Somatosensory System Pathways

• CAM304 covers the neuroanatomy of the somatosensory system

Learning Outcomes

• Expected to describe the central projections of primary sensory pathways in detail
• Must identify and name the nuclei associated with these major sensory pathways
• Expected to predict the impact of lesions on components of these pathways

Somatosensory Pathways

• Posterior (Dorsal) column-medial lemniscal pathway deals with conscious proprioception, 2-point discrimination, touch, and vibration
• Anterolateral system (major component: spinothalamic tract) involves pain (sharp and dull), temperature, and crude touch
• Spinocerebellar pathways handle non-conscious proprioception and touch

Posterior (Dorsal) Column - Medial Lemniscus Pathway

• Primary afferents (1°) are mechano-receptors
• Their cell bodies are in the posterior root ganglia
• These afferents ascend ipsilaterally to the medulla

Spinal Cord

• Fibers are arranged somatotopically
• T6 and below is the fasciculus gracilis
• Above T6 is the fasciculus cuneatus

Medulla

• Primary afferents terminate in the nucleus gracilis and nucleus cuneatus
• Cuneate and Gracile tubercles are swellings on the dorsal surface of the caudal medulla, caudal to the obex of the 4th ventricle
• Internal arcuate fibers cross over to the contralateral side to form the medial lemniscus
• 2nd order neurons are located in the caudal medulla (nuclei gracilis, nuclei cuneatus)

Medial Lemniscus

• In the medulla, the head of the medial lemniscus is posterior, and the legs are anterior
• HAL slips over
• In the pons, the head is medial, and the legs are lateral

Rostral Pons

• Head is medial
• Legs are lateral

Midbrain

• Head is anterior
• Legs are posterior

Medial Lemniscus Organization

• Somatotopic organization exists in the medial lemniscus from the medulla to the thalamus
• Fibers from the head travel in the trigeminal system
• Upper and lower limbs are represented

Thalamus

• 3rd order neurons are in the Ventroposterolateral (VPL) nucleus of the thalamus

Sensory projections from VPL/VPM

• Sensory projections from VPL and VPM of the thalamus travel in the posterior limb of the internal capsule
• They terminate in the somatosensory cortex (VPL/VPM)

Trigeminal Touch Pathway

• This processes proprioceptive and touch input from the face
• 1° cell bodies are in the trigeminal ganglion
• 2° are in the main sensory nucleus of V
• Axons from neurons in the main sensory nucleus of V cross over at the mid-pons and enter the medial lemniscus
• Fibers terminate at the VPM (3° neurons)
• Fibers from VPM project to the primary somatosensory cortex

Somatosensory Cortex

• The somatotopic map of the human somatosensory cortex is also known as the homunculus

Somatosensory Association Cortex

• Further processing of sensory information occurs here
• Stereognosis involves recognizing objects by tactile shape
• Graphesthesia refers to figure writing on the skin

Damage to Posterior Column-Medial Lemniscal Pathway

• Results in impairment of proprioception and discriminative tactile function
• Leads to a distinctive type of ataxia: incoordination of movement due to inadequate sensory feedback

Summary Points

• The two major components of the dorsal column are the fasciculus gracilis and fasciculus cuneatus
• The termination site of these fasciculi is in the medulla
• The formation of the medial lemniscus and the topographical representation within this pathway are important
• Final location of the information in the medial lemniscus is also key