Somatosensory Pathways

Objectives

To describe the course and function of the dorsal column pathways.
To describe the course and function of the spinothalamic pathways.
To describe the course and function of the spinocerebellar pathways.
To describe the course and function of the trigeminal sensory pathways.

Materials

Group Supplies:

· cutouts of cross sections of spinal cord, medulla, pons, cerebellum, thalamus and cerebral cortex

· four colors of string to represent axons

· four colors of beads to represent cell bodies

· Descriptions of the dorsal column parthways and spinothalamic tracts are available as a supplement: Somatosensory and Pain Pathways

Methods

Construct each pathway using a different color. Include the cell bodies, axons, and synaptic sites for each pathway. Label each nucleus and tract as they are relevant.

Use the tables that follow as a guide.

Dorsal Column Pathways

Neuron	Source	Route	Destination

First Order Neurons	spinal nerves and Posterior Root Ganglia from lower body (T7 and below)	Fasciculus Gracilis	Nucleus Gracilis
First Order Neurons	spinal nerves and Posterior Root Ganglia from upper body (T6 and above)	Fasciculus Cuneatus	Nucleus Cuneatus

Second Order Neurons	Nucleus Gracilis	contralateral Medial Lemniscus	contralateral Ventral Posterolateral Thalamus
Second Order Neurons	Nucleus Cuneatus	contralateral Medial Lemniscus	contralateral Ventral Posterolateral Thalamus

Third Order Neurons	contralateral Ventral Posterolateral Thalamus	Internal Capsule	Primary Sensory Cortex

Fourth Order Neurons	Primary Sensory Cortex
Fourth Order Neurons	Primary Sensory Cortex

Spinothalamic Pathways

Neuron	Source	Route	Destination

First Order Neurons	each spinal nerve and Posterior Root Ganglia	Lissauer's Tract	Posterior Horn

Second Order Neurons	Posterior Horn	contrlateral spinothalamic tract	contralateral Ventral Posterolateral Thalamus

Third Order Neurons	contralateral Ventral Posterolateral Thalamus	Internal Capsule	Primary Sensory Cortex

Fourth Order Neurons	Primary Sensory Cortex
Fourth Order Neurons	Primary Sensory Cortex

Trigeminal Sensory Pathways

Neuron	Source	Route	Destination

First Order Neurons	Trigeminal Nerve and Semilunar Ganglion (touch, pressure, vibration)	Pons	Principal Sensory Trigeminal Nucleus
	Trigeminal Nerve and Semilunar Ganglion (pain, temperature, and noxious mechanical stimulation)	Pons	Spinal Trigeminal Nucleus
	Trigeminal Nerve and Semilunar Ganglion (muscle stretch)	Pons	Mesencephalic Trigeminal Nucleus

Second Order Neurons	Principal Sensory Trigeminal Nucleus	contralateral Trigeminal Lemniscus	contralateral Ventral Posterolateral Thalamus
Second Order Neurons	Spinal Trigeminal Nucleus	contralateral Trigeminal Lemniscus	contralateral Ventral Posterolateral Thalamus
	Mesencephalic Trigeminal Nucleus	contralateral Trigeminal Lemniscus	contralateral Ventral Posterolateral Thalamus

Third Order Neurons	contralateral Ventral Posterolateral Thalamus	Internal Capsule	Primary Sensory Cortex

Fourth Order Neurons	Primary Sensory Cortex
Fourth Order Neurons	Primary Sensory Cortex

Discussion

What sensory information is carried by each of the pathways?
Where are the receptive fields for the various pathways?
Do any of the pathways cross over? If so, where?
What is the significance of crossing over?
What is the relationship between the various receptive fields and the somatotopic organization of the cerebral cortex.

Cutouts

Cerebral Cortex (Parietal), Caudal Thalamus

Rostral Midbrain

Caudal Midbrain

Pons

Rostral Medulla

Spinal Cord