Part 1 of Sex and Sensibility: the Physics of the Nervous System

Sensation – it’s not all that sex is about, but it’s a lot. Sure, intimacy is important, and so are trust and communication. But when it comes down to it, one of the best things about sex is that it feels good.

(Listen to the podcast with Text-to-Speech roboreaders Heather and Graham.)

You might wonder why the touch of a fingertip, lips or tongue that is barely detectable on one region of your body causes a surge of pleasure somewhere else. And what is it that makes a caress, pinch, slap, or tickle feel just right at one moment and completely wrong at another? There’s more to it than simple physics, but a look at the nervous system through the eyes of a physicist can help you get a handle on the sources of your sexual pleasure. . .

In order to enjoy sexual sensations your body needs to do at least three things. First it must detect the sensations. Next it has to send information about them to your brain. And finally, it must interpret those sensations as pleasurable. The three components in your body that accomplish these tasks - sensory receptors, nerves, and specialized regions of your brain - are portions of your overall nervous system.

These parts of your body are responsible for much more than making sex pleasurable, of course. The nervous system controls the movement of muscles, both voluntary ones such as your arms and legs and involuntary ones including your intestine and heart. It also monitors the status of your organs and generates thoughts and emotions. In a way, the nervous system is the part of the body that makes us who we are. After all, a person who loses a limb or has a heart, eye, or even face transplant is still the same person.

In effect all the parts of someone’s body could be replaced, but as long as the lump at the end of your spinal cord - which we call the brain - remains intact most of us would feel the essential person that is you is still here. Death, in fact, is defined as the cessation of activity in the brain, regardless of the condition of the rest of the body.

When you have sex or fall in love, revel in a moment of ecstasy or sink into the depths of despair it’s your nervous system that is experiencing all these events. The rest of you - including the bones, muscle, fat, and organs - is only a compilation of components in the vehicle that carries your nervous system around and lets it enjoy the world.

In this week’s edition of The Physics of Sex, we’ll start at the beginning - by looking at the structures in your body that first respond to events around it.

All sensations, sexual or otherwise, start with a sensory receptor. Rods and cones in the retina of your eye react to light, tiny hairs deep inside your ear detect sound, chemical receptors in your nose and on your tongue reveal smells and flavors, and receptors in your skin alert you to cold, heat, pressure and pain. In each case a receptor converts a stimulus into a signal that nerves transmit to other parts of your body.

To a physicist, a sensory receptor is a kind of transducer. Transducers take incoming signals of one type and change them to a convenient form that is easier to transmit or interpret.

Manmade transducers often convert information into electrical signals. A microphone, for example, changes sound into electrical waves before it sends it over metal wires. A speaker, in turn, is a transducer that converts the electrical signal back into something you can hear.

Long before humans thought to build transducers, nature had already discovered essentially the same trick. For example, when cold receptors in skin are exposed to low temperatures they move chemicals around to produce a small electrical voltage.

Heat receptors do the same thing when exposed to elevated temperatures, and receptors on the tongue and in your nose produce voltages in response to chemicals. Mechanical receptors, located primarily in your skin and muscles, produce electrical signals when they are squeezed, stretched, bumped or shaken.

Regardless of their particular sensitivity, all sensory receptor cells are really just small bags of electrically charged fluid. The bags are made of membranes of fatty cells, (much like the soap micelles described in the Physics of Sex chapter on lubrication).

Tiny chemical motors called ion pumps are embedded in the membrane. The pumps move electrically charged ions in and out of the cell. Ions that have excess electrons are negatively charged and ions with missing electrons are positive. The pumps push more of the positive ions out of the cell than into it, so the fluid inside becomes charged from the excess of negative ions that stay behind. The charge is small, roughly 70 thousandths of a volt, which is about twenty times smaller than the charge on a fresh flashlight battery.

When a sensory cell is triggered, pores open up in the cell membrane. This lets positively charged ions flow back into the cell, which makes the electrical voltage inside surge upward. If the stimulus is too mild, the voltage only goes up a tiny bit and the ion channels shut down again to let the pumps restore the cell to its negative resting voltage.

For a larger stimulus, above what is called the triggering threshold, the flow of charge is large enough that it causes more and more of the cell's other ion channels to open, leading to a dramatic voltage change and the cell fires an electrical pulse.

Triggering a sensory cell to fire is a bit like setting off an avalanche on a snow bank. A whisper may not get the snow to break loose, but a shout or a handclap can sometimes be a large enough trigger to send a wall of snow careening down a mountain. Similarly, when some ion channels open up in a sensory cell membrane they induce others to open as well. If too few open up to begin with, they all shut down in a fraction of a second. But if enough open up initially, they trigger an avalanche of ion channels to open.

Mathematically, a sensory cell’s response to stimulation is known as positive feedback. It occurs whenever a stimulus causes an effect that in turn increases the stimulus. Positive feedback typically leads to dramatic events, such as stock market crashes and orgasms, in addition to sensory cell bursts.

Once the flow of positive ions into a sensory cell raises the voltage to 10 millivolts or so, then no more positive ions can squeeze in and the ion channels clamp down. In the same way, an avalanche ceases after all the snow has slid down a valley wall. At this point the sensory cell and the snow filled valley are in their refractory states, which means that they cannot respond to stimulation, at least for a while. In a sensory cell the ion pumps charge the fluid back to a negative voltage and the cell is ready to fire again. Valleys prone to avalanches, however, must wait for snow fall to build up before they can go off once more.

The whole process takes a few thousands of a second in a sensory cell. If the stimulus remains after the cell has completed its cycle, it will fire another pulse.

The positive feedback in a sensory cell ensures that any stimulation, regardless of its strength, will lead to the same electrical burst, provided that the stimulation is above the triggering threshold. (Anything below the threshold - too gentle a touch, too quiet a sound, etc. - will simply go unnoticed.) Nevertheless, it's clear that we can tell the difference between a light touch and a heavy touch. Sensory cells reflect the intensity of a given stimulation by firing a train of electrical pulses. The more intense the stimulus, the more rapid the pulse train. In cases of extreme stimulation, the sensory cells fire immediately upon recovering from their refractory period.

Over time, a receptor exposed to an unchanging stimulus will gradually cease to respond. If you were to record the pulses coming out of a touch receptor, you would see that when it is first stimulated by a firm touch it emits a rapid string of pulses. After a few moments the pulses slow. At this point, the receptor has adapted to the stimulation. Removing the stimulus would then lead to another string of rapid pulses, which again taper off over time.

It is this sort of desensitization that allows you to ignore the touch of clothing against your skin, forget that you have your sunglasses resting on top of your head, and gradually come to tolerate the temperature of a hot shower.

Adaptation to an unchanging sensation can be thought of as a kind of short-term cellular memory. Once a sensory receptor adapts to a stimulus, it acts as though it can’t remember a time when the stimulus wasn’t there. Taking it away is a shock, but the cell soon gets used to the absence of stimulation as the new status quo, and loses all memory of the earlier stimulus. In contrast to the positive feedback avalanche of a firing sensory cell, sensory adaptation is a form of negative feedback.

Despite the term’s pessimistic-sounding name, negative feedback is a good thing most of the time. If a radio is too loud, you turn it down. If it is too quiet, you turn it up. Eventually you'll find a comfortable volume. This is negative feedback. Anything that must remain stable over time needs negative feedback, whether it's your weight, body temperature, emotional state, or even your bank balance.

There must be some flexibility in negative feedback systems to adjust for new situations. For example, there was likely a time when you were young that your appropriate weight was fifty pounds and negative feedback helped you stay close to that number, at least for a while. In order to grow, your body had to allow your weight to increase over the course of months and years. Most negative feedback systems operate this way - working to reduce sudden changes while adapting to gradual shifts. It is as if negative feedback systems have short-term memory and long-term amnesia. In sensory cells, the long-term amnesia results in raising or lowering the cell’s triggering threshold.

Negative feedback helps explain why it’s important to mix things up in bed. Pinching a nipple may be pleasurable for a few moments, but the sensation will fade if it's not varied and the sensory cells have a chance to adapt to the pinch. On the other hand, simply releasing the pressure on a nipple can be pleasurable as the sensory cells react to the change in status.

Varying the type and location of stimulation on a penis or clittoris during oral sex, and switching sexual positions from time to time during lovemaking, take advantage of the short-term memory of sensory receptors.

By focusing on one portion of an erogenous zone, you allow the sensory receptors in other places to adapt to the lack of stimulation, which makes them particularly sensitive when it’s their turn.

A commonly repeated myth claims that women who use vibrators will eventually lose sensation in their genitals. While it’s true that extended vibrator play in a session can lead to numbness, the effect is only a temporary resetting of the triggering threshold of the mechanical sensory receptors. It is certainly possible that intense vibration can rupture cells, but we all have plenty of pain sensors in our genitals that produce clear signals to warn of tissue damage. The intense agony of misusing a vibrator would make most women stop well shy of doing themselves any permanent injury. Thanks to the dense packing of sensory receptors in our erogenous zones, the risks that come with using vibrators are low and the orgasmic benefits are very high. In fact, many women find that using a vibrator helps them learn to orgasm more easily, even when they are not using powered toys.

Sensory adaptation also provides ways to alleviate problems with premature ejaculation in men. One method for increasing a man’s staying power involves repeatedly stimulating him to bring him to the brink of orgasm. Stopping just short of ejaculation and waiting a few moments, and then repeating the procedure several times, raises the triggering threshold of the sensory receptors in the penis to the point where a hair-trigger man can maintain self-control for longer and longer periods. Sex therapists often call this the stop-start method.

It may seem paradoxical that sensory adaptation can enhance sexual sensations under one set of circumstances, and desensitize a man’s penis under another. The key to the different benefits lies in the timing. Consider a hot shower, for example. As you step under the showerhead the steaming water might be barely tolerable. After a minute or so, the pain subsides as your temperature sensors set their pain threshold higher. Provided you remain under the shower, or move away for only brief periods, the water will still feel hot but won’t hurt. If you step out of the shower for several minutes or neglect to run water on your back for a while, the water will sting again when it hits the regions of your skin that have readapted to cooler temperatures.

Temperature sensors in your skin adapt more slowly than touch sensors, so you only have to wait a few seconds for a nipple, penis or clitoris to become re-sensitized after removing its stimulation. Similarly, in helping a man to stave off premature ejaculation it’s important not to entirely cease stimulating him for too long as he nears orgasm. Just ramp it back briefly. Otherwise you may end up speeding things along rather than slowing them down.

There are other ways to manipulate sensory receptors. You can use an ice cube to set temperature thresholds in your skin very low, say by cooling down a nipple, and then sucking on it to give your lover a sensory roller coaster ride. You can achieve a similar effect chemically with oils and lubricants that make your skin feel hot or icy cold (sometimes in rapid succession). Cooling and heating liquids and lubes contain compounds such as camphor and menthol that cause skin sensors to overreact to small temperature changes. If your partner breathes gently onto skin exposed to such a chemical, the warm breath will feel oddly hot, and if your lover blows vigorously on your skin, the rapidly moving air will feel surprisingly cold. In each case their breath is actually very close to body temperature, but the slight difference in skin temperature is amplified by the effect of camphor or menthol on the sensory receptors.

Once a sensory cell has fired in response to a stimulus, the signal is passed to nerve cells. But that is a topic that will have to wait for now. To learn about the physics of the nerves that connect to your sensory cells, be sure to come back next week to read part 2 of Sex and Sensibility: the Physics of the Nervous System.