OBJECTIVES
Blair Turner, Ph.D.
Salvatore Rapisardi, Ph.D.
Define:
Practice tracing the borders of dermatomes V1-V3 and C2-C5.
How are dermatomes and receptive fields related?
How is peripheral innervation density related to receptive field size?
What is meant by two point discrimination?
How is it related to peripheral innervation density and receptive field size?
How does peripheral innervation density distort the dermatomal
What is meant by the Fine Touch System?
Be able to trace its connections to the cerebral cortex.
What is meant by the Pain/Temperature/Coarse Touch System?
Be able to trace its connections to the cerebral cortex.
LECTURE
There are two systems that bring sensory information from the skin, muscles and joints of the body to the spinal cord and brainstem, and from there to the brain. We will refer to the first as the Fine Touch System and the second as the Pain/Temperature/Coarse Touch System. They differ from each other both functionally and anatomically. In this course both systems will be classifies as General Somatic Afferent (GSA).
The Fine Touch System. This system is also called the discriminative touch system. It carries tactile information in a manner that allows us to discriminate between two points on the skin that are very close together. It also carries information about pressure from the skin and proprioceptive information from the joints and muscles.
The receptors of this system are specialize or free nerve endings that are located in the skin, skeletal muscles and joints of the body. The receptors are at the endings of the sensory axons of peripheral nerves. These axons travel in the nerves and eventually join a spinal nerve or nerve of the face.
The primary sensory neurons are unipolar neurons each of which has a cell body and a long bifurcating axon. The cell bodies are located in the dorsal root ganglia of the spinal cord, or the trigeminal ganglion of the head. The peripheral branch of the axon travels in a peripheral nerve to innervate a small section of skin. The central branch of the axon enters the spinal cord via a dorsal root, or brainstem and terminates there on cell bodies of other neurons that relay the touch information up to the cerebral cortex. These second order relays cross to the other side of the central nervous system. These neurons end in the thalamus on a third order relay, which sends the touch/pressure information to the postcentral gyrus of the parietal lobe.
The Pain/Temperature/Coarse Touch System. This system carries pain and temperature as well as tactile information, which we refer to as crude or non-discriminative touch because if two points of contact on the skin are close together this system registers them as one. Compared to the Fine Touch System, the cell bodies of these neurons are smaller, the axons are thinner in diameter and do not conduct as quickly.
There are no known specialized receptors for this system, the free nerve endings of the peripheral axons being sensitive to pain and temperature.
The primary sensory neurons are smaller than those of the Fine Touch System, but otherwise the location of their cell bodies and the distribution of their peripheral axons is similar. However, the termination of the central branch of this axon differs from the Fine Touch System, ending on cell bodies of second order relays that are located in different parts of the spinal cord or brainstem. Again, the second order relays cross to the other side of the central nervous system. Many of these terminate in the central core of the brainstem (the reticular activating system). The rest of the second order neurons end in the thalamus on a third order relay. Some of these third order relays travel to the postcentral gyrus of the parietal lobe.
Fill in the following table, from graphic information on the rest of this page, to help you compare the key neural features of the sensory systems for the body, face, and oral cavity.
Receptor and Modality Functional Component |
Name of Dermatome |
Location of Cell Body |
Site of Termination of Primary Afferent |
Location of Representation on Postcentral Gyrus |
| Fine touch/ pressure | ||||
| Body | ||||
| Face | ||||
| Oral cavity | ||||
| Coarse touch/ pain/temperature | ||||
| Body | ||||
| Face | ||||
| Oral cavity |
From the home page of this web, review the definition of dermatome from the Neuroglossary.
DERMATOMES OF THE HEAD AND NECK
The following figure shows the pathways over which sensory information from the surface of the body is relayed to the contralateral cerebral hemisphere. The site of the third order relay (the thalamus) has been omitted for this course.
Now return to the home page of this web and read the definition of "Receptive Field". Note also the list of the nerves supplying each dermatome on the right of the following figure. Identify them on the cadaver.
The following figure shows the pathways over which sensory information from the surface of the face is relayed to the contralateral cerebral hemisphere. The site of the third order relay (the thalamus) has been omitted for this course.
INNERVATION OF THE ORAL CAVITY
The following figure shows the pathways over which sensory information from the surface of the oral cavity is relayed to the contralateral cerebral hemisphere. The site of the third order relay (the thalamus) has been omitted for this course.
Now return again to the home page of this web and read the definition of "Receptive Field", paying particular attention to the last half of it. Now study the figure below, which displays a functional map of the body surface on the postcentral gyrus that shows the relative amount of cortex devoted to each region. Note that the rostral dermatomes of the body as we see it (see first figure on this page) are located ventrally on the postcentral gyrus, and caudal dermatomes dorsally. What does this mean? Notice also that the functional dermatomal map is distorted. How is this explained? Use your answer to explain how the dermatomal map of the body as we see it (refer again to the first figure on this page) is a distortion of an underlying neural reality. Therefore, how useful is the definition of "dermatome" presented in the Neuroglossary?
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