Somatic Sensory Pathways

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  • 0:01 Somatic Senses
  • 1:05 Ascending Pathways
  • 3:26 Descending Pathways
  • 5:19 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

How does information travel between body and brain? In this lesson, we'll explore somatic sensory pathways, including ascending and descending tracts, afferent and efferent nerves, and how they work together in the body.

Somatic Senses

Patrick has a problem. He had an accident, and he can't feel anything from his right leg or foot. If you tickle his left foot, he laughs, but his right foot doesn't get any reaction from him. Likewise, if his left leg brushes up against something hot, he pulls it away and says, 'Ouch!' But, if his right leg brushes up against something hot, he doesn't feel it and so doesn't pull it away.

Somatic senses are the senses that have to do with touch. Tickling and pain, like on Patrick's legs, are somatic senses, but so are other things that you might not think of right away, like temperature and movement. Somatosensory pathways relay information between the brain and nerve cells in the skin and organs. For example, these pathways are how Patrick knows that someone is tickling his left foot. But, why doesn't he feel tickling on his right foot?

To find out, let's look closer at somatosensory pathways, including the difference between ascending and descending pathways.

Ascending Pathways

Patrick can't really feel anything in his right leg or foot. As we've seen, this can be a problem, like when his right leg brushes up against something hot, and Patrick doesn't feel it, so he doesn't pull his right leg away, and risks injury.

The problem that Patrick is experiencing is with his ascending somatosensory pathway, which is sometimes called the afferent pathway. This is a series of nerves that send information to the brain from the body. Think about the word ascending, which means going up, and you can remember that the ascending pathway sends information up to the brain.

The nerves the connect the body to the spinal cord and the spinal cord to the brain are called afferent nerves, and they send information from the body to the brain. You can remember afferent pathway and afferent nerves by thinking about the letter a: ascending and afferent both start with a, and they are the same somatosensory pathway.

Let's look at an example of the ascending pathway. In most people, an afferent pathway might send sensory information from the right leg to the brain so that they understand what their right leg is experiencing. Of course, for Patrick, that particular ascending pathway isn't working correctly, even though the afferent pathway for his left leg is working fine.

How does the ascending pathway normally work? Information goes from the body part to the spinal cord. From there, it goes up the spinal cord to the brain. As it enters the brain, it shifts to the opposite side, and then goes all the way up to the top of the brain where it settles in the somatosensory cortex, or the part of the brain dedicated to somatic sensory information.

Let's look at that in Patrick's body. His left leg (the one that works normally) might send information to his spinal cord about how scratchy his wool pants are. This information then travels up Patrick's spinal cord. Just as it enters his brain, it crosses over to the right side of his brain and then goes up into the somatosensory cortex, where his brain registers that the sensation he's feeling is scratchiness because of the wool pants.

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