LAB HANDOUT:

THE CRICKET CERCAL SENSORY SYSTEM:

Slide #1:
This image shows a cut away view of the central nervous system (CNS) of the common house cricket, Acheta domesticus. The CNS consists of a chain of ganglia that extend from the head of the animal (the brain), through the thorax and terminate in the abdominal ganglion. The neurons that make up the cercal sensory system are located in the terminal abdominal ganglion. The sensory neurons are located in the cerci, each neuron innervates a single mechanosensory hair. Their axons project into the CNS and terminate in the terminal abdominal ganglion.

Slide #2:
The animal has two sensory appendages located at the tip of its abdomen, called cerci. Each cercus has about 1000 mechanoreceptors on it which are very sensitive to air currents. Each mechanoreceptor consists of a long flexible hair lodged in a cuticular socket (B). The mechanics of the socket constrain the movements of the hair in a single plane. Each mechanoreceptor is innervated by a single sensory neuron. Activity in the sensory neuron is modulated up or down as a function of stimulus direction. Directional tuning curves for 3 individual sensory neurons are shown in panel D. A mechanoreceptor sensitive to stimuli directed at the front of the animal moves in a plane parallel to the axis of the cercus (front-back). Stimuli from the front increase the firing rate of the neuron, stimuli from the back decrease the firing rate of the neuron.

The axons of the primary sensory neurons travel to the central nervous system through the cercal nerve and terminate in the terminal ganglion (panel C). The primary sensory afferents arborize in particular locations of the ganglion according to their directional sensitivity. All cells with a particular directional sensitivity, regardless of their position on the cercus, arborize in the same location. Cells with a different directional sensitivity arborize in a different area. This pattern of arborization results in a topographic map of stimulus direction within the terminal ganglion. This map is a functional representation of the direction of the stimulus with respect to the animal's body.

Slide #3:
Scanning electron micrograph of a cricket cercus

This image shows a small region of the cercus. The filiform mechanoreceptors can be identified as long hairs lodged in elliptical sockets. The hairs are free to move back and forth within a socket. These hairs vary in length as well as diameter. The length of a filiform hair determines its sensitivity to the velocity of the air currents. Long hairs are most sensitive to low velocity air currents, short hairs to lower velocity air currents. Other mechanoreceptors on the cercus are lodged in tight sockets. These hairs are sensitive to touch, but not air currents.

ACTIVITY #1 -- Analyzing the distribution of filiform hairs of different lengths.

  1. Find the shortest filiform hair in the image and record its length on a piece of scratch paper. (do this by simply tracing the length of the hair on the paper). Use this measure to estimate the length of all other filiform hairs on the cercus (set the smallest hair length equal to 1). Any hair that extends out of the image can be estimated. these hairs as 4+, or 5+ etc. Draw a graph below plotting the number of hairs vs. hair length.

    Draw your graph here:
  2. From this small sample, are there equal numbers of hairs of different lengths on the cercus? Which length hair is most prevalent?
  3. How does the diameter of the hair vary with hair length?
Slide #4
Sensory neurons project to specific locations in terminal ganglion according to their directional tuning properties.

A. This image shows 8 sensory neurons (4 mirror image pairs) in the terminal abdominal ganglion. Each neuron has a specific shape and terminates in a specific location in the terminal ganglion. The sensory neurons have been color coded according to their directional tuning properties. The color of the neuron indicates the stimulus direction it is most sensitive to with respect to the animals body.

B. This image shows the coordinate system of the animal's body and the color coding of stimulus direction. A stimulus directed at the head of the animal would be indicated as yellow, one directed at the rear would be indicated as blue. Therefore, a neuron colored red is most sensitive to wind directed at the upper left quadrant of the animal's body. A neuron colored light blue is most sensitive to wind directed at the lower right quadrant of the animals body.

Slide #5
Spatial relationships between individual sensory neurons

This image shows the same sets of sensory afferents in various combinations. Note that each sensory neuron has a mirror image twin on the opposite side of the ganglion.

Slide #6
Spatial relationships between sensory neurons on the left side of the ganglion.

This image shows the same four sensory neurons from three different perspectives: a dorsal view, a sagittal view and a horizontal view. Note the differences in anatomical overlap between the cells in the 3 different views.

ACTIVITY #2 -- The relationship between sensory neuron arbor position and directional tuning.

Use Slides 4, 5, and 6 to answer the following questions:

  1. Each afferent has a mirror image twin on the opposite cercus. Are these cells tuned to the same wind directions? What directions are they tuned to? Give at least two examples.
  2. With respect to the coordinate system of the cercus, mirror image sensory neurons are tuned to the same stimulus direction. Why?
  3. What is the relationship between directional tuning and anatomical overlap between different sensory neurons from the same cercus? Which sets of cells overlap and which do not? Give specific examples. (Note, you must look at several different views of the same cells to determine whether they overlap or not).

Slide #7
The map of stimulus direction formed by sensory afferents in the cricket cercal sensory system.

This image shows the ensemble projection pattern of sensory afferents in the terminal ganglion. Panel A shows the location of the map inside the terminal ganglion, panel B shows an enlarged view of the map. Panel C shows the projection pattern of sensory neurons from the left side of the ganglion from a different view.

ACTIVITY #3 -- The representation of stimulus direction in the terminal ganglion.

Use Slide #7 to answer the following questions:
  1. Describe the map of stimulus direction as either a continuous or patchy representation of direction with respect to body coordinates. Explain your choice.
  2. Sensory cells with similar directional tuning innervate filiform hairs at different locations on the cercus. Considering this fact, would you characterize the map of stimulus direction a topographic map? Why or why not?

Slide #8
Predicting the direction of a stimulus from activity patterns.

This image shows 4 different predicted activity patterns. Yellow indicates an increase in activity at that region in the map and blue a decrease in activity. Each pattern is a prediction of the activity pattern in the map in response to a stimulus directed at the animals body.

ACTIVITY #4 -- Predicting stimulus direction

Using the image in the center as a key, predict the direction of the stimulus for each of the four patterns. For example, a stimulus directed at the front of the animal should activate all cells tuned to that direction ( i.e. those colored yellow, orange or green in the center image).

Pattern A.

Pattern B.

Pattern C.

Pattern D.

Slide #9
Primary sensory interneurons in the cricket cercal system:

This image shows three identified sensory interneurons in the cricket cercal sensory system. Each interneuron has a specific shape and directional tuning to air current stimuli. These cells are all located in the terminal ganglion.

Slide #10
Interneurons 10-3 and 10-2 in their correct spatial relationships. Note the regions where the dendrites overlap.

Slide #11
Interneurons 10-3 and 9-3 in their correct spatial relationships.

Slide #12
Interneurons 10-2 and 9-3 in their correct spatial relationships.

Slide #13
Distribution of excitatory inputs to Interneuron 10-2.

This image shows Interneuron 10-2 in its correct spatial relationship to the map of stimulus direction. The dendrites of the interneuron are color coded according to the directional tuning of the afferents that provide excitatory input to it. The interneuron sums these excitatory inputs to derive its own directional tuning properties.

Slide #14
Distribution of excitatory inputs to Interneuron 10-3.

This image shows Interneuron 10-3 in its correct spatial relationship to the map of stimulus direction. The dendrites of the interneuron are color coded according to the directional tuning of the afferents that provide excitatory input to it. Note that it receives input from a different set of sensory neurons than Interneuron 10-2.

Slide #15
Distribution of excitatory inputs of Interneuron 11-1.

This image shows Interneuron 10-3 in its correct spatial relationship to the map of stimulus direction. The dendrites of the interneuron are color coded according to the directional tuning of the afferents that provide excitatory input to it.

ACTIVITY #5 -- Predicting the directional tuning of primary sensory interneurons.

Using Slides #9-15 answer the following questions:

  1. What stimulus direction(s) is Interneuron 10-2 most sensitive to? Explain your answer.
  2. What stimulus direction (s) is Interneuron 10-3 most sensitive to? Explain your answer.
  3. What stimulus direction (s) is Interneuron 11-1 most sensitive to?
  4. Using the images in slides 10,11, and 12 predict which stimulus directions Interneuron 9-3 is most sensitive to. Which interneurons directional tuning is most similar to Interneuron 9-3? Explain your logic.

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