Anatomy

The brain functions as the information control center for the entire body. The spinal cord forms at the base of the brain and functions to relay messages to and from the brain to the rest of the body. The spinal cord is about the width of a human finger and transmits information in both directions.
 
When a person wants to open and close a hand, the brain sends messages that travel to the spinal cord and eventually down nerves that leave the spinal cord and travel to the muscles. 

If damage occurs to the spinal cord, electrical messages will not be able to travel from the brain to the muscles. Also, sensations such as hot, cold, pain and pleasure from areas of the body cannot travel up the spinal cord to reach the brain.

Vertebrae

The spinal cord is delicate and it must be protected within a strong spine. The spine is made up of 29 individual bones collectively called vertebrae. Individually, each vertebra forms a ring of bone with a hollow center. The vertebrae stack on top of each other and the hollow center performs a tunnel through which the spinal cord travels. Each vertebra has a hole on the right and left side called a foramen. Nerves exit the foramen as they leave the spinal cord and travel to other parts of the body such as muscles, organs and skin. 
 
The entire spine is divided into four regions named cervical, thoracic, lumbar, and sacral. These regions are often abbreviated and referred to by the first letter of the word. For example, “C” represents cervical, “T” represents thoracic, “L” represents lumbar, and “S” represents sacral. There are seven cervical vertebrae; the first cervical (C1) vertebra is in between the base of the skull and the second cervical (C2) vertebrae. The fifth cervical vertebra from the top is referred to as C5 and so on. There are 12 thoracic vertebrae; each also has a connection to a rib. There are five lumbar vertebrae with some variation in individuals, who may have four or six. The five sacral vertebrae are all fused but do have a central open canal area where the nerves continue to travel down the spinal column and out the foramen.

Disks
The vertebrae are connected to each other by disks that function as shock absorbers. There is no flexibility within the bony vertebra, but the disk allows flexibility and movement of the spine. People need this flexibility since to allow the spine to turn and bend.  
 

Ligaments
Ligaments are composed of strong but flexible material (fibrous bands or sheets of connective tissue) that connects two or more bones or cartilage. They help stabilize the 29 vertebrae and protect the spinal cord while allowing for movement.  

Spinal Cord
The spinal cord is a long, circular structure about the size of a small garden hose. It starts at the base of the brain and travels down the spinal column to about the 12th thoracic or first lumbar vertebrae (T12 to L1). The very end of the spinal cord forms in a structure that looks like a cone and is called the conus medullaris. From the end of the spinal cord many nerves come off and continue to travel down the spinal column until they reach a specific vertebra where they leave through a hole called a foramen This group of nerves is referred to as the cauda equina, which means “horse's tail” in Latin.

 

The spinal cord, like the vertebrae, has sections named cervical, thoracic, lumbar and sacral. The sections of the spinal cord are named by the spinal nerve that leaves the spinal cord and exits through a specific vertebra. For example, the area of the spinal cord that is referred to as C7 (seventh cervical) is named because of the nerve that comes off of the spinal cord at that level and exits through the hole or foramen of the seventh cervical vertebra. The L3 (third lumbar) nerve leaves the spinal cord in the area of the conus medullaris and travels down as part of the cauda equina until it exits through the foramen of the third lumbar vertebra. Unlike the cervical and upper thoracic region where the spinal nerve comes off the spinal cord and almost immediately travels through and exits the vertebra, the lower thoracic, lumbar and sacral spinal cord has spinal nerves that leave the spinal cord and travel down the spinal column before exiting the vertebra. This occurs because the end of the spinal cord is located much higher than the end of the vertebral column.

The spinal cord carries messages both up and down through spinal tracts. Spinal tracts are like lanes of the highway that only travel in one direction. Similar to that of a highway, there are many spinal tracts within the spinal cord traveling in opposite directions at the same time. Because they are in specific “lanes,” they do not get mixed up or interfere with each other. This allows us to be able to purposefully move a part of our body while sensing the same area or at the same exact time.

Below is a drawing of a cross-section of the spinal cord with a butterfly appearance in the middle of the spinal cord. The inside of the spinal cord (butterfly appearance) is referred to as the gray matter and contains nerve connections and sprouts between nerves. This is also where nerves eventually exit or enter the spinal cord. The outside area of the spinal cord (area surrounding the butterfly) is referred to as the white matter. This is the area that contains the spinal tracts traveling up and down the spinal cord. There is a very well organized arrangement of spinal tracts that are grouped into specific areas based on what function they are responsible for. For example, there is a very specific area that carries nerve fibers involved in the feeling of pain and temperature. This specific tract within the spinal cord is called the spinothalamic tract. Other spinal tracts carry nerve fibers that cause a leg to move and still another area is responsible for the ability to feel vibrations.

 

Autonomic Nervous System
The autonomic nervous system (ANS) can be thought of as the “automatic” nervous system because it works without our knowledge of it. The role of the autonomic nervous system is to maintain normal functions of the body that operate involuntarily (without our input). For example, it oversees and regulates heart rate, blood pressure, breathing, digestion and temperature control of the body.

The autonomic nervous system has two parts, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The neurons (nerve cells) of the sympathetic nervous system are located in the spinal cord from the first thoracic (T1) to the second lumbar (L2) level. The neurons of the parasympathetic nervous system are located in the brainstem and in the sacral region (S2-4) of the spinal cord.

Sympathetic Nervous System
The sympathetic nervous system can be thought of as the “fight or flight” nervous system. It is activated by stress. It causes increased heart rate, blood pressure, blood flow to the muscles and widening of the pupils. For example, if a mouse is being attacked by a cat, the mouse's sympathetic nervous system is fully activated. Everything is being done to help the mouse escape. The mouse’s vision improves as its pupils dilate; its increased heart rate, blood pressure and blood flow to the muscles give it the greatest physical ability to escape. The same reactions occur in humans when the sympathetic nervous system is activated.

Parasympathetic Nervous System
The parasympathetic nervous system can be thought of as the “rest and digestion” nervous system. Activation of this system causes a decrease in heart rate and blood pressure but an increase in the movement of the intestines. All of the functions of the parasympathetic nervous system aid in saving or creating energy for use.

Most organs in the body are supplied by both the sympathetic and parasympathetic nervous system.

Spinal Cord Blood Supply

Spinal Cord Arteries
The spinal cord has a blood supply from two major arteries; they are named by their location on the spinal cord: anterior refers to front and posterior refers to back. On the front surface of the spinal cord there is one anterior spinal artery that lies in the midline region. On the back side of the spinal cord there are two posterior spinal arteries that lie on the surface of the spinal cord just to the left and the right of the midline. Both of these arteries travel up and down the length of the spinal cord and give off small branches to supply the spinal cord with nutrients and oxygen. The anterior spinal artery supplies the front 2/3 of the spinal cord, while the posterior spinal arteries supply the back 1/3 of the spinal cord. Radicular arteries are smaller arteries that arise from the spinal arteries and travel in along the spinal nerves to each segment of the spinal cord.

Spinal Cord Veins
Six veins run along the surface of the spinal cord (three on the front and three along the back side of the cord). These veins function to drain blood from the spinal cord that was supplied by arteries

 

Spinal Nerves
There are 31 pairs of spinal nerves that come off the spinal cord. The spinal nerves are named by their association with the vertebra that they exit before branching off to the rest of the body. In the cervical region the spinal nerves exit above the vertebra for which they are named. For example, C1 spinal nerve leaves the spinal cord and exits through the foramen above the first cervical vertebra (C1) and the C7 spinal nerve exits above the C7 cervical vertebra. A difference takes place between the C7 and T1 vertebrae. Here the nerve between these two vertebrae is named C8 although there is no C8 vertebra. This leads to a change in the rest of the naming of the spinal nerves. As mentioned, the spinal nerve above T1 is called C8. The spinal nerve that exits below T1 is named the T1 spinal nerve. In all of the vertebrae from T1 and below, the spinal nerve is named for the vertebra above it. For example, the T8 spinal nerves exit between the T8 and T9 vertebrae (the T8 vertebra is above the T8 nerves), and this is the naming process for all of the thoracic, lumbar and sacral nerves.

Each spinal nerve carries fibers that travel both to and away from the spinal cord. The fibers that travel in a direction to the spinal cord from the rest of the body are referred to as afferent. These types of fibers are responsible for feeling such as, hot, cold, vibration and position of an arm or leg. The fibers that travel in a direction from the spinal cord to a muscle, skin or organ are referred to as efferent. Nerves that travel to a muscle and make it move when receiving a message from the brain are a type of efferent nerve that is called a motor nerve.