Spinal cord and peripheral nervous system
The spinal cord runs from the medulla through the backbone. 12 pairs of cranial nerves and 31 pairs of spinal nerves form the peripheral nervous system.
The spinal cord
The spinal cord is a long, tube-like part of the central nervous system. It starts at the medulla oblongata and runs down inside the vertebral column (backbone). The vertebrae protect it, and meninges and CSF cover it. In a cross-section, the grey matter is inside and shaped like a butterfly (or the letter H). The white matter is outside. This is the opposite of the brain. A narrow central canal filled with CSF runs through the middle. The spinal cord carries messages up to the brain and orders down to the body. It is also the centre for reflex actions.
Nerves of the peripheral nervous system
| Nerve | Number | Comes from | Kind |
|---|---|---|---|
| Cranial nerves | 12 pairs | Brain | Sensory, motor or mixed (e.g. the optic nerve is sensory, for sight) |
| Spinal nerves | 31 pairs: cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1 | Spinal cord | All mixed: sensory and motor fibres together |
Kinds of nerves
- Sensory (afferent) nerves carry messages from sense organs to the brain or spinal cord, e.g. the optic nerve.
- Motor (efferent) nerves carry orders from the brain or spinal cord to muscles and glands.
- Mixed nerves have both sensory and motor fibres. All 31 pairs of spinal nerves are mixed.
- Each spinal nerve joins the cord by two roots: a dorsal root (sensory, with a swelling called the dorsal root ganglion) and a ventral root (motor).
How to draw the T.S. of the spinal cord
- 1Draw a large oval. Label white matter.
- 2Inside, draw a butterfly (H) shape. Label grey matter.
- 3Put a small dot in the centre. Label central canal.
- 4Draw a thin line around the oval. Label meninges.
- 5On the upper (back) side, draw a nerve root with a small swelling. Label dorsal root and dorsal root ganglion.
- 6On the lower (front) side, draw another root. Label ventral root. Join both roots outside into one spinal nerve.
Spinal cord injuries in road accidents can stop messages to the legs, leaving them unable to move. Helmets and seat belts save lives, and bending your knees when you lift a heavy water pot protects your backbone.
2 marks: “How many pairs of cranial and spinal nerves are there in humans?” 12 pairs of cranial nerves and 31 pairs of spinal nerves.
2 marks: “Why are spinal nerves called mixed nerves?” Each has both sensory fibres (dorsal root) and motor fibres (ventral root).
4 marks: “Describe the structure and functions of the spinal cord.” Starts at the medulla, runs in the vertebral column; covered by meninges and CSF; inner H-shaped grey matter, outer white matter, central canal; 31 pairs of spinal nerves; carries impulses to and from the brain; reflex centre.
12 from the head, 31 down the back. Spinal groups: 8, 12, 5, 5, 1.