Neurons are brain and nerve cells that transmit electrical activity around the brain and to and from your body, sending messages and allowing you to think. There are several types:
- Sensory neurons: these carry signals from the outer parts of your body (periphery) into the central nervous system (brain and spinal cord); these help you sense the world around you.
- Motor neurons: also known as motoneurons, these also carry signals but from the central nervous system to the outer parts (muscles, skin, glands) of your body; these are involved when you decide to move.
- Interneurons: these connect various neurons within the brain and spinal cord together, meaning you can coordinate your senses, thoughts, and actions together.
Neurons are similar to normal cells in the body in that they have a cell membrane, nucleus, and contain genes, but they are different in their structure. They have tree like branches called dendrites, which bring electrical signals to the cell. And other finger like structures called axons, which carry messages away from the cell
Although all fundamentally the same, neurons vary in size and shape dependent on their function. For example, 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 whereas sensory neurons have dendrites on both ends, connected by a long axon with a cell body in the middle.
The simplest type of neural pathway is a monosynaptic (single connection) reflex pathway, like the knee-jerk reflex. When the doctor taps 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.
There are also different classifications of neuron, decided by the number of extensions that come out of the cell body or soma. A bipolar neuron has two processes extending from the cell body (examples: retinal cells, olfactory epithelium cells). A pseudounipolar cell (example: dorsal root ganglion cells) has 2 axons rather than an axon and dendrite. One axon extends centrally toward the spinal cord, the other axon extends toward the skin or muscle. Multipolar neurons have many processes that extend from the cell body. However, each neuron has only one axon (examples: spinal motor neurons, pyramidal neurons, Purkinje cells).