstem in the lateral funiculus of the white matter of the spinal cord. At the brainstem, the tract merges with other tracts into the medial lemniscus to reach the ventral group of thalamic nuclei in the caudoventral thalamic nuclei.

The cell body of the third neuron is located in this ventral group of nuclei. The axon of the third neuron relays information via the internal capsule to the somatosensory area of the cerebral cortex.

2. The spinoreticular tract

The spinoreticular tract involves multi-neurone chains, and it located on both...

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