Topographic Maps

Transcript

0:00 – 0:30  [Basic Definition]  A topographic map relates specific attributes of sensation or motor control to specific subregions of a structure in the nervous system.  For example, a topographic map of the primary motor cortex shows the body parts that move when neurons in specific areas of the primary motor cortex are excited. Similarly, a topographic map of the primary visual cortex shows the locations where neurons increase activity when visual stimuli are presented at specific locations in the visual field.  In addition to the cerebral cortex, topographic maps are found throughout the nervous system in areas such as the spinal cord and retina.

0:30-2:30  

So Penfield found the same order of body parts across the motor cortex, really across all the patients he stimulated in. That was the most strikingly consistent thing. And in his famous 1937 paper, he drew the first what they call a homunculus. We talked about a homunculus in the somatic sensation lecture – this is a map of the body parts across the motor cortex.

Medial is up here and then lateral is over here. And you can see the the lower limbs, the trunk, the upper limbs and the hands on the face with the pharynx here, the throat, the tongue, and then the larynx here that controls vocalization. That order of body parts was remarkably consistent across patients. So it seemed like there was this really reproducible body map across the motor cortex

Now this is how that map was portrayed to me in the textbook that I first learned about motor cortex from. You can see the lower limb here extending onto that medial surface. The trunk up here, the arm, this giant region for the hand and then the face and the tongue.

So this was the map. But you know, when I saw maps like this in when I was an undergrad, I was like, oh, that’s crazy. There’s this like a point-to-point map of body parts across the motor cortex. Images like this really give the illusion that each point on the cortex is corresponding to a different point of the body.

Obviously, muscles are not points, right? It’s a muscle. But still, it would make you think that the feet muscles are over here and the calf muscles are over here and the thigh muscles are over here. The only problem is that is not actually true. And Penfield actually knew that. He knew that the maps were actually blurry and the areas corresponding to different body parts overlapped.

Hearing that word shouldn’t be a surprise to you, because I stress that about all the maps that we learn about in this class. They’re not perfect. They’re blurry. Development hasn’t made them super precise for whatever evolutionary reason. But images like this really gave people the misimpression that there was a perfect map. But  we now better appreciate that the map is quite blurry.

2:30-3:00  [Parallel Vocabulary] In introductory classes you learned about many topographic maps in the sensory and motor systems.  Here are a few examples… In the auditory system, neurons at specific locations respond to specific frequencies of sound – this is called a tonotopy.  In the somatosensory system, neurons at specific locations respond to signals from specific parts of the body – this is called a somatotopy.  In the visual system, neurons at specific locations respond to light from specific parts of the visual field and this is called a retinotopy.  In all three examples, stimulus properties are coded by the location of the active neurons.

3:00-4:00  [Here’s a real world example]  Detailed knowledge of topographic maps plays an important role in clinical neuroscience. You’ve probably seen a doctor on TV examine a patient by asking the patient to track a moving finger, by tapping the knee to check for a small reflexive kick, and by asking the patient to say “Ahhh”.  But did you know that these procedures are all used to test neural function? These tests along with several others are collectively called the neurologic exam, which can be combined with knowledge of topographic maps, in the motor and sensory systems, to precisely pinpoint the location of an injury affecting the nervous system. For example, throughout the motor system, there are somatotopic maps in areas such as the primary motor cortex, brainstem white matter, and spinal cord gray matter.  If there are abnormal reflexes in a part of the body, or if muscles on the left and right side have drastically different strength, these abnormalities would be revealed by the neurologic exam, and knowledge of topographic maps could be used to infer that a specific subregions of a specific structure has likely been injured.

4:00-6:00 [Follow along with this example]

6:00-6:30 [Here are a few readings to help you review]
1) Neuroscience Exploring the Brain (Bear)

• Chapters 8-14: “Sensory and Motor Systems”

2) Principles of Human Physiology (Stanfield)

• Chapter 10: “The Nervous System: Sensory Systems”

Media attributions

Homunculus by adrigu is licensed under an Attribution 2.0 Generic license (CC by 2.0).