Mechanoreceptors (Theme C)

Question 1

Why are extracellular action potentials biphasic?

Answer 1

Due to the relative position of the electrode to the location of the Na + influx

Question 2

Extracellular action potentials are different sizes. What do smaller APs represent?

Answer 2

Smaller diameter axons

Question 3

The amplitude of an intracellular AP is all-or-nothing. What is the peak amplitude primarily determined by?

Answer 3

The sodium reversal potential

Question 4

Conduction velocity in cockroach unmyelinated sensory axons is 5-10m/s. Why is this faster than in mammalian unmyelinated axons?

Answer 4

Cockroach axons are of larger diameter than mammalian axons

Question 5

What information does sensory adaptation provide the cockroach with?

Answer 5

Dynamic information about the rate of deflection of the sensory spine

Question 6

What mechanism may contribute to sensory adaptation?

Answer 6

Inactivation of voltage-gated Na+ channels in the mechanoreceptor membrane

Question 7

What is the difference between intracellular & extracellular APs (mechanoreceptor practical)?

Answer 7

Intracellular APs are all the same size - all-or-nothing principle

Extracellular APs are different sizes - are graded

Question 8

What are the 2 factors that influence the size of the extracellular AP?

Answer 8

1. Axon diameter - the larger the diameter, the larger the AP
2. Distance to the electrode - closer to the electrode, the larger the AP

Question 9

Extracellular (biphasic) APs can be positive-going then negative-going or negative-going then positive-going. Why is this?

(Mechanoreceptor practical)

Answer 9

Depends on the location of the reference & recording electrodes

• If recording electrode is closest to spine → trace will go negative then positive
• If reference electrode is closest to spine → trace will go positive then negative

Question 10

At rest, would you see any APs from the cockroach spine?

Answer 10

Yes - spontaneous APs

Question 11

What do you record when you touch a single spine (on the cockroach leg)?

Answer 11

• Record burst of APs, all the same size - as you are only activating 1 spine
• i.e., the axon coming out of the mechanoreceptor doesn’t change its distance from the electrode / diameter
• Spontaneous APs still occurring

Question 12

What do you record when you tap the bench / blow on the cockroach leg?

Answer 12

• Record burst of APs, different sizes - because you are activating & recording different spines
• Because each axon will be a different distance from recording electrode / diameter

Question 13

What process does the ‘touch & hold’ stimulus demonstrate on the cockroach leg?

Answer 13

Sensory adaptation

Question 14

What happens in the cockroach when the leg spine moves?

Answer 14

When the spine moves → dendrite of the neurone is distorted → opening mechanically-gated ion channels in the dendrite → influx +ve charge → receptor potential generated → triggers AP

Question 15

The amplitude of a compound AP is influenced what what factors?

Answer 15

1. The size of the axon: larger diameter axons produce greater amplitude APs
2. The proximity of the axon to the recording electrodes: those that lie closer produce greater APs

Question 16

What does CAP stand for?

Answer 16

Compound Action potential

Question 17

How to calculate CV from biphasic AP

Answer 17

1. Select clear biphasic AP
2. Measure the time taken (ms) from peak-to-peak
3. Measure the distance (mm) between the electrodes
4. CV = d/t (m/s)

Question 18

Compare the rates of adaptation in response to the different spines? How much do the rates vary?

Answer 18

Some spines & their nerves adapt more quickly than others.
This provides a variety of info to the cockroach about its environment.

Question 19

Describe the change in activity (in terms of amplitude and frequency) of the sensory nerve during a press-and-hold stimulation

Answer 19

Initially, the frequency and amplitude of CAPs is high. As the neurons adapt to the stimulation, both the amplitude and frequency decreases.

Question 20

What advantage would sensory neurons with different rates of adaptation provide to the cockroach?

Answer 20

Neural adaptation, or sensory adaptation, is a change over time in the responsiveness of the sensory system to a constant stimulus. This allows the organism to “focus” on changes in their environment.

By having neurons that each adapt differently, the organism may interpret more complex environmental stimuli.

Question 21

Did different degrees of deflection affect the AP response, even though you moved the spine the same direction in each instance?

Answer 21

Generally, moving the spines 180° should produce larger responses.

When the spine is deflected 180° towards the proximal segment of the leg the edge of the socket surrounding the spine acts like a fulcrum, causing greater extension of the dendrite and hence greater activation

Question 22

Larger degrees of deflection of the cockroach spines generally produce larger AP responses. What is the potential advantage of this?

Answer 22

This helps the cockroach to sense the direction of potential dangers and mount a rapid escape response when appropriate.

It also helps the cockroach to ignore stimuli from other directions that may merely signal the spine resting against a surface.