Somatic Sensory Pathways

Two Pathways:– 3 neurons in pathway

• 1st, 2nd and 3rd order neurons

(from PNS through CNS)

– 2nd fiber crosses over(ipsilateral to contralateral)

1. dorsal column / medial lemniscus system

2. anterolateral system

Somatic Sensory Pathways

Fig. 13.4

Fig. 7- 2 Ganong

Dorsal Column / Medial Lemniscus

System

• Signals ascend spinal cord via the dorsal (white) columns.– large myelinated fibers

(types I and II)

• fine touch and proprioception– rapid transmission – high degree of spatial

orientationFig. 13.5

Posterior Column / Medial Lemniscus System

• 1st order sensory fibers ascend ipsilaterally in dorsal (posterior) columns to the medulla, and synapse in the medulla.

• 2nd order fibers cross over in the medulla and travel through the brain stem via the medial lemniscus to the thalamus.

• 3rd order fibers project to the somatosensory areas of the cerebral cortex.– post central gyrus of the parietal lobe

Anterolateral System

• Signals ascend the spinal cord via the anterior and lateral spinothalamic tracts.

• small myelinated and unmyelinated fibers

(types III and IV)

• crude touch, pain, temperature– slow transmission– high volume of

information – low spatial orientation Fig. 13.5

Anterolateral System

• 1st order sensory neurons enter spinal cord and synapse in the posterior horn.

• 2nd order fibers cross over in the spinal cord, and ascend spinal cord to the thalamus via the anterior (ventral) and lateral spinothalamic tracts.

• 3rd order fibers project to the somatosensory areas of the cerebral cortex.– post central gyrus of the parietal lobe

Parietal LobePost Central Gyrus

• homunculus– visual representation of

the human body on the cerebral cortex

• Because of crossover, the left side of the body projects to the right side of the cerebrum and vice versa.

Fig. 14.21

Somatic Sensation

• Perception depends less on the source of the signal and more on what part of the brain is stimulated.– 2 examples from pain perception

• phantom pain• referred pain

Phantom Pain

• Amputees feel pain in their (missing) limbs.

– filling of cerebral cortical “vacuum” by other afferent signals?

– activation of cut nerve endings?

Referred Pain

• Pain from viscera is perceived as coming from the skin.– e.g., heart attack: pain in

left arm and shoulder

• related to dermatomes

Fig. 16.4

Fig. 7-7 Ganong

Regulation of Pain Sensation

• Gate Theory– In the dorsal horn, the

synapse between the first and second order neuron is a gateway for pain.

– Pain may be blocked by “closing” this gate.

• counter irritants• brain’s analgesia

system: one of its sites of action

collateralcollateralfrom touchfrom touchreceptorreceptor

gate

Fig. 16.5

Fig. 7-2 Ganong

Sensory Collaterals to the

Brain Stem Stimulate the

Reticular Activating System

Fig. 16.3

Subconscious Proprioception

• via spinocerebellar pathways

Fig. 13.4

Video AssignmentThe Brain – Parts 1 and 2

• Part 1: The Enlightened Machine• Part 2: Vision and Movement• DVD and VHS versions are available.• Each part lasts about an hour.• See p.13 of Lecture Outlines for

checkout information. • As you watch the videos, pay attention

to who has what diseases.