The nervous system encompasses the defining characteristics of what it means to be human–it is in charge of our consciousness, cognition, behavior and memories. It acts as our controlling, regulatory, and communicating system, with three main functions: sensory, integrative, and motor. Sensory receptors detect stimuli: external (environmental factors) and internal (pressure, pH, CO2 concentration, electrolytes). Sensory input is converted into electrical signals called nerve impulses that are transmitted to the brain. This is when integration occurs: these impulses are brought together to create sensations, to produce thoughts, or to add to memory; decisions are made each moment based on the sensory input. Lastly, motor output is the commands that are sent to the rest of the body to perform some action.
Within the nervous system, there are two types of cells: neurons, which are highly specialized for their function of transmitting impulses, and glial cells (aka neuroglia), which provide structural support to neurons, are nonconductive, and are by far the more numerous of the two. Running through the structure of neurons: the cell body is similar to normal cells (contains a nucleus and, most organelles); the dendrites are cytoplasmic extensions that transmit impulses to the cell body (they are called afferent: they transmit inward). Dendrites are usually short and branching–higher surface area to receive signals. The axon carries the impulse away from the cell body (efferent: away). Neurons can be either afferent (these are sensory and have long dendrites and relatively short axons), efferent (these are motor, with short dendrites and long axons), or interneurons (association neurons, are located entirely within the central nervous systemCNS where they connect afferent and efferent neurons). A synapse is where an axon connects to another cell to pass the neural impulse, but does not involve a physical connection between two cells, rather a small gap is present where vesicles move through from one cell to another. it is just a little chasm between the two ports rather than a physical connection. The impulse triggers the release of chemicals (neurotransmitters) from the end of the axon, which cross the synapse to bind to the receiver cell’s membrane and activate it in a specified way depending on the neurotransmitter. You may have heard of white vs gray matter before; white matter is involved in transmitting information, and so the axon is in this category if it is myelinated (meaning it is insulated in a way to speed up impulse transmission). Unmyelinated axons, cell bodies, and dendrites are made up of gray matter.
The nervous system has two main parts: the central and peripheral nervous systems. In the central nervous system (CNS), the main organs are the brain and spinal cord. The CNS is like the command center: it receives signals from the peripheries, processes/interprets them, makes decisions about them, and then sends directions back out to the peripheries for things it wants done. The brain and spinal cord are both encased in bone because they are so essential to our functioning. The spinal cord can generate commands but for involuntary processes only (reflexes); its main function is to pass information from the brain to the rest of the body. The peripheral nervous system (PNS) consists of two components: nerves (tissue made up of neurons and glial cells) and ganglia (collections of nerve cell bodies outside CNS). The PNS has afferent and efferent divisions (i.e. a division that sends signals inwards to the CNS and one that receives information from the CNS). The efferent is divided into the somatic NS: supplies motor impulses to the skeletal muscles, permits conscious control; and the autonomic NS: supplies motor impulses to cardiac muscle, to smooth muscle, and to glandular epithelium, and allows for involuntary or automatic control. The ANS is divided into the sympathetic NS (commands for “fight and flight” response in stressful situations) and parasympathetic NS (controls “rest and digest” states, helps in bodily deescalation of situationstries to conserve energy).
To tie all of these elements together, let’s walk through an example: holding your hand over a flame. The story begins in your peripheral nervous system: the sensory receptors in your palm and fingertips specialized for detecting temperature (called thermoreceptors) will become activated, sending action potentials inwards towards the brain. These impulses travel along nerves, leaving one neuron through its axon and entering the next through the dendrites. Eventually they reach the spinal cord, officially entering the central nervous system. They move up the spinal cord, reaching the brainstem, which is at the base of your brain. There is a special area in the brainstem called the thalamus whose job it is to relay the received electrical signals to the correct area of the brain for processing; so, the thalamus redirects our heat impulses to the somatosensory cortex, as well as important areas for processing a potentially dangerous interaction with fire, where it can then be understood by us. An interesting note: if the flame becomes too hot, we will automatically jerk our hand away from the fire in what is called a reflex. The original sensory signal did not need to reach the brain in order for the motor activity of moving away to be initiated; it only had to go to the spinal cord, as mentioned earlier, in order to save valuable time and make a quick “decision” for self preservation.
Why do we need to know this? Because in order to understand neurotechnology, we need to understand the physiological bases upon which it is built. An really cool example of nanotechnology is one that enables a huge part of the nervous system to be bypassed–namelty the peripheral nervous system. Through EEG devices, we can measure electrical activity in the brain. This is useful in cases for people with motor dysfunction or paralysis, meaning they cannot execute motor control over their body. However, using EEG, scientists can interpret the electrical signals within someone’s brain for their intended movements, and then bypass the latter portion of the nervous system’s responsibilities. IWhat I mean is that instead of sending the electrical impulses generated in the brain to the peripheral nervous system to make, say, the hand move, scientists can translate these signals into code that makes a robotic limb move in the same way you would want your limb to move.