Spinal Cord Anatomy
- Spinal Cord Overview
- Ascending and Descending Spinal Tracts
- Spinal Nerves
- Spinal Cord Level Numbering
- Diagram of Spinal Cord Nerves and Levels
Spinal Cord Overview
The spinal cord is connected to the brain and is about the diameter of a human finger. From the brain the spinal cord descends down the middle of the back and is surrounded and protected by the bony vertebral column. The spinal cord is surrounded by a clear fluid called cerebral spinal fluid (CSF), that acts as a cushion to protect the delicate nerve tissues against damage from banging against the inside of the vertebrae.
The anatomy of the spinal cord itself, consists of millions of nerve fibres which transmit electrical information to and from the limbs, trunk and organs of the body, back to and from the brain. The nerves which exit the spinal cord in the upper section, the neck, control breathing and the arms. The nerves which exit the spinal cord in the mid and lower section of the back, control the trunk and legs, as well as bladder, bowel and sexual function.
The nerves which carry information from the brain to muscles are called motor neurones. The nerves which carry information from the body back to the brain are called sensory neurones. Sensory neurones carry information to the brain about skin temperature, touch, pain and joint position.
The brain and spinal cord are referred to as the central nervous system (CNS), whilst the nerves connecting the spinal cord to the body are referred to as the peripheral nervous system (PNS).
Ascending and Descending Spinal Tracts
The nerves within the spinal cord are grouped together in different bundles called ascending and descending tracts.
- Ascending tracts within the spinal cord carry sensory information from the body upwards to the brain such as touch, skin temperature, pain and joint position.
- Descending tracts within the spinal cord carry information from the brain downwards to initiate movement and control body functions.
Nerves called the spinal nerves or nerve roots, branch off the spinal cord and pass out through a hole in each of the vertebrae called the foramen. These nerves carry information from the spinal cord to the rest of the body, and from the body back up to the brain.
There are four main groups of spinal nerves, which exit different levels of the spinal cord.
These are in descending order down the vertebral column:
- Cervical Nerves "C" : (nerves in the neck) supply movement and feeling to the arms, neck and upper trunk. Also control breathing.
- Thoracic Nerves "T" : (nerves in the upper back) supply the trunk and abdomen.
- Lumbar Nerves "L" and Sacral Nerves "S" : (nerves in the lower back) supply the legs, the bladder, bowel and sexual organs.
Spinal Cord Level Numbering System
The spinal nerves carry information to and from different levels (segments) in the spinal cord. Both the nerves and the segments in the spinal cord are numbered in a similar way to the vertebrae. The point at which the spinal cord ends is called the conus medullaris, and is the terminal end of the spinal cord. It occurs near lumbar nerves L1 and L2. After the spinal cord terminates, the spinal nerves continue as a bundle of nerves called the cauda equina. The upper end of the conus medullaris is usually not well defined.
There are 31 pairs of spinal nerves which branch off from the spinal cord. In the cervical region of the spinal cord, the spinal nerves exit above the vertebrae. A change occurs with the C7 vertebra however where the C8 spinal nerve exits the vertebra below the C7 vertebra. Therefore, there is an 8th cervical spinal nerve even though there is no 8th cervical vertebra. From the 1st thoracic vertebra downwards, all spinal nerves exit below their equivalent numbered vertebrae.
The spinal nerves which leave the spinal cord are numbered according to the vertebra at which they exit the spinal column. So, the spinal nerve T4, exits the spinal column through the foramen in the 4th thoracic vertebra. The spinal nerve L5 leaves the spinal cord from the conus medullaris, and travels along the cauda equina until it exits the 5th lumbar vertebra.
The level of the spinal cord segments do not relate exactly to the level of the vertebral bodies i.e. damage to the bone at a particular level e.g. L5 vertebrae does not necessarily mean damage to the spinal cord at the same spinal nerve level.
Spinal Cord Nerve Levels
Diagram showing the relationship between spinal nerve roots and vertebrae