Spinal nerves - overview
Spinal nerves are part of your Peripheral Nervous System and are responsible for voluntary movement and sensation.
You have 31 pairs of spinal nerves that enter and leave your spinal cord at each vertebral level. A large number of these nerves are concentrated in your conus medullaris. These are referred to as the cauda equina (horse’s tail).
Your Peripheral Nervous System also contains autonomic nerves and these are responsible for involuntary functions of the body. Autonomic nerves assist with the functioning of organs such as your heart, intestines, bladder, bowel and sexual organs.
The autonomic nerves are made up of parasympathetic and sympathetic. They work together to keep the body functions in balance. Sympathetic response increases activity and parasympathetic response decreases activity (for example heart rate up and down). The diagram below shows which areas contain autonomic nerves.
Because there are many spinal and autonomic nerves in the end of your spinal cord, it is called a major reflex centre. Reflex (unconscious) activity takes place between your spinal cord and skeletal muscles, as well as your bladder, bowel and sexual organs.
White and grey matter
Your spinal cord contains both white matter and grey matter. The white matter is nerves that are covered with myelin, which is made of fat and protein, making them white. The grey matter neurons have no myelin covering, so they look grey. They each do different jobs.
Grey matter
The grey matter forms a butterfly-shaped area in the central part of your spinal cord. The back part of the wing of the butterfly shape is referred to as the posterior (dorsal grey horn). This area receives information from receptors or sensors in your skeletal muscles, organs and glands. This sensory information is passed up your spinal tracts to the brain (ascending).
The front part of the wing of the butterfly shape is referred to as the anterior (ventral grey horn). Spinal nerves exiting here pass on information received from the brain to your skeletal muscles (descending). This is referred to as motor information. Your autonomic nerves pass information to your organs, glands and blood vessels.
White matter
The white matter in your spinal cord contains spinal tracts. These are large bundles of nerves that carry information up and down your spinal cord.
There are a number of spinal tracts in your spinal cord, and as shown in the diagram below, some are sensory (ascending) and some are motor (descending).
Some of your spinal tracts are:
- Corticospinal tracts. These are descending or motor tracts, which convey nerve impulses from your brain to various skeletal muscles. These impulses cause controlled and precise movements of your body. This tract also conveys nerve impulses to your external anal and bladder sphincter muscles, allowing you conscious control. Nerves that make up the descending or motor tracts in the spinal cord are referred to as upper motor neurons.
- Spinothalamic tracts. These are ascending or sensory tracts. They convey information to your brain about your body temperature and pain.
- Posterior tracts. These are large ascending or sensory tracts at the back of your spinal cord. These tracts take messages from your skin to your brain about touch and pressure. They also convey information about vibrations that your body experiences, awareness of body position, and movement of your muscles and joints.
The following detailed diagram shows the complexity of the makeup of the spinal cord, with white and grey matter and spinal tracts.
