Our bodies are electro-chemical stews, teeming with bacteria, raging with hormones, and all fired up by the central nervous system. This amazing system turns thoughts into deeds as neurons process brain signals.
A neuron is simply a single nerve cell. A nerve cell transmits electrical signals throughout the body. Sensory neurons receive input signals and from other neurons.
The sensory neuron then sends electrical output signals to muscle neurons (called motoneurons or motor neurons) as well as other neurons. An interneuron signals another neuron.
Think of the body’s nervous system as electrical wiring that connects the brain’s command center with all of your muscles and organs. If you want to move your foot, your nerves don’t simply signal “foot, move.” A series of electrical impulses called action potentials travel to different foot muscles and power the physical response.
The way neurons work is fascinating. Like other cells, they are composed of a cell body with a nucleus, an organelle bounded by a double membrane. An organelle is a structure found inside a cell – think of it as a cell’s internal organ. This specialized subunit within a cell has a specific function.
But neurons have some special features that give them their zip:
- Dendrites receive signals from neighboring neurons (like a radio antenna)
- Axons transmit signals over a distance (like telephone wires)
- Axon terminals transmit signals to other neuron dendrites or tissues (like a radio transmitter)
- Myelin sheaths speed up signal transmission along an axon
The main job of the cell membrane is to maintain a beneficial environment within the interior of a cell. This is done by controlling the passage of molecules and ions into or out of the cell.
Ions are charged atoms – the number of positively-charged protons is different from the number of negatively-charged electrons. The cell membrane uses chemical proteins to help ions pass through the cell’s barrier membrane.
The double-layered cell membrane is composed of phospholipids. Each phospholipid molecule contains a phosphate head and two lipid (fatty) tails. The phosphates form the inner and outer boundaries of the cell membrane and the lipid tails fill the space in between.
Cell membrane lipids are hydrophobic – they repel water and substances dissolved in water such as sodium, potassium and calcium ions. Think of them as a protective moat between a castle’s outer and inner walls.
Transmembrane proteins dot the cell membrane and create pores (like portholes on a ship) that can help ions and other molecules avoid the lipid barrier and pass through the cell’s membrane to its nucleus.
Ion channels are a type of transmembrane protein that let a high rate of ions flow, powered by the electrochemical gradient across the cell membrane. Differences in ion concentration on either side of the membrane create a gradient (or differential). Ions move naturally from areas of higher concentration to areas of lower concentration, seeking to balance the gradient.
Different channels are specific to different ions. Some channels can move more than one type of ion, operating like microscopic gates that open or close to allow an ion to pass through. These passive gates that require no cellular energy to function are either open or closed, never in an intermediate state.
To review, the selective permeability of cell membranes is due principally to ion channels, proteins that allow only certain kinds of ions to cross the membrane in the direction of their concentration gradients.
The ion concentration gradients are established by proteins called active transporters that actively move ions into or out of cells against their concentration gradients.
Ion channels and active transporters work against each other, generating several kinds of energy potentials: the resting membrane potential, action potentials, and the synaptic potentials and receptor potentials that trigger action potentials.
When there is a higher concentration of positively charged ions outside a cell compared to its interior a large concentration gradient exists. The same is also true in the opposite condition when there are more positively charged ions inside the cell than outside.
Both positively and negatively charged ions move in the direction that would balance or even out the gradient.
Neurons have a negative concentration gradient most of the time, with more positively charged ions outside than inside the cell. This regular state of a negative concentration gradient is called resting membrane potential. During the resting state, all of the gated ion channels are closed.
Action potentials – electrical impulses that transmit signals throughout the body – are temporary shifts, from negative to positive, in the neuron’s membrane potential caused by ions suddenly flowing in and out of the neuron. Action potentials are either on or off (like a light switch).
This complex yet mechanically straightforward system of neural electrical signaling and cellular chemical balancing is what animates us. Isn’t nature marvelous?