Long Sensory Pathways (Somatic Sensation)
Dental Neuroanatomy; 1/31/13
David A. Morton, Ph.D.
Objectives

Describe the sensory modalities mediated by the ALS and draw the ALS pathway from the
periphery to the cortex locating the first, second, third order neurons, and the crossing
(decussation).

Identify cranial nerve motor nuclei that are near the ALS pathway and that might be supplied by
the same artery.

Define a dermatome and explain why they are useful. Know the dermatomes represented at the
level of the back of the head, shoulder, thumb, middle finger, small finger, nipple, umbilicus,
inguinal ligament, big toe, small toe and anus.

Explain the sensory loss with a pathological enlargement of the central canal at the level of C 5,6,7.
Why might there be atrophy of the hand muscles?

Define the term somatotopic and explain how this applies to the cerebral cortex.

Explain why there is a larger area of the postcentral gyrus processing information from the hand
than from the foot.

A vascular lesion of which cerebral artery would result in loss of somatic sensation from the hand?
From the foot?

Locate the ALS tract on cross section of cord, medulla, pons and midbrain.

Compare and contrast the differences and similarities between the ALS and DCML pathways. Do
this from memory using the drawing from the Haines atlas diagram at the end of today’s notes.

Explain the term sensory dissociation and explain why the concept is useful.

Name the arteries that supply the DCML pathway in the: spinal cord, medulla, pons, and cerebral
cortex.

Explain how you would distinguish between a peripheral sensory loss and a central sensory loss.
How would they differ if they occurred in different regions such as peripheral nerve, a root, spinal
cord, medulla, midbrain, thalamus and cerebral cortex.

Explain why an extradural tumor pressing on the left side of the spinal cord at spinal level T8 only
produced loss of pain and temperature and not other sensations from the leg.
1
Somatic Sensation Pathway Overview

How many neurons are involved in somatic sensation?

What structures are involved in these pathways?

Will decussation occur? If so, where?

Describe the neurons involved:
Somatic Sensation

General sensation.
2
Anterior Lateral System (ALS)

1° Order neuron
o Location of cell body.
o Location of synapse.
o Collaterals.
o Reflex connections.
3
Anterior Lateral System (ALS)

2° Order neuron
o Location of cell body.
o Decussation.
o Course of axons.
o Location of synapse.
4
Anterior Lateral System (ALS)

3° Order neuron
o Location of cell body.
o Course of axons.
o Location of synapse.
5
Primary Somatosensory Cortex

Brodmann’s areas (1, 2, 3)

Somatotopic organization.

Homunculus.

Contrast cortex area for hand to elbow.

A vascular lesion of which cerebral artery would result in loss of somatic sensation for hand? foot?
6
Anterior Lateral System (ALS) - Trace
7
Dorsal Column-Medial Lemniscus (DCML)

1° Order neuron
o Location of cell body.
o Location of synapse.
o Collaterals.
o Reflex connections.
8
Dorsal Column-Medial Lemniscus (DCML)

2° Order neuron

Location of cell body.

Decussation.

Course of axons.

Location of synapse.
9
Dorsal Column-Medial Lemniscus (DCML)

3° Order neuron

Location of cell body.

Course of axons.

Location of synapse.
10
Dorsal Column-Medial Lemniscus (DCML) - Trace
11
ALS and DCML

Locate the ALS and DCML on the following sections:
Spinal cord
Medulla
12
Pons
Midbrain
13
Diencephalon (Thalamus)
Cerebrum
14