The spinal cord extends through hollow openings in each vertebra in your back. It contains various nerve cell bodies (gray matter) and nerve processes or axons (white matter) that run to and from the brain and outward to the body. The peripheral nerves enter and exit through openings in each vertebra. Within the vertebra, each nerve separates into dorsal roots (sensory nerve cell processes and cell bodies) and ventral roots (motor nerve cell processes). The autonomic nerve cell bodies lie along a chain that runs parallel with the spinal cord and inside the vertebrae, while their axons exit in the spinal nerve sheaths.
The brain, spinal cord and nerves consist of more than 100 billion nerve cells, called neurons. Neurons gather and transmit electrochemical signals. They have the same characteristics and parts as other cells, but the electrochemical aspect lets them transmit signals over long distances (up to several feet or a few meters) and pass messages to each other.
Neurons have three basic parts:
- Cell body: This main part has all of the necessary components of the cell, such as the nucleus (which contains DNA), endoplasmic reticulum and ribosomes (for building proteins) and mitochondria (for making energy). If the cell body dies, the neuron dies. Cell bodies are grouped together in clusters called ganglia, which are located in various parts of the brain and spinal cord.
- Axons: These long, thin, cable-like projections of the cell carry electrochemical messages (nerve impulses or action potentials) along the length of the cell. Depending upon the type of neuron, axons can be covered with a thin layer of myelin, like an insulated electrical wire. Myelin is made of fat, and it helps to speed transmission of a nerve impulse down a long axon. Myelinated neurons are typically found in the peripheral nerves (sensory and motor neurons), while nonmyelinated neurons are found within the brain and spinal cord.
- Dendrites or nerve endings: These small, branchlike projections of the cell make connections to other cells and allow the neuron to talk with other cells or perceive the environment. Dendrites can be located on one or both ends of the cell.
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.
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.
In peripheral and autonomic nerves, the axons get bundled into groups, based on where they're coming from and going to. The bundles are covered by various membranes (fasciculi). Tiny blood vessels travel through the nerves to supply the tissues with oxygen and remove waste. Most peripheral nerves travel near major arteries deep within limbs and close to the bones.
Next, we'll learn about neural pathways.