Basic Neuron Types

Neurons come in many sizes. For example, a single sensory neuron from your fingertip has an axon that extends the length of your arm, while neurons within the brain may extend only a few millimeters. Neurons have different shapes depending on what they do. Motor neurons that control muscle contractions have a cell body on one end, a long axon in the middle and dendrites on the other end; sensory neurons have dendrites on both ends, connected by a long axon with a cell body in the middle.

Some types of neurons: motoneuron (a), sensory neuron (b), cortical pyramidal cell (c)

Neurons also vary with respect to their functions:

• Sensory neurons carry signals from the outer parts of your body (periphery) into the central nervous system.

• Motor neurons (motoneurons) carry signals from the central nervous system to the outer parts (muscles, skin, glands) of your body.

• Receptors sense the environment (chemicals, light, sound, touch) and encode this information into electrochemical messages that are transmitted by sensory neurons.

• Interneurons connect various neurons within the brain and spinal cord.

The simplest type of neural pathway is a monosynaptic (single connection) reflex pathway, like the knee-jerk reflex. When the doctor taps the right 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. The response is a muscular jerk that happens quickly and does not involve your brain. Humans have lots of hard-wired reflexes like this, but as tasks become more complex, the pathway "circuitry" gets more complicated and the brain gets involved.