Try this experiment at home with a friend: close your eyes and have your friend slap you across the face (probably don’t choose a friend who would take some sort of sick pleasure in hitting you as hard as possible). Wait a minute or so and then repeat the experiment with your friend softly caressing your face instead. Now, compare these two experiences and how you perceived them. The slap and the caress both involved someone else touching the same area of skin, alerting sensory neurons beneath the skin and causing an electrochemical impulse to move through the nervous system towards the brain. We know, from last week’s post, that neurons either fire or they don’t—there’s no subtle modulation of that signal at the electrical level. So then how does the nervous system differentiate between a slap and a caress? How does it modulate your perception of those events and your response to them? The answer to these questions lies in the intricate web of chemical and electrical interactions between neurons that forms the uniquely flexible and precise nervous system.
As humans, we perceive and process a huge volume of information each second—both consciously and subconsciously. We experience, process, and record input from the world through our senses—hearing, sight, taste, smell, and touch. But our nervous system also gathers a lot of simultaneous information from within our own bodies—like signals from the stomach telling our brain whether we are hungry or full. Once all that information is processed, the nervous system is also responsible for directing our response to the initial stimulus. This can be an action we consciously decide—like slapping our friend in retaliation or responding with a calm thumbs up—or it can be an involuntary response—like our stomach rumbling. All together the nervous system is able to receive, coordinate, and respond to a nonstop barrage of these signals throughout our day, allowing us to be (mostly) functional human beings.
The nervous system can be broken up into two general parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS is the vast web of nerves and sensory organs that collects and transmits signals from the body and the outside world to the brain and relays back the brain’s response. The CNS is made up of the brain and spinal cord that act as the nervous system’s main processing hub for translating signals, storing information, and formulating decisions. We’ll delve deeper into the central nervous system in a couple weeks when we discuss the architecture of the brain. For now, let’s take a look at how an expansive network of peripheral nerves precisely translates external signals into electrochemical information.
The peripheral nervous system is further broken down into the autonomic nervous system and the somatic nervous system. The autonomic nervous system governs the internal maintenance of the body along with all the involuntary mechanisms that keep the body functioning. These actions are all subconscious, meaning you do not have to actively think about keeping your heart beating regularly (which is a good thing for those of us with no sense of rhythm). The autonomic nervous system is further broken up into two opposing divisions—the sympathetic division and the parasympathetic division. In the case of a threat—like suddenly being slapped—the sympathetic division is responsible for ramping all your body systems up to either fight the threat or run away. Heart rate and blood pressure increase, glucose and adrenaline are released into the bloodstream, pupils dilate to enhance perception, sweat glands release more sweat to keep the body cool, and breathing speeds up to ensure the blood stays oxygenated. When the threat is neutralized, the opposing parasympathetic division is responsible for returning all these systems to normal operations. Together, the sympathetic and parasympathetic divisions work together to maintain the careful homeostasis of bodily functions.
The somatic nervous system is primarily responsible for all the sensations and actions that we are consciously aware of—like the sensation of getting slapped or the action of slapping your friend back. This system includes both afferent sensory nerves and efferent motor nerves. Motor neurons connect to skeletal muscles and coordinate muscle contraction. Sensory neurons extend throughout the body connecting the various sensory organs (including the largest sensory organ, the skin) to the spinal cord. The structure of sensory neurons and sensory receptors depends upon their function. In the eye, highly specialized photoreceptor cells detect changes in light and transmit that information to the optic nerve. Chemoreceptors in the nose and mouth translate chemical signals into sensations of taste and smell. Sound waves cause minute vibrations of the hair cells in the inner ear that trigger neuron depolarization. Other receptors in the inner ear are also responsible for our sense of balance. There are sensory neurons that interact with the muscles to help maintain our proprioception and kinesthesia—our internal sense of where the various parts of our body are in space and how they are moving.
Each of the senses listed above could warrant its own dedicated blog post, but let’s just take a look at the senses of touch that began this inquiry. Our sense of touch is really a broad category of senses that involve skin and muscle receptors. Throughout the skin, there are thermoreceptors that detect changes in external temperature, mechanoreceptors that detect light pressure and vibration, and nociceptors that detect chemicals involved with tissue damage. In addition to the mechanoreceptors, there are separate receptors for detecting deeper pressure touches and higher frequency vibrations. There are even hair follicle receptors for detecting the slight movement of the hairs on our skin. There are stretch receptors that detect the movement and contraction of muscles in the body. All of these receptors work together to form a unified and highly sensitive sense of touch.
So, when your friend slaps you, the high-frequency vibration receptors are activated (presuming it’s a decent slap). The kinetic energy of the hit may also impart some thermal energy (heat) that your thermoreceptors pick up. And there might even be some damage to the external tissues that pings your pain receptors. On the other hand, the soft caress activates an entirely different set of receptors—the light touch receptors and the hair follicle receptors in the cheek (yes, everyone has hair on their cheek—some people just have finer, peach fuzz hair there). The activation of these different sets of receptors changes the kinds of signals that get sent to the brain, which changes your perception of those events and your response. But how exactly does the sensory neuron translate the information picked up by a particular receptor into electrical information that can travel to the brain? Next week, we’ll look at how the nervous system uses neurotransmitters and a special binary code to convert this nuanced information into pure electricity.
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