The simplest type of neural pathway is a monosynaptic (single connection) reflex pathway, like in the knee-jerk reflex. When the doctor taps a certain spot on your knee with a rubber hammer, receptors send a signal into the spinal cord through a sensory neuron. The sensory neuron passes the message to a motor neuron that controls your leg muscles. Nerve impulses travel down the motor neuron and stimulate the appropriate leg muscle to contract. Nerve impulses also travel to the opposing leg muscle to inhibit contraction so that it relaxes (this pathway involves interneurons). The response is a quick muscular jerk that does not involve your brain. Humans have lots of hardwired reflexes like this, but as tasks become more complex, the pathway "circuitry" gets more complicated and the brain gets involved.
We have talked about nerve signals and mentioned that they are electrochemical in nature, but what does that mean?
To understand how neurons transmit signals, we must first look at the structure of the cell membrane. The cell membrane is made of fats or lipids called phospholipids. Each phospholipid has an electrically charged head that sticks near water and two polar tails that avoid water. The phospholipids arrange themselves in a two-layer lipid sandwich with the polar heads sticking into water and the polar tails sticking near each other. In this configuration, they form a barrier that separates the inside of the cell from the outside and that does not permit water-soluble or charged particles (like ions) from moving through it.
So how do charged particles get into cells? We'll find out on the next page.